draft-ietf-httpbis-p4-conditional-26.txt   draft-ietf-httpbis-p4-conditional-latest.txt 
HTTPbis Working Group R. Fielding, Ed. HTTPbis Working Group R. Fielding, Ed.
Internet-Draft Adobe Internet-Draft Adobe
Obsoletes: 2616 (if approved) J. Reschke, Ed. Obsoletes: 2616 (if approved) J. Reschke, Ed.
Intended status: Standards Track greenbytes Intended status: Standards Track greenbytes
Expires: August 10, 2014 February 6, 2014 Expires: December 3, 2014 June 2014
Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests
draft-ietf-httpbis-p4-conditional-26 draft-ietf-httpbis-p4-conditional-latest
Abstract Abstract
The Hypertext Transfer Protocol (HTTP) is a stateless application- The Hypertext Transfer Protocol (HTTP) is a stateless application-
level protocol for distributed, collaborative, hypertext information level protocol for distributed, collaborative, hypertext information
systems. This document defines HTTP/1.1 conditional requests, systems. This document defines HTTP/1.1 conditional requests,
including metadata header fields for indicating state changes, including metadata header fields for indicating state changes,
request header fields for making preconditions on such state, and request header fields for making preconditions on such state, and
rules for constructing the responses to a conditional request when rules for constructing the responses to a conditional request when
one or more preconditions evaluate to false. one or more preconditions evaluate to false.
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Discussion of this draft takes place on the HTTPBIS working group Discussion of this draft takes place on the HTTPBIS working group
mailing list (ietf-http-wg@w3.org), which is archived at 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 D.2. _This is a temporary document for the purpose of tracking the
editorial changes made during the AUTH48 (RFC publication) phase._
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 August 10, 2014. This Internet-Draft will expire on December 3, 2014.
Copyright Notice Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the Copyright (c) 2014 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
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1.1. Conformance and Error Handling . . . . . . . . . . . . . . 4 1.1. Conformance and Error Handling . . . . . . . . . . . . . . 4
1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 4 1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 4
2. Validators . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Validators . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Weak versus Strong . . . . . . . . . . . . . . . . . . . . 5 2.1. Weak versus Strong . . . . . . . . . . . . . . . . . . . . 5
2.2. Last-Modified . . . . . . . . . . . . . . . . . . . . . . 7 2.2. Last-Modified . . . . . . . . . . . . . . . . . . . . . . 7
2.2.1. Generation . . . . . . . . . . . . . . . . . . . . . . 7 2.2.1. Generation . . . . . . . . . . . . . . . . . . . . . . 7
2.2.2. Comparison . . . . . . . . . . . . . . . . . . . . . . 8 2.2.2. Comparison . . . . . . . . . . . . . . . . . . . . . . 8
2.3. ETag . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.3. ETag . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.3.1. Generation . . . . . . . . . . . . . . . . . . . . . . 10 2.3.1. Generation . . . . . . . . . . . . . . . . . . . . . . 10
2.3.2. Comparison . . . . . . . . . . . . . . . . . . . . . . 10 2.3.2. Comparison . . . . . . . . . . . . . . . . . . . . . . 10
2.3.3. Example: Entity-tags Varying on Content-Negotiated 2.3.3. Example: Entity-Tags Varying on Content-Negotiated
Resources . . . . . . . . . . . . . . . . . . . . . . 11 Resources . . . . . . . . . . . . . . . . . . . . . . 11
2.4. When to Use Entity-tags and Last-Modified Dates . . . . . 12 2.4. When to Use Entity-Tags and Last-Modified Dates . . . . . 12
3. Precondition Header Fields . . . . . . . . . . . . . . . . . . 13 3. Precondition Header Fields . . . . . . . . . . . . . . . . . . 13
3.1. If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.1. If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.2. If-None-Match . . . . . . . . . . . . . . . . . . . . . . 14 3.2. If-None-Match . . . . . . . . . . . . . . . . . . . . . . 14
3.3. If-Modified-Since . . . . . . . . . . . . . . . . . . . . 15 3.3. If-Modified-Since . . . . . . . . . . . . . . . . . . . . 15
3.4. If-Unmodified-Since . . . . . . . . . . . . . . . . . . . 16 3.4. If-Unmodified-Since . . . . . . . . . . . . . . . . . . . 16
3.5. If-Range . . . . . . . . . . . . . . . . . . . . . . . . . 18 3.5. If-Range . . . . . . . . . . . . . . . . . . . . . . . . . 18
4. Status Code Definitions . . . . . . . . . . . . . . . . . . . 18 4. Status Code Definitions . . . . . . . . . . . . . . . . . . . 18
4.1. 304 Not Modified . . . . . . . . . . . . . . . . . . . . . 18 4.1. 304 Not Modified . . . . . . . . . . . . . . . . . . . . . 18
4.2. 412 Precondition Failed . . . . . . . . . . . . . . . . . 18 4.2. 412 Precondition Failed . . . . . . . . . . . . . . . . . 18
5. Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . 19 5. Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . 19
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7.1. Status Code Registration . . . . . . . . . . . . . . . . . 21 7.1. Status Code Registration . . . . . . . . . . . . . . . . . 21
7.2. Header Field Registration . . . . . . . . . . . . . . . . 21 7.2. Header Field Registration . . . . . . . . . . . . . . . . 21
8. Security Considerations . . . . . . . . . . . . . . . . . . . 22 8. Security Considerations . . . . . . . . . . . . . . . . . . . 22
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
10.1. Normative References . . . . . . . . . . . . . . . . . . . 23 10.1. Normative References . . . . . . . . . . . . . . . . . . . 23
10.2. Informative References . . . . . . . . . . . . . . . . . . 23 10.2. Informative References . . . . . . . . . . . . . . . . . . 23
Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 23 Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 23
Appendix B. Imported ABNF . . . . . . . . . . . . . . . . . . . . 24 Appendix B. Imported ABNF . . . . . . . . . . . . . . . . . . . . 24
Appendix C. Collected ABNF . . . . . . . . . . . . . . . . . . . 24 Appendix C. Collected ABNF . . . . . . . . . . . . . . . . . . . 24
Appendix D. Change Log (to be removed by RFC Editor before Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
publication) . . . . . . . . . . . . . . . . . . . . 25
D.1. Since draft-ietf-httpbis-p4-conditional-24 . . . . . . . . 25
D.2. Since draft-ietf-httpbis-p4-conditional-25 . . . . . . . . 25
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction 1. Introduction
Conditional requests are HTTP requests [Part2] that include one or Conditional requests are HTTP requests [RFC7231] that include one or
more header fields indicating a precondition to be tested before more header fields indicating a precondition to be tested before
applying the method semantics to the target resource. This document applying the method semantics to the target resource. This document
defines the HTTP/1.1 conditional request mechanisms in terms of the defines the HTTP/1.1 conditional request mechanisms in terms of the
architecture, syntax notation, and conformance criteria defined in architecture, syntax notation, and conformance criteria defined in
[Part1]. [RFC7230].
