HTTP/1.1, part 4: Conditional Requests
draft-ietf-httpbis-p4-conditional-20
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Roy Fielding
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Yves Lafon
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Julian Reschke
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2012-07-16
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HTTPbis Working Group R. Fielding, Ed.
Internet-Draft Adobe
Obsoletes: 2616 (if approved) Y. Lafon, Ed.
Intended status: Standards Track W3C
Expires: January 17, 2013 J. Reschke, Ed.
greenbytes
July 16, 2012
HTTP/1.1, part 4: Conditional Requests
draft-ietf-httpbis-p4-conditional-20
Abstract
The Hypertext Transfer Protocol (HTTP) is an application-level
protocol for distributed, collaborative, hypertext information
systems. This document defines HTTP/1.1 conditional requests,
including metadata header fields for indicating state changes,
request header fields for making preconditions on such state, and
rules for constructing the responses to a conditional request when
one or more preconditions evaluate to false.
Editorial Note (To be removed by RFC Editor)
Discussion of this draft takes place on the HTTPBIS working group
mailing list (ietf-http-wg@w3.org), which is archived at
<http://lists.w3.org/Archives/Public/ietf-http-wg/>.
The current issues list is at
<http://tools.ietf.org/wg/httpbis/trac/report/3> and related
documents (including fancy diffs) can be found at
<http://tools.ietf.org/wg/httpbis/>.
The changes in this draft are summarized in Appendix D.1.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 17, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
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Without obtaining an adequate license from the person(s) controlling
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not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Conformance and Error Handling . . . . . . . . . . . . . . 4
1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 5
2. Validators . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Weak versus Strong . . . . . . . . . . . . . . . . . . . . 6
2.2. Last-Modified . . . . . . . . . . . . . . . . . . . . . . 7
2.2.1. Generation . . . . . . . . . . . . . . . . . . . . . . 8
2.2.2. Comparison . . . . . . . . . . . . . . . . . . . . . . 8
2.3. ETag . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.3.1. Generation . . . . . . . . . . . . . . . . . . . . . . 10
2.3.2. Comparison . . . . . . . . . . . . . . . . . . . . . . 11
2.3.3. Example: Entity-tags varying on Content-Negotiated
Resources . . . . . . . . . . . . . . . . . . . . . . 11
2.4. Rules for When to Use Entity-tags and Last-Modified
Dates . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3. Precondition Header Fields . . . . . . . . . . . . . . . . . . 14
3.1. If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.2. If-None-Match . . . . . . . . . . . . . . . . . . . . . . 15
3.3. If-Modified-Since . . . . . . . . . . . . . . . . . . . . 16
3.4. If-Unmodified-Since . . . . . . . . . . . . . . . . . . . 17
3.5. If-Range . . . . . . . . . . . . . . . . . . . . . . . . . 18
4. Status Code Definitions . . . . . . . . . . . . . . . . . . . 18
4.1. 304 Not Modified . . . . . . . . . . . . . . . . . . . . . 18
4.2. 412 Precondition Failed . . . . . . . . . . . . . . . . . 19
5. Precedence . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
6.1. Status Code Registration . . . . . . . . . . . . . . . . . 20
6.2. Header Field Registration . . . . . . . . . . . . . . . . 21
7. Security Considerations . . . . . . . . . . . . . . . . . . . 21
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
9.1. Normative References . . . . . . . . . . . . . . . . . . . 22
9.2. Informative References . . . . . . . . . . . . . . . . . . 22
Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 22
Appendix B. Imported ABNF . . . . . . . . . . . . . . . . . . . . 23
Appendix C. Collected ABNF . . . . . . . . . . . . . . . . . . . 23
Appendix D. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 24
D.1. Since draft-ietf-httpbis-p4-conditional-19 . . . . . . . . 24
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
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1. Introduction
Conditional requests are HTTP requests [Part2] that include one or
more header fields indicating a precondition to be tested before
applying the method semantics to the target resource. Each
precondition is based on metadata that is expected to change if the
selected representation of the target resource is changed. This
document defines the HTTP/1.1 conditional request mechanisms in terms
of the architecture, syntax notation, and conformance criteria
defined in [Part1].
Conditional GET requests are the most efficient mechanism for HTTP
cache updates [Part6]. Conditionals can also be applied to state-
changing methods, such as PUT and DELETE, to prevent the "lost
update" problem: one client accidentally overwriting the work of
another client that has been acting in parallel.
