HTTP Working Group R. Fielding
INTERNET-DRAFT UC Irvine
<draft-ietf-http-v11-spec-rev-00.txt> J. Gettys
J. C. Mogul
DEC
H. Frystyk
T. Berners-Lee
MIT/LCS
Expires January 30, 1998 July 30, 1997
Hypertext Transfer Protocol -- HTTP/1.1
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are
working documents of the Internet Engineering Task Force
(IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of
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To learn the current status of any Internet-Draft, please
check the "1id-abstracts.txt" listing contained in the
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Coast).
Distribution of this document is unlimited. Please send
comments to the HTTP working group at <http-
wg@cuckoo.hpl.hp.com>. Discussions of the working group are
archived at <URL:http://www.ics.uci.edu/pub/ietf/http/>.
General discussions about HTTP and the applications which
use HTTP should take place on the <www-talk@w3.org> mailing
list.
Abstract
The Hypertext Transfer Protocol (HTTP) is an application-
level protocol for distributed, collaborative, hypermedia
information systems. It is a generic, stateless, object-
oriented protocol which can be used for many tasks, such as
name servers and distributed object management systems,
through extension of its request methods. A feature of HTTP
is the typing and negotiation of data representation,
allowing systems to be built independently of the data being
transferred.
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HTTP has been in use by the World-Wide Web global
information initiative since 1990. This specification
defines the protocol referred to as "HTTP/1.1".
The issues list for HTTP/1.1 can be found at:
http://www.w3.org/Protocols/HTTP/Issues/.
This draft does not resolve all open issues in the HTTP/1.1
specification requiring closure before HTTP/1.1 goes to
draft standard. It does, however, close most of them, and
note where in the document there are still significant
issues under discussion. The best way to view this document
is to get a copy of the Word 97 document found at:
http://www.w3.org/Protocols/HTTP/1.1/diff-v11-
RFC2068to08.doc; all issues are noted as comments in the
source document, with hyperlinks to the Issues list.
The most significant outstanding issue is OPTIONS; there is
a separate internet draft on the topic that you should
review NOT incorporated into this draft (though editorial
notes identify where changes may occur). This draft is
draft-ietf-http-options-00.txt.
Also an issue: AGE-CALCULATION; Roy Fielding has issued an
ID on the topic; Jeff Mogul intends to issue a draft as
well.
The editorial group is very interested in feedback on the
sample table of requirements in this draft (issue
REQUIREMENTS, section 1.9). Is it useful? How could it be
improved?
Open or drafting issues not incorporated into this draft
include: REDIRECTS, ENCODING-NOT-CONNEG, DATE_IF_MODIFIED,
403VS404, PUT-RANGE, HOST, AGE-CALCULATION, RE-
AUTHENTICATION-REQUESTED, VARY
Issues incorporated into this draft where there is still
controversy are noted in bold italic with an editor's note.
These are issues: CONTENT-ENCODING, CACHING-CGI.
Issues incorporated into this draft being working group last
called are: AUTH-CHUNKED, RETRY-AFTER, PROXY-REDIRECT
Closed issues incorporated into this draft include: PROXY-
AUTHORIZATION, PROXY-LENGTH, LANGUAGE-TAG, TSPECIALS,
STATUS100, QZERO, RANGE-ERROR, CLARIFY-NO-CACHE, COMMENT,
CONTENT-LOCATION, QUOTED-BACK, CACHE-CONTRA, CACHE-
DIRECTIVE, BYTE-RANGE, LWS-DELIMITER, CRLF, MAX-AGE,
100DATE, DISPOSITION, CHUNKED, CACHING, WARNINGS, VERSION,
PROXY-MAXAGE, CHARSET-WILDCARD, PADDING, CONNECTION, RANGES,
WARNING-8859, SHOULD-8859, X-BYTERANGES, MULTIPLE-TRANSFER-
CODINGS, LINK_HEADER.
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Editorial issues still open include: CLEANUP, UTF-8, URL-
SYNTAX, ENTITY, DOCKDIGEST, 1310_CACHE.
Editorial issues closed include: ACCEPT-RANGES, KEEP-ALIVE,
BNFNAME, KEYWORDS, RESPONSE-VERSION, XREF, COMMON-HEADERS,
NO-CACHE, FIX-REF, PERSIST-CONFUSED, CONNECTION2, GMT-UTC,
PROXY-FORWARD, REFERER-SEC, CHUNK-EXT, REMOVE_19.6,
IDEMPOTENT, REF-SIGCOMM, 1521-OBSOLETE, MESSAGE-BODY
Apologies for the extreme length; Microsoft Word exhibited a
fatal bug whenever trying to adjust margins when converting to
ascii text; therefore, the margins are extreme and the document
very long in ascii. Get the Postscript version off the Issues
list!
Fielding, et al [Page 3]
Table of Contents
HYPERTEXT TRANSFER PROTOCOL -- HTTP/1.1 ...................1
Status of this Memo .......................................1
Abstract ..................................................1
Table of Contents .........................................5
1 Introduction .........................................11
1.1 Purpose ...........................................11
1.2 Requirements ......................................11
1.3 Terminology .......................................12
1.4 Overall Operation .................................15
2 Notational Conventions and Generic Grammar ...........17
2.1 Augmented BNF .....................................17
2.2 Basic Rules .......................................19
3 Protocol Parameters ..................................20
3.1 HTTP Version ......................................20
3.2 Uniform Resource Identifiers ......................22
3.2.1 General Syntax .................................22
3.2.2 http URL .......................................23
3.2.3 URI Comparison .................................24
3.3 Date/Time Formats .................................24
3.3.1 Full Date ......................................24
3.3.2 Delta Seconds ..................................25
3.4 Character Sets ....................................25
3.5 Content Codings ...................................26
3.6 Transfer Codings ..................................27
3.7 Media Types .......................................29
3.7.1 Canonicalization and Text Defaults .............30
3.7.2 Multipart Types ................................31
3.8 Product Tokens ....................................31
3.9 Quality Values ....................................32
3.10 Language Tags .....................................32
3.11 Entity Tags .......................................33
3.12 Range Units .......................................33
4 HTTP Message .........................................34
4.1 Message Types .....................................34
4.2 Message Headers ...................................34
4.3 Message Body ......................................35
4.4 Message Length ....................................36
4.5 General Header Fields .............................37
5 Request ..............................................38
5.1 Request-Line ......................................38
5.1.1 Method .........................................38
5.1.2 Request-URI ....................................39
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5.2 The Resource Identified by a Request ..............41
5.3 Request Header Fields .............................41
6 Response .............................................42
6.1 Status-Line .......................................43
6.1.1 Status Code and Reason Phrase ..................43
6.2 Response Header Fields ............................45
7 Entity ...............................................45
7.1 Entity Header Fields ..............................46
7.2 Entity Body .......................................46
7.2.1 Type ...........................................47
7.2.2 Length .........................................47
8 Connections ..........................................47
8.1 Persistent Connections ............................47
8.1.1 Purpose ........................................47
8.1.2 Overall Operation ..............................48
8.1.3 Proxy Servers ..................................49
8.1.4 Practical Considerations .......................50
8.2 Message Transmission Requirements .................51
9 Method Definitions ...................................54
9.1 Safe and Idempotent Methods .......................55
9.1.1 Safe Methods ...................................55
9.1.2 Idempotent Methods .............................55
9.2 OPTIONS ...........................................56
9.3 GET ...............................................56
9.4 HEAD ..............................................57
9.5 POST ..............................................57
9.6 PUT ...............................................58
9.7 DELETE ............................................59
9.8 TRACE .............................................60
10 Status Code Definitions ............................60
10.1 Informational 1xx .................................61
10.1.1 100 Continue ...................................61
10.1.2 101 Switching Protocols ........................61
10.2 Successful 2xx ....................................62
10.2.1 200 OK .........................................62
10.2.2 201 Created ....................................62
10.2.3 202 Accepted ...................................62
10.2.4 203 Non-Authoritative Information ..............63
10.2.5 204 No Content .................................63
10.2.6 205 Reset Content ..............................63
10.2.7 206 Partial Content ............................63
10.3 Redirection 3xx ...................................64
10.3.1 300 Multiple Choices ...........................64
10.3.2 301 Moved Permanently ..........................65
10.3.3 302 Moved Temporarily ..........................65
10.3.4 303 See Other ..................................66
10.3.5 304 Not Modified ...............................66
10.3.6 305 Use Proxy ..................................67
10.4 Client Error 4xx ..................................68
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10.4.1 400 Bad Request ................................68
10.4.2 401 Unauthorized ...............................68
10.4.3 402 Payment Required ...........................69
10.4.4 403 Forbidden ..................................69
10.4.5 404 Not Found ..................................69
10.4.6 405 Method Not Allowed .........................69
10.4.7 406 Not Acceptable .............................69
10.4.8 407 Proxy Authentication Required ..............70
10.4.9 408 Request Timeout ............................70
10.4.10 ...............................409 Conflict 70
10.4.11 ...................................410 Gone 71
10.4.12 ........................411 Length Required 71
10.4.13 ....................412 Precondition Failed 71
10.4.14 ...............413 Request Entity Too Large 71
10.4.15 ...................414 Request-URI Too Long 72
10.4.16 .................415 Unsupported Media Type 72
10.5 Server Error 5xx ..................................73
10.5.1 500 Internal Server Error ......................73
10.5.2 501 Not Implemented ............................73
10.5.3 502 Bad Gateway ................................73
10.5.4 503 Service Unavailable ........................73
10.5.5 504 Gateway Timeout ............................73
10.5.6 505 HTTP Version Not Supported .................74
11 Access Authentication ..............................74
11.1 Basic Authentication Scheme .......................76
11.2 Digest Authentication Scheme ......................77
12 Content Negotiation ................................77
12.1 Server-driven Negotiation .........................78
12.2 Agent-driven Negotiation ..........................79
12.3 Transparent Negotiation ...........................79
13 Caching in HTTP ....................................80
13.1.1 Cache Correctness ..............................81
13.1.2 Warnings .......................................82
13.1.3 Cache-control Mechanisms .......................83
13.1.4 Explicit User Agent Warnings ...................84
13.1.5 Exceptions to the Rules and Warnings ...........84
13.1.6 Client-controlled Behavior .....................85
13.2 Expiration Model ..................................85
13.2.1 Server-Specified Expiration ....................85
13.2.2 Heuristic Expiration ...........................86
13.2.3 Age Calculations ...............................86
13.2.4 Expiration Calculations ........................89
13.2.5 Disambiguating Expiration Values ...............90
13.2.6 Disambiguating Multiple Responses ..............90
13.3 Validation Model ..................................91
13.3.1 Last-modified Dates ............................92
13.3.2 Entity Tag Cache Validators ....................92
13.3.3 Weak and Strong Validators .....................92
13.3.4 Rules for When to Use Entity Tags and Last-
modified Dates ........................................95
13.3.5 Non-validating Conditionals ....................96
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13.4 Response Cachability ..............................96
13.5 Constructing Responses From Caches ................97
13.5.1 End-to-end and Hop-by-hop Headers ..............97
13.5.2 Non-modifiable Headers .........................98
13.5.3 Combining Headers ..............................99
13.5.4 Combining Byte Ranges .........................100
13.6 Caching Negotiated Responses .....................101
13.7 Shared and Non-Shared Caches .....................102
13.8 Errors or Incomplete Response Cache Behavior .....102
13.9 Side Effects of GET and HEAD .....................102
13.10 Invalidation After Updates or Deletions .........103
13.11 Write-Through Mandatory .........................103
13.12 Cache Replacement ...............................104
13.13 History Lists ...................................104
14 Header Field Definitions ..........................105
14.1 Accept ...........................................105
14.2 Accept-Charset ...................................107
14.3 Accept-Encoding ..................................108
14.4 Accept-Language ..................................109
14.5 Accept-Ranges ....................................110
14.6 Age ..............................................111
14.7 Allow ............................................111
14.8 Authorization ....................................112
14.9 Cache-Control ....................................113
14.9.1 What is Cachable ..............................114
14.9.2 What May be Stored by Caches ..................115
14.9.3 Modifications of the Basic Expiration Mechanism116
14.9.4 Cache Revalidation and Reload Controls ........118
14.9.5 No-Transform Directive ........................120
14.9.6 Cache Control Extensions ......................121
14.10 Connection ......................................122
14.11 Content-Base ....................................123
14.12 Content-Encoding ................................123
14.13 Content-Language ................................124
14.14 Content-Length ..................................125
14.15 Content-Location ................................125
14.16 Content-MD5 .....................................126
14.17 Content-Range ...................................127
14.18 Content-Type ....................................129
14.19 Date ............................................130
14.20 ETag ............................................131
14.21 Expires .........................................131
14.22 From ............................................132
14.23 Host ............................................133
14.24 If-Modified-Since ...............................134
14.25 If-Match ........................................135
14.26 If-None-Match ...................................136
14.27 If-Range ........................................137
14.28 If-Unmodified-Since .............................138
14.29 Last-Modified ...................................138
14.30 Location ........................................139
14.31 Max-Forwards ....................................140
14.32 Pragma ..........................................140
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14.33 Proxy-Authenticate ..............................141
14.34 Proxy-Authorization .............................142
14.35 Public ..........................................142
14.36 Range ...........................................143
14.36.1 ...............................Byte Ranges 143
14.36.2 ..................Range Retrieval Requests 144
14.37 Referer .........................................145
14.38 Retry-After .....................................145
14.39 Server ..........................................146
14.40 Transfer-Encoding ...............................146
14.41 Upgrade .........................................147
14.42 User-Agent ......................................148
14.43 Vary ............................................148
14.44 Via .............................................150
14.45 Warning .........................................151
14.46 WWW-Authenticate ................................154
15 Security Considerations ...........................157
15.1 Authentication of Clients ........................157
15.2 Offering a Choice of Authentication Schemes ......159
15.3 Abuse of Server Log Information ..................159
15.4 Transfer of Sensitive Information ................160
15.5 Attacks Based On File and Path Names .............160
15.6 Personal Information .............................161
15.7 Privacy Issues Connected to Accept Headers .......161
15.8 DNS Spoofing .....................................162
15.9 Location Headers and Spoofing ....................163
16 Acknowledgments ...................................165
17 References ........................................166
18 Authors' Addresses ................................170
19 Appendices ........................................171
19.1 Internet Media Type message/http .................171
19.2 Internet Media Type multipart/byteranges .........172
19.3 Tolerant Applications ............................173
19.4 Differences Between HTTP Entities and RFC 2045
Entities ...............................................173
19.4.1 Conversion to Canonical Form ..................174
19.4.2 Conversion of Date Formats ....................174
19.4.3 Introduction of Content-Encoding ..............175
19.4.4 No Content-Transfer-Encoding ..................175
19.4.5 HTTP Header Fields in Multipart Body-Parts ....175
19.4.6 Introduction of Transfer-Encoding .............175
19.4.7 MIME-Version ..................................176
19.5 Changes from HTTP/1.0 ............................176
19.5.1 Changes to Simplify Multi-homed Web Servers and
Conserve IP Addresses ................................176
19.6 Additional Features ..............................177
19.6.1 Additional Request Methods ....................178
19.6.2 Additional Header Field Definitions ...........178
19.7 Compatibility with Previous Versions .............178
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19.7.1 Compatibility with HTTP/1.0 Persistent Connections179
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1 Introduction
1.1 Purpose
The Hypertext Transfer Protocol (HTTP) is an application-
level protocol for distributed, collaborative, hypermedia
information systems. HTTP has been in use by the World-Wide
Web global information initiative since 1990. The first
version of HTTP, referred to as HTTP/0.9, was a simple
protocol for raw data transfer across the Internet.
HTTP/1.0, as defined by RFC 1945 [6], improved the protocol
by allowing messages to be in the format of MIME-like
messages, containing metainformation about the data
transferred and modifiers on the request/response semantics.
However, HTTP/1.0 does not sufficiently take into
consideration the effects of hierarchical proxies, caching,
the need for persistent connections, and virtual hosts. In
addition, the proliferation of incompletely-implemented
applications calling themselves "HTTP/1.0" has necessitated
a protocol version change in order for two communicating
applications to determine each other's true capabilities.
This specification defines the protocol referred to as
"HTTP/1.1". This protocol includes more stringent
requirements than HTTP/1.0 in order to ensure reliable
implementation of its features.
Practical information systems require more functionality
than simple retrieval, including search, front-end update,
and annotation. HTTP allows an open-ended set of methods
that indicate the purpose of a request. It builds on the
discipline of reference provided by the Uniform Resource
Identifier (URI) [3], as a location (URL) [4] or name (URN)
[20], for indicating the resource to which a method is to be
applied. Messages are passed in a format similar to that
used by Internet mail [9] as defined by the Multipurpose
Internet Mail Extensions (MIME) [7].
HTTP is also used as a generic protocol for communication
between user agents and proxies/gateways to other Internet
systems, including those supported by the SMTP [16], NNTP
[13], FTP [18], Gopher [2], and WAIS [10] protocols. In this
way, HTTP allows basic hypermedia access to resources
available from diverse applications.
1.2 Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
"SHALL NOT", "SHOULD", SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" in this document are to be interpreted as
described in RFC 2119 [34].
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An implementation is not compliant if it fails to satisfy
one or more of the MUST requirements for the protocols it
implements. An implementation that satisfies all the MUST
and all the SHOULD requirements for its protocols is said to
be "unconditionally compliant"; one that satisfies all the
MUST requirements but not all the SHOULD requirements for
its protocols is said to be "conditionally compliant."
1.3 Terminology
This specification uses a number of terms to refer to the
roles played by participants in, and objects of, the HTTP
communication.
connection
A transport layer virtual circuit established between two
programs for the purpose of communication.
message
The basic unit of HTTP communication, consisting of a
structured sequence of octets matching the syntax defined
in section 4 and transmitted via the connection.
request
An HTTP request message, as defined in section 5.
response
An HTTP response message, as defined in section 6.
resource
A network data object or service that can be identified
by a URI, as defined in section 3.2. Resources may be
available in multiple representations (e.g. multiple
languages, data formats, size, resolutions) or vary in
other ways.
entity
The information transferred as the payload of a request
or response. An entity consists of metainformation in the
form of entity-header fields and content in the form of
an entity-body, as described in section 7.
representation
An entity included with a response that is subject to
content negotiation, as described in section 12. There
may exist multiple representations associated with a
particular response status.
content negotiation
The mechanism for selecting the appropriate
representation when servicing a request, as described in
section 12. The representation of entities in any
response can be negotiated (including error responses).
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variant
A resource may have one, or more than one,
representation(s) associated with it at any given
instant. Each of these representations is termed a
`variant.' Use of the term `variant' does not necessarily
imply that the resource is subject to content
negotiation.
client
A program that establishes connections for the purpose of
sending requests.
user agent
The client which initiates a request. These are often
browsers, editors, spiders (web-traversing robots), or
other end user tools.
server
An application program that accepts connections in order
to service requests by sending back responses. Any given
program may be capable of being both a client and a
server; our use of these terms refers only to the role
being performed by the program for a particular
connection, rather than to the program's capabilities in
general. Likewise, any server may act as an origin
server, proxy, gateway, or tunnel, switching behavior
based on the nature of each request.
origin server
The server on which a given resource resides or is to be
created.
proxy
An intermediary program which acts as both a server and a
client for the purpose of making requests on behalf of
other clients. Requests are serviced internally or by
passing them on, with possible translation, to other
servers. A proxy must implement both the client and
server requirements of this specification.
gateway
A server which acts as an intermediary for some other
server. Unlike a proxy, a gateway receives requests as if
it were the origin server for the requested resource; the
requesting client may not be aware that it is
communicating with a gateway.
tunnel
An intermediary program which is acting as a blind relay
between two connections. Once active, a tunnel is not
considered a party to the HTTP communication, though the
tunnel may have been initiated by an HTTP request. The
tunnel ceases to exist when both ends of the relayed
connections are closed.
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cache
A program's local store of response messages and the
subsystem that controls its message storage, retrieval,
and deletion. A cache stores cachable responses in order
to reduce the response time and network bandwidth
consumption on future, equivalent requests. Any client or
server may include a cache, though a cache cannot be used
by a server that is acting as a tunnel.
cachable
A response is cachable if a cache is allowed to store a
copy of the response message for use in answering
subsequent requests. The rules for determining the
cachability of HTTP responses are defined in section 13.
Even if a resource is cachable, there may be additional
constraints on whether a cache can use the cached copy
for a particular request.
first-hand
A response is first-hand if it comes directly and without
unnecessary delay from the origin server, perhaps via one
or more proxies. A response is also first-hand if its
validity has just been checked directly with the origin
server.
explicit expiration time
The time at which the origin server intends that an
entity should no longer be returned by a cache without
further validation.
heuristic expiration time
An expiration time assigned by a cache when no explicit
expiration time is available.
age
The age of a response is the time since it was sent by,
or successfully validated with, the origin server.
freshness lifetime
The length of time between the generation of a response
and its expiration time.
fresh
A response is fresh if its age has not yet exceeded its
freshness lifetime.
stale
A response is stale if its age has passed its freshness
lifetime.
semantically transparent
A cache behaves in a "semantically transparent" manner,
with respect to a particular response, when its use
affects neither the requesting client nor the origin
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server, except to improve performance. When a cache is
semantically transparent, the client receives exactly the
same response (except for hop-by-hop headers) that it
would have received had its request been handled directly
by the origin server.
validator
A protocol element (e.g., an entity tag or a Last-
Modified time) that is used to find out whether a cache
entry is an equivalent copy of an entity.
1.4 Overall Operation
The HTTP protocol is a request/response protocol. A client
sends a request to the server in the form of a request
method, URI, and protocol version, followed by a MIME-like
message containing request modifiers, client information,
and possible body content over a connection with a server.
The server responds with a status line, including the
message's protocol version and a success or error code,
followed by a MIME-like message containing server
information, entity metainformation, and possible entity-
body content. The relationship between HTTP and MIME is
described in appendix 19.4.
Most HTTP communication is initiated by a user agent and
consists of a request to be applied to a resource on some
origin server. In the simplest case, this may be
accomplished via a single connection (v) between the user
agent (UA) and the origin server (O).
request chain ------------------------>
UA -------------------v------------------- O
<----------------------- response chain
A more complicated situation occurs when one or more
intermediaries are present in the request/response chain.
There are three common forms of intermediary: proxy,
gateway, and tunnel. A proxy is a forwarding agent,
receiving requests for a URI in its absolute form, rewriting
all or part of the message, and forwarding the reformatted
request toward the server identified by the URI. A gateway
is a receiving agent, acting as a layer above some other
server(s) and, if necessary, translating the requests to the
underlying server's protocol. A tunnel acts as a relay point
between two connections without changing the messages;
tunnels are used when the communication needs to pass
through an intermediary (such as a firewall) even when the
intermediary cannot understand the contents of the messages.
request chain -------------------------------------->
UA -----v----- A -----v----- B -----v----- C -----v----- O
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<------------------------------------- response chain
The figure above shows three intermediaries (A, B, and C)
between the user agent and origin server. A request or
response message that travels the whole chain will pass
through four separate connections. This distinction is
important because some HTTP communication options may apply
only to the connection with the nearest, non-tunnel
neighbor, only to the end-points of the chain, or to all
connections along the chain. Although the diagram is linear,
each participant may be engaged in multiple, simultaneous
communications. For example, B may be receiving requests
from many clients other than A, and/or forwarding requests
to servers other than C, at the same time that it is
handling A's request.
Any party to the communication which is not acting as a
tunnel may employ an internal cache for handling requests.
The effect of a cache is that the request/response chain is
shortened if one of the participants along the chain has a
cached response applicable to that request. The following
illustrates the resulting chain if B has a cached copy of an
earlier response from O (via C) for a request which has not
been cached by UA or A.
request chain ---------->
UA -----v----- A -----v----- B - - - - - - C - - - - - - O
<--------- response chain
Not all responses are usefully cachable, and some requests
may contain modifiers which place special requirements on
cache behavior. HTTP requirements for cache behavior and
cachable responses are defined in section 13.
In fact, there are a wide variety of architectures and
configurations of caches and proxies currently being
experimented with or deployed across the World Wide Web;
these systems include national hierarchies of proxy caches
to save transoceanic bandwidth, systems that broadcast or
multicast cache entries, organizations that distribute
subsets of cached data via CD-ROM, and so on. HTTP systems
are used in corporate intranets over high-bandwidth links,
and for access via PDAs with low-power radio links and
intermittent connectivity. The goal of HTTP/1.1 is to
support the wide diversity of configurations already
deployed while introducing protocol constructs that meet the
needs of those who build web applications that require high
reliability and, failing that, at least reliable indications
of failure.
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HTTP communication usually takes place over TCP/IP
connections. The default port is TCP 80[19], but other ports
can be used. This does not preclude HTTP from being
implemented on top of any other protocol on the Internet, or
on other networks. HTTP only presumes a reliable transport;
any protocol that provides such guarantees can be used; the
mapping of the HTTP/1.1 request and response structures onto
the transport data units of the protocol in question is
outside the scope of this specification.
In HTTP/1.0, most implementations used a new connection for
each request/response exchange. In HTTP/1.1, a connection
may be used for one or more request/response exchanges,
although connections may be closed for a variety of reasons
(see section 8.1).
2 Notational Conventions and Generic Grammar
2.1 Augmented BNF
All of the mechanisms specified in this document are
described in both prose and an augmented Backus-Naur Form
(BNF) similar to that used by RFC 822 [9]. Implementers will
need to be familiar with the notation in order to understand
this specification. The augmented BNF includes the following
constructs:
name = definition
The name of a rule is simply the name itself (without
any enclosing "<" and ">") and is separated from its
definition by the equal "=" character. Whitespace is
only significant in that indentation of continuation
lines is used to indicate a rule definition that spans
more than one line. Certain basic rules are in
uppercase, such as SP, LWS, HT, CRLF, DIGIT, ALPHA,
etc. Angle brackets are used within definitions
whenever their presence will facilitate discerning the
use of rule names.
"literal"
Quotation marks surround literal text. Unless stated
otherwise, the text is case-insensitive.
rule1 | rule2
Elements separated by a bar ("|") are alternatives,
e.g., "yes | no" will accept yes or no.
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(rule1 rule2)
Elements enclosed in parentheses are treated as a
single element. Thus, "(elem (foo | bar) elem)" allows
the token sequences "elem foo elem" and
"elem bar elem".
*rule
The character "*" preceding an element indicates
repetition. The full form is "<n>*<m>element"
indicating at least <n> and at most <m> occurrences of
element. Default values are 0 and infinity so that
"*(element)" allows any number, including zero;
"1*element" requires at least one; and "1*2element"
allows one or two.
[rule]
Square brackets enclose optional elements; "[foo bar]"
is equivalent to "*1(foo bar)".
N rule
Specific repetition: "<n>(element)" is equivalent to
"<n>*<n>(element)"; that is, exactly <n> occurrences of
(element). Thus 2DIGIT is a 2-digit number, and 3ALPHA
is a string of three alphabetic characters.
#rule
A construct "#" is defined, similar to "*", for
defining lists of elements. The full form is
"<n>#<m>element " indicating at least <n> and at most
<m> elements, each separated by one or more commas
(",") and optional linear whitespace (LWS). This makes
the usual form of lists very easy; a rule such as
"( *LWS element *( *LWS "," *LWS element )) " can be
shown as "1#element". Wherever this construct is used,
null elements are allowed, but do not contribute to the
count of elements present. That is, "(element), ,
(element) " is permitted, but counts as only two
elements. Therefore, where at least one element is
required, at least one non-null element must be
present. Default values are 0 and infinity so that
"#element" allows any number, including zero;
"1#element" requires at least one; and "1#2element"
allows one or two.
; comment
A semi-colon, set off some distance to the right of
rule text, starts a comment that continues to the end
of line. This is a simple way of including useful notes
in parallel with the specifications.
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implied *LWS
The grammar described by this specification is word-
based. Except where noted otherwise, linear whitespace
(LWS) can be included between any two adjacent words
(token or quoted-string), and between adjacent tokens
and separators, without changing the interpretation of
a field. At least one delimiter (LWS and/or separators)
must exist between any two tokens, since they would
otherwise be interpreted as a single token.
2.2 Basic Rules
The following rules are used throughout this specification
to describe basic parsing constructs. The US-ASCII coded
character set is defined by ANSI X3.4-1986 [21].
OCTET = <any 8-bit sequence of data>
CHAR = <any US-ASCII character (octets 0 - 127)>
UPALPHA = <any US-ASCII uppercase letter "A".."Z">
LOALPHA = <any US-ASCII lowercase letter "a".."z">
ALPHA = UPALPHA | LOALPHA
DIGIT = <any US-ASCII digit "0".."9">
CTL = <any US-ASCII control character
(octets 0 - 31) and DEL (127)>
CR = <US-ASCII CR, carriage return (13)>
LF = <US-ASCII LF, linefeed (10)>
SP = <US-ASCII SP, space (32)>
HT = <US-ASCII HT, horizontal-tab (9)>
<"> = <US-ASCII double-quote mark (34)>
HTTP/1.1 defines the sequence CR LF as the end-of-line
marker for all protocol elements except the entity-body (see
appendix 19.3 for tolerant applications). The end-of-line
marker within an entity-body is defined by its associated
media type, as described in section 3.7.
CRLF = CR LF
HTTP/1.1 headers can be folded onto multiple lines if the
continuation line begins with a space or horizontal tab. All
linear white space, including folding, has the same
semantics as SP.
LWS = [CRLF] 1*( SP | HT )
The TEXT rule is only used for descriptive field contents
and values that are not intended to be interpreted by the
message parser. Words of *TEXT may contain characters from
character sets other than ISO 8859-1 [22] only when encoded
according to the rules of RFC 2047 [14].
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TEXT = <any OCTET except CTLs,
but including LWS>
Hexadecimal numeric characters are used in several protocol
elements.
HEX = "A" | "B" | "C" | "D" | "E" | "F"
| "a" | "b" | "c" | "d" | "e" | "f" | DIGIT
Many HTTP/1.1 header field values consist of words separated
by LWS or special characters. These special characters MUST
be in a quoted string to be used within a parameter value.
token = 1*<any CHAR except CTLs or separators>
separators = "(" | ")" | "<" | ">" | "@"
| "," | ";" | ":" | "\" | <">
| "/" | "[" | "]" | "?" | "="
| "{" | "}" | SP | HT
Comments can be included in some HTTP header fields by
surrounding the comment text with parentheses. Comments are
only allowed in fields containing "comment" as part of their
field value definition. In all other fields, parentheses are
considered part of the field value.
comment = "(" *( ctext | quoted-pair | comment ) ")"
ctext = <any TEXT excluding "(" and ")">
A string of text is parsed as a single word if it is quoted
using double-quote marks.
quoted-string = ( <"> *(qdtext | quoted-pair ) <"> )
qdtext = <any TEXT except <">>
The backslash character ("\") may be used as a single-
character quoting mechanism only within quoted-string and
comment constructs.
quoted-pair = "\" CHAR
3 Protocol Parameters
3.1 HTTP Version
HTTP uses a "<major>.<minor>" numbering scheme to indicate
versions of the protocol. The protocol versioning policy is
intended to allow the sender to indicate the format of a
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message and its capacity for understanding further HTTP
communication, rather than the features obtained via that
communication. No change is made to the version number for
the addition of message components which do not affect
communication behavior or which only add to extensible field
values. The <minor> number is incremented when the changes
made to the protocol add features which do not change the
general message parsing algorithm, but which may add to the
message semantics and imply additional capabilities of the
sender. The <major> number is incremented when the format of
a message within the protocol is changed. See RFC 2145 [36]
for a fuller explanation.
The version of an HTTP message is indicated by an HTTP-
Version field in the first line of the message.
HTTP-Version = "HTTP" "/" 1*DIGIT "." 1*DIGIT
Note that the major and minor numbers MUST be treated as
separate integers and that each may be incremented higher
than a single digit. Thus, HTTP/2.4 is a lower version than
HTTP/2.13, which in turn is lower than HTTP/12.3. Leading
zeros MUST be ignored by recipients and MUST NOT be sent.
Applications sending Request or Response messages, as
defined by this specification, MUST include an HTTP-Version
of "HTTP/1.1". Use of this version number indicates that the
sending application is at least conditionally compliant with
this specification.
The HTTP version of an application is the highest HTTP
version for which the application is at least conditionally
compliant.
Proxy and gateway applications must be careful when
forwarding messages in protocol versions different from that
of the application. Since the protocol version indicates the
protocol capability of the sender, a proxy/gateway MUST
never send a message with a version indicator which is
greater than its actual version; if a higher version request
is received, the proxy/gateway MUST either downgrade the
request version, respond with an error, or switch to tunnel
behavior. Requests with a version lower than that of the
proxy/gateway's version MAY be upgraded before being
forwarded; the proxy/gateway's response to that request MUST
be in the same major version as the request.
Note: Converting between versions of HTTP may involve
modification of header fields required or forbidden by
the versions involved.
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3.2 Uniform Resource Identifiers
URIs have been known by many names: WWW addresses, Universal
Document Identifiers, Universal Resource Identifiers [3],
and finally the combination of Uniform Resource Locators
(URL) [4] and Names (URN) [20]. As far as HTTP is concerned,
Uniform Resource Identifiers are simply formatted strings
which identify--via name, location, or any other
characteristic--a resource.
3.2.1 General Syntax
URIs in HTTP can be represented in absolute form or relative
to some known base URI [11], depending upon the context of
their use. The two forms are differentiated by the fact that
absolute URIs always begin with a scheme name followed by a
colon.
URI = ( absoluteURI | relativeURI ) [ "#" fragment ]
absoluteURI = scheme ":" *( uchar | reserved )
relativeURI = net_path | abs_path | rel_path
net_path = "//" net_loc [ abs_path ]
abs_path = "/" rel_path
rel_path = [ path ] [ ";" params ] [ "?" query ]
path = fsegment *( "/" segment )
fsegment = 1*pchar
segment = *pchar
params = param *( ";" param )
param = *( pchar | "/" )
scheme = 1*( ALPHA | DIGIT | "+" | "-" | "." )
net_loc = *( pchar | ";" | "?" )
query = *( uchar | reserved )
fragment = *( uchar | reserved )
pchar = uchar | ":" | "@" | "&" | "=" | "+"
uchar = unreserved | escape
unreserved = ALPHA | DIGIT | safe | extra | national
escape = "%" HEX HEX
reserved = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+"
extra = "!" | "*" | "'" | "(" | ")" | ","
safe = "$" | "-" | "_" | "."
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unsafe = CTL | SP | <"> | "#" | "%" | "<" | ">"
national = <any OCTET excluding ALPHA, DIGIT,
reserved, extra, safe, and unsafe>
For definitive information on URL syntax and semantics, see
RFC 1738 [4] and RFC 1808 [11]. The BNF above includes
national characters not allowed in valid URLs as specified
by RFC 1738, since HTTP servers are not restricted in the
set of unreserved characters allowed to represent the
rel_path part of addresses, and HTTP proxies may receive
requests for URIs not defined by RFC 1738.
The HTTP protocol does not place any a priori limit on the
length of a URI. Servers MUST be able to handle the URI of
any resource they serve, and SHOULD be able to handle URIs
of unbounded length if they provide GET-based forms that
could generate such URIs. A server SHOULD return 414
(Request-URI Too Long) status if a URI is longer than the
server can handle (see section 10.4.15).
Note: Servers should be cautious about depending on URI
lengths above 255 bytes, because some older client or
proxy implementations may not properly support these
lengths.
3.2.2 http URL
The "http" scheme is used to locate network resources via
the HTTP protocol. This section defines the scheme-specific
syntax and semantics for http URLs.
http_URL = "http:" "//" host [ ":" port ] [ abs_path ]
host = <A legal Internet host domain name or IP
address (in dotted-decimal form),
as defined by Section 2.1 of RFC 1123>
port = *DIGIT
If the port is empty or not given, port 80 is assumed. The
semantics are that the identified resource is located at the
server listening for TCP connections on that port of that
host, and the Request-URI for the resource is abs_path. The
use of IP addresses in URL's SHOULD be avoided whenever
possible (see RFC 1900 [24]). If the abs_path is not present
in the URL, it MUST be given as "/" when used as a Request-
URI for a resource (section 5.1.2).
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3.2.3 URI Comparison
When comparing two URIs to decide if they match or not, a
client SHOULD use a case-sensitive octet-by-octet comparison
of the entire URIs, with these exceptions:
. A port that is empty or not given is equivalent to the
default port for that URI;
. Comparisons of host names MUST be case-insensitive;
. Comparisons of scheme names MUST be case-insensitive;
. An empty abs_path is equivalent to an abs_path of "/".