Conditional GET requests are the most efficient mechanism for HTTP Conditional GET requests are the most efficient mechanism for HTTP
cache updates [Part6]. Conditionals can also be applied to state- cache updates [RFC7234]. Conditionals can also be applied to state-
changing methods, such as PUT and DELETE, to prevent the "lost changing methods, such as PUT and DELETE, to prevent the "lost
update" problem: one client accidentally overwriting the work of update" problem: one client accidentally overwriting the work of
another client that has been acting in parallel. another client that has been acting in 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 a "selected representation" assume that the mapping of requests to a "selected representation"
(Section 3 of [Part2]) will be consistent over time if the server (Section 3 of [RFC7231]) will be consistent over time if the server
intends to take advantage of conditionals. Regardless, if the intends to take advantage of conditionals. Regardless, if the
mapping is inconsistent and the server is unable to select the mapping is inconsistent and the server is unable to select the
appropriate representation, then no harm will result when the appropriate representation, then no harm will result when the
precondition evaluates to false. precondition evaluates to false.
The conditional request preconditions defined by this specification The conditional request preconditions defined by this specification
(Section 3) are evaluated when applicable to the recipient (Section 3) are evaluated when applicable to the recipient
(Section 5) according to their order of precedence (Section 6). (Section 5) according to their order of precedence (Section 6).
1.1. Conformance and Error Handling 1.1. Conformance and Error Handling
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
Conformance criteria and considerations regarding error handling are Conformance criteria and considerations regarding error handling are
defined in Section 2.5 of [Part1]. defined in Section 2.5 of [RFC7230].
1.2. Syntax Notation 1.2. Syntax Notation
This specification uses the Augmented Backus-Naur Form (ABNF) This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234] with a list extension, defined in Section 7 of notation of [RFC5234] with a list extension, defined in Section 7 of
[Part1], that allows for compact definition of comma-separated lists [RFC7230], that allows for compact definition of comma-separated
using a '#' operator (similar to how the '*' operator indicates lists using a '#' operator (similar to how the '*' operator indicates
repetition). Appendix B describes rules imported from other repetition). Appendix B describes rules imported from other
documents. Appendix C shows the collected grammar with all list documents. Appendix C shows the collected grammar with all list
operators expanded to standard ABNF notation. operators expanded to standard ABNF notation.
2. 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 (Section 2.2) and opaque entity tags modification dates (Section 2.2) and opaque entity tags
(Section 2.3). Additional metadata that reflects resource state has (Section 2.3). Additional metadata that reflects resource state has
been defined by various extensions of HTTP, such as WebDAV [RFC4918], been defined by various extensions of HTTP, such as Web Distributed
that are beyond the scope of this specification. A resource metadata Authoring and Versioning (WebDAV, [RFC4918]), that are beyond the
value is referred to as a "validator" when it is used within a scope of this specification. A resource metadata value is referred
precondition. to as a "validator" when it is used within a precondition.
2.1. Weak versus Strong 2.1. Weak versus Strong
Validators come in two flavors: strong or weak. Weak validators are Validators come in two flavors: strong or weak. Weak validators are
easy to generate but are far less useful for comparisons. Strong easy to generate but are far less useful for comparisons. Strong
validators are ideal for comparisons but can be very difficult (and validators are ideal for comparisons but can be very difficult (and
occasionally impossible) to generate efficiently. Rather than impose occasionally impossible) to generate efficiently. Rather than impose
that all forms of resource adhere to the same strength of validator, that all forms of resource adhere to the same strength of validator,
HTTP exposes the type of validator in use and imposes restrictions on HTTP exposes the type of validator in use and imposes restrictions on
when weak validators can be used as preconditions. when weak validators can be used as preconditions.
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not need to be recalculated every time a validation request is not need to be recalculated every time a validation request is
received. However, if a resource has distinct representations that received. However, if a resource has distinct representations that
differ only in their metadata, such as might occur with content differ only in their metadata, such as might occur with content
negotiation over media types that happen to share the same data negotiation over media types that happen to share the same data
format, then the origin server needs to incorporate additional format, then the origin server needs to incorporate additional
information in the validator to distinguish those representations. information in the validator to distinguish those representations.