Conditional request preconditions are based on the state of the
target resource as a whole (its current value set) or the state as
observed in a previously obtained representation (one value in that
set). A resource might have multiple current representations, each
with its own observable state. The conditional request mechanisms
assume that the mapping of requests to corresponding representations
will be consistent over time if the server intends to take advantage
of conditionals. Regardless, if the mapping is inconsistent and the
server is unable to select the appropriate representation, then no
harm will result when the precondition evaluates to false.
We use the term "selected representation" to refer to the current
representation of the target resource that would have been selected
in a successful response if the same request had used the method GET
and had excluded all of the conditional request header fields. The
conditional request preconditions are evaluated by comparing the
values provided in the request header fields to the current metadata
for the selected representation.
1.1. Conformance and Error Handling
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This specification targets conformance criteria according to the role
of a participant in HTTP communication. Hence, HTTP requirements are
placed on senders, recipients, clients, servers, user agents,
intermediaries, origin servers, proxies, gateways, or caches,
depending on what behavior is being constrained by the requirement.
See Section 2 of [Part1] for definitions of these terms.
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The verb "generate" is used instead of "send" where a requirement
differentiates between creating a protocol element and merely
forwarding a received element downstream.
An implementation is considered conformant if it complies with all of
the requirements associated with the roles it partakes in HTTP. Note
that SHOULD-level requirements are relevant here, unless one of the
documented exceptions is applicable.
This document also uses ABNF to define valid protocol elements
(Section 1.2). In addition to the prose requirements placed upon
them, senders MUST NOT generate protocol elements that do not match
the grammar defined by the ABNF rules for those protocol elements
that are applicable to the sender's role. If a received protocol
element is processed, the recipient MUST be able to parse any value
that would match the ABNF rules for that protocol element, excluding
only those rules not applicable to the recipient's role.
Unless noted otherwise, a recipient MAY attempt to recover a usable
protocol element from an invalid construct. HTTP does not define
specific error handling mechanisms except when they have a direct
impact on security, since different applications of the protocol
require different error handling strategies. For example, a Web
browser might wish to transparently recover from a response where the
Location header field doesn't parse according to the ABNF, whereas a
systems control client might consider any form of error recovery to
be dangerous.
1.2. Syntax Notation
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234] with the list rule extension defined in Section
1.2 of [Part1]. Appendix B describes rules imported from other
documents. Appendix C shows the collected ABNF with the list rule
expanded.
2. Validators
This specification defines two forms of metadata that are commonly
used to observe resource state and test for preconditions:
modification dates (Section 2.2) and opaque entity tags
(Section 2.3). Additional metadata that reflects resource state has
been defined by various extensions of HTTP, such as WebDAV [RFC4918],
that are beyond the scope of this specification. A resource metadata
value is referred to as a "validator" when it is used within a
precondition.
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2.1. Weak versus Strong
Validators come in two flavors: strong or weak. Weak validators are
easy to generate but are far less useful for comparisons. Strong
validators are ideal for comparisons but can be very difficult (and
occasionally impossible) to generate efficiently. Rather than impose
that all forms of resource adhere to the same strength of validator,
HTTP exposes the type of validator in use and imposes restrictions on
when weak validators can be used as preconditions.
A "strong validator" is a representation metadata value that MUST be
changed to a new, previously unused or guaranteed unique, value
whenever a change occurs to the representation data such that a
change would be observable in the payload body of a 200 (OK) response
to GET.
A strong validator MAY be changed for other reasons, such as when a
semantically significant part of the representation metadata is
changed (e.g., Content-Type), but it is in the best interests of the
origin server to only change the value when it is necessary to
invalidate the stored responses held by remote caches and authoring
tools. A strong validator MUST be unique across all representations
of a given resource, such that no two representations of that
resource share the same validator unless their payload body would be
identical.
Cache entries might persist for arbitrarily long periods, regardless
of expiration times. Thus, a cache might attempt to validate an
entry using a validator that it obtained in the distant past. A
strong validator MUST be unique across all versions of all
representations associated with a particular resource over time.
However, there is no implication of uniqueness across representations
of different resources (i.e., the same strong validator might be in
use for representations of multiple resources at the same time and
does not imply that those representations are equivalent).
There are a variety of strong validators used in practice. The best
are based on strict revision control, wherein each change to a
representation always results in a unique node name and revision
identifier being assigned before the representation is made
accessible to GET. A collision-resistant hash function applied to
the representation data is also sufficient if the data is available
prior to the response header fields being sent and the digest does
not need to be recalculated every time a validation request is
received. However, if a resource has distinct representations that
differ only in their metadata, such as might occur with content
negotiation over media types that happen to share the same data
format, then the origin server SHOULD incorporate additional
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information in the validator to distinguish those representations and
avoid confusing cache behavior.
