HTTPbis Working Group R. Fielding, Ed.
Internet-Draft Day Software
Obsoletes: 2616 (if approved) J. Gettys
Intended status: Standards Track Alcatel-Lucent
Expires: February 5, 2011 J. Mogul
HP
H. Frystyk
Microsoft
L. Masinter
Adobe Systems
P. Leach
Microsoft
T. Berners-Lee
W3C/MIT
Y. Lafon, Ed.
W3C
J. Reschke, Ed.
greenbytes
August 4, 2010
HTTP/1.1, part 4: Conditional Requests
draft-ietf-httpbis-p4-conditional-11
Abstract
The Hypertext Transfer Protocol (HTTP) is an application-level
protocol for distributed, collaborative, hypermedia information
systems. HTTP has been in use by the World Wide Web global
information initiative since 1990. 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). 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 C.12.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
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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 5, 2011.
Copyright Notice
Copyright (c) 2010 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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Contributions published or made publicly available before November
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 4
1.2.1. Core Rules . . . . . . . . . . . . . . . . . . . . . . 5
1.2.2. ABNF Rules defined in other Parts of the
Specification . . . . . . . . . . . . . . . . . . . . 5
2. Entity-Tags . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Example: Entity-tags varying on Content-Negotiated
Resources . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Status Code Definitions . . . . . . . . . . . . . . . . . . . 7
3.1. 304 Not Modified . . . . . . . . . . . . . . . . . . . . . 7
3.2. 412 Precondition Failed . . . . . . . . . . . . . . . . . 7
4. Weak and Strong Validators . . . . . . . . . . . . . . . . . . 8
5. Rules for When to Use Entity-tags and Last-Modified Dates . . 10
6. Header Field Definitions . . . . . . . . . . . . . . . . . . . 12
6.1. ETag . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.2. If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.3. If-Modified-Since . . . . . . . . . . . . . . . . . . . . 14
6.4. If-None-Match . . . . . . . . . . . . . . . . . . . . . . 16
6.5. If-Unmodified-Since . . . . . . . . . . . . . . . . . . . 17
6.6. Last-Modified . . . . . . . . . . . . . . . . . . . . . . 18
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
7.1. Status Code Registration . . . . . . . . . . . . . . . . . 19
7.2. Header Field Registration . . . . . . . . . . . . . . . . 19
8. Security Considerations . . . . . . . . . . . . . . . . . . . 19
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
10.1. Normative References . . . . . . . . . . . . . . . . . . . 19
10.2. Informative References . . . . . . . . . . . . . . . . . . 20
Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 20
Appendix B. Collected ABNF . . . . . . . . . . . . . . . . . . . 21
Appendix C. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 21
C.1. Since RFC2616 . . . . . . . . . . . . . . . . . . . . . . 21
C.2. Since draft-ietf-httpbis-p4-conditional-00 . . . . . . . . 22
C.3. Since draft-ietf-httpbis-p4-conditional-01 . . . . . . . . 22
C.4. Since draft-ietf-httpbis-p4-conditional-02 . . . . . . . . 22
C.5. Since draft-ietf-httpbis-p4-conditional-03 . . . . . . . . 22
C.6. Since draft-ietf-httpbis-p4-conditional-04 . . . . . . . . 23
C.7. Since draft-ietf-httpbis-p4-conditional-05 . . . . . . . . 23
C.8. Since draft-ietf-httpbis-p4-conditional-06 . . . . . . . . 23
C.9. Since draft-ietf-httpbis-p4-conditional-07 . . . . . . . . 23
C.10. Since draft-ietf-httpbis-p4-conditional-08 . . . . . . . . 23
C.11. Since draft-ietf-httpbis-p4-conditional-09 . . . . . . . . 23
C.12. Since draft-ietf-httpbis-p4-conditional-10 . . . . . . . . 24
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
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1. Introduction
This document defines HTTP/1.1 response metadata for indicating
potential changes to payload content, including modification time
stamps and opaque entity-tags, and the HTTP conditional request
mechanisms that allow preconditions to be placed on a request method.
