HTTPbis Working GroupR. Fielding, Editor
Internet-DraftAdobe
Obsoletes: 2616 (if approved)J. Gettys
Intended status: Standards TrackAlcatel-Lucent
Expires: September 15, 2011J. Mogul
HP
H. Frystyk
Microsoft
L. Masinter
Adobe
P. Leach
Microsoft
T. Berners-Lee
W3C/MIT
Y. Lafon, Editor
W3C
J. Reschke, Editor
greenbytes
March 14, 2011

HTTP/1.1, part 4: Conditional Requests
draft-ietf-httpbis-p4-conditional-13

Abstract

The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document is Part 4 of the seven-part specification that defines the protocol referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616. Part 4 defines request header fields for indicating conditional requests and the rules for constructing responses to those requests.

Editorial Note (To be removed by RFC Editor)

Discussion of this draft should take place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org). The current issues list is at <http://tools.ietf.org/wg/httpbis/trac/report/3> and related documents (including fancy diffs) can be found at <http://tools.ietf.org/wg/httpbis/>.

The changes in this draft are summarized in Appendix C.14.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress”.

This Internet-Draft will expire on September 15, 2011.

Copyright Notice

Copyright © 2011 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.


Table of Contents

1. Introduction

This document defines HTTP/1.1 response metadata for indicating potential changes to payload content, including modification time stamps and opaque entity-tags, and the HTTP conditional request mechanisms that allow preconditions to be placed on a request method. Conditional GET requests allow for efficient cache updates. Other conditional request methods are used to protect against overwriting or misunderstanding the state of a resource that has been changed unbeknownst to the requesting client.

This document is currently disorganized in order to minimize the changes between drafts and enable reviewers to see the smaller errata changes. A future draft will reorganize the sections to better reflect the content. In particular, the sections on resource metadata will be discussed first and then followed by each conditional request header field, concluding with a definition of precedence and the expectation of ordering strong validator checks before weak validator checks. It is likely that more content from [Part6] will migrate to this part, where appropriate. The current mess reflects how widely dispersed these topics and associated requirements had become in [RFC2616].

1.1 Requirements

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

An implementation is not compliant if it fails to satisfy one or more of the "MUST" or "REQUIRED" level requirements for the protocols it implements. An implementation that satisfies all the "MUST" or "REQUIRED" level and all the "SHOULD" level requirements for its protocols is said to be "unconditionally compliant"; one that satisfies all the "MUST" level requirements but not all the "SHOULD" level requirements for its protocols is said to be "conditionally compliant".

1.2 Syntax Notation

This specification uses the ABNF syntax defined in Section 1.2 of [Part1] (which extends the syntax defined in [RFC5234] with a list rule). Appendix B shows the collected ABNF, with the list rule expanded.

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

1.2.1 Core Rules

The core rules below are defined in Section 1.2.2 of [Part1]:

  quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
  OWS           = <OWS, defined in [Part1], Section 1.2.2>

1.2.2 ABNF Rules defined in other Parts of the Specification

The ABNF rules below are defined in other parts:

  HTTP-date     = <HTTP-date, defined in [Part1], Section 6.1>

2. Entity-Tags

Entity-tags are used for comparing two or more representations of the same resource. HTTP/1.1 uses entity-tags in the ETag (Section 6.1), If-Match (Section 6.2), If-None-Match (Section 6.4), and If-Range (Section 5.3 of [Part5]) header fields. The definition of how they are used and compared as cache validators is in Section 4. An entity-tag consists of an opaque quoted string, possibly prefixed by a weakness indicator.

  entity-tag = [ weak ] opaque-tag
  weak       = %x57.2F ; "W/", case-sensitive
  opaque-tag = quoted-string

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 only if the representations are equivalent and could be substituted for each other with no significant change in semantics. A weak entity-tag can only be used for weak comparison.