In contrast, a "weak validator" is a representation metadata value
that might not be changed for every change to the representation
data. This weakness might be due to limitations in how the value is
calculated, such as clock resolution or an inability to ensure
uniqueness for all possible representations of the resource, or due
to a desire by the resource owner to group representations by some
self-determined set of equivalency rather than unique sequences of
data. An origin server SHOULD change a weak entity-tag whenever it
considers prior representations to be unacceptable as a substitute
for the current representation. In other words, a weak entity-tag
ought to change whenever the origin server wants caches to invalidate
old responses.
For example, the representation of a weather report that changes in
content every second, based on dynamic measurements, might be grouped
into sets of equivalent representations (from the origin server's
perspective) with the same weak validator in order to allow cached
representations to be valid for a reasonable period of time (perhaps
adjusted dynamically based on server load or weather quality).
Likewise, a representation's modification time, if defined with only
one-second resolution, might be a weak validator if it is possible
for the representation to be modified twice during a single second
and retrieved between those modifications.
A "use" of a validator occurs when either a client generates a
request and includes the validator in a precondition or when a server
compares two validators. Weak validators are only usable in contexts
that do not depend on exact equality of a representation's payload
body. Strong validators are usable and preferred for all conditional
requests, including cache validation, partial content ranges, and
"lost update" avoidance.
2.2. Last-Modified
The "Last-Modified" header field indicates the date and time at which
the origin server believes the selected representation was last
modified.
Last-Modified = HTTP-date
An example of its use is
Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
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2.2.1. Generation
Origin servers SHOULD send Last-Modified for any selected
representation for which a last modification date can be reasonably
and consistently determined, since its use in conditional requests
and evaluating cache freshness ([Part6]) results in a substantial
reduction of HTTP traffic on the Internet and can be a significant
factor in improving service scalability and reliability.
A representation is typically the sum of many parts behind the
resource interface. The last-modified time would usually be the most
recent time that any of those parts were changed. How that value is
determined for any given resource is an implementation detail beyond
the scope of this specification. What matters to HTTP is how
recipients of the Last-Modified header field can use its value to
make conditional requests and test the validity of locally cached
responses.
An origin server SHOULD obtain the Last-Modified value of the
representation as close as possible to the time that it generates the
Date field value for its response. This allows a recipient to make
an accurate assessment of the representation's modification time,
especially if the representation changes near the time that the
response is generated.
An origin server with a clock MUST NOT send a Last-Modified date that
is later than the server's time of message origination (Date). If
the last modification time is derived from implementation-specific
metadata that evaluates to some time in the future, according to the
origin server's clock, then the origin server MUST replace that value
with the message origination date. This prevents a future
modification date from having an adverse impact on cache validation.
An origin server without a clock MUST NOT assign Last-Modified values
to a response unless these values were associated with the resource
by some other system or user with a reliable clock.
2.2.2. Comparison
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:
o The validator is being compared by an origin server to the actual
current validator for the representation and,
o That origin server reliably knows that the associated
representation did not change twice during the second covered by
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the presented validator.
or
o The validator is about to be used by a client in an If-Modified-
Since, If-Unmodified-Since header field, because the client has a
cache entry, or If-Range for the associated representation, and
o That cache entry includes a Date value, which gives the time when
the origin server sent the original response, and
o The presented Last-Modified time is at least 60 seconds before the
Date value.
or
o The validator is being compared by an intermediate cache to the
validator stored in its cache entry for the representation, and
o That cache entry includes a Date value, which gives the time when
the origin server sent the original response, and
o The presented Last-Modified time is at least 60 seconds before the
Date value.
This method relies on the fact that if two different responses were
sent by the origin server during the same second, but both had the
same Last-Modified time, then at least one of those responses would
have a Date value equal to its Last-Modified time. The arbitrary 60-
second limit guards against the possibility that the Date and Last-
Modified values are generated from different clocks, or at somewhat
different times during the preparation of the response. An
implementation MAY use a value larger than 60 seconds, if it is
believed that 60 seconds is too short.
2.3. ETag
The "ETag" header field provides the current entity-tag for the
selected representation. An entity-tag is an opaque validator for
differentiating between multiple representations of the same
resource, regardless of whether those multiple representations are
due to resource state changes over time, content negotiation
resulting in multiple representations being valid at the same time,
or both. An entity-tag consists of an opaque quoted string, possibly
prefixed by a weakness indicator.
