HTTPbis Working Group R. Fielding, Ed.
Internet-Draft Adobe
Obsoletes: 2616 (if approved) J. Gettys
Intended status: Standards Track Alcatel-Lucent
Expires: February 25, 2012 J. Mogul
HP
H. Frystyk
Microsoft
L. Masinter
Adobe
P. Leach
Microsoft
T. Berners-Lee
W3C/MIT
Y. Lafon, Ed.
W3C
J. Reschke, Ed.
greenbytes
August 24, 2011
HTTP/1.1, part 4: Conditional Requests
draft-ietf-httpbis-p4-conditional-16
Abstract
The Hypertext Transfer Protocol (HTTP) is an application-level
protocol for distributed, collaborative, hypertext information
systems. HTTP has been in use by the World Wide Web global
information initiative since 1990. This document is Part 4 of the
seven-part specification that defines the protocol referred to as
"HTTP/1.1" and, taken together, obsoletes RFC 2616.
Part 4 defines request header fields for indicating conditional
requests and the rules for constructing responses to those requests.
Editorial Note (To be removed by RFC Editor)
Discussion of this draft should take place on the HTTPBIS working
group mailing list (ietf-http-wg@w3.org), which is archived at
.
The current issues list is at
and related
documents (including fancy diffs) can be found at
.
The changes in this draft are summarized in Appendix C.17.
Fielding, et al. Expires February 25, 2012 [Page 1]
Internet-Draft HTTP/1.1, Part 4 August 2011
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
material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 25, 2012.
Copyright Notice
Copyright (c) 2011 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
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 5
Fielding, et al. Expires February 25, 2012 [Page 2]
Internet-Draft HTTP/1.1, Part 4 August 2011
1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 6
2. Validators . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Weak versus Strong . . . . . . . . . . . . . . . . . . . . 6
2.2. Last-Modified . . . . . . . . . . . . . . . . . . . . . . 8
2.2.1. Generation . . . . . . . . . . . . . . . . . . . . . . 8
2.2.2. Comparison . . . . . . . . . . . . . . . . . . . . . . 9
2.3. ETag . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.3.1. Generation . . . . . . . . . . . . . . . . . . . . . . 11
2.3.2. Comparison . . . . . . . . . . . . . . . . . . . . . . 11
2.3.3. Example: Entity-tags varying on Content-Negotiated
Resources . . . . . . . . . . . . . . . . . . . . . . 12
2.4. Rules for When to Use Entity-tags and Last-Modified
Dates . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3. Precondition Header Fields . . . . . . . . . . . . . . . . . . 14
3.1. If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.2. If-None-Match . . . . . . . . . . . . . . . . . . . . . . 15
3.3. If-Modified-Since . . . . . . . . . . . . . . . . . . . . 17
3.4. If-Unmodified-Since . . . . . . . . . . . . . . . . . . . 18
3.5. If-Range . . . . . . . . . . . . . . . . . . . . . . . . . 19
4. Status Code Definitions . . . . . . . . . . . . . . . . . . . 19
4.1. 304 Not Modified . . . . . . . . . . . . . . . . . . . . . 19
4.2. 412 Precondition Failed . . . . . . . . . . . . . . . . . 20
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
5.1. Status Code Registration . . . . . . . . . . . . . . . . . 20
5.2. Header Field Registration . . . . . . . . . . . . . . . . 20
6. Security Considerations . . . . . . . . . . . . . . . . . . . 20
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
8.1. Normative References . . . . . . . . . . . . . . . . . . . 21
8.2. Informative References . . . . . . . . . . . . . . . . . . 21
Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 22
Appendix B. Collected ABNF . . . . . . . . . . . . . . . . . . . 22
Appendix C. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 23
C.1. Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . . 23
C.2. Since draft-ietf-httpbis-p4-conditional-00 . . . . . . . . 23
C.3. Since draft-ietf-httpbis-p4-conditional-01 . . . . . . . . 23
C.4. Since draft-ietf-httpbis-p4-conditional-02 . . . . . . . . 23
C.5. Since draft-ietf-httpbis-p4-conditional-03 . . . . . . . . 23
C.6. Since draft-ietf-httpbis-p4-conditional-04 . . . . . . . . 24