An entity-tag MUST be unique across all versions of all representations associated with a particular resource. A given entity-tag value MAY be used for representations obtained by requests on different URIs. The use of the same entity-tag value in conjunction with representations obtained by requests on different URIs does not imply the equivalence of those representations.

2.1 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. Status Code Definitions

3.1 304 Not Modified

If the client has performed a conditional GET request and access is allowed, but the document has not been modified, the server SHOULD respond with this status code. The 304 response MUST NOT contain a message-body, and thus is always terminated by the first empty line after the header fields.

A 304 response MUST include a Date header field (Section 9.3 of [Part1]) unless its omission is required by Section 9.3.1 of [Part1]. If a 200 response to the same request would have included any of the header fields Cache-Control, Content-Location, ETag, Expires, Last-Modified, or Vary, then those same header fields MUST be sent in a 304 response.

Since the goal of a 304 response is to minimize information transfer when the recipient already has one or more cached representations, the response SHOULD NOT include representation metadata other than the above listed fields unless said metadata exists for the purpose of guiding cache updates (e.g., future HTTP extensions).

If a 304 response includes an entity-tag that indicates a representation not currently cached, then the recipient MUST NOT use the 304 to update its own cache. If that conditional request originated with an outbound client, such as a user agent with its own cache sending a conditional GET to a shared proxy, then the 304 response MAY be forwarded to the outbound client. Otherwise, disregard the response and repeat the request without the conditional.

If a cache uses a received 304 response to update a cache entry, the cache MUST update the entry to reflect any new field values given in the response.

3.2 412 Precondition Failed

The precondition given in one or more of the header fields evaluated to false when it was tested on the server. This response code allows the client to place preconditions on the current resource metadata (header field data) and thus prevent the requested method from being applied to a resource other than the one intended.

4. Weak and Strong Validators

Since both origin servers and caches will compare two validators to decide if they represent 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 this validator a "strong validator".

However, there might be cases when a server prefers to change the validator only on semantically significant changes, and not when insignificant aspects of the representation change. A validator that does not always change when the representation changes is a "weak validator".

An entity-tag is normally a strong validator, but the protocol provides a mechanism to tag an entity-tag as "weak". One can think of a strong validator as one that changes whenever the sequence of bits in a representation changes, while a weak value changes whenever the meaning of a representation changes. Alternatively, 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 semantically equivalent representations.

A "use" of a validator is either when a client generates a request and includes the validator in a validating header field, or when a server compares two validators.

Strong validators are usable in any context. Weak validators are only usable in contexts that do not depend on exact equality of a representation. For example, either kind is usable for a normal conditional GET. However, only a strong validator is usable for a sub-range retrieval, since otherwise the client might end up with an internally inconsistent representation.

Clients MUST NOT use weak validators in range requests ([Part5]).

The only function that HTTP/1.1 defines on validators is comparison. There are two validator comparison functions, depending on whether the comparison context allows the use of weak validators or not:

The example below shows the results for a set of entity-tag pairs, and both the weak and strong comparison function results:

ETag 1ETag 2Strong ComparisonWeak Comparison
W/"1"W/"1"no matchmatch
W/"1"W/"2"no matchno match
W/"1""1"no matchmatch
"1""1"matchmatch

An entity-tag is strong unless it is explicitly tagged as weak. Section 2 gives the syntax for entity-tags.

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

or

or

This method relies on the fact that if two different responses were sent by the origin server during the same second, but both had the same Last-Modified time, then at least one of those responses would have a Date value equal to its Last-Modified time. The arbitrary 60-second limit guards against the possibility that the Date and Last-Modified values are generated from different clocks, or at somewhat different times during the preparation of the response. An implementation MAY use a value larger than 60 seconds, if it is believed that 60 seconds is too short.

If a client wishes to perform a sub-range retrieval on a value for which it has only a Last-Modified time and no opaque validator, it MAY do this only if the Last-Modified time is strong in the sense described here.

A cache or origin server receiving a conditional range request ([Part5]) MUST use the strong comparison function to evaluate the condition.