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ETag = entity-tag
entity-tag = [ weak ] opaque-tag
weak = %x57.2F ; "W/", case-sensitive
opaque-tag = DQUOTE *etagc DQUOTE
etagc = %x21 / %x23-7E / obs-text
; VCHAR except double quotes, plus obs-text
Note: Previously, opaque-tag was defined to be a quoted-string
([RFC2616], Section 3.11), thus some recipients might perform
backslash unescaping. Servers therefore ought to avoid backslash
characters in entity tags.
An entity-tag can be more reliable for validation than a modification
date in situations where it is inconvenient to store modification
dates, where the one-second resolution of HTTP date values is not
sufficient, or where modification dates are not consistently
maintained.
Examples:
ETag: "xyzzy"
ETag: W/"xyzzy"
ETag: ""
An entity-tag can be either a weak or strong validator, with strong
being the default. If an origin server provides an entity-tag for a
representation and the generation of that entity-tag does not satisfy
the requirements for a strong validator (Section 2.1), then that
entity-tag MUST be marked as weak by prefixing its opaque value with
"W/" (case-sensitive).
2.3.1. Generation
The principle behind entity-tags is that only the service author
knows the implementation of a resource well enough to select the most
accurate and efficient validation mechanism for that resource, and
that any such mechanism can be mapped to a simple sequence of octets
for easy comparison. Since the value is opaque, there is no need for
the client to be aware of how each entity-tag is constructed.
For example, a resource that has implementation-specific versioning
applied to all changes might use an internal revision number, perhaps
combined with a variance identifier for content negotiation, to
accurately differentiate between representations. Other
implementations might use a collision-resistant hash of
representation content, a combination of various filesystem
attributes, or a modification timestamp that has sub-second
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resolution.
Origin servers SHOULD send ETag for any selected representation for
which detection of changes can be reasonably and consistently
determined, since the entity-tag's use in conditional requests and
evaluating cache freshness ([Part6]) can result in a substantial
reduction of HTTP network traffic and can be a significant factor in
improving service scalability and reliability.
2.3.2. Comparison
There are two entity-tag comparison functions, depending on whether
the comparison context allows the use of weak validators or not:
o The strong comparison function: in order to be considered equal,
both opaque-tags MUST be identical character-by-character, and
both MUST NOT be weak.
o The weak comparison function: in order to be considered equal,
both opaque-tags MUST be identical character-by-character, but
either or both of them MAY be tagged as "weak" without affecting
the result.
The example below shows the results for a set of entity-tag pairs,
and both the weak and strong comparison function results:
+--------+--------+-------------------+-----------------+
| ETag 1 | ETag 2 | Strong Comparison | Weak Comparison |
+--------+--------+-------------------+-----------------+
| W/"1" | W/"1" | no match | match |
| W/"1" | W/"2" | no match | no match |
| W/"1" | "1" | no match | match |
| "1" | "1" | match | match |
+--------+--------+-------------------+-----------------+
2.3.3. Example: Entity-tags varying on Content-Negotiated Resources
Consider a resource that is subject to content negotiation (Section 8
of [Part2]), and where the representations returned upon a GET
request vary based on the Accept-Encoding request header field
(Section 9.3 of [Part2]):
>> Request:
GET /index HTTP/1.1
Host: www.example.com
Accept-Encoding: gzip
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In this case, the response might or might not use the gzip content
coding. If it does not, the response might look like:
>> Response:
HTTP/1.1 200 OK
Date: Thu, 26 Mar 2010 00:05:00 GMT
ETag: "123-a"
Content-Length: 70
Vary: Accept-Encoding
Content-Type: text/plain
Hello World!
Hello World!
Hello World!
Hello World!
Hello World!
An alternative representation that does use gzip content coding would
be:
>> Response:
HTTP/1.1 200 OK
Date: Thu, 26 Mar 2010 00:05:00 GMT
ETag: "123-b"
Content-Length: 43
Vary: Accept-Encoding
Content-Type: text/plain
Content-Encoding: gzip
...binary data...
Note: Content codings are a property of the representation, so
therefore an entity-tag of an encoded representation has to be
distinct from an unencoded representation to prevent conflicts
during cache updates and range requests. In contrast, transfer
codings (Section 4 of [Part1]) apply only during message transfer
and do not require distinct entity-tags.
2.4. Rules for When to Use Entity-tags and Last-Modified Dates
We adopt a set of rules and recommendations for origin servers,
clients, and caches regarding when various validator types ought to
be used, and for what purposes.
