HTTP Working Group R. Fielding, UC Irvine
INTERNET-DRAFT H. Frystyk, MIT/LCS
<draft-ietf-http-v11-spec-02.txt> T. Berners-Lee, MIT/LCS
J. Gettys, DEC
Jeffrey C. Mogul, DEC
Expires September 23, 1996 April 23, 1996
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 six months
and may be updated, replaced, or made obsolete by other documents at any
time. It is inappropriate to use Internet-Drafts as reference material
or to cite them other than as _work in progress_.
To learn the current status of any Internet-Draft, please check the
_1id-abstracts.txt_ listing contained in the Internet-Drafts Shadow
Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
ftp.isi.edu (US West 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.
NOTE: This specification is for discussion purposes only. It is
not claimed to represent the consensus of the HTTP working
group, and contains a number of proposals that either have not
been discussed or are controversial. The working group is
discussing significant changes in many areas, including -
support for caching, persistent connections, range retrieval,
content negotiation, MIME compatibility, authentication, timing
of the PUT operation.
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 (commands). 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_.
Note to Readers of This Document
This document is still organized to minimize changes from the previous
draft, to ease reviewers work in finding new material (and because the
editor has not had time to reorganize it).. However, the current
organization is now quite poor for new readers of this document. We
recommend that new readers of this document not read it in the current
order of presentation, but may want to skip ahead after reading sections
1-9 and read sections 11, 12 13 and 14 before reading section 10 which
defines the header field definitions. Section 10 itself is now also not
in alphabetical order, again, to avoid renumbering sections to be able
to easily compare between drafts.
If you are reading the version of this document showing revision markup,
note that we've tried to preserve significant changes from the previous
version, though a few changes may have slipped through unmarked. We make
no guarantees that all changes have revision marks, though we've tried
to preserve them as an aid to those who wish to check a specific change
has been reflected in this draft.
Note that some sections are still marked as SLUSHY and a few are marked
FLUID; these are still undergoing drafting.
Note that text in bold in the text are as yet incompletely resolved
issues. Opinions are solicited_
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Table of Contents
HYPERTEXT TRANSFER PROTOCOL -- HTTP/1.1................................1
Status of this Memo....................................................1
Abstract...............................................................1
Note to Readers of This Document.......................................2
Table of Contents......................................................3
1. Introduction........................................................9
1.1 Purpose ..........................................................9
1.2 Requirements .....................................................9
1.3 Terminology .....................................................10
1.4 Overall Operation ...............................................12
1.4 HTTP and MIME ...................................................14
2. Notational Conventions and Generic Grammar.........................14
2.1 Augmented BNF ...................................................14
2.2 Basic Rules .....................................................16
3. Protocol Parameters................................................18
3.1 HTTP Version ....................................................18
3.2 Uniform Resource Identifiers ....................................19
3.2.1 General Syntax ...............................................19
3.2.2 http URL .....................................................21
3.3 Date/Time Formats ...............................................22
3.3.1 Full Date ....................................................22
3.3.2 Delta Seconds ................................................24
3.4 Character Sets ..................................................24
3.5 Content Codings .................................................25
3.6 Transfer Codings ................................................26
3.7 Media Types .....................................................27
3.7.1 Canonicalization and Text Defaults ...........................28
3.7.2 Multipart Types ..............................................29
3.8 Product Tokens ..................................................29
3.9 Quality Values ..................................................30
3.10 Language Tags ..................................................30
3.12 Full Date Values ...............................................31
3.13 Opaque Validators ..............................................31
3.14 Variant IDs ....................................................32
3.15 Validator Sets .................................................32
3.16 Variant Sets ...................................................32
3.17 HTTP Protocol Parameters Related to Ranges .....................32
3.17.1SLUSHY Range Units ...........................................32
3.17.2 SLUSHY Byte Ranges ..........................................33
3.17.3 SLUSHY: Content Ranges ......................................34
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4. HTTP Message.......................................................35
4.1 Message Types ...................................................35
4.2 Message Headers .................................................36
4.3 General Header Fields ...........................................37
5. Request............................................................38
5.1 Request-Line ....................................................38
5.1.1 Method .......................................................38
5.1.2 Request-URI ..................................................39
5.2 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 ..........................................46
7. Entity.............................................................46
7.1 Entity Header Fields ............................................46
7.2 Entity Body .....................................................47
7.2.1 Type .........................................................48
7.2.2 Length .......................................................48
8. Method Definitions.................................................49
8.1 OPTIONS .........................................................49
8.2 GET .............................................................50
8.3 HEAD ............................................................50
8.4 POST ............................................................51
8.4.1 SLUSHY: Entity Transmission Requirements .....................52
8.5 PUT .............................................................53
8.9 DELETE ..........................................................54
8.12 TRACE ..........................................................54
9. Status Code Definitions............................................55
9.1 Informational 1xx ...............................................55
9.2 Successful 2xx ..................................................56
9.3 Redirection 3xx .................................................58
9.4 Client Error 4xx ................................................60
9.5 Server Error 5xx ................................................63
10. Header Field Definitions..........................................65
10.1 Accept .........................................................65
10.2 Accept-Charset .................................................67
10.3 Accept-Encoding ................................................67
10.4 Accept-Language ................................................68
10.5 Allow ..........................................................69
10.6 Authorization ..................................................70
10.7 Cache-Control ..................................................70
Check: is this true? ...............................................72
10.7.1 SLUSHY: Restrictions on What is Cachable ....................72
10.7.2 Restrictions On What May be Stored by a Cache ...............73
10.7.3 Modifications of the Basic Expiration Mechanism .............73
10.7.4 SLUSHY: Controls over cache revalidation and reload .........74
10.7.5 FLUID: Restrictions on use count and demographic reporting ..76
10.7.6 Miscellaneous restrictions ..................................77
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10.8 Connection .....................................................77
10.8.1 Persist ......................................................78
10.9 Content-Base ...................................................78
10.10 Content-Encoding ..............................................78
10.11 Content-Language ..............................................79
10.12 Content-Length ................................................80
10.13 Content-MD5 ...................................................80
10.14 SLUSHY Content-Range ..........................................82
10.14.1 MIME multipart/byteranges content-type .....................82
10.14.2 Additional rules for Content-Range .........................83
10.15 Content-Type ..................................................83
10.16 Content-Location ..............................................84
10.17 Date ..........................................................84
10.19 SLUSHY Expires ................................................85
10.20 Via ...........................................................86
10.21 From ..........................................................88
10.22 Host ..........................................................88
10.23 If-Modified-Since .............................................89
10.25 Last-Modified .................................................90
10.27 Location ......................................................91
10.29 Pragma ........................................................91
10.30 Proxy-Authenticate ............................................92
10.31 Proxy-Authorization ...........................................92
10.32 Public ........................................................93
10.33 Range .........................................................93
10.34 Referer .......................................................94
10.36 Retry-After ...................................................95
10.37 Server ........................................................95
10.38 Title .........................................................95
10.39 Transfer Encoding .............................................96
10.41 Upgrade .......................................................96
10.43 User-Agent ....................................................97
10.44 WWW-Authenticate ..............................................98
10.45 Max-Forwards ..................................................98
10.46 Age ...........................................................99
10.47 CVal ..........................................................99
10.48 If-Invalid ....................................................99
10.49 If-Valid .....................................................100
10.50 If-Unmodified-Since ..........................................101
10.51 Warning ......................................................102
10.52 Vary .........................................................103
10.53 Alternates ...................................................106
10.54 SLUSHY: Accept-Ranges ........................................107
10.55 SLUSHY: Range-If .............................................107
11. Access Authentication............................................108
11.1 Basic Authentication Scheme ...................................109
11.2 Digest Authentication Scheme ..................................110
12. Content Negotiation..............................................111
12.1 Negotiation facilities defined in this specification .........111
13 Caching in HTTP...................................................112
13.1 Semantic Transparency .........................................112
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13.2 Expiration Model ..............................................113
13.2.1 Server-Specified Expiration ................................113
13.2.2 Limitations on the Effect of Expiration Times ..............114
13.2.3 Heuristic Expiration .......................................114
13.2.4 Client-controlled Behavior .................................114
13.2.5 Exceptions to the Rules and Warnings .......................115
13.2.6 Age Calculations ...........................................115
13.2.7 Expiration Calculations ....................................117
13.2.8 UT Mandatory ................................................118
13.3 Validation Model ..............................................118
13.3.1 Last-modified Dates ........................................119
13.3.2 Opaque Validators ..........................................119
13.3.3 Weak and Strong Validators .................................120
13.3.4 Rules for When to Use Opaque Validators and Last-modified
Dates .............................................................122
13.3.5 SLUSHY: Non-validating conditionals ........................123
13.3.6 FLUID: Other Issues ........................................123
13.4 Cache-control Mechanisms ......................................123
13.5 Warnings ......................................................124
13.6 Explicit Indications Regarding User-specified Overrides .......124
13.7 Constructing Responses From Caches ............................125
13.7.1 End-to-end and Hop-by-hop Headers ..........................125
13.7.2 Non-modifiable Headers .....................................126
13.7.3 Combining Headers ..........................................126
13.7.4 Combining Byte Ranges ......................................126
13.7.5 SLUSHY: Scope of Expiration ................................127
13.8 Caching and Content Negotiation ...............................127
13.8.1 Use of the Vary header .....................................127
13.8.2 SLUSHY: Use of the Alternates header .......................128
13.8.3 Use of Variant-IDs .........................................128
13.8.4 Use of Selecting Opaque Validators .........................129
13.10 Shared and Non-Shared Caches .................................130
13.11 SLUSHY: Miscellaneous Considerations .........................130
13.11.1 Detecting Firsthand Responses .............................130
13.11.2 Disambiguating Expiration values ..........................130
13.11.3 Disambiguating Multiple Responses .........................131
13.12 SLUSHY: Cache Keys ...........................................131
13.12.1 Non-varying Resources .....................................132
13.12.2 SLUSHY: Varying Resources .................................132
13.12.3 SLUSHY: Key-Matching Procedure ............................133
13.12.4 Canonicalization of URIs ..................................134
13.13 FLUID: Cache-Related Problems Not Addressed in HTTP/1.1 ......134
13.14 Cache Operation When Receiving Errors or Incomplete Responses 134
13.14.1 Caching and Status Codes ..................................135
13.14.2 Handling of Retry-After ...................................135
13.15 FLUID: Compatibility With Earlier Versions of HTTP ...........135
13.16 SLUSHY: Side Effects of GET and HEAD .........................135
13.17 SLUSHY: Invalidation After Updates or Deletions ..............136
13.18 Write-Through Mandatory ......................................136
13.19 Interoperability of Varying Resources with HTTP/1.0 Proxy
Caches .............................................................136
13.20 Cache Replacement for Varying Resources ......................137
13.22 FLUID: Network Partitions ....................................138
13.23 FLUID: Caching of Negative Responses .........................138
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13.24 History Lists ................................................138
14 Persistent Connections............................................138
14.1 Purpose .......................................................138
14.2 Overall Operation .............................................139
14.2.3 Negotiation ................................................139
14.2.4 Pipe-lining ................................................139
14.2.5 Delimiting Entity-Bodies ...................................139
14.3 Proxy Servers .................................................140
14.4 Interaction with Security Protocols ...........................140
14.5 Practical Considerations ......................................140
15. Security Considerations..........................................141
15.1 Authentication of Clients .....................................141
15.2 Safe Methods ..................................................142
15.3 Abuse of Server Log Information ...............................143
15.4 Transfer of Sensitive Information .............................143
15.5 Attacks Based On File and Path Names ..........................143
15.6 Personal Information ..........................................144
15.7 Privacy issues connected to Accept headers ....................144
15.8 DNS Spoofing ..................................................145
15.9 SLUSHY: Location Headers and Spoofing .........................145
16. Acknowledgments..................................................145
17. References.......................................................147
18. Authors' Addresses...............................................150
Appendices...........................................................151
A. Internet Media Type message/http..................................151
B. Tolerant Applications.............................................152
C. Differences Between HTTP Bodies and RFC 1521 Internet Message Bodies
.....................................................................152
C.1 Conversion to Canonical Form ...................................153
C.2 Conversion of Date Formats .....................................153
C.3 Introduction of Content-Encoding ...............................153
C.4 No Content-Transfer-Encoding ...................................154
C.5 HTTP Header Fields in Multipart Body-Parts .....................154
C.6 Introduction of Transfer-Encoding ..............................154
C.7 MIME-Version ...................................................155
D. Changes from HTTP/1.0.............................................155
D.1 Changes to Simplify Multi-homed Web Servers and Conserve IP
Addresses ..........................................................155
E. Additional Features...............................................156
E.1 Additional Request Methods .....................................156
E.1.1 PATCH .......................................................156
E.1.2 LINK ........................................................157
E.1.3 UNLINK ......................................................157
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E.2 Additional Header Field Definitions ............................157
E.2.1 Content-Version .............................................157
E.2.2 Derived-From ................................................158
E.2.3 Link ........................................................158
E.2.4 URI .........................................................159
E.2.5 Compatibility with HTTP/1.0 Persistent Connections ..........160
F.1 Compatibility with Previous Versions ...........................160
G. Proxy Cache Implementation Guidelines ...........................161
G.1 Support for Content Negotiation by Proxy Caches ................161
G.2 Propagation of Changes in Opaque Selection ....................163
G.3 SLUSHY: State ..................................................163
G.4 FLUID: Cache Replacement Algorithms ............................163
G.5 FLUID: Bypassing in Caching Hierarchies ........................164
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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 xxxx [6], improved the protocol by allowing messages to be in the
format of MIME-like entities, containing metainformation about the data
transferred and modifiers on the request/response semantics. However,
HTTP/1.0 does not sufficiently take into consideration the effect of
hierarchical proxies and caching, the desire for persistent connections
and virtual hosts, and a number of other details that slipped through
the cracks of existing implementations. 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 is backwards-compatible with HTTP/1.0, but includes more
stringent requirements 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 on which a method is to be
applied. Messages are passed in a format similar to that used by
Internet Mail [9] and 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 protocols, such as SMTP
[16], NNTP [13], FTP [18], Gopher [2], and WAIS [10], allowing basic
hypermedia access to resources available from diverse applications and
simplifying the implementation of user agents.
1.2 Requirements
This specification uses the same words as RFC 1123 [8] for defining the
significance of each particular requirement. These words are:
MUST
This word or the adjective _required_ means that the item is an
absolute requirement of the specification.
SHOULD
This word or the adjective _recommended_ means that there may exist
valid reasons in particular circumstances to ignore this item, but
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the full implications should be understood and the case carefully
weighed before choosing a different course.
MAY
This word or the adjective _optional_ means that this item is truly
optional. One vendor may choose to include the item because a
particular marketplace requires it or because it enhances the
product, for example; another vendor may omit the same item.
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
application 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
(Section 3.2).
entity
A particular representation, rendition, encoding, or presentation
of a resource. Resources not supporting content negotiation are
bound to a single entity. Resources supporting content negotiation
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are bound to a set of one or more entities, whose membership may
vary over time.
entity instance
The definite value of an entity at a given
point in time. The HTTP protocol transfers
entity instances in request or response
messages. An entity instance is transferred as
metainformation in the form of entity headers
and content in the form of an entity body.
client
An application 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.
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, with possible
translation, on to other servers. A proxy MUST interpret and, if
necessary, rewrite a request message before forwarding it. Proxies
are often used as client-side portals through network firewalls and
as helper applications for handling requests via protocols not
implemented by the user agent.
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. Gateways are
often used as server-side portals through network firewalls and as
protocol translators for access to resources stored on non-HTTP
systems.
tunnel
A tunnel is an intermediary program which is acting as a blind
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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. Tunnels are
used when a portal is necessary and the intermediary cannot, or
should not, interpret the relayed communication.
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 while it is acting as a tunnel.
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.
1.4 Overall Operation
The HTTP protocol is based on a request/response paradigm. 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 body content.
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
parts 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
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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
<------------------------------------- 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 MUST 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 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.
On the Internet, HTTP communication generally 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.
However, HTTP/1.1 implementations SHOULD implement persistent
connections (See section 14). Both clients and servers MUST be capable
of handling cases where either party closes the connection prematurely,
due to user action, automated time-out, or program failure. In any case,
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the closing of the connection by either or both parties always
terminates the current request, regardless of its status.
1.4 HTTP and MIME
HTTP/1.1 uses many of the constructs defined for MIME, as defined in RFC
1521 [7]. Appendix C describes the ways in which the context of HTTP
allows for different use of Internet Media Types than is typically found
in Internet mail, and gives the rationale for those differences.
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 ("I") are alternatives, e.g., "yes |
no" will accept yes or no.
(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
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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.
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 delimiters (tspecials), without
changing the interpretation of a field. At least one delimiter
(tspecials) MUST exist between any two tokens, since they would
otherwise be interpreted as a single token. However, applications
SHOULD attempt to follow _common form_ when generating HTTP
constructs, since there exist some implementations that fail to
accept anything beyond the common forms.
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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
[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 octet sequence CR LF as the end-of-line marker for
all protocol elements except the Entity-Body (see Appendix B 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 whitespace,
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 octets from character sets other than US-ASCII only
when encoded according to the rules of RFC 1522 [14].
TEXT = <any OCTET except CTLs,
but including LWS>
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Recipients of header field TEXT containing octets outside the US-ASCII
character set range MAY assume that they represent ISO-8859-1 characters
if there is no other encoding indicated by an RFC 1522 mechanism.
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.
word = token | quoted-string
token = 1*<any CHAR except CTLs or tspecials>
tspecials = "(" | ")" | "<" | ">" | "@"
| "," | ";" | ":" | "\" | <">
| "/" | "[" | "]" | "?" | "="
| "{" | "}" | 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 | 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) <"> )
qdtext = <any CHAR except <"> and CTLs,
but including LWS>
The backslash character (_\_) may be used as a single-character quoting
mechanism only within quoted-string and comment constructs.
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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 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.
The version of an HTTP message is indicated by an HTTP-Version field in
the first line of the message. If the protocol version is not specified,
the recipient MUST assume that the message is in the simple HTTP/0.9
format [6].
HTTP-Version = "HTTP" "/" 1*DIGIT "." 1*DIGIT
Note that the major and minor numbers SHOULD 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 SHOULD be ignored by recipients and never
generated by senders.
Applications sending Full-Request or Full-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.