These rules allow HTTP/1.1 caches and clients to safely perform sub-range retrievals on values that have been obtained from HTTP/1.0 servers.

5. Rules for When to Use Entity-tags and Last-Modified Dates

We adopt a set of rules and recommendations for origin servers, clients, and caches regarding when various validator types ought to be used, and for what purposes.

HTTP/1.1 origin servers:

In other words, the preferred behavior for an HTTP/1.1 origin server is to send both a strong entity-tag and a Last-Modified value.

In order to be legitimate, a strong entity-tag MUST change whenever the associated representation changes in any way. A weak entity-tag SHOULD change whenever the associated representation changes in a semantically significant way.

Note: In order to provide semantically transparent caching, an origin server must avoid reusing a specific strong entity-tag value for two different representations, or reusing a specific weak entity-tag value for two semantically different representations. Cache entries might persist for arbitrarily long periods, regardless of expiration times, so it might be inappropriate to expect that a cache will never again attempt to validate an entry using a validator that it obtained at some point in the past.

HTTP/1.1 clients:

An HTTP/1.1 origin server, upon receiving a conditional request that includes both a Last-Modified date (e.g., in an If-Modified-Since or If-Unmodified-Since header field) and one or more entity-tags (e.g., in an If-Match, If-None-Match, or If-Range header field) as cache validators, MUST NOT return a response status code of 304 (Not Modified) unless doing so is consistent with all of the conditional header fields in the request.

An HTTP/1.1 caching proxy, upon receiving a conditional request that includes both a Last-Modified date and one or more entity-tags as cache validators, MUST NOT return a locally cached response to the client unless that cached response is consistent with all of the conditional header fields in the request.

6. Header Field Definitions

This section defines the syntax and semantics of HTTP/1.1 header fields related to conditional requests.

6.1 ETag

The "ETag" header field provides the current value of the entity-tag (see Section 2) for one representation of the target resource. An entity-tag is intended for use as a resource-local identifier for differentiating between representations of the same resource that vary over time or via content negotiation (see Section 4).

  ETag   = "ETag" ":" OWS ETag-v
  ETag-v = entity-tag

Examples:

  ETag: "xyzzy"
  ETag: W/"xyzzy"
  ETag: ""

An entity-tag provides an "opaque" cache validator that allows for more reliable validation than modification dates in situations where it is inconvenient to store modification dates, where the one-second resolution of HTTP date values is not sufficient, or where the origin server wishes to avoid certain paradoxes that might arise from the use of modification dates.

The principle behind entity-tags is that only the service author knows the semantics of a resource well enough to select an appropriate cache validation mechanism, and the specification of any validator comparison function more complex than byte-equality would open up a can of worms. Thus, comparisons of any other header fields (except Last-Modified, for compatibility with HTTP/1.0) are never used for purposes of validating a cache entry.

6.2 If-Match

The "If-Match" header field is used to make a request method conditional. A client that has one or more representations previously obtained from the resource can verify that one of those representations is current by including a list of their associated entity-tags in the If-Match header field.

This allows efficient updates of cached information with a minimum amount of transaction overhead. It is also used when updating resources, to prevent inadvertent modification of the wrong version of a resource. As a special case, the value "*" matches any current representation of the resource.

  If-Match   = "If-Match" ":" OWS If-Match-v
  If-Match-v = "*" / 1#entity-tag

If any of the entity-tags match the entity-tag of the representation that would have been returned in the response to a similar GET request (without the If-Match header field) on that resource, or if "*" is given and any current representation exists for that resource, then the server MAY perform the requested method as if the If-Match header field did not exist.

If none of the entity-tags match, or if "*" is given and no current representation exists, the server MUST NOT perform the requested method, and MUST return a 412 (Precondition Failed) response. This behavior is most useful when the client wants to prevent an updating request method, such as PUT, from modifying a resource that has changed since the client last retrieved it.

