HTTPbis Working Group R. Fielding, Ed.
Internet-Draft Adobe
Obsoletes: 2616 (if approved) J. Reschke, Ed.
Intended status: Standards Track greenbytes
Expires: January 16, 2014 July 15, 2013
Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests
draft-ietf-httpbis-p4-conditional-23
Abstract
The Hypertext Transfer Protocol (HTTP) is an application-level
protocol for distributed, collaborative, hypertext information
systems. This document defines HTTP/1.1 conditional requests,
including metadata header fields for indicating state changes,
request header fields for making preconditions on such state, and
rules for constructing the responses to a conditional request when
one or more preconditions evaluate to false.
Editorial Note (To be removed by RFC Editor)
Discussion of this draft takes place on the HTTPBIS working group
mailing list (ietf-http-wg@w3.org), which is archived at
.
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 D.4.
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
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on January 16, 2014.
Copyright Notice
Copyright (c) 2013 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Conformance and Error Handling . . . . . . . . . . . . . . 4
1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 4
2. Validators . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Weak versus Strong . . . . . . . . . . . . . . . . . . . . 5
2.2. Last-Modified . . . . . . . . . . . . . . . . . . . . . . 7
2.2.1. Generation . . . . . . . . . . . . . . . . . . . . . . 7
2.2.2. Comparison . . . . . . . . . . . . . . . . . . . . . . 8
2.3. ETag . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.3.1. Generation . . . . . . . . . . . . . . . . . . . . . . 10
2.3.2. Comparison . . . . . . . . . . . . . . . . . . . . . . 10
2.3.3. Example: Entity-tags Varying on Content-Negotiated
Resources . . . . . . . . . . . . . . . . . . . . . . 11
2.4. When to Use Entity-tags and Last-Modified Dates . . . . . 12
3. Precondition Header Fields . . . . . . . . . . . . . . . . . . 13
3.1. If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.2. If-None-Match . . . . . . . . . . . . . . . . . . . . . . 14
3.3. If-Modified-Since . . . . . . . . . . . . . . . . . . . . 15
3.4. If-Unmodified-Since . . . . . . . . . . . . . . . . . . . 16
3.5. If-Range . . . . . . . . . . . . . . . . . . . . . . . . . 16
4. Status Code Definitions . . . . . . . . . . . . . . . . . . . 17
4.1. 304 Not Modified . . . . . . . . . . . . . . . . . . . . . 17
4.2. 412 Precondition Failed . . . . . . . . . . . . . . . . . 17
5. Evaluation and Precedence . . . . . . . . . . . . . . . . . . 18
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
6.1. Status Code Registration . . . . . . . . . . . . . . . . . 20
6.2. Header Field Registration . . . . . . . . . . . . . . . . 20
7. Security Considerations . . . . . . . . . . . . . . . . . . . 21
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
9.1. Normative References . . . . . . . . . . . . . . . . . . . 21
9.2. Informative References . . . . . . . . . . . . . . . . . . 22
Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 22
Appendix B. Imported ABNF . . . . . . . . . . . . . . . . . . . . 22
Appendix C. Collected ABNF . . . . . . . . . . . . . . . . . . . 23
Appendix D. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 23
D.1. Since draft-ietf-httpbis-p4-conditional-19 . . . . . . . . 24
D.2. Since draft-ietf-httpbis-p4-conditional-20 . . . . . . . . 24
D.3. Since draft-ietf-httpbis-p4-conditional-21 . . . . . . . . 24
D.4. Since draft-ietf-httpbis-p4-conditional-22 . . . . . . . . 25
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
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1. Introduction
Conditional requests are HTTP requests [Part2] that include one or
more header fields indicating a precondition to be tested before
applying the method semantics to the target resource. This document
defines the HTTP/1.1 conditional request mechanisms in terms of the
architecture, syntax notation, and conformance criteria defined in
[Part1].
Conditional GET requests are the most efficient mechanism for HTTP
cache updates [Part6]. Conditionals can also be applied to state-
changing methods, such as PUT and DELETE, to prevent the "lost
update" problem: one client accidentally overwriting the work of
another client that has been acting in parallel.