Proxy and gateway applications MUST be careful in forwarding requests
that are received in a format different than that of the application's
native HTTP version. 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 native 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
application's native format MAY be upgraded before being forwarded; the
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proxy/gateway's response to that request MUST follow the server
requirements listed above.
Note: Converting between versions of HTTP may involve addition
or deletion of headers required or forbidden by the version
involved. It is likely more involved than just changing the
version indicator.
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 network 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.
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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 = "$" | "-" | "_" | "." | "+"
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.
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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 a status code of
if a URI is longer than the server can handle. See section 9.4.
Note: Servers SHOULD be cautious about depending on URI lengths
above 255 bytes, because some older client or proxy 414 Request-URI Too Large
implementations may not properly support these.
All client and proxy implementations MUST be able to handle a URI of
any finite length.
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).
Note: Although the HTTP protocol is independent of the transport
layer protocol, the http URL only identifies resources by their
TCP location, and thus non-TCP resources MUST be identified by
some other URI scheme.
The canonical form for _http_ URLs is obtained by converting any UPALPHA
characters in host to their LOALPHA equivalent (hostnames are case-
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insensitive), eliding the [ ":" port ] if the port is 80, and replacing
an empty abs_path with _/_.
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, made obsolete 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, though they MUST only generate the RFC 1123 format for
representing date/time stamps in HTTP message fields.
Note: Recipients of date values are encouraged to be robust in
accepting date values that may have been generated by non-HTTP
applications, as is sometimes the case when retrieving or
posting messages via proxies/gateways to SMTP or NNTP.
All HTTP date/time stamps MUST be represented in Universal Time (UT),
also known as Greenwich Mean Time (GMT), without exception. This is
indicated in the first two formats by the inclusion of _GMT_ as the
three-letter abbreviation for time zone, and SHOULD be assumed when
reading the asctime format.
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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.
Additional rules for requirements on parsing and representation of dates
and other potential problems with date representations 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.
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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. This format SHOULD only be used to represent short
time periods or periods that cannot start until receipt of the message.
delta-seconds = 1*DIGIT
3.4 Character Sets
HTTP uses the same definition of the term _character set_ as that
described for MIME:
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]. However, because that registry does not define a single,
consistent token for each character set, we define here the preferred
names for those character sets most likely to be used with HTTP
entities. These character sets include those registered by RFC 1521 [7]
-- the US-ASCII [21] and ISO-8859 [22] character sets -- and other names
specifically recommended for use within MIME charset parameters.
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charset = "US-ASCII"
| "ISO-8859-1" | "ISO-8859-2" | "ISO-8859-3"
| "ISO-8859-4" | "ISO-8859-5" | "ISO-8859-6"
| "ISO-2022-JP" | "ISO-2022-JP-2" | "ISO-2022-KR"
| "UNICODE-1-1" | "UNICODE-1-1-UTF-7" | "UNICODE-1-1-UTF-8"
| token
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.
_ _ is more commonly Although HTTP allows an arbitrary token to be used as a charset value, | "ISO-8859-7" | "ISO-8859-8" | "ISO-8859-9" any token that has a predefined value within the IANA Character Set Note: This use of the term character set
referred to as a _character encoding._ However, since HTTP and
MIME share the same registry, it is important that the
terminology also be shared.
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.
3.5 Content Codings
Content coding values indicate an encoding transformation that has been
or can be applied to a resource. Content codings are primarily used to
allow a document to be compressed or encrypted without losing the
identity of its underlying media type. Typically, the resource is stored
in this encoding and only decoded before rendering or analogous usage.
content-coding = "gzip" | "x-gzip" | "compress" | "x-compress" | token
Note: For historical reasons, HTTP applications SHOULD consider
_x-gzip_ and
_x-compress_ to be equivalent to _gzip_ and _compress_,
respectively.
All content-coding values are case-insensitive. HTTP/1.1 uses content-
coding values in the Accept-Encoding (Section 10.3) and Content-Encoding
(Section 10.10) header fields. Although the value describes the content-
coding, what is more important is that it indicates what decoding
mechanism will be required to remove the encoding. Note that a single
program MAY be capable of decoding multiple content-coding formats. Two
values are defined by this specification:
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gzip
An encoding format produced by the file compression program _gzip_
(GNU zip) developed by Jean-loup Gailly[25]. This format is
typically a Lempel-Ziv coding (LZ77) with a 32 bit CRC.
compress
The encoding format produced by the 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.
HTTP defines a registration process which uses the Internet Assigned
Numbers Authority (IANA) as a central registry for content-coding value
tokens. Additional content-coding value tokens beyond the four defined
in this document (gzip x-gzip compress x-compress) SHOULD be registered
with the IANA. To allow interoperability between clients and servers,
specifications of the content coding algorithms used to implement a new
value SHOULD be publicly available and adequate for independent
implementation, and MUST 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 resource.
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 10.39).
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.
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All HTTP/1.1 applications MUST be able to receive and decode the
_chunked_ transfer coding , and MUST ignore chunked extensions they do
not understand. 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 footer 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
"0" CRLF
footer
CRLF
chunk = chunk-size [ chunk-ext ] CRLF
chunk-data CRLF
chunk-size = hex-no-zero *HEX
chunk-ext = *( ";" chunk-ext-name [ "=" chunk-ext-value ] )
chunk-ext-name = token
chunk-ext-val = token | quoted-string
chunk-data = chunk-size(OCTET)
footer = *< Content-MD5 and future headers that specify
they are allowed in footer>>
hex-no-zero = <HEX excluding "0">
Note that the chunks are ended by a zero-sized chunk, followed by the
footer and terminated by an empty line. An example process for decoding
a Chunked-Body is presented in Appendix C.5.
3.7 Media Types
HTTP uses Internet Media Types [17] in the Content-Type (Section 10.15)
and Accept (Section 10.1) header fields in order to provide open and
extensible data typing and type negotiation.
media-type = type "/" subtype *( ";" parameter )
type = token
subtype = token
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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. LWS MUST NOT be generated between the
type and subtype, nor between an attribute and its value. Upon receipt
of a media type with an unrecognized parameter, a user agent SHOULD
treat the media type as if the unrecognized parameter and its value were
not present.
Some older HTTP applications do not recognize media type parameters.
HTTP/1.1 applications SHOULD only use media type parameters when they
are necessary to define the content of a message.
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 MUST be represented in the
appropriate canonical form prior to its transmission. If the body has
been encoded with a Content-Encoding, the underlying data SHOULD be in
canonical form prior to being encoded.
Media subtypes of the _text_ type use CRLF as the text line break when
in canonical form. However, HTTP allows the transport of text media with
plain CR or LF alone representing a line break when used consistently
within the 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 media 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
SHOULD NOT be substituted for CRLF within any of the HTTP control
structures (such as header fields and multipart boundaries).
The _charset_ parameter is used with some media types to define the
character set (Section 3.4) of the data. When no explicit charset
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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 in order to be
consistently interpreted by the recipient.
Note: Many current HTTP servers provide data using charsets
other than _ISO-8859-1_ without proper labeling. This situation
reduces interoperability and is not recommended. To compensate
for this, some HTTP user agents provide a configuration option
to allow the user to change the default interpretation of the
media type character set when no charset parameter is given.
3.7.2 Multipart Types
MIME provides for a number of _multipart_ types -- encapsulations of one
or more entities within a single message's Entity-Body. All multipart
types share a common syntax, as defined in Section 7.2.1 of RFC 1521 [7]
, 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
1521, the epilogue of any multipart message MUST be empty; HTTP
applications MUST NOT transmit the epilogue even if the original
resource contains an epilogue.
In HTTP, multipart body-parts MAY contain header fields which are
significant to the meaning of that part.
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 via a simple product token, with an optional slash and
version designator. 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.
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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 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. The weights are normalized to a real number in
the range 0 through 1, where 0 is the minimum and 1 the maximum value.
In order to discourage misuse of this feature, 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 ] )
| ( "." 0*3DIGIT )
| ( "1" [ "." 0*3("0") ] )
_Quality values_ is a slight misnomer, since these values actually
measure relative degradation in perceived quality. Thus, a value of
_0.8_ represents a 20% degradation from the optimum rather than a
statement of 80% 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
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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 namespace 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, but examples of tags which
could be registered in future.
3.12 Full Date Values
Contents moved to section 3.3.
3.13 Opaque Validators
Opaque validators are quoted strings whose internal structure is not
visible to clients or caches.
opaque-validator = strong-opaque-validator | weak-opaque-validator
| null-validator
strong-opaque-validator = quoted-string
weak-opaque-validator = quoted-string "/W"
null-validator = <"> <">
Note that the _/W_ tag is considered part of a weak opaque
validator; it MUST NOT be removed by any cache or client.
There are two comparison functions on opaque validators:
. 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, except for the presence
or absence of a _weak_ tag.
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The weak comparison function MAY be used for simple (non-subrange) GET
requests. The strong comparison function MUST be used in all other
cases.
The null validator is a special value, defined as never matching the
current validator of an existing resource, and always matching the
_current_ validator of a resource that does not exist.
3.14 Variant IDs
Variant-IDs are used to identify specific entities (variants) of a
varying resource; see section 13.8.3 for how they are used.
variant-id = quoted-string
Variant-IDs are compared using string octet-equality; case is
significant.
3.15 Validator Sets
Validator sets are used for doing conditional retrievals on varying
resources; see section 13.8.4.
validator-set = 1#validator-set-item
validator-set-item = opaque-validator
3.16 Variant Sets
Validator sets are used for doing conditional retrievals on varying
resources; see section 13.8.3.
variant-set = 1#variant-set-item
variant-set-item = opaque-validator ";" variant-id
3.17 HTTP Protocol Parameters Related to Ranges
This section defines certain HTTP protocol parameters used in range
requests and related responses.
3.17.1SLUSHY Range Units
A resource may be broken down into subranges according to various
structural units.
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bytes-unit = "bytes"
The only range unit defined by HTTP/1.1 is . HTTP/1.1 range-unit = bytes-unit other-range-unit _bytes_
implementations may ignore ranges specified using other units. other-
range-unit = token
3.17.2 SLUSHY 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 that
would be transferred by the protocol if no transfer-encoding were being
applied.
This means that if Content-encoding is applied to the data, the
byte range specification applies to the resulting content-
encoded byte stream, not to the unencoded byte stream. It also
means that if the entity-body's media-type is a composite type
(e.g., multipart/* and message/rfc822), then the composite's
body-parts may have their own content-encoding and content-
transfer-encoding, and the byte range applies to the result of
the those encodings.
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.
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If the last-byte-pos value is absent, it is assumed to be equal to the
current length of the entity in bytes.
If the last-byte-pos value is larger than the current length of the
entity, it is assumed to be equal to the current length of the entity.
suffix-byte-range-spec = "-" suffix-length
suffix-length = 1*DIGIT
A suffix-byte-range-spec is used to specify the suffix of the entity, of
a length given by the suffix-length value. (That is, this form
specifies the last N bytes of an entity.) If the entity is shorter than
the specified suffix-length, the entire entity is used.
Examples of byte-ranges-specifier values (assuming an entity 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
3.17.3 SLUSHY: Content Ranges
When a server returns a partial response to a client, it must describe
both the extent of the range covered by the response, and the length of
the entire entity.
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
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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 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.
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
4. HTTP Message
4.1 Message Types
HTTP messages consist of requests from client to server and responses
from server to client.
HTTP-message = Full-Request ; HTTP/1.1 messages
| Full-Response
| NULL-Request
A NULL-Request (an empty line where a request would normally be
expected) MUST be ignored. Clients SHOULD NOT send a NULL-Request, but
there are some error and testing circumstances in which a NULL-Request
might be sent by mistake and MUST NOT cause failure on the server.
NULL-Request = CRLF
Full-Request and Full-Response use the generic message format of RFC 822
[9] for transferring entities. Both messages may include optional header
fields (also known as _headers_) and an entity body. The entity body is
separated from the headers by a null line (i.e., a line with nothing
preceding the CRLF).
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Full-Request = Request-Line ; Section 5.1
*( General-Header ; Section 4.3
| Request-Header ; Section 5.2
| Entity-Header ) ; Section 7.1
CRLF
[ Entity-Body ] ; Section 7.2
Full-Response = Status-Line ; Section 6.1
*( General-Header ; Section 4.3
| Response-Header ; Section 6.2
| Entity-Header ) ; Section 7.1
CRLF
[ Entity-Body ] ; Section 7.2
4.2 Message Headers
HTTP header fields, which include (Section 4.3), Request-
Header ( General-Header (Section 5.2), 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.
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HTTP-header = field-name ":" [ field-value ] CRLF
and consisting of either *TEXT or combinations
The order in which header fields with differing field names are received
_ field-name = token field-value = *( field-content | LWS ) field-content = <the OCTETs making up the field-value of token, tspecials, and quoted-string> 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 HTTP-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. Thus,
the order in which multiple header fields with the same field-name are
received may be significant to the interpretation of the combined field-
value.
4.3 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 headers apply only to the message being
transmitted.
General-Header = Cache-Control ; Section 10.8
| Connection ; Section 10.9
| Date ; Section 10.17
| Via ; Section 10.20
| Keep-Alive ; Section 10.24
| Pragma ; Section 10.29
| Upgrade ; Section 10.41
General header field names can be extended reliably only in combination
with a change in the protocol version. However, new or experimental
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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.
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. For
backwards compatibility with the more limited HTTP/0.9 protocol, there
are two valid formats for an HTTP request:
Full-Request = Request-Line ; Section 5.1
*( General-Header ; Section 4.3
| Request-Header ; Section 5.2
| Entity-Header ) ; Section 7.1
CRLF
[ Entity-Body ] ; Section 7.2
NULL-Request = CRLF
A NULL-Request MUST be ignored.
5.1 Request-Line Request = Full-Request | NULL-Request
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.
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Method = "OPTIONS" ;
| "GET" ;
| "HEAD" ; Section 8.3
| "POST" ; Section 8.4
| "PUT" ; Section 8.5
| "DELETE" ; |
"TRACE" ; Section 8.12
| extension-method
extension-method = token
The list of methods acceptable by a specific resource can be specified
Allow ). However, the client is always Section 8.1 Section 8.2 in an header field (Section 10.5
notified through the return code of the response whether a method is
currently allowed on a specific resource, as this 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 10.32).
The methods GET and HEAD MUST be supported by all general-purpose
servers. Servers which provide Last-Modified dates for resources MUST
also support the conditional GET method. 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 8.
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
To allow for transition to absoluteURIs in all requests in future
versions of HTTP, HTTP/1.1 servers MUST accept the absoluteURI form in
requests, even though HTTP/1.1 clients will not normally generate them.
Versions of HTTP after HTTP/1.1 may require absoluteURIs everywhere,
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after HTTP/1.1 or later have become the dominant implementations. 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 only allowed to an origin server if the client
knows the server supports HTTP/1.1 or later. If the absoluteURI form is
used, any Host request-header included with the request MUST be ignored.
The absoluteURI form is required when the request is being made to a
proxy. The proxy is requested to forward the request and return the
response. If the request is GET or HEAD and a prior response is cached,
the proxy may use the cached message if it passes any restrictions in
the Cache-Control and Expires header fields. 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
The most common form of Request-URI is that used to identify a resource
on an origin server or gateway. In this case, only the absolute path of
the URI is transmitted (see Section 3.2.1, abs_path). 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 Full-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).
If a proxy receives a request without any path in the Request-URI and
the method used 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
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would be forwarded by the proxy as
OPTIONS * HTTP/1.1
_www.ics.uci.edu_.
is transmitted as an encoded string, where some after connecting to port 8001 of host The Request-URI
characters may be escaped using the _% HEX HEX_ encoding defined by RFC
1738 [4]. The origin server MUST decode the Request-URI in order to
properly interpret the request. 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 _*_. Illegal Request-URIs
SHOULD be responded to with an appropriate status code. (Proxies MAY
transform the Request-URI for internal processing purposes, but SHOULD
NOT send such a transformed Request-URI in forwarded requests.
Transformations for use in cache updates and lookups are subject to
additional requirements; see section 13 on caching. The main reason for
this rule is to make sure that the form of Request-URIs is well
specified, to enable future extensions without fear that they will break
in the face of some rewritings. Another is that one consequence of
rewriting the Request-URI is that integrity or authentication checks by
the server may fail; since rewriting MUST be avoided in this case, it
may as well be proscribed in general.
Note: servers writers SHOULD be aware that some existing proxies
do some rewriting.
5.2 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 (procedure)
invocation.
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Request-Header = Accept ; Section 10.1
| Accept-Charset ; Section 10.2
| Accept-Encoding ; Section 10.3
| Accept-Language ; Section 10.4
| Authorization ; Section 10.6
| From ; Section 10.21
| Host ; Section 10.22
| If-Modified-Since ; Section 10.23
| Proxy-Authorization ; Section 10.31
| Range ; Section 10.33
| Referer ; Section 10.34
| User-Agent ; Section 10.43
| Max-Forwards ; Section 10.45
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 in
the form of an HTTP response message.
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Response = Full-Response
Full-Response = Status-Line ; Section 6.1
*( General-Header ; Section 4.3
| Response-Header ; Section 6.2
| Entity-Header ) ; Section 7.1
CRLF
[ Entity-Body ] ; Section 7.2
6.1 Status-Line
The first line of a Full-Response message is the Status-Line, consisting
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
element is a 3-digit integer result code of the attempt of the protocol version followed by a numeric status code and its The Status-Code
to understand and satisfy the request. 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
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. 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. These
codes are fully defined in Section 9.
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Status-Code = "100" ; Continue
| "101" ; Switching Protocols
| "200" ; OK
| "201" ; Created
| "202" ; Accepted
| "203" ; Non-Authoritative Information
| "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" ; None Acceptable
| "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
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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.
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 = Location ; Section 10.27
| Proxy-Authenticate ; Section 10.30
| Public ; Section 10.32
| Retry-After ; Section 10.36
| Server ; Section 10.37
| WWW-Authenticate ; Section 10.44
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
Full-Request and Full-Response messages MAY transfer an entity within
some requests and responses. An entity consists of Entity-Header fields
and (usually) an Entity-Body. In this section, both sender and recipient
refer to either the client or the server, depending on who sends and who
receives the entity.
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.