Characters other than those in the "reserved" and "unsafe"
sets (see section 3.2) are equivalent to their ""%" HEX HEX"
encodings.
For example, the following three URIs are equivalent:
http://abc.com:80/~smith/home.html
http://ABC.com/%7Esmith/home.html
http://ABC.com:/%7esmith/home.html
3.3 Date/Time Formats
3.3.1 Full Date
HTTP applications have historically allowed three different
formats for the representation of date/time stamps:
Sun, 06 Nov 1994 08:49:37 GMT ; RFC 822, updated by RFC 1123
Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036
Sun Nov 6 08:49:37 1994 ; ANSI C's asctime() format
The first format is preferred as an Internet standard and
represents a fixed-length subset of that defined by RFC 1123
[8] (an update to RFC 822 [9]). The second format is in
common use, but is based on the obsolete RFC 850 [12] date
format and lacks a four-digit year. HTTP/1.1 clients and
servers that parse the date value MUST accept all three
formats (for compatibility with HTTP/1.0), though they MUST
only generate the RFC 1123 format for representing HTTP-date
values in header fields.
Note: Recipients of date values are encouraged to be
robust in accepting date values that may have been sent
by non-HTTP applications, as is sometimes the case when
retrieving or posting messages via proxies/gateways to
SMTP or NNTP.
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All HTTP date/time stamps MUST be represented in Greenwich
Mean Time (GMT), without exception. For the purposes of
HTTP, GMT is exactly equal to UTC (Coordinated Universal
Time). This is indicated in the first two formats by the
inclusion of "GMT" as the three-letter abbreviation for time
zone, and MUST be assumed when reading the asctime format.
HTTP-date = rfc1123-date | rfc850-date | asctime-date
rfc1123-date = wkday "," SP date1 SP time SP "GMT"
rfc850-date = weekday "," SP date2 SP time SP "GMT"
asctime-date = wkday SP date3 SP time SP 4DIGIT
date1 = 2DIGIT SP month SP 4DIGIT
; day month year (e.g., 02 Jun 1982)
date2 = 2DIGIT "-" month "-" 2DIGIT
; day-month-year (e.g., 02-Jun-82)
date3 = month SP ( 2DIGIT | ( SP 1DIGIT ))
; month day (e.g., Jun 2)
time = 2DIGIT ":" 2DIGIT ":" 2DIGIT
; 00:00:00 - 23:59:59
wkday = "Mon" | "Tue" | "Wed"
| "Thu" | "Fri" | "Sat" | "Sun"
weekday = "Monday" | "Tuesday" | "Wednesday"
| "Thursday" | "Friday" | "Saturday"
| "Sunday"
month = "Jan" | "Feb" | "Mar" | "Apr"
| "May" | "Jun" | "Jul" | "Aug"
| "Sep" | "Oct" | "Nov" | "Dec"
Note: HTTP requirements for the date/time stamp format
apply only to their usage within the protocol stream.
Clients and servers are not required to use these
formats for user presentation, request logging, etc.
3.3.2 Delta Seconds
Some HTTP header fields allow a time value to be specified
as an integer number of seconds, represented in decimal,
after the time that the message was received.
delta-seconds = 1*DIGIT
3.4 Character Sets
HTTP uses the same definition of the term "character set" as
that described for MIME:
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The term "character set" is used in this document to
refer to a method used with one or more tables to
convert a sequence of octets into a sequence of
characters. Note that unconditional conversion in the
other direction is not required, in that not all
characters may be available in a given character set
and a character set may provide more than one sequence
of octets to represent a particular character. This
definition is intended to allow various kinds of
character encodings, from simple single-table mappings
such as US-ASCII to complex table switching methods
such as those that use ISO 2022's techniques. However,
the definition associated with a MIME character set
name MUST fully specify the mapping to be performed
from octets to characters. In particular, use of
external profiling information to determine the exact
mapping is not permitted.
Note: This use of the term "character set" is more
commonly referred to as a "character encoding."
However, since HTTP and MIME share the same registry,
it is important that the terminology also be shared.
HTTP character sets are identified by case-insensitive
tokens. The complete set of tokens is defined by the IANA
Character Set registry [19].
charset = token
Although HTTP allows an arbitrary token to be used as a
charset value, any token that has a predefined value within
the IANA Character Set registry [19] MUST represent the
character set defined by that registry. Applications SHOULD
limit their use of character sets to those defined by the
IANA registry.
3.5 Content Codings
Content coding values indicate an encoding transformation
that has been or can be applied to an entity. Content
codings are primarily used to allow a document to be
compressed or otherwise usefully transformed without losing
the identity of its underlying media type and without loss
of information. Frequently, the entity is stored in coded
form, transmitted directly, and only decoded by the
recipient.
content-coding = token
All content-coding values are case-insensitive. HTTP/1.1
uses content-coding values in the Accept-Encoding (section
14.3) and Content-Encoding (section 14.12) header fields.
Although the value describes the content-coding, what is
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more important is that it indicates what decoding mechanism
will be required to remove the encoding.
The Internet Assigned Numbers Authority (IANA) acts as a
registry for content-coding value tokens. Initially, the
registry contains the following tokens:
gzip An encoding format produced by the file compression
program "gzip" (GNU zip) as described in RFC 1952 [25].
This format is a Lempel-Ziv coding (LZ77) with a 32 bit
CRC.
compress
The encoding format produced by the common UNIX file
compression program "compress". This format is an
adaptive Lempel-Ziv-Welch coding (LZW).
Note: Use of program names for the identification of
encoding formats is not desirable and should be
discouraged for future encodings. Their use here is
representative of historical practice, not good design.
For compatibility with previous implementations of
HTTP, applications should consider "x-gzip" and "x-
compress" to be equivalent to "gzip" and "compress"
respectively.
deflate The "zlib" format defined in RFC 1950 [31] in
combination with the "deflate" compression mechanism
described in RFC 1951 [29].
identity
The default (identity) encoding; the use of no
transformation whatsoever. This content-coding is used
only in the Accept-Encoding header, and SHOULD NOT be
used in Content-Encoding header.
New content-coding value tokens should be registered; to
allow interoperability between clients and servers,
specifications of the content coding algorithms needed to
implement a new value should be publicly available and
adequate for independent implementation, and conform to the
purpose of content coding defined in this section.
3.6 Transfer Codings
Transfer coding values are used to indicate an encoding
transformation that has been, can be, or may need to be
applied to an entity-body in order to ensure "safe
transport" through the network. This differs from a content
coding in that the transfer coding is a property of the
message, not of the original entity.
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transfer-coding = "chunked" | transfer-extension
transfer-extension = token
All transfer-coding values are case-insensitive. HTTP/1.1
uses transfer coding values in the Transfer-Encoding header
field (section 14.40).
Transfer codings are analogous to the Content-Transfer-
Encoding values of MIME [7], which were designed to enable
safe transport of binary data over a 7-bit transport
service. However, safe transport has a different focus for
an 8bit-clean transfer protocol. In HTTP, the only unsafe
characteristic of message-bodies is the difficulty in
determining the exact body length (section 7.2.2), or the
desire to encrypt data over a shared transport.
The chunked encoding modifies the body of a message in order
to transfer it as a series of chunks, each with its own size
indicator, followed by an optional trailer containing
entity-header fields. This allows dynamically-produced
content to be transferred along with the information
necessary for the recipient to verify that it has received
the full message.
Chunked-Body = *chunk
last-chunk
trailer
CRLF
chunk = chunk-size [ chunk-extension ] CRLF
chunk-data CRLF
chunk-size = 1*HEX
last-chunk = 1*("0") [ chunk-extension ] CRLF
chunk-extension= *( ";" chunk-ext-name [ "="
chunk-ext-value ] )
chunk-ext-name = token
chunk-ext-val = token | quoted-string
chunk-data = chunk-size(OCTET)
trailer = *entity-header
The chunk-size field is a string of hex digits indicating
the size of the chunk. The chunked encoding is ended by any
chunk whose size is zero, followed by the trailer, which is
terminated by an empty line. The purpose of the trailer is
to provide an efficient way to supply information about an
entity that is generated dynamically. Applications MUST NOT
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send header fields in the trailer which are not explicitly
defined as being appropriate for the trailer.
The Content-MD5 header (section 14.16) is appropriate for
the trailer.
The Authentication-Info header defined by RFC 2069 [32] (An
Extension to HTTP: Digest Access Authentication), or its
successor is appropriate for the trailer.
An example process for decoding a Chunked-Body is presented
in appendix 19.4.6.
All HTTP/1.1 applications MUST be able to receive and decode
the "chunked" transfer coding, and MUST ignore chunk-
extension extensions they do not understand. A server which
receives an entity-body with a transfer-coding it does not
understand SHOULD return 501 (Unimplemented), and close the
connection. A server MUST NOT send transfer-codings to an
HTTP/1.0 client.
3.7 Media Types
HTTP uses Internet Media Types [17] in the Content-Type
(section 14.18) and Accept (section 14.1) header fields in
order to provide open and extensible data typing and type
negotiation.
media-type = type "/" subtype *( ";" parameter )
type = token
subtype = token
Parameters may follow the type/subtype in the form of
attribute/value pairs.
parameter = attribute "=" value
attribute = token
value = token | quoted-string
The type, subtype, and parameter attribute names are case-
insensitive. Parameter values may or may not be case-
sensitive, depending on the semantics of the parameter name.
Linear white space (LWS) MUST NOT be used between the type
and subtype, nor between an attribute and its value. User
agents that recognize the media-type MUST process (or
arrange to be processed by any external applications used to
process that type/subtype by the user agent) the parameters
for that MIME type as described by that type/subtype
definition to the and inform the user of any problems
discovered.
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Note: some older HTTP applications do not recognize
media type parameters. When sending data to older HTTP
applications, implementations should only use media
type parameters when they are required by that
type/subtype definition.
Media-type values are registered with the Internet Assigned
Number Authority (IANA [19]). The media type registration
process is outlined in RFC 1590 [17]. Use of non-registered
media types is discouraged.
3.7.1 Canonicalization and Text Defaults
Internet media types are registered with a canonical form.
In general, an entity-body transferred via HTTP messages
MUST be represented in the appropriate canonical form prior
to its transmission; the exception is "text" types, as
defined in the next paragraph.
When in canonical form, media subtypes of the "text" type
use CRLF as the text line break. HTTP relaxes this
requirement and allows the transport of text media with
plain CR or LF alone representing a line break when it is
done consistently for an entire entity-body. HTTP
applications MUST accept CRLF, bare CR, and bare LF as being
representative of a line break in text media received via
HTTP. In addition, if the text is represented in a character
set that does not use octets 13 and 10 for CR and LF
respectively, as is the case for some multi-byte character
sets, HTTP allows the use of whatever octet sequences are
defined by that character set to represent the equivalent of
CR and LF for line breaks. This flexibility regarding line
breaks applies only to text media in the entity-body; a bare
CR or LF MUST NOT be substituted for CRLF within any of the
HTTP control structures (such as header fields and multipart
boundaries).
If an entity-body is encoded with a Content-Encoding, the
underlying data MUST be in a form defined above prior to
being encoded.
The "charset" parameter is used with some media types to
define the character set (section 3.4) of the data. When no
explicit charset parameter is provided by the sender, media
subtypes of the "text" type are defined to have a default
charset value of "ISO-8859-1" when received via HTTP. Data
in character sets other than "ISO-8859-1" or its subsets
MUST be labeled with an appropriate charset value. See
section 19.8.2 for compatibility problems.
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3.7.2 Multipart Types
MIME provides for a number of "multipart" types --
encapsulations of one or more entities within a single
message-body. All multipart types share a common syntax, as
defined in section 5.1.1 of RFC 2046 [40], and MUST include
a boundary parameter as part of the media type value. The
message body is itself a protocol element and MUST therefore
use only CRLF to represent line breaks between body-parts.
Unlike in RFC 2046, the epilogue of any multipart message
MUST be empty; HTTP applications MUST NOT transmit the
epilogue (even if the original multipart contains an
epilogue).
In HTTP, multipart body-parts MAY contain header fields
which are significant to the meaning of that part. A
Content-Location header field (section 14.15) SHOULD be
included in the body-part of each enclosed entity that can
be identified by a URL.
In general, an HTTP user agent SHOULD follow the same or
similar behavior as a MIME user agent would upon receipt of
a multipart type. If an application receives an unrecognized
multipart subtype, the application MUST treat it as being
equivalent to "multipart/mixed".
Note: The "multipart/form-data" type has been
specifically defined for carrying form data suitable
for processing via the POST request method, as
described in RFC 1867 [15].
3.8 Product Tokens
Product tokens are used to allow communicating applications
to identify themselves by software name and version. Most
fields using product tokens also allow sub-products which
form a significant part of the application to be listed,
separated by whitespace. By convention, the products are
listed in order of their significance for identifying the
application.
product = token ["/" product-version]
product-version = token
Examples:
User-Agent: CERN-LineMode/2.15 libwww/2.17b3
Server: Apache/0.8.4
Product tokens should be short and to the point -- use of
them for advertising or other non-essential information is
explicitly forbidden. Although any token character may
appear in a product-version, this token SHOULD only be used
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for a version identifier (i.e., successive versions of the
same product SHOULD only differ in the product-version
portion of the product value).
3.9 Quality Values
HTTP content negotiation (section 12) uses short "floating
point" numbers to indicate the relative importance
("weight") of various negotiable parameters. A weight is
normalized to a real number in the range 0 through 1, where
0 is the minimum and 1 the maximum value. If a parameter has
a quality value of 0, then content with this parameter is
`not acceptable' for the client. HTTP/1.1 applications MUST
NOT generate more than three digits after the decimal point.
User configuration of these values SHOULD also be limited in
this fashion.
qvalue = ( "0" [ "." 0*3DIGIT ] )
| ( "1" [ "." 0*3("0") ] )
"Quality values" is a misnomer, since these values merely
represent relative degradation in desired quality.
3.10 Language Tags
A language tag identifies a natural language spoken,
written, or otherwise conveyed by human beings for
communication of information to other human beings. Computer
languages are explicitly excluded. HTTP uses language tags
within the Accept-Language and Content-Language fields.
The syntax and registry of HTTP language tags is the same as
that defined by RFC 1766 [1]. In summary, a language tag is
composed of 1 or more parts: A primary language tag and a
possibly empty series of subtags:
language-tag = primary-tag *( "-" subtag )
primary-tag = 1*8ALPHA
subtag = 1*8ALPHA
Whitespace is not allowed within the tag and all tags are
case-insensitive. The name space of language tags is
administered by the IANA. Example tags include:
en, en-US, en-cockney, i-cherokee, x-pig-latin
where any two-letter primary-tag is an ISO 639 language
abbreviation and any two-letter initial subtag is an ISO
3166 country code. (The last three tags above are not
registered tags; all but the last are examples of tags which
could be registered in future.)
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3.11 Entity Tags
Entity tags are used for comparing two or more entities from
the same requested resource. HTTP/1.1 uses entity tags in
the ETag (section 14.20), If-Match (section 14.25), If-None-
Match (section 14.26), and If-Range (section 14.27) header
fields. The definition of how they are used and compared as
cache validators is in section 13.3.3. An entity tag
consists of an opaque quoted string, possibly prefixed by a
weakness indicator.
entity-tag = [ weak ] opaque-tag
weak = "W/"
opaque-tag = quoted-string
A "strong entity tag" may be shared by two entities of a
resource only if they are equivalent by octet equality.
A "weak entity tag," indicated by the "W/" prefix, may be
shared by two entities of a resource only if the entities
are equivalent and could be substituted for each other with
no significant change in semantics. A weak entity tag can
only be used for weak comparison.
An entity tag MUST be unique across all versions of all
entities associated with a particular resource. A given
entity tag value may be used for entities obtained by
requests on different URIs without implying anything about
the equivalence of those entities.
3.12 Range Units
HTTP/1.1 allows a client to request that only part (a range
of) the response entity be included within the response.
HTTP/1.1 uses range units in the Range (section 14.36) and
Content-Range (section 14.17) header fields. An entity may
be broken down into subranges according to various
structural units.
range-unit = bytes-unit | other-range-unit
bytes-unit = "bytes"
other-range-unit = token
The only range unit defined by HTTP/1.1 is "bytes". HTTP/1.1
implementations may ignore ranges specified using other
units. HTTP/1.1 has been designed to allow implementations
of applications that do not depend on knowledge of ranges.
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4 HTTP Message
4.1 Message Types
HTTP messages consist of requests from client to server and
responses from server to client.
HTTP-message = Request | Response ; HTTP/1.1 messages
Request (section 5) and Response (section 6) messages use
the generic message format of RFC 822 [9] for transferring
entities (the payload of the message). Both types of message
consist of a start-line, one or more header fields (also
known as "headers"), an empty line (i.e., a line with
nothing preceding the CRLF) indicating the end of the header
fields, and an optional message-body.
generic-message = start-line
*message-header
CRLF
[ message-body ]
start-line = Request-Line | Status-Line
In the interest of robustness, servers SHOULD ignore any
empty line(s) received where a Request-Line is expected. In
other words, if the server is reading the protocol stream at
the beginning of a message and receives a CRLF first, it
should ignore the CRLF.
Note: certain buggy HTTP/1.0 client implementations
generate an extra CRLF's after a POST request. To
restate what is explicitly forbidden by the BNF, an
HTTP/1.1 client must not preface or follow a request
with an extra CRLF.
4.2 Message Headers
HTTP header fields, which include general-header (section
4.5), request-header (section 5.3), response-header (section
6.2), and entity-header (section 7.1) fields, follow the
same generic format as that given in Section 3.1 of RFC 822 [9].
Each header field consists of a name followed by a
colon (":") and the field value. Field names are case-
insensitive. The field value may be preceded by any amount
of LWS, though a single SP is preferred. Header fields can
be extended over multiple lines by preceding each extra line
with at least one SP or HT. Applications SHOULD follow
"common form" when generating HTTP constructs, since there
might exist some implementations that fail to accept
anything beyond the common forms.
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message-header = field-name ":" [ field-value ] CRLF
field-name = token
field-value = *( field-content | LWS )
field-content = <the OCTETs making up the field-value
and consisting of either *TEXT or combinations
of token, separators, and quoted-string>
The order in which header fields with differing field names
are received is not significant. However, it is "good
practice" to send general-header fields first, followed by
request-header or response-header fields, and ending with
the entity-header fields.
Multiple message-header fields with the same field-name may
be present in a message if and only if the entire field-
value for that header field is defined as a comma-separated
list [i.e., #(values)]. It MUST be possible to combine the
multiple header fields into one "field-name: field-value"
pair, without changing the semantics of the message, by
appending each subsequent field-value to the first, each
separated by a comma. The order in which header fields with
the same field-name are received is therefore significant to
the interpretation of the combined field value, and thus a
proxy MUST NOT change the order of these field values when a
message is forwarded.
4.3 Message Body
The message-body (if any) of an HTTP message is used to
carry the entity-body associated with the request or
response. The message-body differs from the entity-body only
when a transfer coding has been applied, as indicated by the
Transfer-Encoding header field (section 14.40).
message-body = entity-body
| <entity-body encoded as per Transfer-Encoding>
Transfer-Encoding MUST be used to indicate any transfer
codings applied by an application to ensure safe and proper
transfer of the message. Transfer-Encoding is a property of
the message, not of the entity, and thus can be added or
removed by any application along the request/response chain.
The rules for when a message-body is allowed in a message
differ for requests and responses.
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The presence of a message-body in a request is signaled by
the inclusion of a Content-Length or Transfer-Encoding
header field in the request's message-headers. A message-
body MUST NOT be included in a request if the specification
of the request method (section 5.1.1) does not allow sending
an entity-body in requests.
For response messages, whether or not a message-body is
included with a message is dependent on both the request
method and the response status code (section 6.1.1). All
responses to the HEAD request method MUST NOT include a
message-body, even though the presence of entity-header
fields might lead one to believe they do. All 1xx
(informational), 204 (no content), and 304 (not modified)
responses MUST NOT include a message-body. All other
responses do include a message-body, although it may be of
zero length.
4.4 Message Length
When a message-body is included with a message, the length
of that body is determined by one of the following (in order
of precedence):
1. Any response message which MUST NOT include a message-
body (such as the 1xx, 204, and 304 responses and any
response to a HEAD request) is always terminated by the
first empty line after the header fields, regardless of
the entity-header fields present in the message.
2. If a Transfer-Encoding header field (section 14.40) is
present and indicates that the "chunked" transfer
coding has been applied, then the length is defined by
the chunked encoding (section 3.6).
3. If a Content-Length header field (section 14.14) is
present, its value in bytes represents the length of
the message-body.
4. If the message uses the media type
"multipart/byteranges", which is self-delimiting, then
that defines the length. This media type MUST NOT be
used unless the sender knows that the recipient can
parse it; the presence in a request of a Range header
with multiple byte-range specifiers implies that the
client can parse multipart/byteranges responses.
5. By the server closing the connection. (Closing the
connection cannot be used to indicate the end of a
request body, since that would leave no possibility for
the server to send back a response.)
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For compatibility with HTTP/1.0 applications, HTTP/1.1
requests containing a message-body MUST include a valid
Content-Length header field unless the server is known to be
HTTP/1.1 compliant. If a request contains a message-body and
a Content-Length is not given, the server SHOULD respond
with 400 (bad request) if it cannot determine the length of
the message, or with 411 (length required) if it wishes to
insist on receiving a valid Content-Length.
All HTTP/1.1 applications that receive entities MUST accept
the "chunked" transfer coding (section 3.6), thus allowing
this mechanism to be used for messages when the message
length cannot be determined in advance.
Messages MUST NOT include both a Content-Length header field
and the "chunked" transfer coding. If both are received, the
Content-Length MUST be ignored.
When a Content-Length is given in a message where a message-
body is allowed, its field value MUST exactly match the
number of OCTETs in the message-body. HTTP/1.1 user agents
MUST notify the user when an invalid length is received and
detected.
4.5 General Header Fields
There are a few header fields which have general
applicability for both request and response messages, but
which do not apply to the entity being transferred. These
header fields apply only to the message being transmitted.
general-header = Cache-Control ; Section 14.9
| Connection ; Section 14.10
| Date ; Section 14.19
| Pragma ; Section 14.32
| Transfer-Encoding ; Section 14.40
| Upgrade ; Section 14.41
| Via ; Section 14.44
General-header field names can be extended reliably only in
combination with a change in the protocol version. However,
new or experimental header fields may be given the semantics
of general header fields if all parties in the communication
recognize them to be general-header fields. Unrecognized
header fields are treated as entity-header fields.
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5 Request
A request message from a client to a server includes, within
the first line of that message, the method to be applied to
the resource, the identifier of the resource, and the
protocol version in use.
Request = Request-Line ; Section 5.1
*( general-header ; Section 4.5
| request-header ; Section 5.3
| entity-header ) ; Section 7.1
CRLF
[ message-body ] ; Section 4.3
5.1 Request-Line
The Request-Line begins with a method token, followed by the
Request-URI and the protocol version, and ending with CRLF.
The elements are separated by SP characters. No CR or LF are
allowed except in the final CRLF sequence.
Request-Line = Method SP Request-URI SP HTTP-Version CRLF
5.1.1 Method
The Method token indicates the method to be performed on the
resource identified by the Request-URI. The method is case-
sensitive.
Method = "OPTIONS" ; Section 9.2
| "GET" ; Section 9.3
| "HEAD" ; Section 9.4
| "POST" ; Section 9.5
| "PUT" ; Section 9.6
| "DELETE" ; Section 9.7
| "TRACE" ; Section 9.8
| extension-method
extension-method = token
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The list of methods allowed by a resource can be specified
in an Allow header field (section 14.7). The return code of
the response always notifies the client whether a method is
currently allowed on a resource, since the set of allowed
methods can change dynamically. Servers SHOULD return the
status code 405 (Method Not Allowed) if the method is known
by the server but not allowed for the requested resource,
and 501 (Not Implemented) if the method is unrecognized or
not implemented by the server. The list of methods known by
a server can be listed in a Public response-header field
(section 14.35).
The methods GET and HEAD MUST be supported by all general-
purpose servers. All other methods are optional; however, if
the above methods are implemented, they MUST be implemented
with the same semantics as those specified in section 9.
5.1.2 Request-URI
The Request-URI is a Uniform Resource Identifier (section
3.2) and identifies the resource upon which to apply the
request.
Request-URI = "*" | absoluteURI | abs_path
The three options for Request-URI are dependent on the
nature of the request. The asterisk "*" means that the
request does not apply to a particular resource, but to the
server itself, and is only allowed when the method used does
not necessarily apply to a resource. One example would be
OPTIONS * HTTP/1.1
The absoluteURI form is required when the request is being
made to a proxy. The proxy is requested to forward the
request or service it from a valid cache, and return the
response. Note that the proxy MAY forward the request on to
another proxy or directly to the server specified by the
absoluteURI. In order to avoid request loops, a proxy MUST
be able to recognize all of its server names, including any
aliases, local variations, and the numeric IP address. An
example Request-Line would be:
GET http://www.w3.org/pub/WWW/TheProject.html HTTP/1.1
To allow for transition to absoluteURIs in all requests in
future versions of HTTP, all HTTP/1.1 servers MUST accept
the absoluteURI form in requests, even though HTTP/1.1
clients will only generate them in requests to proxies.
The most common form of Request-URI is that used to identify
a resource on an origin server or gateway. In this case the
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absolute path of the URI MUST be transmitted (see section
3.2.1, abs_path) as the Request-URI, and the network
location of the URI (net_loc) MUST be transmitted in a Host
header field. For example, a client wishing to retrieve the
resource above directly from the origin server would create
a TCP connection to port 80 of the host "www.w3.org" and
send the lines:
GET /pub/WWW/TheProject.html HTTP/1.1
Host: www.w3.org
followed by the remainder of the Request. Note that the
absolute path cannot be empty; if none is present in the
original URI, it MUST be given as "/" (the server root).
Editorial note: The proposed changes to OPTIONS will remove
the following down to ***END***. See draft-ietf-http-options-00.txt
If a proxy receives a request without any path in the
Request-URI and the method specified is capable of
supporting the asterisk form of request, then the last proxy
on the request chain MUST forward the request with "*" as
the final Request-URI. For example, the request
OPTIONS http://www.ics.uci.edu:8001 HTTP/1.1
would be forwarded by the proxy as
OPTIONS * HTTP/1.1
Host: www.ics.uci.edu:8001
after connecting to port 8001 of host "www.ics.uci.edu".
***END***
The Request-URI is transmitted in the format specified in
section 3.2.1. The origin server MUST decode the Request-URI
in order to properly interpret the request. Servers SHOULD
respond to invalid Request-URIs with an appropriate status
code.
In requests that they forward, proxies MUST NOT rewrite the
"abs_path" part of a Request-URI in any way except as noted
above to replace a null abs_path with "*", no matter what
the proxy does in its internal implementation.
Note: The "no rewrite" rule prevents the proxy from
changing the meaning of the request when the origin
server is improperly using a non-reserved URL character
for a reserved purpose. Implementers should be aware
that some pre-HTTP/1.1 proxies have been known to
rewrite the Request-URI.
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5.2 The Resource Identified by a Request
HTTP/1.1 origin servers SHOULD be aware that the exact
resource identified by an Internet request is determined by
examining both the Request-URI and the Host header field.
An origin server that does not allow resources to differ by
the requested host MAY ignore the Host header field value.
(But see section 19.5.1 for other requirements on Host
support in HTTP/1.1.)
An origin server that does differentiate resources based on
the host requested (sometimes referred to as virtual hosts
or vanity hostnames) MUST use the following rules for
determining the requested resource on an HTTP/1.1 request:
1. If Request-URI is an absoluteURI, the host is part
of the Request-URI. Any Host header field value in the
request MUST be ignored.
2. If the Request-URI is not an absoluteURI, and the
request includes a Host header field, the host is
determined by the Host header field value.
3. If the host as determined by rule 1 or 2 is not a
valid host on the server, the response MUST be a 400
(Bad Request) error message.
Recipients of an HTTP/1.0 request that lacks a Host header
field MAY attempt to use heuristics (e.g., examination of
the URI path for something unique to a particular host) in
order to determine what exact resource is being requested.
5.3 Request Header Fields
The request-header fields allow the client to pass
additional information about the request, and about the
client itself, to the server. These fields act as request
modifiers, with semantics equivalent to the parameters on a
programming language method invocation.
request-header = Accept ; Section 14.1
| Accept-Charset ; Section 14.2
| Accept-Encoding ; Section 14.3
| Accept-Language ; Section 14.4
| Authorization ; Section 14.8
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| Expect ; Section 14.47
| From ; Section 14.22
| Host ; Section 14.23
| If-Modified-Since ; Section 14.24
| If-Match ; Section 14.25
| If-None-Match ; Section 14.26
| If-Range ; Section 14.27
| If-Unmodified-Since ; Section 14.28
| Max-Forwards ; Section 14.31
| Proxy-Authorization ; Section 14.34
| Range ; Section 14.36
| Referer ; Section 14.37
| User-Agent ; Section 14.42
Request-header field names can be extended reliably only in
combination with a change in the protocol version. However,
new or experimental header fields MAY be given the semantics
of request-header fields if all parties in the communication
recognize them to be request-header fields. Unrecognized
header fields are treated as entity-header fields.
6 Response
After receiving and interpreting a request message, a server
responds with an HTTP response message.
Response = Status-Line ; Section 6.1
*( general-header ; Section 4.5
| response-header ; Section 6.2
| entity-header ) ; Section 7.1
CRLF
[ message-body ] ; Section 7.2
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6.1 Status-Line
The first line of a Response message is the Status-Line,
consisting of the protocol version followed by a numeric
status code and its associated textual phrase, with each
element separated by SP characters. No CR or LF is allowed
except in the final CRLF sequence.
Status-Line = HTTP-Version SP Status-Code SP
Reason-Phrase CRLF
6.1.1 Status Code and Reason Phrase
The Status-Code element is a 3-digit integer result code of
the attempt to understand and satisfy the request. These
codes are fully defined in section 10. The Reason-Phrase is
intended to give a short textual description of the Status-
Code. The Status-Code is intended for use by automata and
the Reason-Phrase is intended for the human user. The client
is not required to examine or display the Reason-Phrase.
The first digit of the Status-Code defines the class of
response. The last two digits do not have any categorization
role. There are 5 values for the first digit:
. 1xx: Informational - Request received, continuing
process
. 2xx: Success - The action was successfully received,
understood, and accepted
. 3xx: Redirection - Further action must be taken in
order to complete the request
. 4xx: Client Error - The request contains bad syntax or
cannot be fulfilled
. 5xx: Server Error - The server failed to fulfill an
apparently valid request
The individual values of the numeric status codes defined
for HTTP/1.1, and an example set of corresponding Reason-
Phrase's, are presented below. The reason phrases listed
here are only recommended -- they may be replaced by local
equivalents without affecting the protocol.
Status-Code = "100" ; Continue
| "101" ; Switching Protocols
| "200" ; OK
| "201" ; Created
| "202" ; Accepted
| "203" ; Non-Authoritative
Information
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| "204" ; No Content
| "205" ; Reset Content
| "206" ; Partial Content
| "300" ; Multiple Choices
| "301" ; Moved Permanently
| "302" ; Moved Temporarily
| "303" ; See Other
| "304" ; Not Modified
| "305" ; Use Proxy
| "400" ; Bad Request
| "401" ; Unauthorized
| "402" ; Payment Required
| "403" ; Forbidden
| "404" ; Not Found
| "405" ; Method Not Allowed
| "406" ; Not Acceptable
| "407" ; Proxy Authentication Required
| "408" ; Request Time-out
| "409" ; Conflict
| "410" ; Gone
| "411" ; Length Required
| "412" ; Precondition Failed
| "413" ; Request Entity Too Large
| "414" ; Request-URI Too Large
| "415" ; Unsupported Media Type
| "416" ; Requested range not valid
| "500" ; Internal Server Error
| "501" ; Not Implemented
| "502" ; Bad Gateway
| "503" ; Service Unavailable
| "504" ; Gateway Time-out
| "505" ; HTTP Version not supported
| extension-code
extension-code = 3DIGIT
Reason-Phrase = *<TEXT, excluding CR, LF>
HTTP status codes are extensible. HTTP applications are not
required to understand the meaning of all registered status
codes, though such understanding is obviously desirable.
However, applications MUST understand the class of any
status code, as indicated by the first digit, and treat any
unrecognized response as being equivalent to the x00 status
code of that class, with the exception that an unrecognized
response MUST NOT be cached. For example, if an unrecognized
status code of 431 is received by the client, it can safely
assume that there was something wrong with its request and
treat the response as if it had received a 400 status code.
In such cases, user agents SHOULD present to the user the
entity returned with the response, since that entity is
likely to include human-readable information which will
explain the unusual status.
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6.2 Response Header Fields
The response-header fields allow the server to pass
additional information about the response which cannot be
placed in the Status-Line. These header fields give
information about the server and about further access to the
resource identified by the Request-URI.
response-header = Accept-Ranges ; Section 14.5
| Age ; Section 14.6
| Location ; Section 14.30
| Proxy-Authenticate ; Section 14.33
| Public ; Section 14.35
| Retry-After ; Section 14.38
| Server ; Section 14.39
| Set-Proxy ; Section 14.48
| Vary ; Section 14.43
| Warning ; Section 14.45
| WWW-Authenticate ; Section 14.46
Response-header field names can be extended reliably only in
combination with a change in the protocol version. However,
new or experimental header fields MAY be given the semantics
of response-header fields if all parties in the
communication recognize them to be response-header fields.
Unrecognized header fields are treated as entity-header
fields.
7 Entity
Request and Response messages MAY transfer an entity if not
otherwise restricted by the request method or response
status code. An entity consists of entity-header fields and
an entity-body, although some responses will only include
the entity-headers.
In this section, both sender and recipient refer to either
the client or the server, depending on who sends and who
receives the entity.
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7.1 Entity Header Fields
Entity-header fields define optional metainformation about
the entity-body or, if no body is present, about the
resource identified by the request.
entity-header = Allow ; Section 14.7
| Content-Base ; Section 14.11
| Content-Encoding ; Section 14.12
| Content-Language ; Section 14.13
| Content-Length ; Section 14.14
| Content-Location ; Section 14.15
| Content-MD5 ; Section 14.16
| Content-Range ; Section 14.17
| Content-Type ; Section 14.18
| ETag ; Section 14.20
| Expires ; Section 14.21
| Last-Modified ; Section 14.29
| extension-header
extension-header = message-header
The extension-header mechanism allows additional entity-
header fields to be defined without changing the protocol,
but these fields cannot be assumed to be recognizable by the
recipient. Unrecognized header fields SHOULD be ignored by
the recipient and MUST be forwarded by proxies.
7.2 Entity Body
The entity-body (if any) sent with an HTTP request or
response is in a format and encoding defined by the entity-
header fields.
entity-body = *OCTET
An entity-body is only present in a message when a message-
body is present, as described in section 4.3. The entity-
body is obtained from the message-body by decoding any
Transfer-Encoding that may have been applied to ensure safe
and proper transfer of the message.
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7.2.1 Type
When an entity-body is included with a message, the data
type of that body is determined via the header fields
Content-Type and Content-Encoding. These define a two-layer,
ordered encoding model:
entity-body := Content-Encoding( Content-Type( data ) )
Content-Type specifies the media type of the underlying
data. Content-Encoding may be used to indicate any
additional content codings applied to the data, usually for
the purpose of data compression, that are a property of the
requested resource. There is no default encoding.
Any HTTP/1.1 message containing an entity-body SHOULD
include a Content-Type header field defining the media type
of that body. If and only if the media type is not given by
a Content-Type field, the recipient MAY attempt to guess the
media type via inspection of its content and/or the name
extension(s) of the URL used to identify the resource. If
the media type remains unknown, the recipient SHOULD treat
it as type "application/octet-stream".
7.2.2 Length
The length of an entity-body is the length of the message-
body after any transfer codings have been removed. Section
4.4 defines how the length of a message-body is determined.
8 Connections
8.1 Persistent Connections
8.1.1 Purpose
Prior to persistent connections, a separate TCP connection
was established to fetch each URL, increasing the load on
HTTP servers and causing congestion on the Internet. The use
of inline images and other associated data often require a
client to make multiple requests of the same server in a
short amount of time. Analyses of these performance problems
are available [30]; analysis and results from a prototype
implementation are in [26]. Implementation experience and
measurements of actual HTTP/1.1 (RFC 2068) implementations
show good results [39].