Conditional request preconditions are based on the state of the
target resource as a whole (its current value set) or the state as
observed in a previously obtained representation (one value in that
set). A resource might have multiple current representations, each
with its own observable state. The conditional request mechanisms
assume that the mapping of requests to a "selected representation"
(Section 3 of [Part2]) 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.
The conditional request preconditions defined by this specification
are evaluated by comparing the validators provided in the conditional
request header fields to the current validators for the selected
representation in the order defined by Section 5.
1.1. Conformance and Error Handling
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Conformance criteria and considerations regarding error handling are
defined in Section 2.5 of [Part1].
1.2. Syntax Notation
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234] with the list rule extension defined in Section
1.2 of [Part1]. Appendix B describes rules imported from other
documents. Appendix C shows the collected ABNF with the list rule
expanded.
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2. Validators
This specification defines two forms of metadata that are commonly
used to observe resource state and test for preconditions:
modification dates (Section 2.2) and opaque entity tags
(Section 2.3). Additional metadata that reflects resource state has
been defined by various extensions of HTTP, such as WebDAV [RFC4918],
that are beyond the scope of this specification. A resource metadata
value is referred to as a ""validator"" when it is used within a
precondition.
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 representation metadata that changes value
whenever a change occurs to the representation data that would be
observable in the payload body of a 200 (OK) response to GET.
A strong validator might change for other reasons, such as when a
semantically significant part of the representation metadata is
changed (e.g., Content-Type), but it is in the best interests of the
origin server to only change the value when it is necessary to
invalidate the stored responses held by remote caches and authoring
tools. A strong validator is unique across all representations of a
given resource, such that no two representations of that resource can
share the same validator unless their representation data is
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 is 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
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identifier being assigned before the representation is made
accessible to GET. A collision-resistant hash function applied to
the representation data is also sufficient if the data is available
prior to the response header fields being sent and the digest does
not need to be recalculated every time a validation request is
received. However, if a resource has distinct representations that
differ only in their metadata, such as might occur with content
negotiation over media types that happen to share the same data
format, then the origin server SHOULD incorporate additional
information in the validator to distinguish those representations.
In contrast, a "weak validator" is representation metadata that might
not change for every change to the representation data. This
weakness might be due to limitations in how the value is calculated,
such as clock resolution or an inability to ensure uniqueness for all
possible representations of the resource, or due to a desire by the
resource owner to group representations by some self-determined set
of equivalency rather than unique sequences of data. An origin
server SHOULD change a weak entity-tag whenever it considers prior
representations to be unacceptable as a substitute for the current
representation. In other words, a weak entity-tag ought to change
whenever the origin server wants caches to invalidate old responses.
For example, the representation of a weather report that changes in
content every second, based on dynamic measurements, might be grouped
into sets of equivalent representations (from the origin server's
perspective) with the same weak validator in order to allow cached
representations to be valid for a reasonable period of time (perhaps
adjusted dynamically based on server load or weather quality).
Likewise, a representation's modification time, if defined with only
one-second resolution, might be a weak validator if it is possible
for the representation to be modified twice during a single second
and retrieved between those modifications.
Likewise, a validator is weak if it is shared by two or more
representations of a given resource at the same time, unless those
representations have identical representation data. For example, if
the origin server sends the same validator for a representation with
a gzip content coding applied as it does for a representation with no
content coding, then that validator is weak. However, two
simultaneous representations might share the same strong validator if
they differ only in the representation metadata, such as when two
different media types are available for the same representation data.
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 the representation data.
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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 in a response provides a timestamp
indicating the date and time at which the origin server believes the
selected representation was last modified, as determined at the
conclusion of handling the request.
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,
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
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modification date from having an adverse impact on cache validation.
An origin server without a clock MUST NOT assign Last-Modified values
to a response unless these values were associated with the resource
by some other system or user with a reliable clock.