Conditional GET requests allow for efficient cache updates. Other
conditional request methods are used to protect against overwriting
or misunderstanding the state of a resource that has been changed
unbeknownst to the requesting client.
This document is currently disorganized in order to minimize the
changes between drafts and enable reviewers to see the smaller errata
changes. The next draft will reorganize the sections to better
reflect the content. In particular, the sections on resource
metadata will be discussed first and then followed by each
conditional request-header, concluding with a definition of
precedence and the expectation of ordering strong validator checks
before weak validator checks. It is likely that more content from
[Part6] will migrate to this part, where appropriate. The current
mess reflects how widely dispersed these topics and associated
requirements had become in [RFC2616].
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".
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),
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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).
1.2.1. Core Rules
The core rules below are defined in Section 1.2.2 of [Part1]:
quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
OWS = <OWS, defined in [Part1], Section 1.2.2>
1.2.2. ABNF Rules defined in other Parts of the Specification
The ABNF rules below are defined in other parts:
HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>
2. Entity-Tags
Entity-tags are used for comparing two or more representations of the
same resource. HTTP/1.1 uses entity-tags in the ETag (Section 6.1),
If-Match (Section 6.2), If-None-Match (Section 6.4), and If-Range
(Section 5.3 of [Part5]) header fields. The definition of how they
are used and compared as cache validators is in Section 4. An
entity-tag consists of an opaque quoted string, possibly prefixed by
a weakness indicator.
entity-tag = [ weak ] opaque-tag
weak = %x57.2F ; "W/", case-sensitive
opaque-tag = quoted-string
A "strong entity-tag" MAY be shared by two representations of a
resource only if they are equivalent by octet equality.
A "weak entity-tag", indicated by the "W/" prefix, MAY be shared by
two representations of a resource only if the representations are
equivalent and could be substituted for each other with no
significant change in semantics. A weak entity-tag can only be used
for weak comparison.
An entity-tag MUST be unique across all versions of all
representations associated with a particular resource. A given
entity-tag value MAY be used for representations obtained by requests
on different URIs. The use of the same entity-tag value in
conjunction with representations obtained by requests on different
URIs does not imply the equivalence of those representations.
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2.1. 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:
>> 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...
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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.
3. Status Code Definitions
3.1. 304 Not Modified
If the client has performed a conditional GET request and access is
allowed, but the document has not been modified, the server SHOULD
respond with this status code. The 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 a 304 response includes an entity-tag that indicates a
representation not currently cached, then the recipient MUST NOT use
the 304 to update its own cache. If that 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,
disregard the response and repeat the request without the
conditional.
If a cache uses a received 304 response to update a cache entry, the
cache MUST update the entry to reflect any new field values given in
the response.
3.2. 412 Precondition Failed
The precondition given in one or more of the request-header fields
evaluated to false when it was tested on the server. This response
code allows the client to place preconditions on the current resource
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metadata (header field data) and thus prevent the requested method
from being applied to a resource other than the one intended.
4. Weak and Strong Validators
Since both origin servers and caches will compare two validators to
decide if they represent the same or different representations, one
normally would expect that if the representation (including both
representation header fields and representation body) changes in any
way, then the associated validator would change as well. If this is
true, then we call this validator a "strong validator".
However, there might be cases when a server prefers to change the
validator only on semantically significant changes, and not when
insignificant aspects of the representation change. A validator that
does not always change when the representation changes is a "weak
validator".
An entity-tag is normally a strong validator, but the protocol
provides a mechanism to tag an entity-tag as "weak". One can think
of a strong validator as one that changes whenever the sequence of
bits in a representation changes, while a weak value changes whenever
the meaning of a representation changes. Alternatively, one can
think of a strong validator as part of an identifier for a specific
representation, whereas a weak validator is part of an identifier for
a set of semantically equivalent representations.
Note: One example of a strong validator is an integer that is
incremented in stable storage every time a representation is
changed.
