wdiff rfc7232v1.txt rfc7232.txt

Internet Engineering Task Force (IETF) R. Fielding, Ed.
Request for Comments: 7232 Adobe
Obsoletes: 2616 J. Reschke, Ed.
Category: Standards Track greenbytes
ISSN: 2070-1721 May June 2014

Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests

Abstract

The Hypertext Transfer Protocol (HTTP) is a stateless 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.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7232.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 ....................................................4
1.1. Conformance and Error Handling . . . . . . . . . . . . . . 4 .............................4
1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 4 ............................................4
2. Validators . . . . . . . . . . . . . . . . . . . . . . . . . . 5 ......................................................5
2.1. Weak versus Strong . . . . . . . . . . . . . . . . . . . . 5 .........................................5
2.2. Last-Modified . . . . . . . . . . . . . . . . . . . . . . 7 ..............................................7
2.2.1. Generation . . . . . . . . . . . . . . . . . . . . . . 7 ..........................................7
2.2.2. Comparison . . . . . . . . . . . . . . . . . . . . . . 8 ..........................................8
2.3. ETag . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 .......................................................9
2.3.1. Generation . . . . . . . . . . . . . . . . . . . . . . 10 .........................................10
2.3.2. Comparison . . . . . . . . . . . . . . . . . . . . . . 10 .........................................10
2.3.3. Example: Entity-Tags Varying on
Content-Negotiated Resources . . . . . . . . . . . . . . . . . . . . . . 11 .......................11
2.4. When to Use Entity-Tags and Last-Modified Dates . . . . . 12 ...........12
3. Precondition Header Fields . . . . . . . . . . . . . . . . . . 13 .....................................13
3.1. If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 13 ..................................................13
3.2. If-None-Match . . . . . . . . . . . . . . . . . . . . . . 14 .............................................14
3.3. If-Modified-Since . . . . . . . . . . . . . . . . . . . . 15 .........................................16
3.4. If-Unmodified-Since . . . . . . . . . . . . . . . . . . . 16 .......................................17
3.5. If-Range . . . . . . . . . . . . . . . . . . . . . . . . . 18 ..................................................18
4. Status Code Definitions . . . . . . . . . . . . . . . . . . . 18 ........................................18
4.1. 304 Not Modified . . . . . . . . . . . . . . . . . . . . . 18 ..........................................18
4.2. 412 Precondition Failed . . . . . . . . . . . . . . . . . 18 ...................................19
5. Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . 19 .....................................................19
6. Precedence . . . . . . . . . . . . . . . . . . . . . . . . . . 19 .....................................................20
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 ............................................22
7.1. Status Code Registration . . . . . . . . . . . . . . . . . 21 ..................................22
7.2. Header Field Registration . . . . . . . . . . . . . . . . 21 .................................22
8. Security Considerations . . . . . . . . . . . . . . . . . . . 22 ........................................22
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22 ................................................23
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23 ....................................................24
10.1. Normative References . . . . . . . . . . . . . . . . . . . 23 .....................................24
10.2. Informative References . . . . . . . . . . . . . . . . . . 23 ...................................24
Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 23 .................................25
Appendix B. Imported ABNF . . . . . . . . . . . . . . . . . . . . 24 .........................................25
Appendix C. Collected ABNF . . . . . . . . . . . . . . . . . . . 24 ........................................26
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 .............................................................27

1. Introduction

Conditional requests are HTTP requests [RFC7231] 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
[RFC7230].

Conditional GET requests are the most efficient mechanism for HTTP
cache updates [RFC7234]. Conditionals can also be applied to state-
changing
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 [RFC7231]) 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
(Section 3) are evaluated when applicable to the recipient
(Section 5) according to their order of precedence (Section 6).

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 [RFC7230].

1.2. Syntax Notation

This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234] with a list extension, defined in Section 7 of
[RFC7230], that allows for compact definition of comma-separated
lists using a '#' operator (similar to how the '*' operator indicates
repetition). Appendix B describes rules imported from other
documents. Appendix C shows the collected grammar with all list
operators expanded to standard ABNF notation.

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 Web Distributed
Authoring and Versioning (WebDAV) [RFC4918], (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 reasons other than a change to
the representation data, 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 change only change the
value when it is necessary to invalidate the stored responses held by
remote caches and authoring tools.

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
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 needs to 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, an inability to ensure uniqueness for all
possible representations of the resource, or a desire of 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.