2.2.2. Comparison
A Last-Modified time, when used as a validator in a request, is
implicitly weak unless it is possible to deduce that it is strong,
using the following rules:
o The validator is being compared by an origin server to the actual
current validator for the representation and,
o That origin server reliably knows that the associated
representation did not change twice during the second covered by
the presented validator.
or
o The validator is about to be used by a client in an If-Modified-
Since, If-Unmodified-Since header field, because the client has a
cache entry, or If-Range for the associated representation, and
o That cache entry includes a Date value, which gives the time when
the origin server sent the original response, and
o The presented Last-Modified time is at least 60 seconds before the
Date value.
or
o The validator is being compared by an intermediate cache to the
validator stored in its cache entry for the representation, and
o That cache entry includes a Date value, which gives the time when
the origin server sent the original response, and
o The presented Last-Modified time is at least 60 seconds before the
Date value.
This method relies on the fact that if two different responses were
sent by the origin server during the same second, but both had the
same Last-Modified time, then at least one of those responses would
have a Date value equal to its Last-Modified time. The arbitrary 60-
second limit guards against the possibility that the Date and Last-
Modified values are generated from different clocks, or at somewhat
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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 in a response provides the current entity-tag
for the selected representation, as determined at the conclusion of
handling the request. 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 = DQUOTE *etagc DQUOTE
etagc = %x21 / %x23-7E / obs-text
; VCHAR except double quotes, plus obs-text
Note: Previously, opaque-tag was defined to be a quoted-string
([RFC2616], Section 3.11), thus some recipients might perform
backslash unescaping. Servers therefore ought to avoid backslash
characters in entity tags.
An entity-tag can be more reliable for validation than a modification
date in situations where it is inconvenient to store modification
dates, where the one-second resolution of HTTP date values is not
sufficient, or where modification dates are not consistently
maintained.
Examples:
ETag: "xyzzy"
ETag: W/"xyzzy"
ETag: ""
An entity-tag can be either a weak or strong validator, with strong
being the default. If an origin server provides an entity-tag for a
representation and the generation of that entity-tag does not satisfy
all of the characteristics of a strong validator (Section 2.1), then
the origin server MUST mark the entity-tag as weak by prefixing its
opaque value with "W/" (case-sensitive).
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2.3.1. Generation
The principle behind entity-tags is that only the service author
knows the implementation of a resource well enough to select the most
accurate and efficient validation mechanism for that resource, and
that any such mechanism can be mapped to a simple sequence of octets
for easy comparison. Since the value is opaque, there is no need for
the client to be aware of how each entity-tag is constructed.
For example, a resource that has implementation-specific versioning
applied to all changes might use an internal revision number, perhaps
combined with a variance identifier for content negotiation, to
accurately differentiate between representations. Other
implementations might use a collision-resistant hash of
representation content, a combination of various filesystem
attributes, or a modification timestamp that has sub-second
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 "Strong comparison": two entity-tags are equivalent if both are
not weak and their opaque-tags match character-by-character.
o "Weak comparison": two entity-tags are equivalent if their opaque-
tags match character-by-character, regardless of either or both
being tagged as "weak".
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 |
+--------+--------+-------------------+-----------------+
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2.3.3. Example: Entity-tags Varying on Content-Negotiated Resources
Consider a resource that is subject to content negotiation (Section
3.4 of [Part2]), and where the representations sent in response to a
GET request vary based on the Accept-Encoding request header field
(Section 5.3.4 of [Part2]):
>> 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: Fri, 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: Fri, 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 has to be
distinct from an unencoded representation to prevent conflicts
during cache updates and range requests. In contrast, transfer
codings (Section 4 of [Part1]) apply only during message transfer
and do not require distinct entity-tags.
2.4. 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.
In 200 (OK) responses to GET or HEAD, an origin server:
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 origin server is to
send both a strong entity-tag and a Last-Modified value in successful
responses to a retrieval request.
A client:
o MUST use that entity-tag in any cache-conditional request (using
If-Match or If-None-Match) if an entity-tag has been provided by
the origin server.
o SHOULD use the Last-Modified value in non-subrange cache-
conditional requests (using If-Modified-Since) if only a Last-
Modified value has been provided by the origin server.
o MAY use the Last-Modified value in subrange cache-conditional
requests (using If-Unmodified-Since) if only a Last-Modified value
has been provided by an HTTP/1.0 origin server. The user agent
SHOULD provide a way to disable this, in case of difficulty.
o SHOULD use both validators in cache-conditional requests if both
an entity-tag and a Last-Modified value have been provided by the
origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to
respond appropriately.