A representation's modification time, if defined with only one-
second resolution, could be a weak validator, since it is possible
that the representation might be modified twice during a single
second.
Support for weak validators is optional. However, weak validators
allow for more efficient caching of equivalent objects; for
example, a hit counter on a site is probably good enough if it is
updated every few days or weeks, and any value during that period
is likely "good enough" to be equivalent.
A "use" of a validator is either when a client generates a request
and includes the validator in a validating header field, or when a
server compares two validators.
Strong validators are usable in any context. Weak validators are
only usable in contexts that do not depend on exact equality of a
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representation. For example, either kind is usable for a normal
conditional GET. However, only a strong validator is usable for a
sub-range retrieval, since otherwise the client might end up with an
internally inconsistent representation.
Clients MUST NOT use weak validators in range requests ([Part5]).
The only function that HTTP/1.1 defines on validators is comparison.
There are two validator comparison functions, depending on whether
the comparison context allows the use of weak validators or not:
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 |
+--------+--------+-------------------+-----------------+
An entity-tag is strong unless it is explicitly tagged as weak.
Section 2 gives the syntax for entity-tags.
A Last-Modified time, when used as a validator in a request, is
implicitly weak unless it is possible to deduce that it is strong,
using the following rules:
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
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o The validator is about to be used by a client in an If-Modified-
Since or If-Unmodified-Since header, because the client has a
cache entry for the associated 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.
If a client wishes to perform a sub-range retrieval on a value for
which it has only a Last-Modified time and no opaque validator, it
MAY do this only if the Last-Modified time is strong in the sense
described here.
A cache or origin server receiving a conditional range request
([Part5]) MUST use the strong comparison function to evaluate the
condition.
These rules allow HTTP/1.1 caches and clients to safely perform sub-
range retrievals on values that have been obtained from HTTP/1.0
servers.
5. 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
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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,
unless the risk of a breakdown in semantic transparency that could
result from using this date in an If-Modified-Since header would
lead to serious problems.
In other words, the preferred behavior for an HTTP/1.1 origin server
is to send both a strong entity-tag and a Last-Modified value.
In order to be legal, a strong entity-tag MUST change whenever the
associated representation changes in any way. A weak entity-tag
SHOULD change whenever the associated representation changes in a
semantically significant way.
Note: In order to provide semantically transparent caching, an
origin server must avoid reusing a specific strong entity-tag
value for two different representations, or reusing a specific
weak entity-tag value for two semantically different
representations. Cache entries might persist for arbitrarily long
periods, regardless of expiration times, so it might be
inappropriate to expect that a cache will never again attempt to
validate an entry using a validator that it obtained at some point
in the past.
HTTP/1.1 clients:
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.
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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 will ignore entity-tags. Generally,
last-modified values received or used by these systems will
support transparent and efficient caching, and so HTTP/1.1 origin
servers should provide Last-Modified values. In those rare cases
where the use of a Last-Modified value as a validator by an
HTTP/1.0 system could result in a serious problem, then HTTP/1.1
origin servers should not provide one.
6. Header Field Definitions
This section defines the syntax and semantics of HTTP/1.1 header
fields related to conditional requests.
6.1. ETag
The "ETag" response-header field provides the current value of the
entity-tag (see Section 2) for one representation of the target
resource. An entity-tag is intended for use as a resource-local
identifier for differentiating between representations of the same
resource that vary over time or via content negotiation (see
Section 4).
ETag = "ETag" ":" OWS ETag-v
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ETag-v = entity-tag
Examples:
ETag: "xyzzy"
ETag: W/"xyzzy"
ETag: ""
An entity-tag provides an "opaque" cache validator that allows for
more reliable validation than modification dates in situations where
it is inconvenient to store modification dates, where the one-second
resolution of HTTP date values is not sufficient, or where the origin
server wishes to avoid certain paradoxes that might arise from the
use of modification dates.