Strong validators are usable for all conditional requests, including
cache validation, partial content ranges, and "lost update"
avoidance. Weak validators are only usable when the client does not
require exact equality with previously obtained representation data,
such as when validating a cache entry or limiting a web traversal to
recent changes.

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

An origin server 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 ([RFC7234]) results in a substantial
reduction of HTTP traffic on the Internet and can be a significant
factor in improving service scalability and reliability.

A representation is typically the sum of many parts behind the
resource interface. The last-modified time would usually be the most
recent time that any of those parts were changed. How that value is
determined for any given resource is an implementation detail beyond
the scope of this specification. What matters to HTTP is how
recipients of the Last-Modified header field can use its value to
make conditional requests and test the validity of locally cached
responses.

An origin server SHOULD obtain the Last-Modified value of the
representation as close as possible to the time that it generates the
Date field value for its response. This allows a recipient to make
an accurate assessment of the representation's modification time,
especially if the representation changes near the time that the
response is generated.

An origin server with a clock MUST NOT send a Last-Modified date that
is later than the server's time of message origination (Date). If
the last modification time is derived from implementation-specific
metadata that evaluates to some time in the future, according to the
origin server's clock, then the origin server MUST replace that value
with the message origination date. This prevents a future
modification date from having an adverse impact on cache validation.

An origin server without a clock MUST NOT assign Last-Modified values
to a response unless these values were associated with the resource
by some other system or user with a reliable clock.

2.2.2. Comparison

A Last-Modified time, when used as a validator in a request, is
implicitly weak unless it is possible to deduce that it is strong,
using the following rules:

o The validator is being compared by an origin server to the actual
current validator for the representation and,

o That origin server reliably knows that the associated
representation did not change twice during the second covered by
the presented validator.

o The validator is about to be used by a client in an If-Modified-
Since
If-Modified-Since, If-Unmodified-Since, or If-Unmodified-Since If-Range header field,
because the client has a cache entry, or If-Range 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.

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
60-second limit guards against the possibility that the Date and Last-
Modified
Last-Modified values are generated from different clocks or at
somewhat different times during the preparation of the response. An
implementation MAY use a value larger than 60 seconds, if it is
believed that 60 seconds is too short.

2.3. ETag

The "ETag" header field 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).

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 file attributes, or
a modification timestamp that has sub-second resolution.

An origin server SHOULD send an 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 ([RFC7234]) 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
or not the comparison context allows the use of weak validators:

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
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 |
+--------+--------+-------------------+-----------------+

2.3.3. Example: Entity-Tags Varying on Content-Negotiated Resources

Consider a resource that is subject to content negotiation (Section
3.4 of [RFC7231]), 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 [RFC7231]):

>> 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...

Note: Content codings are a property of the representation data,
so a strong entity-tag for a content-encoded representation has to
be distinct from the entity tag of an unencoded representation to
prevent potential conflicts during cache updates and range
requests. In contrast, transfer codings (Section 4 of [RFC7230])
apply only during message transfer and do not result in distinct
entity-tags.

2.4. When to Use Entity-Tags and Last-Modified Dates

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 send that entity-tag in any cache validation request (using
If-Match or If-None-Match) if an entity-tag has been provided by
the origin server.

o SHOULD send the Last-Modified value in non-subrange cache
validation requests (using If-Modified-Since) if only a Last-
Modified
Last-Modified value has been provided by the origin server.

o MAY send the Last-Modified value in subrange cache validation
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 send both validators in cache validation 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.

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. Section 6 defines the order of
evaluation when more than one precondition is present.

3.1. If-Match

The "If-Match" header field makes the request method conditional on
the recipient origin server either having at least one current
representation of the target resource, when the field-value is "*",
or having a current representation of the target resource that has an
entity-tag matching a member of the list of entity-tags provided in
the field-value.

An origin server MUST use the strong comparison function when
comparing entity-tags for If-Match (Section 2.3.2), since the client
intends this precondition to prevent the method from being applied if
there have been any changes to the representation data.

If-Match = "*" / 1#entity-tag

Examples:

If-Match: "xyzzy"
If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-Match: *

If-Match is most often used with state-changing methods (e.g., POST,
PUT, DELETE) to prevent accidental overwrites when multiple user
agents might be acting in parallel on the same resource (i.e., to
prevent the "lost update" problem). It can also be used with safe
methods to abort a request if the selected representation does not
match one already stored (or partially stored) from a prior request.