C.7. Since draft-ietf-httpbis-p4-conditional-05 . . . . . . . . 24
C.8. Since draft-ietf-httpbis-p4-conditional-06 . . . . . . . . 24
C.9. Since draft-ietf-httpbis-p4-conditional-07 . . . . . . . . 24
C.10. Since draft-ietf-httpbis-p4-conditional-08 . . . . . . . . 24
C.11. Since draft-ietf-httpbis-p4-conditional-09 . . . . . . . . 25
C.12. Since draft-ietf-httpbis-p4-conditional-10 . . . . . . . . 25
C.13. Since draft-ietf-httpbis-p4-conditional-11 . . . . . . . . 25
C.14. Since draft-ietf-httpbis-p4-conditional-12 . . . . . . . . 25
Fielding, et al. Expires February 25, 2012 [Page 3]
Internet-Draft HTTP/1.1, Part 4 August 2011
C.15. Since draft-ietf-httpbis-p4-conditional-13 . . . . . . . . 25
C.16. Since draft-ietf-httpbis-p4-conditional-14 . . . . . . . . 26
C.17. Since draft-ietf-httpbis-p4-conditional-15 . . . . . . . . 26
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Fielding, et al. Expires February 25, 2012 [Page 4]
Internet-Draft HTTP/1.1, Part 4 August 2011
1. Introduction
This document defines the HTTP/1.1 conditional request mechanisms,
including both metadata for indicating/observing changes in resource
representations and request header fields that specify preconditions
on that metadata be checked before performing the request method.
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. Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
An implementation is not compliant if it fails to satisfy one or more
of the "MUST" or "REQUIRED" level requirements for the protocols it
implements. An implementation that satisfies all the "MUST" or
"REQUIRED" level and all the "SHOULD" level requirements for its
protocols is said to be "unconditionally compliant"; one that
satisfies all the "MUST" level requirements but not all the "SHOULD"
level requirements for its protocols is said to be "conditionally
compliant".
Fielding, et al. Expires February 25, 2012 [Page 5]
Internet-Draft HTTP/1.1, Part 4 August 2011
1.2. Syntax Notation
This specification uses the ABNF syntax defined in Section 1.2 of
[Part1] (which extends the syntax defined in [RFC5234] with a list
rule). Appendix B shows the collected ABNF, with the list rule
expanded.
The following core rules are included by reference, as defined in
[RFC5234], Appendix B.1: 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), VCHAR (any visible USASCII character),
and WSP (whitespace).
The ABNF rules below are defined in [Part1]:
OWS =
quoted-string =
HTTP-date =
2. Validators
This specification defines two forms of metadata that are commonly
used to observe resource state and test for preconditions:
modification dates and opaque entity tags. 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.
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 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
Fielding, et al. Expires February 25, 2012 [Page 6]
Internet-Draft HTTP/1.1, Part 4 August 2011
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 cryptographic 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 a server
SHOULD incorporate additional 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. A weak entity-tag SHOULD change whenever the origin server
considers prior representations to be unacceptable as a substitute
for the current representation. In other words, a weak entity-tag
SHOULD 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
Fielding, et al. Expires February 25, 2012 [Page 7]
Internet-Draft HTTP/1.1, Part 4 August 2011
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
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,
Fielding, et al. Expires February 25, 2012 [Page 8]
Internet-Draft HTTP/1.1, Part 4 August 2011
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.
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
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
Fielding, et al. Expires February 25, 2012 [Page 9]
Internet-Draft HTTP/1.1, Part 4 August 2011
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.
ETag = entity-tag
entity-tag = [ weak ] opaque-tag
weak = %x57.2F ; "W/", case-sensitive
opaque-tag = quoted-string
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).