The principle behind entity-tags is that only the service author
knows the semantics of a resource well enough to select an
appropriate cache validation mechanism, and the specification of any
validator comparison function more complex than byte-equality would
open up a can of worms. Thus, comparisons of any other headers
(except Last-Modified, for compatibility with HTTP/1.0) are never
used for purposes of validating a cache entry.
6.2. If-Match
The "If-Match" request-header field is used to make a request method
conditional. A client that has one or more representations
previously obtained from the resource can verify that one of those
representations is current by including a list of their associated
entity-tags in the If-Match header field.
This allows efficient updates of cached information with a minimum
amount of transaction overhead. It is also used when updating
resources, to prevent inadvertent modification of the wrong version
of a resource. As a special case, the value "*" matches any current
representation of the resource.
If-Match = "If-Match" ":" OWS If-Match-v
If-Match-v = "*" / 1#entity-tag
If any of the entity-tags match the entity-tag of the representation
that would have been returned in the response to a similar GET
request (without the If-Match header) on that resource, or if "*" is
given and any current representation exists for that resource, then
the server MAY perform the requested method as if the If-Match header
field did not exist.
If none of the entity-tags match, or if "*" is given and no current
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representation exists, the server MUST NOT perform the requested
method, and MUST return a 412 (Precondition Failed) response. This
behavior is most useful when the client wants to prevent an updating
method, such as PUT, from modifying a resource that has changed since
the client last retrieved it.
If the request would, without the If-Match header field, result in
anything other than a 2xx or 412 status code, then the If-Match
header MUST be ignored.
The meaning of "If-Match: *" is that the method SHOULD be performed
if the representation selected by the origin server (or by a cache,
possibly using the Vary mechanism, see Section 3.5 of [Part6])
exists, and MUST NOT be performed if the representation does not
exist.
A request intended to update a resource (e.g., a PUT) MAY include an
If-Match header field to signal that the request method MUST NOT be
applied if the representation corresponding to the If-Match value (a
single entity-tag) is no longer a representation of that resource.
This allows the user to indicate that they do not wish the request to
be successful if the resource has been changed without their
knowledge. Examples:
If-Match: "xyzzy"
If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-Match: *
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.
6.3. If-Modified-Since
The "If-Modified-Since" request-header field is used to make a
request method conditional by date: if the representation that would
have been transferred in a 200 response to a GET request has not been
modified since the time specified in this field, then do not perform
the method; instead, respond as detailed below.
If-Modified-Since = "If-Modified-Since" ":" OWS
If-Modified-Since-v
If-Modified-Since-v = HTTP-date
An example of the field is:
If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
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A GET method with an If-Modified-Since header and no Range header
requests that the representation be transferred only if it has been
modified since the date given by the If-Modified-Since header. The
algorithm for determining this includes the following cases:
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 representation has been modified since the If-Modified-
Since date, the response is exactly the same as for a normal GET.
3. If the 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 request-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 instead of a date taken from the Last-Modified header 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.
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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.
6.4. If-None-Match
The "If-None-Match" request-header field is used to make a request
method conditional. A client that has one or more representations
previously obtained from the resource can verify that none of those
representations is current by including a list of their associated
entity-tags in the If-None-Match header field.
This allows efficient updates of cached information with a minimum
amount of transaction overhead. It is also used to prevent a method
(e.g., PUT) from inadvertently modifying an existing resource when
the client believes that the resource does not exist.
As a special case, the value "*" matches any current representation
of the resource.
If-None-Match = "If-None-Match" ":" OWS If-None-Match-v
If-None-Match-v = "*" / 1#entity-tag
If any of the entity-tags match the entity-tag of the representation
that would have been returned in the response to a similar GET
request (without the If-None-Match header) on that resource, or if
"*" is given and any current representation exists for that resource,
then the server MUST NOT perform the requested method, unless
required to do so because the resource's modification date fails to
match that supplied in an If-Modified-Since header field in the
request. Instead, if the request method was GET or HEAD, the server
SHOULD respond with a 304 (Not Modified) response, including the
cache-related header fields (particularly ETag) of one of the
representations that matched. 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 MUST be ignored. (See Section 5 for a discussion of
server behavior when both If-Modified-Since and If-None-Match appear
in the same request.)
