draft-ietf-httpbis-p4-conditional-15.txt   draft-ietf-httpbis-p4-conditional-16.txt 
HTTPbis Working Group R. Fielding, Ed. HTTPbis Working Group R. Fielding, Ed.
Internet-Draft Adobe Internet-Draft Adobe
Obsoletes: 2616 (if approved) J. Gettys Obsoletes: 2616 (if approved) J. Gettys
Intended status: Standards Track Alcatel-Lucent Intended status: Standards Track Alcatel-Lucent
Expires: January 12, 2012 J. Mogul Expires: February 25, 2012 J. Mogul
HP HP
H. Frystyk H. Frystyk
Microsoft Microsoft
L. Masinter L. Masinter
Adobe Adobe
P. Leach P. Leach
Microsoft Microsoft
T. Berners-Lee T. Berners-Lee
W3C/MIT W3C/MIT
Y. Lafon, Ed. Y. Lafon, Ed.
W3C W3C
J. Reschke, Ed. J. Reschke, Ed.
greenbytes greenbytes
July 11, 2011 August 24, 2011
HTTP/1.1, part 4: Conditional Requests HTTP/1.1, part 4: Conditional Requests
draft-ietf-httpbis-p4-conditional-15 draft-ietf-httpbis-p4-conditional-16
Abstract Abstract
The Hypertext Transfer Protocol (HTTP) is an application-level The Hypertext Transfer Protocol (HTTP) is an application-level
protocol for distributed, collaborative, hypermedia information protocol for distributed, collaborative, hypertext information
systems. HTTP has been in use by the World Wide Web global systems. HTTP has been in use by the World Wide Web global
information initiative since 1990. This document is Part 4 of the information initiative since 1990. This document is Part 4 of the
seven-part specification that defines the protocol referred to as seven-part specification that defines the protocol referred to as
"HTTP/1.1" and, taken together, obsoletes RFC 2616. Part 4 defines "HTTP/1.1" and, taken together, obsoletes RFC 2616.
request header fields for indicating conditional requests and the
rules for constructing responses to those requests. Part 4 defines request header fields for indicating conditional
requests and the rules for constructing responses to those requests.
Editorial Note (To be removed by RFC Editor) Editorial Note (To be removed by RFC Editor)
Discussion of this draft should take place on the HTTPBIS working Discussion of this draft should take place on the HTTPBIS working
group mailing list (ietf-http-wg@w3.org), which is archived at group mailing list (ietf-http-wg@w3.org), which is archived at
<http://lists.w3.org/Archives/Public/ietf-http-wg/>. <http://lists.w3.org/Archives/Public/ietf-http-wg/>.
The current issues list is at The current issues list is at
<http://tools.ietf.org/wg/httpbis/trac/report/3> and related <http://tools.ietf.org/wg/httpbis/trac/report/3> and related
documents (including fancy diffs) can be found at documents (including fancy diffs) can be found at
<http://tools.ietf.org/wg/httpbis/>. <http://tools.ietf.org/wg/httpbis/>.
The changes in this draft are summarized in Appendix C.16. The changes in this draft are summarized in Appendix C.17.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 12, 2012. This Internet-Draft will expire on February 25, 2012.
Copyright Notice Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
skipping to change at page 3, line 5 skipping to change at page 3, line 5
outside the IETF Standards Process, and derivative works of it may outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other it for publication as an RFC or to translate it into languages other
than English. than English.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 5 1.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 5
1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 6 1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 6
2. Resource State Metadata (Validators) . . . . . . . . . . . . . 6 2. Validators . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Last-Modified . . . . . . . . . . . . . . . . . . . . . . 6 2.1. Weak versus Strong . . . . . . . . . . . . . . . . . . . . 6
2.1.1. Generation . . . . . . . . . . . . . . . . . . . . . . 6 2.2. Last-Modified . . . . . . . . . . . . . . . . . . . . . . 8
2.1.2. Comparison . . . . . . . . . . . . . . . . . . . . . . 7 2.2.1. Generation . . . . . . . . . . . . . . . . . . . . . . 8
2.2. ETag . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.2.2. Comparison . . . . . . . . . . . . . . . . . . . . . . 9
2.2.1. Generation . . . . . . . . . . . . . . . . . . . . . . 9 2.3. ETag . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.2.2. Weak versus Strong . . . . . . . . . . . . . . . . . . 9 2.3.1. Generation . . . . . . . . . . . . . . . . . . . . . . 11
2.2.3. Comparison . . . . . . . . . . . . . . . . . . . . . . 11 2.3.2. Comparison . . . . . . . . . . . . . . . . . . . . . . 11
2.2.4. Rules for When to Use Entity-tags and 2.3.3. Example: Entity-tags varying on Content-Negotiated
Last-Modified Dates . . . . . . . . . . . . . . . . . 11 Resources . . . . . . . . . . . . . . . . . . . . . . 12
2.2.5. Example: Entity-tags varying on Content-Negotiated 2.4. Rules for When to Use Entity-tags and Last-Modified
Resources . . . . . . . . . . . . . . . . . . . . . . 13 Dates . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3. Precondition Header Fields . . . . . . . . . . . . . . . . . . 14 3. Precondition Header Fields . . . . . . . . . . . . . . . . . . 14
3.1. If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.1. If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.2. If-None-Match . . . . . . . . . . . . . . . . . . . . . . 15 3.2. If-None-Match . . . . . . . . . . . . . . . . . . . . . . 15