If the request would, without the If-Match header field, result in anything other than a 2xx or 412 status code, then the If-Match header field MUST be ignored.

The meaning of "If-Match: *" is that the request method SHOULD be performed if the representation selected by the origin server (or by a cache, possibly using the Vary mechanism, see Section 3.5 of [Part6]) exists, and MUST NOT be performed if the representation does not exist.

A request intended to update a resource (e.g., a PUT) MAY include an If-Match header field to signal that the request method MUST NOT be applied if the representation corresponding to the If-Match value (a single entity-tag) is no longer a representation of that resource. This allows the user to indicate that they do not wish the request to be successful if the resource has been changed without their knowledge. Examples:

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

The result of a request having both an If-Match header field and either an If-None-Match or an If-Modified-Since header fields is undefined by this specification.

6.3 If-Modified-Since

The "If-Modified-Since" header field is used to make a request method conditional by date: if the representation that would have been transferred in a 200 response to a GET request has not been modified since the time specified in this field, then do not perform the method; instead, respond as detailed below.

  If-Modified-Since   = "If-Modified-Since" ":" OWS
                        If-Modified-Since-v
  If-Modified-Since-v = HTTP-date

An example of the field is:

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

A GET method with an If-Modified-Since header field and no Range header field requests that the representation be transferred only if it has been modified since the date given by the If-Modified-Since header field. The algorithm for determining this includes the following cases:

  1. If the request would normally result in anything other than a 200 (OK) status code, or if the passed If-Modified-Since date is invalid, the response is exactly the same as for a normal GET. A date which is later than the server's current time is invalid.
  2. If the representation has been modified since the If-Modified-Since date, the response is exactly the same as for a normal GET.
  3. If the representation has not been modified since a valid If-Modified-Since date, the server SHOULD return a 304 (Not Modified) response.

The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead.

The result of a request having both an If-Modified-Since header field and either an If-Match or an If-Unmodified-Since header fields is undefined by this specification.

6.4 If-None-Match

The "If-None-Match" header field is used to make a request method conditional. A client that has one or more representations previously obtained from the resource can verify that none of those representations is current by including a list of their associated entity-tags in the If-None-Match header field.

This allows efficient updates of cached information with a minimum amount of transaction overhead. It is also used to prevent a request method (e.g., PUT) from inadvertently modifying an existing resource when the client believes that the resource does not exist.

As a special case, the value "*" matches any current representation of the resource.

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

If any of the entity-tags match the entity-tag of the representation that would have been returned in the response to a similar GET request (without the If-None-Match header field) on that resource, or if "*" is given and any current representation exists for that resource, then the server MUST NOT perform the requested method, unless required to do so because the resource's modification date fails to match that supplied in an If-Modified-Since header field in the request. Instead, if the request method was GET or HEAD, the server SHOULD respond with a 304 (Not Modified) response, including the cache-related header fields (particularly ETag) of one of the representations that matched. For all other request methods, the server MUST respond with a 412 (Precondition Failed) status code.

If none of the entity-tags match, then the server MAY perform the requested method as if the If-None-Match header field did not exist, but MUST also ignore any If-Modified-Since header field(s) in the request. That is, if no entity-tags match, then the server MUST NOT return a 304 (Not Modified) response.

If the request would, without the If-None-Match header field, result in anything other than a 2xx or 304 status code, then the If-None-Match header field MUST be ignored. (See Section 5 for a discussion of server behavior when both If-Modified-Since and If-None-Match appear in the same request.)

The meaning of "If-None-Match: *" is that the request method MUST NOT be performed if the representation selected by the origin server (or by a cache, possibly using the Vary mechanism, see Section 3.5 of [Part6]) exists, and SHOULD be performed if the representation does not exist. This feature is intended to be useful in preventing races between PUT operations.

Examples:

  If-None-Match: "xyzzy"
  If-None-Match: W/"xyzzy"
  If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
  If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
  If-None-Match: *

The result of a request having both an If-None-Match header field and either an If-Match or an If-Unmodified-Since header fields is undefined by this specification.