HTTP/1.1 origin servers:
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o SHOULD send an entity-tag validator unless it is not feasible to
generate one.
o MAY send a weak entity-tag instead of a strong entity-tag, if
performance considerations support the use of weak entity-tags, or
if it is unfeasible to send a strong entity-tag.
o SHOULD send a Last-Modified value if it is feasible to send one.
In other words, the preferred behavior for an HTTP/1.1 origin server
is to send both a strong entity-tag and a Last-Modified value.
HTTP/1.1 clients:
o MUST use that entity-tag in any cache-conditional request (using
If-Match or If-None-Match) if an entity-tag has been provided by
the origin server.
o SHOULD use the Last-Modified value in non-subrange cache-
conditional requests (using If-Modified-Since) if only a Last-
Modified value has been provided by the origin server.
o MAY use the Last-Modified value in subrange cache-conditional
requests (using If-Unmodified-Since) if only a Last-Modified value
has been provided by an HTTP/1.0 origin server. The user agent
SHOULD provide a way to disable this, in case of difficulty.
o SHOULD use both validators in cache-conditional requests if both
an entity-tag and a Last-Modified value have been provided by the
origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to
respond appropriately.
An HTTP/1.1 origin server, upon receiving a conditional request that
includes both a Last-Modified date (e.g., in an If-Modified-Since or
If-Unmodified-Since header field) and one or more entity-tags (e.g.,
in an If-Match, If-None-Match, or If-Range header field) as cache
validators, MUST NOT return a response status code of 304 (Not
Modified) unless doing so is consistent with all of the conditional
header fields in the request.
An HTTP/1.1 caching proxy, upon receiving a conditional request that
includes both a Last-Modified date and one or more entity-tags as
cache validators, MUST NOT return a locally cached response to the
client unless that cached response is consistent with all of the
conditional header fields in the request.
Note: The general principle behind these rules is that HTTP/1.1
servers and clients ought to transmit as much non-redundant
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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 might ignore entity-tags. Generally,
last-modified values received or used by these systems will
support transparent and efficient caching, and so HTTP/1.1 origin
servers still ought to provide Last-Modified values.
3. Precondition Header Fields
This section defines the syntax and semantics of HTTP/1.1 header
fields for applying preconditions on requests. Section 5 defines the
order of evaluation when more than one precondition is present in a
request.
3.1. If-Match
The "If-Match" header field can be used to make a request method
conditional on the current existence or value of an entity-tag for
one or more representations of the target resource.
If-Match is generally useful for resource update requests, such as
PUT requests, as a means for protecting against accidental overwrites
when multiple clients are acting in parallel on the same resource
(i.e., the "lost update" problem). An If-Match field-value of "*"
places the precondition on the existence of any current
representation for the target resource.
If-Match = "*" / 1#entity-tag
The If-Match condition is met if and only if any of the entity-tags
listed in the If-Match field value match the entity-tag of the
selected representation for the target resource (as per
Section 2.3.2), or if "*" is given and any current representation
exists for the target resource.
If the condition is met, the server MAY perform the request method as
if the If-Match header field was not present.
Origin servers MUST NOT perform the requested method if the condition
is not met; instead they MUST respond with the 412 (Precondition
Failed) status code.
Proxy servers using a cached response as the selected representation
MUST NOT perform the requested method if the condition is not met;
instead, they MUST forward the request towards the origin server.
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If the request would, without the If-Match header field, result in
anything other than a 2xx (Successful) or 412 (Precondition Failed)
status code, then the If-Match header field MUST be ignored.
Examples:
If-Match: "xyzzy"
If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-Match: *
3.2. If-None-Match
The "If-None-Match" header field can be used to make a request method
conditional on not matching any of the current entity-tag values for
representations of the target resource.
If-None-Match is primarily used in conditional GET requests to enable
efficient updates of cached information with a minimum amount of
transaction overhead. A client that has one or more representations
previously obtained from the target resource can send If-None-Match
with a list of the associated entity-tags in the hope of receiving a
304 (Not Modified) response if at least one of those representations
matches the selected representation.
If-None-Match can also be used with a value of "*" to prevent an
unsafe request method (e.g., PUT) from inadvertently modifying an
existing representation of the target resource when the client
believes that the resource does not have a current representation.
This is a variation on the "lost update" problem that might arise if
more than one client attempts to create an initial representation for
the target resource.
If-None-Match = "*" / 1#entity-tag
The If-None-Match condition is met if and only if none of the entity-
tags listed in the If-None-Match field value match the entity-tag of
the selected representation for the target resource (as per
Section 2.3.2), or if "*" is given and no current representation
exists for that resource.
