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Network Working Group R. Fielding, Ed.
Internet-Draft Day Software
Obsoletes: 2616 (if approved) J. Gettys
Intended status: Standards Track One Laptop per Child
Expires: December 19, 2008 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
June 17, 2008
HTTP/1.1, part 4: Conditional Requests
draft-ietf-httpbis-p4-conditional-03
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
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This Internet-Draft will expire on December 19, 2008.
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Internet-Draft HTTP/1.1, Part 4 June 2008
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://www.tools.ietf.org/wg/httpbis/trac/report/11> and related
documents (including fancy diffs) can be found at
<http://www.tools.ietf.org/wg/httpbis/>.
The changes in this draft are summarized in Appendix B.4.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 4
2. Notational Conventions and Generic Grammar . . . . . . . . . . 4
3. Entity Tags . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Status Code Definitions . . . . . . . . . . . . . . . . . . . 5
4.1. 304 Not Modified . . . . . . . . . . . . . . . . . . . . . 5
4.2. 412 Precondition Failed . . . . . . . . . . . . . . . . . 6
5. Weak and Strong Validators . . . . . . . . . . . . . . . . . . 6
6. Rules for When to Use Entity Tags and Last-Modified Dates . . 9
7. Header Field Definitions . . . . . . . . . . . . . . . . . . . 11
7.1. ETag . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.2. If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.3. If-Modified-Since . . . . . . . . . . . . . . . . . . . . 13
7.4. If-None-Match . . . . . . . . . . . . . . . . . . . . . . 14
7.5. If-Unmodified-Since . . . . . . . . . . . . . . . . . . . 15
7.6. Last-Modified . . . . . . . . . . . . . . . . . . . . . . 16
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
8.1. Message Header Registration . . . . . . . . . . . . . . . 17
9. Security Considerations . . . . . . . . . . . . . . . . . . . 17
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
11.1. Normative References . . . . . . . . . . . . . . . . . . . 18
11.2. Informative References . . . . . . . . . . . . . . . . . . 18
Appendix A. Compatibility with Previous Versions . . . . . . . . 18
A.1. Changes from RFC 2616 . . . . . . . . . . . . . . . . . . 18
Appendix B. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 19
B.1. Since RFC2616 . . . . . . . . . . . . . . . . . . . . . . 19
B.2. Since draft-ietf-httpbis-p4-conditional-00 . . . . . . . . 19
B.3. Since draft-ietf-httpbis-p4-conditional-01 . . . . . . . . 19
B.4. Since draft-ietf-httpbis-p4-conditional-02 . . . . . . . . 19
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
Intellectual Property and Copyright Statements . . . . . . . . . . 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."
2. Notational Conventions and Generic Grammar
This specification uses the ABNF syntax defined in Section 2.1 of
[Part1] and the core rules defined in Section 2.2 of [Part1]:
[[abnf.dep: ABNF syntax and basic rules will be adopted from RFC
5234, see <http://tools.ietf.org/wg/httpbis/trac/ticket/36>.]]
quoted-string = <quoted-string, defined in [Part1], Section 2.2>
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The ABNF rules below are defined in other parts:
HTTP-date = <HTTP-date, defined in [Part1], Section 3.3.1>
3. Entity Tags
Entity tags are used for comparing two or more entities from the same
requested resource. HTTP/1.1 uses entity tags in the ETag
(Section 7.1), If-Match (Section 7.2), If-None-Match (Section 7.4),
and If-Range (Section 6.3 of [Part5]) header fields. The definition
of how they are used and compared as cache validators is in
Section 5. An entity tag consists of an opaque quoted string,
possibly prefixed by a weakness indicator.
entity-tag = [ weak ] opaque-tag
weak = "W/"
opaque-tag = quoted-string
A "strong entity tag" MAY be shared by two entities of a resource
only if they are equivalent by octet equality.
A "weak entity tag," indicated by the "W/" prefix, MAY be shared by
two entities of a resource only if the entities are equivalent and
could be substituted for each other with no significant change in
semantics. A weak entity tag can only be used for weak comparison.
An entity tag MUST be unique across all versions of all entities
associated with a particular resource. A given entity tag value MAY
be used for entities obtained by requests on different URIs. The use
of the same entity tag value in conjunction with entities obtained by
requests on different URIs does not imply the equivalence of those
entities.
4. Status Code Definitions
4.1. 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.