In contrast, a "weak validator" is representation metadata that might In contrast, a "weak validator" is representation metadata that might
not change for every change to the representation data. This not change for every change to the representation data. This
weakness might be due to limitations in how the value is calculated, weakness might be due to limitations in how the value is calculated,
such as clock resolution or an inability to ensure uniqueness for all such as clock resolution, an inability to ensure uniqueness for all
possible representations of the resource, or due to a desire by the possible representations of the resource, or a desire of the resource
resource owner to group representations by some self-determined set owner to group representations by some self-determined set of
of equivalency rather than unique sequences of data. An origin equivalency rather than unique sequences of data. An origin server
server SHOULD change a weak entity-tag whenever it considers prior SHOULD change a weak entity-tag whenever it considers prior
representations to be unacceptable as a substitute for the current representations to be unacceptable as a substitute for the current
representation. In other words, a weak entity-tag ought to change representation. In other words, a weak entity-tag ought to change
whenever the origin server wants caches to invalidate old responses. whenever the origin server wants caches to invalidate old responses.
For example, the representation of a weather report that changes in For example, the representation of a weather report that changes in
content every second, based on dynamic measurements, might be grouped content every second, based on dynamic measurements, might be grouped
into sets of equivalent representations (from the origin server's into sets of equivalent representations (from the origin server's
perspective) with the same weak validator in order to allow cached perspective) with the same weak validator in order to allow cached
representations to be valid for a reasonable period of time (perhaps representations to be valid for a reasonable period of time (perhaps
adjusted dynamically based on server load or weather quality). adjusted dynamically based on server load or weather quality).
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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
2.2.1. Generation 2.2.1. Generation
An origin server SHOULD send Last-Modified for any selected An origin server 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 ([RFC7234]) 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
recent time that any of those parts were changed. How that value is recent time that any of those parts were changed. How that value is
determined for any given resource is an implementation detail beyond determined for any given resource is an implementation detail beyond
the scope of this specification. What matters to HTTP is how the scope of this specification. What matters to HTTP is how
recipients of the Last-Modified header field can use its value to recipients of the Last-Modified header field can use its value to
make conditional requests and test the validity of locally cached make conditional requests and test the validity of locally cached
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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, If-Unmodified-Since header field, because the client has a Since, If-Unmodified-Since, or If-Range header field, because the
cache entry, or If-Range for the associated representation, and client has a cache entry 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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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.
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.
2.3. ETag 2.3. ETag
The "ETag" header field in a response provides the current entity-tag The "ETag" header field in a response provides the current entity-tag
for the selected representation, as determined at the conclusion of for the selected representation, as determined at the conclusion of
handling the request. An entity-tag is an opaque validator for handling the request. An entity-tag is an opaque validator for
differentiating between multiple representations of the same differentiating between multiple representations of the same
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ETag = entity-tag ETag = entity-tag
entity-tag = [ weak ] opaque-tag entity-tag = [ weak ] opaque-tag
weak = %x57.2F ; "W/", case-sensitive weak = %x57.2F ; "W/", case-sensitive
opaque-tag = DQUOTE *etagc DQUOTE opaque-tag = DQUOTE *etagc DQUOTE
etagc = %x21 / %x23-7E / obs-text etagc = %x21 / %x23-7E / obs-text
; VCHAR except double quotes, plus obs-text ; VCHAR except double quotes, plus obs-text
Note: Previously, opaque-tag was defined to be a quoted-string Note: Previously, opaque-tag was defined to be a quoted-string
([RFC2616], Section 3.11), thus some recipients might perform ([RFC2616], Section 3.11); thus, some recipients might perform
backslash unescaping. Servers therefore ought to avoid backslash backslash unescaping. Servers therefore ought to avoid backslash
characters in entity tags. characters in entity tags.
An entity-tag can be more reliable for validation than a modification An entity-tag can be more reliable for validation than a modification
date in situations where it is inconvenient to store modification date in situations where it is inconvenient to store modification
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:
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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
combined with a variance identifier for content negotiation, to combined with a variance identifier for content negotiation, to
accurately differentiate between representations. Other accurately differentiate between representations. Other
implementations might use a collision-resistant hash of implementations might use a collision-resistant hash of
representation content, a combination of various file attributes, or representation content, a combination of various file attributes, or
a modification timestamp that has sub-second resolution. a modification timestamp that has sub-second resolution.
An origin server SHOULD send ETag for any selected representation for An origin server SHOULD send an ETag for any selected representation
which detection of changes can be reasonably and consistently for 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 ([RFC7234]) 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.
2.3.2. Comparison 2.3.2. 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: or not the comparison context allows the use of weak validators:
o Strong comparison: two entity-tags are equivalent if both are not o Strong comparison: two entity-tags are equivalent if both are not
weak and their opaque-tags match character-by-character. weak and their opaque-tags match character-by-character.
o Weak comparison: two entity-tags are equivalent if their opaque- o Weak comparison: two entity-tags are equivalent if their opaque-
tags match character-by-character, regardless of either or both tags match character-by-character, regardless of either or both
being tagged as "weak". being tagged as "weak".