3.3. If-Modified-Since . . . . . . . . . . . . . . . . . . . . 16 3.3. If-Modified-Since . . . . . . . . . . . . . . . . . . . . 17
3.4. If-Unmodified-Since . . . . . . . . . . . . . . . . . . . 18 3.4. If-Unmodified-Since . . . . . . . . . . . . . . . . . . . 18
3.5. If-Range . . . . . . . . . . . . . . . . . . . . . . . . . 18 3.5. If-Range . . . . . . . . . . . . . . . . . . . . . . . . . 19
4. Status Code Definitions . . . . . . . . . . . . . . . . . . . 18 4. Status Code Definitions . . . . . . . . . . . . . . . . . . . 19
4.1. 304 Not Modified . . . . . . . . . . . . . . . . . . . . . 18 4.1. 304 Not Modified . . . . . . . . . . . . . . . . . . . . . 19
4.2. 412 Precondition Failed . . . . . . . . . . . . . . . . . 19 4.2. 412 Precondition Failed . . . . . . . . . . . . . . . . . 20
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
5.1. Status Code Registration . . . . . . . . . . . . . . . . . 19 5.1. Status Code Registration . . . . . . . . . . . . . . . . . 20
5.2. Header Field Registration . . . . . . . . . . . . . . . . 20 5.2. Header Field Registration . . . . . . . . . . . . . . . . 20
6. Security Considerations . . . . . . . . . . . . . . . . . . . 20 6. Security Considerations . . . . . . . . . . . . . . . . . . . 20
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
8.1. Normative References . . . . . . . . . . . . . . . . . . . 20 8.1. Normative References . . . . . . . . . . . . . . . . . . . 21
8.2. Informative References . . . . . . . . . . . . . . . . . . 21 8.2. Informative References . . . . . . . . . . . . . . . . . . 21
Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 21 Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 22
Appendix B. Collected ABNF . . . . . . . . . . . . . . . . . . . 22 Appendix B. Collected ABNF . . . . . . . . . . . . . . . . . . . 22
Appendix C. Change Log (to be removed by RFC Editor before Appendix C. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 22 publication) . . . . . . . . . . . . . . . . . . . . 23
C.1. Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . . 22 C.1. Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . . 23
C.2. Since draft-ietf-httpbis-p4-conditional-00 . . . . . . . . 22 C.2. Since draft-ietf-httpbis-p4-conditional-00 . . . . . . . . 23
C.3. Since draft-ietf-httpbis-p4-conditional-01 . . . . . . . . 23 C.3. Since draft-ietf-httpbis-p4-conditional-01 . . . . . . . . 23
C.4. Since draft-ietf-httpbis-p4-conditional-02 . . . . . . . . 23 C.4. Since draft-ietf-httpbis-p4-conditional-02 . . . . . . . . 23
C.5. Since draft-ietf-httpbis-p4-conditional-03 . . . . . . . . 23 C.5. Since draft-ietf-httpbis-p4-conditional-03 . . . . . . . . 23
C.6. Since draft-ietf-httpbis-p4-conditional-04 . . . . . . . . 23 C.6. Since draft-ietf-httpbis-p4-conditional-04 . . . . . . . . 24
C.7. Since draft-ietf-httpbis-p4-conditional-05 . . . . . . . . 24 C.7. Since draft-ietf-httpbis-p4-conditional-05 . . . . . . . . 24
C.8. Since draft-ietf-httpbis-p4-conditional-06 . . . . . . . . 24 C.8. Since draft-ietf-httpbis-p4-conditional-06 . . . . . . . . 24
C.9. Since draft-ietf-httpbis-p4-conditional-07 . . . . . . . . 24 C.9. Since draft-ietf-httpbis-p4-conditional-07 . . . . . . . . 24
C.10. Since draft-ietf-httpbis-p4-conditional-08 . . . . . . . . 24 C.10. Since draft-ietf-httpbis-p4-conditional-08 . . . . . . . . 24
C.11. Since draft-ietf-httpbis-p4-conditional-09 . . . . . . . . 24 C.11. Since draft-ietf-httpbis-p4-conditional-09 . . . . . . . . 25
C.12. Since draft-ietf-httpbis-p4-conditional-10 . . . . . . . . 24 C.12. Since draft-ietf-httpbis-p4-conditional-10 . . . . . . . . 25
C.13. Since draft-ietf-httpbis-p4-conditional-11 . . . . . . . . 25 C.13. Since draft-ietf-httpbis-p4-conditional-11 . . . . . . . . 25
C.14. Since draft-ietf-httpbis-p4-conditional-12 . . . . . . . . 25 C.14. Since draft-ietf-httpbis-p4-conditional-12 . . . . . . . . 25
C.15. Since draft-ietf-httpbis-p4-conditional-13 . . . . . . . . 25 C.15. Since draft-ietf-httpbis-p4-conditional-13 . . . . . . . . 25
C.16. Since draft-ietf-httpbis-p4-conditional-14 . . . . . . . . 25 C.16. Since draft-ietf-httpbis-p4-conditional-14 . . . . . . . . 26
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 C.17. Since draft-ietf-httpbis-p4-conditional-15 . . . . . . . . 26
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction 1. Introduction
This document defines the HTTP/1.1 conditional request mechanisms, This document defines the HTTP/1.1 conditional request mechanisms,
including both response metadata that can be used to indicate or including both metadata for indicating/observing changes in resource
observe changes to resource state and request header fields that representations and request header fields that specify preconditions
specify preconditions to be checked before performing the action on that metadata be checked before performing the request method.
given by the request method. Conditional GET requests are the most Conditional GET requests are the most efficient mechanism for HTTP
efficient mechanism for HTTP cache updates [Part6]. Conditionals can cache updates [Part6]. Conditionals can also be applied to state-
also be applied to state-changing methods, such as PUT and DELETE, to changing methods, such as PUT and DELETE, to prevent the "lost
prevent the "lost update" problem: one client accidentally update" problem: one client accidentally overwriting the work of
overwriting the work of another client that has been acting in another client that has been acting in parallel.
parallel.