Fielding, et al. Expires February 25, 2012 [Page 10]
Internet-Draft HTTP/1.1, Part 4 August 2011
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 stored hash of representation content, a
combination of various filesystem attributes, or a modification
timestamp that has sub-second 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 |
Fielding, et al. Expires February 25, 2012 [Page 11]
Internet-Draft HTTP/1.1, Part 4 August 2011
| "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 5
of [Part3]), and where the representations returned upon a GET
request vary based on the Accept-Encoding request header field
(Section 6.3 of [Part3]):
>> Request:
GET /index HTTP/1.1
Host: www.example.com
Accept-Encoding: gzip
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:
Fielding, et al. Expires February 25, 2012 [Page 12]
Internet-Draft HTTP/1.1, Part 4 August 2011
>> 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 must be
distinct from an unencoded representation to prevent conflicts
during cache updates and range requests. In contrast, transfer
codings (Section 6.2 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:
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.
Fielding, et al. Expires February 25, 2012 [Page 13]
Internet-Draft HTTP/1.1, Part 4 August 2011
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
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 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.
3. Precondition Header Fields
This section defines the syntax and semantics of HTTP/1.1 header
fields for applying preconditions on requests.
3.1. If-Match
The "If-Match" header field MAY 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
Fielding, et al. Expires February 25, 2012 [Page 14]
Internet-Draft HTTP/1.1, Part 4 August 2011
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
If any of the entity-tags listed in the If-Match field value match
(as per Section 2.3.2) the entity-tag of the selected representation
for the target resource, or if "*" is given and any current
representation exists for the target resource, then the server MAY
perform the request method as if the If-Match header field was not
present.
If none of the entity-tags match, or if "*" is given and no current
representation exists, the server MUST NOT perform the requested
method. Instead, the server MUST respond with the 412 (Precondition
Failed) status code.
If the request would, without the If-Match header field, result in
anything other than a 2xx or 412 status code, then the If-Match
header field MUST be ignored.
Examples:
If-Match: "xyzzy"
If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-Match: *
The result of a request having both an If-Match header field and
either an If-None-Match or an If-Modified-Since header fields is
undefined by this specification.
3.2. If-None-Match
The "If-None-Match" header field MAY 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 response if at
least one of those representations matches the selected
representation.
Fielding, et al. Expires February 25, 2012 [Page 15]
Internet-Draft HTTP/1.1, Part 4 August 2011
If-None-Match MAY 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
If any of the entity-tags listed in the If-None-Match field-value
match (as per Section 2.3.2) the entity-tag of the selected
representation, or if "*" is given and any current representation
exists for that resource, then 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.
If none of the entity-tags match, then 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 or 304 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: *
The result of a request having both an If-None-Match header field and
either an If-Match or an If-Unmodified-Since header fields is
undefined by this specification.
Fielding, et al. Expires February 25, 2012 [Page 16]
Internet-Draft HTTP/1.1, Part 4 August 2011
3.3. If-Modified-Since
The "If-Modified-Since" header field MAY be used to make a request
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.
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-
Fielding, et al. Expires February 25, 2012 [Page 17]
Internet-Draft HTTP/1.1, Part 4 August 2011
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.
The result of a request having both an If-Modified-Since header field
and either an If-Match or an If-Unmodified-Since header fields is
undefined by this specification.
3.4. If-Unmodified-Since
The "If-Unmodified-Since" header field MAY 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
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
field) would result in anything other than a 2xx or 412 status code,
the If-Unmodified-Since header field SHOULD be ignored.
If the specified date is invalid, the header field MUST be ignored.
The result of a request having both an If-Unmodified-Since header
field and either an If-None-Match or an If-Modified-Since header
fields is undefined by this specification.
Fielding, et al. Expires February 25, 2012 [Page 18]
Internet-Draft HTTP/1.1, Part 4 August 2011
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 9.3 of
[Part1]) unless its omission is required by Section 9.3.1 of [Part1].
If a 200 response to the same request would have included any of the
header fields Cache-Control, Content-Location, ETag, Expires, Last-
Modified, 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
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 the
outbound 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.