If the condition is not met, the server MUST NOT perform the
requested method. Instead, if the request method was GET or HEAD,
the server SHOULD respond with a 304 (Not Modified) status code,
including the cache-related header fields (particularly ETag) of the
selected representation that has a matching entity-tag. For all
other request methods, the server MUST respond with a 412
(Precondition Failed) status code.
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If the condition is met, the server MAY perform the requested method
as if the If-None-Match header field did not exist, but MUST also
ignore any If-Modified-Since header field(s) in the request. That
is, if no entity-tags match, then the server MUST NOT return a 304
(Not Modified) response.
If the request would, without the If-None-Match header field, result
in anything other than a 2xx (Successful) or 304 (Not Modified)
status code, then the If-None-Match header field MUST be ignored.
(See Section 2.4 for a discussion of server behavior when both If-
Modified-Since and If-None-Match appear in the same request.)
Examples:
If-None-Match: "xyzzy"
If-None-Match: W/"xyzzy"
If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
If-None-Match: *
3.3. If-Modified-Since
The "If-Modified-Since" header field can be used with GET or HEAD to
make the method conditional by modification date: if the selected
representation has not been modified since the time specified in this
field, then do not perform the request method; instead, respond as
detailed below.
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 field and no Range
header field requests that the selected representation be transferred
only if it has been modified since the date given by the If-Modified-
Since header field. The algorithm for determining this includes the
following cases:
1. If the request would normally result in anything other than a 200
(OK) status code, 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.
2. If the selected representation has been modified since the If-
Modified-Since date, the response is exactly the same as for a
normal GET.
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3. If the selected representation has not been modified since a
valid If-Modified-Since date, the server SHOULD 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: The Range header field modifies the meaning of If-Modified-
Since; see Section 5.4 of [Part5] for full details.
Note: If-Modified-Since times are interpreted by the server, whose
clock might not be synchronized with the client.
Note: When handling an If-Modified-Since header field, some
servers will use an exact date comparison function, rather than a
less-than function, for deciding whether to send a 304 (Not
Modified) response. To get best results when sending an If-
Modified-Since header field for cache validation, clients are
advised to use the exact date string received in a previous Last-
Modified header field whenever possible.
Note: If a client uses an arbitrary date in the If-Modified-Since
header field instead of a date taken from the Last-Modified header
field for the same request, the client needs to be aware that this
date is interpreted in the server's understanding of time.
Unsynchronized clocks and rounding problems, due to the different
encodings of time between the client and server, are concerns.
This includes the possibility of race conditions if the document
has changed between the time it was first requested and the If-
Modified-Since date of a subsequent request, and the possibility
of clock-skew-related problems if the If-Modified-Since date is
derived from the client's clock without correction to the server's
clock. Corrections for different time bases between client and
server are at best approximate due to network latency.
3.4. If-Unmodified-Since
The "If-Unmodified-Since" header field can be used to make a request
method conditional by modification date: if the selected
representation has been modified since the time specified in this
field, then the server MUST NOT perform the requested operation and
MUST instead respond with the 412 (Precondition Failed) status code.
If the selected representation has not been modified since the time
specified in this field, the server SHOULD perform the request method
as if the If-Unmodified-Since header field were not present.
If-Unmodified-Since = HTTP-date
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An example of the field is:
If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
If a request normally (i.e., in absence of the If-Unmodified-Since
header field) would result in anything other than a 2xx (Successful)
or 412 (Precondition Failed) status code, the If-Unmodified-Since
header field SHOULD be ignored.
If the specified date is invalid, the header field MUST be ignored.
3.5. If-Range
The "If-Range" header field provides a special conditional request
mechanism that is similar to If-Match and If-Unmodified-Since but
specific to HTTP range requests. If-Range is defined in Section 5.3
of [Part5].
4. Status Code Definitions
4.1. 304 Not Modified
The 304 status code indicates that a conditional GET request has been
received and would have resulted in a 200 (OK) response if it were
not for the fact that the condition has evaluated to false. In other
words, there is no need for the server to transfer a representation
of the target resource because the client's request indicates that it
already has a valid representation, as indicated by the 304 response
header fields, and is therefore redirecting the client to make use of
that stored representation as if it were the payload of a 200
response. The 304 response MUST NOT contain a message-body, and thus
is always terminated by the first empty line after the header fields.
A 304 response MUST include a Date header field (Section 9.10 of
[Part2]) unless the origin server does not have a clock that can
provide a reasonable approximation of the current time. If a 200
(OK) response to the same request would have included any of the
header fields Cache-Control, Content-Location, ETag, Expires, or
Vary, then those same header fields MUST be sent in a 304 response.