Conditional request preconditions are based on the state of the Conditional request preconditions are based on the state of the
target resource as a whole (its current value set) or the state as target resource as a whole (its current value set) or the state as
observed in a previously obtained representation (one value in that observed in a previously obtained representation (one value in that
set). A resource might have multiple current representations, each set). A resource might have multiple current representations, each
with its own observable state. The conditional request mechanisms with its own observable state. The conditional request mechanisms
assume that the mapping of requests to corresponding representations assume that the mapping of requests to corresponding representations
will be consistent over time if the server intends to take advantage will be consistent over time if the server intends to take advantage
of conditionals. Regardless, if the mapping is inconsistent and the of conditionals. Regardless, if the mapping is inconsistent and the
server is unable to select the appropriate representation, then no server is unable to select the appropriate representation, then no
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rule). Appendix B shows the collected ABNF, with the list rule rule). Appendix B shows the collected ABNF, with the list rule
expanded. expanded.
The following core rules are included by reference, as defined in The following core rules are included by reference, as defined in
[RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF
(CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote), (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 HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
sequence of data), SP (space), VCHAR (any visible USASCII character), sequence of data), SP (space), VCHAR (any visible USASCII character),
and WSP (whitespace). and WSP (whitespace).
The ABNF rules below are defined in other parts: The ABNF rules below are defined in [Part1]:
quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
OWS = <OWS, defined in [Part1], Section 1.2.2> OWS = <OWS, defined in [Part1], Section 1.2.2>
quoted-string = <quoted-string, defined in [Part1], Section 3.2.3>
HTTP-date = <HTTP-date, defined in [Part1], Section 6.1> HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>
2. Resource State Metadata (Validators) 2. Validators
This specification defines two forms of metadata that are commonly This specification defines two forms of metadata that are commonly
used to observe resource state and test for preconditions: used to observe resource state and test for preconditions:
modification dates and opaque entity tags. Additional metadata that modification dates and opaque entity tags. Additional metadata that
reflects resource state has been defined by various extensions of reflects resource state has been defined by various extensions of
HTTP, such as WebDAV [RFC4918], that are beyond the scope of this HTTP, such as WebDAV [RFC4918], that are beyond the scope of this
specification. A resource metadata value is referred to as a specification. A resource metadata value is referred to as a
"validator" when it is used within a precondition. "validator" when it is used within a precondition.
2.1. Last-Modified 2.1. Weak versus Strong
Validators come in two flavors: strong or weak. Weak validators are
easy to generate but are far less useful for comparisons. Strong
validators are ideal for comparisons but can be very difficult (and
occasionally impossible) to generate efficiently. Rather than impose
that all forms of resource adhere to the same strength of validator,
HTTP exposes the type of validator in use and imposes restrictions on
when weak validators can be used as preconditions.
A "strong validator" is a representation metadata value that MUST be
changed to a new, previously unused or guaranteed unique, value
whenever a change occurs to the representation data such that a
change would be observable in the payload body of a 200 response to
GET. A strong validator MAY be changed for other reasons, such as
when a semantically significant part of the representation metadata
is changed (e.g., Content-Type), but it is in the best interests of
the origin server to only change the value when it is necessary to
invalidate the stored responses held by remote caches and authoring
tools. A strong validator MUST be unique across all representations
of a given resource, such that no two representations of that
resource share the same validator unless their payload body would be
identical.
Cache entries might persist for arbitrarily long periods, regardless
of expiration times. Thus, a cache might attempt to validate an
entry using a validator that it obtained in the distant past. A
strong validator MUST be unique across all versions of all
representations associated with a particular resource over time.
However, there is no implication of uniqueness across representations
of different resources (i.e., the same strong validator might be in
use for representations of multiple resources at the same time and
does not imply that those representations are equivalent).
There are a variety of strong validators used in practice. The best
are based on strict revision control, wherein each change to a
representation always results in a unique node name and revision
identifier being assigned before the representation is made
accessible to GET. A cryptographic hash function applied to the
representation data is also sufficient if the data is available prior
to the response header fields being sent and the digest does not need
to be recalculated every time a validation request is received.
However, if a resource has distinct representations that differ only
in their metadata, such as might occur with content negotiation over
media types that happen to share the same data format, then a server
SHOULD incorporate additional information in the validator to
distinguish those representations and avoid confusing cache behavior.
In contrast, a "weak validator" is a representation metadata value
that might not be changed for every change to the representation
data. This weakness might be due to limitations in how the value is
calculated, such as clock resolution or an inability to ensure
uniqueness for all possible representations of the resource, or due
to a desire by the resource owner to group representations by some
self-determined set of equivalency rather than unique sequences of
data. A weak entity-tag SHOULD change whenever the origin server
considers prior representations to be unacceptable as a substitute
for the current representation. In other words, a weak entity-tag
SHOULD change whenever the origin server wants caches to invalidate
old responses.
For example, the representation of a weather report that changes in
content every second, based on dynamic measurements, might be grouped
into sets of equivalent representations (from the origin server's
perspective) with the same weak validator in order to allow cached
representations to be valid for a reasonable period of time (perhaps
adjusted dynamically based on server load or weather quality).
Likewise, a representation's modification time, if defined with only
one-second resolution, might be a weak validator if it is possible
for the representation to be modified twice during a single second
and retrieved between those modifications.