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3. Precondition Header Fields
This section defines the syntax and semantics of HTTP/1.1 header
fields for applying preconditions on requests. Section 5 defines
when the preconditions are applied and the order of evaluation when
more than one precondition is present.
3.1. If-Match
The "If-Match" header field can be used to make a request method
conditional on the current existence or value of an entity-tag for
one or more representations of the target resource.
If-Match is generally useful for resource update requests, such as
PUT requests, as a means for protecting against accidental overwrites
when multiple clients are acting in parallel on the same resource
(i.e., the "lost update" problem). An If-Match field-value of "*"
places the precondition on the existence of any current
representation for the target resource.
If-Match = "*" / 1#entity-tag
The If-Match condition is met if and only if any of the entity-tags
listed in the If-Match field value match the entity-tag of the
selected representation using the weak comparison function (as per
Section 2.3.2), or if "*" is given and any current representation
exists for the target resource.
If the condition is met, the server MAY perform the request method.
Origin servers MUST NOT perform the requested method if the condition
is not met; instead they MUST respond with the 412 (Precondition
Failed) status code.
Proxy servers using a cached response as the selected representation
MUST NOT perform the requested method if the condition is not met;
instead, they MUST forward the request towards the origin server.
Examples:
If-Match: "xyzzy"
If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-Match: *
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3.2. If-None-Match
The "If-None-Match" header field can be used to make a request method
conditional on not matching any of the current entity-tag values for
representations of the target resource.
If-None-Match is primarily used in conditional GET requests to enable
efficient updates of cached information with a minimum amount of
transaction overhead. A client that has one or more representations
previously obtained from the target resource can send If-None-Match
with a list of the associated entity-tags in the hope of receiving a
304 (Not Modified) response if at least one of those representations
matches the selected representation.
If-None-Match can also be used with a value of "*" to prevent an
unsafe request method (e.g., PUT) from inadvertently modifying an
existing representation of the target resource when the client
believes that the resource does not have a current representation
(Section 4.2.1 of [Part2]). This is a variation on the "lost update"
problem that might arise if more than one client attempts to create
an initial representation for the target resource.
If-None-Match = "*" / 1#entity-tag
The If-None-Match condition is met if and only if none of the entity-
tags listed in the If-None-Match field value match the entity-tag of
the selected representation using the weak comparison function (as
per Section 2.3.2), or if "*" is given and no current representation
exists for that resource.
If the condition is not met, the server MUST NOT perform the
requested method. Instead, if the request method was GET or HEAD,
the server SHOULD respond with a 304 (Not Modified) status code,
including the cache-related header fields (particularly ETag) of the
selected representation that has a matching entity-tag. For all
other request methods, the server MUST respond with a 412
(Precondition Failed) status code when the condition is not met.
If the condition is met, the server MAY perform the requested method
and MUST ignore any If-Modified-Since header field(s) in the request.
That is, if no entity-tags match, then the server MUST NOT send a 304
(Not Modified) response.
Examples:
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If-None-Match: "xyzzy"
If-None-Match: W/"xyzzy"
If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
If-None-Match: *
3.3. If-Modified-Since
The "If-Modified-Since" header field can be used with GET or HEAD to
make the method conditional by modification date: if the selected
representation has not been modified since the time specified in this
field, then do not perform the request method; instead, respond as
detailed below.
If-Modified-Since = HTTP-date
An example of the field is:
If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
A GET method with an If-Modified-Since header field and no Range
header field requests that the selected representation be transferred
only if it has been modified since the date given by the If-Modified-
Since header field. The algorithm for determining this includes the
following cases:
1. If the request would normally result in anything other than a 200
(OK) status code, or if the passed If-Modified-Since date is
invalid, the response is exactly the same as for a normal GET. A
date that 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 send a 304 (Not
Modified) response.