The response MUST include the following header fields:
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o Date, unless its omission is required by Section 8.3.1 of [Part1]
If a clockless origin server obeys these rules, and proxies and
clients add their own Date to any response received without one (as
already specified by [RFC2068], Section 14.19), caches will operate
correctly.
o ETag and/or Content-Location, if the header would have been sent
in a 200 response to the same request
o Expires, Cache-Control, and/or Vary, if the field-value might
differ from that sent in any previous response for the same
variant
If the conditional GET used a strong cache validator (see Section 5),
the response SHOULD NOT include other entity-headers. Otherwise
(i.e., the conditional GET used a weak validator), the response MUST
NOT include other entity-headers; this prevents inconsistencies
between cached entity-bodies and updated headers.
If a 304 response indicates an entity not currently cached, then the
cache MUST disregard the response and repeat the request without the
conditional.
If a cache uses a received 304 response to update a cache entry, the
cache MUST update the entry to reflect any new field values given in
the response.
4.2. 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
metainformation (header field data) and thus prevent the requested
method from being applied to a resource other than the one intended.
5. Weak and Strong Validators
Since both origin servers and caches will compare two validators to
decide if they represent the same or different entities, one normally
would expect that if the entity (the entity-body or any entity-
headers) changes in any way, then the associated validator would
change as well. If this is true, then we call this validator a
"strong validator."
However, there might be cases when a server prefers to change the
validator only on semantically significant changes, and not when
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insignificant aspects of the entity change. A validator that does
not always change when the resource changes is a "weak validator."
Entity tags are normally "strong validators," but the protocol
provides a mechanism to tag an entity tag as "weak." One can think
of a strong validator as one that changes whenever the bits of an
entity changes, while a weak value changes whenever the meaning of an
entity changes. Alternatively, one can think of a strong validator
as part of an identifier for a specific entity, while a weak
validator is part of an identifier for a set of semantically
equivalent entities.
Note: One example of a strong validator is an integer that is
incremented in stable storage every time an entity is changed.
An entity's modification time, if represented with one-second
resolution, could be a weak validator, since it is possible that
the resource 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 an
entity. For example, either kind is usable for a conditional GET of
a full entity. However, only a strong validator is usable for a sub-
range retrieval, since otherwise the client might end up with an
internally inconsistent entity.
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 validators MUST be identical in every way, and both MUST NOT
be weak.
o The weak comparison function: in order to be considered equal,
both validators MUST be identical in every way, but either or both
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of them MAY be tagged as "weak" without affecting the result.
An entity tag is strong unless it is explicitly tagged as weak.
Section 3 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 entity and,
o That origin server reliably knows that the associated entity 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 or If-Unmodified-Since header, because the client has a
cache entry for the associated entity, 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 entity, 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.
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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.
6. 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,
unless the risk of a breakdown in semantic transparency that could
result from using this date in an If-Modified-Since header would
lead to serious problems.
In other words, the preferred behavior for an HTTP/1.1 origin server
is to send both a strong entity tag and a Last-Modified value.
In order to be legal, a strong entity tag MUST change whenever the
associated entity value changes in any way. A weak entity tag SHOULD
change whenever the associated entity changes in a semantically
significant way.
Note: in order to provide semantically transparent caching, an
origin server must avoid reusing a specific strong entity tag
value for two different entities, or reusing a specific weak
entity tag value for two semantically different entities. Cache
entries might persist for arbitrarily long periods, regardless of
expiration times, so it might be inappropriate to expect that a
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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 If an entity tag has been provided by the origin server, MUST use
that entity tag in any cache-conditional request (using If-Match
or If-None-Match).
o If only a Last-Modified value has been provided by the origin
server, SHOULD use that value in non-subrange cache-conditional
requests (using If-Modified-Since).
o If only a Last-Modified value has been provided by an HTTP/1.0
origin server, MAY use that value in subrange cache-conditional
requests (using If-Unmodified-Since:). The user agent SHOULD
provide a way to disable this, in case of difficulty.
o If both an entity tag and a Last-Modified value have been provided
by the origin server, SHOULD use both validators in cache-
conditional requests. This allows both HTTP/1.0 and HTTP/1.1
caches to respond appropriately.
An HTTP/1.1 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 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 should transmit as much non-redundant
information as is available in their responses and requests.
HTTP/1.1 systems receiving this information will make the most
conservative assumptions about the validators they receive.
HTTP/1.0 clients and caches will ignore entity tags. Generally,
last-modified values received or used by these systems will
support transparent and efficient caching, and so HTTP/1.1 origin
servers should provide Last-Modified values. In those rare cases
where the use of a Last-Modified value as a validator by an
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HTTP/1.0 system could result in a serious problem, then HTTP/1.1
origin servers should not provide one.
7. Header Field Definitions
This section defines the syntax and semantics of HTTP/1.1 header
fields related to conditional requests.
For entity-header fields, both sender and recipient refer to either
the client or the server, depending on who sends and who receives the
entity.