A "use" of a validator occurs when either a client generates a
request and includes the validator in a precondition or when a server
compares two validators. Weak validators are only usable in contexts
that do not depend on exact equality of a representation's payload
body. Strong validators are usable and preferred for all conditional
requests, including cache validation, partial content ranges, and
"lost update" avoidance.
2.2. Last-Modified
The "Last-Modified" header field indicates the date and time at which The "Last-Modified" header field indicates the date and time at which
the origin server believes the selected representation was last the origin server believes the selected representation was last
modified. modified.
Last-Modified = HTTP-date Last-Modified = HTTP-date
An example of its use is An example of its use is
Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
2.1.1. Generation 2.2.1. Generation
Origin servers SHOULD send Last-Modified for any selected Origin servers SHOULD send Last-Modified for any selected
representation for which a last modification date can be reasonably representation for which a last modification date can be reasonably
and consistently determined, since its use in conditional requests and consistently determined, since its use in conditional requests
and evaluating cache freshness ([Part6]) results in a substantial and evaluating cache freshness ([Part6]) results in a substantial
reduction of HTTP traffic on the Internet and can be a significant reduction of HTTP traffic on the Internet and can be a significant
factor in improving service scalability and reliability. factor in improving service scalability and reliability.
A representation is typically the sum of many parts behind the A representation is typically the sum of many parts behind the
resource interface. The last-modified time would usually be the most resource interface. The last-modified time would usually be the most
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response is generated. response is generated.
An origin server with a clock MUST NOT send a Last-Modified date that 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 is later than the server's time of message origination (Date). If
the last modification time is derived from implementation-specific the last modification time is derived from implementation-specific
metadata that evaluates to some time in the future, according to the metadata that evaluates to some time in the future, according to the
origin server's clock, then the origin server MUST replace that value origin server's clock, then the origin server MUST replace that value
with the message origination date. This prevents a future with the message origination date. This prevents a future
modification date from having an adverse impact on cache validation. modification date from having an adverse impact on cache validation.
2.1.2. Comparison 2.2.2. Comparison
A Last-Modified time, when used as a validator in a request, is 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, implicitly weak unless it is possible to deduce that it is strong,
using the following rules: using the following rules:
o The validator is being compared by an origin server to the actual o The validator is being compared by an origin server to the actual
current validator for the representation and, current validator for the representation and,
o That origin server reliably knows that the associated o That origin server reliably knows that the associated
representation did not change twice during the second covered by representation did not change twice during the second covered by
the presented validator. the presented validator.
or or
o The validator is about to be used by a client in an If-Modified- o The validator is about to be used by a client in an If-Modified-
Since or If-Unmodified-Since header field, because the client has Since, If-Unmodified-Since header field, because the client has a
a cache entry for the associated representation, and cache entry, or If-Range for the associated representation, and
o That cache entry includes a Date value, which gives the time when o That cache entry includes a Date value, which gives the time when
the origin server sent the original response, and the origin server sent the original response, and
o The presented Last-Modified time is at least 60 seconds before the o The presented Last-Modified time is at least 60 seconds before the
Date value. Date value.
or or
o The validator is being compared by an intermediate cache to the o The validator is being compared by an intermediate cache to the
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This method relies on the fact that if two different responses were 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 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 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- have a Date value equal to its Last-Modified time. The arbitrary 60-
second limit guards against the possibility that the Date and Last- second limit guards against the possibility that the Date and Last-
Modified values are generated from different clocks, or at somewhat Modified values are generated from different clocks, or at somewhat
different times during the preparation of the response. An different times during the preparation of the response. An
implementation MAY use a value larger than 60 seconds, if it is implementation MAY use a value larger than 60 seconds, if it is
believed that 60 seconds is too short. believed that 60 seconds is too short.
2.2. ETag 2.3. ETag
The ETag header field provides the current entity-tag for the The ETag header field provides the current entity-tag for the
selected representation. An entity-tag is an opaque validator for selected representation. An entity-tag is an opaque validator for
differentiating between multiple representations of the same differentiating between multiple representations of the same
resource, regardless of whether those multiple representations are resource, regardless of whether those multiple representations are
due to resource state changes over time, content negotiation due to resource state changes over time, content negotiation
resulting in multiple representations being valid at the same time, resulting in multiple representations being valid at the same time,
or both. An entity-tag consists of an opaque quoted string, possibly or both. An entity-tag consists of an opaque quoted string, possibly
prefixed by a weakness indicator. prefixed by a weakness indicator.
skipping to change at page 9, line 11 skipping to change at page 10, line 42
dates, where the one-second resolution of HTTP date values is not dates, where the one-second resolution of HTTP date values is not
sufficient, or where modification dates are not consistently sufficient, or where modification dates are not consistently
maintained. maintained.
Examples: Examples:
ETag: "xyzzy" ETag: "xyzzy"
ETag: W/"xyzzy" ETag: W/"xyzzy"
ETag: "" ETag: ""
2.2.1. Generation An entity-tag can be either a weak or strong validator, with strong
being the default. If an origin server provides an entity-tag for a
representation and the generation of that entity-tag does not satisfy
the requirements for a strong validator (Section 2.1), then that
entity-tag MUST be marked as weak by prefixing its opaque value with
"W/" (case-sensitive).
2.3.1. Generation
The principle behind entity-tags is that only the service author The principle behind entity-tags is that only the service author
knows the implementation of a resource well enough to select the most knows the implementation of a resource well enough to select the most
accurate and efficient validation mechanism for that resource, and accurate and efficient validation mechanism for that resource, and
that any such mechanism can be mapped to a simple sequence of octets 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 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. the client to be aware of how each entity-tag is constructed.