Since the goal of a 304 response is to minimize information transfer
when the recipient already has one or more cached representations,
the response SHOULD NOT include representation metadata other than
the above listed fields unless said metadata exists for the purpose
of guiding cache updates (e.g., future HTTP extensions).
If the recipient of a 304 response does not have a cached
representation corresponding to the entity-tag indicated by the 304
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response, then the recipient MUST NOT use the 304 to update its own
cache. If this conditional request originated with an outbound
client, such as a user agent with its own cache sending a conditional
GET to a shared proxy, then the 304 response MAY be forwarded to that
client. Otherwise, the recipient MUST disregard the 304 response and
repeat the request without any preconditions.
If a cache uses a received 304 response to update a cache entry, the
cache MUST update the entry to reflect any new field values given in
the response.
4.2. 412 Precondition Failed
The 412 status code indicates that one or more preconditions given in
the request header fields evaluated to false when tested on the
server. This response code allows the client to place preconditions
on the current resource state (its current representations and
metadata) and thus prevent the request method from being applied if
the target resource is in an unexpected state.
5. Precedence
When more than one conditional request header field is present in a
request, the order in which the fields are evaluated becomes
important. In practice, the fields defined in this document are
consistently implemented in a single, logical order, due to the fact
that entity tags are presumed to be more accurate than date
validators. For example, the only reason to send both If-Modified-
Since and If-None-Match in the same GET request is to support
intermediary caches that might not have implemented If-None-Match, so
it makes sense to ignore the If-Modified-Since when entity tags are
understood and available for the selected representation.
The general rule of conditional precedence is that exact match
conditions are evaluated before cache-validating conditions and,
within that order, last-modified conditions are only evaluated if the
corresponding entity tag condition is not present (or not applicable
because the selected representation does not have an entity tag).
Specifically, the fields defined by this specification are evaluated
as follows:
1. When If-Match is present, evaluate it:
* if true, continue to step 3
* if false, respond 412 (Precondition Failed)
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2. When If-Match is not present and If-Unmodified-Since is present,
evaluate it:
* if true, continue to step 3
* if false, respond 412 (Precondition Failed)
3. When the method is GET and both Range and If-Range are present,
evaluate it:
* if the validator matches, respond 206 (Partial Content)
* if the validator does not match, respond 200 (OK)
4. When If-None-Match is present, evaluate it:
* if true, all conditions are met
* if false for GET/HEAD, respond 304 (Not Modified)
* if false for other methods, respond 412 (Precondition Failed)
5. When the method is GET or HEAD, If-None-Match is not present, and
If-Modified-Since is present, evaluate it:
* if true, all conditions are met
* if false, respond 304 (Not Modified)
Any extension to HTTP/1.1 that defines additional conditional request
header fields ought to define its own expectations regarding the
order for evaluating such fields in relation to those defined in this
document and other conditionals that might be found in practice.
6. IANA Considerations
6.1. Status Code Registration
The HTTP Status Code Registry located at
<http://www.iana.org/assignments/http-status-codes> shall be updated
with the registrations below:
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+-------+---------------------+-------------+
| Value | Description | Reference |
+-------+---------------------+-------------+
| 304 | Not Modified | Section 4.1 |
| 412 | Precondition Failed | Section 4.2 |
+-------+---------------------+-------------+
6.2. Header Field Registration
The Message Header Field Registry located at <http://www.iana.org/
assignments/message-headers/message-header-index.html> shall be
updated with the permanent registrations below (see [RFC3864]):
+---------------------+----------+----------+-------------+
| Header Field Name | Protocol | Status | Reference |
+---------------------+----------+----------+-------------+
| ETag | http | standard | Section 2.3 |
| If-Match | http | standard | Section 3.1 |
| If-Modified-Since | http | standard | Section 3.3 |
| If-None-Match | http | standard | Section 3.2 |
| If-Unmodified-Since | http | standard | Section 3.4 |
| Last-Modified | http | standard | Section 2.2 |
+---------------------+----------+----------+-------------+
The change controller is: "IETF (iesg@ietf.org) - Internet
Engineering Task Force".
7. Security Considerations
No additional security considerations have been identified beyond
those applicable to HTTP in general [Part1].
The validators defined by this specification are not intended to
ensure the validity of a representation, guard against malicious
changes, or detect man-in-the-middle attacks. At best, they enable
more efficient cache updates and optimistic concurrent writes when
all participants are behaving nicely. At worst, the conditions will
fail and the client will receive a response that is no more harmful
than an HTTP exchange without conditional requests.