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
and both the weak and strong comparison function results: 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 |
+--------+--------+-------------------+-----------------+ +--------+--------+-------------------+-----------------+
2.3.3. Example: Entity-tags Varying on Content-Negotiated Resources 2.3.3. Example: Entity-Tags Varying on Content-Negotiated Resources
Consider a resource that is subject to content negotiation (Section Consider a resource that is subject to content negotiation (Section
3.4 of [Part2]), and where the representations sent in response to a 3.4 of [RFC7231]), and where the representations sent in response to
GET request vary based on the Accept-Encoding request header field a GET request vary based on the Accept-Encoding request header field
(Section 5.3.4 of [Part2]): (Section 5.3.4 of [RFC7231]):
>> Request: >> Request:
GET /index HTTP/1.1 GET /index HTTP/1.1
Host: www.example.com Host: www.example.com
Accept-Encoding: gzip Accept-Encoding: gzip
In this case, the response might or might not use the gzip content In this case, the response might or might not use the gzip content
coding. If it does not, the response might look like: coding. If it does not, the response might look like:
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Vary: Accept-Encoding Vary: Accept-Encoding
Content-Type: text/plain Content-Type: text/plain
Content-Encoding: gzip Content-Encoding: gzip
...binary data... ...binary data...
Note: Content codings are a property of the representation data, Note: Content codings are a property of the representation data,
so a strong entity-tag for a content-encoded representation has to so a strong entity-tag for a content-encoded representation has to
be distinct from the entity tag of an unencoded representation to be distinct from the entity tag of an unencoded representation to
prevent potential conflicts during cache updates and range prevent potential conflicts during cache updates and range
requests. In contrast, transfer codings (Section 4 of [Part1]) requests. In contrast, transfer codings (Section 4 of [RFC7230])
apply only during message transfer and do not result in distinct apply only during message transfer and do not result in distinct
entity-tags. entity-tags.
2.4. When to Use Entity-tags and Last-Modified Dates 2.4. When to Use Entity-Tags and Last-Modified Dates
In 200 (OK) responses to GET or HEAD, an origin server: In 200 (OK) responses to GET or HEAD, an origin server:
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.
o MAY send a weak entity-tag instead of a strong entity-tag, if o MAY send a weak entity-tag instead of a strong entity-tag, if
performance considerations support the use of weak entity-tags, or performance considerations support the use of weak entity-tags, or
if it is unfeasible to send a strong entity-tag. if it is unfeasible to send a strong entity-tag.
skipping to change at page 13, line 50 skipping to change at page 13, line 50
An origin server that receives an If-Match header field MUST evaluate An origin server that receives an If-Match header field MUST evaluate
the condition prior to performing the method (Section 5). If the the condition prior to performing the method (Section 5). If the
field-value is "*", the condition is false if the origin server does field-value is "*", the condition is false if the origin server does
not have a current representation for the target resource. If the 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 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 of the listed tags match the entity-tag of the selected
representation. representation.
An origin server MUST NOT perform the requested method if a received An origin server MUST NOT perform the requested method if a received
If-Match condition evaluates to false; instead the origin server MUST If-Match condition evaluates to false; instead, the origin server
respond with either: a) the 412 (Precondition Failed) status code; MUST respond with either a) the 412 (Precondition Failed) status code
or, b) one of the 2xx (Successful) status codes if the origin server 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 has verified that a state change is being requested and the final
state is already reflected in the current state of the target state is already reflected in the current state of the target
resource (i.e., the change requested by the user agent has already resource (i.e., the change requested by the user agent has already
succeeded, but the user agent might not be aware of it, perhaps succeeded, but the user agent might not be aware of it, perhaps
because the prior response was lost or a compatible change was made because the prior response was lost or a compatible change was made
by some other user agent). In the latter case, the origin server 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 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 verify that the request is a duplicate of an immediately prior change
made by the same user agent. made by the same user agent.
skipping to change at page 15, line 5 skipping to change at page 15, line 5
stored responses that have entity-tags, the client SHOULD generate an stored responses that have entity-tags, the client SHOULD generate an
If-None-Match header field containing a list of those entity-tags If-None-Match header field containing a list of those entity-tags
when making a GET request; this allows recipient servers to send a when making a GET request; this allows recipient servers to send a
304 (Not Modified) response to indicate when one of those stored 304 (Not Modified) response to indicate when one of those stored
responses matches the selected representation. responses matches the selected representation.
If-None-Match can also be used with a value of "*" to prevent an If-None-Match can also be used with a value of "*" to prevent an
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
(Section 4.2.1 of [Part2]). This is a variation on the "lost update" (Section 4.2.1 of [RFC7231]). This is a variation on the "lost
problem that might arise if more than one client attempts to create update" problem that might arise if more than one client attempts to
an initial representation for the target resource. create an initial representation for the target resource.
An origin server that receives an If-None-Match header field MUST An origin server that receives an If-None-Match header field MUST
evaluate the condition prior to performing the method (Section 5). evaluate the condition prior to performing the method (Section 5).
If the field-value is "*", the condition is false if the origin If the field-value is "*", the condition is false if the origin
server has a current representation for the target resource. If the 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 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 of the listed tags match the entity-tag of the selected
representation. representation.
An origin server MUST NOT perform the requested method if the An origin server MUST NOT perform the requested method if the
condition evaluates to false; instead, the origin server MUST respond condition evaluates to false; instead, the origin server MUST respond
with either a) the 304 (Not Modified) status code if the request with either a) the 304 (Not Modified) status code if the request
method is GET or HEAD; or, b) the 412 (Precondition Failed) status method is GET or HEAD or b) the 412 (Precondition Failed) status code
code for all other request methods. for all other request methods.
Requirements on cache handling of a received If-None-Match header Requirements on cache handling of a received If-None-Match header
field are defined in Section 4.3.2 of [Part6]. field are defined in Section 4.3.2 of [RFC7234].