For example, a resource that has implementation-specific versioning For example, a resource that has implementation-specific versioning
applied to all changes might use an internal revision number, perhaps applied to all changes might use an internal revision number, perhaps
skipping to change at page 9, line 35 skipping to change at page 11, line 29
combination of various filesystem attributes, or a modification combination of various filesystem attributes, or a modification
timestamp that has sub-second resolution. timestamp that has sub-second resolution.
Origin servers SHOULD send ETag for any selected representation for Origin servers SHOULD send ETag for any selected representation for
which detection of changes can be reasonably and consistently which detection of changes can be reasonably and consistently
determined, since the entity-tag's use in conditional requests and determined, since the entity-tag's use in conditional requests and
evaluating cache freshness ([Part6]) can result in a substantial evaluating cache freshness ([Part6]) can result in a substantial
reduction of HTTP network traffic and can be a significant factor in reduction of HTTP network traffic and can be a significant factor in
improving service scalability and reliability. improving service scalability and reliability.
2.2.2. Weak versus Strong 2.3.2. Comparison
Since both origin servers and caches will compare two validators to
decide if they indicate the same or different representations, one
normally would expect that if the representation (including both
representation header fields and representation body) changes in any
way, then the associated validator would change as well. If this is
true, then we call that validator a "strong validator". One example
of a strong validator is an integer that is incremented in stable
storage every time a representation is changed.
However, there might be cases when a server prefers to change the
validator only when it desires cached representations to be
invalidated. 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) 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). A validator
that does not always change when the representation changes is a
"weak validator".
One can think of a strong validator as part of an identifier for a
specific representation, whereas a weak validator is part of an
identifier for a set of equivalent representations (where this notion
of equivalence is entirely governed by the origin server and beyond
the scope of this specification).
An entity-tag is normally a strong validator, but the protocol
provides a mechanism to tag an entity-tag as "weak".
A representation's modification time, if defined with only one-
second resolution, could be a weak validator, since it is possible
that the representation might be modified twice during a single
second.
Support for weak validators is optional. However, weak validators
allow for more efficient caching of equivalent objects; for
example, a hit counter on a site is probably good enough if it is
updated every few days or weeks, and any value during that period
is likely "good enough" to be equivalent.
A strong entity-tag MUST change whenever the associated
representation changes in any way. A weak entity-tag SHOULD change
whenever the origin server considers prior representations to be
unacceptable as a substitute for the current representation. In
other words, a weak entity tag SHOULD change whenever the origin
server wants caches to invalidate old responses.
A "strong entity-tag" MAY be shared by two representations of a
resource only if they are equivalent by octet equality.
A "weak entity-tag", indicated by the "W/" prefix, MAY be shared by
two representations of a resource. A weak entity-tag can only be
used for weak comparison.
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 entity-tag MUST be unique across all versions of all
representations associated with a particular resource over time.
However, there is no implication of uniqueness across entity-tags of
different resources (i.e., the same entity-tag value might be in use
for representations of multiple resources at the same time and does
not imply that those representations are equivalent).
2.2.3. Comparison
There are two entity-tag comparison functions, depending on whether There are two entity-tag comparison functions, depending on whether
the comparison context allows the use of weak validators or not: the comparison context allows the use of weak validators or not:
o The strong comparison function: in order to be considered equal, o The strong comparison function: in order to be considered equal,
both opaque-tags MUST be identical character-by-character, and both opaque-tags MUST be identical character-by-character, and
both MUST NOT be weak. both MUST NOT be weak.
o The weak comparison function: in order to be considered equal, o The weak comparison function: in order to be considered equal,
both opaque-tags MUST be identical character-by-character, but both opaque-tags MUST be identical character-by-character, but
either or both of them MAY be tagged as "weak" without affecting either or both of them MAY be tagged as "weak" without affecting
the result. the result.
A "use" of a validator is either when a client generates a request
and includes the validator in a precondition, or when a server
compares two validators.
Strong validators are usable in any context. Weak validators are
only usable in contexts that do not depend on exact equality of a
representation. For example, either kind is usable for a normal
conditional GET.
The example below shows the results for a set of entity-tag pairs, The example below shows the results for a set of entity-tag pairs,
and both the weak and strong comparison function results: and both the weak and strong comparison function results:
+--------+--------+-------------------+-----------------+ +--------+--------+-------------------+-----------------+
| ETag 1 | ETag 2 | Strong Comparison | Weak Comparison | | ETag 1 | ETag 2 | Strong Comparison | Weak Comparison |
+--------+--------+-------------------+-----------------+ +--------+--------+-------------------+-----------------+
| W/"1" | W/"1" | no match | match | | W/"1" | W/"1" | no match | match |
| W/"1" | W/"2" | no match | no match | | W/"1" | W/"2" | no match | no match |
| W/"1" | "1" | no match | match | | W/"1" | "1" | no match | match |
| "1" | "1" | match | match | | "1" | "1" | match | match |
+--------+--------+-------------------+-----------------+ +--------+--------+-------------------+-----------------+
An entity-tag is strong unless it is explicitly tagged as weak. 2.3.3. Example: Entity-tags varying on Content-Negotiated Resources
2.2.4. Rules for When to Use Entity-tags and Last-Modified Dates Consider a resource that is subject to content negotiation (Section 5
of [Part3]), and where the representations returned upon a GET
request vary based on the Accept-Encoding request header field
(Section 6.3 of [Part3]):
>> Request:
GET /index HTTP/1.1
Host: www.example.com
Accept-Encoding: gzip
In this case, the response might or might not use the gzip content
coding. If it does not, the response might look like:
>> Response:
HTTP/1.1 200 OK
Date: Thu, 26 Mar 2010 00:05:00 GMT
ETag: "123-a"
Content-Length: 70
Vary: Accept-Encoding
Content-Type: text/plain
Hello World!