7.1. ETag
The ETag response-header field provides the current value of the
entity tag for the requested variant. The headers used with entity
tags are described in Sections 7.2 and 7.4 of this document, and in
Section 6.3 of [Part5]. The entity tag MAY be used for comparison
with other entities from the same resource (see Section 5).
ETag = "ETag" ":" entity-tag
Examples:
ETag: "xyzzy"
ETag: W/"xyzzy"
ETag: ""
The ETag response-header field value, an entity tag, provides for an
"opaque" cache validator. This might allow more reliable validation
in situations where it is inconvenient to store modification dates,
where the one-second resolution of HTTP date values is not
sufficient, or where the origin server wishes to avoid certain
paradoxes that 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.
7.2. If-Match
The If-Match request-header field is used with a method to make it
conditional. A client that has one or more entities previously
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obtained from the resource can verify that one of those entities is
current by including a list of their associated entity tags in the
If-Match header field. Entity tags are defined in Section 3. The
purpose of this feature is to allow efficient updates of cached
information with a minimum amount of transaction overhead. It is
also used, on updating requests, to prevent inadvertent modification
of the wrong version of a resource. As a special case, the value "*"
matches any current entity of the resource.
If-Match = "If-Match" ":" ( "*" | 1#entity-tag )
If any of the entity tags match the entity tag of the entity that
would have been returned in the response to a similar GET request
(without the If-Match header) on that resource, or if "*" is given
and any current entity exists for that resource, then the server MAY
perform the requested method as if the If-Match header field did not
exist.
A server MUST use the strong comparison function (see Section 5) to
compare the entity tags in If-Match.
If none of the entity tags match, or if "*" is given and no current
entity exists, the server MUST NOT perform the requested method, and
MUST return a 412 (Precondition 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, 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 16.5 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 entity corresponding to the If-Match value (a single
entity tag) is no longer a representation of that resource. This
allows the user to indicate that they do not wish the request to be
successful if the resource has been changed without their knowledge.
Examples:
If-Match: "xyzzy"
If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
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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.
7.3. If-Modified-Since
The If-Modified-Since request-header field is used with a method to
make it conditional: if the requested variant has not been modified
since the time specified in this field, an entity will not be
returned from the server; instead, a 304 (Not Modified) response will
be returned without any message-body.
If-Modified-Since = "If-Modified-Since" ":" HTTP-date
An example of the field is:
If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
A GET method with an If-Modified-Since header and no Range header
requests that the identified entity be transferred only if it has
been modified since the date given by the If-Modified-Since header.
The algorithm for determining this includes the following cases:
1. If the request would normally result in anything other than a 200
(OK) status, or if the passed If-Modified-Since date is invalid,
the response is exactly the same as for a normal GET. A date
which is later than the server's current time is invalid.
2. If the variant has been modified since the If-Modified-Since
date, the response is exactly the same as for a normal GET.
3. If the variant 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 6.4 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
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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 should be aware of the fact that this
date is interpreted in the server's understanding of time. The
client should consider unsynchronized clocks and rounding problems
due to the different encodings of time between the client and
server. This includes the possibility of race conditions if the
document has changed between the time it was first requested and
the If-Modified-Since date of a subsequent request, and the
possibility of clock-skew-related problems if the If-Modified-
Since date is derived from the client's clock without correction
to the server's clock. Corrections for different time bases
between client and server are at best approximate due to network
latency.
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.
7.4. If-None-Match
The If-None-Match request-header field is used with a method to make
it conditional. A client that has one or more entities previously
obtained from the resource can verify that none of those entities is
current by including a list of their associated entity tags in the
If-None-Match header field. The purpose of this feature is to allow
efficient updates of cached information with a minimum amount of
transaction overhead. It is also used 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 entity of the
resource.
If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-tag )
If any of the entity tags match the entity tag of the entity that
would have been returned in the response to a similar GET request
(without the If-None-Match header) on that resource, or if "*" is
given and any current entity exists for that resource, then the
server MUST NOT perform the requested method, unless required to do
so because the resource's modification date fails to match that
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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 entities that
matched. For all other request methods, the server MUST respond with
a status of 412 (Precondition Failed).
See Section 5 for rules on how to determine if two entity tags match.
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, then the If-None-Match
header MUST be ignored. (See Section 6 for a discussion of server
behavior when both If-Modified-Since and If-None-Match appear in the
same request.)
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 16.5 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.
7.5. If-Unmodified-Since
The If-Unmodified-Since request-header field is used with a method to
make it conditional. If the requested resource has not been modified
since the time specified in this field, the server SHOULD perform the
requested operation as if the If-Unmodified-Since header were not
present.
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If the requested variant has been modified since the specified time,
the server MUST NOT perform the requested operation, and MUST return
a 412 (Precondition Failed).