Alternatives have also been explored, for example, T/TCP
[27].
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Persistent HTTP connections have a number of advantages:
. By opening and closing fewer TCP connections, CPU time
is saved, and memory used for TCP protocol control
blocks is also saved.
. HTTP requests and responses can be pipelined on a
connection. Pipelining allows a client to make multiple
requests without waiting for each response, allowing a
single TCP connection to be used much more efficiently,
with much lower elapsed time.
. Network congestion is reduced by reducing the number of
packets caused by TCP opens, and by allowing TCP
sufficient time to determine the congestion state of
the network.
. HTTP can evolve more gracefully; since errors can be
reported without the penalty of closing the TCP
connection. Clients using future versions of HTTP might
optimistically try a new feature, but if communicating
with an older server, retry with old semantics after an
error is reported.
HTTP implementations SHOULD implement persistent
connections.
8.1.2 Overall Operation
A significant difference between HTTP/1.1 and earlier
versions of HTTP is that persistent connections are the
default behavior of any HTTP connection. That is, unless
otherwise indicated, the client may assume that the server
will maintain a persistent connection.
Persistent connections provide a mechanism by which a client
and a server can signal the close of a TCP connection. This
signaling takes place using the Connection header field.
Once a close has been signaled, the client MUST not send any
more requests on that connection.
8.1.2.1 Negotiation
An HTTP/1.1 server MAY assume that a HTTP/1.1 client intends
to maintain a persistent connection unless a Connection
header including the connection-token "close" was sent in
the request. If the server chooses to close the connection
immediately after sending the response, it SHOULD send a
Connection header including the connection-token close.
An HTTP/1.1 client MAY expect a connection to remain open,
but would decide to keep it open based on whether the
response from a server contains a Connection header with the
connection-token close. In case the client does not want to
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maintain a connection for more than that request, it SHOULD
send a Connection header including the connection-token
close.
If either the client or the server sends the close token in
the Connection header, that request becomes the last one for
the connection.
Clients and servers SHOULD NOT assume that a persistent
connection is maintained for HTTP versions less than 1.1
unless it is explicitly signaled. See section 19.7.1 for
more information on backwards compatibility with HTTP/1.0
clients.
In order to remain persistent, all messages on the
connection must have a self-defined message length (i.e.,
one not defined by closure of the connection), as described
in section 4.4.
8.1.2.2 Pipelining
A client that supports persistent connections MAY "pipeline"
its requests (i.e., send multiple requests without waiting
for each response). A server MUST send its responses to
those requests in the same order that the requests were
received.
Clients which assume persistent connections and pipeline
immediately after connection establishment SHOULD be
prepared to retry their connection if the first pipelined
attempt fails. If a client does such a retry, it MUST NOT
pipeline before it knows the connection is persistent.
Clients MUST also be prepared to resend their requests if
the server closes the connection before sending all of the
corresponding responses.
Clients SHOULD NOT pipeline requests using non-idempotent
methods or non-idempotent sequences of methods (see section
9.1.2). Otherwise, a premature termination of the transport
connection may lead toindeterminate results. A client
wishing to send a non-idempotent request SHOULD wait to send
that request until it has received the response status for
the previous request.
8.1.3 Proxy Servers
It is especially important that proxies correctly implement
the properties of the Connection header field as specified
in 14.2.1.
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The proxy server MUST signal persistent connections
separately with its clients and the origin servers (or other
proxy servers) that it connects to. Each persistent
connection applies to only one transport link.
A proxy server MUST NOT establish a persistent connection
with an HTTP/1.0 client (but see section 19.7.1.1 for
information about the Keep-Alive header implemented by many
HTTP/1.0 clients).
8.1.4 Practical Considerations
Servers will usually have some time-out value beyond which
they will no longer maintain an inactive connection. Proxy
servers might make this a higher value since it is likely
that the client will be making more connections through the
same server. The use of persistent connections places no
requirements on the length of this time-out for either the
client or the server.
When a client or server wishes to time-out it SHOULD issue a
graceful close on the transport connection. Clients and
servers SHOULD both constantly watch for the other side of
the transport close, and respond to it as appropriate. If a
client or server does not detect the other side's close
promptly it could cause unnecessary resource drain on the
network.
A client, server, or proxy MAY close the transport
connection at any time. For example, a client MAY have
started to send a new request at the same time that the
server has decided to close the "idle" connection. From the
server's point of view, the connection is being closed while
it was idle, but from the client's point of view, a request
is in progress.
This means that clients, servers, and proxies MUST be able
to recover from asynchronous close events. Client software
SHOULD reopen the transport connection and retransmit the
aborted sequence of requests without user interaction so
long as the request sequence is idempotent (see section
9.1.2);.Non-idempotent methods or sequences MUST NOT be
automatically retried, although user agents MAY offer a
human operator the choice of retrying the request(s).
However, this automatic retry SHOULD NOT be repeated if the
second request fails.
Servers SHOULD always respond to at least one request per
connection, if at all possible. Servers SHOULD NOT close a
connection in the middle of transmitting a response, unless
a network or client failure is suspected.
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Clients that use persistent connections SHOULD limit the
number of simultaneous connections that they maintain to a
given server. A single-user client SHOULD maintain AT MOST 2
connections with any server or proxy. A proxy SHOULD use up
to 2*N connections to another server or proxy, where N is
the number of simultaneously active users. These guidelines
are intended to improve HTTP response times and avoid
congestion of the Internet or other networks.
8.2 Message Transmission Requirements
8.2.1 Persistent connections and flow control
HTTP/1.1 servers SHOULD maintain persistent connections and
use TCP's flow control mechanisms to resolve temporary
overloads, rather than terminating connections with the
expectation that clients will retry. The latter technique
can exacerbate network congestion.
8.2.2 Monitoring connections for error status messages
An HTTP/1.1 (or later) client sending a message-body SHOULD
monitor the network connection for an error status while it
is transmitting the request. If the client sees an error
status, it SHOULD immediately cease transmitting the body.
If the body is being sent using a "chunked" encoding
(section 3.6), a zero length chunk and empty trailer MAY be
used to prematurely mark the end of the message. If the body
was preceded by a Content-Length header, the client MUST
close the connection.
8.2.3 Automatic retrying of requests
If a user agent sees the transport connection close before
it receives a final response to its request, if the request
method is idempotent (see section 9.1.2), the user agent
SHOULD retry the request without user interaction. If the
request method is not idempotent, the user agent SHOULD NOT
retry the request without user confirmation. (Confirmation
by user-agent software with semantic understanding of the
application MAY substitute for user confirmation.)
8.2.4 Use of the 100 (Continue) status
The purpose of the 100 (Continue) status (see section
10.1.1) is to allow an end-client that is sending a request
message with a request body to determine if the origin
server is willing to accept the request (based on the
request headers) before the client sends the request body.
In some cases, it may either be inappropriate or highly
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inefficient for the client to send the body if the server
will reject the message without looking at the body.
Requirements for HTTP/1.1 clients:
. If a client will wait for a 100 (Continue) response
before sending the request body, it MUST send an Expect
request-header field (section 14.47) with the "100-
continue" expectation.
. A client MUST NOT send an Expect request-header field
(section 14.47) with the "100-continue" expectation if
it does not intend to send a request body.
Note: Because of the presence of older implementations,
the protocol allows ambiguous situations in which a
client may send "Expect: 100-continue" without
receiving either a 419 (Expectation Failed) status or
a 100 (Continue) status. Therefore, when a client sends
this header field to an origin server (possibly via a
proxy) from which it has never seen a 100 (Continue)
status, the client should not wait for an indefinite or
lengthy period before sending the request body.
Requirements for HTTP/1.1 origin servers:
. Upon receiving a request which includes an Expect
request-header field with the "100-continue"
expectation, an origin server MUST either respond with
100 (Continue) status and continue to read from the
input stream, or respond with an error status. The
origin server MUST NOT wait for the request body before
sending the 100 (Continue) response. If it responds
with an error status, it MAY close the transport
connection or it MAY continue to read and discard the
rest of the request. It MUST NOT perform the requested
method if it returns an error status.
. An origin server SHOULD NOT send a 100 (Continue)
response if the request message does not include an
Expect request-header field with the "100-continue"
expectation, and MUST NOT send a 100 (Continue)
response if such a request comes from an HTTP/1.0 (or
earlier) client.
. An origin server MAY omit a 100 (Continue) response if
has already received some or all of the request body
for the corresponding request.
. An origin server that sends a 100 (Continue) response
MUST ultimately send a final status code, once the
request body is received and processed, unless it
terminates the transport connection prematurely.
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. If an origin server receives a request that does not
include an Expect request-header field with the "100-
continue" expectation, the request includes a request
body, and the server responds with an error status
before reading the entire request body from the
transport connection, then the server SHOULD NOT close
the transport connection until it has read the entire
request, or until the client closes the connection.
Otherwise, the client may not reliably receive the
response message. However, this requirement should not
be construed as preventing a server from defending
itself against denial-of-service attacks, or from badly
broken client implementations.
For compatibility with RFC 2068, a server MAY send a 100
(Continue) status in response to an HTTP/1.1 PUT or POST
request that does not include an Expect request-header field
with the "100-continue" expectation. This exception, the
purpose of which is to minimize any client processing delays
associated with an undeclared wait for 100 (Continue)
status, applies only to HTTP/1.1 requests, and not to
requests with any other HTTP-version value.
Requirements for HTTP/1.1 proxies:
. If a proxy receives a request that includes an Expect
request-header field with the "100-continue"
expectation, and the proxy either knows that the next-
hop server complies with HTTP/1.1 or higher, or does
not know the HTTP version of the next-hop server, it
MUST forward the request, including the Expect header
field.
. If the proxy knows that the version of the next-hop
server is HTTP/1.0 or lower, it MUST NOT forward the
request, and it MUST respond with a 419 (Expectation
Failed) status.
. Proxies SHOULD maintain a cache recording the HTTP
version numbers received from recently-referenced next-
hop servers.
. A proxy MUST NOT forward a 100 (Continue) response if
the request message was received from an HTTP/1.0 (or
earlier) client and did not include an Expect request-
header field with the "100-continue" expectation. This
requirement overrides the general rule for forwarding
of 1xx responses (see section 10.1).
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8.2.5 Client behavior if server prematurely closes
connection
If an HTTP/1.1 client sends a request which includes a
request body, but which does not include an Expect request-
header field with the "100-continue" expectation, and if the
client is not directly connected to an HTTP/1.1 origin
server, and if the the client sees the connection close
before receiving any status from the server, the client
SHOULD retry the request, subject to the restrictions in
section 8.2.3. If the client does retry this request, it MAY
use the following "binary exponential backoff" algorithm to
be assured of obtaining a reliable response:
1.Initiate a new connection to the server
2.Transmit the request-headers
3.Initialize a variable R to the estimated round-trip
time to the server (e.g., based on the time it took to
establish the connection), or to a constant value of 5
seconds if the round-trip time is not available.
4.Compute T = R * (2**N), where N is the number of
previous retries of this request.
5.Wait either for an error response from the server, or
for T seconds (whichever comes first)
6.If no error response is received, after T seconds
transmit the body of the request.
7.If client sees that the connection is closed
prematurely, repeat from step 1 until the request is
accepted, an error response is received, or the user
becomes impatient and terminates the retry process.
If at any point an error status is received, the client
. SHOULD NOT continue and
. SHOULD close the connection if it has not completed
sending the request message.
9 Method Definitions
The set of common methods for HTTP/1.1 is defined below.
Although this set can be expanded, additional methods cannot
be assumed to share the same semantics for separately
extended clients and servers.
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The Host request-header field (section 14.23) MUST accompany
all HTTP/1.1 requests.
9.1 Safe and Idempotent Methods
9.1.1 Safe Methods
Implementers should be aware that the software represents
the user in their interactions over the Internet, and should
be careful to allow the user to be aware of any actions they
may take which may have an unexpected significance to
themselves or others.
In particular, the convention has been established that the
GET and HEAD methods should never have the significance of
taking an action other than retrieval. These methods should
be considered "safe." This allows user agents to represent
other methods, such as POST, PUT and DELETE, in a special
way, so that the user is made aware of the fact that a
possibly unsafe action is being requested.
Naturally, it is not possible to ensure that the server does
not generate side-effects as a result of performing a GET
request; in fact, some dynamic resources consider that a
feature. The important distinction here is that the user did
not request the side-effects, so therefore cannot be held
accountable for them.
9.1.2 Idempotent Methods
Methods may also have the property of "idempotence" in that
(aside from error or expiration issues) the side-effects of
N > 0 identical requests is the same as for a single
request. The methods GET, HEAD, PUT and DELETE share this
property. Also, the methods OPTIONS and TRACE should have no
side effects, and so are inherently idempotent.
However, it is possible that a sequence of several requests
is non-idempotent, even if all of the methods executed in
that sequence is idempotent. (A sequence is idempotent if a
single execution of the entire sequence always yields a
result that is not changed by a reexecution of all, or part,
of that sequence.) For example, a sequence is non-
idempotent if its result depends on a value that is later
modified in the same sequence.
A sequence that never has side effects is idempotent, by
definition (provided that no concurrent operations are being
executed on the same set of resources).
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9.2 OPTIONS
The OPTIONS method represents a request for information
about the communication options available on the
request/response chain identified by the Request-URI. This
method allows the client to determine the options and/or
requirements associated with a resource, or the capabilities
of a server, without implying a resource action or
initiating a resource retrieval.
Editorial note: The proposed changes to OPTIONS will change
the following down to ***END***. See draft-ietf-http-
options-00.txt.
Unless the server's response is an error, the response MUST
NOT include entity information other than what can be
considered as communication options (e.g., Allow is
appropriate, but Content-Type is not). Responses to this
method are not cachable.
If the Request-URI is an asterisk ("*"), the OPTIONS request
is intended to apply to the server as a whole. A 200
response SHOULD include any header fields which indicate
optional features implemented by the server (e.g., Public),
including any extensions not defined by this specification,
in addition to any applicable general or response-header
fields. As described in section 5.1.2, an "OPTIONS *"
request can be applied through a proxy by specifying the
destination server in the Request-URI without any path
information.
If the Request-URI is not an asterisk, the OPTIONS request
applies only to the options that are available when
communicating with that resource. A 200 response SHOULD
include any header fields which indicate optional features
implemented by the server and applicable to that resource
(e.g., Allow), including any extensions not defined by this
specification, in addition to any applicable general or
response-header fields. If the OPTIONS request passes
through a proxy, the proxy MUST edit the response to exclude
those options which apply to a proxy's capabilities and
which are known to be unavailable through that proxy.
***END***
9.3 GET
The GET method means retrieve whatever information (in the
form of an entity) is identified by the Request-URI. If the
Request-URI refers to a data-producing process, it is the
produced data which shall be returned as the entity in the
response and not the source text of the process, unless that
text happens to be the output of the process.
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The semantics of the GET method change to a "conditional
GET" if the request message includes an If-Modified-Since,
If-Unmodified-Since, If-Match, If-None-Match, or If-Range
header field. A conditional GET method requests that the
entity be transferred only under the circumstances described
by the conditional header field(s). The conditional GET
method is intended to reduce unnecessary network usage by
allowing cached entities to be refreshed without requiring
multiple requests or transferring data already held by the
client.
The semantics of the GET method change to a "partial GET" if
the request message includes a Range header field. A partial
GET requests that only part of the entity be transferred, as
described in section 14.36. The partial GET method is
intended to reduce unnecessary network usage by allowing
partially-retrieved entities to be completed without
transferring data already held by the client.
The response to a GET request is cachable if and only if it
meets the requirements for HTTP caching described in section
13.
See section 15.11 for security considerations when used for
forms.
9.4 HEAD
The HEAD method is identical to GET except that the server
MUST NOT return a message-body in the response. The
metainformation contained in the HTTP headers in response to
a HEAD request SHOULD be identical to the information sent
in response to a GET request. This method can be used for
obtaining metainformation about the entity implied by the
request without transferring the entity-body itself. This
method is often used for testing hypertext links for
validity, accessibility, and recent modification.
The response to a HEAD request may be cachable in the sense
that the information contained in the response may be used
to update a previously cached entity from that resource. If
the new field values indicate that the cached entity differs
from the current entity (as would be indicated by a change
in Content-Length, Content-MD5, ETag or Last-Modified), then
the cache MUST treat the cache entry as stale.
9.5 POST
The POST method is used to request that the destination
server accept the entity enclosed in the request as a new
subordinate of the resource identified by the Request-URI in
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the Request-Line. POST is designed to allow a uniform method
to cover the following functions:
. Annotation of existing resources;
. Posting a message to a bulletin board, newsgroup,
mailing list, or similar group of articles;
. Providing a block of data, such as the result of
submitting a form, to a data-handling process;
. Extending a database through an append operation.
The actual function performed by the POST method is
determined by the server and is usually dependent on the
Request-URI. The posted entity is subordinate to that URI in
the same way that a file is subordinate to a directory
containing it, a news article is subordinate to a newsgroup
to which it is posted, or a record is subordinate to a
database.
The action performed by the POST method might not result in
a resource that can be identified by a URI. In this case,
either 200 (OK) or 204 (No Content) is the appropriate
response status, depending on whether or not the response
includes an entity that describes the result.
If a resource has been created on the origin server, the
response SHOULD be 201 (Created) and contain an entity which
describes the status of the request and refers to the new
resource, and a Location header (see section 14.30).
Responses to this method are not cachable, unless the
response includes appropriate Cache-Control or Expires
header fields. However, the 303 (See Other) response can be
used to direct the user agent to retrieve a cachable
resource.
POST requests must obey the message transmission
requirements set out in section 8.2.
See section 15.11 for security considerations.
9.6 PUT
Editor's note: Paul Leach is circulating changes that would
define how to do PUTs with byte ranges; the spec is
currently silent on the topic.
The PUT method requests that the enclosed entity be stored
under the supplied Request-URI. If the Request-URI refers to
an already existing resource, the enclosed entity SHOULD be
considered as a modified version of the one residing on the
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origin server. If the Request-URI does not point to an
existing resource, and that URI is capable of being defined
as a new resource by the requesting user agent, the origin
server can create the resource with that URI. If a new
resource is created, the origin server MUST inform the user
agent via the 201 (Created) response. If an existing
resource is modified, either the 200 (OK) or 204 (No
Content) response codes SHOULD be sent to indicate
successful completion of the request. If the resource could
not be created or modified with the Request-URI, an
appropriate error response SHOULD be given that reflects the
nature of the problem. The recipient of the entity MUST NOT
ignore any Content-* (e.g. Content-Range) headers that it
does not understand or implement and MUST return a 501 (Not
Implemented) response in such cases.
If the request passes through a cache and the Request-URI
identifies one or more currently cached entities, those
entries should be treated as stale. Responses to this method
are not cachable.
The fundamental difference between the POST and PUT requests
is reflected in the different meaning of the Request-URI.
The URI in a POST request identifies the resource that will
handle the enclosed entity. That resource may be a data-
accepting process, a gateway to some other protocol, or a
separate entity that accepts annotations. In contrast, the
URI in a PUT request identifies the entity enclosed with the
request -- the user agent knows what URI is intended and the
server MUST NOT attempt to apply the request to some other
resource. If the server desires that the request be applied
to a different URI, it MUST send a 301 (Moved Permanently)
response; the user agent MAY then make its own decision
regarding whether or not to redirect the request.
A single resource MAY be identified by many different URIs.
For example, an article may have a URI for identifying "the
current version" which is separate from the URI identifying
each particular version. In this case, a PUT request on a
general URI may result in several other URIs being defined
by the origin server.
HTTP/1.1 does not define how a PUT method affects the state
of an origin server.
PUT requests must obey the message transmission requirements
set out in section 8.2.
9.7 DELETE
The DELETE method requests that the origin server delete the
resource identified by the Request-URI. This method MAY be
overridden by human intervention (or other means) on the
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origin server. The client cannot be guaranteed that the
operation has been carried out, even if the status code
returned from the origin server indicates that the action
has been completed successfully. However, the server SHOULD
not indicate success unless, at the time the response is
given, it intends to delete the resource or move it to an
inaccessible location.
A successful response SHOULD be 200 (OK) if the response
includes an entity describing the status, 202 (Accepted) if
the action has not yet been enacted, or 204 (No Content) if
the response is OK but does not include an entity.
If the request passes through a cache and the Request-URI
identifies one or more currently cached entities, those
entries should be treated as stale. Responses to this method
are not cachable.
9.8 TRACE
The TRACE method is used to invoke a remote, application-
layer loop-back of the request message. The final recipient
of the request SHOULD reflect the message received back to
the client as the entity-body of a 200 (OK) response. The
final recipient is either the origin server or the first
proxy or gateway to receive a Max-Forwards value of zero (0)
in the request (see section 14.31). A TRACE request MUST NOT
include an entity.
TRACE allows the client to see what is being received at the
other end of the request chain and use that data for testing
or diagnostic information. The value of the Via header field
(section 14.44) is of particular interest, since it acts as
a trace of the request chain. Use of the Max-Forwards header
field allows the client to limit the length of the request
chain, which is useful for testing a chain of proxies
forwarding messages in an infinite loop.
If successful, the response SHOULD contain the entire
request message in the entity-body, with a Content-Type of
"message/http". Responses to this method MUST NOT be cached.
10 Status Code Definitions
Each Status-Code is described below, including a description
of which method(s) it can follow and any metainformation
required in the response.
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10.1 Informational 1xx
This class of status code indicates a provisional response,
consisting only of the Status-Line and optional headers, and
is terminated by an empty line. There are no required
headers for this class of status codes. Since HTTP/1.0 did
not define any 1xx status codes, servers MUST NOT send a 1xx
response to an HTTP/1.0 client except under experimental
conditions.
A client MUST be prepared to accept one or more 1xx status
responses prior to a regular response, even if the client
does not expect a 100 (Continue) status message. Unexpected
1xx status responses MAY be ignored by a user agent.
Proxies MUST forward 1xx responses, unless the connection
between the proxy and its client has been closed, or unless
the proxy itself requested the generation of the 1xx
response. (For example, if a proxy adds a "Expect: 100-
continue" field when it forwards a request, then it need not
forward the corresponding 100 (Continue) response(s).)
10.1.1 100 Continue
The client may continue with its request. This interim
response is used to inform the client that the initial part
of the request has been received and has not yet been
rejected by the server. The client SHOULD continue by
sending the remainder of the request or, if the request has
already been completed, ignore this response. The server
MUST send a final response after the request has been
completed. See section 8.2.4 for detailed discussion of the
use and handling of this status code.
10.1.2 101 Switching Protocols
The server understands and is willing to comply with the
client's request, via the Upgrade message header field
(section 14.41), for a change in the application protocol
being used on this connection. The server will switch
protocols to those defined by the response's Upgrade header
field immediately after the empty line which terminates the
101 response.
The protocol should only be switched when it is advantageous
to do so. For example, switching to a newer version of HTTP
is advantageous over older versions, and switching to a
real-time, synchronous protocol may be advantageous when
delivering resources that use such features.
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10.2 Successful 2xx
This class of status code indicates that the client's
request was successfully received, understood, and accepted.
10.2.1 200 OK
The request has succeeded. The information returned with the
response is dependent on the method used in the request, for
example:
GET an entity corresponding to the requested resource is
sent in the response;
HEAD the entity-header fields corresponding to the requested
resource are sent in the response without any message-
body;
POST an entity describing or containing the result of the
action;
TRACE an entity containing the request message as received
by the end server.
10.2.2 201 Created
The request has been fulfilled and resulted in a new
resource being created. The newly created resource can be
referenced by the URI(s) returned in the entity of the
response, with the most specific URL for the resource given
by a Location header field. The origin server MUST create
the resource before returning the 201 status code. If the
action cannot be carried out immediately, the server should
respond with 202 (Accepted) response instead.
10.2.3 202 Accepted
The request has been accepted for processing, but the
processing has not been completed. The request MAY or MAY
NOT eventually be acted upon, as it MAY be disallowed when
processing actually takes place. There is no facility for
re-sending a status code from an asynchronous operation such
as this.
The 202 response is intentionally non-committal. Its purpose
is to allow a server to accept a request for some other
process (perhaps a batch-oriented process that is only run
once per day) without requiring that the user agent's
connection to the server persist until the process is
completed. The entity returned with this response SHOULD
include an indication of the request's current status and
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either a pointer to a status monitor or some estimate of
when the user can expect the request to be fulfilled.
10.2.4 203 Non-Authoritative Information
The returned metainformation in the entity-header is not the
definitive set as available from the origin server, but is
gathered from a local or a third-party copy. The set
presented MAY be a subset or superset of the original
version. For example, including local annotation information
about the resource MAY result in a superset of the
metainformation known by the origin server. Use of this
response code is not required and is only appropriate when
the response would otherwise be 200 (OK).
10.2.5 204 No Content
The server has fulfilled the request but there is no new
information to send back. If the client is a user agent, it
SHOULD NOT change its document view from that which caused
the request to be sent. This response is primarily intended
to allow input for actions to take place without causing a
change to the user agent's active document view. The
response MAY include new metainformation in the form of
entity-headers, which SHOULD apply to the document currently
in the user agent's active view.
The 204 response MUST NOT include a message-body, and thus
is always terminated by the first empty line after the
header fields.
10.2.6 205 Reset Content
The server has fulfilled the request and the user agent
SHOULD reset the document view which caused the request to
be sent. This response is primarily intended to allow input
for actions to take place via user input, followed by a
clearing of the form in which the input is given so that the
user can easily initiate another input action. The response
MUST NOT include an entity.
10.2.7 206 Partial Content
The server has fulfilled the partial GET request for the
resource. The request must have included a Range header
field (section 14.36) indicating the desired range , and may
have included an If-Range header field (section 14.27) to
make the request conditional.
The response MUST include the following header fields:
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. Either a Content-Range header field (section 14.17)
indicating the range included with this response, or a
multipart/byteranges Content-Type including Content-
Range fields for each part. If multipart/byteranges is
not used, the Content-Length header field in the
response MUST match the actual number of OCTETs
transmitted in the message-body.
. Date
. ETag and/or Content-Location, if the header would have
been sent in a 200 response to the same request
. Expires, Cache-Control, and/or Vary, if the field-value
might differ from that sent in any previous response
for the same variant
If the 206 response is the result of an If-Range request
that used a strong cache validator (see section 13.3.3), the
response SHOULD NOT include other entity-headers. If the
response is the result of an If-Range request that used a
weak validator, the response MUST NOT include other entity-
headers; this prevents inconsistencies between cached
entity-bodies and updated headers. Otherwise, the response
MUST include all of the entity-headers that would have been
returned with a 200 (OK) response to the same request.
A cache MUST NOT combine a 206 response with other
previously cached content if the ETag or Last-Modified
headers do not match exactly, see 13.5.4.
A cache that does not support the Range and Content-Range
headers MUST NOT cache 206 (Partial) responses.
10.3 Redirection 3xx
This class of status code indicates that further action
needs to be taken by the user agent in order to fulfill the
request. The action required MAY be carried out by the user
agent without interaction with the user if and only if the
method used in the second request is GET or HEAD. A user
agent SHOULD NOT automatically redirect a request more than
5 times, since such redirections usually indicate an
infinite loop.
10.3.1 300 Multiple Choices
The requested resource corresponds to any one of a set of
representations, each with its own specific location, and
agent-driven negotiation information (section 12) is being
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provided so that the user (or user agent) can select a
preferred representation and redirect its request to that
location.
Unless it was a HEAD request, the response SHOULD include an
entity containing a list of resource characteristics and
location(s) from which the user or user agent can choose the
one most appropriate. The entity format is specified by the
media type given in the Content-Type header field. Depending
upon the format and the capabilities of the user agent,
selection of the most appropriate choice may be performed
automatically. However, this specification does not define
any standard for such automatic selection.
If the server has a preferred choice of representation, it
SHOULD include the specific URL for that representation in
the Location field; user agents MAY use the Location field
value for automatic redirection. This response is cachable
unless indicated otherwise.
10.3.2 301 Moved Permanently
The requested resource has been assigned a new permanent URI
and any future references to this resource SHOULD be done
using one of the returned URIs. Clients with link editing
capabilities SHOULD automatically re-link references to the
Request-URI to one or more of the new references returned by
the server, where possible. This response is cachable unless
indicated otherwise.
If the new URI is a location, its URL SHOULD be given by the
Location field in the response. Unless the request method
was HEAD, the entity of the response SHOULD contain a short
hypertext note with a hyperlink to the new URI(s).
If the 301 status code is received in response to a request
other than GET or HEAD, the user agent MUST NOT
automatically redirect the request unless it can be
confirmed by the user, since this might change the
conditions under which the request was issued.
Note: When automatically redirecting a POST request
after receiving a 301 status code, some existing
HTTP/1.0 user agents will erroneously change it into a
GET request.
10.3.3 302 Moved Temporarily
The requested resource resides temporarily under a different
URI. Since the redirection may be altered on occasion, the
client SHOULD continue to use the Request-URI for future
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requests. This response is only cachable if indicated by a
Cache-Control or Expires header field.
If the new URI is a location, its URL SHOULD be given by the
Location field in the response. Unless the request method
was HEAD, the entity of the response SHOULD contain a short
hypertext note with a hyperlink to the new URI(s).
If the 302 status code is received in response to a request
other than GET or HEAD, the user agent MUST NOT
automatically redirect the request unless it can be
confirmed by the user, since this might change the
conditions under which the request was issued.
Note: When automatically redirecting a POST request
after receiving a 302 status code, some existing
HTTP/1.0 user agents will erroneously change it into a
GET request.
10.3.4 303 See Other
The response to the request can be found under a different
URI and SHOULD be retrieved using a GET method on that
resource. This method exists primarily to allow the output
of a POST-activated script to redirect the user agent to a
selected resource. The new URI is not a substitute reference
for the originally requested resource. The 303 response is
not cachable, but the response to the second (redirected)
request MAY be cachable.
If the new URI is a location, its URL SHOULD be given by the
Location field in the response. Unless the request method
was HEAD, the entity of the response SHOULD contain a short
hypertext note with a hyperlink to the new URI(s).
10.3.5 304 Not Modified
If the client has performed a conditional GET request and
access is allowed, but the document has not been modified,
the server SHOULD respond with this status code. The
response MUST NOT contain a message-body.
The response MUST include the following header fields:
. Date, unless its omission is required by section
14.19.1
If a clockless origin server obeys these rules, and
proxies and clients add their own Date to any response
received without one (as already specified by [RFC 2068],
section 14.19), caches will operate correctly.
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. ETag and/or Content-Location, if the header would have
been sent in a 200 response to the same request
. Expires, Cache-Control, and/or Vary, if the field-value
might differ from that sent in any previous response
for the same variant
If the conditional GET used a strong cache validator (see
section 13.3.3), the response SHOULD NOT include other
entity-headers. Otherwise (i.e., the conditional GET used a
weak validator), the response MUST NOT include other entity-
headers; this prevents inconsistencies between cached
entity-bodies and updated headers.
If a 304 response indicates an entity not currently cached,
then the cache MUST disregard the response and repeat the
request without the conditional.
If a cache uses a received 304 response to update a cache
entry, the cache MUST update the entry to reflect any new
field values given in the response.
The 304 response MUST NOT include a message-body, and thus
is always terminated by the first empty line after the
header fields.
10.3.6 305 Use Proxy
The 305 is generated by an origin server to indicate that
the client, or proxy, should use a proxy to access the
requested resource.
The request SHOULD be accompanied by a Set-Proxy response
header indicating what proxy is to be used. The client will
parse the Set-Proxy header as defined below to decide how
long and for what URLs it should use the specified proxy.
If the 305 response is not accompanied by a Set-Proxy
header, it MUST be accompanied by a Location header. The
Location header will specify a URL to the proxy.
If both headers are present in the response, the client
SHOULD only use the Set-Proxy header only.
10.3.7 306 Switch Proxy
The 306 response is generated by a proxy server to indicate
that the client or proxy should use the information in the
accompanying Set-Proxy header to choose a proxy for
subsequent requests.
The 306 response code MUST be accompanied by the Set-Proxy
response header. The client or proxy will parse the Set-
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Proxy header to determine which proxy to use, how long to
use it, and for which URLs to use it.
The scope in the Set-Proxy header is considered an optional
advisory. The client or proxy may choose to ignore it, and
use it for just this request, for all requests, or for a
scope previously or implicitly defined by another
configuration method or autoconfiguration system.
10.4 Client Error 4xx
The 4xx class of status code is intended for cases in which
the client seems to have erred. Except when responding to a
HEAD request, the server SHOULD include an entity containing
an explanation of the error situation, and whether it is a
temporary or permanent condition. These status codes are
applicable to any request method. User agents SHOULD display
any included entity to the user.
Note: If the client is sending data, a server
implementation using TCP should be careful to ensure
that the client acknowledges receipt of the packet(s)
containing the response, before the server closes the
input connection. If the client continues sending data
to the server after the close, the server's TCP stack
will send a reset packet to the client, which may erase
the client's unacknowledged input buffers before they
can be read and interpreted by the HTTP application.
10.4.1 400 Bad Request
The request could not be understood by the server due to
malformed syntax. The client SHOULD NOT repeat the request
without modifications.
10.4.2 401 Unauthorized
The request requires user authentication. The response MUST
include a WWW-Authenticate header field (section 14.46)
containing a challenge applicable to the requested resource.
The client MAY repeat the request with a suitable
Authorization header field (section 14.8). If the request
already included Authorization credentials, then the 401
response indicates that authorization has been refused for
those credentials. If the 401 response contains the same
challenge as the prior response, and the user agent has
already attempted authentication at least once, then the
user SHOULD be presented the entity that was given in the
response, since that entity MAY include relevant diagnostic
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information. HTTP access authentication is explained in
section 11.
10.4.3 402 Payment Required
This code is reserved for future use.
Editor's Note: Henrik Frystyk will be drafting language to
deal with 403 vs. 404. Issue. Current wording says:
Description for "404 Not found" says "403 Forbidden" can be
used instead. As Ari Luotonen points out - this should be
the other way round .
10.4.4 403 Forbidden
The server understood the request, but is refusing to
fulfill it. Authorization will not help and the request
SHOULD NOT be repeated. If the request method was not HEAD
and the server wishes to make public why the request has not
been fulfilled, it SHOULD describe the reason for the
refusal in the entity. This status code is commonly used
when the server does not wish to reveal exactly why the
request has been refused, or when no other response is
applicable.
10.4.5 404 Not Found
The server has not found anything matching the Request-URI.
No indication is given of whether the condition is temporary
or permanent. If the server does not wish to make this
information available to the client, the status code 403
(Forbidden) can be used instead. The 410 (Gone) status code
SHOULD be used if the server knows, through some internally
configurable mechanism, that an old resource is permanently
unavailable and has no forwarding address.
10.4.6 405 Method Not Allowed
The method specified in the Request-Line is not allowed for
the resource identified by the Request-URI. The response
MUST include an Allow header containing a list of valid
methods for the requested resource.
10.4.7 406 Not Acceptable
The resource identified by the request is only capable of
generating response entities which have content
characteristics not acceptable according to the accept
headers sent in the request.
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Unless it was a HEAD request, the response SHOULD include an
entity containing a list of available entity characteristics
and location(s) from which the user or user agent can choose
the one most appropriate. The entity format is specified by
the media type given in the Content-Type header field.
Depending upon the format and the capabilities of the user
agent, selection of the most appropriate choice may be
performed automatically. However, this specification does
not define any standard for such automatic selection.
Note: HTTP/1.1 servers are allowed to return responses
which are not acceptable according to the accept
headers sent in the request. In some cases, this may
even be preferable to sending a 406 response. User
agents are encouraged to inspect the headers of an
incoming response to determine if it is acceptable. If
the response could be unacceptable, a user agent SHOULD
temporarily stop receipt of more data and query the
user for a decision on further actions.
10.4.8 407 Proxy Authentication Required
This code is similar to 401 (Unauthorized), but indicates
that the client MUST first authenticate itself with the
proxy. The proxy MUST return a Proxy-Authenticate header
field (section 14.33) containing a challenge applicable to
the proxy for the requested resource. The client MAY repeat
the request with a suitable Proxy-Authorization header field
(section 14.34). HTTP access authentication is explained in
section 11.