The two purposes of this feature are to allow efficient updates of
cached information, with a minimum amount of transaction overhead,
and to limit the scope of a web traversal to resources that have
recently changed.
When used for cache updates, a cache will typically use the value of
the cached message's Last-Modified field to generate the field value
of If-Modified-Since. This behavior is most interoperable for cases
where clocks are poorly synchronized or when the server has chosen to
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only honor exact timestamp matches (due to a problem with Last-
Modified dates that appear to go "back in time" when the origin
server's clock is corrected or a representation is restored from an
archived backup). However, caches occasionally generate the field
value based on other data, such as the Date header field of the
cached message or the local clock time that the message was received,
particularly when the cached message does not contain a Last-Modified
field.
When used for limiting the scope of retrieval to a recent time
window, a user agent will generate an If-Modified-Since field value
based on either its own local clock or a Date header field received
from the server during a past run. Origin servers that choose an
exact timestamp match based on the selected representation's Last-
Modified field will not be able to help the user agent limit its data
transfers to only those changed during the specified window.
Note: If a client uses an arbitrary date in the If-Modified-Since
header field instead of a date taken from a Last-Modified or Date
header field from the origin server, the client ought to be aware
that its date will be interpreted according to the server's
understanding of time.
3.4. If-Unmodified-Since
The "If-Unmodified-Since" header field can be used to make a request
method conditional by modification date: if the selected
representation has been modified since the time specified in this
field, then the server MUST NOT perform the requested operation and
MUST instead respond with the 412 (Precondition Failed) status code.
If the selected representation has not been modified since the time
specified in this field, the server MAY perform the request.
If-Unmodified-Since = HTTP-date
An example of the field is:
If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
A server MUST ignore the If-Unmodified-Since header field if the
received value is not a valid HTTP-date.
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 range requests. If-Range is defined in Section 3.2 of
[Part5].
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4. Status Code Definitions
4.1. 304 Not Modified
The "304 (Not Modified)" status code indicates that a conditional GET
or HEAD 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
request indicates that the client, which made the request
conditional, already has a valid representation; the server is
therefore redirecting the client to make use of that stored
representation as if it were the payload of a 200 (OK) response.
The server generating a 304 response MUST generate any of the
following header fields that would have been sent in a 200 (OK)
response to the same request: Cache-Control, Content-Location, ETag,
Expires, and Vary.
Since the goal of a 304 response is to minimize information transfer
when the recipient already has one or more cached representations, a
sender SHOULD NOT generate representation metadata other than the
above listed fields unless said metadata exists for the purpose of
guiding cache updates (e.g., Last-Modified might be useful if the
response does not have an ETag field).
Requirements on a cache that receives a 304 response are defined in
Section 4.2.1 of [Part6]. If the 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 proxy SHOULD forward the
304 response to that client.
A 304 response cannot contain a message-body; it is always terminated
by the first empty line after the header fields.
4.2. 412 Precondition Failed
The "412 (Precondition Failed)" 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.
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5. Evaluation and Precedence
For each conditional request, a server MUST evaluate the request
preconditions after it has successfully performed its normal request
checks (i.e., just before it would perform the action associated with
the request method). Preconditions are ignored if the server
determines that an error or redirect response applies before they are
evaluated. Otherwise, the evaluation depends on both the method
semantics and the choice of conditional.
A conditional request header field that is designed specifically for
cache validation, which includes If-None-Match and If-Modified-Since
when used in a GET or HEAD request, allows cached representations to
be refreshed without repeatedly transferring data already held by the
client. Evaluating to false is thus an indication that the client
can continue to use its local copy of the selected representation, as
indicated by the server generating a 304 (Not Modified) response that
includes only those header fields useful for refreshing the cached
representation.
All other conditionals are intended to signal failure when the
precondition evaluates to false. For example, an If-Match
conditional sent with a state-changing method (e.g., POST, PUT,
DELETE) is intended to prevent the request from taking effect on the
target resource if the resource state does not match the expected
state. In other words, evaluating the condition to false means that
the resource has been changed by some other client, perhaps by
another user attempting to edit the same resource, and thus
preventing the request from being applied saves the client from
overwriting some other client's work. This result is indicated by
the server generating a 412 (Precondition Failed) response.