8. Acknowledgments
See Section 9 of [Part1].
9. References
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9.1. Normative References
[Part1] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"HTTP/1.1, part 1: Message Routing and Syntax"",
draft-ietf-httpbis-p1-messaging-20 (work in progress),
July 2012.
[Part2] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"HTTP/1.1, part 2: Semantics and Payloads",
draft-ietf-httpbis-p2-semantics-20 (work in progress),
July 2012.
[Part5] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"HTTP/1.1, part 5: Range Requests",
draft-ietf-httpbis-p5-range-20 (work in progress),
July 2012.
[Part6] Fielding, R., Ed., Lafon, Y., Ed., Nottingham, M., Ed.,
and J. Reschke, Ed., "HTTP/1.1, part 6: Caching",
draft-ietf-httpbis-p6-cache-20 (work in progress),
July 2012.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
9.2. Informative References
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004.
[RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed
Authoring and Versioning (WebDAV)", RFC 4918, June 2007.
Appendix A. Changes from RFC 2616
Allow weak entity-tags in all requests except range requests
(Sections 2.1 and 3.2).
Change ETag header field ABNF not to use quoted-string, thus avoiding
escaping issues. (Section 2.3)
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Change ABNF productions for header fields to only define the field
value. (Section 3)
Appendix B. Imported ABNF
The following core rules are included by reference, as defined in
Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return),
CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any
8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII
character).
The rules below are defined in [Part1]:
OWS = <OWS, defined in [Part1], Section 3.2.1>
obs-text = <obs-text, defined in [Part1], Section 3.2.4>
The rules below are defined in other parts:
HTTP-date = <HTTP-date, defined in [Part2], Section 5.1>
Appendix C. Collected ABNF
ETag = entity-tag
HTTP-date = <HTTP-date, defined in [Part2], Section 5.1>
If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
entity-tag ] ) )
If-Modified-Since = HTTP-date
If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
entity-tag ] ) )
If-Unmodified-Since = HTTP-date
Last-Modified = HTTP-date
OWS = <OWS, defined in [Part1], Section 3.2.1>
entity-tag = [ weak ] opaque-tag
etagc = "!" / %x23-7E ; '#'-'~'
/ obs-text
obs-text = <obs-text, defined in [Part1], Section 3.2.4>
opaque-tag = DQUOTE *etagc DQUOTE
weak = %x57.2F ; W/
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Appendix D. Change Log (to be removed by RFC Editor before publication)
Changes up to the first Working Group Last Call draft are summarized
in <http://tools.ietf.org/html/
draft-ietf-httpbis-p4-conditional-19#appendix-C>.
D.1. Since draft-ietf-httpbis-p4-conditional-19
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/241>: "Need to
clarify eval order/interaction of conditional headers"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/354>: "ETags and
Conditional Requests"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/361>: "ABNF
requirements for recipients"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/363>: "Rare cases"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/365>: "Conditional
Request Security Considerations"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/371>: "If-Modified-
Since lacks definition for method != GET"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/372>: "refactor
conditional header field descriptions"
Index
3
304 Not Modified (status code) 18
4
412 Precondition Failed (status code) 19
E
ETag header field 9
G
Grammar
entity-tag 10
ETag 10
etagc 10
If-Match 14
If-Modified-Since 16
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If-None-Match 15
If-Unmodified-Since 17
Last-Modified 7
opaque-tag 10
weak 10
H
Header Fields
ETag 9
If-Match 14
If-Modified-Since 16
If-None-Match 15
If-Unmodified-Since 17
Last-Modified 7
I
If-Match header field 14
If-Modified-Since header field 16
If-None-Match header field 15
If-Unmodified-Since header field 17
L
Last-Modified header field 7
M
metadata 5
S
selected representation 4
Status Codes
304 Not Modified 18
412 Precondition Failed 19
V
validator 5
strong 6
weak 6
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Authors' Addresses
Roy T. Fielding (editor)
Adobe Systems Incorporated
345 Park Ave
San Jose, CA 95110
USA
EMail: fielding@gbiv.com
URI: http://roy.gbiv.com/
Yves Lafon (editor)
World Wide Web Consortium
W3C / ERCIM
2004, rte des Lucioles
Sophia-Antipolis, AM 06902
France
EMail: ylafon@w3.org
URI: http://www.raubacapeu.net/people/yves/
Julian F. Reschke (editor)
greenbytes GmbH
Hafenweg 16
Muenster, NW 48155
Germany
EMail: julian.reschke@greenbytes.de
URI: http://greenbytes.de/tech/webdav/
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