6.5 If-Unmodified-Since

The "If-Unmodified-Since" header field is used to make a request method conditional. If the representation that would have been transferred in a 200 response to a GET request on the same resource has not been modified since the time specified in this field, the server SHOULD perform the requested operation as if the If-Unmodified-Since header field were not present.

If the representation has been modified since the specified time, the server MUST NOT perform the requested operation, and MUST return a 412 (Precondition Failed).

  If-Unmodified-Since   = "If-Unmodified-Since" ":" OWS
                          If-Unmodified-Since-v
  If-Unmodified-Since-v = HTTP-date

An example of the field is:

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

If the request normally (i.e., without the If-Unmodified-Since header field) would result in anything other than a 2xx or 412 status code, the If-Unmodified-Since header field SHOULD be ignored.

If the specified date is invalid, the header field is ignored.

The result of a request having both an If-Unmodified-Since header field and either an If-None-Match or an If-Modified-Since header fields is undefined by this specification.

6.6 Last-Modified

The "Last-Modified" header field indicates the date and time at which the origin server believes the representation was last modified.

  Last-Modified   = "Last-Modified" ":" OWS Last-Modified-v
  Last-Modified-v = HTTP-date

An example of its use is

  Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT

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

An origin server MUST NOT send a Last-Modified date which is later than the server's time of message origination. In such cases, where the resource's last modification would indicate some time in the future, the server MUST replace that date with the message origination date.

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

HTTP/1.1 servers SHOULD send Last-Modified whenever feasible.

The Last-Modified header field value is often used as a cache validator. In simple terms, a cache entry is considered to be valid if the representation has not been modified since the Last-Modified value.

7. IANA Considerations

7.1 Status Code Registration

The HTTP Status Code Registry located at <http://www.iana.org/assignments/http-status-codes> shall be updated with the registrations below:

ValueDescriptionReference
304Not ModifiedSection 3.1
412Precondition FailedSection 3.2

7.2 Header Field Registration

The Message Header Field Registry located at <http://www.iana.org/assignments/message-headers/message-header-index.html> shall be updated with the permanent registrations below (see [RFC3864]):

Header Field NameProtocolStatusReference
ETaghttpstandardSection 6.1
If-MatchhttpstandardSection 6.2
If-Modified-SincehttpstandardSection 6.3
If-None-MatchhttpstandardSection 6.4
If-Unmodified-SincehttpstandardSection 6.5
Last-ModifiedhttpstandardSection 6.6

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

8. Security Considerations

No additional security considerations have been identified beyond those applicable to HTTP in general [Part1].

9. Acknowledgments

10. References

10.1 Normative References

[Part1]Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 1: URIs, Connections, and Message Parsing”, Internet-Draft draft-ietf-httpbis-p1-messaging-13 (work in progress), March 2011.
[Part3]Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 3: Message Payload and Content Negotiation”, Internet-Draft draft-ietf-httpbis-p3-payload-13 (work in progress), March 2011.
[Part5]Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 5: Range Requests and Partial Responses”, Internet-Draft draft-ietf-httpbis-p5-range-13 (work in progress), March 2011.
[Part6]Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., Nottingham, M., Ed., and J. Reschke, Ed., “HTTP/1.1, part 6: Caching”, Internet-Draft draft-ietf-httpbis-p6-cache-13 (work in progress), March 2011.
[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

[RFC2616]Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1”, RFC 2616, June 1999.
[RFC3864]Klyne, G., Nottingham, M., and J. Mogul, “Registration Procedures for Message Header Fields”, BCP 90, RFC 3864, September 2004.