The conditional request header fields defined by this specification
are ignored for request methods that never involve the selection or
modification of a selected representation (e.g., CONNECT, OPTIONS,
and TRACE). Other conditional request header fields, defined by
extensions to HTTP, might place conditions on the state of the target
resource in general, or on a group of resources. For instance, the
If header field in WebDAV can make a request conditional on various
aspects (such as locks) of multiple resources ([RFC4918], Section
10.4).
When more than one conditional request header field is present in a
request, the order in which the fields are evaluated becomes
important. In practice, the fields defined in this document are
consistently implemented in a single, logical order, due to the fact
that entity tags are presumed to be more accurate than date
validators. For example, the only reason to send both If-Modified-
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Since and If-None-Match in the same GET request is to support
intermediary caches that might not have implemented If-None-Match, so
it makes sense to ignore the If-Modified-Since when entity tags are
understood and available for the selected representation.
The general rule of conditional precedence is that exact match
conditions are evaluated before cache-validating conditions and,
within that order, last-modified conditions are only evaluated if the
corresponding entity tag condition is not present (or not applicable
because the selected representation does not have an entity tag).
Specifically, the fields defined by this specification are evaluated
as follows:
1. When If-Match is present, evaluate it:
* if true, continue to step 3
* if false, respond 412 (Precondition Failed)
2. When If-Match is not present and If-Unmodified-Since is present,
evaluate it:
* if true, continue to step 3
* if false, respond 412 (Precondition Failed)
3. When If-None-Match is present, evaluate it:
* if true, continue to step 5
* if false for GET/HEAD, respond 304 (Not Modified)
* if false for other methods, respond 412 (Precondition Failed)
4. When the method is GET or HEAD, If-None-Match is not present, and
If-Modified-Since is present, evaluate it:
* if true, continue to step 5
* if false, respond 304 (Not Modified)
5. When the method is GET and both Range and If-Range are present,
evaluate If-Range:
* if the validator matches and the Range specification is
applicable to the selected representation, respond 206
(Partial Content) [Part5]
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6. Otherwise,
* all conditions are met, so perform the requested action and
respond according to its success or failure.
Any extension to HTTP/1.1 that defines additional conditional request
header fields ought to define its own expectations regarding the
order for evaluating such fields in relation to those defined in this
document and other conditionals that might be found in practice.
6. IANA Considerations
6.1. Status Code Registration
The HTTP Status Code Registry located at
shall be updated
with the registrations below:
+-------+---------------------+-------------+
| Value | Description | Reference |
+-------+---------------------+-------------+
| 304 | Not Modified | Section 4.1 |
| 412 | Precondition Failed | Section 4.2 |
+-------+---------------------+-------------+
6.2. Header Field Registration
HTTP header fields are registered within the Message Header Field
Registry maintained at .
This document defines the following HTTP header fields, so their
associated registry entries shall be updated according to the
permanent registrations below (see [BCP90]):
+---------------------+----------+----------+-------------+
| 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".
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7. Security Considerations
This section is meant to inform developers, information providers,
and users of known security concerns specific to the HTTP/1.1
conditional request mechanisms. More general security considerations
are addressed in HTTP messaging [Part1] and semantics [Part2].
The validators defined by this specification are not intended to
ensure the validity of a representation, guard against malicious
changes, or detect man-in-the-middle attacks. At best, they enable
more efficient cache updates and optimistic concurrent writes when
all participants are behaving nicely. At worst, the conditions will
fail and the client will receive a response that is no more harmful
than an HTTP exchange without conditional requests.
An entity-tag can be abused in ways that create privacy risks. For
example, a site might deliberately construct a semantically invalid
entity-tag that is unique to the user or user agent, send it in a
cacheable response with a long freshness time, and then read that
entity-tag in later conditional requests as a means of re-identifying
that user or user agent. Such an identifying tag would become a
persistent identifier for as long as the user agent retained the
original cache entry. User agents that cache representations ought
to ensure that the cache is cleared or replaced whenever the user
performs privacy-maintaining actions, such as clearing stored cookies
or changing to a private browsing mode.