Hello World!
Hello World!
Hello World!
Hello World!
An alternative representation that does use gzip content coding would
be:
>> Response:
HTTP/1.1 200 OK
Date: Thu, 26 Mar 2010 00:05:00 GMT
ETag: "123-b"
Content-Length: 43
Vary: Accept-Encoding
Content-Type: text/plain
Content-Encoding: gzip
...binary data...
Note: Content codings are a property of the representation, so
therefore an entity-tag of an encoded representation must be
distinct from an unencoded representation to prevent conflicts
during cache updates and range requests. In contrast, transfer
codings (Section 6.2 of [Part1]) apply only during message
transfer and do not require distinct entity-tags.
2.4. Rules for When to Use Entity-tags and Last-Modified Dates
We adopt a set of rules and recommendations for origin servers, We adopt a set of rules and recommendations for origin servers,
clients, and caches regarding when various validator types ought to clients, and caches regarding when various validator types ought to
be used, and for what purposes. be used, and for what purposes.
HTTP/1.1 origin servers: HTTP/1.1 origin servers:
o SHOULD send an entity-tag validator unless it is not feasible to o SHOULD send an entity-tag validator unless it is not feasible to
generate one. generate one.
skipping to change at page 13, line 13 skipping to change at page 14, line 43
conservative assumptions about the validators they receive. conservative assumptions about the validators they receive.
HTTP/1.0 clients and caches might ignore entity-tags. Generally, HTTP/1.0 clients and caches might ignore entity-tags. Generally,
last-modified values received or used by these systems will last-modified values received or used by these systems will
support transparent and efficient caching, and so HTTP/1.1 origin support transparent and efficient caching, and so HTTP/1.1 origin
servers should provide Last-Modified values. In those rare cases servers should provide Last-Modified values. In those rare cases
where the use of a Last-Modified value as a validator by an where the use of a Last-Modified value as a validator by an
HTTP/1.0 system could result in a serious problem, then HTTP/1.1 HTTP/1.0 system could result in a serious problem, then HTTP/1.1
origin servers should not provide one. origin servers should not provide one.
2.2.5. Example: Entity-tags varying on Content-Negotiated Resources
Consider a resource that is subject to content negotiation (Section 5
of [Part3]), and where the representations returned upon a GET
request vary based on the Accept-Encoding request header field
(Section 6.3 of [Part3]):
>> Request:
GET /index HTTP/1.1
Host: www.example.com
Accept-Encoding: gzip
In this case, the response might or might not use the gzip content
coding. If it does not, the response might look like:
>> Response:
HTTP/1.1 200 OK
Date: Thu, 26 Mar 2010 00:05:00 GMT
ETag: "123-a"
Content-Length: 70
Vary: Accept-Encoding
Content-Type: text/plain
Hello World!
Hello World!
Hello World!
Hello World!
Hello World!
An alternative representation that does use gzip content coding would
be:
>> Response:
HTTP/1.1 200 OK
Date: Thu, 26 Mar 2010 00:05:00 GMT
ETag: "123-b"
Content-Length: 43
Vary: Accept-Encoding
Content-Type: text/plain
Content-Encoding: gzip
...binary data...
Note: Content codings are a property of the representation, so
therefore an entity-tag of an encoded representation must be
distinct from an unencoded representation to prevent conflicts
during cache updates and range requests. In contrast, transfer
codings (Section 6.2 of [Part1]) apply only during message
transfer and do not require distinct entity-tags.
3. Precondition Header Fields 3. Precondition Header Fields
This section defines the syntax and semantics of HTTP/1.1 header This section defines the syntax and semantics of HTTP/1.1 header
fields for applying preconditions on requests. fields for applying preconditions on requests.
3.1. If-Match 3.1. If-Match
The "If-Match" header field MAY be used to make a request method The "If-Match" header field MAY be used to make a request method
conditional on the current existence or value of an entity-tag for conditional on the current existence or value of an entity-tag for
one or more representations of the target resource. If-Match is one or more representations of the target resource. If-Match is
generally useful for resource update requests, such as PUT requests, generally useful for resource update requests, such as PUT requests,
as a means for protecting against accidental overwrites when multiple as a means for protecting against accidental overwrites when multiple
clients are acting in parallel on the same resource (i.e., the "lost clients are acting in parallel on the same resource (i.e., the "lost
update" problem). An If-Match field-value of "*" places the update" problem). An If-Match field-value of "*" places the
precondition on the existence of any current representation for the precondition on the existence of any current representation for the
target resource. target resource.
If-Match = "*" / 1#entity-tag If-Match = "*" / 1#entity-tag
If any of the entity-tags listed in the If-Match field value match If any of the entity-tags listed in the If-Match field value match
(as per Section 2.2.3) the entity-tag of the selected representation (as per Section 2.3.2) the entity-tag of the selected representation
for the target resource, or if "*" is given and any current for the target resource, or if "*" is given and any current
representation exists for the target resource, then the server MAY representation exists for the target resource, then the server MAY
perform the request method as if the If-Match header field was not perform the request method as if the If-Match header field was not
present. present.