3.3. If-Modified-Since 3.3. If-Modified-Since
The "If-Modified-Since" header field makes a GET or HEAD request The "If-Modified-Since" header field makes a GET or HEAD request
method conditional on the selected representation's modification date method conditional on the selected representation's modification date
being more recent than the date provided in the field-value. being more recent than the date provided in the field-value.
Transfer of the selected representation's data is avoided if that Transfer of the selected representation's data is avoided if that
data has not changed. data has not changed.
If-Modified-Since = HTTP-date If-Modified-Since = HTTP-date
An example of the field is: An example of the field is:
If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
A recipient MUST ignore If-Modified-Since if the request contains an A recipient MUST ignore If-Modified-Since if the request contains an
If-None-Match header field; the condition in If-None-Match is If-None-Match header field; the condition in If-None-Match is
considered to be a more accurate replacement for the condition in If- considered to be a more accurate replacement for the condition in If-
Modified-Since and the two are only combined for the sake of Modified-Since, and the two are only combined for the sake of
interoperating with older intermediaries that might not implement If- interoperating with older intermediaries that might not implement If-
None-Match. None-Match.
A recipient MUST ignore the If-Modified-Since header field if the 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 received field-value is not a valid HTTP-date, or if the request
method is neither GET nor HEAD. method is neither GET nor HEAD.
A recipient MUST interpret an If-Modified-Since field-value's A recipient MUST interpret an If-Modified-Since field-value's
timestamp in terms of the origin server's clock. timestamp in terms of the origin server's clock.
If-Modified-Since is typically used for two distinct purposes: 1) to If-Modified-Since is typically used for two distinct purposes: 1) to
allow efficient updates of a cached representation that does not have 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 an entity-tag and 2) to limit the scope of a web traversal to
resources that have recently changed. resources that have recently changed.
When used for cache updates, a cache will typically use the value of 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 the cached message's Last-Modified field to generate the field value
of If-Modified-Since. This behavior is most interoperable for cases of If-Modified-Since. This behavior is most interoperable for cases
where clocks are poorly synchronized or when the server has chosen to where clocks are poorly synchronized or when the server has chosen to
only honor exact timestamp matches (due to a problem with Last- only honor exact timestamp matches (due to a problem with Last-
Modified dates that appear to go "back in time" when the origin Modified dates that appear to go "back in time" when the origin
server's clock is corrected or a representation is restored from an server's clock is corrected or a representation is restored from an
archived backup). However, caches occasionally generate the field archived backup). However, caches occasionally generate the field
skipping to change at page 16, line 42 skipping to change at page 16, line 42
An origin server that receives an If-Modified-Since header field An origin server that receives an If-Modified-Since header field
SHOULD evaluate the condition prior to performing the method SHOULD evaluate the condition prior to performing the method
(Section 5). The origin server SHOULD NOT perform the requested (Section 5). The origin server SHOULD NOT perform the requested
method if the selected representation's last modification date is method if the selected representation's last modification date is
earlier than or equal to the date provided in the field-value; earlier than or equal to the date provided in the field-value;
instead, the origin server SHOULD generate a 304 (Not Modified) instead, the origin server SHOULD generate a 304 (Not Modified)
response, including only those metadata that are useful for response, including only those metadata that are useful for
identifying or updating a previously cached response. identifying or updating a previously cached response.
Requirements on cache handling of a received If-Modified-Since header Requirements on cache handling of a received If-Modified-Since header
field are defined in Section 4.3.2 of [Part6]. field are defined in Section 4.3.2 of [RFC7234].
3.4. If-Unmodified-Since 3.4. If-Unmodified-Since
The "If-Unmodified-Since" header field makes the request method The "If-Unmodified-Since" header field makes the request method
conditional on the selected representation's last modification date conditional on the selected representation's last modification date
being earlier than or equal to the date provided in the field-value. 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 This field accomplishes the same purpose as If-Match for cases where
the user agent does not have an entity-tag for the representation. the user agent does not have an entity-tag for the representation.
If-Unmodified-Since = HTTP-date If-Unmodified-Since = HTTP-date
An example of the field is: An example of the field is:
If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
A recipient MUST ignore If-Unmodified-Since if the request contains A recipient MUST ignore If-Unmodified-Since if the request contains
an If-Match header field; the condition in If-Match is considered to an If-Match header field; the condition in If-Match is considered to
be a more accurate replacement for the condition in If-Unmodified- be a more accurate replacement for the condition in If-Unmodified-
Since and the two are only combined for the sake of interoperating Since, and the two are only combined for the sake of interoperating
with older intermediaries that might not implement If-Match. with older intermediaries that might not implement If-Match.
A recipient MUST ignore the If-Unmodified-Since header field if the A recipient MUST ignore the If-Unmodified-Since header field if the
received field-value is not a valid HTTP-date. received field-value is not a valid HTTP-date.
A recipient MUST interpret an If-Unmodified-Since field-value's A recipient MUST interpret an If-Unmodified-Since field-value's
timestamp in terms of the origin server's clock. timestamp in terms of the origin server's clock.
If-Unmodified-Since is most often used with state-changing methods If-Unmodified-Since is most often used with state-changing methods
(e.g., POST, PUT, DELETE) to prevent accidental overwrites when (e.g., POST, PUT, DELETE) to prevent accidental overwrites when
skipping to change at page 17, line 36 skipping to change at page 17, line 36
does not supply entity-tags with its representations (i.e., to does not supply entity-tags with its representations (i.e., to
prevent the "lost update" problem). It can also be used with safe prevent the "lost update" problem). It can also be used with safe
methods to abort a request if the selected representation does not methods to abort a request if the selected representation does not
match one already stored (or partially stored) from a prior request. match one already stored (or partially stored) from a prior request.