Fielding, et al. Expires February 25, 2012 [Page 19]
Internet-Draft HTTP/1.1, Part 4 August 2011
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. IANA Considerations
5.1. Status Code Registration
The HTTP Status Code Registry located at
shall be updated
with the registrations below:
+-------+---------------------+-------------+
| Value | Description | Reference |
+-------+---------------------+-------------+
| 304 | Not Modified | Section 4.1 |
| 412 | Precondition Failed | Section 4.2 |
+-------+---------------------+-------------+
5.2. Header Field Registration
The Message Header Field Registry located at 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".
6. Security Considerations
No additional security considerations have been identified beyond
those applicable to HTTP in general [Part1].
Fielding, et al. Expires February 25, 2012 [Page 20]
Internet-Draft HTTP/1.1, Part 4 August 2011
7. Acknowledgments
See Section 12 of [Part1].
8. References
8.1. Normative References
[Part1] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections,
and Message Parsing", draft-ietf-httpbis-p1-messaging-16
(work in progress), August 2011.
[Part3] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
and J. Reschke, Ed., "HTTP/1.1, part 3: Message Payload
and Content Negotiation", draft-ietf-httpbis-p3-payload-16
(work in progress), August 2011.
[Part5] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and
Partial Responses", draft-ietf-httpbis-p5-range-16 (work
in progress), August 2011.
[Part6] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
Nottingham, M., Ed., and J. Reschke, Ed., "HTTP/1.1, part
6: Caching", draft-ietf-httpbis-p6-cache-16 (work in
progress), August 2011.
[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.
8.2. 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.
Fielding, et al. Expires February 25, 2012 [Page 21]
Internet-Draft HTTP/1.1, Part 4 August 2011
[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 ABNF productions for header fields to only define the field
value. (Section 3)
Appendix B. Collected ABNF
ETag = entity-tag
HTTP-date =
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 =
entity-tag = [ weak ] opaque-tag
opaque-tag = quoted-string
quoted-string =
weak = %x57.2F ; W/
ABNF diagnostics:
; ETag defined but not used
; If-Match defined but not used
; If-Modified-Since defined but not used
; If-None-Match defined but not used
; If-Unmodified-Since defined but not used
; Last-Modified defined but not used
Fielding, et al. Expires February 25, 2012 [Page 22]
Internet-Draft HTTP/1.1, Part 4 August 2011
Appendix C. Change Log (to be removed by RFC Editor before publication)
C.1. Since RFC 2616
Extracted relevant partitions from [RFC2616].
C.2. Since draft-ietf-httpbis-p4-conditional-00
Closed issues:
o : "Normative and
Informative references"
Other changes:
o Move definitions of 304 and 412 condition codes from Part2.
C.3. Since draft-ietf-httpbis-p4-conditional-01
Ongoing work on ABNF conversion
():
o Add explicit references to BNF syntax and rules imported from
other parts of the specification.
C.4. Since draft-ietf-httpbis-p4-conditional-02
Closed issues:
o : "Weak ETags on
non-GET requests"
Ongoing work on IANA Message Header Field Registration
():
o Reference RFC 3984, and update header field registrations for
header fields defined in this document.
C.5. Since draft-ietf-httpbis-p4-conditional-03
Closed issues:
o : "Examples for
ETag matching"
o : "'entity
value' undefined"
Fielding, et al. Expires February 25, 2012 [Page 23]
Internet-Draft HTTP/1.1, Part 4 August 2011
o : "bogus 2068
Date header reference"
C.6. Since draft-ietf-httpbis-p4-conditional-04
Ongoing work on ABNF conversion
():
o Use "/" instead of "|" for alternatives.
o Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
whitespace ("OWS") and required whitespace ("RWS").
o Rewrite ABNFs to spell out whitespace rules, factor out header
field value format definitions.
C.7. Since draft-ietf-httpbis-p4-conditional-05
Final work on ABNF conversion
():
o Add appendix containing collected and expanded ABNF, reorganize
ABNF introduction.
C.8. Since draft-ietf-httpbis-p4-conditional-06
Closed issues:
o : "case-
sensitivity of etag weakness indicator"
C.9. Since draft-ietf-httpbis-p4-conditional-07
Closed issues:
o : "Weak ETags on
non-GET requests" (If-Match still was defined to require strong
matching)
o : "move IANA
registrations for optional status codes"
C.10. Since draft-ietf-httpbis-p4-conditional-08
No significant changes.