If none of the entity-tags match, or if "*" is given and no current If none of the entity-tags match, or if "*" is given and no current
representation exists, the server MUST NOT perform the requested representation exists, the server MUST NOT perform the requested
method. Instead, the server MUST respond with the 412 (Precondition method. Instead, the server MUST respond with the 412 (Precondition
Failed) status code. Failed) status code.
skipping to change at page 15, line 44 skipping to change at page 16, line 16
unsafe request method (e.g., PUT) from inadvertently modifying an unsafe request method (e.g., PUT) from inadvertently modifying an
existing representation of the target resource when the client existing representation of the target resource when the client
believes that the resource does not have a current representation. believes that the resource does not have a current representation.
This is a variation on the "lost update" problem that might arise if This is a variation on the "lost update" problem that might arise if
more than one client attempts to create an initial representation for more than one client attempts to create an initial representation for
the target resource. the target resource.
If-None-Match = "*" / 1#entity-tag If-None-Match = "*" / 1#entity-tag
If any of the entity-tags listed in the If-None-Match field-value If any of the entity-tags listed in the If-None-Match field-value
match (as per Section 2.2.3) the entity-tag of the selected match (as per Section 2.3.2) the entity-tag of the selected
representation, or if "*" is given and any current representation representation, or if "*" is given and any current representation
exists for that resource, then the server MUST NOT perform the exists for that resource, then the server MUST NOT perform the
requested method. Instead, if the request method was GET or HEAD, requested method. Instead, if the request method was GET or HEAD,
the server SHOULD respond with a 304 (Not Modified) status code, the server SHOULD respond with a 304 (Not Modified) status code,
including the cache-related header fields (particularly ETag) of the including the cache-related header fields (particularly ETag) of the
selected representation that has a matching entity-tag. For all selected representation that has a matching entity-tag. For all
other request methods, the server MUST respond with a 412 other request methods, the server MUST respond with a 412
(Precondition Failed) status code. (Precondition Failed) status code.
If none of the entity-tags match, then the server MAY perform the If none of the entity-tags match, then the server MAY perform the
requested method as if the If-None-Match header field did not exist, requested method as if the If-None-Match header field did not exist,
but MUST also ignore any If-Modified-Since header field(s) in the but MUST also ignore any If-Modified-Since header field(s) in the
request. That is, if no entity-tags match, then the server MUST NOT request. That is, if no entity-tags match, then the server MUST NOT
return a 304 (Not Modified) response. return a 304 (Not Modified) response.
If the request would, without the If-None-Match header field, result If the request would, without the If-None-Match header field, result
in anything other than a 2xx or 304 status code, then the If-None- in anything other than a 2xx or 304 status code, then the If-None-
Match header field MUST be ignored. (See Section 2.2.4 for a Match header field MUST be ignored. (See Section 2.4 for a
discussion of server behavior when both If-Modified-Since and If- discussion of server behavior when both If-Modified-Since and If-
None-Match appear in the same request.) None-Match appear in the same request.)
Examples: Examples:
If-None-Match: "xyzzy" If-None-Match: "xyzzy"
If-None-Match: W/"xyzzy" If-None-Match: W/"xyzzy"
If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz" If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
If-None-Match: * If-None-Match: *
skipping to change at page 20, line 21 skipping to change at page 20, line 38
5.2. Header Field Registration 5.2. Header Field Registration
The Message Header Field Registry located at <http://www.iana.org/ The Message Header Field Registry located at <http://www.iana.org/
assignments/message-headers/message-header-index.html> shall be assignments/message-headers/message-header-index.html> shall be
updated with the permanent registrations below (see [RFC3864]): updated with the permanent registrations below (see [RFC3864]):
+---------------------+----------+----------+-------------+ +---------------------+----------+----------+-------------+
| Header Field Name | Protocol | Status | Reference | | Header Field Name | Protocol | Status | Reference |
+---------------------+----------+----------+-------------+ +---------------------+----------+----------+-------------+
| ETag | http | standard | Section 2.2 | | ETag | http | standard | Section 2.3 |
| If-Match | http | standard | Section 3.1 | | If-Match | http | standard | Section 3.1 |
| If-Modified-Since | http | standard | Section 3.3 | | If-Modified-Since | http | standard | Section 3.3 |
| If-None-Match | http | standard | Section 3.2 | | If-None-Match | http | standard | Section 3.2 |
| If-Unmodified-Since | http | standard | Section 3.4 | | If-Unmodified-Since | http | standard | Section 3.4 |
| Last-Modified | http | standard | Section 2.1 | | Last-Modified | http | standard | Section 2.2 |
+---------------------+----------+----------+-------------+ +---------------------+----------+----------+-------------+
The change controller is: "IETF (iesg@ietf.org) - Internet The change controller is: "IETF (iesg@ietf.org) - Internet
Engineering Task Force". Engineering Task Force".
6. Security Considerations 6. Security Considerations
No additional security considerations have been identified beyond No additional security considerations have been identified beyond
those applicable to HTTP in general [Part1]. those applicable to HTTP in general [Part1].
7. Acknowledgments 7. Acknowledgments
See Section 12 of [Part1].
8. References 8. References
8.1. Normative References 8.1. Normative References
[Part1] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., [Part1] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections, and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections,
and Message Parsing", draft-ietf-httpbis-p1-messaging-15 and Message Parsing", draft-ietf-httpbis-p1-messaging-16
(work in progress), July 2011. (work in progress), August 2011.
[Part3] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., [Part3] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
and J. Reschke, Ed., "HTTP/1.1, part 3: Message Payload and J. Reschke, Ed., "HTTP/1.1, part 3: Message Payload
and Content Negotiation", draft-ietf-httpbis-p3-payload-15 and Content Negotiation", draft-ietf-httpbis-p3-payload-16
(work in progress), July 2011. (work in progress), August 2011.