10.4.9 408 Request Timeout
The client did not produce a request within the time that
the server was prepared to wait. The client MAY repeat the
request without modifications at any later time.
10.4.10 409 Conflict
The request could not be completed due to a conflict with
the current state of the resource. This code is only allowed
in situations where it is expected that the user might be
able to resolve the conflict and resubmit the request. The
response body SHOULD include enough information for the user
to recognize the source of the conflict. Ideally, the
response entity would include enough information for the
user or user agent to fix the problem; however, that may not
be possible and is not required.
Conflicts are most likely to occur in response to a PUT
request. If versioning is being used and the entity being
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PUT includes changes to a resource which conflict with those
made by an earlier (third-party) request, the server MAY use
the 409 response to indicate that it can't complete the
request. In this case, the response entity SHOULD contain a
list of the differences between the two versions in a format
defined by the response Content-Type.
10.4.11 410 Gone
The requested resource is no longer available at the server
and no forwarding address is known. This condition SHOULD be
considered permanent. Clients with link editing capabilities
SHOULD delete references to the Request-URI after user
approval. If the server does not know, or has no facility to
determine, whether or not the condition is permanent, the
status code 404 (Not Found) SHOULD be used instead. This
response is cachable unless indicated otherwise.
The 410 response is primarily intended to assist the task of
web maintenance by notifying the recipient that the resource
is intentionally unavailable and that the server owners
desire that remote links to that resource be removed. Such
an event is common for limited-time, promotional services
and for resources belonging to individuals no longer working
at the server's site. It is not necessary to mark all
permanently unavailable resources as "gone" or to keep the
mark for any length of time -- that is left to the
discretion of the server owner.
10.4.12 411 Length Required
The server refuses to accept the request without a defined
Content-Length. The client MAY repeat the request if it adds
a valid Content-Length header field containing the length of
the message-body in the request message.
10.4.13 412 Precondition Failed
The precondition given in one or more of the request-header
fields evaluated to false when it was tested on the server.
This response code allows the client to place preconditions
on the current resource metainformation (header field data)
and thus prevent the requested method from being applied to
a resource other than the one intended.
10.4.14 413 Request Entity Too Large
The server is refusing to process a request because the
request entity is larger than the server is willing or able
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to process. The server may close the connection to prevent
the client from continuing the request.
If the condition is temporary, the server SHOULD include a
Retry-After header field to indicate that it is temporary
and after what time the client may try again.
10.4.15 414 Request-URI Too Long
The server is refusing to service the request because the
Request-URI is longer than the server is willing to
interpret. This rare condition is only likely to occur when
a client has improperly converted a POST request to a GET
request with long query information, when the client has
descended into a URL "black hole" of redirection (e.g., a
redirected URL prefix that points to a suffix of itself), or
when the server is under attack by a client attempting to
exploit security holes present in some servers using fixed-
length buffers for reading or manipulating the Request-URI.
10.4.16 415 Unsupported Media Type
The server is refusing to service the request because the
entity of the request is in a format not supported by the
requested resource for the requested method.
10.4.17 416 Requested range not valid
A server SHOULD return a response with this status code if a
request included a Range request-header field (section
14.36), and none of the range-specifier values in this field
overlap the current extent of the selected resource, and the
request did not include an If-Range request-header field.
(For byte-ranges, this means that the first-byte-pos of all
of the byte-range-spec values were greater than the current
length of the selected resource.)
When this status code is returned for a byte-range request,
the response MUST include a Content-Range entity-header
field specifying the current length of the selected resource
(see section 14.17). This response MUST NOT use the
multipart/byteranges content-type.
10.4.18 419 Expectation Failed
The expectation given in an Expect request-header field (see
section 14.47) could not be met by this server, or, if the
server is a proxy, the server has unambiguous evidence that
the request could not be met by the next-hop server
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10.5 Server Error 5xx
Response status codes beginning with the digit "5" indicate
cases in which the server is aware that it has erred or is
incapable of performing the request. Except when responding
to a HEAD request, the server SHOULD include an entity
containing an explanation of the error situation, and
whether it is a temporary or permanent condition. User
agents SHOULD display any included entity to the user. These
response codes are applicable to any request method.
10.5.1 500 Internal Server Error
The server encountered an unexpected condition which
prevented it from fulfilling the request.
10.5.2 501 Not Implemented
The server does not support the functionality required to
fulfill the request. This is the appropriate response when
the server does not recognize the request method and is not
capable of supporting it for any resource.
10.5.3 502 Bad Gateway
The server, while acting as a gateway or proxy, received an
invalid response from the upstream server it accessed in
attempting to fulfill the request.
10.5.4 503 Service Unavailable
The server is currently unable to handle the request due to
a temporary overloading or maintenance of the server. The
implication is that this is a temporary condition which will
be alleviated after some delay. If known, the length of the
delay may be indicated in a Retry-After header. If no Retry-
After is given, the client SHOULD handle the response as it
would for a 500 response.
Note: The existence of the 503 status code does not
imply that a server must use it when becoming
overloaded. Some servers may wish to simply refuse the
connection.
10.5.5 504 Gateway Timeout
The server, while acting as a gateway or proxy, did not
receive a timely response from the upstream server it
accessed in attempting to complete the request.
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10.5.6 505 HTTP Version Not Supported
The server does not support, or refuses to support, the HTTP
protocol version that was used in the request message. The
server is indicating that it is unable or unwilling to
complete the request using the same major version as the
client, as described in section 3.1, other than with this
error message. The response SHOULD contain an entity
describing why that version is not supported and what other
protocols are supported by that server.
10.5.7 506 Redirection Failed
The 506 response is returned when a redirection fails or is
refused by a proxy or client. If the redirection response
included a body, then it SHOULD be included in the 506
response.
This response is returned by a proxy, to a downstream proxy
or client, when it cannot or chooses not to honor a
redirection.
11 Access Authentication
Editor's note: This section (11) will be removed from future
drafts of this document, and combined with Digest
authentication, which will then become a more general
document "Authentication in HTTP".
HTTP provides a simple challenge-response authentication
mechanism which MAY be used by a server to challenge a
client request and by a client to provide authentication
information. It uses an extensible, case-insensitive token
to identify the authentication scheme, followed by a comma-
separated list of attribute-value pairs which carry the
parameters necessary for achieving authentication via that
scheme.
auth-scheme = token
auth-param = token "=" quoted-string
The 401 (Unauthorized) response message is used by an origin
server to challenge the authorization of a user agent. This
response MUST include a WWW-Authenticate header field
containing at least one challenge applicable to the
requested resource. The 407 (Proxy Authentication Required)
response message is used by a proxy to challenge the
authorization of a client and MUST include a Proxy-
Authenticate header field containing a challenge applicable
to the proxy for the requested resource.
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challenge = auth-scheme 1*SP realm *( "," auth-
param )
realm = "realm" "=" realm-value
realm-value = quoted-string
The realm attribute (case-insensitive) is required for all
authentication schemes which issue a challenge. The realm
value (case-sensitive), in combination with the canonical
root URL (see section 5.1.2) of the server being accessed,
defines the protection space. These realms allow the
protected resources on a server to be partitioned into a set
of protection spaces, each with its own authentication
scheme and/or authorization database. The realm value is a
string, generally assigned by the origin server, which may
have additional semantics specific to the authentication
scheme.
A user agent that wishes to authenticate itself with an
origin server--usually, but not necessarily, after receiving
a 401 (Unauthorized)--MAY do so by including an
Authorization header field with the request. A client that
wishes to authenticate itself with a proxy--usually, but not
necessarily, after receiving a 407 (Proxy Authentication
Required)--MAY do so by including a Proxy-Authoraiztion
header field with the request. Both the Authorization field
value and the Proxy-Authorization field value consists of
credentials containing the authentication information of the
client for the realm of the resource being requested.
credentials = basic-credentials
| auth-scheme #auth-param
The domain over which credentials can be automatically
applied by a client is determined by the protection space.
If a prior request has been authorized, the same credentials
MAY be reused for all other requests within that protection
space for a period of time determined by the authentication
scheme, parameters, and/or user preference. Unless otherwise
defined by the authentication scheme, a single protection
space cannot extend outside the scope of its server.
If the origin server does not wish to accept the credentials
sent with a request, it SHOULD return a 401 (Unauthorized)
response. The response MUST include a WWW-Authenticate
header field containing at least one (possibly new)
challenge applicable to the requested resource. If a proxy
does not accept the credentials sent with a request, it
SHOULD return a 407 (Proxy Authentication Required). The
response MUST include a Proxy-Authenticate header field
containing a (possibly new) challenge applicable to the
proxy for the requested resource.
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The HTTP protocol does not restrict applications to this
simple challenge-response mechanism for access
authentication. Additional mechanisms MAY be used, such as
encryption at the transport level or via message
encapsulation, and with additional header fields specifying
authentication information. However, these additional
mechanisms are not defined by this specification.
Proxies MUST be completely transparent regarding user agent
authentication by origin servers. That is, they MUST forward
the WWW-Authenticate and Authorization headers untouched,
and follow the rules found in section 14.8. Both the Proxy-
Authenticate and the Proxy-Authorization header fields are
hop-by-hop headers (see section 13.5.1).
11.1 Basic Authentication Scheme
The "basic" authentication scheme is based on the model that
the client must authenticate itself with a user-ID and a
password for each realm. The realm value should be
considered an opaque string which can only be compared for
equality with other realms on that server. The server will
service the request only if it can validate the user-ID and
password for the protection space of the Request-URI. There
are no optional authentication parameters.
Upon receipt of an unauthorized request for a URI within the
protection space, the origin server MAY respond with a
challenge like the following:
WWW-Authenticate: Basic realm="WallyWorld"
where "WallyWorld" is the string assigned by the server to
identify the protection space of the Request-URI. A proxy
may respond with the same challenge using the Proxy-
Authenticate header field.
To receive authorization, the client sends the userid and
password, separated by a single colon (":") character,
within a base64 [7] encoded string in the credentials.
basic-credentials = "Basic" SP base64-user-pass
base64-user-pass = <base64 [7] encoding of user-pass,
except not limited to 76 char/line>
user-pass = userid ":" password
userid = *<TEXT excluding ":">
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password = *TEXT
Userids might be case sensitive.
If the user agent wishes to send the userid "Aladdin" and
password "open sesame", it would use the following header
field:
Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==
If a client wishes to send the same userid and password to a
proxy, it would use the Proxy-Authorization header field.
See section 15 for security considerations associated with
Basic authentication.
11.2 Digest Authentication Scheme
A digest authentication for HTTP is specified in RFC 2069
[32].
12 Content Negotiation
Most HTTP responses include an entity which contains
information for interpretation by a human user. Naturally,
it is desirable to supply the user with the "best available"
entity corresponding to the request. Unfortunately for
servers and caches, not all users have the same preferences
for what is "best," and not all user agents are equally
capable of rendering all entity types. For that reason, HTTP
has provisions for several mechanisms for "content
negotiation" -- the process of selecting the best
representation for a given response when there are multiple
representations available.
Note: This is not called "format negotiation" because
the alternate representations may be of the same media
type, but use different capabilities of that type, be
in different languages, etc.
Any response containing an entity-body MAY be subject to
negotiation, including error responses.
There are two kinds of content negotiation which are
possible in HTTP: server-driven and agent-driven
negotiation. These two kinds of negotiation are orthogonal
and thus may be used separately or in combination. One
method of combination, referred to as transparent
negotiation, occurs when a cache uses the agent-driven
negotiation information provided by the origin server in
order to provide server-driven negotiation for subsequent
requests.
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12.1 Server-driven Negotiation
If the selection of the best representation for a response
is made by an algorithm located at the server, it is called
server-driven negotiation. Selection is based on the
available representations of the response (the dimensions
over which it can vary; e.g. language, content-coding, etc.)
and the contents of particular header fields in the request
message or on other information pertaining to the request
(such as the network address of the client).
Server-driven negotiation is advantageous when the algorithm
for selecting from among the available representations is
difficult to describe to the user agent, or when the server
desires to send its "best guess" to the client along with
the first response (hoping to avoid the round-trip delay of
a subsequent request if the "best guess" is good enough for
the user). In order to improve the server's guess, the user
agent MAY include request header fields (Accept, Accept-
Language, Accept-Encoding, etc.) which describe its
preferences for such a response.
Server-driven negotiation has disadvantages:
1.It is impossible for the server to accurately determine
what might be "best" for any given user, since that
would require complete knowledge of both the
capabilities of the user agent and the intended use for
the response (e.g., does the user want to view it on
screen or print it on paper?).
2.Having the user agent describe its capabilities in
every request can be both very inefficient (given that
only a small percentage of responses have multiple
representations) and a potential violation of the
user's privacy.
3.It complicates the implementation of an origin server
and the algorithms for generating responses to a
request.
4.It may limit a public cache's ability to use the same
response for multiple user's requests.
HTTP/1.1 includes the following request-header fields for
enabling server-driven negotiation through description of
user agent capabilities and user preferences: Accept
(section 14.1), Accept-Charset (section 14.2), Accept-
Encoding (section 14.3), Accept-Language (section 14.4), and
User-Agent (section 14.42). However, an origin server is not
limited to these dimensions and MAY vary the response based
on any aspect of the request, including information outside
the request-header fields or within extension header fields
not defined by this specification.
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HTTP/1.1 origin servers MUST include an appropriate Vary
header field (section 14.43) in any cachable response based
on server-driven negotiation. The Vary header field
describes the dimensions over which the response might vary
(i.e. the dimensions over which the origin server picks its
"best guess" response from multiple representations).
HTTP/1.1 public caches MUST recognize the Vary header field
when it is included in a response and obey the requirements
described in section 13.6 that describes the interactions
between caching and content negotiation.
12.2 Agent-driven Negotiation
With agent-driven negotiation, selection of the best
representation for a response is performed by the user agent
after receiving an initial response from the origin server.
Selection is based on a list of the available
representations of the response included within the header
fields (this specification reserves the field-name
Alternates, as described in appendix 19.6.1.0) or entity-
body of the initial response, with each representation
identified by its own URI. Selection from among the
representations may be performed automatically (if the user
agent is capable of doing so) or manually by the user
selecting from a generated (possibly hypertext) menu.
Agent-driven negotiation is advantageous when the response
would vary over commonly-used dimensions (such as type,
language, or encoding), when the origin server is unable to
determine a user agent's capabilities from examining the
request, and generally when public caches are used to
distribute server load and reduce network usage.
Agent-driven negotiation suffers from the disadvantage of
needing a second request to obtain the best alternate
representation. This second request is only efficient when
caching is used. In addition, this specification does not
define any mechanism for supporting automatic selection,
though it also does not prevent any such mechanism from
being developed as an extension and used within HTTP/1.1.
HTTP/1.1 defines the 300 (Multiple Choices) and 406 (Not
Acceptable) status codes for enabling agent-driven
negotiation when the server is unwilling or unable to
provide a varying response using server-driven negotiation.
12.3 Transparent Negotiation
Transparent negotiation is a combination of both server-
driven and agent-driven negotiation. When a cache is
supplied with a form of the list of available
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representations of the response (as in agent-driven
negotiation) and the dimensions of variance are completely
understood by the cache, then the cache becomes capable of
performing server-driven negotiation on behalf of the origin
server for subsequent requests on that resource.
Transparent negotiation has the advantage of distributing
the negotiation work that would otherwise be required of the
origin server and also removing the second request delay of
agent-driven negotiation when the cache is able to correctly
guess the right response.
This specification does not define any mechanism for
transparent negotiation, though it also does not prevent any
such mechanism from being developed as an extension and used
within HTTP/1.1. An HTTP/1.1 cache performing transparent
negotiation MUST include a Vary header field in the response
(defining the dimensions of its variance) if it is cachable
to ensure correct interoperation with all HTTP/1.1 clients.
The agent-driven negotiation information supplied by the
origin server SHOULD be included with the transparently
negotiated response.
13 Caching in HTTP
HTTP is typically used for distributed information systems,
where performance can be improved by the use of response
caches. The HTTP/1.1 protocol includes a number of elements
intended to make caching work as well as possible. Because
these elements are inextricable from other aspects of the
protocol, and because they interact with each other, it is
useful to describe the basic caching design of HTTP
separately from the detailed descriptions of methods,
headers, response codes, etc.
Caching would be useless if it did not significantly improve
performance. The goal of caching in HTTP/1.1 is to eliminate
the need to send requests in many cases, and to eliminate
the need to send full responses in many other cases. The
former reduces the number of network round-trips required
for many operations; we use an "expiration" mechanism for
this purpose (see section 13.2). The latter reduces network
bandwidth requirements; we use a "validation" mechanism for
this purpose (see section 13.3).
Requirements for performance, availability, and disconnected
operation require us to be able to relax the goal of
semantic transparency. The HTTP/1.1 protocol allows origin
servers, caches, and clients to explicitly reduce
transparency when necessary. However, because non-
transparent operation may confuse non-expert users, and may
be incompatible with certain server applications (such as
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those for ordering merchandise), the protocol requires that
transparency be relaxed
. only by an explicit protocol-level request when relaxed
by client or origin server
. only with an explicit warning to the end user when
relaxed by cache or client
Therefore, the HTTP/1.1 protocol provides these important
elements:
1.Protocol features that provide full semantic
transparency when this is required by all parties.
2.Protocol features that allow an origin server or user
agent to explicitly request and control non-transparent
operation.
3.Protocol features that allow a cache to attach warnings
to responses that do not preserve the requested
approximation of semantic transparency.
A basic principle is that it must be possible for the
clients to detect any potential relaxation of semantic
transparency.
Note: The server, cache, or client implementer may be
faced with design decisions not explicitly discussed in
this specification. If a decision may affect semantic
transparency, the implementer ought to err on the side
of maintaining transparency unless a careful and
complete analysis shows significant benefits in
breaking transparency.
13.1.1 Cache Correctness
A correct cache MUST respond to a request with the most up-
to-date response held by the cache that is appropriate to
the request (see sections 13.2.5, 13.2.6, and 13.12) which
meets one of the following conditions:
1.It has been checked for equivalence with what the
origin server would have returned by revalidating the
response with the origin server (section 13.3);
2.It is "fresh enough" (see section 13.2). In the default
case, this means it meets the least restrictive
freshness requirement of the client, origin server, and
cache (see section 14.9); if the origin server so
specifies, it is the freshness requirement of the
origin server alone.
If a stored response is not "fresh enough" by the most
restrictive freshness requirement of both the client
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and the origin server, in carefully considered
circumstances the cache may still return the response
with the appropriate Warning header (see section 13.1.5
and 14.45), unless such a response is prohibited (e.g.,
by a "no-store" cache-directive, or by a "no-cache"
cache-request-directive; see section 14.9). \*
MERGEFORMAT \* MERGEFORMAT
3.It is an appropriate 304 (Not Modified), 305 (Proxy
Redirect), or error (4xx or 5xx) response message.
If the cache can not communicate with the origin server,
then a correct cache SHOULD respond as above if the response
can be correctly served from the cache; if not it MUST
return an error or warning indicating that there was a
communication failure.
If a cache receives a response (either an entire response,
or a 304 (Not Modified) response) that it would normally
forward to the requesting client, and the received response
is no longer fresh, the cache SHOULD forward it to the
requesting client without adding a new Warning (but without
removing any existing Warning headers). A cache SHOULD NOT
attempt to revalidate a response simply because that
response became stale in transit; this might lead to an
infinite loop. An user agent that receives a stale response
without a Warning MAY display a warning indication to the
user.
13.1.2 Warnings
Whenever a cache returns a response that is neither first-
hand nor "fresh enough" (in the sense of condition 2 in
section 13.1.1), it must attach a warning to that effect,
using a Warning response-header. This warning allows clients
to take appropriate action.
Warnings may be used for other purposes, both cache-related
and otherwise. The use of a warning, rather than an error
status code, distinguish these responses from true failures.
Warnings come in two categories:
1.Those that describe the freshness or revalidation
status of the response, and so MUST be deleted after a
successful revalidation (see section 13.3 for a
definition of revalidation).
2.Those that describe some aspect of the entity body or
entity headers that are not rectified by a
revalidation; for example, a lossy compression of the
entity bodys. These warnings MUST NOT be deleted after
a successful revalidation.
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Warnings are assigned 3-digit code numbers. The first digit
indicates whether the Warning must or must not be deleted
from a cached response after it is successfully revalidated.
This specification defines the code numbers and meanings of
each currently assigned warning, allowing a client or cache
to take automated action in some (but not all) cases.
HTTP/1.0 caches will cache all Warnings, without deleting
the ones in the first category. Warnings that are passed to
HTTP/1.0 caches carry an extra warning-date field, which
prevents a future HTTP/1.1 recipient from believing an
erroneously cached Warning.
Warnings also carry a warning text. The text may be in any
appropriate natural language (perhaps based on the client's
Accept headers), and include an optional indication of what
character set is used.
Multiple warnings may be attached to a response (either by
the origin server or by a cache), including multiple
warnings with the same code number. For example, a server
may provide the same warning with texts in both English and
Basque.
When multiple warnings are attached to a response, it may
not be practical or reasonable to display all of them to the
user. This version of HTTP does not specify strict priority
rules for deciding which warnings to display and in what
order, but does suggest some heuristics.
The Warning header and the currently defined warnings are
described in section 14.45.
13.1.3 Cache-control Mechanisms
The basic cache mechanisms in HTTP/1.1 (server-specified
expiration times and validators) are implicit directives to
caches. In some cases, a server or client may need to
provide explicit directives to the HTTP caches. We use the
Cache-Control header for this purpose.
The Cache-Control header allows a client or server to
transmit a variety of directives in either requests or
responses. These directives typically override the default
caching algorithms. As a general rule, if there is any
apparent conflict between header values, the most
restrictive interpretation should be applied (that is, the
one that is most likely to preserve semantic transparency).
However, in some cases, Cache-Control directives are
explicitly specified as weakening the approximation of
semantic transparency (for example, "max-stale" or
"public").
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The Cache-Control directives are described in detail in
section 14.9.
13.1.4 Explicit User Agent Warnings
Many user agents make it possible for users to override the
basic caching mechanisms. For example, the user agent may
allow the user to specify that cached entities (even
explicitly stale ones) are never validated. Or the user
agent might habitually add "Cache-Control: max-stale=3600"
to every request. The user should have to explicitly request
either non-transparent behavior, or behavior that results in
abnormally ineffective caching.
If the user has overridden the basic caching mechanisms, the
user agent should explicitly indicate to the user whenever
this results in the display of information that might not
meet the server's transparency requirements (in particular,
if the displayed entity is known to be stale). Since the
protocol normally allows the user agent to determine if
responses are stale or not, this indication need only be
displayed when this actually happens. The indication need
not be a dialog box; it could be an icon (for example, a
picture of a rotting fish) or some other visual indicator.
If the user has overridden the caching mechanisms in a way
that would abnormally reduce the effectiveness of caches,
the user agent should continually display an indication (for
example, a picture of currency in flames) so that the user
does not inadvertently consume excess resources or suffer
from excessive latency.
13.1.5 Exceptions to the Rules and Warnings
In some cases, the operator of a cache may choose to
configure it to return stale responses even when not
requested by clients. This decision should not be made
lightly, but may be necessary for reasons of availability or
performance, especially when the cache is poorly connected
to the origin server. Whenever a cache returns a stale
response, it MUST mark it as such (using a Warning header).
This allows the client software to alert the user that there
may be a potential problem.
It also allows the user agent to take steps to obtain a
first-hand or fresh response. For this reason, a cache
SHOULD NOT return a stale response if the client explicitly
requests a first-hand or fresh one, unless it is impossible
to comply for technical or policy reasons.
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13.1.6 Client-controlled Behavior
While the origin server (and to a lesser extent,
intermediate caches, by their contribution to the age of a
response) are the primary source of expiration information,
in some cases the client may need to control a cache's
decision about whether to return a cached response without
validating it. Clients do this using several directives of
the Cache-Control header.
A client's request may specify the maximum age it is willing
to accept of an unvalidated response; specifying a value of
zero forces the cache(s) to revalidate all responses. A
client may also specify the minimum time remaining before a
response expires. Both of these options increase constraints
on the behavior of caches, and so cannot further relax the
cache's approximation of semantic transparency.
A client may also specify that it will accept stale
responses, up to some maximum amount of staleness. This
loosens the constraints on the caches, and so may violate
the origin server's specified constraints on semantic
transparency, but may be necessary to support disconnected
operation, or high availability in the face of poor
connectivity.
13.2 Expiration Model
13.2.1 Server-Specified Expiration
HTTP caching works best when caches can entirely avoid
making requests to the origin server. The primary mechanism
for avoiding requests is for an origin server to provide an
explicit expiration time in the future, indicating that a
response may be used to satisfy subsequent requests. In
other words, a cache can return a fresh response without
first contacting the server.
Our expectation is that servers will assign future explicit
expiration times to responses in the belief that the entity
is not likely to change, in a semantically significant way,
before the expiration time is reached. This normally
preserves semantic transparency, as long as the server's
expiration times are carefully chosen.
The expiration mechanism applies only to responses taken
from a cache and not to first-hand responses forwarded
immediately to the requesting client.
If an origin server wishes to force a semantically
transparent cache to validate every request, it may assign
an explicit expiration time in the past. This means that the
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response is always stale, and so the cache SHOULD validate
it before using it for subsequent requests. See section
14.9.4 for a more restrictive way to force revalidation.
If an origin server wishes to force any HTTP/1.1 cache, no
matter how it is configured, to validate every request, it
should use the "must-revalidate" Cache-Control directive
(see section 14.9).
Servers specify explicit expiration times using either the
Expires header, or the max-age directive of the Cache-
Control header.
An expiration time cannot be used to force a user agent to
refresh its display or reload a resource; its semantics
apply only to caching mechanisms, and such mechanisms need
only check a resource's expiration status when a new request
for that resource is initiated. See section 13.13 for
explanation of the difference between caches and history
mechanisms.
13.2.2 Heuristic Expiration
Since origin servers do not always provide explicit
expiration times, HTTP caches typically assign heuristic
expiration times, employing algorithms that use other header
values (such as the Last-Modified time) to estimate a
plausible expiration time. The HTTP/1.1 specification does
not provide specific algorithms, but does impose worst-case
constraints on their results. Since heuristic expiration
times may compromise semantic transparency, they should be
used cautiously, and we encourage origin servers to provide
explicit expiration times as much as possible.
13.2.3 Age Calculations
In order to know if a cached entry is fresh, a cache needs
to know if its age exceeds its freshness lifetime. We
discuss how to calculate the latter in section 13.2.4; this
section describes how to calculate the age of a response or
cache entry.
In this discussion, we use the term "now" to mean "the
current value of the clock at the host performing the
calculation." Hosts that use HTTP, but especially hosts
running origin servers and caches, should use NTP [28] or
some similar protocol to synchronize their clocks to a
globally accurate time standard.
Also note that HTTP/1.1 requires origin servers to send a
Date header with every response, giving the time at which
the response was generated. We use the term "date_value" to
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denote the value of the Date header, in a form appropriate
for arithmetic operations.
HTTP/1.1 uses the Age response-header to help convey age
information between caches. The Age header value is the
sender's estimate of the amount of time since the response
was generated at the origin server. In the case of a cached
response that has been revalidated with the origin server,
the Age value is based on the time of revalidation, not of
the original response.
In essence, the Age value is the sum of the time that the
response has been resident in each of the caches along the
path from the origin server, plus the amount of time it has
been in transit along network paths.
We use the term "age_value" to denote the value of the Age
header, in a form appropriate for arithmetic operations.
A response's age can be calculated in two entirely
independent ways:
1.now minus date_value, if the local clock is reasonably
well synchronized to the origin server's clock. If the
result is negative, the result is replaced by zero.
2.age_value, if all of the caches along the response path
implement HTTP/1.1.
Given that we have two independent ways to compute the age
of a response when it is received, we can combine these as
corrected_received_age = max(now - date_value,
age_value)
and as long as we have either nearly synchronized clocks or
all-HTTP/1.1 paths, one gets a reliable (conservative)
result.
Note that this correction is applied at each HTTP/1.1 cache
along the path, so that if there is an HTTP/1.0 cache in the
path, the correct received age is computed as long as the
receiving cache's clock is nearly in sync. We don't need
end-to-end clock synchronization (although it is good to
have), and there is no explicit clock synchronization step.
Because of network-imposed delays, some significant interval
may pass from the time that a server generates a response
and the time it is received at the next outbound cache or
client. If uncorrected, this delay could result in
improperly low ages.
Because the request that resulted in the returned Age value
must have been initiated prior to that Age value's
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generation, we can correct for delays imposed by the network
by recording the time at which the request was initiated.
Then, when an Age value is received, it MUST be interpreted
relative to the time the request was initiated, not the time
that the response was received. This algorithm results in
conservative behavior no matter how much delay is
experienced. So, we compute:
corrected_initial_age = corrected_received_age
+ (now - request_time)
where "request_time" is the time (according to the local
clock) when the request that elicited this response was
sent.
Summary of age calculation algorithm, when a cache receives
a response:
/*
* age_value
* is the value of Age: header received by the cache with
* this response.
* date_value
* is the value of the origin server's Date: header
* request_time
* is the (local) time when the cache made the request
* that resulted in this cached response
* response_time
* is the (local) time when the cache received the
* response
* now
* is the current (local) time
*/
apparent_age = max(0, response_time - date_value);
corrected_received_age = max(apparent_age, age_value);
response_delay = response_time - request_time;
corrected_initial_age = corrected_received_age + response_delay;
resident_time = now - response_time;
current_age = corrected_initial_age + resident_time;
When a cache sends a response, it must add to the
corrected_initial_age the amount of time that the response
was resident locally. It must then transmit this total age,
using the Age header, to the next recipient cache.
Note that a client cannot reliably tell that a response
is first-hand, but the presence of an Age header
indicates that a response is definitely not first-hand.
Also, if the Date in a response is earlier than the
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client's local request time, the response is probably
not first-hand (in the absence of serious clock skew).
13.2.4 Expiration Calculations
In order to decide whether a response is fresh or stale, we
need to compare its freshness lifetime to its age. The age
is calculated as described in section 13.2.3; this section
describes how to calculate the freshness lifetime, and to
determine if a response has expired. In the discussion
below, the values can be represented in any form appropriate
for arithmetic operations.
We use the term "expires_value" to denote the value of the
Expires header. We use the term "max_age_value" to denote an
appropriate value of the number of seconds carried by the
max-age directive of the Cache-Control header in a response
(see section 14.10.
The max-age directive takes priority over Expires, so if
max-age is present in a response, the calculation is simply:
freshness_lifetime = max_age_value
Otherwise, if Expires is present in the response, the
calculation is:
freshness_lifetime = expires_value - date_value
Note that neither of these calculations is vulnerable to
clock skew, since all of the information comes from the
origin server.
If neither Expires nor Cache-Control: max-age or s-maxage
(see section 14.9.3) appears in the response, and the
response does not include other restrictions on caching, the
cache MAY compute a freshness lifetime using a heuristic. If
the value is greater than 24 hours, the cache must attach
Warning 13 to any response whose age is more than 24 hours
if such warning has not already been added.
Also, if the response does have a Last-Modified time, the
heuristic expiration value SHOULD be no more than some
fraction of the interval since that time. A typical setting
of this fraction might be 10%.
The calculation to determine if a response has expired is
quite simple:
response_is_fresh = (freshness_lifetime > current_age)
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13.2.5 Disambiguating Expiration Values
Because expiration values are assigned optimistically, it is
possible for two caches to contain fresh values for the same
resource that are different.
If a client performing a retrieval receives a non-first-hand
response for a request that was already fresh in its own
cache, and the Date header in its existing cache entry is
newer than the Date on the new response, then the client MAY
ignore the response. If so, it MAY retry the request with a
"Cache-Control: max-age=0" directive (see section 14.9), to
force a check with the origin server.
If a cache has two fresh responses for the same
representation with different validators, it MUST use the
one with the more recent Date header. This situation may
arise because the cache is pooling responses from other
caches, or because a client has asked for a reload or a
revalidation of an apparently fresh cache entry.
13.2.6 Disambiguating Multiple Responses
Because a client may be receiving responses via multiple
paths, so that some responses flow through one set of caches
and other responses flow through a different set of caches,
a client may receive responses in an order different from
that in which the origin server sent them. We would like the
client to use the most recently generated response, even if
older responses are still apparently fresh.
Neither the entity tag nor the expiration value can impose
an ordering on responses, since it is possible that a later
response intentionally carries an earlier expiration time.
However, the HTTP/1.1 specification requires the
transmission of Date headers on every response, and the Date
values are ordered to a granularity of one second.
When a client tries to revalidate a cache entry, and the
response it receives contains a Date header that appears to
be older than the one for the existing entry, then the
client SHOULD repeat the request unconditionally, and
include
Cache-Control: max-age=0
to force any intermediate caches to validate their copies
directly with the origin server, or
Cache-Control: no-cache
to force any intermediate caches to obtain a new copy from
the origin server.
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If the Date values are equal, then the client may use either
response (or may, if it is being extremely prudent, request
a new response). Servers MUST NOT depend on clients being
able to choose deterministically between responses generated
during the same second, if their expiration times overlap.
13.3 Validation Model
When a cache has a stale entry that it would like to use as
a response to a client's request, it first has to check with
the origin server (or possibly an intermediate cache with a
fresh response) to see if its cached entry is still usable.
We call this "validating" the cache entry. Since we do not
want to have to pay the overhead of retransmitting the full
response if the cached entry is good, and we do not want to
pay the overhead of an extra round trip if the cached entry
is invalid, the HTTP/1.1 protocol supports the use of
conditional methods.
The key protocol features for supporting conditional methods
are those concerned with "cache validators." When an origin
server generates a full response, it attaches some sort of
validator to it, which is kept with the cache entry. When a
client (user agent or proxy cache) makes a conditional
request for a resource for which it has a cache entry, it
includes the associated validator in the request.
The server then checks that validator against the current
validator for the entity, and, if they match, it responds
with a special status code (usually, 304 (Not Modified)) and
no entity-body. Otherwise, it returns a full response
(including entity-body). Thus, we avoid transmitting the
full response if the validator matches, and we avoid an
extra round trip if it does not match.
Note: the comparison functions used to decide if
validators match are defined in section 13.3.3.
In HTTP/1.1, a conditional request looks exactly the same as
a normal request for the same resource, except that it
carries a special header (which includes the validator) that
implicitly turns the method (usually, GET) into a
conditional.
The protocol includes both positive and negative senses of
cache-validating conditions. That is, it is possible to
request either that a method be performed if and only if a
validator matches or if and only if no validators match.
Note: a response that lacks a validator may still be
cached, and served from cache until it expires, unless
this is explicitly prohibited by a Cache-Control
directive. However, a cache cannot do a conditional
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retrieval if it does not have a validator for the
entity, which means it will not be refreshable after it
expires.
13.3.1 Last-modified Dates
The Last-Modified entity-header field value is often used as
a cache validator. In simple terms, a cache entry is
considered to be valid if the entity has not been modified
since the Last-Modified value.
13.3.2 Entity Tag Cache Validators
The ETag entity-header field value, an entity tag, provides
for an "opaque" cache validator. This may allow more
reliable validation in situations where it is inconvenient
to store modification dates, where the one-second resolution
of HTTP date values is not sufficient, or where the origin
server wishes to avoid certain paradoxes that may arise from
the use of modification dates.
Entity Tags are described in section 3.11. The headers used
with entity tags are described in sections 14.20, 14.25,
14.26 and 14.43.
13.3.3 Weak and Strong Validators
Since both origin servers and caches will compare two
validators to decide if they represent the same or different
entities, one normally would expect that if the entity (the
entity-body or any entity-headers) changes in any way, then
the associated validator would change as well. If this is
true, then we call this validator a "strong validator."
However, there may be cases when a server prefers to change
the validator only on semantically significant changes, and
not when insignificant aspects of the entity change. A
validator that does not always change when the resource
changes is a "weak validator."
Entity tags are normally "strong validators," but the
protocol provides a mechanism to tag an entity tag as
"weak." One can think of a strong validator as one that
changes whenever the bits of an entity changes, while a weak
value changes whenever the meaning of an entity changes.
Alternatively, one can think of a strong validator as part
of an identifier for a specific entity, while a weak
validator is part of an identifier for a set of semantically
equivalent entities.
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Note: One example of a strong validator is an integer
that is incremented in stable storage every time an
entity is changed.
An entity's modification time, if represented with one-
second resolution, could be a weak validator, since it
is possible that the resource may be modified twice
during a single second.