An origin server that receives an If-Unmodified-Since header field An origin server that receives an If-Unmodified-Since header field
MUST evaluate the condition prior to performing the method MUST evaluate the condition prior to performing the method
(Section 5). The origin server MUST NOT perform the requested method (Section 5). The origin server MUST NOT perform the requested method
if the selected representation's last modification date is more if the selected representation's last modification date is more
recent than the date provided in the field-value; instead the origin recent than the date provided in the field-value; instead the origin
server MUST respond with either: a) the 412 (Precondition Failed) server MUST respond with either a) the 412 (Precondition Failed)
status code; or, b) one of the 2xx (Successful) status codes if the 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 origin server has verified that a state change is being requested and
the final state is already reflected in the current state of the the final state is already reflected in the current state of the
target resource (i.e., the change requested by the user agent has target resource (i.e., the change requested by the user agent has
already succeeded, but the user agent might not be aware of that already succeeded, but the user agent might not be aware of that
because the prior response message was lost or a compatible change because the prior response message was lost or a compatible change
was made by some other user agent). In the latter case, the origin 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 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 it can verify that the request is a duplicate of an immediately prior
change made by the same user agent. change made by the same user agent.
The If-Unmodified-Since header field can be ignored by caches and The If-Unmodified-Since header field can be ignored by caches and
intermediaries because it is not applicable to a stored response. intermediaries because it is not applicable to a stored response.
3.5. If-Range 3.5. If-Range
The "If-Range" header field provides a special conditional request The "If-Range" header field provides a special conditional request
mechanism that is similar to the If-Match and If-Unmodified-Since mechanism that is similar to the If-Match and If-Unmodified-Since
header fields but instructs the recipient to ignore the Range header header fields but that instructs the recipient to ignore the Range
field if the validator doesn't match, resulting in transfer of the header field if the validator doesn't match, resulting in transfer of
new selected representation instead of a 412 response. If-Range is the new selected representation instead of a 412 (Precondition
defined in Section 3.2 of [Part5]. Failed) response. If-Range is defined in Section 3.2 of [RFC7233].
4. Status Code Definitions 4. Status Code Definitions
4.1. 304 Not Modified 4.1. 304 Not Modified
The 304 (Not Modified) status code indicates that a conditional GET The 304 (Not Modified) status code indicates that a conditional GET
or HEAD request has been received and would have resulted in a 200 or HEAD request has been received and would have resulted in a 200
(OK) response if it were not for the fact that the condition has (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 evaluated to false. In other words, there is no need for the server
to transfer a representation of the target resource because the to transfer a representation of the target resource because the
request indicates that the client, which made the request request indicates that the client, which made the request
conditional, already has a valid representation; the server is conditional, already has a valid representation; the server is
therefore redirecting the client to make use of that stored therefore redirecting the client to make use of that stored
representation as if it were the payload of a 200 (OK) response. representation as if it were the payload of a 200 (OK) response.
The server generating a 304 response MUST generate any of the The server generating a 304 response MUST generate any of the
following header fields that would have been sent in a 200 (OK) following header fields that would have been sent in a 200 (OK)
response to the same request: Cache-Control, Content-Location, Date, response to the same request: Cache-Control, Content-Location, Date,
ETag, Expires, and Vary. ETag, Expires, and Vary.
Since the goal of a 304 response is to minimize information transfer Since the goal of a 304 response is to minimize information transfer
when the recipient already has one or more cached representations, a when the recipient already has one or more cached representations, a
sender SHOULD NOT generate representation metadata other than the sender SHOULD NOT generate representation metadata other than the
above listed fields unless said metadata exists for the purpose of above listed fields unless said metadata exists for the purpose of
guiding cache updates (e.g., Last-Modified might be useful if the guiding cache updates (e.g., Last-Modified might be useful if the
response does not have an ETag field). response does not have an ETag field).
Requirements on a cache that receives a 304 response are defined in Requirements on a cache that receives a 304 response are defined in
Section 4.3.4 of [Part6]. If the conditional request originated with Section 4.3.4 of [RFC7234]. If the conditional request originated
an outbound client, such as a user agent with its own cache sending a with an outbound client, such as a user agent with its own cache
conditional GET to a shared proxy, then the proxy SHOULD forward the sending a conditional GET to a shared proxy, then the proxy SHOULD
304 response to that client. forward the 304 response to that client.
A 304 response cannot contain a message-body; it is always terminated A 304 response cannot contain a message-body; it is always terminated
by the first empty line after the header fields. by the first empty line after the header fields.
4.2. 412 Precondition Failed 4.2. 412 Precondition Failed
The 412 (Precondition Failed) status code indicates that one or more The 412 (Precondition Failed) status code indicates that one or more
conditions given in the request header fields evaluated to false when conditions given in the request header fields evaluated to false when
tested on the server. This response code allows the client to place tested on the server. This response code allows the client to place
preconditions on the current resource state (its current preconditions on the current resource state (its current
representations and metadata) and thus prevent the request method representations and metadata) and, thus, prevent the request method
from being applied if the target resource is in an unexpected state. from being applied if the target resource is in an unexpected state.
5. Evaluation 5. Evaluation
Except when excluded below, a recipient cache or origin server MUST Except when excluded below, a recipient cache or origin server MUST
evaluate received request preconditions after it has successfully evaluate received request preconditions after it has successfully
performed its normal request checks and just before it would perform performed its normal request checks and just before it would perform
the action associated with the request method. A server MUST ignore the action associated with the request method. A server MUST ignore
all received preconditions if its response to the same request all received preconditions if its response to the same request
without those conditions would have been a status code other than a without those conditions would have been a status code other than a
2xx or 412 (Precondition Failed). In other words, redirects and 2xx (Successful) or 412 (Precondition Failed). In other words,
failures take precedence over the evaluation of preconditions in redirects and failures take precedence over the evaluation of
conditional requests. preconditions in conditional requests.