An origin server that receives an If-Match header field MUST evaluate
the condition prior to performing the method (Section 5). If the
field-value is "*", the condition is false if the origin server does
not have a current representation for the target resource. If the
field-value is a list of entity-tags, the condition is false if none
of the listed tags match the entity-tag of the selected
representation.

An origin server MUST NOT perform the requested method if a received
If-Match condition evaluates to false; instead, the origin server
MUST respond with either: either a) the 412 (Precondition Failed) status code
or b) one of the 2xx (Successful) status codes if the origin server
has verified that a state change is being requested and the final
state is already reflected in the current state of the target
resource (i.e., the change requested by the user agent has already
succeeded, but the user agent might not be aware of it, perhaps
because the prior response was lost or a compatible change was made
by some other user agent). In the latter case, the origin server
MUST NOT send a validator header field in the response unless it can
verify that the request is a duplicate of an immediately prior change
made by the same user agent.

The If-Match header field can be ignored by caches and intermediaries
because it is not applicable to a stored response.

3.2. If-None-Match

The "If-None-Match" header field makes the request method conditional
on a recipient cache or origin server either not having any current
representation of the target resource, when the field-value is "*",
or having a selected representation with an entity-tag that does not
match any of those listed in the field-value.

A recipient MUST use the weak comparison function when comparing
entity-tags for If-None-Match (Section 2.3.2), since weak entity-tags
can be used for cache validation even if there have been changes to
the representation data.

If-None-Match = "*" / 1#entity-tag
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: *

If-None-Match is primarily used in conditional GET requests to enable
efficient updates of cached information with a minimum amount of
transaction overhead. When a client desires to update one or more
stored responses that have entity-tags, the client SHOULD generate an
If-None-Match header field containing a list of those entity-tags
when making a GET request; this allows recipient servers to send a
304 (Not Modified) response to indicate when one of those stored
responses 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 [RFC7231]). 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.

An origin server that receives an If-None-Match header field MUST
evaluate the condition prior to performing the method (Section 5).
If the field-value is "*", the condition is false if the origin
server has a current representation for the target resource. If the
field-value is a list of entity-tags, the condition is false if one
of the listed tags match the entity-tag of the selected
representation.

An origin server MUST NOT perform the requested method if the
condition evaluates to false; instead, the origin server MUST respond
with either a) the 304 (Not Modified) status code if the request
method is GET or HEAD or b) the 412 (Precondition Failed) status code
for all other request methods.

Requirements on cache handling of a received If-None-Match header
field are defined in Section 4.3.2 of [RFC7234].

3.3. If-Modified-Since

The "If-Modified-Since" header field makes a GET or HEAD request
method conditional on the selected representation's modification date
being more recent than the date provided in the field-value.
Transfer of the selected representation's data is avoided if that
data has not changed.

If-Modified-Since = HTTP-date

An example of the field is:

If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT

A recipient MUST ignore If-Modified-Since if the request contains an
If-None-Match header field; the condition in If-None-Match is
considered to be a more accurate replacement for the condition in If-
Modified-Since
If-Modified-Since, and the two are only combined for the sake of
interoperating with older intermediaries that might not implement If-
None-Match.
If-None-Match.

A recipient MUST ignore the If-Modified-Since header field if the
received field-value is not a valid HTTP-date, or if the request
method is neither GET nor HEAD.

A recipient MUST interpret an If-Modified-Since field-value's
timestamp in terms of the origin server's clock.

If-Modified-Since is typically used for two distinct purposes: 1) to
allow efficient updates of a cached representation that does not have
an entity-tag and 2) 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
only honor exact timestamp matches (due to a problem with Last-
Modified
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 in a prior response. Origin servers that choose an
exact timestamp match based on the selected representation's Last-
Modified
Last-Modified field will not be able to help the user agent limit its
data transfers to only those changed during the specified window.

An origin server that receives an If-Modified-Since header field
SHOULD evaluate the condition prior to performing the method
(Section 5). The origin server SHOULD NOT perform the requested
method if the selected representation's last modification date is
earlier than or equal to the date provided in the field-value;
instead, the origin server SHOULD generate a 304 (Not Modified)
response, including only those metadata that are useful for
identifying or updating a previously cached response.

Requirements on cache handling of a received If-Modified-Since header
field are defined in Section 4.3.2 of [RFC7234].