Support for weak validators is optional; however, weak
validators allow for more efficient caching of
equivalent objects; for example, a hit counter on a
site is probably good enough if it is updated every few
days or weeks, and any value during that period is
likely "good enough" to be equivalent.
A "use" of a validator is either when a client generates a
request and includes the validator in a validating header
field, or when a server compares two validators.
Strong validators are usable in any context. Weak validators
are only usable in contexts that do not depend on exact
equality of an entity. For example, either kind is usable
for a conditional GET of a full entity. However, only a
strong validator is usable for a sub-range retrieval, since
otherwise the client may end up with an internally
inconsistent entity.
The only function that the HTTP/1.1 protocol defines on
validators is comparison. There are two validator comparison
functions, depending on whether the comparison context
allows the use of weak validators or not:
. The strong comparison function: in order to be
considered equal, both validators must be identical in
every way, and neither may be weak.
. The weak comparison function: in order to be considered
equal, both validators must be identical in every way,
but either or both of them may be tagged as "weak"
without affecting the result.
The weak comparison function MAY be used for simple (non-
subrange) GET requests. The strong comparison function MUST
be used in all other cases.
An entity tag is strong unless it is explicitly tagged as
weak. Section 3.11 gives the syntax for entity tags.
A Last-Modified time, when used as a validator in a request,
is implicitly weak unless it is possible to deduce that it
is strong, using the following rules:
. The validator is being compared by an origin server to
the actual current validator for the entity and,
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. That origin server reliably knows that the associated
entity did not change twice during the second covered
by the presented validator.
or
. The validator is about to be used by a client in an If-
Modified-Since or If-Unmodified-Since header, because
the client has a cache entry for the associated entity,
and
. That cache entry includes a Date value, which gives the
time when the origin server sent the original response,
and
. The presented Last-Modified time is at least 60 seconds
before the Date value.
or
. The validator is being compared by an intermediate
cache to the validator stored in its cache entry for
the entity, and
. That cache entry includes a Date value, which gives the
time when the origin server sent the original response,
and
. The presented Last-Modified time is at least 60 seconds
before the Date value.
This method relies on the fact that if two different
responses were sent by the origin server during the same
second, but both had the same Last-Modified time, then at
least one of those responses would have a Date value equal
to its Last-Modified time. The arbitrary 60-second limit
guards against the possibility that the Date and Last-
Modified values are generated from different clocks, or at
somewhat different times during the preparation of the
response. An implementation may use a value larger than 60
seconds, if it is believed that 60 seconds is too short.
If a client wishes to perform a sub-range retrieval on a
value for which it has only a Last-Modified time and no
opaque validator, it may do this only if the Last-Modified
time is strong in the sense described here.
A cache or origin server receiving a cache-conditional
request, other than a full-body GET request, MUST use the
strong comparison function to evaluate the condition.
These rules allow HTTP/1.1 caches and clients to safely
perform sub-range retrievals on values that have been
obtained from HTTP/1.0 servers.
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13.3.4 Rules for When to Use Entity Tags and Last-modified
Dates
We adopt a set of rules and recommendations for origin
servers, clients, and caches regarding when various
validator types should be used, and for what purposes.
HTTP/1.1 origin servers:
. SHOULD send an entity tag validator unless it is not
feasible to generate one.
. MAY send a weak entity tag instead of a strong entity
tag, if performance considerations support the use of
weak entity tags, or if it is unfeasible to send a
strong entity tag.
. SHOULD send a Last-Modified value if it is feasible to
send one, unless the risk of a breakdown in semantic
transparency that could result from using this date in
an If-Modified-Since header would lead to serious
problems.
In other words, the preferred behavior for an HTTP/1.1
origin server is to send both a strong entity tag and a
Last-Modified value.
In order to be legal, a strong entity tag MUST change
whenever the associated entity value changes in any way. A
weak entity tag SHOULD change whenever the associated entity
changes in a semantically significant way.
Note: in order to provide semantically transparent
caching, an origin server must avoid reusing a specific
strong entity tag value for two different entities, or
reusing a specific weak entity tag value for two
semantically different entities. Cache entries may
persist for arbitrarily long periods, regardless of
expiration times, so it may be inappropriate to expect
that a cache will never again attempt to validate an
entry using a validator that it obtained at some point
in the past.
HTTP/1.1 clients:
. If an entity tag has been provided by the origin
server, MUST use that entity tag in any cache-
conditional request (using If-Match or If-None-Match).
. If only a Last-Modified value has been provided by the
origin server, SHOULD use that value in non-subrange
cache-conditional requests (using If-Modified-Since).
. If only a Last-Modified value has been provided by an
HTTP/1.0 origin server, MAY use that value in subrange
cache-conditional requests (using If-Unmodified-
Since:). The user agent should provide a way to disable
this, in case of difficulty.
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. If both an entity tag and a Last-Modified value have
been provided by the origin server, SHOULD use both
validators in cache-conditional requests. This allows
both HTTP/1.0 and HTTP/1.1 caches to respond
appropriately.
An HTTP/1.1 cache, upon receiving a request, MUST use the
most restrictive validator when deciding whether the
client's cache entry matches the cache's own cache entry.
This is only an issue when the request contains both an
entity tag and a last-modified-date validator (If-Modified-
Since or If-Unmodified-Since).
A note on rationale: The general principle behind these
rules is that HTTP/1.1 servers and clients should
transmit as much non-redundant information as is
available in their responses and requests. HTTP/1.1
systems receiving this information will make the most
conservative assumptions about the validators they
receive.
HTTP/1.0 clients and caches will ignore entity tags.
Generally, last-modified values received or used by
these systems will support transparent and efficient
caching, and so HTTP/1.1 origin servers should provide
Last-Modified values. In those rare cases where the use
of a Last-Modified value as a validator by an HTTP/1.0
system could result in a serious problem, then HTTP/1.1
origin servers should not provide one.
13.3.5 Non-validating Conditionals
The principle behind entity tags is that only the service
author knows the semantics of a resource well enough to
select an appropriate cache validation mechanism, and the
specification of any validator comparison function more
complex than byte-equality would open up a can of worms.
Thus, comparisons of any other headers (except Last-
Modified, for compatibility with HTTP/1.0) are never used
for purposes of validating a cache entry.
13.4 Response Cachability
Unless specifically constrained by a Cache-Control (section
14.9) directive, a caching system may always store a
successful response (see section 13.8) as a cache entry, may
return it without validation if it is fresh, and may return
it after successful validation. If there is neither a cache
validator nor an explicit expiration time associated with a
response, we do not expect it to be cached, but certain
caches may violate this expectation (for example, when
little or no network connectivity is available). A client
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can usually detect that such a response was taken from a
cache by comparing the Date header to the current time.
Note that some HTTP/1.0 caches are known to violate
this expectation without providing any Warning.
However, in some cases it may be inappropriate for a cache
to retain an entity, or to return it in response to a
subsequent request. This may be because absolute semantic
transparency is deemed necessary by the service author, or
because of security or privacy considerations. Certain
Cache-Control directives are therefore provided so that the
server can indicate that certain resource entities, or
portions thereof, may not be cached regardless of other
considerations.
Note that section 14.8 normally prevents a shared cache from
saving and returning a response to a previous request if
that request included an Authorization header.
A response received with a status code of 200, 203, 206,
300, 301 or 410 may be stored by a cache and used in reply
to a subsequent request, subject to the expiration
mechanism, unless a Cache-Control directive prohibits
caching. However, a cache that does not support the Range
and Content-Range headers MUST NOT cache 206 (Partial
Content) responses.
A response received with any other status code MUST NOT be
returned in a reply to a subsequent request unless there are
Cache-Control directives or another header(s) that
explicitly allow it. For example, these include the
following: an Expires header (section 14.21); a "max-age",
"s-maxage" "must-revalidate", "proxy-revalidate", "public"
or "private" Cache-Control directive (section 14.9).
13.5 Constructing Responses From Caches
The purpose of an HTTP cache is to store information
received in response to requests, for use in responding to
future requests. In many cases, a cache simply returns the
appropriate parts of a response to the requester. However,
if the cache holds a cache entry based on a previous
response, it may have to combine parts of a new response
with what is held in the cache entry.
13.5.1 End-to-end and Hop-by-hop Headers
For the purpose of defining the behavior of caches and non-
caching proxies, we divide HTTP headers into two categories:
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. End-to-end headers, which must be transmitted to the
ultimate recipient of a request or response. End-to-end
headers in responses must be stored as part of a cache
entry and transmitted in any response formed from a
cache entry.
. Hop-by-hop headers, which are meaningful only for a
single transport-level connection, and are not stored
by caches or forwarded by proxies.
The following HTTP/1.1 headers are hop-by-hop headers:
. Connection
. Keep-Alive
. Public
. Proxy-Authenticate
. Transfer-Encoding
. Upgrade
All other headers defined by HTTP/1.1 are end-to-end
headers.
Hop-by-hop headers introduced in future versions of HTTP
MUST be listed in a Connection header, as described in
section 14.10.
13.5.2 Non-modifiable Headers
Some features of the HTTP/1.1 protocol, such as Digest
Authentication, depend on the value of certain end-to-end
headers. A cache or non-caching proxy SHOULD NOT modify an
end-to-end header unless the definition of that header
requires or specifically allows that.
A cache or non-caching proxy MUST NOT modify any of the
following fields in a request or response, nor may it add
any of these fields if not already present:
. Content-Location
. Content-MD5
. ETag
. Last-Modified
A cache or non-caching proxy MUST NOT modify any of the
following fields in a response:
. Expires
. Content-Length
but it may add any of these fields if not already present.
If an Expires header is added, it MUST be given a field-
value identical to that of the Date header in that response.
If a Content-Length header is added, it MUST correctly
reflect the length of the entity-body.
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Note: a typical reason for adding the Content-Length header
is that the origin server sent the content chunked encoded.
A cache or non-caching proxy MUST NOT modify or add any of
the following fields in a response that contains the no-
transform Cache-Control directive, or in any request:
. Content-Encoding
. Content-Length
. Content-Range
. Content-Type
A cache or non-caching proxy MAY modify or add these fields
in a response that does not include no-transform, but if it
does so, it MUST add a Warning 14 (Transformation applied)
if one does not already appear in the response.
Warning: unnecessary modification of end-to-end headers
may cause authentication failures if stronger
authentication mechanisms are introduced in later
versions of HTTP. Such authentication mechanisms may
rely on the values of header fields not listed here.
13.5.3 Combining Headers
When a cache makes a validating request to a server, and the
server provides a 304 (Not Modified) response or a 206
(Partial Content) response, the cache must construct a
response to send to the requesting client.
In the status code is 304 (Not Modified), the cache uses the
entity-body stored in the cache entry as the entity-body of
this outgoing response. If the status code is 206 (Partial
Content) and the ETag or Last-Modified headers match
exactly, see 13.5.4, the cache may combine the contents
stored in the cache entry with the new contents received in
the response and use the result as the entity-body of this
outgoing response, see 13.5.4.
The end-to-end headers stored in the cache entry are used
for the constructed response, except that
. any stored Warning headers with warn-code 1XX (see
section 14.45) are deleted from the cache entry and the
forwarded response.
. any stored Warning headers with warn-code 2XX are
retained in the cache entry and the forwarded response.
. any end-to-end headers provided in the 304 or 206
response MUST replace the corresponding headers from
the cache entry.
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Unless the cache decides to remove the cache entry, it MUST
also replace the end-to-end headers stored with the cache
entry with corresponding headers received in the incoming
response.
In other words, the set of end-to-end headers received in
the incoming response overrides all corresponding end-to-end
headers stored with the cache entry (except for stored
Warning headers with warn-code 1XX, which are deleted even
if not overridden).
If a header field-name in the incoming response matches more
than one header in the cache entry, all such old headers are
replaced.
Note: this rule allows an origin server to use a 304
(Not Modified) or a 206 (Partial Content) response to
update any header associated with a previous response
for the same entity or sub-ranges thereof, although it
might not always be meaningful or correct to do so.
This rule does not allow an origin server to use a 304
(Not Modified) or a 206 (Partial Content) response to
entirely delete a header that it had provided with a
previous response.
13.5.4 Combining Byte Ranges
A response may transfer only a subrange of the bytes of an
entity-body, either because the request included one or more
Range specifications, or because a connection was broken
prematurely. After several such transfers, a cache may have
received several ranges of the same entity-body.
If a cache has a stored non-empty set of subranges for an
entity, and an incoming response transfers another subrange,
the cache MAY combine the new subrange with the existing set
if both the following conditions are met:
. Both the incoming response and the cache entry must
have a cache validator.
. The two cache validators must match using the strong
comparison function (see section 13.3.3).
If either requirement is not meant, the cache must use only
the most recent partial response (based on the Date values
transmitted with every response, and using the incoming
response if these values are equal or missing), and must
discard the other partial information.
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13.6 Caching Negotiated Responses
Use of server-driven content negotiation (section 12), as
indicated by the presence of a Vary header field in a
response, alters the conditions and procedure by which a
cache can use the response for subsequent requests.
A server MUST use the Vary header field (section 14.43) to
inform a cache of what header field dimensions are used to
select among multiple representations of a cachable
response. A cache may use the selected representation (the
entity included with that particular response) for replying
to subsequent requests on that resource only when the
subsequent requests have the same or equivalent values for
all header fields specified in the Vary response-header.
Requests with a different value for one or more of those
header fields would be forwarded toward the origin server.
If an entity tag was assigned to the representation, the
forwarded request SHOULD be conditional and include the
entity tags in an If-None-Match header field from all its
cache entries for the Request-URI. This conveys to the
server the set of entities currently held by the cache, so
that if any one of these entities matches the requested
entity, the server can use the ETag header in its 304 (Not
Modified) response to tell the cache which entry is
appropriate. If the entity-tag of the new response matches
that of an existing entry, the new response SHOULD be used
to update the header fields of the existing entry, and the
result MUST be returned to the client.
The Vary header field may also inform the cache that the
representation was selected using criteria not limited to
the request-headers; in this case, a cache MUST NOT use the
response in a reply to a subsequent request unless the cache
relays the new request to the origin server in a conditional
request and the server responds with 304 (Not Modified),
including an entity tag or Content-Location that indicates
which entity should be used.
If any of the existing cache entries contains only partial
content for the associated entity, its entity-tag SHOULD NOT
be included in the If-None-Match header unless the request
is for a range that would be fully satisfied by that entry.
If a cache receives a successful response whose Content-
Location field matches that of an existing cache entry for
the same Request-URI, whose entity-tag differs from that of
the existing entry, and whose Date is more recent than that
of the existing entry, the existing entry SHOULD NOT be
returned in response to future requests, and should be
deleted from the cache.
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13.7 Shared and Non-Shared Caches
For reasons of security and privacy, it is necessary to make
a distinction between "shared" and "non-shared" caches. A
non-shared cache is one that is accessible only to a single
user. Accessibility in this case SHOULD be enforced by
appropriate security mechanisms. All other caches are
considered to be "shared." Other sections of this
specification place certain constraints on the operation of
shared caches in order to prevent loss of privacy or failure
of access controls.
13.8 Errors or Incomplete Response Cache Behavior
A cache that receives an incomplete response (for example,
with fewer bytes of data than specified in a Content-Length
header) may store the response. However, the cache MUST
treat this as a partial response. Partial responses may be
combined as described in section 13.5.4; the result might be
a full response or might still be partial. A cache MUST NOT
return a partial response to a client without explicitly
marking it as such, using the 206 (Partial Content) status
code. A cache MUST NOT return a partial response using a
status code of 200 (OK).
If a cache receives a 5xx response while attempting to
revalidate an entry, it may either forward this response to
the requesting client, or act as if the server failed to
respond. In the latter case, it MAY return a previously
received response unless the cached entry includes the
"must-revalidate" Cache-Control directive (see section
14.9).
13.9 Side Effects of GET and HEAD
Unless the origin server explicitly prohibits the caching of
their responses, the application of GET and HEAD methods to
any resources SHOULD NOT have side effects that would lead
to erroneous behavior if these responses are taken from a
cache. They may still have side effects, but a cache is not
required to consider such side effects in its caching
decisions. Caches are always expected to observe an origin
server's explicit restrictions on caching.
EDITOR's Note: Roy Fielding is opposed to the change
represented by the following two paragraphs:
Some HTTP/1.0 cache operators have found that it is
dangerous to cache and reuse without revalidation responses
to requests for URLs that include any of the strings "cgi-
bin", "htbin", or "?". Applications have traditionally used
these URLs in conjunction with operations with significant
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side effects for GET or HEAD methods. However, if such a
response includes an explicit, future, expiration time, then
this implies that the response may be cached and reused
without revalidation until it expires. If such a response
includes a Last-Modified or Etag header, this implies that
the response may be reused after revalidation (or without
revalidation if explicitly fresh).
A cache MUST NOT assign a heuristic expiration time to a
response for a URL that includes the strings "htbin", "cgi-
bin", or "?" in its rel_path part. If such a response does
not carry an explicit expiration time, it must be treated
as if it expires immediately.
13.10 Invalidation After Updates or Deletions
The effect of certain methods at the origin server may cause
one or more existing cache entries to become non-
transparently invalid. That is, although they may continue
to be "fresh," they do not accurately reflect what the
origin server would return for a new request.
There is no way for the HTTP protocol to guarantee that all
such cache entries are marked invalid. For example, the
request that caused the change at the origin server may not
have gone through the proxy where a cache entry is stored.
However, several rules help reduce the likelihood of
erroneous behavior.
In this section, the phrase "invalidate an entity" means
that the cache should either remove all instances of that
entity from its storage, or should mark these as "invalid"
and in need of a mandatory revalidation before they can be
returned in response to a subsequent request.
Some HTTP methods may invalidate an entity. This is either
the entity referred to by the Request-URI, or by the
Location or Content-Location headers (if present). These
methods are:
. PUT
. DELETE
. POST
In order to prevent denial of service attacks, an
invalidation based on the URI in a Location or Content-
Location header MUST only be performed if the host part is
the same as in the Request-URI.
13.11 Write-Through Mandatory
All methods that may be expected to cause modifications to
the origin server's resources MUST be written through to the
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origin server. This currently includes all methods except
for GET and HEAD. A cache MUST NOT reply to such a request
from a client before having transmitted the request to the
inbound server, and having received a corresponding response
from the inbound server. This does not prevent a proxy cache
from sending a 100 (Continue) response before the inbound
server has sent its final reply.
The alternative (known as "write-back" or "copy-back"
caching) is not allowed in HTTP/1.1, due to the difficulty
of providing consistent updates and the problems arising
from server, cache, or network failure prior to write-back.
13.12 Cache Replacement
If a new cachable (see sections 14.9.2, 13.2.5, 13.2.6 and
13.8) response is received from a resource while any
existing responses for the same resource are cached, the
cache SHOULD use the new response to reply to the current
request. It may insert it into cache storage and may, if it
meets all other requirements, use it to respond to any
future requests that would previously have caused the old
response to be returned. If it inserts the new response into
cache storage it should follow the rules in section 13.5.3.
Note: a new response that has an older Date header
value than existing cached responses is not cachable.
13.13 History Lists
User agents often have history mechanisms, such as "Back"
buttons and history lists, which can be used to redisplay an
entity retrieved earlier in a session.
History mechanisms and caches are different. In particular
history mechanisms SHOULD NOT try to show a semantically
transparent view of the current state of a resource. Rather,
a history mechanism is meant to show exactly what the user
saw at the time when the resource was retrieved.
By default, an expiration time does not apply to history
mechanisms. If the entity is still in storage, a history
mechanism should display it even if the entity has expired,
unless the user has specifically configured the agent to
refresh expired history documents.
This should not be construed to prohibit the history
mechanism from telling the user that a view may be stale.
Note: if history list mechanisms unnecessarily prevent
users from viewing stale resources, this will tend to
force service authors to avoid using HTTP expiration
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controls and cache controls when they would otherwise
like to. Service authors may consider it important that
users not be presented with error messages or warning
messages when they use navigation controls (such as
BACK) to view previously fetched resources. Even though
sometimes such resources ought not to cached, or ought
to expire quickly, user interface considerations may
force service authors to resort to other means of
preventing caching (e.g. "once-only" URLs) in order not
to suffer the effects of improperly functioning history
mechanisms.
14 Header Field Definitions
This section defines the syntax and semantics of all
standard HTTP/1.1 header fields. For entity-header fields,
both sender and recipient refer to either the client or the
server, depending on who sends and who receives the entity.
14.1 Accept
The Accept request-header field can be used to specify
certain media types which are acceptable for the response.
Accept headers can be used to indicate that the request is
specifically limited to a small set of desired types, as in
the case of a request for an in-line image.
Accept = "Accept" ":"
#( media-range [ accept-params ] )
media-range = ( "*/*"
| ( type "/" "*" )
| ( type "/" subtype )
) *( ";" parameter )
accept-params = ";" "q" "=" qvalue
*( accept-extension )
accept-extension = ";" token [ "="
( token | quoted-string ) ]
The asterisk "*" character is used to group media types into
ranges, with "*/*" indicating all media types and "type/*"
indicating all subtypes of that type. The media-range MAY
include media type parameters that are applicable to that
range.
Each media-range MAY be followed by one or more accept-
params, beginning with the "q" parameter for indicating a
relative quality factor. The first "q" parameter (if any)
separates the media-range parameter(s) from the accept-
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params. Quality factors allow the user or user agent to
indicate the relative degree of preference for that media-
range, using the qvalue scale from 0 to 1 (section 3.9). The
default value is q=1.
Note: Use of the "q" parameter name to separate media
type parameters from Accept extension parameters is due
to historical practice. Although this prevents any
media type parameter named "q" from being used with a
media range, such an event is believed to be unlikely
given the lack of any "q" parameters in the IANA media
type registry and the rare usage of any media type
parameters in Accept. Future media types should be
discouraged from registering any parameter named "q".
The example
Accept: audio/*; q=0.2, audio/basic
SHOULD be interpreted as "I prefer audio/basic, but send me
any audio type if it is the best available after an 80%
mark-down in quality."
If no Accept header field is present, then it is assumed
that the client accepts all media types. If an Accept header
field is present, and if the server cannot send a response
which is acceptable according to the combined Accept field
value, then the server SHOULD send a 406 (not acceptable)
response.
A more elaborate example is
Accept: text/plain; q=0.5, text/html,
text/x-dvi; q=0.8, text/x-c
Verbally, this would be interpreted as "text/html and
text/x-c are the preferred media types, but if they do not
exist, then send the text/x-dvi entity, and if that does not
exist, send the text/plain entity."
Media ranges can be overridden by more specific media ranges
or specific media types. If more than one media range
applies to a given type, the most specific reference has
precedence. For example,
Accept: text/*, text/html, text/html;level=1, */*
have the following precedence:
1) text/html;level=1
2) text/html
3) text/*
4) */*
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The media type quality factor associated with a given type
is determined by finding the media range with the highest
precedence which matches that type. For example,
Accept: text/*;q=0.3, text/html;q=0.7,
text/html;level=1,
text/html;level=2;q=0.4, */*;q=0.5
would cause the following values to be associated:
text/html;level=1 = 1
text/html = 0.7
text/plain = 0.3
image/jpeg = 0.5
text/html;level=2 = 0.4
text/html;level=3 = 0.7
Note: A user agent may be provided with a default set
of quality values for certain media ranges. However,
unless the user agent is a closed system which cannot
interact with other rendering agents, this default set
should be configurable by the user.
14.2 Accept-Charset
The Accept-Charset request-header field can be used to
indicate what character sets are acceptable for the
response. This field allows clients capable of understanding
more comprehensive or special-purpose character sets to
signal that capability to a server which is capable of
representing documents in those character sets. The ISO-
8859-1 character set can be assumed to be acceptable to all
user agents.
Accept-Charset = "Accept-Charset" ":"
1#( ( charset | "*" [ ";" "q" "=" qvalue ]
)
Character set values are described in section 3.4. Each
charset may be given an associated quality value which
represents the user's preference for that charset. The
default value is q=1. An example is
Accept-Charset: iso-8859-5, unicode-1-1;q=0.8
The special value "*", if present in the Accept-Charset
field, matches every character set (including ISO-8859-1)
which is not mentioned elsewhere in the Accept-Charset
field. If no "*" is present in an Accept-Charset field,
then all character sets not explicitly mentioned get a
quality value of 0, except for ISO-8859-1, which gets a
quality value of 1 if not explicitly mentioned.
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If no Accept-Charset header is present, the default is that
any character set is acceptable. If an Accept-Charset header
is present, and if the server cannot send a response which
is acceptable according to the Accept-Charset header, then
the server SHOULD send an error response with the 406 (not
acceptable) status code, though the sending of an
unacceptable response is also allowed.
14.3 Accept-Encoding
Editor's note: there is still some nervousness about
introducing q values into accept encoding; concrete examples
of implementations which will break are needed to make the
case against them, as q values are common across other
Accept-* headers. This does seem the cleanest solution to
the problem right now.
The Accept-Encoding request-header field is similar to
Accept, but restricts the content-coding (section 3.5) that
are acceptable in the response.
Accept-Encoding = "Accept-Encoding" ":"
1#( codings [ ";" "q" "=" qvalue ] )
codings = ( content-codings | "*" )
Examples of its use are:
Accept-Encoding: compress, gzip
Accept-Encoding:
Accept-Encoding: *
Accept-Encoding: compress;q=0.5, gzip;q=1.0
Accept-Encoding: gzip=1.0; identity=0.5; *;q=0
A server tests whether a content-coding is acceptable,
according to an Accept-Encoding field, using these rules:
1. If the content-coding is one of the content-codings
listed in the Accept-Encoding field, then it is acceptable,
unless it is accompanied by a qvalue of 0. (As defined in
section 3.9, a qvalue of 0 means "not acceptable.")
2. The special "*" symbol in an Accept-Encoding field
matches any available content-coding not explicitly listed
in the header field.
3. If multiple content-codings are acceptable, then the
acceptable content-coding with the highest non-zero qvalue
is preferred.
4. The "identity" content-coding is always acceptable,
unless specifically refused because the Accept-Encoding
field includes "identity;q=0", or because the field includes
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"*;q=0" and does not explictly include the "identity"
content-coding. If the Accept-Encoding field-value is
empty, then only the "identity" encoding is acceptable.
If an Accept-Encoding field is present in a request, and if
the server cannot send a response which is acceptable
according to the Accept-Encoding header, then the server
SHOULD send an error response with the 406 (Not Acceptable)
status code.
If no Accept-Encoding field is present in a request, the
server MAY assume that the client will accept any content
coding. In this case, if "identity" is one of the available
content-codings, then the server SHOULD use the "identity"
content-coding, unless it has additional information that a
different content-coding is meaningful to the client.
Note: If the request does not include an Accept-Encoding
field, and if the "identity" content-coding is unavailable,
then preference should be given to content-codings commonly
understood by HTTP/1.0 clients (i.e., "gzip" and
"compress"); some older clients improperly display messages
sent with other content-encodings. The server may also make
this decision based on information about the particular
user-agent or client.
14.4 Accept-Language
The Accept-Language request-header field is similar to
Accept, but restricts the set of natural languages that are
preferred as a response to the request.
Accept-Language = "Accept-Language" ":"
1#( language-range [ ";" "q" "=" qvalue ] )
language-range = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) | "*" )
Each language-range MAY be given an associated quality value
which represents an estimate of the user's preference for
the languages specified by that range. The quality value
defaults to "q=1". For example,
Accept-Language: da, en-gb;q=0.8, en;q=0.7
would mean: "I prefer Danish, but will accept British
English and other types of English." A language-range
matches a language-tag if it exactly equals the tag, or if
it exactly equals a prefix of the tag such that the first
tag character following the prefix is "-". The special range
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"*", if present in the Accept-Language field, matches every
tag not matched by any other range present in the Accept-
Language field.
Note: This use of a prefix matching rule does not imply
that language tags are assigned to languages in such a
way that it is always true that if a user understands a
language with a certain tag, then this user will also
understand all languages with tags for which this tag
is a prefix. The prefix rule simply allows the use of
prefix tags if this is the case.
The language quality factor assigned to a language-tag by
the Accept-Language field is the quality value of the
longest language-range in the field that matches the
language-tag. If no language-range in the field matches the
tag, the language quality factor assigned is 0. If no
Accept-Language header is present in the request, the server
SHOULD assume that all languages are equally acceptable. If
an Accept-Language header is present, then all languages
which are assigned a quality factor greater than 0 are
acceptable.
It may be contrary to the privacy expectations of the user
to send an Accept-Language header with the complete
linguistic preferences of the user in every request. For a
discussion of this issue, see section 15.7.
Note: As intelligibility is highly dependent on the
individual user, it is recommended that client
applications make the choice of linguistic preference
available to the user. If the choice is not made
available, then the Accept-Language header field must
not be given in the request.
Note: When making the choice of linguistic preference
available to the user, implementors should take into
account the fact that users are not familiar with the
details of language matching as described above, and
should provide appropriate guidance. As an example,
users may assume that on selecting "en-gb", they will
be served any kind of English document if British
English is not available. A user agent may suggest in
such a case to add "en" to get the best matching
behaviour.
14.5 Accept-Ranges
The Accept-Ranges response-header field allows the server to
indicate its acceptance of range requests for a resource:
Accept-Ranges = "Accept-Ranges" ":" acceptable-ranges
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acceptable-ranges = 1#range-unit | "none"
Origin servers that accept byte-range requests MAY send
Accept-Ranges: bytes
but are not required to do so. Clients MAY generate byte-
range requests without having received this header for the
resource involved.
Servers that do not accept any kind of range request for a
resource MAY send
Accept-Ranges: none
to advise the client not to attempt a range request.
14.6 Age
The Age response-header field conveys the sender's estimate
of the amount of time since the response (or its
revalidation) was generated at the origin server. A cached
response is "fresh" if its age does not exceed its freshness
lifetime. Age values are calculated as specified in section
13.2.3.
Age = "Age" ":" age-value
age-value = delta-seconds
Age values are non-negative decimal integers, representing
time in seconds.
If a cache receives a value larger than the largest positive
integer it can represent, or if any of its age calculations
overflows, it MUST transmit an Age header with a value of
2147483648 (2^31). HTTP/1.1 caches MUST send an Age header
in every response. Caches SHOULD use an arithmetic type of
at least 31 bits of range.
14.7 Allow
Editors Note: The OPTIONS changes would cause possible
changes to Allow and/or Public for consistency with each
other and with section 9.2 (OPTIONS).
The Allow entity-header field lists the set of methods
supported by the resource identified by the Request-URI. The
purpose of this field is strictly to inform the recipient of
valid methods associated with the resource. An Allow header
field MUST be present in a 405 (Method Not Allowed)
response.
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Allow = "Allow" ":" 1#method
Example of use:
Allow: GET, HEAD, PUT
This field cannot prevent a client from trying other
methods. However, the indications given by the Allow header
field value SHOULD be followed. The actual set of allowed
methods is defined by the origin server at the time of each
request.
The Allow header field MAY be provided with a PUT request to
recommend the methods to be supported by the new or modified
resource. The server is not required to support these
methods and SHOULD include an Allow header in the response
giving the actual supported methods.
A proxy MUST NOT modify the Allow header field even if it
does not understand all the methods specified, since the
user agent MAY have other means of communicating with the
origin server.
The Allow header field does not indicate what methods are
implemented at the server level. Servers MAY use the Public
response-header field (section 14.35) to describe what
methods are implemented on the server as a whole.
14.8 Authorization
A user agent that wishes to authenticate itself with a
server--usually, but not necessarily, after receiving a 401
response--MAY do so by including an Authorization request-
header field with the request. The Authorization field value
consists of credentials containing the authentication
information of the user agent for the realm of the resource
being requested.
Authorization = "Authorization" ":" credentials
HTTP access authentication is described in section 11. If a
request is authenticated and a realm specified, the same
credentials SHOULD be valid for all other requests within
this realm.
When a shared cache (see section 13.7) receives a request
containing an Authorization field, it MUST NOT return the
corresponding response as a reply to any other request,
unless one of the following specific exceptions holds:
1.If the response includes the "s-maxage" Cache-Control
directive, the cache MAY use that response in replying
to a subsequent request. But (if the specified maximum
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age has passed) a proxy cache MUST first revalidate it
with the origin server, using the request-headers from
the new request to allow the origin server to
authenticate the new request. (This is the defined
behavior for proxy-maxage.) If the response includes
"proxy-maxage=0", the proxy MUST always revalidate it
before re-using it.
2.If the response includes the "must-revalidate" Cache-
Control directive, the cache MAY use that response in
replying to a subsequent request. But if the response
is stale, all caches MUST first revalidate it with the
origin server, using the request-headers from the new
request to allow the origin server to authenticate the
new request.
3.If the response includes the "public" Cache-Control
directive, it may be returned in reply to any
subsequent request.
14.9 Cache-Control
The Cache-Control general-header field is used to specify
directives that MUST be obeyed by all caching mechanisms
along the request/response chain. The directives specify
behavior intended to prevent caches from adversely
interfering with the request or response. These directives
typically override the default caching algorithms. Cache
directives are unidirectional in that the presence of a
directive in a request does not imply that the same
directive should be given in the response.
Note that HTTP/1.0 caches may not implement Cache-
Control and may only implement Pragma: no-cache (see
section 14.32).
Cache directives must be passed through by a proxy or
gateway application, regardless of their significance to
that application, since the directives may be applicable to
all recipients along the request/response chain. It is not
possible to specify a cache-directive for a specific cache.
Cache-Control = "Cache-Control" ":" 1#cache-
directive
cache-directive = cache-request-directive
| cache-response-directive
cache-request-directive =
"no-cache"
| "no-store"
| "max-age" "=" delta-seconds
| "max-stale" [ "=" delta-seconds ]
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| "min-fresh" "=" delta-seconds
| "no-transform"
| "only-if-cached"
| cache-extension
cache-response-directive =
"public"
| "private" [ "=" <"> 1#field-name <"> ]
| "no-cache" [ "=" <"> 1#field-name <"> ]
| "no-store"
| "no-transform"
| "must-revalidate"
| "proxy-revalidate"
| "max-age" "=" delta-seconds
| "s-maxage" "=" delta-seconds
| cache-extension
cache-extension = token [ "=" ( token
| quoted-string ) ]
When a directive appears without any 1#field-name parameter,
the directive applies to the entire request or response.
When such a directive appears with a 1#field-name parameter,
it applies only to the named field or fields, and not to the
rest of the request or response. This mechanism supports
extensibility; implementations of future versions of the
HTTP protocol may apply these directives to header fields
not defined in HTTP/1.1.
The cache-control directives can be broken down into these
general categories:
. Restrictions on what is cachable; these may only be
imposed by the origin server.
. Restrictions on what may be stored by a cache; these
may be imposed by either the origin server or the user
agent.
. Modifications of the basic expiration mechanism; these
may be imposed by either the origin server or the user
agent.
. Controls over cache revalidation and reload; these may
only be imposed by a user agent.
. Control over transformation of entities.
. Extensions to the caching system.
14.9.1 What is Cachable
By default, a response is cachable if the requirements of
the request method, request header fields, and the response
status indicate that it is cachable. Section 13.4 summarizes
these defaults for cachability. The following Cache-Control
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response directives allow an origin server to override the
default cachability of a response:
public
Indicates that the response is cachable by any cache,
even if it would normally be non-cachable or cachable
only within a non-shared cache. (See also Authorization,
section 14.8, for additional details.)
private
Indicates that all or part of the response message is
intended for a single user and MUST NOT be cached by a
shared cache. This allows an origin server to state that
the specified parts of the response are intended for only
one user and are not a valid response for requests by
other users. A private (non-shared) cache may cache the
response.
Note: This usage of the word private only controls
where the response may be cached, and cannot ensure the
privacy of the message content.
no-cache
If the no-cache directive does not specify a field-name,
then a cache MUST NOT use the response to satisfy a
subsequent request without successful revalidation with
the origin server. This allows an origin server to
prevent caching even by caches that have been configured
to return stale responses to client requests.
If the no-cache directive does specify one or more field-
names, then a cache MAY use the response to satisfy a
subsequent request, subject to any other restrictions on
caching. However, the specified field-name(s) MUST NOT be
sent in the response to a subsequent request without
successful revalidation with the origin server. This
allows an origin server to prevent the re-use of certain
header fields in a response, while still allowing caching
of the rest of the response.
Note: Most HTTP/1.0 caches will not recognize or obey
this directive.