A server that is not the origin server for the target resource and 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 cannot act as a cache for requests on the target resource MUST NOT
evaluate the conditional request header fields defined by this evaluate the conditional request header fields defined by this
specification, and MUST forward them if the request is forwarded, specification, and it MUST forward them if the request is forwarded,
since the generating client intends that they be evaluated by a since the generating client intends that they be evaluated by a
server that can provide a current representation. Likewise, a server server that can provide a current representation. Likewise, a server
MUST ignore the conditional request header fields defined by this MUST ignore the conditional request header fields defined by this
specification when received with a request method that does not specification when received with a request method that does not
involve the selection or modification of a selected representation, involve the selection or modification of a selected representation,
such as CONNECT, OPTIONS, or TRACE. such as CONNECT, OPTIONS, or TRACE.
Conditional request header fields that are defined by extensions to Conditional request header fields that are defined by extensions to
HTTP might place conditions on all recipients, on the state of the HTTP might place conditions on all recipients, on the state of the
target resource in general, or on a group of resources. For target resource in general, or on a group of resources. For
skipping to change at page 21, line 7 skipping to change at page 21, line 7
* if true, continue to step 5 * if true, continue to step 5
* if false, respond 304 (Not Modified) * if false, respond 304 (Not Modified)
5. When the method is GET and both Range and If-Range are present, 5. When the method is GET and both Range and If-Range are present,
evaluate the If-Range precondition: evaluate the If-Range precondition:
* if the validator matches and the Range specification is * if the validator matches and the Range specification is
applicable to the selected representation, respond 206 applicable to the selected representation, respond 206
(Partial Content) [Part5] (Partial Content) [RFC7233]
6. Otherwise, 6. Otherwise,
* all conditions are met, so perform the requested action and * all conditions are met, so perform the requested action and
respond according to its success or failure. respond according to its success or failure.
Any extension to HTTP/1.1 that defines additional conditional request Any extension to HTTP/1.1 that defines additional conditional request
header fields ought to define its own expectations regarding the header fields ought to define its own expectations regarding the
order for evaluating such fields in relation to those defined in this order for evaluating such fields in relation to those defined in this
document and other conditionals that might be found in practice. document and other conditionals that might be found in practice.
7. IANA Considerations 7. IANA Considerations
7.1. Status Code Registration 7.1. Status Code Registration
The HTTP Status Code Registry located at The "Hypertext Transfer Protocol (HTTP) Status Code Registry" located
<http://www.iana.org/assignments/http-status-codes> shall be updated at <http://www.iana.org/assignments/http-status-codes> has been
with the registrations below: updated with the registrations below:
+-------+---------------------+-------------+ +-------+---------------------+-------------+
| Value | Description | Reference | | Value | Description | Reference |
+-------+---------------------+-------------+ +-------+---------------------+-------------+
| 304 | Not Modified | Section 4.1 | | 304 | Not Modified | Section 4.1 |
| 412 | Precondition Failed | Section 4.2 | | 412 | Precondition Failed | Section 4.2 |
+-------+---------------------+-------------+ +-------+---------------------+-------------+
7.2. Header Field Registration 7.2. Header Field Registration
HTTP header fields are registered within the Message Header Field HTTP header fields are registered within the "Message Headers"
Registry maintained at <http://www.iana.org/assignments/ registry maintained at
message-headers/message-header-index.html>. <http://www.iana.org/assignments/message-headers/>.
This document defines the following HTTP header fields, so their This document defines the following HTTP header fields, so their
associated registry entries shall be updated according to the associated registry entries have been updated according to the
permanent registrations below (see [BCP90]): permanent registrations below (see [BCP90]):
+---------------------+----------+----------+-------------+ +---------------------+----------+----------+-------------+
| Header Field Name | Protocol | Status | Reference | | Header Field Name | Protocol | Status | Reference |
+---------------------+----------+----------+-------------+ +---------------------+----------+----------+-------------+
| ETag | http | standard | Section 2.3 | | 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 |
skipping to change at page 22, line 24 skipping to change at page 22, line 24
+---------------------+----------+----------+-------------+ +---------------------+----------+----------+-------------+
The change controller is: "IETF (iesg@ietf.org) - Internet The change controller is: "IETF (iesg@ietf.org) - Internet
Engineering Task Force". Engineering Task Force".
8. Security Considerations 8. Security Considerations
This section is meant to inform developers, information providers, This section is meant to inform developers, information providers,
and users of known security concerns specific to the HTTP conditional and users of known security concerns specific to the HTTP conditional
request mechanisms. More general security considerations are request mechanisms. More general security considerations are
addressed in HTTP messaging [Part1] and semantics [Part2]. addressed in HTTP "Message Syntax and Routing" [RFC7230] and
"Semantics and Content" [RFC7231].
The validators defined by this specification are not intended to The validators defined by this specification are not intended to
ensure the validity of a representation, guard against malicious ensure the validity of a representation, guard against malicious
changes, or detect man-in-the-middle attacks. At best, they enable changes, or detect man-in-the-middle attacks. At best, they enable
more efficient cache updates and optimistic concurrent writes when more efficient cache updates and optimistic concurrent writes when
all participants are behaving nicely. At worst, the conditions will all participants are behaving nicely. At worst, the conditions will
fail and the client will receive a response that is no more harmful fail and the client will receive a response that is no more harmful
than an HTTP exchange without conditional requests. than an HTTP exchange without conditional requests.