Fielding, et al. Expires February 25, 2012 [Page 24]
Internet-Draft HTTP/1.1, Part 4 August 2011
C.11. Since draft-ietf-httpbis-p4-conditional-09
No significant changes.
C.12. Since draft-ietf-httpbis-p4-conditional-10
Closed issues:
o : "Clarify
'Requested Variant'"
o : "Clarify
entity / representation / variant terminology"
o : "consider
removing the 'changes from 2068' sections"
C.13. Since draft-ietf-httpbis-p4-conditional-11
None.
C.14. Since draft-ietf-httpbis-p4-conditional-12
Closed issues:
o : "Header
Classification"
C.15. Since draft-ietf-httpbis-p4-conditional-13
Closed issues:
o : "If-* and
entities"
o : "Definition of
validator weakness"
o : "untangle
ABNFs for header fields"
o : "ETags and
Quotes"
Fielding, et al. Expires February 25, 2012 [Page 25]
Internet-Draft HTTP/1.1, Part 4 August 2011
C.16. Since draft-ietf-httpbis-p4-conditional-14
None.
C.17. Since draft-ietf-httpbis-p4-conditional-15
Closed issues:
o : "If-Range
should be listed when dicussing contexts where L-M can be
considered strong"
Index
3
304 Not Modified (status code) 19
4
412 Precondition Failed (status code) 20
E
ETag header field 10
G
Grammar
entity-tag 10
ETag 10
If-Match 15
If-Modified-Since 17
If-None-Match 16
If-Unmodified-Since 18
Last-Modified 8
opaque-tag 10
weak 10
H
Header Fields
ETag 10
If-Match 14
If-Modified-Since 17
If-None-Match 15
If-Unmodified-Since 18
Last-Modified 8
I
If-Match header field 14
If-Modified-Since header field 17
If-None-Match header field 15
Fielding, et al. Expires February 25, 2012 [Page 26]
Internet-Draft HTTP/1.1, Part 4 August 2011
If-Unmodified-Since header field 18
L
Last-Modified header field 8
M
metadata 6
S
selected representation 5
Status Codes
304 Not Modified 19
412 Precondition Failed 20
V
validator 6
strong 6
weak 6
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/
Jim Gettys
Alcatel-Lucent Bell Labs
21 Oak Knoll Road
Carlisle, MA 01741
USA
EMail: jg@freedesktop.org
URI: http://gettys.wordpress.com/
Fielding, et al. Expires February 25, 2012 [Page 27]
Internet-Draft HTTP/1.1, Part 4 August 2011
Jeffrey C. Mogul
Hewlett-Packard Company
HP Labs, Large Scale Systems Group
1501 Page Mill Road, MS 1177
Palo Alto, CA 94304
USA
EMail: JeffMogul@acm.org
Henrik Frystyk Nielsen
Microsoft Corporation
1 Microsoft Way
Redmond, WA 98052
USA
EMail: henrikn@microsoft.com
Larry Masinter
Adobe Systems Incorporated
345 Park Ave
San Jose, CA 95110
USA
EMail: LMM@acm.org
URI: http://larry.masinter.net/
Paul J. Leach
Microsoft Corporation
1 Microsoft Way
Redmond, WA 98052
EMail: paulle@microsoft.com
Tim Berners-Lee
World Wide Web Consortium
MIT Computer Science and Artificial Intelligence Laboratory
The Stata Center, Building 32
32 Vassar Street
Cambridge, MA 02139
USA
EMail: timbl@w3.org
URI: http://www.w3.org/People/Berners-Lee/
Fielding, et al. Expires February 25, 2012 [Page 28]
Internet-Draft HTTP/1.1, Part 4 August 2011
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
Phone: +49 251 2807760
Fax: +49 251 2807761
EMail: julian.reschke@greenbytes.de
URI: http://greenbytes.de/tech/webdav/
Fielding, et al. Expires February 25, 2012 [Page 29]