14.9.2 What May be Stored by Caches
The purpose of the no-store directive is to prevent the
inadvertent release or retention of sensitive information
(for example, on backup tapes). The no-store directive
applies to the entire message, and may be sent either in a
response or in a request. If sent in a request, a cache MUST
NOT store any part of either this request or any response to
it. If sent in a response, a cache MUST NOT store any part
of either this response or the request that elicited it.
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This directive applies to both non-shared and shared caches.
"MUST NOT store" in this context means that the cache MUST
NOT intentionally store the information in non-volatile
storage, and MUST make a best-effort attempt to remove the
information from volatile storage as promptly as possible
after forwarding it.
Even when this directive is associated with a response,
users may explicitly store such a response outside of the
caching system (e.g., with a "Save As" dialog). History
buffers may store such responses as part of their normal
operation.
The purpose of this directive is to meet the stated
requirements of certain users and service authors who are
concerned about accidental releases of information via
unanticipated accesses to cache data structures. While the
use of this directive may improve privacy in some cases, we
caution that it is NOT in any way a reliable or sufficient
mechanism for ensuring privacy. In particular, malicious or
compromised caches may not recognize or obey this directive;
and communications networks may be vulnerable to
eavesdropping.
14.9.3 Modifications of the Basic Expiration Mechanism
The expiration time of an entity may be specified by the
origin server using the Expires header (see section 14.21).
Alternatively, it may be specified using the max-age
directive in a response. When the "max-age" directive is
present in a cached response, the response is stale if its
current age is greater than the age value given (in seconds)
at the time of a new request for that resource. The "max-
age" directive on a response implies that the response is
cachable (i.e., "public") unless some other, more
restrictive cache directive is also present.
If a response includes both an Expires header and a max-age
directive, the max-age directive overrides the Expires
header, even if the Expires header is more restrictive. This
rule allows an origin server to provide, for a given
response, a longer expiration time to an HTTP/1.1 (or later)
cache than to an HTTP/1.0 cache. This may be useful if
certain HTTP/1.0 caches improperly calculate ages or
expiration times, perhaps due to desynchronized clocks.
Many HTTP/1.0 cache implementations will treat an Expires
value that is less than or equal to the response Date value
as being equivalent to the Cache-Control response directive
"no-cache". If an HTTP/1.1 cache receives such a response,
and the response does not include a Cache-Control header
field, it SHOULD consider the response to be non-cachable in
order to retain compatibility with HTTP/1.0 servers.
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Note: An origin server might wish to use a relatively
new HTTP cache control feature, such as the "private"
directive, on a network including older caches that do
not understand that feature. The origin server will
need to combine the new feature with an Expires field
whose value is less than or equal to the Date value.
This will prevent older caches from improperly caching
the response.
If a response includes a s-maxage directive, then for a
shared cache (but not for a private cache), the maximum age
specified by this directive overrides the maximum age
specified by either the max-age directive or the Expires
header. The s-maxage directive also implies the semantics
of the proxy-revalidate directive (see section 14.9.4),
i.e., that the shared cache MUST NOT use the entry after it
becomes stale to respond to a subsequent request without
first revalidating it with the origin server. The s-maxage
directive is always ignored by a private cache.
Note: most older caches, not compliant with this
specification, do not implement any Cache-Control
directives. An origin server wishing to use a Cache-
Control directive that restricts, but does not prevent,
caching by an HTTP/1.1-compliant cache may exploit the
requirement that the max-age directive overrides the
Expires header, and the fact that non-HTTP/1.1-
compliant caches do not observe the max-age directive.
Other directives allow an user agent to modify the basic
expiration mechanism. These directives may be specified on a
request:
max-age
Indicates that the client is willing to accept a response
whose age is no greater than the specified time in
seconds. Unless max-stale directive is also included, the
client is not willing to accept a stale response.
min-fresh
Indicates that the client is willing to accept a response
whose freshness lifetime is no less than its current age
plus the specified time in seconds. That is, the client
wants a response that will still be fresh for at least
the specified number of seconds.
max-stale
Indicates that the client is willing to accept a response
that has exceeded its expiration time. If max-stale is
assigned a value, then the client is willing to accept a
response that has exceeded its expiration time by no more
than the specified number of seconds. If no value is
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assigned to max-stale, then the client is willing to
accept a stale response of any age.
If a cache returns a stale response, either because of a
max-stale directive on a request, or because the cache is
configured to override the expiration time of a response,
the cache MUST attach a Warning header to the stale
response, using Warning 10 (Response is stale).
Note: A cache may be configured to return stale
responses without validation, but only if this does not
conflict with any MUST-level requirements concerning
cache validation (e.g., a "must-revalidate" Cache-
Control directive).
If both the new request and the cached entry include "max-
age" directives, then the lesser of the two values is used
for determining the freshness of the cached entry for that
request.
14.9.4 Cache Revalidation and Reload Controls
Sometimes an user agent may want or need to insist that a
cache revalidate its cache entry with the origin server (and
not just with the next cache along the path to the origin
server), or to reload its cache entry from the origin
server. End-to-end revalidation may be necessary if either
the cache or the origin server has overestimated the
expiration time of the cached response. End-to-end reload
may be necessary if the cache entry has become corrupted for
some reason.
End-to-end revalidation may be requested either when the
client does not have its own local cached copy, in which
case we call it "unspecified end-to-end revalidation", or
when the client does have a local cached copy, in which case
we call it "specific end-to-end revalidation."
The client can specify these three kinds of action using
Cache-Control request directives:
End-to-end reload
The request includes a "no-cache" Cache-Control directive
or, for compatibility with HTTP/1.0 clients, "Pragma: no-
cache".
No field names may be included with the no-cache
directive in a request. The server MUST NOT use a cached
copy when responding to such a request.
Specific end-to-end revalidation
The request includes a "max-age=0" Cache-Control
directive, which forces each cache along the path to the
origin server to revalidate its own entry, if any, with
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the next cache or server. The initial request includes a
cache-validating conditional with the client's current
validator.
Unspecified end-to-end revalidation
The request includes "max-age=0" Cache-Control directive,
which forces each cache along the path to the origin
server to revalidate its own entry, if any, with the next
cache or server. The initial request does not include a
cache-validating conditional; the first cache along the
path (if any) that holds a cache entry for this resource
includes a cache-validating conditional with its current
validator.
When an intermediate cache is forced, by means of a max-
age=0 directive, to revalidate its own cache entry, and the
client has supplied its own validator in the request, the
supplied validator may differ from the validator currently
stored with the cache entry. In this case, the cache may use
either validator in making its own request without affecting
semantic transparency.
However, the choice of validator may affect performance. The
best approach is for the intermediate cache to use its own
validator when making its request. If the server replies
with 304 (Not Modified), then the cache should return its
now validated copy to the client with a 200 (OK) response.
If the server replies with a new entity and cache validator,
however, the intermediate cache should compare the returned
validator with the one provided in the client's request,
using the strong comparison function. If the client's
validator is equal to the origin server's, then the
intermediate cache simply returns 304 (Not Modified).
Otherwise, it returns the new entity with a 200 (OK)
response.
If a request includes the no-cache directive, it should not
include min-fresh, max-stale, or max-age.
In some cases, such as times of extremely poor network
connectivity, a client may want a cache to return only those
responses that it currently has stored, and not to reload or
revalidate with the origin server. To do this, the client
may include the only-if-cached directive in a request. If it
receives this directive, a cache SHOULD either respond using
a cached entry that is consistent with the other constraints
of the request, or respond with a 504 (Gateway Timeout)
status. However, if a group of caches is being operated as a
unified system with good internal connectivity, such a
request MAY be forwarded within that group of caches.
Because a cache may be configured to ignore a server's
specified expiration time, and because a client request may
include a max-stale directive (which has a similar effect),
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the protocol also includes a mechanism for the origin server
to require revalidation of a cache entry on any subsequent
use. When the must-revalidate directive is present in a
response received by a cache, that cache MUST NOT use the
entry after it becomes stale to respond to a subsequent
request without first revalidating it with the origin
server. (I.e., the cache must do an end-to-end revalidation
every time, if, based solely on the origin server's Expires
or max-age value, the cached response is stale.)
The must-revalidate directive is necessary to support
reliable operation for certain protocol features. In all
circumstances an HTTP/1.1 cache MUST obey the must-
revalidate directive; in particular, if the cache cannot
reach the origin server for any reason, it MUST generate a
504 (Gateway Timeout) response.
Servers should send the must-revalidate directive if and
only if failure to revalidate a request on the entity could
result in incorrect operation, such as a silently unexecuted
financial transaction. Recipients MUST NOT take any
automated action that violates this directive, and MUST NOT
automatically provide an unvalidated copy of the entity if
revalidation fails.
Although this is not recommended, user agents operating
under severe connectivity constraints may violate this
directive but, if so, MUST explicitly warn the user that an
unvalidated response has been provided. The warning MUST be
provided on each unvalidated access, and SHOULD require
explicit user confirmation.
The proxy-revalidate directive has the same meaning as the
must-revalidate directive, except that it does not apply to
non-shared user agent caches. It can be used on a response
to an authenticated request to permit the user's cache to
store and later return the response without needing to
revalidate it (since it has already been authenticated once
by that user), while still requiring proxies that service
many users to revalidate each time (in order to make sure
that each user has been authenticated). Note that such
authenticated responses also need the public cache control
directive in order to allow them to be cached at all.
14.9.5 No-Transform Directive
Implementers of intermediate caches (proxies) have found it
useful to convert the media type of certain entity bodies. A
proxy might, for example, convert between image formats in
order to save cache space or to reduce the amount of traffic
on a slow link. HTTP has to date been silent on these
transformations.
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Serious operational problems have already occurred, however,
when these transformations have been applied to entity
bodies intended for certain kinds of applications. For
example, applications for medical imaging, scientific data
analysis and those using end-to-end authentication, all
depend on receiving an entity body that is bit for bit
identical to the original entity-body.
Therefore, if a message includes the no-transform directive,
an intermediate cache or proxy MUST NOT change those headers
that are listed in section 13.5.2 as being subject to the
no-transform directive. This implies that the cache or
proxy must not change any aspect of the entity-body that is
specified by these headers.
14.9.6 Cache Control Extensions
The Cache-Control header field can be extended through the
use of one or more cache-extension tokens, each with an
optional assigned value. Informational extensions (those
which do not require a change in cache behavior) may be
added without changing the semantics of other directives.
Behavioral extensions are designed to work by acting as
modifiers to the existing base of cache directives. Both the
new directive and the standard directive are supplied, such
that applications which do not understand the new directive
will default to the behavior specified by the standard
directive, and those that understand the new directive will
recognize it as modifying the requirements associated with
the standard directive. In this way, extensions to the
Cache-Control directives can be made without requiring
changes to the base protocol.
This extension mechanism depends on a HTTP cache obeying all
of the cache-control directives defined for its native HTTP-
version, obeying certain extensions, and ignoring all
directives that it does not understand.
For example, consider a hypothetical new response directive
called "community" which acts as a modifier to the "private"
directive. We define this new directive to mean that, in
addition to any non-shared cache, any cache which is shared
only by members of the community named within its value may
cache the response. An origin server wishing to allow the
"UCI" community to use an otherwise private response in
their shared cache(s) may do so by including
Cache-Control: private, community="UCI"
A cache seeing this header field will act correctly even if
the cache does not understand the "community" cache-
extension, since it will also see and understand the
"private" directive and thus default to the safe behavior.
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Unrecognized cache-directives MUST be ignored; it is assumed
that any cache-directive likely to be unrecognized by an
HTTP/1.1 cache will be combined with standard directives (or
the response's default cachability) such that the cache
behavior will remain minimally correct even if the cache
does not understand the extension(s).
14.10 Connection
The Connection general-header field allows the sender to
specify options that are desired for that particular
connection and MUST NOT be communicated by proxies over
further connections.
The Connection header has the following grammar:
Connection-header = "Connection" ":" 1#(connection-token)
connection-token = token
HTTP/1.1 proxies MUST parse the Connection header field
before a message is forwarded and, for each connection-token
in this field, remove any header field(s) from the message
with the same name as the connection-token. Connection
options are signaled by the presence of a connection-token
in the Connection header field, not by any corresponding
additional header field(s), since the additional header
field may not be sent if there are no parameters associated
with that connection option.
Message headers listed in the Connection header MUST NOT
include end-to-end headers, such as Cache-Control.
HTTP/1.1 defines the "close" connection option for the
sender to signal that the connection will be closed after
completion of the response. For example,
Connection: close
in either the request or the response header fields
indicates that the connection should not be considered
`persistent' (section 8.1) after the current
request/response is complete.
HTTP/1.1 applications that do not support persistent
connections MUST include the "close" connection option in
every message.
A system receiving an HTTP/1.0 (or lower-version) message
that includes a Connection header MUST, for each connection-
token in this field, remove and ignore any header field(s)
from the message with the same name as the connection-token.
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This protects against mistaken forwarding of such header
fields by pre-HTTP/1.1 proxies.
14.11 Content-Base
The Content-Base entity-header field may be used to specify
the base URI for resolving relative URLs within the entity.
This header field is described as Base in RFC 1808[11],
which is expected to be revised.
Content-Base = "Content-Base" ":" absoluteURI
If no Content-Base field is present, the base URI of an
entity is defined either by its Content-Location (if that
Content-Location URI is an absolute URI) or the URI used to
initiate the request, in that order of precedence. Note,
however, that the base URI of the contents within the
entity-body may be redefined within that entity-body.
14.12 Content-Encoding
The Content-Encoding entity-header field is used as a
modifier to the media-type. When present, its value
indicates what additional content codings have been applied
to the entity-body, and thus what decoding mechanisms MUST
be applied in order to obtain the media-type referenced by
the Content-Type header field. Content-Encoding is primarily
used to allow a document to be compressed without losing the
identity of its underlying media type.
Content-Encoding = "Content-Encoding" ":" 1#content-coding
Content codings are defined in section 3.5. An example of
its use is
Content-Encoding: gzip
The Content-Encoding is a characteristic of the entity
identified by the Request-URI. Typically, the entity-body is
stored with this encoding and is only decoded before
rendering or analogous usage. However, a proxy MAY modify
the content-coding if the new coding is known to be
acceptable to the recipient, unless the "no-transform"
Cache-Control directive is present in the message.
If the content-coding of an entity is not "identity", then
the response MUST including a Content-Encoding entity-header
(section 14.12) that lists the non-identity content-
coding(s) used.
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If the content-coding of an entity in a request message is
not acceptable to the origin server, the server SHOULD
respond with a status code of 415 (Unsupported Media Type).
If multiple encodings have been applied to an entity, the
content codings MUST be listed in the order in which they
were applied. Additional information about the encoding
parameters MAY be provided by other entity-header fields not
defined by this specification.
14.13 Content-Language
The Content-Language entity-header field describes the
natural language(s) of the intended audience for the
enclosed entity. Note that this may not be equivalent to all
the languages used within the entity-body.
Content-Language = "Content-Language" ":" 1#language-tag
Language tags are defined in section Error! Reference source
not found.. The primary purpose of Content-Language is to
allow a user to identify and differentiate entities
according to the user's own preferred language. Thus, if the
body content is intended only for a Danish-literate
audience, the appropriate field is
Content-Language: da
If no Content-Language is specified, the default is that the
content is intended for all language audiences. This may
mean that the sender does not consider it to be specific to
any natural language, or that the sender does not know for
which language it is intended.
Multiple languages MAY be listed for content that is
intended for multiple audiences. For example, a rendition of
the "Treaty of Waitangi," presented simultaneously in the
original Maori and English versions, would call for
Content-Language: mi, en
However, just because multiple languages are present within
an entity does not mean that it is intended for multiple
linguistic audiences. An example would be a beginner's
language primer, such as "A First Lesson in Latin," which is
clearly intended to be used by an English-literate audience.
In this case, the Content-Language should only include "en".
Content-Language may be applied to any media type -- it is
not limited to textual documents.
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14.14 Content-Length
The Content-Length entity-header field indicates the size of
the message-body, in decimal number of octets, sent to the
recipient or, in the case of the HEAD method, the size of
the entity-body that would have been sent had the request
been a GET.
Content-Length = "Content-Length" ":" 1*DIGIT
An example is
Content-Length: 3495
Applications SHOULD use this field to indicate the size of
the message-body to be transferred, regardless of the media
type of the entity. It must be possible for the recipient to
reliably determine the end of HTTP/1.1 requests containing
an entity-body, e.g., because the request has a valid
Content-Length field, uses Transfer-Encoding: chunked or a
multipart body.
Any Content-Length greater than or equal to zero is a valid
value. Section 4.4 describes how to determine the length of
a message-body if a Content-Length is not given.
Note: The meaning of this field is significantly
different from the corresponding definition in MIME,
where it is an optional field used within the
"message/external-body" content-type. In HTTP, it
SHOULD be sent whenever the message's length can be
determined prior to being transferred.
14.15 Content-Location
The Content-Location entity-header field MAY be used to
supply the resource location for the entity enclosed in the
message when that entity is accessible from a location
separate from the requested resource's URI.. In the case
where a resource has multiple entities associated with it,
and those entities actually have separate locations by which
they might be individually accessed, the server should
provide a Content-Location for the particular variant which
is returned. In addition, a server SHOULD provide a Content-
Location for the resource corresponding to the response
entity.
Content-Location = "Content-Location" ":"
( absoluteURI | relativeURI )
If no Content-Base header field is present, the value of
Content-Location also defines the base URL for the entity
(see section 14.11).
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The Content-Location value is not a replacement for the
original requested URI; it is only a statement of the
location of the resource corresponding to this particular
entity at the time of the request. Future requests MAY use
the Content-Location URI if the desire is to identify the
source of that particular entity.
A cache cannot assume that an entity with a Content-Location
different from the URI used to retrieve it can be used to
respond to later requests on that Content-Location URI.
However, the Content-Location can be used to differentiate
between multiple entities retrieved from a single requested
resource, as described in section 13.6.
If the Content-Location is a relative URI, the URI is
interpreted relative to any Content-Base URI provided in the
response. If no Content-Base is provided, the relative URI
is interpreted relative to the Request-URI.
14.16 Content-MD5
The Content-MD5 entity-header field, as defined in RFC 1864
[23], is an MD5 digest of the entity-body for the purpose of
providing an end-to-end message integrity check (MIC) of the
entity-body. (Note: a MIC is good for detecting accidental
modification of the entity-body in transit, but is not proof
against malicious attacks.)
Content-MD5 = "Content-MD5" ":" md5-digest
md5-digest = <base64 of 128 bit MD5 digest as per RFC 1864>
The Content-MD5 header field may be generated by an origin
server to function as an integrity check of the entity-body.
Only origin servers may generate the Content-MD5 header
field; proxies and gateways MUST NOT generate it, as this
would defeat its value as an end-to-end integrity check. Any
recipient of the entity-body, including gateways and
proxies, MAY check that the digest value in this header
field matches that of the entity-body as received.
The MD5 digest is computed based on the content of the
entity-body, including any Content-Encoding that has been
applied, but not including any Transfer-Encoding that may
have been applied to the message-body. If the message is
received with a Transfer-Encoding, that encoding must be
removed prior to checking the Content-MD5 value against the
received entity.
This has the result that the digest is computed on the
octets of the entity-body exactly as, and in the order that,
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they would be sent if no Transfer-Encoding were being
applied.
HTTP extends RFC 1864 to permit the digest to be computed
for MIME composite media-types (e.g., multipart/* and
message/rfc822), but this does not change how the digest is
computed as defined in the preceding paragraph.
Note: There are several consequences of this. The
entity-body for composite types may contain many body-
parts, each with its own MIME and HTTP headers
(including Content-MD5, Content-Transfer-Encoding, and
Content-Encoding headers). If a body-part has a
Content-Transfer-Encoding or Content-Encoding header,
it is assumed that the content of the body-part has had
the encoding applied, and the body-part is included in
the Content-MD5 digest as is -- i.e., after the
application. The Transfer-Encoding header field is not
allowed within body-parts.
Note: while the definition of Content-MD5 is exactly
the same for HTTP as in RFC 1864 for MIME entity-
bodies, there are several ways in which the application
of Content-MD5 to HTTP entity-bodies differs from its
application to MIME entity-bodies. One is that HTTP,
unlike MIME, does not use Content-Transfer-Encoding,
and does use Transfer-Encoding and Content-Encoding.
Another is that HTTP more frequently uses binary
content types than MIME, so it is worth noting that, in
such cases, the byte order used to compute the digest
is the transmission byte order defined for the type.
Lastly, HTTP allows transmission of text types with any
of several line break conventions and not just the
canonical form using CRLF. Conversion of all line
breaks to CRLF should not be done before computing or
checking the digest: the line break convention used in
the text actually transmitted should be left unaltered
when computing the digest.
14.17 Content-Range
Editor's note: Paul Leach is circulating changes that would
define how to do PUTs with byte ranges; the spec is
currently silent on the topic
The Content-Range entity-header is sent with a partial
entity-body to specify where in the full entity-body the
partial body should be inserted. It SHOULD indicate the
total length of the full entity-body, unless length this is
unknown or difficult to determine.
Content-Range = "Content-Range" ":" content-range-spec
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content-range-spec = byte-content-range-spec | "*"
byte-content-range-spec = bytes-unit SP first-byte-pos "-"
last-byte-pos "/"
( entity-length | "*" )
entity-length = 1*DIGIT
The asterisk "*" character means that the entity-length is
unknown at the time when the response was generated.
Unlike byte-ranges-specifier values, a byte-content-range-
spec may only specify one range, and must contain absolute
byte positions for both the first and last byte of the
range.
A byte-content-range-spec whose last-byte-pos value is less
than its first-byte-pos value, or whose entity-length value
is less than or equal to its last-byte-pos value, is
invalid. The recipient of an invalid byte-content-range-spec
MUST ignore it and any content transferred along with it.
A server sending a response with status code 416 (Requested
range not valid) SHOULD include a Content-range field with
a content-range-spec of "*". The entity-length specifies
the current length of the selected resource. A response
with status code 206 (Partial Content) MUST NOT include a
Content-range field with a content-range-spec of "*".
Examples of byte-content-range-spec values, assuming that
the entity contains a total of 1234 bytes:
. The first 500 bytes:
bytes 0-499/1234
. The second 500 bytes:
bytes 500-999/1234
. All except for the first 500 bytes:
bytes 500-1233/1234
. The last 500 bytes:
bytes 734-1233/1234
When an HTTP message includes the content of a single range
(for example, a response to a request for a single range, or
to a request for a set of ranges that overlap without any
holes), this content is transmitted with a Content-Range
header, and a Content-Length header showing the number of
bytes actually transferred. For example,
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HTTP/1.1 206 Partial content
Date: Wed, 15 Nov 1995 06:25:24 GMT
Last-modified: Wed, 15 Nov 1995 04:58:08 GMT
Content-Range: bytes 21010-47021/47022
Content-Length: 26012
Content-Type: image/gif
When an HTTP message includes the content of multiple ranges
(for example, a response to a request for multiple non-
overlapping ranges), these are transmitted as a multipart
MIME message. The multipart MIME content-type used for this
purpose is defined in this specification to be
"multipart/byteranges". See appendix 19.2 for its
definition. See appendix 19.8.3 for a compatibility issue.
A client that cannot decode a MIME multipart/byteranges
message should not ask for multiple byte-ranges in a single
request.
When a client requests multiple byte-ranges in one request,
the server SHOULD return them in the order that they
appeared in the request.
If the server ignores a byte-range-spec because it is
syntactically invalid, the server should treat the request
as if the invalid Range header field did not exist.
(Normally, this means return a 200 response containing the
full entity).
If the server receives a request (other than one including
an If-Range request-header field) with an unsatisfiable
Range request-header field (that is, all of whose byte-
range-spec values have a first-byte-pos value greater than
the current length of the selected resource), it SHOULD
return a response code of 416 (Requested range not valid)
(section 10.4.17).
Note: clients cannot depend on servers to send a 416
(Requested range not valid) response instead of a 200
(OK) response for an unsatisfiable Range request-
header, since not all servers implement this request-
header.
14.18 Content-Type
The Content-Type entity-header field indicates the media
type of the entity-body sent to the recipient or, in the
case of the HEAD method, the media type that would have been
sent had the request been a GET.
Content-Type = "Content-Type" ":" media-type
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Media types are defined in section 3.7. An example of the
field is
Content-Type: text/html; charset=ISO-8859-4
Further discussion of methods for identifying the media type
of an entity is provided in section 7.2.1.
14.19 Date
The Date general-header field represents the date and time
at which the message was originated, having the same
semantics as orig-date in RFC 822. The field value is an
HTTP-date, as described in section 3.3.1; it MUST be sent in
RFC1123 [8]-date format.
Date = "Date" ":" HTTP-date
An example is
Date: Tue, 15 Nov 1994 08:12:31 GMT
Origin servers MUST include a Date header field in all
responses, except in these cases:
1.If the response status code is 100 (Continue) or 101
(Switching Protocols), the response MAY include a Date
header field, at the server's option.
2.If the response status code conveys a server error,
e.g. 500 (Internal Server Error) or 503 (Service
Unavailable), and it is inconvenient or impossible to
generate a valid Date.
3.If the server does not have a clock that can provide a
reasonable approximation of the current time, its
responses MUST NOT include a Date header field. In
this case, the rules in section 14.19.1 MUST be
followed.
A received message that does not have a Date header field
MUST be assigned one by the recipient if the message will be
cached by that recipient or gatewayed via a protocol which
requires a Date. An HTTP implementation without a clock
MUST NOT cache responses without revalidating them on every
use. An HTTP cache, especially a shared cache, SHOULD use a
mechanism, such as NTP [28], to synchronize its clock with a
reliable external standard.
Clients SHOULD only send a Date header field in messages
that include an entity-body, as in the case of the PUT and
POST requests, and even then it is optional. A client
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without a clock MUST NOT send a Date header field in a
request.
In theory, the date SHOULD represent the moment just before
the entity is generated. In practice, the date can be
generated at any time during the message origination without
affecting its semantic value.
14.19.1 Clockless Origin Server Operation
Some origin server implementations may not have a clock
available. An origin server without a clock MUST NOT assign
Expires or Last-Modified values to a response, unless these
values were associated with the resource by a system or user
with a reliable clock. It MAY assign an Expires value that
is known, at or before server configuration time, to be in
the past (this allows "pre-expiration" of responses without
storing separate Expires values for each resource).
14.20 ETag
The ETag entity-header field defines the entity tag for the
associated entity. The headers used with entity tags are
described in sections 14.20, 14.25, 14.26 and 14.43. The
entity tag may be used for comparison with other entities
from the same resource (see section 13.3.2).
ETag = "ETag" ":" entity-tag
Examples:
ETag: "xyzzy"
ETag: W/"xyzzy"
ETag: ""
14.21 Expires
The Expires entity-header field gives the date/time after
which the response should be considered stale. A stale cache
entry may not normally be returned by a cache (either a
proxy cache or an user agent cache) unless it is first
validated with the origin server (or with an intermediate
cache that has a fresh copy of the entity). See section 13.2
for further discussion of the expiration model.
The presence of an Expires field does not imply that the
original resource will change or cease to exist at, before,
or after that time.
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The format is an absolute date and time as defined by HTTP-
date in section 3.3; it MUST be in RFC1123-date format:
Expires = "Expires" ":" HTTP-date
An example of its use is
Expires: Thu, 01 Dec 1994 16:00:00 GMT
Note: if a response includes a Cache-Control field with
the max-age directive, that directive overrides the
Expires field.
HTTP/1.1 clients and caches MUST treat other invalid date
formats, especially including the value "0", as in the past
(i.e., "already expired").
To mark a response as "already expired," an origin server
should use an Expires date that is equal to the Date header
value. (See the rules for expiration calculations in section
13.2.4.)
To mark a response as "never expires," an origin server
should use an Expires date approximately one year from the
time the response is sent. HTTP/1.1 servers should not send
Expires dates more than one year in the future.
The presence of an Expires header field with a date value of
some time in the future on an response that otherwise would
by default be non-cacheable indicates that the response is
cachable, unless indicated otherwise by a Cache-Control
header field (section 14.9).
14.22 From
The From request-header field, if given, SHOULD contain an
Internet e-mail address for the human user who controls the
requesting user agent. The address SHOULD be machine-usable,
as defined by mailbox in RFC 822 [9] (as updated by RFC 1123
[8]):
From = "From" ":" mailbox
An example is:
From: webmaster@w3.org
This header field MAY be used for logging purposes and as a
means for identifying the source of invalid or unwanted
requests. It SHOULD NOT be used as an insecure form of
access protection. The interpretation of this field is that
the request is being performed on behalf of the person
given, who accepts responsibility for the method performed.
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In particular, robot agents SHOULD include this header so
that the person responsible for running the robot can be
contacted if problems occur on the receiving end.
The Internet e-mail address in this field MAY be separate
from the Internet host which issued the request. For
example, when a request is passed through a proxy the
original issuer's address SHOULD be used.
Note: The client SHOULD not send the From header field
without the user's approval, as it may conflict with
the user's privacy interests or their site's security
policy. It is strongly recommended that the user be
able to disable, enable, and modify the value of this
field at any time prior to a request.
14.23 Host
Editor's note: not yet drafted is a change that would allow
a proxy to add a host header if not present, but not change
it if it is already present.
The Host request-header field specifies the Internet host
and port number of the resource being requested, as obtained
from the original URL given by the user or referring
resource (generally an HTTP URL, as described in section
3.2.2). The Host field value MUST represent the network
location of the origin server or gateway given by the
original URL. This allows the origin server or gateway to
differentiate between internally-ambiguous URLs, such as the
root "/" URL of a server for multiple host names on a single
IP address.
Host = "Host" ":" host [ ":" port ] ; Section
3.2.2
A "host" without any trailing port information implies the
default port for the service requested (e.g., "80" for an
HTTP URL). For example, a request on the origin server for
<http://www.w3.org/pub/WWW/> MUST include:
GET /pub/WWW/ HTTP/1.1
Host: www.w3.org
A client MUST include a Host header field in all HTTP/1.1
request messages on the Internet (i.e., on any message
corresponding to a request for a URL which includes an
Internet host address for the service being requested). If
the Host field is not already present, an HTTP/1.1 proxy
MUST add a Host field to the request message prior to
forwarding it on the Internet. All Internet-based HTTP/1.1
servers MUST respond with a 400 status code to any HTTP/1.1
request message which lacks a Host header field.
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See sections 5.2 and 19.5.1 for other requirements relating
to Host.
14.24 If-Modified-Since
Editor's Note: issue DATE-IF-MODIFIED this still needs some
advice to implementers that would suggest that an HTTP
client ought to act as if the If-Modified-Since headers that
it sends *for cache validation* are going to be interpreted
as "If-Modification-date-does-not-match-exactly".
The If-Modified-Since request-header field is used with the
GET method to make it conditional: if the requested variant
has not been modified since the time specified in this
field, an entity will not be returned from the server;
instead, a 304 (not modified) response will be returned
without any message-body.
If-Modified-Since = "If-Modified-Since" ":" HTTP-date
An example of the field is:
If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
A GET method with an If-Modified-Since header and no Range
header requests that the identified entity be transferred
only if it has been modified since the date given by the If-
Modified-Since header. The algorithm for determining this
includes the following cases:
a) If the request would normally result in anything other
than a 200 (OK) status, or if the passed If-Modified-
Since date is invalid, the response is exactly the same
as for a normal GET. A date which is later than the
server's current time is invalid.
b) If the variant has been modified since the If-Modified-
Since date, the response is exactly the same as for a
normal GET.
c) If the variant has not been modified since a valid If-
Modified-Since date, the server MUST return a 304 (Not
Modified) response.
The purpose of this feature is to allow efficient updates of
cached information with a minimum amount of transaction
overhead.
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Note that the Range request-header field modifies the
meaning of If-Modified-Since; see section 14.36 for
full details.
Note that If-Modified-Since times are interpreted by
the server, whose clock may not be synchronized with
the client.
Note that if a client uses an arbitrary date in the If-
Modified-Since header instead of a date taken from the Last-
Modified header for the same request, the client should be
aware of the fact that this date is interpreted in the
server's understanding of time. The client should consider
unsynchronized clocks and rounding problems due to the
different encodings of time between the client and server.
This includes the possibility of race conditions if the
document has changed between the time it was first requested
and the If-Modified-Since date of a subsequent request, and
the possibility of clock-skew-related problems if the If-
Modified-Since date is derived from the client's clock
without correction to the server's clock. Corrections for
different time bases between client and server are at best
approximate due to network latency.
14.25 If-Match
The If-Match request-header field is used with a method to
make it conditional. A client that has one or more entities
previously obtained from the resource can verify that one of
those entities is current by including a list of their
associated entity tags in the If-Match header field. The
purpose of this feature is to allow efficient updates of
cached information with a minimum amount of transaction
overhead. It is also used, on updating requests, to prevent
inadvertent modification of the wrong version of a resource.
As a special case, the value "*" matches any current entity
of the resource.
If-Match = "If-Match" ":" ( "*" | 1#entity-tag )
If any of the entity tags match the entity tag of the entity
that would have been returned in the response to a similar
GET request (without the If-Match header) on that resource,
or if "*" is given and any current entity exists for that
resource, then the server MAY perform the requested method
as if the If-Match header field did not exist.
A server MUST use the strong comparison function (see
section 3.11) to compare the entity tags in If-Match.
If none of the entity tags match, or if "*" is given and no
current entity exists, the server MUST NOT perform the
requested method, and MUST return a 412 (Precondition
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Failed) response. This behavior is most useful when the
client wants to prevent an updating method, such as PUT,
from modifying a resource that has changed since the client
last retrieved it.
If the request would, without the If-Match header field,
result in anything other than a 2xx status, then the If-
Match header MUST be ignored.
The meaning of "If-Match: *" is that the method SHOULD be
performed if the representation selected by the origin
server (or by a cache, possibly using the Vary mechanism,
see section 14.43) exists, and MUST NOT be performed if the
representation does not exist.
A request intended to update a resource (e.g., a PUT) MAY
include an If-Match header field to signal that the request
method MUST NOT be applied if the entity corresponding to
the If-Match value (a single entity tag) is no longer a
representation of that resource. This allows the user to
indicate that they do not wish the request to be successful
if the resource has been changed without their knowledge.
Examples:
If-Match: "xyzzy"
If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-Match: *
14.26 If-None-Match
The If-None-Match request-header field is used with a method
to make it conditional. A client that has one or more
entities previously obtained from the resource can verify
that none of those entities is current by including a list
of their associated entity tags in the If-None-Match header
field. The purpose of this feature is to allow efficient
updates of cached information with a minimum amount of
transaction overhead. It is also used, on updating requests,
to prevent inadvertent modification of a resource which was
not known to exist.
As a special case, the value "*" matches any current entity
of the resource.
If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-
tag )
If any of the entity tags match the entity tag of the entity
that would have been returned in the response to a similar
GET request (without the If-None-Match header) on that
resource, or if "*" is given and any current entity exists
for that resource, then the server MUST NOT perform the
requested method. Instead, if the request method was GET or
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HEAD, the server SHOULD respond with a 304 (Not Modified)
response, including the cache-related entity-header fields
(particularly ETag) of one of the entities that matched. For
all other request methods, the server MUST respond with a
status of 412 (Precondition Failed).
See section 13.3.3 for rules on how to determine if two
entity tags match. The weak comparison function can only be
used with GET or HEAD requests.
If none of the entity tags match, or if "*" is given and no
current entity exists, then the server MAY perform the
requested method as if the If-None-Match header field did
not exist.
If the request would, without the If-None-Match header
field, result in anything other than a 2xx status, then the
If-None-Match header MUST be ignored.
The meaning of "If-None-Match: *" is that the method MUST
NOT be performed if the representation selected by the
origin server (or by a cache, possibly using the Vary
mechanism, see section 14.43) exists, and SHOULD be
performed if the representation does not exist. This feature
may be useful in preventing races between PUT operations.
Examples:
If-None-Match: "xyzzy"
If-None-Match: W/"xyzzy"
If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
If-None-Match: *
14.27 If-Range
If a client has a partial copy of an entity in its cache,
and wishes to have an up-to-date copy of the entire entity
in its cache, it could use the Range request-header with a
conditional GET (using either or both of If-Unmodified-Since
and If-Match.) However, if the condition fails because the
entity has been modified, the client would then have to make
a second request to obtain the entire current entity-body.