An entity-tag can be abused in ways that create privacy risks. For An entity-tag can be abused in ways that create privacy risks. For
skipping to change at page 22, line 48 skipping to change at page 22, line 49
entity-tag in later conditional requests as a means of re-identifying entity-tag in later conditional requests as a means of re-identifying
that user or user agent. Such an identifying tag would become a that user or user agent. Such an identifying tag would become a
persistent identifier for as long as the user agent retained the persistent identifier for as long as the user agent retained the
original cache entry. User agents that cache representations ought original cache entry. User agents that cache representations ought
to ensure that the cache is cleared or replaced whenever the user to ensure that the cache is cleared or replaced whenever the user
performs privacy-maintaining actions, such as clearing stored cookies performs privacy-maintaining actions, such as clearing stored cookies
or changing to a private browsing mode. or changing to a private browsing mode.
9. Acknowledgments 9. Acknowledgments
See Section 10 of [Part1]. See Section 10 of [RFC7230].
10. References 10. References
10.1. Normative References 10.1. Normative References
[Part1] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [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", Protocol (HTTP/1.1): Message Syntax and Routing",
draft-ietf-httpbis-p1-messaging-26 (work in progress), draft-ietf-httpbis-p1-messaging-latest (work in progress),
February 2014. June 2014.
[Part2] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", Protocol (HTTP/1.1): Semantics and Content",
draft-ietf-httpbis-p2-semantics-26 (work in progress), draft-ietf-httpbis-p2-semantics-latest (work in progress),
February 2014. June 2014.
[Part5] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., [RFC7233] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"Hypertext Transfer Protocol (HTTP/1.1): Range Requests", "Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
draft-ietf-httpbis-p5-range-26 (work in progress), draft-ietf-httpbis-p5-range-latest (work in progress),
February 2014. June 2014.
[Part6] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching", Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
draft-ietf-httpbis-p6-cache-26 (work in progress), draft-ietf-httpbis-p6-cache-latest (work in progress),
February 2014. June 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
10.2. Informative References 10.2. Informative References
[BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration [BCP90] 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.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
skipping to change at page 24, line 23 skipping to change at page 24, line 23
Appendix B. Imported ABNF Appendix B. Imported ABNF
The following core rules are included by reference, as defined in The following core rules are included by reference, as defined in
Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return), Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return),
CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double 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 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 8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII
character). character).
The rules below are defined in [Part1]: The rules below are defined in [RFC7230]:
OWS = <OWS, defined in [Part1], Section 3.2.3> OWS = <OWS, see [RFC7230], Section 3.2.3>
obs-text = <obs-text, defined in [Part1], Section 3.2.6> obs-text = <obs-text, see [RFC7230], Section 3.2.6>
The rules below are defined in other parts: The rules below are defined in other parts:
HTTP-date = <HTTP-date, defined in [Part2], Section 7.1.1.1> HTTP-date = <HTTP-date, see [RFC7231], Section 7.1.1.1>
Appendix C. Collected ABNF Appendix C. Collected ABNF
In the collected ABNF below, list rules are expanded as per Section In the collected ABNF below, list rules are expanded as per Section
1.2 of [Part1]. 1.2 of [RFC7230].
ETag = entity-tag ETag = entity-tag
HTTP-date = <HTTP-date, defined in [Part2], Section 7.1.1.1> HTTP-date = <HTTP-date, see [RFC7231], Section 7.1.1.1>
If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
entity-tag ] ) ) entity-tag ] ) )
If-Modified-Since = HTTP-date If-Modified-Since = HTTP-date
If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
entity-tag ] ) ) entity-tag ] ) )
If-Unmodified-Since = HTTP-date If-Unmodified-Since = HTTP-date
Last-Modified = HTTP-date Last-Modified = HTTP-date
OWS = <OWS, defined in [Part1], Section 3.2.3> OWS = <OWS, see [RFC7230], Section 3.2.3>
entity-tag = [ weak ] opaque-tag entity-tag = [ weak ] opaque-tag
etagc = "!" / %x23-7E ; '#'-'~' etagc = "!" / %x23-7E ; '#'-'~'
/ obs-text / obs-text
obs-text = <obs-text, defined in [Part1], Section 3.2.6> obs-text = <obs-text, see [RFC7230], Section 3.2.6>
opaque-tag = DQUOTE *etagc DQUOTE opaque-tag = DQUOTE *etagc DQUOTE
weak = %x57.2F ; W/ weak = %x57.2F ; W/
Appendix D. Change Log (to be removed by RFC Editor before publication)
Changes up to the IETF Last Call draft are summarized in <http://
tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-24#appendix-D>.
D.1. Since draft-ietf-httpbis-p4-conditional-24
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/518>: "APPSDIR
review of draft-ietf-httpbis-p4-conditional-24"
D.2. Since draft-ietf-httpbis-p4-conditional-25
Closed issues:
o <http://tools.ietf.org/wg/httpbis/trac/ticket/538>: "add
'stateless' to Abstract"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/542>: "improve
introduction of list rule"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/549>: "augment
security considerations with pointers to current research"
Index Index
3 3
304 Not Modified (status code) 18 304 Not Modified (status code) 18
4 4
412 Precondition Failed (status code) 18 412 Precondition Failed (status code) 18
E E
ETag header field 9 ETag header field 9
 End of changes. 64 change blocks. 
134 lines changed or deleted 107 lines changed or added

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