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Entity-Header = Allow ; Section 10.5
| Content-Base ; Section 10.9
| Content-Encoding ; Section 10.10
| Content-Language ; Section 10.11
| Content-Length ; Section 10.12
| Content-Location ; Section 10.16
| Content-MD5 ; Section 10.13
| Content-Range ; Section 10.14
| Content-Type ; Section 10.15
| Expires ; Section 10.19
| Last-Modified ; Section 10.25
| Title ; Section 10.38
| Transfer-Encoding ; Section 10.39
| extension-header
extension-header = HTTP-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 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 included with a request message only when the request
method calls for one. The presence of an entity body in a request is
signaled by the inclusion of a Content-Length and/or Content-Type header
field in the request message headers.
For response messages, whether or not an entity body is included with a
message is dependent on both the request method and the response code.
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All responses to the HEAD request method MUST not include a body, even
though the presence of entity header fields may lead one to believe they
do. All 1xx (informational), 204 (no content), and 304 (not modified)
responses MUST not include a body. All other responses MUST include an
entity body or a Content-Length header field defined with a value of
zero (0).
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, Content-Encoding,
and Transfer-Encoding. These define a three-layer, ordered encoding
model:
entity-body :=
Transfer-Encoding( Content-Encoding( Content-Type( data ) ) )
The default for both encodings is none (i.e., the identity function).
Content-Type specifies the media type of the underlying data. Content-
Encoding may be used to indicate any additional content codings applied
to the type, usually for the purpose of data compression, that are a
property of the resource requested. Transfer-Encoding may be used to
indicate any additional transfer codings applied by an application to
ensure safe and proper transfer of the message. Note that Transfer-
Encoding is a property of the message, not of the resource.
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 header, 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
When an entity body is included with a message, the length of that body
may be determined in one of several ways. If a Content-Length header
field is present, its value in bytes represents the length of the entity
body. Otherwise, the body length is determined by the Transfer-Encoding
(if the _chunked_ transfer coding has been applied) or by the server
closing the connection.
Note: Any response message which MUST NOT include an entity 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.
Closing the connection cannot be used to indicate the end of a request
body, since it leaves no possibility for the server to send back a
response. For compatibility with HTTP/1.0 applications, HTTP/1.1
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requests containing an entity body MUST include a valid Content-Length
header field unless the server is known to be HTTP/1.1 compliant.
HTTP/1.1 servers MUST accept the _chunked_ transfer coding (Section 3.6
), thus allowing this mechanism to be used for a request when Content-
Length is unknown.
If a request contains an entity body and Content-Length is not
specified, the server SHOULD respond with 400 (bad request) if it cannot
determine the length of the request message's content, or with 411
(length required) if it wishes to insist on receiving a valid Content-
Length.
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 an entity body is
allowed, its field value MUST exactly match the number of OCTETs in the
entity body. HTTP/1.1 user agents MUST notify the user when an invalid
length is received and detected.
8. 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.
The Host request-header field (Section 10.22) MUST accompany all
HTTP/1.1 requests.
8.1 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.
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) and MUST
include a Content-Length with a value of zero (0). 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
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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 known to be unavailable
through that proxy.
8.2 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.
The semantics of the GET method change to a _conditional GET_ if the
request message includes an If-Modified-Since header field. A
conditional GET method requests that the identified resource be
transferred only if it has been modified since the date given by the If-
Modified-Since header, as described in Section 10.23. 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 identified resource be transferred, as described in
Section 10.33. 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 may be cachable if and only if it meets
the requirements for HTTP caching described in Section 13.
8.3 HEAD
The HEAD method is identical to GET except that the server MUST not
return any Entity-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 resource identified by the
Request-URI without transferring the Entity-Body itself. This method is
often used for testing hypertext links for validity, accessibility, and
recent modification.
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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 resource (as would be
indicated by a change in Content-Length, Content-MD5, or Content-
Version), then the cache MUST discard the cached entity.
There is no _conditional HEAD_ or _partial HEAD_ request analogous to
those associated with the GET method. If an If-Modified-Since and/or
Range header field is included with a HEAD request, they SHOULD be
ignored.
8.4 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 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
[5], 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.
For compatibility with HTTP/1.0 applications, all POST requests MUST
include a valid Content-Length header field unless the server is known
to be HTTP/1.1 compliant. When sending a POST request to an HTTP/1.1
server, a client MUST use a valid Content-Length or the _chunked_
Transfer-Encoding. The server SHOULD respond with a 400 (bad request)
message if it cannot determine the length of the request message's
content, or with 411 (length required) if it wishes to insist on
receiving a valid Content-Length.
A successful POST does not require that the entity be created as a
resource on the origin server or made accessible for future reference.
That is, 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.
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If a resource has been created on the origin server, the response SHOULD
be 201 (created) and contain an entity (preferably of type _text/html_)
which describes the status of the request and refers to the new
resource.
Responses to this method are not cachable. However, the 303 (see other)
response can be used to direct the user agent to retrieve a cachable
resource.
POST requests must obey the entity transmission requirements set out in
section 8.4.1.
8.4.1 SLUSHY: Entity Transmission Requirements
The following rules apply to any method that is subject to the two-phase
mechanism.
Upon receiving such a method from an HTTP/1.1 (or later) client, an
HTTP/1.1 (or later) server immediately either respond with _100
Continue_ and continue to read from the input stream, or respond with an
error status. If it responds with an error status, it MAY close the
transport (TCP) connection or it MAY continue to read and discard the
rest of the request. It MUST not perform the requested action if
returns an error status.
HTTP/1.1 servers are encouraged to 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.
An HTTP/1.1 (or later) client doing a PUT-like method SHOULD monitor the
network connection for an error status while it is transmitting the body
of the request including any encoding mechanism used to transmit the
body. If the client sees an error status, it SHOULD immediately cease
transmitting the body. If the body was proceeded by a Content-length
header, the client MUST either close the connection or if the body is
being sent using a Chunked encoding, use a 0 length chunk, to mark the
end of the message.
An HTTP/1.1 (or later) client MUST be prepared to accept a 100 Continue
status followed by a regular response.
An HTTP/1.1 (or later) client that sees the connection close before
receiving any status from the server SHOULD retry the request, but if it
does so, it MUST use the two-phase mechanism. In the two-phase
mechanism, the client first sends the request headers, then waits for
the server to respond with either a 100 Continue, in which case the
client SHOULD continue, or an error status, in which case the client
MUST NOT continue and MUST close the connection if it has not already
completed sending the full request body including any encoding mechanism
used to transmit the body.
If the client knows that the server is an HTTP/1.1 (or later) server,
because of the server protocol version returned with a previous request
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on the same persistent connection [alternatively: within the past <N>
hours], it MUST wait for a response. If the client believes that the
server is a 1.0 or earlier server, it SHOULD continue transmitting
its request after waiting at least [5] seconds for a status response.
An HTTP/1.1 (or later) client that sees the connection close after
receiving a _100 Continue_ but before receiving any other status SHOULD
retry the request, and need not use the two-phase method (but MAY do so
if this simplifies the implementation).
An HTTP/1.1 (or later) server that receives a request from a 1.0 (or
earlier) client MUST NOT transmit the _100 Continue_ response; it SHOULD
either wait for the request to be completed normally (thus avoiding an
interrupted request) or close the connection prematurely.
8.5 PUT
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 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.
If the request passes through a cache and the Request-URI identifies a
currently cached entity, that entity MUST be removed from the cache.
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 as
an appendage. 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.
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For compatibility with HTTP/1.0 applications, all PUT requests MUST
include a valid Content-Length header field unless the server is known
to be HTTP/1.1 compliant. When sending a PUT request to an HTTP/1.1
server, a client MUST use a valid Content-Length or the _chunked_
Transfer-Encoding. The server SHOULD respond with a 400 (bad request)
message if it cannot determine the length of the request message's
content, or with 411 (length required) if it wishes to insist on
receiving a valid Content-Length.
The actual method for determining how the resource is placed, and what
happens to its predecessor, is defined entirely by the origin server. If
the entity being PUT was derived from an existing resource which
included a Content-Version header field, the new entity MUST include a
Derived-From header field corresponding to the value of the original
Content-Version header field. Multiple Derived-From values may be
included if the entity was derived from multiple resources with Content-
Version information. Applications are encouraged to use these fields for
constructing versioning relationships and resolving version conflicts.
PUT requests must obey the entity transmission requirements set out in
section 8.4.1.
8.9 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 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 a
currently cached entity, that entity MUST be removed from the cache.
Responses to this method are not cachable.
8.12 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 10.45). A TRACE request MUST NOT include an
entity body and MUST include a Content-Length header field with a value
of zero (0).
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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 10.20) 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_,
_application/http_, or _text/plain_. Responses to this method MUST NOT
be cached.
9. 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.
9.1 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. 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.
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.
101 Switching Protocols
The server understands and is willing to comply with the client's
request, via the Upgrade message header field (Section 10.41), 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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9.2 Successful 2xx
This class of status code indicates that the client's request was
successfully received, understood, and accepted.
200 OK
The request has succeeded. The information returned with the response is
dependent on the method used in the request, as follows:
GET
an entity corresponding to the requested resource is sent in the
response;
HEAD
the response MUST only contain the header information and no Entity-
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;
otherwise,
an entity describing the result of the action;
If the entity corresponds to a resource, the response MAY include a
Content-Location header field giving the actual location of that
specific resource for later reference.
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 SHOULD
create the resource before using this Status-Code. If the action cannot
be carried out immediately, the server MUST include in the response body
a description of when the resource will be available; otherwise, the
server SHOULD respond with 202 (accepted).
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
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completed. The entity returned with this response SHOULD include an
indication of the request's current status and either a pointer to a
status monitor or some estimate of when the user can expect the request
to be fulfilled.
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).
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 generated. 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 an entity body, and thus is always
terminated by the first empty line after the header fields.
205 Reset Content
The server has fulfilled the request and the user agent SHOULD reset the
document view which caused the request to be generated. 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 include a Content-Length with a value of zero (0) and no entity
body.
206 Partial Content
The server has fulfilled the partial GET request for the resource. The
request MUST have included a Range header field (Section 10.33)
indicating the desired range. The response MUST include a Content-Range
header field (Section 10.14) indicating the range included with this
response. All entity header fields in the response MUST describe the
partial entity transmitted rather than what would have been transmitted
in a full response. In particular, the Content-Length header field in
the response MUST match the actual number of OCTETs transmitted in the
entity body. It is assumed that the client already has the complete
entity's header field data.
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207 Range Out Of Bounds
The server has determined that the requested range(s) are not present in
the requested resource, and so there is no content to return. This
status code should be handled by the client the same as 204 No Content.
This could be a compatibility problem if there is an installed
base. If treating this status code as the generic 2xx code by
such implementations would lead to an error, it will have to be
replace by 204.
9.3 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.
300 Multiple Choices
This status code is reserved for future use by a planned content
negotiation mechanism. HTTP/1.1 user agents receiving a 300 response
which includes a Location header field can treat this response as they
would treat a 303 (See Other) response. If no Location header field is
included, the appropriate action is to display the entity enclosed in
the response to the user.
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 MUST be given by the Location
field in the response. Unless it was a HEAD request, the Entity-Body 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.
302 Moved Temporarily
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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 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 MUST be given by the Location
field in the response. Unless it was a HEAD request, the Entity-Body 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.
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 resource is not
a update reference for the original Request-URI. The 303 response is not
cachable, but the response to the second request MAY be cachable.
If the new URI is a location, its URL MUST be given by the Location
field in the response. Unless it was a HEAD request, the Entity-Body of
the response SHOULD contain a short hypertext note with a hyperlink to
the new URI(s).
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.
304 Not Modified
If the client has performed a conditional GET request and access is
allowed, but the document has not been modified since the date and time
specified in the If-Modified-Since field, the server MUST respond with
this status code and not send an Entity-Body to the client. Header
fields contained in the response SHOULD only include information which
is relevant to cache managers or which MAY have changed independently of
the entity's Last-Modified date. Examples of relevant header fields
include: Date, Server, Content-Length, Content-MD5, Content-Version,
Cache-Control and Expires.
A cache SHOULD update its cached entity to reflect any new field values
given in the 304 response. If the new field values indicate that the
cached entity differs from the current resource (as would be indicated
by a change in Content-Length, Content-MD5, or Content-Version), then
the cache MUST disregard the 304 response and repeat the request without
an If-Modified-Since field.
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The 304 response MUST NOT include an entity body, and thus is always
terminated by the first empty line after the header fields.
305 Use Proxy
The requested resource MUST be accessed through the proxy given by the
Location field in the response. In other words, this is a proxy
redirect.
9.4 Client Error 4xx
The 4xx class of status code is intended for cases in which the client
seems to have erred. If the client has not completed the request when a
4xx code is received, it SHOULD immediately cease sending data to the
server. 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.
Note: If the client is sending data, server implementations on
TCP SHOULD be careful to ensure that the client acknowledges
receipt of the packet(s) containing the response prior to
closing the input connection. If the client continues sending
data to the server after the close, the server's controller 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.
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.
401 Unauthorized
The request requires user authentication. The response MUST include a
WWW-Authenticate header field (Section 10.44) containing a challenge
applicable to the requested resource. The client MAY repeat the request
with a suitable Authorization header field (Section 10.6). 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
information. HTTP access authentication is explained in Section 11.
402 Payment Required
This code is reserved for future use.
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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 body. 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.
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.
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.
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.
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 over 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 is not acceptable, user
agents SHOULD interrupt the receipt of the response if doing so would
save network resources. If it is unknown whether an incoming response
would be acceptable, a user agent SHOULD temporarily stop receipt of
more data and query the user for a decision on further
actions.
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 10.30) 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 10.31). HTTP access authentication is explained in Section 11.
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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.
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 MAY 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 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.
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.
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 entity body in the
request message.
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
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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.
413 Request Entity Too Large
The server is refusing to process a request because it considers the
request entity to be larger than it is willing or able to process. The
server SHOULD close the connection if that is necessary to prevent the
client from continuing the request.
If the client manages to read the 413 response, it MUST honor it and
SHOULD reflect it to the user.
If this restriction is considered temporary, the server MAY include a
Retry-After header field to indicate that it is temporary and after what
time the client MAY try again.
414 Request-URI Too Large
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.
415 Unsupported Media Type
The server is refusing to service the request because the entity body of
the request is in a format not supported by the requested resource for
the requested method.
416 None Acceptable
This status code is reserved for future use by a planned content
negotiation mechanism. HTTP/1.1 user agents receiving a 416 response
which includes a Location header can treat this response as they would
treat a 303 (See Other) response. If no Location header is included, the
appropriate action is to display the entity enclosed in the response to
the user.
9.5 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. If the client has not completed the request when
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a 5xx code is received, it SHOULD immediately cease sending data to the
server. 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 response codes
are applicable to any request method and there are no required header
fields.
500 Internal Server Error
The server encountered an unexpected condition which prevented it from
fulfilling the request.
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.
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.
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.
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.
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.
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10. 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.
10.1 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.
The field MAY be folded onto several lines and more than one occurrence
of the field is allowed, with the semantics being the same as if all the
entries had been in one field value.
Accept = "Accept" ":" #(
media-range
[ ( ":" | ";" )
range-parameter
*( ";" range-parameter ) ]
| extension-token )
media-range = ( "*/*"
| ( type "/" "*" )
| ( type "/" subtype )
) *( ";" parameter )
range-parameter = ( "q" "=" qvalue )
| extension-range-parameter
extension-range-parameter = ( token "=" token )
extension-token = token
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 range-parameter q is used to indicate the
media type quality factor for the range, which represents the user's
preference for that range of media types. The default value is q=1. In
Accept headers generated by HTTP/1.1 clients, the character separating
media-ranges from range-parameters SHOULD be a _:_. HTTP/1.1 servers
SHOULD be tolerant of use of the _;_ separator by HTTP/1.0 clients.
The example
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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 is present, then it is assumed that the client
accepts all media types. If Accept headers are present, and if the
server cannot send a response which is acceptable according to the
Accept headers, 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.
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) */*
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,
*/*:q=0.5
would cause the following values to be associated:
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text/html;level=1 = 1
image/jpeg = 0.5
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 text/html = 0.7 text/plain = 0.3
agents, this default set SHOULD be configurable by the user.
10.2 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
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.
10.3 Accept-Encoding
The Accept-Encoding request-header field is similar to Accept, but
restricts the content-coding values (Section 3.5) which are acceptable
in the response.
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Accept-Encoding = "Accept-Encoding" ":"
#( content-coding )
An example of its use is
Accept-Encoding: compress, gzip
If no Accept-Encoding header is present in a request, the server MAY
assume that the client will accept any content coding. If an Accept-
Encoding header is present, 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.
10.4 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 comprehension of the languages
specified by that range. The quality value defaults to _q=1_ (100%
comprehension).For example,
Accept-Language: da, en-gb;q=0.8, en;q=0.7
would mean: _I prefer Danish, but will accept British English (with 80%
comprehension) and other types of English(with 70% comprehension)._ A
language-range matches a language-tag if it exactly equals the tag, or
if it exactly equals a prefix (a sub-sequence starting at the first
character) of the tag such that the first tag character following the
prefix is _-_. The special range _*_, if present in the Accept-Language
field, matches every tag not matched by any other ranges 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
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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-range. 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. If the server cannot generate a
response for an audience capable of understanding at least one
acceptable language, it can send a response that uses one or more un-
accepted languages.
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 14.7
.
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.
10.5 Allow
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. The Allow header field is not permitted in a request
using the POST method, and thus SHOULD be ignored if it is received as
part of a POST entity.
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.
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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 10.32) to describe what methods are implemented on the server
as a whole.
10.6 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 13.10) 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 _proxy-revalidate_ Cache-Control
directive, the cache MAY use that response in replying to a
subsequent request, but 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.
2. If the response includes the _must-revalidate_ Cache-Control
directive, the cache MAY use that response in replying to a
subsequent request, but 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.
10.7 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
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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.
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 = "public"
| "private" [ "=" <"> 1#field-name <"> ]
| "no-cache" [ "=" <"> 1#field-name <"> ]
| "no-store"
| "no-transform"
| "must-revalidate"
| "proxy-revalidate"
| "only-if-cached"
| "max-age" "=" delta-seconds
| "max-stale" "=" delta-seconds
| "min-fresh" "=" delta-seconds
| "min-vers" "=" HTTP-Version
and perhaps
| "max-uses" "=" 1*DIGIT
| "use-count" "=" 1*DIGIT
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 end-user client.
. Modifications of the basic expiration mechanism; these may be
imposed by either the origin server or the end-user client.