The If-Range header allows a client to "short-circuit" the
second request. Informally, its meaning is `if the entity is
unchanged, send me the part(s) that I am missing; otherwise,
send me the entire new entity.'
If-Range = "If-Range" ":" ( entity-tag | HTTP-date )
If the client has no entity tag for an entity, but does have
a Last-Modified date, it may use that date in a If-Range
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header. (The server can distinguish between a valid HTTP-
date and any form of entity-tag by examining no more than
two characters.) The If-Range header should only be used
together with a Range header, and must be ignored if the
request does not include a Range header, or if the server
does not support the sub-range operation.
If the entity tag given in the If-Range header matches the
current entity tag for the entity, then the server
SHOULDprovide the specified sub-range of the entity using a
206 (Partial content) response. If the entity tag does not
match, then the server SHOULDreturn the entire entity using
a 200 (OK) response.
14.28 If-Unmodified-Since
The If-Unmodified-Since request-header field is used with a
method to make it conditional. If the requested resource has
not been modified since the time specified in this field,
the server should perform the requested operation as if the
If-Unmodified-Since header were not present.
If the requested variant has been modified since the
specified time, the server MUST NOT perform the requested
operation, and MUST return a 412 (Precondition Failed).
If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date
An example of the field is:
If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
If the request normally (i.e., without the If-Unmodified-
Since header) would result in anything other than a 2xx
status, the If-Unmodified-Since header should be ignored.
If the specified date is invalid, the header is ignored.
14.29 Last-Modified
The Last-Modified entity-header field indicates the date and
time at which the origin server believes the variant was
last modified.
Last-Modified = "Last-Modified" ":" HTTP-date
An example of its use is
Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
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The exact meaning of this header field depends on the
implementation of the origin server and the nature of the
original resource. For files, it may be just the file system
last-modified time. For entities with dynamically included
parts, it may be the most recent of the set of last-modify
times for its component parts. For database gateways, it may
be the last-update time stamp of the record. For virtual
objects, it may be the last time the internal state changed.
An origin server MUST NOT send a Last-Modified date which is
later than the server's time of message origination. In such
cases, where the resource's last modification would indicate
some time in the future, the server MUST replace that date
with the message origination date.
An origin server should obtain the Last-Modified value of
the entity as close as possible to the time that it
generates the Date value of its response. This allows a
recipient to make an accurate assessment of the entity's
modification time, especially if the entity changes near the
time that the response is generated.
HTTP/1.1 servers SHOULD send Last-Modified whenever
feasible.
14.30 Location
In RFC 2068, the Location header was used to indicate the
proxy setting. Its use is DEPRECATED by the Set-Proxy
header in the context of a 305 response. All new
implementations MUST send the Set-proxy header.
Implementations MAY send the Location header so as to allow
backward compatibility.
If the Location header is specified, it should contain a URI
of the proxy. If the Set-Proxy header is not specified, the
client should use this proxy for just one request, and only
for the originally requested exact URL.
The Location response-header field is used to redirect the
recipient to a location other than the Request-URI for
completion of the request or identification of a new
resource. For 201 (Created) responses, the Location is that
of the new resource which was created by the request. For
3xx responses, the location SHOULD indicate the server's
preferred URL for automatic redirection to the resource. The
field value consists of a single absolute URL.
Location = "Location" ":" absoluteURI
An example is
Location: http://www.w3.org/pub/WWW/People.html
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Note: The Content-Location header field (section 14.15)
differs from Location in that the Content-Location
identifies the original location of the entity enclosed
in the request. It is therefore possible for a response
to contain header fields for both Location and Content-
Location. Also see section 13.10 for cache requirements
of some methods.
14.31 Max-Forwards
Editor's note: The OPTIONS changes would allow Max-Forward
with OPTIONS, not just with TRACE.
The Max-Forwards request-header field may be used with the
TRACE method (section 14.31) to limit the number of proxies
or gateways that can forward the request to the next inbound
server. This can be useful when the client is attempting to
trace a request chain which appears to be failing or looping
in mid-chain.
Max-Forwards = "Max-Forwards" ":" 1*DIGIT
The Max-Forwards value is a decimal integer indicating the
remaining number of times this request message may be
forwarded.
Each proxy or gateway recipient of a TRACE request
containing a Max-Forwards header field SHOULD check and
update its value prior to forwarding the request. If the
received value is zero (0), the recipient SHOULD NOT forward
the request; instead, it SHOULD respond as the final
recipient with a 200 (OK) response containing the received
request message as the response entity-body (as described in
section 9.8). If the received Max-Forwards value is greater
than zero, then the forwarded message SHOULD contain an
updated Max-Forwards field with a value decremented by one
(1).
The Max-Forwards header field SHOULD be ignored for all
other methods defined by this specification and for any
extension methods for which it is not explicitly referred to
as part of that method definition.
14.32 Pragma
The Pragma general-header field is used to include
implementation-specific directives that may apply to any
recipient along the request/response chain. All pragma
directives specify optional behavior from the viewpoint of
the protocol; however, some systems MAY require that
behavior be consistent with the directives.
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Pragma = "Pragma" ":" 1#pragma-directive
pragma-directive = "no-cache" | extension-pragma
extension-pragma = token [ "=" ( token
| quoted-string ) ]
When the no-cache directive is present in a request message,
an application SHOULD forward the request toward the origin
server even if it has a cached copy of what is being
requested. This pragma directive has the same semantics as
the no-cache cache-directive (see section 14.9) and is
defined here for backwards compatibility with HTTP/1.0.
Clients SHOULD include both header fields when a no-cache
request is sent to a server not known to be HTTP/1.1
compliant.
Pragma directives MUST be passed through by a proxy or
gateway application, regardless of their significance to
that application, since the directives may be applicable to
all recipients along the request/response chain. It is not
possible to specify a pragma for a specific recipient;
however, any pragma directive not relevant to a recipient
SHOULD be ignored by that recipient.
HTTP/1.1 clients SHOULD NOT send the Pragma request-header.
HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the
client had sent "Cache-Control: no-cache". No new Pragma
directives will be defined in HTTP.
14.33 Proxy-Authenticate
The Proxy-Authenticate response-header field MUST be
included as part of a 407 (Proxy Authentication Required)
response. The field value consists of a challenge that
indicates the authentication scheme and parameters
applicable to the proxy for this Request-URI.
Proxy-Authenticate = "Proxy-Authenticate" ":"
challenge
The HTTP access authentication process is described in
section 11. Unlike WWW-Authenticate, the Proxy-Authenticate
header field applies only to the current connection and
SHOULD NOT be passed on to downstream clients. However, an
intermediate proxy may need to obtain its own credentials by
requesting them from the downstream client, which in some
circumstances will appear as if the proxy is forwarding the
Proxy-Authenticate header field.
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14.34 Proxy-Authorization
The Proxy-Authorization request-header field allows the
client to identify itself (or its user) to a proxy which
requires authentication. The Proxy-Authorization field value
consists of credentials containing the authentication
information of the user agent for the proxy and/or realm of
the resource being requested.
Proxy-Authorization = "Proxy-Authorization" ":"
credentials
The HTTP access authentication process is described in
section 11. Unlike Authorization, the Proxy-Authorization
header field applies only to the next outbound proxy that
demanded authentication using the Proxy-Authenticate field.
When multiple proxies are used in a chain, the Proxy-
Authorization header field is consumed by the first outbound
proxy that was expecting to receive credentials. A proxy MAY
relay the credentials from the client request to the next
proxy if that is the mechanism by which the proxies
cooperatively authenticate a given request.
14.35 Public
Editors Note: The OPTIONS changes would cause possible
changes to Allow and/or Public for consistency with each
other and with section 9.2 (OPTIONS )
The Public response-header field lists the set of methods
supported by the server. The purpose of this field is
strictly to inform the recipient of the capabilities of the
server regarding unusual methods. The methods listed may or
may not be applicable to the Request-URI; the Allow header
field (section 14.7) MAY be used to indicate methods allowed
for a particular URI.
Public = "Public" ":" 1#method
Example of use:
Public: OPTIONS, MGET, MHEAD, GET, HEAD
This header field applies only to the server directly
connected to the client (i.e., the nearest neighbor in a
chain of connections). If the response passes through a
proxy, the proxy MUST either remove the Public header field
or replace it with one applicable to its own capabilities.
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14.36 Range
14.36.1 Byte Ranges
Since all HTTP entities are represented in HTTP messages as
sequences of bytes, the concept of a byte range is
meaningful for any HTTP entity. (However, not all clients
and servers need to support byte-range operations.)
Byte range specifications in HTTP apply to the sequence of
bytes in the entity-body (not necessarily the same as the
message-body).
A byte range operation may specify a single range of bytes,
or a set of ranges within a single entity.
ranges-specifier = byte-ranges-specifier
byte-ranges-specifier = bytes-unit "=" byte-range-set
byte-range-set = 1#( byte-range-spec
| suffix-byte-range-spec )
byte-range-spec = first-byte-pos "-" [last-byte-pos]
first-byte-pos = 1*DIGIT
last-byte-pos = 1*DIGIT
The first-byte-pos value in a byte-range-spec gives the
byte-offset of the first byte in a range. The last-byte-pos
value gives the byte-offset of the last byte in the range;
that is, the byte positions specified are inclusive. Byte
offsets start at zero.
If the last-byte-pos value is present, it must be greater
than or equal to the first-byte-pos in that byte-range-spec,
or the byte-range-spec is invalid. The recipient of an
invalid byte-range-spec must ignore it.
If the last-byte-pos value is absent, or if the value is
greater than or equal to the current length of the entity-
body, last-byte-pos is taken to be equal to one less than
the current length of the entity-body in bytes.
By its choice of last-byte-pos, a client can limit the
number of bytes retrieved without knowing the size of the
entity.
suffix-byte-range-spec = "-" suffix-length
suffix-length = 1*DIGIT
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A suffix-byte-range-spec is used to specify the suffix of
the entity-body, of a length given by the suffix-length
value. (That is, this form specifies the last N bytes of an
entity-body.) If the entity is shorter than the specified
suffix-length, the entire entity-body is used.
Examples of byte-ranges-specifier values (assuming an
entity-body of length 10000):
. The first 500 bytes (byte offsets 0-499, inclusive):
bytes=0-499
. The second 500 bytes (byte offsets 500-999, inclusive):
bytes=500-999
. The final 500 bytes (byte offsets 9500-9999,
inclusive):
bytes=-500
. Or
bytes=9500-
. The first and last bytes only (bytes 0 and 9999):
bytes=0-0,-1
. Several legal but not canonical specifications of the
second 500 bytes (byte offsets 500-999, inclusive):
bytes=500-600,601-999
bytes=500-700,601-999
14.36.2 Range Retrieval Requests
HTTP retrieval requests using conditional or unconditional
GET methods may request one or more sub-ranges of the
entity, instead of the entire entity, using the Range
request header, which applies to the entity returned as the
result of the request:
Range = "Range" ":" ranges-specifier
A server MAY ignore the Range header. However, HTTP/1.1
origin servers and intermediate caches SHOULD support byte
ranges when possible, since Range supports efficient
recovery from partially failed transfers, and supports
efficient partial retrieval of large entities.
If the server supports the Range header and the specified
range or ranges are appropriate for the entity:
. The presence of a Range header in an unconditional GET
modifies what is returned if the GET is otherwise
successful. In other words, the response carries a
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status code of 206 (Partial Content) instead of 200
(OK).
. The presence of a Range header in a conditional GET (a
request using one or both of If-Modified-Since and If-
None-Match, or one or both of If-Unmodified-Since and
If-Match) modifies what is returned if the GET is
otherwise successful and the condition is true. It does
not affect the 304 (Not Modified) response returned if
the conditional is false.
In some cases, it may be more appropriate to use the If-
Range header (see section 14.27) in addition to the Range
header.
If a proxy that supports ranges receives a Range request,
forwards the request to an inbound server, and receives an
entire entity in reply, it SHOULD only return the requested
range to its client. It SHOULD store the entire received
response in its cache, if that is consistent with its cache
allocation policies.
14.37 Referer
The Referer[sic] request-header field allows the client to
specify, for the server's benefit, the address (URI) of the
resource from which the Request-URI was obtained (the
"referrer", although the header field is misspelled.) The
Referer request-header allows a server to generate lists of
back-links to resources for interest, logging, optimized
caching, etc. It also allows obsolete or mistyped links to
be traced for maintenance. The Referer field MUST NOT be
sent if the Request-URI was obtained from a source that does
not have its own URI, such as input from the user keyboard.
Referer = "Referer" ":" ( absoluteURI |
relativeURI )
Example:
Referer:
http://www.w3.org/hypertext/DataSources/Overview.html
If the field value is a partial URI, it SHOULD be
interpreted relative to the Request-URI. The URI MUST NOT
include a fragment.See section 15.11 for security
considerations.
14.38 Retry-After
The Retry-After response-header field can be used with a 503
(Service Unavailable) response to indicate how long the
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service is expected to be unavailable to the requesting
client. This field MAY also be used with any 3xx
(Redirection) response to indicate the minimum time the
user-agent should wait before issuing the redirected
request. The value of this field can be either an HTTP-date
or an integer number of seconds (in decimal) after the time
of the response.
Retry-After = "Retry-After" ":" ( HTTP-date
| delta-seconds )
Two examples of its use are
Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
Retry-After: 120
In the latter example, the delay is 2 minutes.
14.39 Server
The Server response-header field contains information about
the software used by the origin server to handle the
request. The field can contain multiple product tokens
(section 3.8) and comments identifying the server and any
significant subproducts. The product tokens are listed in
order of their significance for identifying the application.
Server = "Server" ":" 1*( product | comment )
Example:
Server: CERN/3.0 libwww/2.17
If the response is being forwarded through a proxy, the
proxy application MUST NOT modify the Server response-
header. Instead, it SHOULD include a Via field (as described
in section 14.44).
Note: Revealing the specific software version of the
server may allow the server machine to become more
vulnerable to attacks against software that is known to
contain security holes. Server implementers are
encouraged to make this field a configurable option.
14.40 Transfer-Encoding
The Transfer-Encoding general-header field indicates what
(if any) type of transformation has been applied to the
message body in order to safely transfer it between the
sender and the recipient. This differs from the Content-
Encoding in that the transfer coding is a property of the
message, not of the entity.
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Transfer-Encoding = "Transfer-Encoding" ":"
1#transfer-coding
Transfer codings are defined in section 3.6. An example is:
Transfer-Encoding: chunked
If multiple encodings have been applied to an entity, the
transfer codings MUST be listed in the order in which they
were applied. Additional information about the encoding
parameters MAY be provided by other entity-header fields not
defined by this specification.
Many older HTTP/1.0 applications do not understand the
Transfer-Encoding header.
14.41 Upgrade
The Upgrade general-header allows the client to specify what
additional communication protocols it supports and would
like to use if the server finds it appropriate to switch
protocols. The server MUST use the Upgrade header field
within a 101 (Switching Protocols) response to indicate
which protocol(s) are being switched.
Upgrade = "Upgrade" ":" 1#product
For example,
Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
The Upgrade header field is intended to provide a simple
mechanism for transition from HTTP/1.1 to some other,
incompatible protocol. It does so by allowing the client to
advertise its desire to use another protocol, such as a
later version of HTTP with a higher major version number,
even though the current request has been made using
HTTP/1.1. This eases the difficult transition between
incompatible protocols by allowing the client to initiate a
request in the more commonly supported protocol while
indicating to the server that it would like to use a
"better" protocol if available (where "better" is determined
by the server, possibly according to the nature of the
method and/or resource being requested).
The Upgrade header field only applies to switching
application-layer protocols upon the existing transport-
layer connection. Upgrade cannot be used to insist on a
protocol change; its acceptance and use by the server is
optional. The capabilities and nature of the application-
layer communication after the protocol change is entirely
dependent upon the new protocol chosen, although the first
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action after changing the protocol MUST be a response to the
initial HTTP request containing the Upgrade header field.
The Upgrade header field only applies to the immediate
connection. Therefore, the upgrade keyword MUST be supplied
within a Connection header field (section 14.10) whenever
Upgrade is present in an HTTP/1.1 message.
The Upgrade header field cannot be used to indicate a switch
to a protocol on a different connection. For that purpose,
it is more appropriate to use a 301, 302, 303, or 305
redirection response.
This specification only defines the protocol name "HTTP" for
use by the family of Hypertext Transfer Protocols, as
defined by the HTTP version rules of section 3.1 and future
updates to this specification. Any token can be used as a
protocol name; however, it will only be useful if both the
client and server associate the name with the same protocol.
14.42 User-Agent
The User-Agent request-header field contains information
about the user agent originating the request. This is for
statistical purposes, the tracing of protocol violations,
and automated recognition of user agents for the sake of
tailoring responses to avoid particular user agent
limitations. User agents SHOULD include this field with
requests. The field can contain multiple product tokens
(section 3.8) and comments identifying the agent and any
subproducts which form a significant part of the user agent.
By convention, the product tokens are listed in order of
their significance for identifying the application.
User-Agent = "User-Agent" ":" 1*( product | comment )
Example:
User-Agent: CERN-LineMode/2.15 libwww/2.17b3
14.43 Vary
Editors Note: Henrik Frystyk has drafted language to fix an
editorial problem (VARY) around Vary. Vary is really cache
advice. The description varies throughout the spec. Time
did not permit me to incorporate a final version of these
changes.
The Vary response-header field is used by a server to signal
that the response entity was selected from the available
representations of the response using server-driven
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negotiation (section 12). Field-names listed in Vary headers
are those of request-headers. The Vary field value indicates
either that the given set of header fields encompass the
dimensions over which the representation might vary, or that
the dimensions of variance are unspecified ("*") and thus
may vary over any aspect of future requests.
Vary = "Vary" ":" ( "*" | 1#field-name )
An HTTP/1.1 server MUST include an appropriate Vary header
field with any cachable response that is subject to server-
driven negotiation. Doing so allows a cache to properly
interpret future requests on that resource and informs the
user agent about the presence of negotiation on that
resource. A server SHOULD include an appropriate Vary header
field with a non-cachable response that is subject to
server-driven negotiation, since this might provide the user
agent with useful information about the dimensions over
which the response might vary.
The set of header fields named by the Vary field value is
known as the "selecting" request-headers.
When the cache receives a subsequent request whose Request-
URI specifies one or more cache entries including a Vary
header, the cache MUST NOT use such a cache entry to
construct a response to the new request unless all of the
headers named in the cached Vary header are present in the
new request, and all of the stored selecting request-headers
from the previous request match the corresponding headers in
the new request.
The selecting request-headers from two requests are defined
to match if and only if the selecting request-headers in the
first request can be transformed to the selecting request-
headers in the second request by adding or removing linear
whitespace (LWS) at places where this is allowed by the
corresponding BNF, and/or combining multiple message-header
fields with the same field name following the rules about
message headers in section 4.2.
A Vary field value of "*" signals that unspecified
parameters, possibly other than the contents of request-
header fields (e.g., the network address of the client),
play a role in the selection of the response representation.
Subsequent requests on that resource can only be properly
interpreted by the origin server, and thus a cache MUST
forward a (possibly conditional) request even when it has a
fresh response cached for the resource. See section 13.6 for
use of the Vary header by caches.
A Vary field value consisting of a list of field-names
signals that the representation selected for the response is
based on a selection algorithm which considers ONLY the
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listed request-header field values in selecting the most
appropriate representation. A cache MAY assume that the same
selection will be made for future requests with the same
values for the listed field names, for the duration of time
in which the response is fresh.
The field-names given are not limited to the set of standard
request-header fields defined by this specification. Field
names are case-insensitive.
14.44 Via
The Via general-header field MUST be used by gateways and
proxies to indicate the intermediate protocols and
recipients between the user agent and the server on
requests, and between the origin server and the client on
responses. It is analogous to the "Received" field of RFC
822 [9] and is intended to be used for tracking message
forwards, avoiding request loops, and identifying the
protocol capabilities of all senders along the
request/response chain.
Via = "Via" ":" 1#( received-protocol received-by
[ comment ] )
received-protocol = [ protocol-name "/" ] protocol-version
protocol-name = token
protocol-version = token
received-by = ( host [ ":" port ] ) | pseudonym
pseudonym = token
The received-protocol indicates the protocol version of the
message received by the server or client along each segment
of the request/response chain. The received-protocol version
is appended to the Via field value when the message is
forwarded so that information about the protocol
capabilities of upstream applications remains visible to all
recipients.
The protocol-name is optional if and only if it would be
"HTTP". The received-by field is normally the host and
optional port number of a recipient server or client that
subsequently forwarded the message. However, if the real
host is considered to be sensitive information, it MAY be
replaced by a pseudonym. If the port is not given, it MAY be
assumed to be the default port of the received-protocol.
Multiple Via field values represent each proxy or gateway
that has forwarded the message. Each recipient MUST append
its information such that the end result is ordered
according to the sequence of forwarding applications.
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Comments MAY be used in the Via header field to identify the
software of the recipient proxy or gateway, analogous to the
User-Agent and Server header fields. However, all comments
in the Via field are optional and MAY be removed by any
recipient prior to forwarding the message.
For example, a request message could be sent from an
HTTP/1.0 user agent to an internal proxy code-named "fred",
which uses HTTP/1.1 to forward the request to a public proxy
at nowhere.com, which completes the request by forwarding it
to the origin server at www.ics.uci.edu. The request
received by www.ics.uci.edu would then have the following
Via header field:
Via: 1.0 fred, 1.1 nowhere.com (Apache/1.1)
Proxies and gateways used as a portal through a network
firewall SHOULD NOT, by default, forward the names and ports
of hosts within the firewall region. This information SHOULD
only be propagated if explicitly enabled. If not enabled,
the received-by host of any host behind the firewall SHOULD
be replaced by an appropriate pseudonym for that host.
For organizations that have strong privacy requirements for
hiding internal structures, a proxy MAY combine an ordered
subsequence of Via header field entries with identical
received-protocol values into a single such entry. For
example,
Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
could be collapsed to
Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
Applications SHOULD NOT combine multiple entries unless they
are all under the same organizational control and the hosts
have already been replaced by pseudonyms. Applications MUST
NOT combine entries which have different received-protocol
values.
14.45 Warning
The Warning response-header field is used to carry
additional information about the status of a response which
may not be reflected by the response status code. This
information is typically, though not exclusively, used to
warn about a possible lack of semantic transparency from
caching operations.
Warning headers are sent with responses using:
Warning = "Warning" ":" 1#warning-value
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warning-value = warn-code SP warn-agent SP warn-text
[SP warn-date]
warn-code = 3DIGIT
warn-agent = ( host [ ":" port ] ) | pseudonym
; the name or pseudonym of the server adding
; the Warning header, for use in debugging
warn-text = quoted-string
warn-date = <"> HTTP-date <">
A response may carry more than one Warning header.
The warn-text should be in a natural language and character
set that is most likely to be intelligible to the human user
receiving the response. This decision may be based on any
available knowledge, such as the location of the cache or
user, the Accept-Language field in a request, the Content-
Language field in a response, etc. The default language is
English and the default character set is ISO-8859-1.
If a character set other than ISO-8859-1 is used, it MUST be
encoded in the warn-text using the method described in RFC
2047 [14].
Any server or cache may add Warning headers to a response.
New Warning headers should be added after any existing
Warning headers. A cache MUST NOT delete any Warning header
that it received with a response. However, if a cache
successfully validates a cache entry, it SHOULD remove any
Warning headers previously attached to that entry except as
specified for specific Warning codes. It MUST then add any
Warning headers received in the validating response. In
other words, Warning headers are those that would be
attached to the most recent relevant response.
When multiple Warning headers are attached to a response,
the user agent SHOULD display as many of them as possible,
in the order that they appear in the response. If it is not
possible to display all of the warnings, the user agent
should follow these heuristics:
. Warnings that appear early in the response take
priority over those appearing later in the response.
. Warnings in the user's preferred character set take
priority over warnings in other character sets but with
identical warn-codes and warn-agents.
Systems that generate multiple Warning headers should order
them with this user agent behavior in mind.
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The warn-code consists of three digits. The first digit
indicates whether the Warning MUST or MUST NOT be deleted
from a stored cache entry after a successful revalidation:
1XX Warnings that describe the freshness or revalidation
status of the response, and so MUST be deleted after a
successful revalidation.
2XX Warnings that describe some aspect of the entity body
or entity headers that is not rectified by a
revalidation, and which MUST NOT be deleted after a
successful revalidation.
This is a list of the currently-defined warn-codes, each
with a recommended warn-text in English, and a description
of its meaning.
110 Response is stale
MUST be included whenever the returned response is stale.
111 Revalidation failed
MUST be included if a cache returns a stale response
because an attempt to revalidate the response failed, due
to an inability to reach the server.
112 Disconnected operation
SHOULD be included if the cache is intentionally
disconnected from the rest of the network for a period of
time.
113 Heuristic expiration
MUST be included if the cache heuristically chose a
freshness lifetime greater than 24 hours and the
response's age is greater than 24 hours.
199 Miscellaneous warning
The warning text may include arbitrary information to be
presented to a human user, or logged. A system receiving
this warning MUST NOT take any automated action.
214 Transformation applied
MUST be added by an intermediate cache or proxy if it
applies any transformation changing the content-coding
(as specified in the Content-Encoding header) or media-
type (as specified in the Content-Type header) of the
response, unless this Warning code already appears in the
response.
299 Miscellaneous persistent warning
The warning text may include arbitrary information to be
presented to a human user, or logged. A system receiving
this warning MUST NOT take any automated action.
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If an implementation sends a response with one or more
Warning headers to a client whose version is HTTP/1.0 or
lower, then the sender MUST include a warn-date in each
warning-value.
If an implementation receives a response with a warning-
value that includes a warn-date, and that warn-date is
different from the Date value in the response, then that
warning-value MUST be deleted from the message before
storing, forwarding, or using it. If all of the warning-
values are deleted for this reason, the Warning header MUST
be deleted as well.
14.46 WWW-Authenticate
The WWW-Authenticate response-header field MUST be included
in 401 (Unauthorized) response messages. The field value
consists of at least one challenge that indicates the
authentication scheme(s) and parameters applicable to the
Request-URI.
WWW-Authenticate = "WWW-Authenticate" ":" 1#challenge
The HTTP access authentication process is described in
section 11. User agents MUST take special care in parsing
the WWW-Authenticate field value if it contains more than
one challenge, or if more than one WWW-Authenticate header
field is provided, since the contents of a challenge may
itself contain a comma-separated list of authentication
parameters.
14.47 Expect
The Expect request-header field is used to indicate that
particular server behaviors are required by the client. A
server that does not understand or is unable to comply with
any of the expectation values in the Expect field of a
request MUST respond with appropriate error status.
Expect = "Expect" ":" 1#expectation
expectation = "100-continue" | expectation-extension
expectation-extension = token [ "="
( token | quoted-string ) *expect-params ]
expect-params = ";" token [ = ( token | quoted-string ) ]
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The server SHOULD respond with a 419 (Expectation Failed)
status if any of the expectations cannot be met.
This header field is defined with extensible syntax to allow
for future extensions. If a server receives a request
containing an Expect field that includes an expectation-
extension that it does not support, it MUST respond with a
419 (Expectation Failed) status.
14.47.1 Expect 100-continue
When the "100-continue" expectation is present on a request
that includes a body, the requesting client will wait after
sending the request headers before sending the content-body.
In this case, the server MUST conform to the requirements of
section 8.2.4: it MUST either send a 100 (Continue) status,
or an error status, after receiving the "Expect: 100-
continue" request header.
If a proxy receives a request with the "100-continue"
expectation, and the proxy either knows that the next-hop
server complies with HTTP/1.1 or higher, or does not know
the HTTP version of the next-hop server, it MUST forward the
request, including the Expect header field. If the proxy
knows that the version of the next-hop server is HTTP/1.0 or
lower, it MUST NOT forward the request, and it MUST respond
with a 419 (Expectation Failed) status. Proxies SHOULD
maintain a cache recording the HTTP version numbers received
from recently-referenced next-hop servers.
Note: Because of the presence of older implementations,
the protocol allows ambiguous situations in which a
client may send "Expect: 100-continue" without
receiving either a 419 (Expectation Failed) status or a
100 (Continue) status. Therefore, when a client sends
this header field to an origin server (possibly via a
proxy) from which it has never seen a 100 (Continue)
status, the client should not wait for an indefinite or
lengthy period before sending the request body.
14.48 Set-Proxy
The Set-Proxy response-header is used to carry information
to redirect a client to use a different proxy.
Set-Proxy: "Set-Proxy" ":" action [ ";" 1#parameters ]
parameters = ( "scope" "=" scopePattern ) |
( proxyURI "=" URI ) | lifetime
lifetime = ( "seconds" "=" integer )
| ( "hits" "=" integer )
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action = ( "direct" | "ipl" | "set" )
scopePattern = "*" | "-" | URIpattern
URIpattern = character | "*"
character = Any character legal in the definition
of a URL/URI in the context of RFC2068
An example header:
Set-proxy: set ; proxyURI = "http://proxy.me.com:8080/",
scope="http://", seconds=5
Scope Meaning: all URLS beginning with "http://"
Another example header:
Set-Proxy: set ; proxyURI = "http://proxy.me.com:8080/",
scope="http://*.ups.com/", seconds=5
Scope meaning: all URLS beginning which are for hosts in the
ups.com domain.
The action response directive specifies the type or mode of
the change.
direct
Attempt to connect directly, with no proxy
ipl
Initial Program Load, the client or proxy should attempt
to revert back to its default or initial proxy setting.
This is meant to instruct a client to re-fetch its proxy
configuration, or PAC file. When set, the accompanying
scope field MUST be "*" A client receiving this response
SHOULD prompt the user for confirmation.
If accompanied by a proxyURI parameter, a proxy or client
MAY use the value as a URL containing a configuration to
retrieve. If a client does so, it MUST prompt the user
for confirmation.
set
Set to parameter proxyURI. The client should use the URL
specified for proxyURI as the proxy. If the SET mode is
specified, the parameter, proxyURI, MUST be present.
Scope refers to an expression pattern that specifies which
URIs that are subject to this header setting. URIs should
be matched against the scope with this rule :
The scope "*" means all requests.
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The scope "-" means this EXACT URL ONLY
Otherwise, the URL is compared with the scope in the
following manner.
The scope is a prefix of matching URLs.
The character "*" is allowed in the DNS name portion of a
URL, or in the path portion of the URL, but ONLY when used
with a 306, not a 305.
It matches any sequence of characters except '/'.
This is intended to be a simple matching scheme to allow a
prefix match to take place.
See the examples section in section 15.12.2.
The lifetime parameter specifies how long the specified
proxy should be used. If lifetime is specified as "seconds"
then the proxy setting remains in effect for `integer'
seconds. If lifetime is specified in `hits' then the proxy
setting remains in effect for `integer' transactions.
14.49 Compliance
Editor's note: The OPTIONS changes would introduce a new
"Compliance" header.
14.50 Non-Compliance
Editor's note: The OPTIONS changes would introduce a new
"Compliance" header.
15 Security Considerations
This section is meant to inform application developers,
information providers, and users of the security limitations
in HTTP/1.1 as described by this document. The discussion
does not include definitive solutions to the problems
revealed, though it does make some suggestions for reducing
security risks.
15.1 Authentication of Clients
The Basic authentication scheme is not a secure method of
user authentication, nor does it in any way protect the
entity, which is transmitted in clear text across the
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physical network used as the carrier. HTTP does not prevent
additional authentication schemes and encryption mechanisms
from being employed to increase security or the addition of
enhancements (such as schemes to use one-time passwords) to
Basic authentication.
The most serious flaw in Basic authentication is that it
results in the essentially clear text transmission of the
user's password over the physical network. It is this
problem which Digest Authentication attempts to address.
Because Basic authentication involves the clear text
transmission of passwords it SHOULD never be used (without
enhancements) to protect sensitive or valuable information.
A common use of Basic authentication is for identification
purposes -- requiring the user to provide a user name and
password as a means of identification, for example, for
purposes of gathering accurate usage statistics on a server.
When used in this way it is tempting to think that there is
no danger in its use if illicit access to the protected
documents is not a major concern. This is only correct if
the server issues both user name and password to the users
and in particular does not allow the user to choose his or
her own password. The danger arises because naive users
frequently reuse a single password to avoid the task of
maintaining multiple passwords.
If a server permits users to select their own passwords,
then the threat is not only illicit access to documents on
the server but also illicit access to the accounts of all
users who have chosen to use their account password. If
users are allowed to choose their own password that also
means the server must maintain files containing the
(presumably encrypted) passwords. Many of these may be the
account passwords of users perhaps at distant sites. The
owner or administrator of such a system could conceivably
incur liability if this information is not maintained in a
secure fashion.
Basic Authentication is also vulnerable to spoofing by
counterfeit servers. If a user can be led to believe that he
is connecting to a host containing information protected by
basic authentication when in fact he is connecting to a
hostile server or gateway then the attacker can request a
password, store it for later use, and feign an error. This
type of attack is not possible with Digest Authentication
[32]. Server implementers SHOULD guard against the
possibility of this sort of counterfeiting by gateways or
CGI scripts. In particular it is very dangerous for a server
to simply turn over a connection to a gateway since that
gateway can then use the persistent connection mechanism to
engage in multiple transactions with the client while
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impersonating the original server in a way that is not
detectable by the client.
15.2 Offering a Choice of Authentication Schemes
An HTTP/1.1 server may return multiple challenges with a 401
(Authenticate) response, and each challenge may use a
different scheme. The order of the challenges returned to
the user agent is in the order that the server would prefer
they be chosen. The server should order its challenges with
the "most secure" authentication scheme first. A user agent
should choose as the challenge to be made to the user the
first one that the user agent understands.
When the server offers choices of authentication schemes
using the WWW-Authenticate header, the "security" of the
authentication is only as good as the security of the
weakest of the authentication schemes. A malicious user
could capture the set of challenges and try to authenticate
him/herself using the weakest of the authentication schemes.
Thus, the ordering serves more to protect the user's
credentials than the server's information.
A possible man-in-the-middle (MITM) attack would be to add a
weak authentication scheme to the set of choices, hoping
that the client will use one that exposes the user's
credentials (e.g. password). For this reason, the client
should always use the strongest scheme that it understands
from the choices accepted.
An even better MITM attack would be to remove all offered
choices, and to insert a challenge that requests Basic
authentication. For this reason, user agents that are
concerned about this kind of attack could remember the
strongest authentication scheme ever requested by a server
and produce a warning message that requires user
confirmation before using a weaker one. A particularly
insidious way to mount such a MITM attack would be to offer
a "free" proxy caching service to gullible users.
15.3 Abuse of Server Log Information
A server is in the position to save personal data about a
user's requests which may identify their reading patterns or
subjects of interest. This information is clearly
confidential in nature and its handling may be constrained
by law in certain countries. People using the HTTP protocol
to provide data are responsible for ensuring that such
material is not distributed without the permission of any
individuals that are identifiable by the published results.
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15.4 Transfer of Sensitive Information
Like any generic data transfer protocol, HTTP cannot
regulate the content of the data that is transferred, nor is
there any a priori method of determining the sensitivity of
any particular piece of information within the context of
any given request. Therefore, applications SHOULD supply as
much control over this information as possible to the
provider of that information. Four header fields are worth
special mention in this context: Server, Via, Referer and
From.
Revealing the specific software version of the server may
allow the server machine to become more vulnerable to
attacks against software that is known to contain security
holes. Implementers SHOULD make the Server header field a
configurable option.
Proxies which serve as a portal through a network firewall
SHOULD take special precautions regarding the transfer of
header information that identifies the hosts behind the
firewall. In particular, they SHOULD remove, or replace with
sanitized versions, any Via fields generated behind the
firewall.
The Referer field allows reading patterns to be studied and
reverse links drawn. Although it can be very useful, its
power can be abused if user details are not separated from
the information contained in the Referer. Even when the
personal information has been removed, the Referer field may
indicate a private document's URI whose publication would be
inappropriate.
The information sent in the From field might conflict with
the user's privacy interests or their site's security
policy, and hence it SHOULD NOT be transmitted without the
user being able to disable, enable, and modify the contents
of the field. The user MUST be able to set the contents of
this field within a user preference or application defaults
configuration.
We suggest, though do not require, that a convenient toggle
interface be provided for the user to enable or disable the
sending of From and Referer information.