8. Acknowledgments
See Section 9 of [Part1].
9. References
9.1. Normative References
[Part1] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
draft-ietf-httpbis-p1-messaging-23 (work in progress),
July 2013.
[Part2] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content",
draft-ietf-httpbis-p2-semantics-23 (work in progress),
July 2013.
[Part5] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
draft-ietf-httpbis-p5-range-23 (work in progress),
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July 2013.
[Part6] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
draft-ietf-httpbis-p6-cache-23 (work in progress),
July 2013.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
9.2. Informative References
[BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004.
[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.
[RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed
Authoring and Versioning (WebDAV)", RFC 4918, June 2007.
Appendix A. Changes from RFC 2616
The definition of validator weakness has been expanded and clarified.
(Section 2.1)
Weak entity-tags are now allowed in all requests except range
requests (Sections 2.1 and 3.2).
The ETag header field ABNF has been changed to not use quoted-string,
thus avoiding escaping issues. (Section 2.3)
ETag is defined to provide an entity tag for the selected
representation, thereby clarifying what it applies to in various
situations (such as a PUT response). (Section 2.3)
The precedence for evaluation of conditional requests has been
defined. (Section 5)
Appendix B. Imported ABNF
The following core rules are included by reference, as defined in
Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return),
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CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any
8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII
character).
The rules below are defined in [Part1]:
OWS =
obs-text =
The rules below are defined in other parts:
HTTP-date =
Appendix C. Collected ABNF
In the collected ABNF below, list rules are expanded as per Section
1.2 of [Part1].
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
etagc = "!" / %x23-7E ; '#'-'~'
/ obs-text
obs-text =
opaque-tag = DQUOTE *etagc DQUOTE
weak = %x57.2F ; W/
Appendix D. Change Log (to be removed by RFC Editor before publication)
Changes up to the first Working Group Last Call draft are summarized
in .
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D.1. Since draft-ietf-httpbis-p4-conditional-19
Closed issues:
o : "Need to
clarify eval order/interaction of conditional headers"
o : "Required
headers on 304 and 206"
o : "Optionality
of Conditional Request Support"
o : "ETags and
Conditional Requests"
o : "ABNF
requirements for recipients"
o : "Rare cases"
o : "Conditional
Request Security Considerations"
o : "If-Modified-
Since lacks definition for method != GET"
o : "refactor
conditional header field descriptions"
D.2. Since draft-ietf-httpbis-p4-conditional-20
o Conformance criteria and considerations regarding error handling
are now defined in Part 1.
D.3. Since draft-ietf-httpbis-p4-conditional-21
Closed issues:
o : "Conditional
GET text"
o : "Optionality
of Conditional Request Support"
o : "unclear prose
in definition of 304"
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o : "ETags and
Conneg"
o : "Comparison
function for If-Match and If-None-Match"
o : "304 without
validator"
o : "If-Match and
428"
D.4. Since draft-ietf-httpbis-p4-conditional-22
Closed issues:
o : "explain list
expansion in ABNF appendices"
o : "incorrect
example dates"
Partly resolved issues:
o : "Editorial
suggestions"
Index
3
304 Not Modified (status code) 17
4
412 Precondition Failed (status code) 17
E
ETag header field 9
G
Grammar
entity-tag 9
ETag 9
etagc 9
If-Match 13
If-Modified-Since 15
If-None-Match 14
If-Unmodified-Since 16
Last-Modified 7
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opaque-tag 9
weak 9
I
If-Match header field 13
If-Modified-Since header field 15
If-None-Match header field 14
If-Unmodified-Since header field 16
L
Last-Modified header field 7
M
metadata 5
S
selected representation 4
V
validator 5
strong 5
weak 5
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/
Julian F. Reschke (editor)
greenbytes GmbH
Hafenweg 16
Muenster, NW 48155
Germany
EMail: julian.reschke@greenbytes.de
URI: http://greenbytes.de/tech/webdav/
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