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The meaning of "If-None-Match: *" is that the method MUST NOT be
performed if the representation selected by the origin server (or by
a cache, possibly using the Vary mechanism, see Section 3.5 of
[Part6]) exists, and SHOULD be performed if the representation does
not exist. This feature is intended to be useful in preventing races
between PUT operations.
Examples:
If-None-Match: "xyzzy"
If-None-Match: W/"xyzzy"
If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
If-None-Match: *
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.
6.5. If-Unmodified-Since
The "If-Unmodified-Since" request-header field is used to make a
request method conditional. If the representation that would have
been transferred in a 200 response to a GET request on the same
resource has not been modified since the time specified in this
field, the server SHOULD perform the requested operation as if the
If-Unmodified-Since header were not present.
If the representation has been modified since the specified time, the
server MUST NOT perform the requested operation, and MUST return a
412 (Precondition Failed).
If-Unmodified-Since = "If-Unmodified-Since" ":" OWS
If-Unmodified-Since-v
If-Unmodified-Since-v = HTTP-date
An example of the field is:
If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
If the request normally (i.e., without the If-Unmodified-Since
header) would result in anything other than a 2xx or 412 status code,
the If-Unmodified-Since header SHOULD be ignored.
If the specified date is invalid, the header is 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
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fields is undefined by this specification.
6.6. Last-Modified
The "Last-Modified" header field indicates the date and time at which
the origin server believes the representation was last modified.
Last-Modified = "Last-Modified" ":" OWS Last-Modified-v
Last-Modified-v = HTTP-date
An example of its use is
Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
The exact meaning of this header field depends on the implementation
of the origin server and the nature of the original resource. For
files, it might be just the file system last-modified time. For
representations with dynamically included parts, it might be the most
recent of the set of last-modify times for its component parts. For
database gateways, it might be the last-update time stamp of the
record. For virtual objects, it might be the last time the internal
state changed.
An origin server MUST NOT send a Last-Modified date which is later
than the server's time of message origination. In such cases, where
the resource's last modification would indicate some time in the
future, the server MUST replace that date with the message
origination date.
An origin server SHOULD obtain the Last-Modified value of the
representation as close as possible to the time that it generates the
Date value of its response. This allows a recipient to make an
accurate assessment of the representation's modification time,
especially if the representation changes near the time that the
response is generated.
HTTP/1.1 servers SHOULD send Last-Modified whenever feasible.
The Last-Modified header field value is often used as a cache
validator. In simple terms, a cache entry is considered to be valid
if the representation has not been modified since the Last-Modified
value.
7. IANA Considerations
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7.1. 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:
+-------+---------------------+-------------+
| Value | Description | Reference |
+-------+---------------------+-------------+
| 304 | Not Modified | Section 3.1 |
| 412 | Precondition Failed | Section 3.2 |
+-------+---------------------+-------------+
7.2. 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 6.1 |
| If-Match | http | standard | Section 6.2 |
| If-Modified-Since | http | standard | Section 6.3 |
| If-None-Match | http | standard | Section 6.4 |
| If-Unmodified-Since | http | standard | Section 6.5 |
| Last-Modified | http | standard | Section 6.6 |
+---------------------+----------+----------+-------------+
The change controller is: "IETF (iesg@ietf.org) - Internet
Engineering Task Force".
8. Security Considerations
No additional security considerations have been identified beyond
those applicable to HTTP in general [Part1].
9. Acknowledgments
10. References
10.1. 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-11
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(work in progress), August 2010.
[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-11
(work in progress), August 2010.
[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-11 (work
in progress), August 2010.
[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-11 (work in
progress), August 2010.
[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.
10.2. 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.
Appendix A. Changes from RFC 2616
Allow weak entity-tags in all requests except range requests
(Sections 4 and 6.4).