15.5 Attacks Based On File and Path Names
Implementations of HTTP origin servers SHOULD be careful to
restrict the documents returned by HTTP requests to be only
those that were intended by the server administrators. If an
HTTP server translates HTTP URIs directly into file system
calls, the server MUST take special care not to serve files
that were not intended to be delivered to HTTP clients. For
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example, UNIX, Microsoft Windows, and other operating
systems use ".." as a path component to indicate a directory
level above the current one. On such a system, an HTTP
server MUST disallow any such construct in the Request-URI
if it would otherwise allow access to a resource outside
those intended to be accessible via the HTTP server.
Similarly, files intended for reference only internally to
the server (such as access control files, configuration
files, and script code) MUST be protected from inappropriate
retrieval, since they might contain sensitive information.
Experience has shown that minor bugs in such HTTP server
implementations have turned into security risks.
15.6 Personal Information
HTTP clients are often privy to large amounts of personal
information (e.g. the user's name, location, mail address,
passwords, encryption keys, etc.), and SHOULD be very
careful to prevent unintentional leakage of this information
via the HTTP protocol to other sources. We very strongly
recommend that a convenient interface be provided for the
user to control dissemination of such information, and that
designers and implementers be particularly careful in this
area. History shows that errors in this area are often both
serious security and/or privacy problems, and often generate
highly adverse publicity for the implementer's company.
15.7 Privacy Issues Connected to Accept Headers
Accept request-headers can reveal information about the user
to all servers which are accessed. The Accept-Language
header in particular can reveal information the user would
consider to be of a private nature, because the
understanding of particular languages is often strongly
correlated to the membership of a particular ethnic group.
User agents which offer the option to configure the contents
of an Accept-Language header to be sent in every request are
strongly encouraged to let the configuration process include
a message which makes the user aware of the loss of privacy
involved.
An approach that limits the loss of privacy would be for a
user agent to omit the sending of Accept-Language headers by
default, and to ask the user whether it should start sending
Accept-Language headers to a server if it detects, by
looking for any Vary response-header fields generated by the
server, that such sending could improve the quality of
service.
Elaborate user-customized accept header fields sent in every
request, in particular if these include quality values, can
be used by servers as relatively reliable and long-lived
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user identifiers. Such user identifiers would allow content
providers to do click-trail tracking, and would allow
collaborating content providers to match cross-server click-
trails or form submissions of individual users. Note that
for many users not behind a proxy, the network address of
the host running the user agent will also serve as a long-
lived user identifier. In environments where proxies are
used to enhance privacy, user agents should be conservative
in offering accept header configuration options to end
users. As an extreme privacy measure, proxies could filter
the accept headers in relayed requests. General purpose user
agents which provide a high degree of header configurability
should warn users about the loss of privacy which can be
involved.
15.8 DNS Spoofing
Clients using HTTP rely heavily on the Domain Name Service,
and are thus generally prone to security attacks based on
the deliberate mis-association of IP addresses and DNS
names. Clients need to be cautious in assuming the
continuing validity of an IP number/DNS name association.
In particular, HTTP clients SHOULD rely on their name
resolver for confirmation of an IP number/DNS name
association, rather than caching the result of previous host
name lookups. Many platforms already can cache host name
lookups locally when appropriate, and they SHOULD be
configured to do so. These lookups should be cached,
however, only when the TTL (Time To Live) information
reported by the name server makes it likely that the cached
information will remain useful.
If HTTP clients cache the results of host name lookups in
order to achieve a performance improvement, they MUST
observe the TTL information reported by DNS.
If HTTP clients do not observe this rule, they could be
spoofed when a previously-accessed server's IP address
changes. As network renumbering is expected to become
increasingly common [24], the possibility of this form of
attack will grow. Observing this requirement thus reduces
this potential security vulnerability.
This requirement also improves the load-balancing behavior
of clients for replicated servers using the same DNS name
and reduces the likelihood of a user's experiencing failure
in accessing sites which use that strategy.
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15.9 Location Headers and Spoofing
If a single server supports multiple organizations that do
not trust one another, then it must check the values of
Location and Content-Location headers in responses that are
generated under control of said organizations to make sure
that they do not attempt to invalidate resources over which
they have no authority.
15.10 Content-Disposition Issues
RFC 1806, from which the often implemented Content-
Disposition (see section 19.6.1) header in HTTP is derived,
has a number of very serious security considerations.
Content-Disposition is not part of the HTTP standard, but
since it is widely implemented, we are documenting its use
and risks for implementers. See RFC 1806 [35] for details.
15.11 Encoding Sensitive Information in URL's
Because the source of a link may be private information or
may reveal an otherwise private information source, it is
strongly recommended that the user be able to select whether
or not the Referer field is sent. For example, a browser
client could have a toggle switch for browsing
openly/anonymously, which would respectively enable/disable
the sending of Referer and From information.
Clients SHOULD NOT include a Referer header field in a
(non-secure) HTTP request if the referring page was
transferred with a secure protocol.
Authors of services which use the HTTP protocol SHOULD NOT
use GET based forms for the submission of sensitive data,
because this will cause this data to be encoded in the
request URI. Many existing servers, proxies, and user agents
will log the request URI in some place where it may be
visible to third parties. Servers can use POST based form
submission instead.
15.12 Using 305/306 response codes and 'Set-Proxy' header
Editor's note: This presumes the OPTIONS issue gets closed
quickly and incorporated in the next draft of this document.
15.12.1 Methods
A client or proxy receiving a 305 or 306, should use the
OPTIONS method to determine if the server or proxy it is
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talking to actually is an HTTP/1.1 server supporting set-
proxy header 305 and 306 responses.
15.12.2 Operational Contraints
Both the 305 and 306 response codes are HOP by HOP. A proxy
server MUST not forward a 305 or 306 respose code (unless it
generated the 306). A webserver MUST NOT send a 306 response
under any circumstances. A proxy server MUST NOT generate a
305 response. A client or proxy SHOULD NOT accept a 306 from
a proxy that it learned of via a 305 response code. A client
or proxy MAY maintain state and allow a lifetime to extend
beyond a session or restart. A "Set-Proxy: ipl" SHOULD
override any previous Set-Proxy header. A 305 or 306
response MAY contain a body containing an explanation of the
redirect for clients which do not understand the redirect.
In the absence of any parameter, the following defaults
should be used:
lifetime = this transaction only
scope = this exact URL only
When receiving a 305 response, the client or proxy will
enforce the following rule with respect to the scope.The
scope specified must be more restrictive than the
transformed URL in question based on the rightmost slash in
the URI.
Example: (in order of restrictiveness)
for URI = http://www.ups.com/services/index.html
http://www.ups.com/services/ (allowed)
http://www.ups.com/services/express/ ( allowed )
http://www.ups.com/ (NOT allowed)
Using "*" in a 306 response Set-Proxy header:
The scope may be set to:
http://*.foo.com/
which would apply to all URLs in to domain foo.com
If the scope returned with a 305 response is less
restrictive than the requested URL, the client may reject
the redirection and return 506 Redirection Failed. If the
client wished to honor the redirect, it client MUST prompt
the user for confirmation before accepting the new proxy
setting.
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Since HTTP/1.0 proxies may unknowingly forward a 305 or 306
response code that was generated maliciously or in good
faith, the client must attempt to ascertain if the proxy
with which it is directly communicating is HTTP/1.1 and if
it supports the Set-Proxy header. To determine this, the
client or proxy should use the OPTIONS method to make a
request check for this feature. The extension string should
be HDR='set-proxy', or, should this be defined in the
Standard RFC for HTTP/1.1, then the string should be
RFC='rfcXXXX' in the OPTIONS request.
Great care should be taken when implementing client side
actions based on the 305 or 306. Since older proxies may
unknowingly forward either of these reponses, clients should
be prepared to check the validity. A client or proxy MUST
NOT accept a 305 response from a proxy. A client or proxy
MUST NOT accept a 306 response from an origin server. When
receiving a 306 response from a proxy, the client MUST
verify that the proxy supports the 306 response with an
OPTIONS request.
16 Acknowledgments
This specification makes heavy use of the augmented BNF and
generic constructs defined by David H. Crocker for RFC 822
[9]. Similarly, it reuses many of the definitions provided
by Nathaniel Borenstein and Ned Freed for MIME [7]. We hope
that their inclusion in this specification will help reduce
past confusion over the relationship between HTTP and
Internet mail message formats.
The HTTP protocol has evolved considerably over the past
four years. It has benefited from a large and active
developer community--the many people who have participated
on the www-talk mailing list--and it is that community which
has been most responsible for the success of HTTP and of the
World-Wide Web in general. Marc Andreessen, Robert Cailliau,
Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois
Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen,
Rob McCool, Lou Montulli, Dave Raggett, Tony Sanders, and
Marc VanHeyningen deserve special recognition for their
efforts in defining early aspects of the protocol.
This document has benefited greatly from the comments of all
those participating in the HTTP-WG. In addition to those
already mentioned, the following individuals have
contributed to this specification:
Gary Adams Albert Lunde
Harald Tveit Alvestrand John C. Mallery
Keith Ball Jean-Philippe Martin-Flatin
Brian Behlendorf Larry Masinter
Paul Burchard Mitra
Maurizio Codogno David Morris
Mike Cowlishaw Gavin Nicol
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Roman Czyborra Bill Perry
Michael A. Dolan Jeffrey Perry
David J. Fiander Scott Powers
Alan Freier Owen Rees
Marc Hedlund Luigi Rizzo
Greg Herlihy David Robinson
Koen Holtman Marc Salomon
Alex Hopmann Rich Salz
Bob Jernigan Allan M. Schiffman
Shel Kaphan Jim Seidman
Rohit Khare Chuck Shotton
John Klensin Eric W. Sink
Martijn Koster Simon E. Spero
Alexei Kosut Richard N. Taylor
David M. Kristol Robert S. Thau
Daniel LaLiberte Bill (BearHeart) Weinman
Ben Laurie Francois Yergeau
Paul J. Leach Mary Ellen Zurko
Daniel DuBois
Much of the content and presentation of the caching design
is due to suggestions and comments from individuals
including: Shel Kaphan, Paul Leach, Koen Holtman, David
Morris, and Larry Masinter.
Most of the specification of ranges is based on work
originally done by Ari Luotonen and John Franks, with
additional input from Steve Zilles.
Thanks to the "cave men" of Palo Alto. You know who you are.
Jim Gettys (the current editor of this document) wishes
particularly to thank Roy Fielding, the previous editor of
this document, along with John Klensin, Jeff Mogul, Paul
Leach, Dave Kristol, Koen Holtman, John Franks, Alex
Hopmann, and Larry Masinter for their help.
17 References
[1] Alvestrand, H., "Tags for the identification of languages."
RFC 1766, UNINETT, March 1995.
[2] Anklesaria, F., McCahill, M., Lindner, P., Johnson,
D., Torrey, D., and B. Alberti. "The Internet Gopher Protocol (a
distributed document search and retrieval protocol)", RFC 1436,
University of Minnesota, March 1993.
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INTERNET-DRAFT HTTP/1.1 Wednesday, July 30, 1997
[3] Berners-Lee, T., "Universal Resource Identifiers in WWW:
A Unifying Syntax for the Expression of Names and Addresses of
Objects on the Network as used in the World-Wide Web."
RFC 1630, CERN, June 1994.
[4] Berners-Lee, T., Masinter, L., and M. McCahill. "Uniform
Resource Locators (URL)." RFC 1738, CERN, Xerox PARC,
University of Minnesota, December 1994.
[5] Berners-Lee, T. and D. Connolly. "HyperText Markup
Language Specification - 2.0." RFC 1866, MIT/LCS, November 1995.
[6] Berners-Lee, T., Fielding, R. and H. Frystyk.
"Hypertext Transfer Protocol -- HTTP/1.0." RFC 1945,
MIT/LCS, UC Irvine, May 1996.
[7] Freed, N., and N. Borenstein. "Multipurpose Internet
Mail Extensions (MIME) Part One: Format of Internet
Message Bodies." RFC 2045, Innosoft, First Virtual,
November 1996.
[8] Braden, R., "Requirements for Internet hosts - application
and support." STD 3, RFC 1123, IETF, October 1989.
[9] D. H. Crocker, "Standard for the Format of ARPA Internet
Text Messages." STD 11, RFC 822, UDEL, August 1982.
[10]Davis, F., Kahle, B., Morris, H., Salem, J., Shen, T.,
Wang, R., Sui, J., and M. Grinbaum, "WAIS Interface
Protocol Prototype Functional Specification." (v1.5),
Thinking Machines Corporation, April 1990.
[11] Fielding, R., "Relative Uniform Resource Locators."
RFC 1808, UC Irvine, June 1995.
[12] Horton, M., and R. Adams. "Standard for interchange
of USENET messages." RFC 1036 (Obsoletes RFC 850), AT&T
Bell Laboratories, Center for Seismic Studies, December 1987.
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INTERNET-DRAFT HTTP/1.1 Wednesday, July 30, 1997
[13] Kantor, B. and P. Lapsley. "Network News Transfer Protocol
A Proposed Standard for the Stream-Based Transmission of News."
RFC 977, UC San Diego, UC Berkeley, February 1986.
[14] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text",
RFC 2047, University of Tennessee, November 1996.
[15] Nebel, E., and L. Masinter.
"Form-based File Upload in HTML." RFC 1867, Xerox Corporation,
November 1995.
[16] Postel, J., "Simple Mail Transfer Protocol." STD 10,
RFC 821, USC/ISI, August 1982.
[17] Postel, J., "Media Type Registration Procedure." RFC
2048, USC/ISI, November 1996.
[18] Postel, J. and J. Reynolds.
"File Transfer Protocol (FTP)." STD 9, RFC 959, USC/ISI,
October 1985.
[19] Reynolds, J. and J. Postel. "Assigned Numbers." STD
2, RFC 1700, USC/ISI, October 1994.
[20] Sollins, K. and L. Masinter. "Functional Requirements
for Uniform Resource Names." RFC 1737, MIT/LCS, Xerox
Corporation, December 1994.
[21] US-ASCII. Coded Character Set - 7-Bit American
Standard Code for Information Interchange. Standard ANSI
X3.4-1986, ANSI, 1986.
[22] ISO-8859. International Standard -- Information
Processing -- 8-bit Single-Byte Coded Graphic Character Sets --
Part 1: Latin alphabet No. 1, ISO 8859-1:1987.
Part 2: Latin alphabet No. 2, ISO 8859-2, 1987.
Part 3: Latin alphabet No. 3, ISO 8859-3, 1988.
Part 4: Latin alphabet No. 4, ISO 8859-4, 1988.
Part 5: Latin/Cyrillic alphabet, ISO 8859-5, 1988.
Part 6: Latin/Arabic alphabet, ISO 8859-6, 1987.
Part 7: Latin/Greek alphabet, ISO 8859-7, 1987.
Fielding, et al [Page 168]
INTERNET-DRAFT HTTP/1.1 Wednesday, July 30, 1997
Part 8: Latin/Hebrew alphabet, ISO 8859-8, 1988.
Part 9: Latin alphabet No. 5, ISO 8859-9, 1990.
[23] Meyers, J., and M. Rose. "The Content-MD5 Header Field."
RFC 1864, Carnegie Mellon, Dover Beach Consulting, October, 1995.
[24] Carpenter, B. and Y. Rekhter. "Renumbering Needs Work."
RFC 1900, IAB, February 1996.
[25] Deutsch, P.,
"GZIP file format specification version 4.3." RFC 1952,
Aladdin Enterprises, May, 1996.
[26] Venkata N. Padmanabhan, and Jeffrey C. Mogul.
"Improving HTTP Latency", Computer Networks and ISDN
Systems, v. 28, pp. 25-35, Dec. 1995. Slightly revised
version of paper in Proc. 2nd International WWW
Conference '94: Mosaic and the Web, Oct. 1994, which is
available at http://www.ncsa.uiuc.edu/SDG/IT94/Proceedings
/DDay/mogul/HTTPLatency.html
[27] Joe Touch, John Heidemann, and Katia Obraczka.
"Analysis of HTTP Performance", <URL:
http://www.isi.edu/lsam/publications/http-perf/index.html>,
USC/Information Sciences Institute, June 1996.
[28] Mills, D., "Network Time Protocol, Version 3."
Specification, Implementation and Analysis RFC 1305,
University of Delaware, March, 1992.
[29] Deutsch, P., "DEFLATE Compressed Data Format Specification
version 1.3." RFC 1951, Aladdin Enterprises, May 1996.
[30] S. Spero, "Analysis of HTTP Performance Problems"
<URL:http://sunsite.unc.edu/mdma-release/http-prob.html>.
[31] Deutsch, P. and J-L. Gailly. "ZLIB Compressed Data
Format Specification version 3.3."
RFC 1950, Aladdin Enterprises, Info-ZIP, May 1996.
[32] Franks, J., Hallam-Baker, P., Hostetler, J., Leach, P.,
Luotonen, A., Sink, E., and L. Stewart. "An Extension to
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INTERNET-DRAFT HTTP/1.1 Wednesday, July 30, 1997
HTTP : Digest Access Authentication," RFC 2069, January
1997.
[33] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Berners-Lee, T., "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2068, UC Irvine, Digital Equipment
Corporation, M.I.T., January, 1997.
[34] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, Harvard University, March
1997.
[35] Troost, R., and Dorner, S., "Communicating
Presentation Information in Internet Messages: The
Content-Disposition Header," RFC 1806, New Century
Systems, QUALCOMM, Inc., June 1995.
[36] Mogul, J.C., Fielding, R., Gettys, J, Frystyk, H., "Use
and Interpretation of HTTP Version Numbers", RFC 2145,
Digital Equipment Corporation, U.C. Irvine, M.I.T., May
1997.
[37] Palme, J, "Common Internet Message Headers," RFC
2076, Stockholm University, KTH, February, 1997.
[38] Yergeau, F., "UTF-8, a transformation format of
Unicode and ISO 10646," RFC 2044, Alis Technologies,
October, 1996.
[39] Nielsen, H.F., Gettys, J., Baird-Smith, A.,
Prud'hommeaux, E., Lie, H., and C. Lilley. "Network
Performance Effects of HTTP/1.1, CSS1, and PNG,"
Proceedings of ACM SIGCOMM '97, Cannes France, September
1997.
[40] Freed, N., and N. Borenstein. "Multipurpose Internet
Mail Extensions (MIME) Part Two: Media Types." RFC 2046,
Innosoft, First Virtual, November 1996.
18 Authors' Addresses
Roy T. Fielding
Department of Information and Computer Science
University of California
Irvine, CA 92717-3425, USA
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Fax: +1 (714) 824-4056
Email: fielding@ics.uci.edu
Jim Gettys
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, USA
Fax: +1 (617) 258 8682
Email: jg@w3.org
Jeffrey C. Mogul
Western Research Laboratory
Digital Equipment Corporation
250 University Avenue
Palo Alto, California, 94305, USA
Email: mogul@wrl.dec.com
Henrik Frystyk Nielsen
W3 Consortium
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, USA
Fax: +1 (617) 258 8682
Email: frystyk@w3.org
Tim Berners-Lee
Director, W3 Consortium
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, USA
Fax: +1 (617) 258 8682
Email: timbl@w3.org
19 Appendices
19.1 Internet Media Type message/http
In addition to defining the HTTP/1.1 protocol, this document
serves as the specification for the Internet media type
"message/http". The following is to be registered with IANA [17].
Media Type name: message
Media subtype name: http
Required parameters: none
Optional parameters: version, msgtype
version: The HTTP-Version number of the enclosed message
(e.g., "1.1"). If not present, the version can be
determined from the first line of the body.
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msgtype: The message type -- "request" or "response". If not
present, the type can be determined from the first
line of the body.
Encoding considerations: only "7bit", "8bit", or "binary" are
permitted
Security considerations: none
19.2 Internet Media Type multipart/byteranges
When an HTTP message includes the content of multiple ranges
(for example, a response to a request for multiple non-
overlapping ranges), these are transmitted as a multipart
MIME message. The multipart media type for this purpose is
called "multipart/byteranges".
The multipart/byteranges media type includes two or more
parts, each with its own Content-Type and Content-Range
fields. The parts are separated using a MIME boundary
parameter.
Media Type name: multipart
Media subtype name: byteranges
Required parameters: boundary
Optional parameters: none
Encoding considerations: only "7bit", "8bit", or
"binary" are permitted
Security considerations: none
For example:
HTTP/1.1 206 Partial content
Date: Wed, 15 Nov 1995 06:25:24 GMT
Last-modified: Wed, 15 Nov 1995 04:58:08 GMT
Content-type: multipart/byteranges;
boundary=THIS_STRING_SEPARATES
--THIS_STRING_SEPARATES
Content-type: application/pdf
Content-range: bytes 500-999/8000
...the first range...
--THIS_STRING_SEPARATES
Content-type: application/pdf
Content-range: bytes 7000-7999/8000
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...the second range
--THIS_STRING_SEPARATES--
19.3 Tolerant Applications
Although this document specifies the requirements for the
generation of HTTP/1.1 messages, not all applications will
be correct in their implementation. We therefore recommend
that operational applications be tolerant of deviations
whenever those deviations can be interpreted unambiguously.
Clients SHOULD be tolerant in parsing the Status-Line and
servers tolerant when parsing the Request-Line. In
particular, they SHOULD accept any amount of SP or HT
characters between fields, even though only a single SP is
required.
The line terminator for message-header fields is the
sequence CRLF. However, we recommend that applications, when
parsing such headers, recognize a single LF as a line
terminator and ignore the leading CR.
The character set of an entity-body should be labeled as the
lowest common denominator of the character codes used within
that body, with the exception that no label is preferred
over the labels US-ASCII or ISO-8859-1.
Additional rules for requirements on parsing and encoding of
dates and other potential problems with date encodings
include:
. HTTP/1.1 clients and caches should assume that an RFC-
850 date which appears to be more than 50 years in the
future is in fact in the past (this helps solve the
"year 2000" problem).
. An HTTP/1.1 implementation may internally represent a
parsed Expires date as earlier than the proper value,
but MUST NOT internally represent a parsed Expires date
as later than the proper value.
. All expiration-related calculations must be done in
GMT. The local time zone MUST NOT influence the
calculation or comparison of an age or expiration time.
. If an HTTP header incorrectly carries a date value with
a time zone other than GMT, it must be converted into
GMT using the most conservative possible conversion.
19.4 Differences Between HTTP Entities and RFC 2045 Entities
HTTP/1.1 uses many of the constructs defined for Internet
Mail (RFC 822 [9]) and the Multipurpose Internet Mail
Extensions (MIME [7]) to allow entities to be transmitted in
an open variety of representations and with extensible
mechanisms. However, RFC 2045 discusses mail, and HTTP has a
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few features that are different from those described in RFC
2045. These differences were carefully chosen to optimize
performance over binary connections, to allow greater
freedom in the use of new media types, to make date
comparisons easier, and to acknowledge the practice of some
early HTTP servers and clients.
This appendix describes specific areas where HTTP differs
from RFC 2045. Proxies and gateways to strict MIME
environments SHOULD be aware of these differences and
provide the appropriate conversions where necessary. Proxies
and gateways from MIME environments to HTTP also need to be
aware of the differences because some conversions may be
required.
19.4.1 Conversion to Canonical Form
RFC 2045 requires that an Internet mail entity be converted
to canonical form prior to being transferred, as described
in Appendix G of RFC 2045 [7]. Section 3.7.1 of this
document describes the forms allowed for subtypes of the
"text" media type when transmitted over HTTP. RFC 2045
requires that content with a type of "text" represent line
breaks as CRLF and forbids the use of CR or LF outside of
line break sequences. HTTP allows CRLF, bare CR, and bare LF
to indicate a line break within text content when a message
is transmitted over HTTP.
Where it is possible, a proxy or gateway from HTTP to a
strict RFC 2045 environment SHOULD translate all line breaks
within the text media types described in section 3.7.1 of
this document to the RFC 2045 canonical form of CRLF. Note,
however, that this may be complicated by the presence of a
Content-Encoding and by the fact that HTTP allows the use of
some character sets which do not use octets 13 and 10 to
represent CR and LF, as is the case for some multi-byte
character sets.
19.4.2 Conversion of Date Formats
HTTP/1.1 uses a restricted set of date formats (section
3.3.1) to simplify the process of date comparison. Proxies
and gateways from other protocols SHOULD ensure that any
Date header field present in a message conforms to one of
the HTTP/1.1 formats and rewrite the date if necessary.
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19.4.3 Introduction of Content-Encoding
RFC 2045 does not include any concept equivalent to
HTTP/1.1's Content-Encoding header field. Since this acts as
a modifier on the media type, proxies and gateways from HTTP
to MIME-compliant protocols MUST either change the value of
the Content-Type header field or decode the entity-body
before forwarding the message. (Some experimental
applications of Content-Type for Internet mail have used a
media-type parameter of ";conversions=<content-coding>" to
perform an equivalent function as Content-Encoding. However,
this parameter is not part of RFC 2045.)
19.4.4 No Content-Transfer-Encoding
HTTP does not use the Content-Transfer-Encoding (CTE) field
of RFC 2045. Proxies and gateways from MIME-compliant
protocols to HTTP MUST remove any non-identity CTE ("quoted-
printable" or "base64") encoding prior to delivering the
response message to an HTTP client.
Proxies and gateways from HTTP to MIME-compliant protocols
are responsible for ensuring that the message is in the
correct format and encoding for safe transport on that
protocol, where "safe transport" is defined by the
limitations of the protocol being used. Such a proxy or
gateway SHOULD label the data with an appropriate Content-
Transfer-Encoding if doing so will improve the likelihood of
safe transport over the destination protocol.
19.4.5 HTTP Header Fields in Multipart Body-Parts
In RFC 2045, most header fields in multipart body-parts are
generally ignored unless the field name begins with
"Content-". In HTTP/1.1, multipart body-parts may contain
any HTTP header fields which are significant to the meaning
of that part.
19.4.6 Introduction of Transfer-Encoding
HTTP/1.1 introduces the Transfer-Encoding header field
(section 14.40). Proxies/gateways MUST remove any transfer
coding prior to forwarding a message via a MIME-compliant
protocol.
A process for decoding the "chunked" transfer coding
(section 3.6) can be represented in pseudo-code as:
length := 0
read chunk-size, chunk-extension (if any) and CRLF
while (chunk-size > 0) {
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read chunk-data and CRLF
append chunk-data to entity-body
length := length + chunk-size
read chunk-size and CRLF
}
read entity-header
while (entity-header not empty) {
append entity-header to existing header fields
read entity-header
}
Content-Length := length
Remove "chunked" from Transfer-Encoding
19.4.7 MIME-Version
HTTP is not a MIME-compliant protocol (see appendix 19.4).
However, HTTP/1.1 messages may include a single MIME-Version
general-header field to indicate what version of the MIME
protocol was used to construct the message. Use of the MIME-
Version header field indicates that the message is in full
compliance with the MIME protocol (as defined in RFC
2045[7]). Proxies/gateways are responsible for ensuring full
compliance (where possible) when exporting HTTP messages to
strict MIME environments.
MIME-Version = "MIME-Version" ":" 1*DIGIT "."
1*DIGIT
MIME version "1.0" is the default for use in HTTP/1.1.
However, HTTP/1.1 message parsing and semantics are defined
by this document and not the MIME specification.
19.5 Changes from HTTP/1.0
This section summarizes major differences between versions
HTTP/1.0 and HTTP/1.1.
19.5.1 Changes to Simplify Multi-homed Web Servers and
Conserve IP Addresses
The requirements that clients and servers support the Host
request-header, report an error if the Host request-header
(section 14.23) is missing from an HTTP/1.1 request, and
accept absolute URIs (section 5.1.2) are among the most
important changes defined by this specification.
Older HTTP/1.0 clients assumed a one-to-one relationship of
IP addresses and servers; there was no other established
mechanism for distinguishing the intended server of a
request than the IP address to which that request was
directed. The changes outlined above will allow the
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Internet, once older HTTP clients are no longer common, to
support multiple Web sites from a single IP address, greatly
simplifying large operational Web servers, where allocation
of many IP addresses to a single host has created serious
problems. The Internet will also be able to recover the IP
addresses that have been allocated for the sole purpose of
allowing special-purpose domain names to be used in root-
level HTTP URLs. Given the rate of growth of the Web, and
the number of servers already deployed, it is extremely
important that all implementations of HTTP (including
updates to existing HTTP/1.0 applications) correctly
implement these requirements:
. Both clients and servers MUST support the Host request-
header.
. Host request-headers are required in HTTP/1.1 requests.
. Servers MUST report a 400 (Bad Request) error if an
HTTP/1.1 request does not include a Host request-
header.
. Servers MUST accept absolute URIs.
19.6 Additional Features
RFC 1945 and RFC 2068 document protocol elements used by
some existing HTTP implementations, but not consistently and
correctly across most HTTP/1.1 applications. Implementers
should be aware of these features, but cannot rely upon
their presence in, or interoperability with, other HTTP/1.1
applications. Some of these describe proposed experimental
features, and some describe features that experimental
deployment found lacking that are now addressed in the base
HTTP/1.1 specification.
A number of other headers, such as Content-Disposition and
Title, from SMTP and MIME are also often implemented (see
RFC 2076 [37]).
19.6.1 Content-Disposition
The Content-Disposition response-header field has been
proposed as a means for the origin server to suggest a
default filename if the user requests that the content is
saved to a file. This usage is derived from the definition
of Content-Disposition in RFC 1806 [35].
content-disposition = "Content-Disposition" ":"
disposition-type *( ";" disposition-parm )
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disposition-type = "attachment" | disp-extension-token
disposition-parm = filename-parm | disp-extension-parm
filename-parm = "filename" "=" quoted-string
disp-extension-token = token
disp-extension-parm = token "=" ( token | quoted-string )
An example is
Content-Disposition: attachment; filename="fname.ext"
The receiving user agent should not respect any directory
path information that may seem to be present in the filename
parameter. The filename should be treated as a terminal
component only.
If this header is used in a response with the
application/octet-stream content-type, the implied
suggestion is that the user agent should not display the
response, but directly enter a `save response as..' dialog.
See section 15.10 for Content-Disposition security issues.
19.7 Compatibility with Previous Versions
It is beyond the scope of a protocol specification to
mandate compliance with previous versions. HTTP/1.1 was
deliberately designed, however, to make supporting previous
versions easy. It is worth noting that at the time of
composing this specification, we would expect commercial
HTTP/1.1 servers to:
. recognize the format of the Request-Line for HTTP/0.9,
1.0, and 1.1 requests;
. understand any valid request in the format of HTTP/0.9,
1.0, or 1.1;
. respond appropriately with a message in the same major
version used by the client.
And we would expect HTTP/1.1 clients to:
. recognize the format of the Status-Line for HTTP/1.0
and 1.1 responses;
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. understand any valid response in the format of
HTTP/0.9, 1.0, or 1.1.
For most implementations of HTTP/1.0, each connection is
established by the client prior to the request and closed by
the server after sending the response. A few implementations
implement the Keep-Alive version of persistent connections
described in section 19.7.1.1.
19.7.1 Compatibility with HTTP/1.0 Persistent Connections
Some clients and servers may wish to be compatible with some
previous implementations of persistent connections in
HTTP/1.0 clients and servers. Persistent connections in
HTTP/1.0 must be explicitly negotiated as they are not the
default behavior. HTTP/1.0 experimental implementations of
persistent connections are faulty, and the new facilities in
HTTP/1.1 are designed to rectify these problems. The problem
was that some existing 1.0 clients may be sending Keep-Alive
to a proxy server that doesn't understand Connection, which
would then erroneously forward it to the next inbound
server, which would establish the Keep-Alive connection and
result in a hung HTTP/1.0 proxy waiting for the close on the
response. The result is that HTTP/1.0 clients must be
prevented from using Keep-Alive when talking to proxies.
However, talking to proxies is the most important use of
persistent connections, so that prohibition is clearly
unacceptable. Therefore, we need some other mechanism for
indicating a persistent connection is desired, which is safe
to use even when talking to an old proxy that ignores
Connection. Persistent connections are the default for
HTTP/1.1 messages; we introduce a new keyword (Connection:
close) for declaring non-persistence.
The following describes the original HTTP/1.0 form of
persistent connections.
When it connects to an origin server, an HTTP client MAY
send the Keep-Alive connection-token:
Connection: Keep-Alive
An HTTP/1.0 server would then respond with the Keep-Alive
connection token and the client may proceed with an HTTP/1.0
(or Keep-Alive) persistent connection.
An HTTP/1.1 server may also establish persistent connections
with HTTP/1.0 clients upon receipt of a Keep-Alive
connection token. However, a persistent connection with an
HTTP/1.0 client cannot make use of the chunked transfer-
coding, and therefore MUST use a Content-Length for marking
the ending boundary of each message.
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A client MUST NOT send the Keep-Alive connection token to a
proxy server as HTTP/1.0 proxy servers do not obey the rules
of HTTP/1.1 for parsing the Connection header field.
19.7.1.1 The Keep-Alive Header
When the Keep-Alive connection-token has been transmitted
with a request or a response, a Keep-Alive header field MAY
also be included. The Keep-Alive header field takes the
following form:
Keep-Alive-header = "Keep-Alive" ":" 0# keepalive-param
keepalive-param = param-name "=" value
The Keep-Alive header itself is optional, and is used only
if a parameter is being sent. HTTP/1.1 does not define any
parameters.
If the Keep-Alive header is sent, the corresponding
connection token MUST be transmitted. The Keep-Alive header
MUST be ignored if received without the connection token.
19.8 Backward Compatibility
We (the editorial group) have discussed moving many of the
implementation notes having to do with backward
compatibility (often bug work-arounds) out of the mainline
specification into an appendix. This is mostly a
placeholder in case this work gets done. _ JG.
19.8.1 CRLF's in Quoted Strings
CRLF in a quoted string is legal, but only in a strange way:
as part of a header continuation, as in "part of
a
quoted-string". This is strange, and CRLF's should be
allowed in general, but backward compatibility constraints
mean that they are not allowed in general. .
19.8.2 Missing Content Type
Some HTTP/1.0 software has interpreted a Content-Type header
without charset parameter incorrectly to mean "recipient
should guess." Senders wishing to defeat this behavior MAY
include a charset parameter even when the charset is ISO-
8859-1 and SHOULD do so when it is known that it will not
confuse the recipient.
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Unfortunately, some older HTTP/1.0 clients did not deal
properly with an explicit charset parameter. HTTP/1.1
recipients MUST respect the charset label provided by the
sender; and those user agents that have a provision to
"guess" a charset MUST use the charset from the content-type
field if they support that charset, rather than the
recipient's preference, when initially displaying a
document. See section 3.7.1.
19.8.3 Multipart/x-byteranges
A number of browsers and servers were coded to an early
draft of the byteranges specification to use a media type of
multipart/x-byteranges, which is almost, but not quite
compatible with the version documented in HTTP/1.1.
19.9 Requirements Summary
This section summarizes the requirements of the HTTP/1.1
specification. (Requirements are those aspects of the
protocol defined with the words " "
MUST , "SHOULD", or "MAY.")
This list is not a normative part of the HTTP/1.1
specification, and if there is any conflict between this
listing and another part of the specification, the
statements elsewhere in the specification take absolute
priority.
Requirements are listed in the order that they appear in the
the specification. For each requirement, the list includes
. a very brief summary of the feature; this is meant for
identification purposes only, and must not be used as a
specification of the feature.
. the section of the document in which the feature is
specified
. A column for each of three categories of implementation
(Server, Proxy, and Client), showing whether the listed
feature is a MUST, SHOULD, or MAY requirement. ("MUST
NOT" is abbreviated as "MST NT"; "SHOULD NOT" is
abbreviated as "SH NOT".)
. A column for additional footnotes Note that some
aspects of the protocol may be specified in multiple
sections in separated part of the document.
Editor's Note: this draft of the HTTP/1.1 specification does
not include a requirements summary. A summary will be
provided in a subsequent draft. The format of the list may
change, based on experience with the creation of the list.
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What follows is meant only as an example of the final
listing.
Feature summary Section Server Proxy Client Note
Send From header in 14.22 Na Na MAY 1
requests
From header contains user's 14.22 Na Na SHOULD
email address
From not meant for 14.22 SH SH Na
authentication NOT NOT
User approves sending of 14.22 Na Na SHOULD
From header
Send Host header in 14.23 Na Na MUST 2
requests
Add Host hdr to forwarded 14.23 Na MUST na
HTTP/1.1 req if missing
Require Host header in 14.23 MUST MUST na
HTTP/1.1 requests
Footnotes:
(1) From header SHOULD be sent by robots
(2) Not required on non-Internet networks
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