. Controls over cache revalidation and reload; these may only be
imposed by an end-user client.
. Restrictions on the number of times a cache entry may be used, and
related demographic reporting mechanisms.
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. Miscellaneous restrictions
Caches never add or remove Cache-Control directives to requests or
responses.
Check: is this true?
10.7.1 SLUSHY: Restrictions on What is Cachable
Unless specifically constrained by a Cache-Control directive, a caching
system may always store a successful response 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) as long as
they explicit mark their responses using the Warning mechanism describe
in section 10.51.
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 a
resource value, 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 resources, or portions thereof, may not
be cached regardless of other considerations.
Note that section 10.6 normally prevents a shared cache from saving and
returning a response to a previous request if that request included an
Authorization header.
The following Cache-Control response directives add or remove
restrictions on what is cachable:
public
Overrides the restriction in section 10.6 that prevents a shared
cache from saving and returning a response to a previous request if
that request included an Authorization header. However, any other
constraints on caching still apply.
private
Indicates that all or parts of the response message are 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. applicable to responses and must not be
generated by clients. A private (non-shared) cache may ignore this
directive.
Note: This usage of the word _private_ only controls where the
response may cached, and cannot ensure the privacy of the
message content. Note in particular that HTTP/1.0 caches will
not recognize or obey this directive.
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no-cache
indicates that all or parts of the response message MUST NOT be
cached. This allows an origin server to prevent caching even by
caches that have been configured to return stale responses to client
requests.
Note: HTTP/1.0 caches will not recognize or obey this directive.
TBS: precedence relations between public, private, and no-cache.
10.7.2 Restrictions On What May be Stored by a Cache
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. This directive applies to both non-
shared and shared caches.
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, HTTP/1.0 caches will not
recognize or obey this directive, malicious or compromised caches may
not recognize or obey this directive, and all communications networks
may be vulnerable to eavesdropping.
The _min-vers_ 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
whose HTTP version number is less than the specified version MUST NOT
store any part of either this request or any response to it. If sent in
a response, a cache whose HTTP version number is less than the specified
version MUST NOT store any part of either this response or the request
that elicited it, nor may any cache transmit a stored (non-firsthand)
copy of the response to any client with a lower HTTP version number.
This directive applies to both non-shared and shared caches, and is made
mandatory to allow for future protocol extensions that may affect
caching.
Note that the lowest version that can be sensibly included in a
_min-vers_ directive is HTTP/1.1, since HTTP/1.0 caches do not
obey it.
10.7.3 Modifications of the Basic Expiration Mechanism
The expiration time of a resource may be specified by the origin server
using the Expires header (see section TBS). Alternatively, it may be
specified using the _max-age_ directive in a response.
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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 badly unsynchronized clocks.
Other directives allow an end-user client to modify the basic expiration
mechanism, making it either stricter or looser. 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_
is also included, the client is not willing to accept a stale response.
This directive overrides any policy of the cache.
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 that response
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 by no more than the specified number of
seconds. If a cache returns a stale response in response to such a
request, it MUST mark it as stale using the Warning header.
Note that HTTP/1.0 caches will ignore these directives.
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).
10.7.4 SLUSHY: Controls over cache revalidation and reload
Sometimes an end-user client 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 response value has become corrupted for some reason,
and the fact that its validator is up-to-date is irrelevant.
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:
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End-to-end reload The request includes _Cache-Control: no-cache_ 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 _Cache-Control:
max-age=0_, 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 includes a cache-validating conditional with the
client's current validator.
Unspecified end-to-end revalidation The request includes _Cache-Control:
max-age=0_, 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.
Note that HTTP/1.0 caches will ignore these directives, except
perhaps for _Pragma: no-cache_.
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-body 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-body with a 200 (OK)
response.
If a request includes the _no-cache_ directive, it should not include
_fresh-min_, _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 value 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.
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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, 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 value
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.)
The _must-revalidate_ directive is necessary to support reliable
operation for cookies and certain other 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. Note
that HTTP/1.0 caches will ignore this directive.
Servers should send the _must-revalidate_ directive if and only if
failure to revalidate a request on the entity could result in
significantly 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 user-agent
caches. This directive is meant to support digest authentication.
10.7.5 FLUID: Restrictions on use count and demographic reporting
This section is highly debatable and is likely to be removed to a
separate I.D.
The _max-uses_ response directive allows a cache to use a response at
most a certain limited number of times. For example, _max-uses=10_
means that the response should be returned in reply to the current
request, and may be returned in reply to no more than nine subsequent
requests (subject to other caching constraints), unless revalidated.
A cache may subdivide its remaining use-count among several of its own
clients. For example, if the incoming response includes _max-uses=10_,
the recipient may forward this as two responses, each with _max-uses=5_.
The idea is that the total number of uses allowed in a cache hierarchy
should not exceed the specified limit. (The heuristics a cache uses to
sub-allocate its max-uses value are beyond the scope of the HTTP spec.)
The _use-count_ request directive allows a cache to tell a server how
many times it has actually used the cache entry specified in the
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associated request. If a cache receives a use-count value from one of
its clients, and it has a corresponding cache entry, it should add the
incoming use-count to its local count.
When a cache removes an entry, it MAY first send a HEAD request on the
associated URI, including its use-count value, to inform the server of
the actual use-count. If the server has sent a max-uses limit, the
cache SHOULD perform this notification.
A cache that is willing to perform such notifications and that is
willing to obey the max-uses limit SHOULD send a ``use-count=0''
directive on its first (non-conditional) request on a resource. This
informs the server that the cache intends to use these two directives in
the manner described here.
10.7.6 Miscellaneous restrictions
In certain circumstances, an intermediate cache (proxy) may find it
useful to convert the encoding of an entity body. For example, a proxy
might use a compressed content-coding to transfer the body to a client
on a slow link.
Because end-to-end authentication of entity bodies and/or entity headers
relies on the specific encoding of these values, such transformations
may cause authentication failures. Therefore, an intermediate cache MUST
NOT change the encoding of an entity body if the response includes the
_no-transform_ directive.
10.8 Connection
HTTP version 1.1 provides a new request and response header field called
_Connection_. This header field allows the client and server to specify
options which should only exist over that particular connection and MUST
NOT be communicated by proxies over further connections. The connection
header field MAY have multiple tokens separated by commas (referred to
as connection-tokens).
HTTP version 1.1 proxies MUST parse the Connection header field and for
every connection-token in this field, remove a corresponding header
field from the request before the request is forwarded. The use of a
connection option is specified by the presence of a connection token in
the Connection header field, not by the corresponding additional header
field (which may not be present).
When a client wishes to establish a persistent connection it MUST send a
_Persist_ connection-token:
Connection: persist
The Connection header has the following grammar:
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Connection-header = "Connection" ":"
connection-token 0#( "," connection-token )
When the Persist connection-token has been transmitted with a request or
a response a Persist header field MAY also be included. The 10.8.1 Persist Persist
header field takes the following form:
Persist-header = "Persist" ":" 0#pers-param
pers-param = param-name "=" value
The Persist header itself is optional, and is used only if a parameter
is being sent. HTTP/1.1 does not define any parameters.
If the Persist header is sent, the corresponding connection token MUST
be transmitted. The Persist header MUST be ignored if received without
the connection token.
10.9 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
soon.
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 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.
10.10 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 resource, 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
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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 resource identified by
the Request-URI. Typically, the resource is stored with this encoding
and is only decoded before rendering or analogous usage.
If multiple encodings have been applied to a resource, 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.
10.11 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.
Content-Language = "Content-Language" ":" 1#language-tag
Language tags are defined in Section 3.10. The primary purpose of
Content-Language is to allow a selective consumer to identify and
differentiate resources according to the consumer's own preferred
language. Thus, if the body content is intended only for a Danish-
literate audience, the appropriate field is
Content-Language: dk
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
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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 SHOULD not be
limited to textual documents.
10.12 Content-Length
The Content-Length entity-header field indicates the size of the Entity-
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 Entity-
Body to be transferred, regardless of the media type of the entity. A
valid Content-Length field value is required on all HTTP/1.1 request
messages containing an entity body.
Any Content-Length greater than or equal to zero is a valid value.
Section 7.2.2 describes how to determine the length of an Entity-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 used whenever the entity's length can be
determined prior to being transferred.
10.13 Content-MD5
The Content-MD5 entity-header field is an MD5 digest of the entity-body,
as defined in RFC 1864 [23], for the purpose of providing an end-to-end
message integrity check (MIC) of the entity-body. (Note: an MIC is good
for detecting accidental modification of the entity-body in transit, but
is not proof against malicious attacks.)
ContentMD5 = "Content-MD5" ":" md5-digest
md5-digest = <base64 of 128 bit MD5 digest as per RFC 1864>
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The Content-MD5 header 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. If the entity 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, 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 many 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. Also, the HTTP
Transfer-Encoding header makes no sense within body-parts; if it
is present, it is ignored -- i.e. treated as ordinary text.
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 CR-LF.
Conversion of all line breaks to CR-LF 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.
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10.14 SLUSHY Content-Range
The Content-Range header is sent with a partial entity body to specify
where in the full entity body the partial body should be inserted. It
also indicates the total size of the entity.
Content-Range = "Content-Range" ":" content-range-spec
When an HTTP message includes the content of a single range (for
example, a response to a request for a single range, or to 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,
HTTP/1.0 206 Partial content
Date: Wed, 15 Nov 1995 06:25:24 GMT
Last-modified: Wed, 15 Nov 1995 04:58:08 GMT
Content-range: 21010-47021/47022
Content-length: 26012
Content-type: image/gif
10.14.1 MIME multipart/byteranges content-type
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".
The MIME multipart/byteranges content-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.
For example:
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HTTP/1.0 206 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
...the second range...
--THIS_STRING_SEPARATES_
10.14.2 Additional rules for Content-Range
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 invalid, or
because it specifies a range that starts beyond the end of the entity,
it may omit the corresponding Content-Range field and partial entity
body.
If none of the byte-range-spec values in a request specify part of the
current entity (i.e., start before the last byte of the entity), then
the server should return a status of 207 (Range Out Of Bounds).
10.15 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.
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Content-Type = "Content-Type" ":" media-type
Media types are defined in Section 3.7. An example of the field is
Further discussion of methods for identifying the media type of an
entity is provided in Section 7.2.1. Content-Type: text/html; charset=ISO-8859-4
10.16 Content-Location
The Content-Location entity-header field is used to define the location
of the specific resource associated with the entity enclosed in the
message. A server SHOULD provide a Content-Location if, when including
an entity in response to a GET request on a negotiated resource, the
entity corresponds to a specific, non-negotiated location which can be
accessed via the Content-Location URI. A server SHOULD provide a
Content-Location with any 200 (OK) response which was internally (not
visible to the client) redirected to a resource other than the one
identified by the request and for which correct interpretation of that
resource MAY require knowledge of its actual location. The recipient MAY
make future requests on this location instead of on the Request-URI.
Content-Location = "Content-Location" ":" absoluteURI
If no Content-Base header field is present, the value of Content-
Location also defines the base URL for the entity (see Section 10.9).
Note: Since the Content-Location field re-interprets the source
of an entity, recipients must take care in not allowing a
_spoofed_ location to deny access to the real resource. This is
described in Section 15.9.
10.17 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.
Date = "Date" ":" HTTP-date
An example is
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Date: Tue, 15 Nov 1994 08:12:31 GMT
If a message is received via direct connection with the user agent (in
the case of requests) or the origin server (in the case of responses),
then the date can be assumed to be the current date at the receiving
end. However, since the date--as it is believed by the origin--is
important for evaluating cached responses, origin servers SHOULD always
include a Date header. 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 received message which does
not have a Date header field SHOULD be assigned one by the recipient if
the message will be cached by that recipient or gatewayed via a protocol
which requires a Date.
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.
Note: An earlier version of this document incorrectly specified
that this field SHOULD contain the creation date of the enclosed
Entity-Body. This has been changed to reflect actual (and
proper) usage.
Origin servers MUST send a Date field in every response. However, if a
cache receives a response without a Date field, it SHOULD attach one
with the cache's best estimate of the time at which the response was
originally generated.
The format of the Date is an absolute date and time as defined by HTTP-
date in Section 3.3; it MUST be in RFC1123-date format.
10.19 SLUSHY Expires
The Expires entity-header field gives the date/time after which the
entity should be considered stale. A stale cache entry may not normally
be returned by a cache (either a proxy cache or an end-user cache)
unless it is first validated with the origin server (or with an
intermediate cache that has a fresh copy of the resource). 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.
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
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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 13.2.7.)
To mark a response as _never expires,_ an origin server should use
Expires date approximately one year from the time the response is
generated. HTTP/1.1 servers should not send Expires dates more than one
year in the future.
10.20 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
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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 their information
such that the end result is ordered according to the sequence of
forwarding applications.
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
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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.
Note: The Via header field replaces the Forwarded header field
which was present in earlier drafts of this specification.
10.21 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. 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.
10.22 Host
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
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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 ] ; see Section 3.2.2
_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
The header field MUST be included in all HTTP/1.1 request messages A Host
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.
10.23 If-Modified-Since
The If-Modified-Since request-header field is used with the GET method
to make it conditional: if the requested resource has not been modified
since the time specified in this field, a copy of the resource will not
be returned from the server; instead, a 304 (not modified) response will
be returned without any Entity-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 resource 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:
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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 resource has been modified since the If-Modified-Since date,
the response is exactly the same as for a normal GET.
c)
If the resource 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.
Note that the Range request-header field modifies the meaning of
If-Modified-Since; see section 13.9 for full details.
Note that If-Modified-Since is ignored for varying resources.
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 representations 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 request and the If-Modified-Since date of a subsequent
request, and the possibility of clock-skew-related problems if
the If-Modified-Date 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.
10.25 Last-Modified
The Last-Modified entity-header field indicates the date and time at
which the sender believes the resource was last modified. The exact
semantics of this field are defined in terms of how the recipient SHOULD
interpret it: if the recipient has a copy of this resource which is
older than the date given by the Last-Modified field, that copy SHOULD
be considered stale.
Last-Modified = "Last-Modified" ":" HTTP-date
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An example of its use is
Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
The exact meaning of this header field depends on the implementation of
the sender 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.
10.27 Location
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 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
Note: The Content-Location header field (Section 10.16) differs
from Location in that the former 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.
10.29 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
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from the viewpoint of the protocol; however, some systems MAY require
that behavior be consistent with the directives.
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 Pragma = "Pragma" ":" 1#pragma-directive extension-pragma = token [ "=" word ]
has the same semantics as the _no-cache_ cache-directive (see pragma-directive = "no-cache" | extension-pragma
Section 10.8) 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.
10.30 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-Authentication = "Proxy-Authentication" ":" 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 MUST not be passed on to downstream
clients.
10.31 Proxy-Authorization
The Proxy-Authorization request-header field allows the client to
identify itself (or its user) to a proxy which requires authentication.
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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 applies only to the
current connection and MUST not be passed on to upstream servers. If a
request is authenticated and a realm specified, the same credentials
SHOULD be valid for all other requests within this realm.
10.32 Public
The Public response-header field lists the set of non-standard 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 10.5) SHOULD be used to indicate
methods allowed for a particular URI. This does not prevent a client
from trying other methods. The field value SHOULD not include the
methods predefined for HTTP/1.1 in Section 5.1.1.
Public = "Public" ":" 1#method
Example of use:
Public: OPTIONS, MGET, MHEAD
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.
10.33 Range
HTTP retrieval requests using conditional or unconditional GET methods
may request one or more sub-ranges of the entity, instead of the entire
entity. This is done using the Range request header:
Range = "Range" ":" ranges-specifier
A server MAY ignore the Range header. However, HTTP/1.1 origin servers
and intermediate caches SHOULD support byte ranges whenever possible,
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since this supports efficient recovery from partially failed transfers,
and it supports efficient partial retrieval of large entities.
I 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 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-Invalid, or one or
both of If-Unmodified-Since and If-Valid) 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 Range-If header
(see section 10.104) instead of the Range header.
10.34 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. This 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 a partial URI is given, it SHOULD be interpreted relative to the
Request-URI. The URI MUST not include a fragment.
Note: 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.
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10.36 Retry-After
The Retry-After response-header field can be used with a 503 (service
unavailable) response to indicate how long the service is expected to be
unavailable to the requesting client. 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: Wed, 14 Dec 1994 18:22:54 GMT
Retry-After: 120
In the latter example, the delay is 2 minutes.
10.37 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. By convention, 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 add its data to the product list. Instead, it
SHOULD include a Via field (as described in Section 10.20).
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.
10.38 Title
The Title entity-header field indicates the title of the entity
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Title = "Title" ":" *TEXT
An example of the field is
Title: Hypertext Transfer Protocol -- HTTP/1.1
This field is isomorphic with the <TITLE> element in HTML [5].
10.39 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 original resource.
Transfer-Encoding = "Transfer-Encoding" ":" 1#transfer-coding
Transfer codings are defined in Section 3.6. An example is:
Transfer-Encoding: chunked
Many older HTTP/1.0 applications do not understand the Transfer-Encoding
header.
10.41 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,
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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 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 10.8) 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.
10.43 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. Although it is not required, 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.
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User-Agent = "User-Agent" ":" 1*( product | comment )
Example:
User-Agent: CERN-LineMode/2.15 libwww/2.17b3
10.44 WWW-Authenticate
The WWW-Authenticate response-header field MUST be included in 401
(unauthorized) response messages. The field value consists of at least
one 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 challenge header field is provided, since the contents of a challenge
may itself contain a comma-separated list of authentication parameters.
10.45 Max-Forwards
[JG17]The Max-Forwards general-header field may be used with the TRACE
method (Section 8.12) to limit the number of times that a proxy or
gateway 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 response containing the received request
message as the response entity body (as described in Section 8.12). 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).
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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.
10.46 Age
Caches transmit age values using:
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 NOT
transmit an Age header. Otherwise, HTTP/1.1 caches MUST send an Age
header in every response. Caches SHOULD use a representation with at
least 31 bits of range.
10.47 CVal
The CVal header is used to transmit opaque cache validators in HTTP/1.1
responses.
CVal = "CVal" ":" cval-info
cval-info = opaque-validator [ ";" variant-id ]
Examples:
CVal: "xyzzy"
CVal: "xyzzy"/W
CVal: "xyzzy";3
CVal: "xyzzy"/W;3
CVal: ""
Note that the variant-id is not part of the opaque validator.