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Appendix B. Collected ABNF
ETag = "ETag:" OWS ETag-v
ETag-v = entity-tag
HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>
If-Match = "If-Match:" OWS If-Match-v
If-Match-v = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
entity-tag ] ) )
If-Modified-Since = "If-Modified-Since:" OWS If-Modified-Since-v
If-Modified-Since-v = HTTP-date
If-None-Match = "If-None-Match:" OWS If-None-Match-v
If-None-Match-v = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
entity-tag ] ) )
If-Unmodified-Since = "If-Unmodified-Since:" OWS
If-Unmodified-Since-v
If-Unmodified-Since-v = HTTP-date
Last-Modified = "Last-Modified:" OWS Last-Modified-v
Last-Modified-v = HTTP-date
OWS = <OWS, defined in [Part1], Section 1.2.2>
entity-tag = [ weak ] opaque-tag
opaque-tag = quoted-string
quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
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
Appendix C. Change Log (to be removed by RFC Editor before publication)
C.1. Since RFC2616
Extracted relevant partitions from [RFC2616].
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C.2. Since draft-ietf-httpbis-p4-conditional-00
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/35>: "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
(<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
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 <http://tools.ietf.org/wg/httpbis/trac/ticket/116>: "Weak ETags on
non-GET requests"
Ongoing work on IANA Message Header Registration
(<http://tools.ietf.org/wg/httpbis/trac/ticket/40>):
o Reference RFC 3984, and update header registrations for headers
defined in this document.
C.5. Since draft-ietf-httpbis-p4-conditional-03
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/71>: "Examples for
ETag matching"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/124>: "'entity
value' undefined"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/126>: "bogus 2068
Date header reference"
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C.6. Since draft-ietf-httpbis-p4-conditional-04
Ongoing work on ABNF conversion
(<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
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
value format definitions.
C.7. Since draft-ietf-httpbis-p4-conditional-05
Final work on ABNF conversion
(<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
o Add appendix containing collected and expanded ABNF, reorganize
ABNF introduction.
C.8. Since draft-ietf-httpbis-p4-conditional-06
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/153>: "case-
sensitivity of etag weakness indicator"
C.9. Since draft-ietf-httpbis-p4-conditional-07
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/116>: "Weak ETags on
non-GET requests" (If-Match still was defined to require strong
matching)
o <http://tools.ietf.org/wg/httpbis/trac/ticket/198>: "move IANA
registrations for optional status codes"
C.10. Since draft-ietf-httpbis-p4-conditional-08
No significant changes.
C.11. Since draft-ietf-httpbis-p4-conditional-09
No significant changes.
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C.12. Since draft-ietf-httpbis-p4-conditional-10
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/69>: "Clarify
'Requested Variant'"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/109>: "Clarify
entity / representation / variant terminology"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/220>: "consider
removing the 'changes from 2068' sections"
Index
3
304 Not Modified (status code) 7
4
412 Precondition Failed (status code) 7
E
ETag header 12
G
Grammar
entity-tag 5
ETag 12
ETag-v 12
If-Match 13
If-Match-v 13
If-Modified-Since 14
If-Modified-Since-v 14
If-None-Match 16
If-None-Match-v 16
If-Unmodified-Since 17
If-Unmodified-Since-v 17
Last-Modified 18
Last-Modified-v 18
opaque-tag 5
weak 5
H
Headers
ETag 12
If-Match 13
If-Modified-Since 14
If-None-Match 16
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If-Unmodified-Since 17
Last-Modified 18
I
If-Match header 13
If-Modified-Since header 14
If-None-Match header 16
If-Unmodified-Since header 17
L
Last-Modified header 18
S
Status Codes
304 Not Modified 7
412 Precondition Failed 7
Authors' Addresses
Roy T. Fielding (editor)
Day Software
23 Corporate Plaza DR, Suite 280
Newport Beach, CA 92660
USA
Phone: +1-949-706-5300
Fax: +1-949-706-5305
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/
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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/
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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/
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