draft-ietf-httpbis-p4-conditional-19.txt		draft-ietf-httpbis-p4-conditional-20.txt
	HTTPbis Working Group R. Fielding, Ed.		HTTPbis Working Group R. Fielding, Ed.
	Internet-Draft Adobe		Internet-Draft Adobe
	Obsoletes: 2616 (if approved) Y. Lafon, Ed.		Obsoletes: 2616 (if approved) Y. Lafon, Ed.
	Intended status: Standards Track W3C		Intended status: Standards Track W3C
	Expires: September 13, 2012 J. Reschke, Ed.		Expires: January 17, 2013 J. Reschke, Ed.
	greenbytes		greenbytes
	March 12, 2012		July 16, 2012
	HTTP/1.1, part 4: Conditional Requests		HTTP/1.1, part 4: Conditional Requests
	draft-ietf-httpbis-p4-conditional-19		draft-ietf-httpbis-p4-conditional-20
	Abstract		Abstract
	The Hypertext Transfer Protocol (HTTP) is an application-level		The Hypertext Transfer Protocol (HTTP) is an application-level
	protocol for distributed, collaborative, hypertext information		protocol for distributed, collaborative, hypertext information
	systems. HTTP has been in use by the World Wide Web global		systems. This document defines HTTP/1.1 conditional requests,
	information initiative since 1990. This document is Part 4 of the		including metadata header fields for indicating state changes,
	seven-part specification that defines the protocol referred to as		request header fields for making preconditions on such state, and
	"HTTP/1.1" and, taken together, obsoletes RFC 2616.		rules for constructing the responses to a conditional request when
			one or more preconditions evaluate to false.
	Part 4 defines request header fields for indicating conditional
	requests and the rules for constructing responses to those requests.
	Editorial Note (To be removed by RFC Editor)		Editorial Note (To be removed by RFC Editor)
	Discussion of this draft should take place on the HTTPBIS working		Discussion of this draft takes place on the HTTPBIS working group
	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 C.20.		The changes in this draft are summarized in Appendix D.1.
	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 September 13, 2012.		This Internet-Draft will expire on January 17, 2013.
	Copyright Notice		Copyright Notice
	Copyright (c) 2012 IETF Trust and the persons identified as the		Copyright (c) 2012 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(http://trustee.ietf.org/license-info) in effect on the date of		(http://trustee.ietf.org/license-info) in effect on the date of
	publication of this document. Please review these documents		publication of this document. Please review these documents
	skipping to change at page 2, line 43		skipping to change at page 3, line 11
	not be created outside the IETF Standards Process, except to format		not be created outside the IETF Standards Process, except to format
	it for publication as an RFC or to translate it into languages other		it for publication as an RFC or to translate it into languages other
	than English.		than English.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
	1.1. Conformance and Error Handling . . . . . . . . . . . . . . 4		1.1. Conformance and Error Handling . . . . . . . . . . . . . . 4
	1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 5		1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 5
	2. Validators . . . . . . . . . . . . . . . . . . . . . . . . . . 5		2. Validators . . . . . . . . . . . . . . . . . . . . . . . . . . 5
	2.1. Weak versus Strong . . . . . . . . . . . . . . . . . . . . 5		2.1. Weak versus Strong . . . . . . . . . . . . . . . . . . . . 6
	2.2. Last-Modified . . . . . . . . . . . . . . . . . . . . . . 7		2.2. Last-Modified . . . . . . . . . . . . . . . . . . . . . . 7
	2.2.1. Generation . . . . . . . . . . . . . . . . . . . . . . 7		2.2.1. Generation . . . . . . . . . . . . . . . . . . . . . . 8
	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 . . . . . . . . . . . . . . . . . . . . . . 11		2.3.2. Comparison . . . . . . . . . . . . . . . . . . . . . . 11
	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. Rules for When to Use Entity-tags and Last-Modified		2.4. Rules for When to Use Entity-tags and Last-Modified
	Dates . . . . . . . . . . . . . . . . . . . . . . . . . . 12		Dates . . . . . . . . . . . . . . . . . . . . . . . . . . 12
	3. Precondition Header Fields . . . . . . . . . . . . . . . . . . 14		3. Precondition Header Fields . . . . . . . . . . . . . . . . . . 14
	3.1. If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 14		3.1. If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 14
	3.2. If-None-Match . . . . . . . . . . . . . . . . . . . . . . 15		3.2. If-None-Match . . . . . . . . . . . . . . . . . . . . . . 15
	3.3. If-Modified-Since . . . . . . . . . . . . . . . . . . . . 16		3.3. If-Modified-Since . . . . . . . . . . . . . . . . . . . . 16
	3.4. If-Unmodified-Since . . . . . . . . . . . . . . . . . . . 17		3.4. If-Unmodified-Since . . . . . . . . . . . . . . . . . . . 17
	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
	skipping to change at page 3, line 16		skipping to change at page 3, line 31
	Dates . . . . . . . . . . . . . . . . . . . . . . . . . . 12		Dates . . . . . . . . . . . . . . . . . . . . . . . . . . 12
	3. Precondition Header Fields . . . . . . . . . . . . . . . . . . 14		3. Precondition Header Fields . . . . . . . . . . . . . . . . . . 14
	3.1. If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 14		3.1. If-Match . . . . . . . . . . . . . . . . . . . . . . . . . 14
	3.2. If-None-Match . . . . . . . . . . . . . . . . . . . . . . 15		3.2. If-None-Match . . . . . . . . . . . . . . . . . . . . . . 15
	3.3. If-Modified-Since . . . . . . . . . . . . . . . . . . . . 16		3.3. If-Modified-Since . . . . . . . . . . . . . . . . . . . . 16
	3.4. If-Unmodified-Since . . . . . . . . . . . . . . . . . . . 17		3.4. If-Unmodified-Since . . . . . . . . . . . . . . . . . . . 17