The CVal field is used to transmit a variant-id simply as a
matter of compact representation of responses.
TBS: does the protocol allow the combination of a null validator and a
variant-ID?
10.48 If-Invalid
The If-Invalid request-header field is used with a method to make it
conditional. A client that has a cache entry for the relevant entity
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supplies the associated validator using the If-Invalid header; if this
validator matches the server's current validator for the entity, the
server SHOULD return a 304 (Not modified) response without any Entity-
Body.
If the validators do not match, the server should treat the request as
if the If-Invalid header was not present.
See section 13.3.3 for rules on how to determine if two validators
match.
If the If-Invalid header is used to make a conditional request on
varying resource, it may be used to pass a set of validators. This is
done using the variant-set mechanism if the client has variant IDs for
the corresponding cache entries (see sections 13.8.3 and 3.16), or the
validator-set mechanism if the client has no variant IDs (see sections
13.8.4 and 3.15).
If-Invalid = "If-Invalid" ":" if-invalid-rhs
if-invalid-rhs = variant-set | validator-set
Examples of single-entity form:
If-Invalid: "xyzzy"
If-Invalid: "xyzzy"/W
Examples of multiple-entity form:
If-Invalid: "xyzzy";4
If-Invalid: "xyzzy";3, "r2d2xxxx";5, "c3piozzzz";7
If-Invalid: "xyzzy"/W;3, "r2d2xxxx"/W;5, "c3piozzzz"/W;7
If-Invalid: "xyzzy", "r2d2xxxx", "c3piozzzz"
If the request would, without the If-Invalid header, result in anything
other than a 2xx status, then the If-Invalid header is ignored.
The purpose of this feature is to allow efficient updates of cached
information with a minimum amount of transaction overhead.
10.49 If-Valid
The If-Valid request-header field is used with a method to make it
conditional. A client that has a cache entry for the relevant entity
supplies the associated validator using the If-Valid header; if this
validator matches the server's current validator for the entity, the
server SHOULD perform the requested operation as if the If-Valid header
were not present.
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If the validators do not match, the server MUST NOT perform the
requested operation, and MUST return a 412 (Precondition failed)
response with no Entity-Body. This behavior is most useful when the
client wants to prevent an updating method, such as PUT or POST, from
modifying a resource whose value has changed since the client last
checked it.
When the If-Valid header is used, the server should use the strong
comparison function (see section 3.13) to compare validators.
If the If-Valid header is used to make a conditional request on varying
resource, it may be used to pass a set of validators. This is done using
the variant-set mechanism if the client has variant IDs for the
corresponding cache entries (see sections 13.8.3 and 3.16), or the
validator-set mechanism if the client has no variant IDs (see sections
13.8.4 and 3.15).
If-Valid = "If-Valid" ":" if-valid-rhs
if-valid-rhs = validator-set | variant-set
An updating request (e.g., a PUT or POST) on a multi-entity resource
should include only one variant-set-item, the one associated with the
particular variant whose value is being conditionally updated.
Examples of single-entity form:
- If-Valid: "xyzzy"
- If-Valid: "xyzzy"/W
Examples of multiple-entity form:
- If-Valid: "xyzzy";4
- If-Valid: "xyzzy";3, "r2d2xxxx";5, "c3piozzzz";7
- If-Valid: "xyzzy", "r2d2xxxx", "c3piozzzz"
- If-Valid: "xyzzy"/W;3, "r2d2xxxx"/W;5, "c3piozzzz"/W;7
If the request would, without the If-Valid header, result in anything
other than a 2xx status, then the If-Valid header is ignored.
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 instance of a resource.
10.50 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
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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 resource has been modified since the specified time,
the server MUST NOT perform the requested operation, and MUST return a
412 (Precondition failed) response with no Entity-Body.
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.
10.51 Warning If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date
Warning headers are sent with responses using:
Warning = "Warning" ":" warn-code SP warn-agent SP warn-text
[SP language-tag [SP charset]]
warn-code = 2DIGIT
warn-agent = ( host [ ":" port ] ) | pseudonym
; the name or pseudonym of the server adding
; the Warning header, for use in debugging
warn-text = quoted-string
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-8599-1.
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. 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.
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This needs clarification. Someplace else in the specification, we need
to make a clear distinction between headers that are stored with a cache
entry and those that aren't, and we have to define carefully what
headers are simply deleted when a cache entry is updated. Section 13.7.3
already talks about combining headers, but doesn't provide a way to
remove, say, a _Response is stale_ Warning after a fresh response is
received.
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 language and character set take
priority over warnings in other languages or character sets but
with identical warn-codes and warn-agents.
. TBS
This is a list of the currently-defined warn-codes, each with a
recommended warn-text in English, and a description of its meaning.
10 _Response is stale_
MUST be included whenever the returned response is stale. A cache may
add this warning to any response, but may never remove it until the
response is known to be fresh.
11 _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. A cache may add this warning to any response, but
may never remove it until the response is successfully revalidated.
13 _Disconnected operation_
SHOULD be included if the cache is intentionally disconnected from
the rest of the network for a period of time.
99 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.
TBS XXX anything else?
10.52 Vary
The Vary response-header field is used by an origin server to signal
that the resource identified by the current request is a varying
resource. A varying resource has multiple entities associated with it,
all of which are representations of the content of the resource. If a
GET or HEAD request on a varying resource is received, the origin server
will select one of the associated entities as the entity best matching
the request. Selection of this entity is based on the contents of
particular header fields in the request message, or on other information
pertaining to the request, like the network address of the sending
client.
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If a resource is varying, this has an important effect on cache
management, particularly for caching proxies which service a diverse set
of user agents. All 200 (OK) responses from varying resources MUST
contain at least one Vary header or Alternates header (Section 10.53) to
signal variance.
If no Vary headers and no Alternates headers are present in a 200 (OK)
response, then caches may assume, as long as the response is fresh, that
the resource in question is not varying, and has only one associated
entity. Note however that this entity can still change through time, as
possibly indicated by a Cache-Control response header (section 10.cc).
After selection of the entity best matching the current request, the
origin server will usually generate a 200 (OK) response, but it can also
generate other responses like 206 (Partial Content) or 304 (Not
modified) if headers which modify the semantics of the request, like
Range (Section 10.ran) or If-Valid (Section 10.ifva), are present. An
origin server need not be capable of selecting an entity for every
possible incoming request on a varying resource; it can choose to
generate a 3xx (redirection) or 4xx (client error) type response for
some requests.
In a request message on a varying resource, the selecting request
headers are those request headers whose contents were used by the origin
server to select the entity best matching the request. The Vary header
field specifies the selecting request headers and any other selection
parameters that were used by the origin server.
Vary = "Vary" ":" 1#selection-parameter
selection-parameter = request-header-name
| "{accept-headers}"
| "{other}"
| "{" extension-parameter "}"
request-header-name = field-name
extension-parameter = token
The presence of a request-header-name signals that the request-header
field with this name is selecting. Note that the name need not belong
to a request-header field defined in this specification, and that header
names are case-insensitive. The presence of the _{accept-headers}_
parameter signals that all request headers whose names start with
_accept_ are selecting.
The inclusion of the _{other}_ parameter in a Vary field signals that
parameters other than the contents of request headers, for example the
network address of the sending party, play a role in the selection of
the response.
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Note: This specification allows the origin server to express
that other parameters were used, but does not allow the origin
server to specify the exact nature of these parameters. This is
left to future extensions.
If an extension-parameter unknown to the cache is present in a Vary
header, the cache MUST treat it as the _{other}_ parameter. If multiple
Vary and Alternates header fields are present in a response, these MUST
be combined to give all selecting parameters.
The field name _Host_ MUST never be included into a Vary header; clients
MUST ignore it if it is present. The names of fields which change the
semantics of a GET request, like _Range_ and _If-Valid_ MUST also never
be included, and MUST be ignored when present.
Servers which use access authentication are not obliged to send _Vary:
Authorization_ headers in responses. It MUST be assumed that requests
on authenticated resources can always produce different responses for
different users. Note that servers can signal the absence of
authentication by including a _Cache-Control: public_ header in the
response.
A cache MAY store and refresh 200 (OK) responses from a varying resource
according to the rules in Section 13.7.2. The partial entities in 206
(Partial Content) responses from varying resources MAY also be used by
the cache.
When getting a request on a varying resource, a cache can only return a
cached 200 (OK) response to one of its clients in two particular cases.
First, if a cache gets a request on a varying resource for which it has
cached one or more responses with Vary or Alternates headers, it can
relay that request towards the origin server, adding an If-Invalid
header listing the cval-info values in the CVal headers (Section 10.47)
of the cached responses. If it then gets back a 304 (Not Modified)
response with the cval-info of a cached 200 (OK) response in its CVal
header, it can return this cached 200 (OK) response to its client, after
merging in any of the 304 response headers as specified in Section
13.7.2.
Second, if a cache gets a request on a varying resource, it can return
to its client a cached, fresh 200 (OK) response which has Vary or
Alternates headers, provided that
. the Vary and Alternates headers of this fresh response specify that
only request header fields are selecting parameters,
. the specified selecting request header fields of the current
request match the specified selecting request header fields of a
previous request on the resource relayed towards the origin server,
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. this previous request got a 200 (OK) or 304 (Not Modified) response
which had the same cval-info value in its CVal header as the
cached, fresh 200 (OK) response.
Two sequences of selecting request header fields match if and only if
the first sequence can be transformed into the second sequence by only
adding or removing whitespace at places in fields where this is allowed
according to the syntax rules in this specification.
If a cached 200 (OK) response MAY be returned to a request on a varying
resource which includes a Range request header, then a cache MAY also
use this 200 (OK) response to construct and return a 206 (Partial
Content) response with the requested range.
Note: Implementation of support for the second case above is
mainly interesting in user agent caches, as a user agent cache
will generally have an easy way of determining whether the
sequence of request header fields of the current request equals
the sequence sent in an earlier request on the same resource.
Proxy caches supporting the second case would have to record
diverse sequences of request header fields previously relayed;
the implementation effort associated with this may not be
balanced by a sufficient payoff in traffic savings. A planned
specification of a content negotiation mechanism will define
additional cases in which proxy caches can return a cached 200
(OK) response without contacting the origin server. The
implementation effort associated with support for these
additional cases is expected to have a much better cost/benefit
ratio.
10.53 Alternates
The Alternates response-header field is used by origin servers to signal
that the resource identified by the current request has the capability
to send different responses depending on the accept headers in the
request message. This has an important effect on cache management,
particularly for caching proxies which service a diverse set of user
agents. This effect is covered in Section 10.v.
Alternates = "Alternates" ":" opaque-field
opaque-field = field-value
The Alternates header is included into HTTP/1.1 to make HTTP/1.1 caches
compatible with a planned content negotiation mechanism. HTTP/1.1
allows a future content negotiation standard to define the format of the
Alternates header field-value, as long as the defined format satisfies
the general rules in Section 4.2.
To ensure compatibility with future experimental or standardized
software, caching HTTP/1.1 clients MUST treat all Alternates headers in
a response as synonymous to the following Vary header:
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Vary: {accept-headers}
and follow the caching rules associated with the presence of this Vary
header, as covered in Section 10.v. HTTP/1.1 allows origin servers to
send Alternates headers under experimental conditions.
10.54 SLUSHY: Accept-Ranges
In some cases, a client may want to know if the server accepts range
requests using a certain range unit. The server may indicate its
acceptance of range requests for a resource by providing this header in
a response for that resource:
Accept-Ranges = "Accept-Ranges" ":" acceptable-ranges
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 specific resource involved,
but the server MAY ignore such requests.
Should this say that the server SHOULD send "Accept-Ranges:
bytes", or is MAY good enough
Origin servers that do not accept any kind of range request for a
specific resource MAY send
Accept-Ranges: none
to advise the client not to attempt a range request.
We're still not quite sure why this header is in the protocol.
We gather that Netscape uses it for something, but nobody from
Netscape has even tried to explain to me whether it is necessary
for anything. The only thing we can think of is that a client
would have to know in advance if a server accepted partial-
content PUTs (i.e., PUT+Content-Range), but we don't see any
indication that this is what Netscape wants.
10.55 SLUSHY: Range-If
If a client has a partial copy of an entity in its cache, and wished to
have an up-to-date copy of the entire entity in its cache, it could use
Range request header with a conditional GET (using either of both of If-
Unmodified-Since and If-Valid.) 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.
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The Range-If 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.''
Range-If: if-valid-rhs
The Range-If 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 validator given in the Range-If header matches the current
validator for the entity, then the server should provide the specified
sub-range of the entity using a 206 (Partial content) response. If the
validator does not match, then the server should return the entire
entity using a 200 (OK) response.
This description may need slight modification to deal with(1)
the use of a last-modified date as a validator (but this |can
perhaps be hidden in the definition of if-valid-rhs), and|(2)
its application to multi-entity resources.
11. Access Authentication
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.
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 of the server
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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 a server--usually,
but not necessarily, after receiving a 401 or 411 response--MAY do so by
including an Authorization 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.
credentials = basic-credentials
| auth-scheme *("," auth-param )
The domain over which credentials can be automatically applied by a user
agent 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 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 the (possibly
new) challenge applicable to the requested resource and an entity
explaining the refusal.
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. That is, they MUST forward the WWW-Authenticate and
Authorization headers untouched, and MUST not cache the response to a
request containing Authorization.
HTTP/1.1 allows a client to pass authentication information to and from
a proxy via the Proxy-Authenticate and Proxy-Authorization headers.
11.1 Basic Authentication Scheme
The _basic_ authentication scheme is based on the model that the user
agent 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
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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 server SHOULD 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.
To receive authorization, the client sends the user-ID and password,
separated by a single colon (_:_) character, within a base64 [7] encoded
string in the credentials.
basic-credentials = "Basic" SP basic-cookie
basic-cookie = <base64 [7] encoding of userid-password,
except not limited to 76 char/line>
userid-password = [ token ] ":" *TEXT
If the user agent wishes to send the user-ID _Aladdin_ and password
_open sesame_, it would use the following header field:
Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==
The basic authentication scheme is a non-secure method of filtering
unauthorized access to resources on an HTTP server. It is based on the
assumption that the connection between the client and the server can be
regarded as a trusted carrier. As this is not generally true on an open
network, the basic authentication scheme should be used accordingly. In
spite of this, clients SHOULD implement the scheme in order to
communicate with servers that use it.
11.2 Digest Authentication Scheme
The _digest_ authentication scheme is [currently described in an expired
Internet-Draft, and this description will have to be improved to
reference a new draft or include the old one].
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12. Content Negotiation
A varying resource has multiple entities associated with it, all of
which are representations of the content of the resource. Content
negotiation is the process of selecting the best representation when a
GET or HEAD request is made on the varying resource. HTTP/1.1 has
provisions for two kinds of content negotiation: opaque negotiation and
transparent negotiation.
With opaque negotiation, the selection of the best representation is
done by an algorithm located at the origin server, and unknown to the
proxies and user agents involved. Selection is based on the contents of
particular header fields in the request message, or on other information
pertaining to the request, like the network address of the sending
client. A typical example of opaque negotiation would be the selection
of a text/html response in a particular language based on the contents
of the Accept-Language request header field. A disadvantage of opaque
negotiation is that the request headers may not always contain enough
information to allow for selection. If the Accept header
Accept: text/*: q=0.3, text/html, */*: q=0.5
is sent in a request on a varying resource which has a video/mpeg and a
video/quicktime representation, the selection algorithm in the origin
server will either have to make a default choice, or return an error
response which allows the user to decide on further actions.
With transparent negotiation, the selection of the best representation
is done by a distributed algorithm which can perform computation steps
in the origin server, in proxies, or in the user agent. Transparent
negotiation guarantees that, if the user agent supports the transparent
negotiation algorithm and is correctly configured, the request will
always correctly yield either the video/mpeg representation, the
video/quicktime representation, or an error message indicating that the
resource cannot be displayed by the user agent.
12.1 Negotiation facilities defined in this specification
This specification defines all protocol facilities for opaque
negotiation, but does not define the distributed algorithm for
transparent negotiation. This specification only defines the basic
facilities (Vary, Alternates, Accept) in the core protocol allowing
requests on transparently negotiated resources to be correctly handled
by HTTP/1.1 caches. All other information about transparent content
negotiation is found in a separate document[29].
If a varying resource is opaquely negotiated, successful responses to
requests on the resource will always include a Vary header. If a
varying resource is transparently negotiated, successful responses to
requests on the resource will always include an Alternates header. If a
successful response contains an Alternates header, it will also always
contain a Content-Location header. A future specification may allow a
combination of opaque and transparent negotiation that would lead to the
inclusion of both a Vary header and an Alternates header in a response.
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.
13 Caching in HTTP
The World Wide Web is a distributed system, and so its performance can
be improved by the use of caches. These caches are typically placed at
proxies and in the clients themselves. 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.
13.1 Semantic Transparency
Ideally, an HTTP/1.1 cache would be _semantically transparent._ That is,
use of the cache would not affect either the clients or the servers in
any way except to improve performance. When a client makes a request via
a semantically transparent cache, it receives exactly the same entity
headers and entity body it would have received if it had made the same
request to the origin server, at the same time.
In the real world, requirements for performance, availability, and
disconnected operation require us to relax the goal of semantic
transparency in many cases. 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 those
for ordering merchandise), the protocol requires that transparency may
not be relaxed
. without an explicit protocol-level request (when relaxed by client
or origin server)
. without a means for warning 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 desired by all parties.
2. Protocol features that allow an origin server or end-user client to
explicitly request and control non-transparent operation.
3. Protocol features that allow a cache to attach warnings to
responses that do not preserve semantic transparency.
A basic principle is that it must be possible for the clients to detect
any potential breakdown of semantic transparency.
Caching would be useless if it did not significantly improve performance
in many cases. 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).
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The server, cache, or client implementer may be faced with design
decisions not explicitly discussed in this specification. If 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.
A note on terminology: we say that a resource is _cachable_ if a cache
is allowed to store a copy of this resource, when it arrives in a
response message, and then later use that copy to respond to a
subsequent request. Even if a resource is cachable, there may be
additional constraints on when and whether a cache can use a cached copy
of it.
13.2 Expiration Model
In order to describe the associated mechanisms, we introduce several
terms for describing responses returned by a cache in response to a
client's request:
firsthand
A response is firsthand if it comes directly and without unnecessary
delay from the origin server, perhaps via one or more proxies. A
response is also firsthand 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 generated 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 reached its freshness
lifetime.
stale
A response is stale if its age has passed its freshness lifetime.