3.4. If-Unmodified-Since

The "If-Unmodified-Since" header field makes the request method
conditional on the selected representation's last modification date
being earlier than or equal to the date provided in the field-value.
This field accomplishes the same purpose as If-Match for cases where
the user agent does not have an entity-tag for the representation.

If-Unmodified-Since = HTTP-date

An example of the field is:

If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT

A recipient MUST ignore If-Unmodified-Since if the request contains
an If-Match header field; the condition in If-Match is considered to
be a more accurate replacement for the condition in If-Unmodified-
Since,
If-Unmodified-Since, and the two are only combined for the sake of
interoperating with older intermediaries that might not implement
If-Match.

A recipient MUST ignore the If-Unmodified-Since header field if the
received field-value is not a valid HTTP-date.

A recipient MUST interpret an If-Unmodified-Since field-value's
timestamp in terms of the origin server's clock.

If-Unmodified-Since is most often used with state-changing methods
(e.g., POST, PUT, DELETE) to prevent accidental overwrites when
multiple user agents might be acting in parallel on a resource that
does not supply entity-tags with its representations (i.e., to
prevent the "lost update" problem). It can also be used with safe
methods to abort a request if the selected representation does not
match one already stored (or partially stored) from a prior request.

An origin server that receives an If-Unmodified-Since header field
MUST evaluate the condition prior to performing the method
(Section 5). The origin server MUST NOT perform the requested method
if the selected representation's last modification date is more
recent than the date provided in the field-value; instead the origin
server MUST respond with either a) the 412 (Precondition Failed)
status code or b) one of the 2xx (Successful) status codes if the
origin server has verified that a state change is being requested and
the final state is already reflected in the current state of the
target resource (i.e., the change requested by the user agent has
already succeeded, but the user agent might not be aware of that
because the prior response message was lost or a compatible change
was made by some other user agent). In the latter case, the origin
server MUST NOT send a validator header field in the response unless
it can verify that the request is a duplicate of an immediately prior
change made by the same user agent.

The If-Unmodified-Since header field can be ignored by caches and
intermediaries because it is not applicable to a stored response.

3.5. If-Range

The "If-Range" header field provides a special conditional request
mechanism that is similar to the If-Match and If-Unmodified-Since
header fields but that instructs the recipient to ignore the Range
header field if the validator doesn't match, resulting in transfer of
the new selected representation instead of a 412 (Precondition
Failed) response. If-Range is defined in Section 3.2 of [RFC7233].

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
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 (Not Modified) 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, Date,
ETag, Expires, and Vary.

Since the goal of a 304 (Not Modified) 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 (Not Modified) response are defined in
Section 4.3.4 of [RFC7234]. 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 (Not Modified) response to that client.

A 304 (Not Modified) 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
conditions given in the request header fields evaluated to false when
tested on the server. This response code allows the client to place
preconditions on the current resource state (its current
representations and metadata) and, thus, prevent the request method
from being applied if the target resource is in an unexpected state.

5. Evaluation

Except when excluded below, a recipient cache or origin server MUST
evaluate received request preconditions after it has successfully
performed its normal request checks and just before it would perform
the action associated with the request method. A server MUST ignore
all received preconditions if its response to the same request
without those conditions would have been a status code other than a
2xx (Successful) or 412 (Precondition Failed). In other words,
redirects and failures take precedence over the evaluation of
preconditions in conditional requests.

A server that is not the origin server for the target resource and
cannot act as a cache for requests on the target resource MUST NOT
evaluate the conditional request header fields defined by this
specification, and it MUST forward them if the request is forwarded,
since the generating client intends that they be evaluated by a
server that can provide a current representation. Likewise, a server
MUST ignore the conditional request header fields defined by this
specification when received with a request method that does not
involve the selection or modification of a selected representation,
such as CONNECT, OPTIONS, or TRACE.

Conditional request header fields that are defined by extensions to
HTTP might place conditions on all recipients, 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 of multiple resources, such as locks,
if the recipient understands and implements that field ([RFC4918],
Section 10.4).

Although conditional request header fields are defined as being
usable with the HEAD method (to keep HEAD's semantics consistent with
those of GET), there is no point in sending a conditional HEAD
because a successful response is around the same size as a 304 (Not
Modified) response and more useful than a 412 (Precondition Failed)
response.

6. Precedence

When more than one conditional request header field is present in a
request, the order in which the fields are evaluated becomes
important. In practice, the fields defined in this document are
consistently implemented in a single, logical order, since "lost
update" preconditions have more strict requirements than cache
validation, a validated cache is more efficient than a partial
response, and entity tags are presumed to be more accurate than date
validators.