[Part5] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., [Part5] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and
Partial Responses", draft-ietf-httpbis-p5-range-15 (work Partial Responses", draft-ietf-httpbis-p5-range-16 (work
in progress), July 2011. in progress), August 2011.
[Part6] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., [Part6] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
Nottingham, M., Ed., and J. Reschke, Ed., "HTTP/1.1, part Nottingham, M., Ed., and J. Reschke, Ed., "HTTP/1.1, part
6: Caching", draft-ietf-httpbis-p6-cache-15 (work in 6: Caching", draft-ietf-httpbis-p6-cache-16 (work in
progress), July 2011. progress), August 2011.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008. Specifications: ABNF", STD 68, RFC 5234, January 2008.
8.2. Informative References 8.2. Informative References
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
skipping to change at page 21, line 40 skipping to change at page 22, line 11
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864, Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004. September 2004.
[RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed [RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed
Authoring and Versioning (WebDAV)", RFC 4918, June 2007. Authoring and Versioning (WebDAV)", RFC 4918, June 2007.
Appendix A. Changes from RFC 2616 Appendix A. Changes from RFC 2616
Allow weak entity-tags in all requests except range requests Allow weak entity-tags in all requests except range requests
(Sections 2.2.2 and 3.2). (Sections 2.1 and 3.2).
Change ABNF productions for header fields to only define the field Change ABNF productions for header fields to only define the field
value. (Section 3) value. (Section 3)
Appendix B. Collected ABNF Appendix B. Collected ABNF
ETag = entity-tag ETag = entity-tag
HTTP-date = <HTTP-date, defined in [Part1], Section 6.1> HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>
skipping to change at page 22, line 26 skipping to change at page 22, line 37
If-Unmodified-Since = HTTP-date If-Unmodified-Since = HTTP-date
Last-Modified = HTTP-date Last-Modified = HTTP-date
OWS = <OWS, defined in [Part1], Section 1.2.2> OWS = <OWS, defined in [Part1], Section 1.2.2>
entity-tag = [ weak ] opaque-tag entity-tag = [ weak ] opaque-tag
opaque-tag = quoted-string opaque-tag = quoted-string
quoted-string = <quoted-string, defined in [Part1], Section 1.2.2> quoted-string = <quoted-string, defined in [Part1], Section 3.2.3>
weak = %x57.2F ; W/ weak = %x57.2F ; W/
ABNF diagnostics: ABNF diagnostics:
; ETag defined but not used ; ETag defined but not used
; If-Match defined but not used ; If-Match defined but not used
; If-Modified-Since defined but not used ; If-Modified-Since defined but not used
; If-None-Match defined but not used ; If-None-Match defined but not used
; If-Unmodified-Since defined but not used ; If-Unmodified-Since defined but not used
skipping to change at page 25, line 36 skipping to change at page 26, line 9
o <http://tools.ietf.org/wg/httpbis/trac/ticket/276>: "untangle o <http://tools.ietf.org/wg/httpbis/trac/ticket/276>: "untangle
ABNFs for header fields" ABNFs for header fields"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/269>: "ETags and o <http://tools.ietf.org/wg/httpbis/trac/ticket/269>: "ETags and
Quotes" Quotes"
C.16. Since draft-ietf-httpbis-p4-conditional-14 C.16. Since draft-ietf-httpbis-p4-conditional-14
None. None.
C.17. Since draft-ietf-httpbis-p4-conditional-15
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/304>: "If-Range
should be listed when dicussing contexts where L-M can be
considered strong"
Index Index
3 3
304 Not Modified (status code) 18 304 Not Modified (status code) 19
4 4
412 Precondition Failed (status code) 19 412 Precondition Failed (status code) 20
E E
ETag header field 8 ETag header field 10
G G
Grammar Grammar
entity-tag 8 entity-tag 10
ETag 8 ETag 10
If-Match 14 If-Match 15
If-Modified-Since 16 If-Modified-Since 17
If-None-Match 15 If-None-Match 16
If-Unmodified-Since 18 If-Unmodified-Since 18
Last-Modified 6 Last-Modified 8
opaque-tag 8 opaque-tag 10
weak 8 weak 10
H H
Header Fields Header Fields
ETag 8 ETag 10
If-Match 14 If-Match 14
If-Modified-Since 16 If-Modified-Since 17
If-None-Match 15 If-None-Match 15
If-Unmodified-Since 18 If-Unmodified-Since 18
Last-Modified 6 Last-Modified 8
I I
If-Match header field 14 If-Match header field 14
If-Modified-Since header field 16 If-Modified-Since header field 17
If-None-Match header field 15 If-None-Match header field 15
If-Unmodified-Since header field 18 If-Unmodified-Since header field 18
L L
Last-Modified header field 6 Last-Modified header field 8
M M
metadata 6 metadata 6
S S
selected representation 5 selected representation 5
Status Codes Status Codes
304 Not Modified 18 304 Not Modified 19
412 Precondition Failed 19 412 Precondition Failed 20
V V
validator 6 validator 6
strong 6
weak 6
Authors' Addresses Authors' Addresses
Roy T. Fielding (editor) Roy T. Fielding (editor)
Adobe Systems Incorporated Adobe Systems Incorporated
345 Park Ave 345 Park Ave
San Jose, CA 95110 San Jose, CA 95110
USA USA
EMail: fielding@gbiv.com EMail: fielding@gbiv.com
 End of changes. 57 change blocks. 
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