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
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reached. This normally preserves semantic transparency, as long as the
server's expiration times are carefully chosen.
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 response is always stale, and so the cache
SHOULD validate it before using it for subsequent requests. (Note that a
firsthand response MUST always be returned to the requesting client,
independent of its expiration time, unless the connection to the client
is lost.)
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 10.8).
Servers specify explicit expiration times using either the Expires
header, or the max-age directive of the Cache-Control header.
13.2.2 Limitations on the Effect of Expiration Times
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.
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. 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.
13.2.3 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.
13.2.4 Client-controlled Behavior
While the origin server (and to a lesser extent, intermediate caches)
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
for an unvalidated response; specifying a value of zero forces the
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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 decrease
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 semantic transparency, but may be necessary
to support disconnected operation, or high availability in the face of
poor connectivity.
13.2.5 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
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 to take steps to obtain a firsthand or fresh
response, if the user so desires. For this reason, a cache MUST NOT
return a stale response if the client explicitly requests a first-hand
or fresh one, unless it is impossible to comply.
13.2.6 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 13.2.7; 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._ All HTTP
implementations, but especially origin servers and caches, should use
NTP [RFC1305] 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 denote a representation of
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
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server, plus the amount of time it has been in transit along network
paths.
We use the term _age_value_ to denote a representation of the value of
the Age header, in a form appropriate for arithmetic operations.
An response's age can be calculated in two entirely independent ways:
1. now - date_value, if the local clock is reasonably well
synchronized to the origin server's clock. If the result is
negative, this 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
and as long as we have either nearly synchronized clocks or all-HTTP/1.1 corrected_received_age = max(now - date_value, age_value)
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 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:
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/*
* 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, now - date_value);
corrected_received_age = max(apparent_age, age_value);
response_delay = now - 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
Age header, to the next recipient
cache.
13.2.7 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 13.2.6; this section describes how to calculate the
freshness lifetime, and to determine if a response has expired.
_ transmit this total age, using the We use the term expires_value_ to denote a representation of the value
of the Expires header, in a form appropriate for arithmetic operations.
We use the term _max_age_value_ to denote an appropriate representation
of the number of seconds carried by the max-age directive of the Cache-
Control header in a response (see section 10.8).
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:
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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 appears in the response,
and the response does not include other restrictions on caching, the
cache MAY compute a freshness lifetime using a heuristic. This heuristic
is subject to certain limitations; the minimum value may be zero, and
the maximum value MUST be no more than 24 hours.
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)
13.2.8 UT Mandatory
All expiration-related calculations must be done in Universal Time
(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.
13.3 Validation Model
When a cache has a stale value 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 value 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 value is good, and we do not want to pay the
overhead of an extra round trip if the cached value 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 (end-user or 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 resource, and if they match, it responds with a special status code
(usually, _304 Not Modified_) and no entity body. Otherwise, it returns
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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 the validators match, or if and only
if the validators do not match.
Note: a response that lacks a cache 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 retrieval if it does not have a
cache validator for the entity, which means it will not be
refreshable after it expires.
13.3.1 Last-modified Dates
In HTTP/1.0, the only cache validator is the Last-Modified time carried
by a response. Clients validate entities using the If-Modified-Since
header. In simple terms, a cache entry is considered to be valid if the
actual resource has not been modified since the original response was
generated.
13.3.2 Opaque Validators
HTTP/1.1 introduces the possibility of using an _opaque_ validator, for
situations where the Last-Modified date is not appropriate. This may
include server implementations where it is not convenient to store
modification dates, or where the one-second resolution of HTTP date
values is insufficient, or where the origin server wishes to avoid
certain paradoxes that may arise from the use of modification dates.
An opaque validator is simply a string of octets whose internal
structure is not known to clients or caches. Caches store opaque
validators and return them when making conditional requests. Also, when
a cache receives a conditional request for a resource for which it has a
fresh cache entry, it may compare opaque validators using strict octet-
equality. Otherwise, opaque validators have no semantic value to clients
or caches.
To preserve compatibility with HTTP/1.0 clients and caches, and because
the Last-Modified date may be useful for purposes other than cache
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validation, HTTP/1.1 servers SHOULD send Last-Modified whenever
feasible.
The headers used to convey opaque validators are described in sections
10.47, 10.48, 10.49, and 10.55.
13.3.3 Weak and Strong Validators
Since both origin servers and caches will compare two validator values
to decide if they represent the same or different values for the entire
resource, one normally would expect that if the resource value (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 resource change. A validator that does not
always change when the resource changes is a _weak validator._
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 instance of an entity,
while a weak validator is part of an identifier for a set of
semantically equivalent instances of an entity.
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.
Opaque validators are normally _strong,_ but the protocol provides a
mechanism to tag an opaque validator as _weak._
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 body.
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:
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. 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 should be used for simple (non-subrange)
GET requests. The strong comparison function must be used in all other
cases.
An opaque validator is strong unless it is explicitly tagged as weak.
Section 3.13 gives the syntax for opaque validators.
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,
. 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 include a Date value, which gives the time when
the origin server generated 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 include a Date value, which gives the time when
the origin server generated 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
generated 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.
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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.
This allows HTTP/1.1 caches and clients to safely perform sub-range
retrievals on values that have been obtained from HTTP/1.0 servers.
13.3.4 Rules for When to Use Opaque Validators 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 a strong opaque validator unless performance
considerations support the use of weak opaque validators, or unless
it is unfeasible to send a strong opaque validator.
. MAY send a weak opaque validator instead of a strong one.
. MAY send no opaque validator if it is infeasible to generate one.
. 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 opaque validator and a Last-Modified value.
In order to be legal, a strong opaque validator MUST change whenever the
associated entity value changes in any way. A weak opaque validator
SHOULD change whenever the associated entity value changes in a
semantically significant way.
Note: in order to provide semantically transparent caching, an
origin server should avoid reusing a specific strong opaque
validator value for two different instances of an entity, or
reusing a specific weak opaque validator value for two
semantically different instances of an entity. Caches 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 opaque validator has been provided by the origin server, MUST
use that validator in any cache-conditional request (using If-Valid
or If-Invalid).
. 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 opaque validator 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 opaque validator 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 opaque validators.
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 SLUSHY: Non-validating conditionals
TBS
The principle behind opaque validators 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.
13.3.6 FLUID: Other Issues
TBS: what if no validator present in response?
13.4 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,
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Cache-Control directives are explicitly specified as weakening semantic
transparency (for example, _max-stale_ or _public_).
The Cache-Control directives are described in detail in section 10.7.
13.5 Warnings
Whenever a cache returns a response that is not semantically
transparent, it must attach a warning to that effect, using a Warning
response header. This warning allows clients and user agents 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 are always cachable, because they never weaken the transparency
of a response. This means that warnings can be passed to HTTP/1.0 caches
without danger; such caches will simply pass the warning along as a
entity header in the response.
Warnings are assigned numbers between 0 and 99. This specification
defines the code numbers and meanings of each warning, allowing a client
or cache to take automated action in some (but not all) cases.
Warnings also carry a warning message text in any appropriate natural
language (perhaps based on the client's Accept headers), and an optional
indication of what language and character set are used.
Multiple warning messages 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 10.106.
13.6 Explicit Indications Regarding User-specified Overrides
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_ or _Cache-Control: reload_ to every request. We recognize
that there may be situations which require such overrides, although user
agents SHOULD NOT default to any behavior contrary to the HTTP/1.1
specification. That is, the user should have to explicitly request
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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 resource 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.
13.7 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.
13.7.1 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:
. 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
. Upgrade
. Public
. Proxy-Authenticate
. Transfer-Encoding
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 10.9.
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13.7.2 Non-modifiable Headers
Some features of the HTTP/1.1 protocol, such as Digest Authentication
(see TBS), 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-Type
. Content-Encoding
. Content-Length
. Expires
. Last-Modified
. Content-Range
. Content-Location
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.
13.7.3 Combining Headers
When a cache makes a validating request to a server, and the server
provides a 304 Not Modified response, the cache must construct a
response to send to the requesting client. The cache uses the entity-
body stored in the cache entry as the entity-body of this outgoing
response. It uses the end-to-end headers from the incoming response, not
from the cache entry. Unless it decides to remove the cache entry, it
must also replace the end-to-end headers stored with the cache entry
with those received in the incoming response.
In other words, the complete set of end-to-end headers received in the
incoming response overrides all end-to-end headers stored with the cache
entry. The cache may add Warning headers (see section 10.106) to this
set.
A cache MUST preserve the order of all headers as received in an
incoming response.
These rule allows an origin server to completely control the response
seen by the client of a cache when the cache revalidates an entry, and
may be necessary for preserving semantic transparency or for certain
kinds of security mechanisms or future extensions.
13.7.4 Combining Byte Ranges
A response may transfer only a subrange of the bytes of an entity,
either because the request included one or more Range specifications, or
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because a connection was broken prematurely. After several such
transfers, a cache may have received several ranges of the same entity.
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 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.
13.7.5 SLUSHY: Scope of Expiration
HTTP/1.1's expiration model is that as soon as any variant of a URI
becomes stale, all variants becomes stale as well. Thus, _freshness_
applies to all the variants of URI, rather than any particular variant.
Dates and expires etc. apply to any cached variant that a proxy might
have with a URI and not just the one particular entity.
13.8 Caching and Content Negotiation
The HTTP content negotiation mechanism interacts with caching in several
ways:
. A varying resource (one subject to content negotiation) may be
bound to more than one entity. Each of these entities is called a
_variant_ of the resource.
. The request-URI may be only one part of the cache key.
13.8.1 Use of the Vary header
Origin servers may respond to requests for varying resources use the
Vary header (see section 10.vary for a full description) to inform the
cache which header fields of the request were used to select the variant
returned in the response. A cache can use that response to reply to a
subsequent request only if the two requests not only specify the same
URI, but also have the same value for all headers specified in the Vary
response-header.
The Vary header may also inform the cache that the variant was selected
using criteria not limited to the request headers; in this case, the
response MUST NOT be used in a reply to a subsequent request except if
the cache relays the new request to the origin server in a conditional
request, and the origin server responds with 304 (Not Modified) and
includes the same variant-ID (see 13.8.3).
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13.8.2 SLUSHY: Use of the Alternates header
Origin servers may respond to requests for varying resources with a
status of 300 (Multiple choice), using the Alternates header (see
section 10.alternates) to inform the requesting client that describes
the set of possible choices, including specific URIs for each variant.
Roy says this response also includes a Content-Location header.
In this case, the client may choose one of the available variants and
make a subsequent request using the specific URI for that variant. Since
such an URI is bound to just one entity, the origin server's response to
this request includes neither a Vary header nor an Alternates header,
and a cache may treat it as it would any non-varying resource.
If a cache receives an Alternates header in a response from the origin
server, it should act as if the response carried a "Vary:{accept-
headers}" header. This means that the response may be returned in reply
to a subsequent request with Accept-* headers identical to those in the
current request.
Note that section 13.14.1 prevents caching of 300 (Multiple
choices) responses unless this is explicitly allowed by an
Expires or Cache-control header.
13.8.3 Use of Variant-IDs
A cache stores copies of specific entity instances, not copies of
varying resources per se. Therefore, the URI of a varying resource is
not sufficient for use as a cache key. In certain interactions between a
cache and an origin server, it is convenient to encode the cache key
using a more compact representation than the full set of selecting
request headers. Or, if the selection criteria are not known to the
cache, it may be impossible to express the actual cache key to the
cache. For these reasons, the HTTP protocol provides two different
optional mechanisms to encode a cache key:
. Variant-IDs: an opaque identifier for a specific variant of a
varying resource.
. Selecting opaque validators: a special kind of opaque validator
that is defined to be unique across all variants of a varying
resource.
Variant-IDs are the preferred mechanism, since they generally allow more
efficient management of caches.
If an origin server chooses to use the variant-ID mechanism, it assigns
a variant-ID (see section 3.14) to each distinct variant. This
assignment can only be done by the origin server. It then returns the
appropriate variant-ID with each response that applies to a specific
variant, using the CVal header (see 10.47).
If an origin server provides a variant-ID for any variant of a resource,
it SHOULD provide a variant-ID for all variants of that resource.
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When a cache receives a successful response with a variant-ID, it SHOULD
use this information to replace any existing cache entries for the same
variant of the corresponding URI. That is, it forms a cache key using
the URI of the request and the variant-ID of the response. If this key
matches the key of an existing cache entry, it SHOULD replace the
existing entry with the new response (subject to all of the other rules
on caching). See section 13.12 for more details on update.
When a cache performs a conditional request on a varying resource, and
it has one or more cache entries for the resource that include variant-
IDs, the cache MUST transmit the (cache-validator, variant-ID) tuples in
the conditional request, using the variant-set mechanism (see section
3.16). This tells the server which variants are currently in the
requester's cache.
The client MAY choose to transmit only a subset of the (cache-
validator, variant-ID) tuples corresponding to its cache entries
for this resource.
When a server receives a conditional request that includes a variant-
set, and the server is able to reply with an appropriate variant (either
because it is the origin server, or because it is an intermediate cache
that can properly implement the variant selection algorithm), once it
has selected the variant it should examine the elements of the supplied
variant-set. If one of these matches the variant-ID of the selected
variant, and if the cache validators match, the server SHOULD reply with
a 304 (Not Modified) response, including the variant-ID of the selected
variant. Otherwise, the server should reply as if the request were
unconditional.
The server may optionally use the variant-set information in its
selection algorithm. For example, if the selection algorithm yields
several variants with equal preference, and one of these is already in
the requester's cache, the server could select that variant and avoid an
extra data transfer. This is a performance optimization; otherwise, the
variant-selection mechanism is orthogonal to the variant-ID mechanism.
13.8.4 Use of Selecting Opaque Validators
If the origin server prefers not to provide variant-IDs, it MAY at its
option use the _selecting opaque validator_ mechanism. A selecting
opaque validator is an opaque validator whose value is unique across all
variants of a resource.
If the origin server cannot generate opaque validators that are
guaranteed to be unique across all variants of a varying resource, it
MUST NOT send any opaque validators for that resource.
When a cache receives a successful response with an opaque validator and
no variant-ID, it MAY either replace any cache entries for the resource
with the new response, or it may keep multiple such entries. See
section 13.12 for more details on update.
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When a cache performs a conditional request on a varying resource, and
it has one or more cache entries for the resource that include opaque
validators, the cache SHOULD transmit the set of opaque validators in
the conditional request, using the validator-set mechanism (see section
3.15). This tells the server which variants are currently in the
requester's cache.
The client MAY chose to transmit only a subset of the opaque validators
from its cache entries for this resource.
When a server receives a conditional request that includes a validator-
set, and the server is able to reply with an appropriate variant (either
because it is the origin server, or because it is an intermediate cache
that can properly implement the variant selection algorithm), once it
has selected the variant it should examine the elements of the supplied
validator-set. If one of these matches the cache validator of the
selected variant, the server SHOULD reply with a 304 (Not Modified)
response, including that cache validator. Otherwise, the server should
reply as if the request were unconditional.
13.10 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.
13.11 SLUSHY: Miscellaneous Considerations
This section is somewhat miscellaneous, and its contents might be
shifted to other locations in the document.
13.11.1 Detecting Firsthand Responses
Note that a client can usually tell if a response is firsthand by
comparing the Date to its local request-time, and hoping that the clocks
are not badly skewed.
13.11.2 Disambiguating Expiration values
Because expiration values are assigned optimistically, it is possible
that two caches may contain fresh values for the same resource that are
different.
If a client performing a retrieval receives a non-firsthand response for
a resource 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 10.8), to force
a check with the origin server.
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If a cache that is pooling cached responses from other caches sees two
fresh responses for the same resource with different validators, it
SHOULD use the one with the newer Date header.
13.11.3 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 generated them. We
would like the client to use the most recently generated response, even
if older responses are still apparently fresh.
Neither the opaque validator 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.
If a client performs a request for a resource that it already has in its
cache, and the response it receives contains a Date header that appears
to be older than the one it already has in its cache, 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. This prevents certain paradoxes arising from the use of multiple
caches.
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.
13.12 SLUSHY: Cache Keys
A _cache key_ is a value used to identify a cache entry. HTTP caches
three different kinds of cache keys, for use in different contexts:
. Some subset of the fields stored with a cache entry constitute the
_entry key_ for that entry. These may include the Request-URI,
some request-header fields, and some response-header fields.
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. Some subset of the fields of a response, together with perhaps the
Request-URI, constitute the _update key_ of a response.
. Some subset of the fields of a request, together with the Request-
URI, constitute the _lookup key_ of a request.
When a cache receives a request, it builds a lookup key from that
request, then tries to find (lookup) a cache entry with a matching entry
key according to the key matching procedure in section 13.12.3. If such
a match exists, then the cache can decide (based on the other caching
rules) whether to return that entry in reply to the request.
When a cache receives a response, it builds a update key from that
response, and from the request that elicited it. It uses this key to
find any previously stored entry with a matching entry key. If such an
entry exists, the cache replaces the old entry with the new one.
The term _update_ means to remove the old entry from the cache,
and then to insert the new entry. It does not imply a
modification of an existing entry.
This section describes specifically how the three kinds of keys are
constructed, and how a cache determines if keys match.
13.12.1 Non-varying Resources
When a response is received for a non-varying resource (that is, the
response includes no Vary, Alternates, or Content-Location headers), the
update key for the response is simply the Request-URI of the request
that elicited it: (Request-URI, null). The entry key for the response
is (Request-URI, null, null).
13.12.2 SLUSHY: Varying Resources
If a response includes a Vary header, then we use the notation _sel-hdr-
values_ to denote the canonical form of the headers in the corresponding
request whose field-names are given in the Vary header. If the response
does not include a Vary header, then sel-hdr-values is assigned the null
value. Section 10.52 on Vary defines the canonical form for selecting
headers.
The canonical form of the headers is defined to be a set whose elements
are sequences of request headers with identical field-names. For a
given field-name, the corresponding element is the concatenation of the
request headers with that field-name, in exactly the order that these
fields appear in the request
If the response contains "Vary: {other}", then sel-hdr-values is
assigned a non-null value that is defined as never matching a set of
request headers.