A recipient cache or origin server MUST evaluate the request
preconditions defined by this specification in the following order:

1. When recipient is the origin server and If-Match is present,
evaluate the If-Match precondition:

* if true, continue to step 3

* if false, respond 412 (Precondition Failed) unless it can be
determined that the state-changing request has already
succeeded (see Section 3.1)
2. When recipient is the origin server, If-Match is not present, and
If-Unmodified-Since is present, evaluate the If-Unmodified-Since
precondition:

* if true, continue to step 3

* if false, respond 412 (Precondition Failed) unless it can be
determined that the state-changing request has already
succeeded (see Section 3.4)

3. When If-None-Match is present, evaluate the If-None-Match
precondition:

* 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 the If-Modified-Since
precondition:

* 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 the If-Range precondition:

* if the validator matches and the Range specification is
applicable to the selected representation, respond 206
(Partial Content) [RFC7233]

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.

7. IANA Considerations

7.1. Status Code Registration

The "HTTP "Hypertext Transfer Protocol (HTTP) Status Codes" registry Code Registry" located
at <http://www.iana.org/assignments/http-status-codes> has been
updated with the registrations below:

+-------+---------------------+-------------+
| Value | Description | Reference |
+-------+---------------------+-------------+
| 304 | Not Modified | Section 4.1 |
| 412 | Precondition Failed | Section 4.2 |
+-------+---------------------+-------------+

7.2. Header Field Registration

HTTP header fields are registered within the Message Header Field
Registry "Message Headers"
registry maintained at
<http://www.iana.org/assignments/message-headers/>.

This document defines the following HTTP header fields, so their
associated registry entries have been updated according to the
permanent registrations below (see [BCP90]):

The change controller is: "IETF (iesg@ietf.org) - Internet
Engineering Task Force".

8. Security Considerations

This section is meant to inform developers, information providers,
and users of known security concerns specific to the HTTP conditional
request mechanisms. More general security considerations are
addressed in the HTTP messaging "Message Syntax and Routing" [RFC7230] and semantics [RFC7231]
documents.
"Semantics and Content" [RFC7231].

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.

9. Acknowledgments

See Section 10 of [RFC7230].

10. References

10.1. Normative References

[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.

[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, May June 2014.

[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
May
June 2014.

[RFC7233] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
RFC 7233, May June 2014.

[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, May June 2014.

10.2. 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 clarified.
(Section 2.1). 2.1)

Weak entity-tags are now allowed in all requests except range
requests. (Sections 2.1 and 3.2.) 3.2)

The ETag header field ABNF has been changed to not use quoted-string,
thus avoiding escaping issues issues. (Section 2.3). 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) response). (Section 2.3). 2.3)

The precedence for evaluation of conditional requests has been
defined
defined. (Section 6). 6)

Appendix B. Imported ABNF

The following core rules are included by reference, as defined in
Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return),
CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any
8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII
character).

The rules below are defined in [RFC7230]:

OWS = <OWS, defined in see [RFC7230], Section 3.2.3>
obs-text = <obs-text, defined in see [RFC7230], Section 3.2.6>

The rule rules below is are defined in [RFC7231]: other parts:

HTTP-date = <HTTP-date, defined in see [RFC7231], Section 7.1.1.1>

Appendix C. Collected ABNF

In the collected ABNF below, list rules are expanded as per Section
1.2 of [RFC7230].

ETag = entity-tag

HTTP-date = <HTTP-date, defined in see [RFC7231], Section 7.1.1.1>

If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
entity-tag ] ) )
If-Modified-Since = HTTP-date
If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
entity-tag ] ) )
If-Unmodified-Since = HTTP-date

Last-Modified = HTTP-date

OWS = <OWS, defined in see [RFC7230], Section 3.2.3>

entity-tag = [ weak ] opaque-tag
etagc = "!" / %x23-7E ; '#'-'~'
/ obs-text

obs-text = <obs-text, defined in see [RFC7230], Section 3.2.6>
opaque-tag = DQUOTE *etagc DQUOTE

weak = %x57.2F ; W/

Index

3
304 Not Modified (status code) 18 19

4
412 Precondition Failed (status code) 18

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 17
Last-Modified 7
opaque-tag 9
weak 9

I
If-Match header field 13
If-Modified-Since header field 15 16
If-None-Match header field 14
If-Unmodified-Since header field 16 17

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/