If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date
An example of the field is:
If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
If the request normally (i.e., without the If-Unmodified-Since
header) would result in anything other than a 2xx or 412 status, 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
fields is undefined by this specification.
7.6. Last-Modified
The Last-Modified entity-header field indicates the date and time at
which the origin server believes the variant was last modified.
Last-Modified = "Last-Modified" ":" HTTP-date
An example of its use is
Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
The exact meaning of this header field depends on the implementation
of the origin server and the nature of the original resource. For
files, it may be just the file system last-modified time. For
entities with dynamically included parts, it may be the most recent
of the set of last-modify times for its component parts. For
database gateways, it may be the last-update time stamp of the
record. For virtual objects, it may be the last time the internal
state changed.
An origin server MUST NOT send a Last-Modified date which is later
than the server's time of message origination. In such cases, where
the resource's last modification would indicate some time in the
future, the server MUST replace that date with the message
origination date.
An origin server SHOULD obtain the Last-Modified value of the entity
as close as possible to the time that it generates the Date value of
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its response. This allows a recipient to make an accurate assessment
of the entity's modification time, especially if the entity changes
near the time that the response is generated.
HTTP/1.1 servers SHOULD send Last-Modified whenever feasible.
The Last-Modified entity-header field value is often used as a cache
validator. In simple terms, a cache entry is considered to be valid
if the entity has not been modified since the Last-Modified value.
8. IANA Considerations
8.1. Message Header Registration
The Message Header Registry located at <http://www.iana.org/
assignments/message-headers/message-header-index.html> should be
updated with the permanent registrations below (see [RFC3864]):
+---------------------+----------+----------+-------------+
| Header Field Name | Protocol | Status | Reference |
+---------------------+----------+----------+-------------+
| ETag | http | standard | Section 7.1 |
| If-Match | http | standard | Section 7.2 |
| If-Modified-Since | http | standard | Section 7.3 |
| If-None-Match | http | standard | Section 7.4 |
| If-Unmodified-Since | http | standard | Section 7.5 |
| Last-Modified | http | standard | Section 7.6 |
+---------------------+----------+----------+-------------+
The change controller is: "IETF (iesg@ietf.org) - Internet
Engineering Task Force".
9. Security Considerations
No additional security considerations have been identified beyond
those applicable to HTTP in general [Part1].
10. Acknowledgments
11. References
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11.1. 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-03
(work in progress), June 2008.
[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-03 (work
in progress), June 2008.
[Part6] 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 6: Caching",
draft-ietf-httpbis-p6-cache-03 (work in progress),
June 2008.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
11.2. Informative References
[RFC2068] Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T.
Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
RFC 2068, January 1997.
[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. Compatibility with Previous Versions
A.1. Changes from RFC 2616
Allow weak entity tags in all requests except range requests
(Sections 5 and 7.4).
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Appendix B. Change Log (to be removed by RFC Editor before publication)
B.1. Since RFC2616
Extracted relevant partitions from [RFC2616].
B.2. Since draft-ietf-httpbis-p4-conditional-00
Closed issues:
o <http://www3.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.
B.3. Since draft-ietf-httpbis-p4-conditional-01
Ongoing work on ABNF conversion
(<http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36>):
o Add explicit references to BNF syntax and rules imported from
other parts of the specification.
B.4. Since draft-ietf-httpbis-p4-conditional-02
Closed issues:
o <http://www3.tools.ietf.org/wg/httpbis/trac/ticket/116>: "Weak
ETags on non-GET requests"
Ongoing work on IANA Message Header Registration
(<http://www3.tools.ietf.org/wg/httpbis/trac/ticket/40>):
o Reference RFC 3984, and update header registrations for headers
defined in this document.
Index
3
304 Not Modified (status code) 5
4
412 Precondition Failed (status code) 6
E
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ETag header 11
G
Grammar
entity-tag 5
ETag 11
If-Match 12
If-Modified-Since 13
If-None-Match 14
If-Unmodified-Since 16
Last-Modified 16
opaque-tag 5
weak 5
H
Headers
ETag 11
If-Match 11
If-Modified-Since 13
If-None-Match 14
If-Unmodified-Since 15
Last-Modified 16
I
If-Match header 11
If-Modified-Since header 13
If-None-Match header 14
If-Unmodified-Since header 15
L
Last-Modified header 16
S
Status Codes
304 Not Modified 5
412 Precondition Failed 6
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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
One Laptop per Child
21 Oak Knoll Road
Carlisle, MA 01741
USA
Email: jg@laptop.org
URI: http://www.laptop.org/
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
Fielding, et al. Expires December 19, 2008 [Page 21]
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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/
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/
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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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