When a response is received that includes a variant-ID in a CVal header
(see section 10.102), but no Content-Location header, then the update
key is (Request-URI, variant-ID), and the entry key for the response is
(Request-URI, variant-ID, sel-hdr-values).
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When a response is received that includes a Vary header and an opaque
validator, but no variant-ID or Content-Location header, then the update
key is (Request-URI, opaque-validator), and the entry key for the
response is (Request-URI, opaque-validator, sel-hdr-values).
This rule supports the _selecting opaque validators_ mechanism described
in section 13.8.4. The cache should distinguish between actual variant-
IDs and opaque-validators in the variant-ID element of the entry key; a
non-null opaque-validator in an entry key DOES match a null variant-ID
in a lookup key.
When a response is received that includes both a variant-ID in a CVal
header, and a Content-Location header, then the update key is (content-
location-URI, variant-ID), and the entry key for the response is
(content-location-URI, variant-ID, sel-hdr-values).
When a response is received that includes a Content-Location header but
no variant-ID, then the update key is (content-location-URI, null), and
the entry key for the response is (content-location-URI, null, sel-hdr-
values).
13.12.3 SLUSHY: Key-Matching Procedure
We express entry keys as the tuple (URI, variant-ID, sel-hdr-values), in
which the variant-ID may be null, and the sel-hdr-values may either be
null, or may be a set of request headers.
We express update keys as a tuple (URI, variant-ID), in which the
variant-ID may be null. A update key matches an entry key if both their
URI elements match and their variant-ID elements match. (A null
variant-ID does not match a non-null variant-ID.)
We express lookup keys as a tuple (URI, variant-ID, all-request-
headers), in which the variant-ID may be null. The all-request-headers
element of the tuple is not always used, but is included here as a
notational convenience. A lookup key matches an entry key if both their
URI elements match and their variant-ID elements match, and either
. the sel-hdr-values element of the entry key is null
or
. the sel-hdr-values element of the entry key matches the appropriate
headers in the all-request-headers element of the lookup key,
according to the matching rules in section on Vary, section 10.52.
This description matching algorithm is clearly not the most efficient
implementation of an equivalent algorithm. A cache may use any
algorithm that yields equivalent results. For example, it may use a
hierarchical approach where cache entries are grouped into sets by the
URI and variant-ID, and only if a set includes non-null sel-hdr-values
elements does the cache need to consider the other request headers.
If on a cache lookup there are two or more fresh entries that appear to
match the request, then the one with the most recent Date value MUST be
used.
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13.12.4 Canonicalization of URIs
A cache, when comparing two URIs to decide if they match or not, a cache
MUST use a case-sensitive octet-by-octet comparison of the entire URIs,
with these exceptions:
Following the rules from section 3.2.2:
. A port that is empty or not given is equivalent to port 80.
. 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 except those in the reserved set and the unsafe set (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
13.13 FLUID: Cache-Related Problems Not Addressed in HTTP/1.1
TBS
This section will list a few problems that are NOT addressed in
HTTP/1.1, with the intention of encouraging implementers not to adopt
proprietary solutions inconsistent with possible future protocol
revisions..
. Server-driven invalidation
. Demographics
13.14 Cache Operation When Receiving Errors or Incomplete Responses
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 13.7.4; 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).
A cache that receives a response with a zero-length Entity-body and no
explicit indication that the correct length is zero (such as _Content-
Length: 0_) MUST NOT not store the response. The same rule applies to a
response of any length received without an explicit length indication if
the transport connection was terminated in any unusual way.
If a cache receives a response carrying Retry-After header (see section
10.36), 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, although it MUST follow all of
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the rules applying to stale responses. In particular, it MUST NOT
override the _must-revalidate_ Cache-Control directive (see section
10.7).
13.14.1 Caching and Status Codes
A response received with a status code of 200 or 206 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.
A response received with any other status code MUST not be returned in a
reply to a subsequent request unless it carries at least one of the
following:
. an Expires header
. a max-age Cache-control directive
. a must-revalidate Cache-control directive
. a public Cache-control directive
13.14.2 Handling of Retry-After
If a cache receives a response carrying a Retry-After header (see
section 10.36), 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, although it MUST follow
all of the rules applying to stale responses. In particular, it MUST
not override the _must-revalidate_ Cache-control directive (see section
10.7).
13.15 FLUID: Compatibility With Earlier Versions of HTTP
TBS
If anything should be here, it should be a collection of warnings about
what HTTP/1.1 systems should not assume about HTTP/1.0 systems.
13.16 SLUSHY: 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.
We note one exception to this rule: since some applications have
traditionally used GETs and HEADs with query URLs (those containing a
_?_ in the rel_path part) to perform operations with significant side
effects, caches MUST NOT treat responses to such URLs as fresh unless
the server provides an explicit expiration time.
This specifically means that responses from HTTP/1.0 servers for such
URIs should not be taken from a cache.
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See section 15.2 for related information.
13.17 SLUSHY: 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 invalidate a single entity. This is either the entity
referred to by the Request-URI, or by the Location or Content-Location
response 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.
13.18 Write-Through Mandatory
All methods that may be expected to cause modifications to the origin
server's resources MUST be written through to the 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.
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.
13.19 Interoperability of Varying Resources with HTTP/1.0 Proxy Caches
If the correct handling of responses from a varying resource (Section
10.xxx) by HTTP/1.0 proxy caches in the response chain is important,
HTTP/1.1 origin servers can include the following Expires (Section
10.exp) response header in all responses from the varying resource:
Expires: Thu, 01 Jan 1980 00:00:00 GMT
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If this Expires header is included, the server should usually also
include a Cache-Control header for the benefit of HTTP/1.1 caches, for
example
which overrides the freshness lifetime of zero seconds specified by the
included Cache-Control: max-age=604800 Expires header.
13.20 Cache Replacement for Varying Resources
If a new 200 (OK) response is received from a non-varying resource while
an old 200 (OK) response is cached, caches can delete this old response
from cache memory and insert the new response. For 200 (OK) responses
from varying resources (Section 13.12.3), cache replacement is more
complex.
HTTP/1.1 allows the authors of varying resources to guide cache update
by the inclusion of elements of so-called update keys in the responses
of these resources. The update key of a varying response consists of
two elements, both of which may be empty strings, separated by a
semicolon:
update-key = variant-id ";" absoluteURI
The variant-id element of the update key is the variant-id value in the
CVal header of the response, if a CVal header which such a value is
present, and an empty string otherwise. The absoluteURI element of the
update key is the absolute URI given in, or derived from, the Content-
Location header of the response if present, and an empty string if no
Content-Location header is present.
If a cache has stored in memory a 200 (OK) response with a certain
update key, and receives, from the same resource, a new 200 (OK)
response which has the same update key, this should be interpreted as a
signal from the resource author that the old response can be deleted
from cache memory and replaced by the new response.
The update key mechanism cannot cause deletion from cache memory of old
responses with update keys that will no longer be used. It is expected
that the normal _least recently used_ update heuristics employed by
caches will eventually cause such old responses to be deleted.
All 200 (OK) responses from varying resources should include update key
elements. Resource authors may not assume that caches will be able to
cache responses not including update key elements. If a Vary header is
used to signal variance, the response should include a variant-id value
as the update key element. The Content-Location header should only be
used to supply a update key element if an Alternates header is present
in the response.
13.22 FLUID: Network Partitions
TBS
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There may be enough said elsewhere already, but we haven't checked.
13.23 FLUID: Caching of Negative Responses
TBS
13.24 History Lists
History lists as implemented in many user agents and caches are
different. In particular history lists SHOULD NOT try to show a
semantically transparent view of the current state of a resource.
Rather, a history list is meant to show exactly what the user saw at the
time when the resource was retrieved .
This should not be construed to prohibit the history mechanism from
telling the user that a view may be stale.
14 Persistent Connections
14.1 Purpose
HTTP's greatest strength and its greatest weakness has been its
simplicity. 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 requires a client to make multiple requests
of the same server in a short amount of time. An excellent analysis of
these performance problems is available [2]; analysis and results from a
prototype implementation are in [32, 33].
Persistent HTTP connections have a number of advantages, including:
. By opening and closing TCP fewer connections, CPU time is saved,
and memory used for TCP protocol control blocks is also saved
. HTTP requests and responses can be pipe-lined on a connection.
Pipe-lining 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 were reported.
HTTP implementations SHOULD implement persistent connections.
14.2 Overall Operation
Persistent connections provides a mechanism by which a client and a
server can negotiate the use of a TCP connection for an extended
conversation.. This negotiation takes place using the Connection and
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Persist header fields. Once this option has been negotiated the client
can make multiple HTTP requests over a single transport connection.
14.2.3 Negotiation
To request the use of persistent connections, a client sends a
Connection header with a connection-token _Persist_. If the server
wishes to accept persistent connections it will respond with the same
connection-token. Both the client and server MUST send this connection-
token with every request and response for the duration of the persistent
connection. If either the client or the server omits the Persist token
from the Connection header, that request becomes the last one for the
connection.
A server MUST NOT establish a persistent connection with an HTTP/1.0
client that uses the above form of the Persist header due to problems
with the interactions between 1.1 clients and 1.0 proxy servers (See
section E.2.5 for more information on backwards compatibility with HTTP
1.0 clients).
14.2.4 Pipe-lining
Clients and servers which support persistent connections MAY _pipe-line_
their requests and responses. When pipe-lining, a client will send
multiple requests without waiting for the responses. The server MUST
then send all of the responses in the same order that the requests were
made.
A client MAY pipeline multiple requests immediately if it has previous
knowledge that the server it is connecting to supports persistent
connections. A client MAY assume that a server supports persistent
connections if the same server has accepted persistent connections
within the past 24 hours. Clients which assume persistent connections
and pipeline immediately SHOULD be prepared to retry their connection if
the first pipe-lined attempt fails. If a client does such a retry, it
MUST NOT pipeline without first receiving an explicit Persist token from
the server. Clients MUST also be prepared to resend their requests if
the server closes the connection before sending all of the corresponding
responses.
14.2.5 Delimiting Entity-Bodies
When using persistent connections both the client and the server MUST
mark the exact endings of transmitted entity-bodies using one of the
following three techniques:
1. Send a Content-length field in the header with the exact number of
bytes in the entity-body.
2. Send the message using Chunked transfer encoding as described in
section 3.6. Chunked transfer encoding allows the server to
transmit the data to the client a piece at a time while still
communicating an exact ending of the entity-body.
3. Close the transport connection after the entity body.
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Sending the Content-length is the preferred technique. Chunked encoding
SHOULD be used when the size of the entity-body is not known before
beginning to transmit the entity-body. Finally, the connection MAY be
closed and fall back to non-persistent connections, if neither 1 or 2
are possible.
Clients and servers that support persistent connections MUST correctly
support receiving via all three techniques.
14.3 Proxy Servers
It is especially important that proxies correctly implement the
properties of the Connection header field as specified in 14.2.1.
The proxy server MUST negotiate 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.
14.4 Interaction with Security Protocols
It is expected that the Session extension will operate with both SHTTP
[31] and SSL [32]. When used in conjunction with SHTTP, the SHTTP
request is prepared normally and the persist connection-token is placed
in the outermost request block (the one containing the _Secure_ method).
When used in conjunction with SSL, a SSL session is started as normal
and the first HTTP request made using SSL contains the persistent
connection header.
14.5 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
sides 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.
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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 request without user
interaction. 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.
It is suggested that clients which 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 of 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.
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
As mentioned in Section 11.1, the Basic authentication scheme is not a
secure method of user authentication, nor does it in any way protect the
Entity-Body, which is transmitted in clear text across the 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
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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[26].
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 impersonating the
original server in a way that is not detectable by the client.
15.2 Safe Methods
The writers of client software 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.
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
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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.
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 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
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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 very 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 or Alternates response headers
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 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.
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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 miss-
association of IP addresses and DNS names. The deployment of DNSSEC[27]
should help this situation. In advance of this deployment, however,
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 a 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 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.
15.9 SLUSHY: 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.
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
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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 Harald Tveit Alvestrand
Keith Ball Brian Behlendorf
Paul Burchard Maurizio Codogno
Mike Cowlishaw Roman Czyborra
Michael A. Dolan Jim Gettys
Marc Hedlund Koen Holtman
Alex Hopmann Bob Jernigan
Shel Kaphan Rohit Khare
Martijn Koster Alexei Kosut
David M. Kristol Daniel LaLiberte
Paul J. Leach Albert Lunde
John C. Mallery Jean-Philippe Martin-Flatin
Larry Masinter Mitra
Jeffrey Mogul Gavin Nicol
Bill Perry Jeffrey Perry
Owen Rees Luigi Rizzo
David Robinson Marc Salomon
Rich Salz Jim Seidman
Chuck Shotton Eric W. Sink
Simon E. Spero Richard N. Taylor
Robert S. Thau Francois Yergeau
Mary Ellen Zurko David Morris
Greg Herlihy Scott Powers
Allan M. Schiffman Alan Freier
Bill (BearHeart) Weinman
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, Larry Masinter, and Roy Fielding.
Most of the specification of ranges is based on work originally done by
Ari Luotonen and John Franks, with additional input from Steve Zilles
and Roy Fielding.
XXX need acks for subgroup work.
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17. References
[1]
H. Alvestrand. _Tags for the identification of languages._ RFC 1766,
UNINETT, March 1995.
[2]
F. Anklesaria, M. McCahill, P. Lindner, D. Johnson, D. Torrey, B.
Alberti. _The Internet Gopher Protocol: (a distributed document
search and retrieval protocol)_, RFC 1436, University of Minnesota,
March 1993.
[3]
T. Berners-Lee. _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]
T. Berners-Lee, L. Masinter, M. McCahill.
_Uniform Resource Locators (URL)._ RFC 1738, CERN, Xerox PARC,
University of Minnesota, December 1994.
[5]
T. Berners-Lee, D. Connolly.
_HyperText Markup Language Specification - 2.0._ RFC 1866, MIT/LCS,
November 1995.
[6]
T. Berners-Lee, R. Fielding, H. Frystyk.
"Hypertext Transfer Protocol - HTTP/1.0." Work in Progress (draft-
ietf-http-v10-spec-04.txt), MIT/LCS, UC Irvine, September 1995.
[7]
N. Borenstein, N. Freed.
_MIME (Multipurpose Internet Mail Extensions) Part One: Mechanisms
for Specifying and Describing the Format of Internet Message Bodies."
RFC 1521, Bellcore, Innosoft, September 1993.
[8]
R. Braden.
_Requirements for Internet hosts - application and support._ STD 3,
RFC 1123, IETF, October 1989.
[9]
D. H. Crocker.
Fielding, Frystyk, Berners-Lee, Gettys, and Mogul [Page 147]
INTERNET-DRAFT HTTP/1.1 Monday, April 22, 1996
_Standard for the Format of ARPA Internet Text Messages._ STD 11, RFC
822, UDEL, August 1982.
[10]
F. Davis, B. Kahle, H. Morris, J. Salem, T. Shen, R. Wang, J. Sui, M.
Grinbaum. _WAIS Interface Protocol Prototype Functional
Specification._ (v1.5), Thinking Machines Corporation, April 1990.
[11]
R. Fielding. _Relative Uniform Resource Locators._ RFC 1808, UC
Irvine, June 1995.
[12]
M. Horton, R. Adams. _Standard for interchange of USENET messages._
RFC 1036 (Obsoletes RFC 850), AT&T Bell Laboratories, Center for
Seismic Studies, December 1987.
[13]
B. Kantor, 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]
K. Moore. _MIME (Multipurpose Internet Mail Extensions) Part Two :
Message Header Extensions for Non-ASCII Text._ RFC 1522, University
of Tennessee, September 1993.
[15]
E. Nebel, L. Masinter. _Form-based File Upload in HTML._ RFC 1867,
Xerox Corporation, November 1995.
[16]
J. Postel. _Simple Mail Transfer Protocol._ STD 10, RFC 821, USC/ISI,
August 1982.
[17]
J. Postel. _Media Type Registration Procedure._ RFC 1590, USC/ISI,
March 1994.
[18]
J. Postel, J. K. Reynolds. _File Transfer Protocol (FTP)_ STD 9, RFC
959, USC/ISI, October 1985.
Fielding, Frystyk, Berners-Lee, Gettys, and Mogul [Page 148]
INTERNET-DRAFT HTTP/1.1 Monday, April 22, 1996
[19]
J. Reynolds, J. Postel. _Assigned Numbers._ STD 2, RFC 1700, USC/ISI,
October 1994.
[20]
K. Sollins, 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.
Part 8: Latin/Hebrew alphabet, ISO 8859-8, 1988.
Part 9: Latin alphabet No. 5, ISO 8859-9, 1990.
[23]
Meyers, M. Rose _The Content-MD5 Header Field._ RFC 1864, Carnegie
Mellon, Dover Beach Consulting, October, 1995.
[24]
B. Carpenter, Y. Rekhter, _Renumbering Needs Work_. RFC 1900, IAB,
February 1996.
[25]
Gzip is available from the GNU project at
<URL:ftp://prep.ai.mit.edu/pub/gnu/>. A more formal specification is
currently a work in progress.
[26]
Work In Progress for Digest authentication of the IETF HTTP working
group.
[27]
TBS, Work in progress (XXX should put RFC in here_ )
[28]
Mills, D, _Network Time Protocol, Version 3_, Specification,
Fielding, Frystyk, Berners-Lee, Gettys, and Mogul [Page 149]
INTERNET-DRAFT HTTP/1.1 Monday, April 22, 1996
Implementation and Analysis RFC 1305, University of Delaware, March,
1992.
[29]
Work in progress of the HTTP working group (XXX is this correct
reference for incomplete work?).
[30]
S. Spero. _Analysis of HTTP Performance Problems_
<URL:http://sunsite.unc.edu/mdma-release/http-prob.html>
[31]
E. Rescorla, A. Schiffman _The Secure HyperText Transfer Protocol_
Internet-Draft (work in progress).
[32]
A. Freier, P Karlton, P. Kocher. _SSL Version 3.0" Internet-Draft_
(work in progress).
[33]
Jeffrey C. Mogul. _The Case for Persistent-Connection HTTP_. In
Proc.SIGCOMM '95 Symposium on Communications Architectures and
Protocols, pages 299-313. Cambridge, MA, August, 1995.
[34]
Jeffrey C. Mogul. _The Case for Persistent-Connection HTTP_.