Authors' Addresses

Roy T. Fielding (editor)
Adobe Systems Incorporated
345 Park Ave
San Jose, CA 95110
USA
Email: fielding@gbiv.com
URI: http://roy.gbiv.com/

Jim Gettys
Alcatel-Lucent Bell Labs
21 Oak Knoll Road
Carlisle, MA 01741
USA
Email: jg@freedesktop.org
URI: http://gettys.wordpress.com/

Jeffrey C. Mogul
Hewlett-Packard Company
HP Labs, Large Scale Systems Group
1501 Page Mill Road, MS 1177
Palo Alto, CA 94304
USA
Email: JeffMogul@acm.org

Henrik Frystyk Nielsen
Microsoft Corporation
1 Microsoft Way
Redmond, WA 98052
USA
Email: henrikn@microsoft.com

Larry Masinter
Adobe Systems Incorporated
345 Park Ave
San Jose, CA 95110
USA
Email: LMM@acm.org
URI: http://larry.masinter.net/

Paul J. Leach
Microsoft Corporation
1 Microsoft Way
Redmond, WA 98052
Email: paulle@microsoft.com

Tim Berners-Lee
World Wide Web Consortium
MIT Computer Science and Artificial Intelligence Laboratory
The Stata Center, Building 32
32 Vassar Street
Cambridge, MA 02139
USA
Email: timbl@w3.org
URI: http://www.w3.org/People/Berners-Lee/

Yves Lafon (editor)
World Wide Web Consortium
W3C / ERCIM
2004, rte des Lucioles
Sophia-Antipolis, AM 06902
France
Email: ylafon@w3.org
URI: http://www.raubacapeu.net/people/yves/

Julian F. Reschke (editor)
greenbytes GmbH
Hafenweg 16
Muenster, NW 48155
Germany
Phone: +49 251 2807760
Fax: +49 251 2807761
Email: julian.reschke@greenbytes.de
URI: http://greenbytes.de/tech/webdav/

A. Changes from RFC 2616

Allow weak entity-tags in all requests except range requests (Sections 4 and 6.4).

B. Collected ABNF

ETag = "ETag:" OWS ETag-v
ETag-v = entity-tag

HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>

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

Last-Modified = "Last-Modified:" OWS Last-Modified-v
Last-Modified-v = HTTP-date

OWS = <OWS, defined in [Part1], Section 1.2.2>

entity-tag = [ weak ] opaque-tag

opaque-tag = quoted-string

quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>

weak = %x57.2F ; W/

ABNF diagnostics:

; ETag defined but not used
; If-Match defined but not used
; If-Modified-Since defined but not used
; If-None-Match defined but not used
; If-Unmodified-Since defined but not used
; Last-Modified defined but not used

C. Change Log (to be removed by RFC Editor before publication)

C.1 Since RFC 2616

Extracted relevant partitions from [RFC2616].

C.2 Since draft-ietf-httpbis-p4-conditional-00

Closed issues:

Other changes:

C.3 Since draft-ietf-httpbis-p4-conditional-01

Ongoing work on ABNF conversion (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):

C.4 Since draft-ietf-httpbis-p4-conditional-02

Closed issues:

Ongoing work on IANA Message Header Field Registration (<http://tools.ietf.org/wg/httpbis/trac/ticket/40>):

C.5 Since draft-ietf-httpbis-p4-conditional-03

Closed issues:

C.6 Since draft-ietf-httpbis-p4-conditional-04

Ongoing work on ABNF conversion (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):

C.7 Since draft-ietf-httpbis-p4-conditional-05

Final work on ABNF conversion (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):

C.8 Since draft-ietf-httpbis-p4-conditional-06

Closed issues:

C.9 Since draft-ietf-httpbis-p4-conditional-07

Closed issues:

C.10 Since draft-ietf-httpbis-p4-conditional-08

No significant changes.

C.11 Since draft-ietf-httpbis-p4-conditional-09

No significant changes.

C.12 Since draft-ietf-httpbis-p4-conditional-10

Closed issues:

C.13 Since draft-ietf-httpbis-p4-conditional-11

None.

C.14 Since draft-ietf-httpbis-p4-conditional-12

Closed issues:

Index

3 4 E G H I L P R S