Research, Report 95/4, Digital Equipment Corporation Western Research
Laboratory, May, 1995.,
<URL
:http://www.research.digital.com/wrl/techreports/abstracts/95.4.html>
[35]
Work in progress of the HTTP working group on state management.
18. Authors' Addresses
Roy T. Fielding
Department of Information and Computer Science
University of California
Irvine, CA 92717-3425, USA
Fax: +1 (714) 824-4056
Email: fielding@ics.uci.edu
Henrik Frystyk Nielsen
W3 Consortium
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, USA
Fielding, Frystyk, Berners-Lee, Gettys, and Mogul [Page 150]
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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
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, U.S.A.
Email: mogul@wrl.dec.com
Appendices
These appendices are provided for informational reasons only -- they do
not form a part of the HTTP/1.1 specification.
A. 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].
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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.
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
B. 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
Clients SHOULD be tolerant in parsing the Status-Line and servers
tolerant when parsing the Request-Line. In particular, they SHOULD
characters between fields, even though unambiguously. accept any amount of SP or HT
only a single SP is required.
The line terminator for HTTP-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.
C. Differences Between HTTP Bodies and RFC 1521 Internet Message Bodies
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 1521 discusses mail, and HTTP
has a few features that are different than those described in RFC 1521.
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.
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At the time of this writing, it is expected that RFC 1521 will be
revised. The revisions may include some of the practices found in
HTTP/1.1 but not in RFC 1521.
This appendix describes specific areas where HTTP differs from RFC 1521.
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.
C.1 Conversion to Canonical Form
RFC 1521 requires that an Internet mail entity be converted to canonical
form prior to being transferred, as described in Appendix G of RFC 1521
[7]. Section 3.6.1 of this document describes the forms allowed for
subtypes of the _text_ media type when transmitted over HTTP. RFC 1521
requires that content with a typeof _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 1521
environment SHOULD translate all line breaks within the text media types
described in Section 3.6.1 of this document to the RFC 1521 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.
C.2 Conversion of Date Formats
HTTP/1.1 uses a restricted set of date formats (Section 3.3) 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.
C.3 Introduction of Content-Encoding
RFC 1521 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 1521.)
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C.4 No Content-Transfer-Encoding
HTTP does not use the Content-Transfer-Encoding (CTE) field of RFC 1521.
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.
C.5 HTTP Header Fields in Multipart Body-Parts
In RFC 1521, 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.
C.6 Introduction of Transfer-Encoding
HTTP/1.1 introduces the Transfer-Encoding header field (Section 10.39).
Proxies/gateways MUST remove any transfer coding prior to forwarding a
message via a MIME-compliant protocol. The process for decoding the
_chunked_ transfer coding (Section 3.6) can be represented in pseudo-
code as:
length := 0
read chunk-size and CRLF
while (chunk-size > 0) {
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
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C.7 MIME-Version
HTTP is not a MIME-compliant protocol (see Appendix C). 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 [7]).
Proxies/gateways are responsible for ensuring full compliance (where
possible) when exporting HTTP messages to strict MIME environments.
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.
D. Changes from HTTP/1.0
This section will summarize major differences between versions 1.0 and MIME-Version = "MIME-Version" ":" 1*DIGIT "." 1*DIGIT
1.1 of the Hypertext Transfer Protocol.
D.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 is missing from an
HTTP/1.1 request (Section 10.22), and accept absolute URIs (Section
5.1.2) are among the most important changes from HTTP/1.0.
In HTTP/1.0 there is a one-to-one relationship of IP addresses and
servers. There is no other way to distinguish the intended server of a
request than the IP address to which that request is directed. The
HTTP/1.1 change will allow the Internet, once HTTP/1.0 clients and
servers 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 used for the sole purpose of allowing root-
level domain names to be used in HTTP URLs. Given the rate of growth of
the Web, and the number of servers already deployed, it is extremely
important that implementations of HTTP/1.1 correctly implement these new
requirements:
. both clients and servers MUST support the Host request-header
. Host request-headers are required in HTTP/1.1 requests.
. servers MUST report an error if an HTTP/1.1 request does not
include a Host request-header
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. servers MUST accept absolute URIs
E. Additional Features
This appendix documents 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.
E.1 Additional Request Methods
E.1.1 PATCH
The PATCH method is similar to PUT except that the entity contains a
list of differences between the original version of the resource
identified by the Request-URI and the desired content of the resource
after the PATCH action has been applied. The list of differences is in a
format defined by the media type of the entity (e.g.,
_application/diff_) and MUST include sufficient information to allow the
server to recreate the changes necessary to convert the original version
of the resource to the desired version.
If the request passes through a cache and the Request-URI identifies a
currently cached entity, that entity MUST be removed from the cache.
Responses to this method are not cachable.
For compatibility with HTTP/1.0 applications, all PATCH requests MUST
include a valid Content-Length header field unless the server is known
to be HTTP/1.1 compliant. When sending a PATCH request to an HTTP/1.1
server, a client MUST use a valid Content-Length or the _chunked_
Transfer-Encoding. The server SHOULD respond with a 400 (bad request)
message if it cannot determine the length of the request message's
content, or with 411 (length required) if it wishes to insist on
receiving a valid Content-Length.
The actual method for determining how the patched resource is placed,
and what happens to its predecessor, is defined entirely by the origin
server. If the original version of the resource being patched included a
Content-Version header field, the request entity MUST include a Derived-
From header field corresponding to the value of the original Content-
Version header field. Applications are encouraged to use these fields
for constructing versioning relationships and resolving version
conflicts.
PATCH requests must obey the entity transmission requirements set out in
section 8.4.1.
Caches that implement PATCH should invalidate cached responses as
defined in section 13.17 for PUT.
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E.1.2 LINK
The LINK method establishes one or more Link relationships between the
existing resource identified by the Request-URI and other existing
resources. The difference between LINK and other methods allowing links
to be established between resources is that the LINK method does not
allow any Entity-Body to be sent in the request and does not directly
result in the creation of new resources.
If the request passes through a cache and the Request-URI identifies a
currently cached entity, that entity MUST be removed from the cache.
Responses to this method are not cachable.
Caches that implement LINK should invalidate cached responses as defined
in section 13.17 for PUT.
E.1.3 UNLINK
The UNLINK method removes one or more Link relationships from the
existing resource identified by the Request-URI. These relationships may
have been established using the LINK method or by any other method
supporting the Link header. The removal of a link to a resource does not
imply that the resource ceases to exist or becomes inaccessible for
future references.
If the request passes through a cache and the Request-URI identifies a
currently cached entity, that entity MUST be removed from the cache.
Responses to this method are not cachable.
Caches that implement UNLINK should invalidate cached responses as
defined in section 13.17 for PUT.
E.2 Additional Header Field Definitions
E.2.1 Content-Version
The Content-Version entity-header field defines the version tag
associated with a rendition of an evolving entity. Together with the
Derived-From field described in Section 10.18, it allows a group of
people to work simultaneously on the creation of a work as an iterative
process. The field SHOULD be used to allow evolution of a particular
work along a single path. It SHOULD NOT be used to indicate derived
works or renditions in different representations. It MAY also me used as
an opaque value for comparing a cached entity's version with that of the
current resource.
Content-Version = "Content-Version" ":" quoted-string
Examples of the Content-Version field include:
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Content-Version: "2.1.2"
Content-Version: "Fred 19950116-12:26:48"
Content-Version: "2.5a4-omega7"
The value of the Content-Version field SHOULD be considered opaque to
all parties but the origin server. A user agent MAY suggest a value for
the version of an entity transferred via a PUT request; however, only
the origin server can reliably assign that value.
E.2.2 Derived-From
The Derived-From entity-header field can be used to indicate the version
tag of the resource from which the enclosed entity was derived before
modifications were made by the sender. This field is used to help manage
the process of merging successive changes to a resource, particularly
when such changes are being made in parallel and from multiple sources.
Derived-From = "Derived-From" ":" quoted-string
An example use of the field is:
Derived-From: "2.1.1"
The Derived-From field is required for PUT and PATCH requests if the
entity being sent was previously retrieved from the same URI and a
Content-Version header was included with the entity when it was last
retrieved.
E.2.3 Link
The Link entity-header field provides a means for describing a
relationship between two resources, generally between the requested
resource and some other resource. An entity MAY include multiple Link
values. Links at the metainformation level typically indicate
relationships like hierarchical structure and navigation paths. The Link
field is semantically equivalent to the <LINK> element in HTML [5].
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Link = "Link" ":" #("<" URI ">" *( ";" link-param )
link-param = ( ( "rel" "=" relationship )
| ( "rev" "=" relationship )
| ( "title" "=" quoted-string )
| ( "anchor" "=" <"> URI <"> )
| ( link-extension ) )
link-extension = token [ "=" ( token | quoted-string ) ]
relationship = sgml-name
| ( <"> sgml-name *( SP sgml-name) <"> )
sgml-name = ALPHA *( ALPHA | DIGIT | "." | "-" )
Relationship values are case-insensitive and MAY be extended within the
constraints of the sgml-name syntax. The title parameter MAY be used to
label the destination of a link such that it can be used as
identification within a human-readable menu. The anchor parameter MAY be
used to indicate a source anchor other than the entire current resource,
such as a fragment of this resource or a third resource.
Examples of usage include:
Link: <http://www.cern.ch/TheBook/chapter2>; rel="Previous"
Link: <mailto:timbl@w3.org>; rev="Made"; title="Tim Berners-Lee"
The first example indicates that chapter2 is previous to this resource
in a logical navigation path. The second indicates that the person
responsible for making the resource available is identified by the given
e-mail address.
E.2.4 URI
The URI header field has, in past versions of this specification, been
used as a combination of the existing Location, Content-Location, and
Alternates header fields. Its primary purpose has been to include a list
of additional URIs for the resource, including names and mirror
locations. However, it has become clear that the combination of many
different functions within this single field has been a barrier to
consistently and correctly implementing any of those functions.
Furthermore, we believe that the identification of names and mirror
locations would be better performed via the Link header field. The URI
header field is therefore deprecated in favor of those other fields.
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URI-header = "URI" ":" 1#( "<" URI ">" )
E.2.5 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 version 1.0 clients
and servers.
When connecting to an origin server an HTTP client MAY send the Keep-
Alive connection-token in addition to the Persist connection-token:
Connection: Keep-Alive,Persist
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.
A persistent connection based on the Keep-Alive connection token MUST
only use the _Content-Length_ technique for marking the ending
boundaries of entity-bodies. It MAY use pipe-lining.
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.
E.2.5.1 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.
F.1 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. While we are contemplating a
separate document containing advice to implementers, we feel it worth
noting that at the time of composing this specification, we would expect
commercial HTTP/1.1 servers to::
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. 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;
. 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 E.2.5.1.
G. Proxy Cache Implementation Guidelines
G.1 Support for Content Negotiation by Proxy Caches
The material in appendix G should go into a separate implementation
guide as an informational RFC, rather than in this specification. (since
it mostly describes 3 possible cache implementation strategies possible
within the protocol, rather than just the two protocol facilities
(transparent and opaque)). For the purposes of this (02) draft, we will
leave it in as an appendix as it clarifies some points of how caching
might work in the context of the HTTP/1.1 protocol.
If a resource is varying, this has an important effect on cache
management, particularly for caching proxies which service a diverse set
of user agents. Such proxy caches must correctly handle requests on
varying resources in order not to disturb the negotiation process.
This specification distinguishes six levels of correct support for
content negotiation by proxy caches. The text below describes these
levels, but does not exhaustively list all mechanisms associated with
support on these levels. In particular, mechanisms for handling partial
requests on varying resources are not discussed.
1. A proxy cache providing level 1 support will never store in cache
memory responses from varying resources (such responses always
include at least one Vary header or Alternates header). When
receiving a request on a varying resource, the proxy will thus
always forward the request towards an upstream server. A level 1
proxy cache never makes selection decisions itself.
2. A proxy cache providing level 2 support is able to maintain in
cache memory a mapping from the varying resource URI to a set of
200 (OK) response messages. When receiving a request on the
varying resource, the proxy will forward this request to an
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upstream server after including an If-Invalid request header field
listing the CVal header values of the associated cached 200
responses, as described in Section 10.52. If a 304 (Not Modified)
response is received from the upstream server, the proxy updates,
with the 304 response headers, the stored 200 response which has
the same CVal header field as the 304 response. It then passes
either the updated 200 response or the 304 response on to its
client, the choice depending on the presence and contents of an If-
Invalid header in the original request. If a 200 response is
received from the upstream server, the proxy will update the set of
responses it has for the varying resource by using the cache update
algorithm described in Section 13.20, and pass on the 200 response
to its client.
3. A proxy cache providing level 3 support is able to maintain in
cache memory a mapping from the varying resource URI to a set of
200 (OK) response messages. In addition, it is able to maintain,
for each cached 200 response belonging to the varying resource, a
list of selecting request header sequences. This list of selecting
request header sequences starts with the sequence taken from the
request which initially caused an upstream server to return the
cached 200 response, and continues with any additional sequences
taken from subsequent requests which caused an upstream server to
return a 304 response with a CVal header identical to the CVal
header of that cached 200 response. When receiving a request on
the varying resource, the proxy will iterate over all cached, fresh
200 responses associated with the resource. For each fresh 200
response, it will search the associated list of selecting request
header sequences to see if a match to the headers of the current
request can be found. If a match is found, the proxy will return
the fresh 200 response in question. If no match is found, the
proxy will switch to level 1 behavior and pass on the request to an
upstream server. The response received from the upstream server
may refresh a stale 200 response that was cached for the varying
resource a side effect. XXX previous sentence doesn't make sense_
4. A proxy cache providing level 4 support provides transparent
negotiation services for transparently negotiated resources, and
provides level 1 support for opaquely negotiated resources.
5. A proxy cache providing level 5 support provides transparent
negotiation services for transparently negotiated resources, and
provides level 2 support for opaquely negotiated resources.
6. A proxy cache providing level 6 support provides transparent
negotiation services for transparently negotiated resources, and
provides level 3 support for opaquely negotiated resources.
Note: Implementation of support levels 4 to 6 is only possible
when the planned content negotiation specification [29] is
completed. The level numbers above were assigned to reflect
expected caching efficiency in an environment where the proxy
cache is serving a diverse set of clients. It is expected that
level 4 proxies will be easier to implement than level 3
proxies.
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G.2 Propagation of Changes in Opaque Selection
Level 3 and level 6 proxy caches not only cache the responses from an
opaquely varying resource, they also cache the mappings from request
headers to particular entities computed by the opaque selection
algorithm located at the origin server. If this selection algorithm is
changed by the resource author, for example because a Spanish text
entity is added to a resource which previously only had English and
French entities available, it is important to make the level 3 and 6
caches refresh their cached mappings. This can be done by changing the
CVal header fields sent along with the original English and French
responses. This change will eventually cause the proxies to replace the
old English and French responses in cache memory, along with their
associated lists of selecting request header sequences, by `new' English
and French responses with fresh lists of selecting request header
sequences. In order to guarantee an upper time bound for this update
process, the resource author can include an appropriate Cache-control:
max-age=... directive in the responses from the varying resource.
G.3 SLUSHY: State
This should probably be in the cookie ID, and not in this document at
all.
HTTP implementations often support facilities for state management,
often called _cookies_[35]. Cookies can not be cached by public
(shared) caches, but since public documents may make up part of a
_stateful dialog,_ and in particular the first document in a stateful
dialog may be (for example) a public and cachable home page, servers
that wish to receive the client's cookie on each request, or to issue a
new cookie on requests for a document, must set the document up to
require validation on each request (Cache-Control: must-revalidate)
In general, the cache control headers for responses control what a proxy
has to do. If a document is fresh in a cache, a request containing a
cookie does not have to be forwarded to the origin server, since (by
definition) if the document can be served from a cache the origin server
must have said there are no important side effects at the origin
relating to requests for that document, and so, no changes to the
cookie.
One important state issue bearing on caching is that for conditional
requests that go through to the origin server, for which the origin
server responds with 304 and also with a set-cookie header, caches must
splice the set-cookie sent by the origin server into their own response.
For example, this allows a home page to be cached, but stale, so that
the only traffic to the origin server is to validate the home page,
receiving a 304 and potentially a new cookie.
G.4 FLUID: Cache Replacement Algorithms
TBS
Should go into an implementers Informational RFC.
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G.5 FLUID: Bypassing in Caching Hierarchies
This should also go into an implementers Informational RFC, and become
grist for HTML's mill.
Many HTTP caching configurations involve hierarchies of caches, often
designed to reduce bandwidth requirements rather than improving latency.
However, if a cache at a low level in the hierarchy is sure that the
cache(s) above it do not contain a cache entry to match a given request,
that low-level cache can transmit the request directly to the origin
server. This improves retrieval latency without increasing total
bandwidth requirements (it even eliminates some packet transmissions)
and is entirely appropriate for resources whose values are explicitly
not cached.
We call this technique _request bypassing._ Note that although the
bypassing decision might be done by the ultimate client, in many cases
the use of firewalls or unsophisticated clients means that the decision
must be made by an intermediate-level cache.
In order for request bypassing to work in the most efficient possible
way, the caches must be able to determine from the request whether the
response is likely to be cachable. (It is important to err on the side
of assuming cachability, since the assuming converse could seriously
reduce the effectiveness of the higher-level caches.)
The current HTTP/1.1 draft specification does not include a foolproof
mechanism to mark requests in this way. While we generally do not allow
caching of responses to GET requests for URLs with a _?_ in the rel_path
part (see section 13.16), we also allow the origin server to mark
responses to such queries as cachable. Therefore, any bypassing done
using this heuristic runs the risk of giving up perfectly good
opportunities to cache some resources.
XXX we have discussed various approaches for marking requests, all of
which apparently require some kind of change to HTML to allow the origin
server to pass the marks to the ultimate client. Some people suggest
using special methods that are explicitly always cachable
(_POST_WITH_NO_SIDE_EFFECTS_, or more concisely _POSTC_) or never
cachable (_GET_QUERY_, or more concisely _GETQ_). Others have suggested
adding tags to HTML that would cause subsequent requests to carry some
special sort of header. Neither solution has resulted in a consensus.
An origin server would be able to use POSTC only withHTTP/1.1 clients
and proxies, and so would have to return different HTML forms depending
on the protocol version in the request header. This would also imply
using the proposed Vary: header with some token that indicates _varies
based on request HTTP version,_ since we don't want a cache returning
one of these HTML responses to an HTTP/1.0 client
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