	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 . . . . . . . . . . . . . . . . . 19		4.2. 412 Precondition Failed . . . . . . . . . . . . . . . . . 19
	5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19		5. Precedence . . . . . . . . . . . . . . . . . . . . . . . . . . 19
	5.1. Status Code Registration . . . . . . . . . . . . . . . . . 19		6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
	5.2. Header Field Registration . . . . . . . . . . . . . . . . 19		6.1. Status Code Registration . . . . . . . . . . . . . . . . . 20
	6. Security Considerations . . . . . . . . . . . . . . . . . . . 20		6.2. Header Field Registration . . . . . . . . . . . . . . . . 21
	7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20		7. Security Considerations . . . . . . . . . . . . . . . . . . . 21
	8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20		8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
	8.1. Normative References . . . . . . . . . . . . . . . . . . . 20		9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
	8.2. Informative References . . . . . . . . . . . . . . . . . . 21		9.1. Normative References . . . . . . . . . . . . . . . . . . . 22
	Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 21		9.2. Informative References . . . . . . . . . . . . . . . . . . 22
	Appendix B. Collected ABNF . . . . . . . . . . . . . . . . . . . 22		Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 22
	Appendix C. Change Log (to be removed by RFC Editor before		Appendix B. Imported ABNF . . . . . . . . . . . . . . . . . . . . 23
	publication) . . . . . . . . . . . . . . . . . . . . 22		Appendix C. Collected ABNF . . . . . . . . . . . . . . . . . . . 23
	C.1. Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . . 22		Appendix D. Change Log (to be removed by RFC Editor before
	C.2. Since draft-ietf-httpbis-p4-conditional-00 . . . . . . . . 22		publication) . . . . . . . . . . . . . . . . . . . . 24
	C.3. Since draft-ietf-httpbis-p4-conditional-01 . . . . . . . . 23		D.1. Since draft-ietf-httpbis-p4-conditional-19 . . . . . . . . 24
	C.4. Since draft-ietf-httpbis-p4-conditional-02 . . . . . . . . 23		Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
	C.5. Since draft-ietf-httpbis-p4-conditional-03 . . . . . . . . 23
	C.6. Since draft-ietf-httpbis-p4-conditional-04 . . . . . . . . 23
	C.7. Since draft-ietf-httpbis-p4-conditional-05 . . . . . . . . 24
	C.8. Since draft-ietf-httpbis-p4-conditional-06 . . . . . . . . 24
	C.9. Since draft-ietf-httpbis-p4-conditional-07 . . . . . . . . 24
	C.10. Since draft-ietf-httpbis-p4-conditional-08 . . . . . . . . 24
	C.11. Since draft-ietf-httpbis-p4-conditional-09 . . . . . . . . 24
	C.12. Since draft-ietf-httpbis-p4-conditional-10 . . . . . . . . 24
	C.13. Since draft-ietf-httpbis-p4-conditional-11 . . . . . . . . 25
	C.14. Since draft-ietf-httpbis-p4-conditional-12 . . . . . . . . 25
	C.15. Since draft-ietf-httpbis-p4-conditional-13 . . . . . . . . 25
	C.16. Since draft-ietf-httpbis-p4-conditional-14 . . . . . . . . 25
	C.17. Since draft-ietf-httpbis-p4-conditional-15 . . . . . . . . 25
	C.18. Since draft-ietf-httpbis-p4-conditional-16 . . . . . . . . 25
	C.19. Since draft-ietf-httpbis-p4-conditional-17 . . . . . . . . 26
	C.20. Since draft-ietf-httpbis-p4-conditional-18 . . . . . . . . 26
	Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
	1. Introduction		1. Introduction
	This document defines the HTTP/1.1 conditional request mechanisms,		Conditional requests are HTTP requests [Part2] that include one or
	including both metadata for indicating/observing changes in resource		more header fields indicating a precondition to be tested before
	representations and request header fields that specify preconditions		applying the method semantics to the target resource. Each
	on that metadata be checked before performing the request method.		precondition is based on metadata that is expected to change if the
			selected representation of the target resource is changed. This
			document defines the HTTP/1.1 conditional request mechanisms in terms
			of the architecture, syntax notation, and conformance criteria
			defined in [Part1].
	Conditional GET requests are the most efficient mechanism for HTTP		Conditional GET requests are the most efficient mechanism for HTTP
	cache updates [Part6]. Conditionals can also be applied to state-		cache updates [Part6]. 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
	skipping to change at page 4, line 42		skipping to change at page 4, line 47
	conditional request preconditions are evaluated by comparing the		conditional request preconditions are evaluated by comparing the
	values provided in the request header fields to the current metadata		values provided in the request header fields to the current metadata
	for the selected representation.		for the selected representation.
	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].
	This document defines conformance criteria for several roles in HTTP		This specification targets conformance criteria according to the role
	communication, including Senders, Recipients, Clients, Servers, User-		of a participant in HTTP communication. Hence, HTTP requirements are
	Agents, Origin Servers, Intermediaries, Proxies and Gateways. See		placed on senders, recipients, clients, servers, user agents,
	Section 2 of [Part1] for definitions of these terms.		intermediaries, origin servers, proxies, gateways, or caches,
			depending on what behavior is being constrained by the requirement.
			See Section 2 of [Part1] for definitions of these terms.
			The verb "generate" is used instead of "send" where a requirement
			differentiates between creating a protocol element and merely
			forwarding a received element downstream.
	An implementation is considered conformant if it complies with all of		An implementation is considered conformant if it complies with all of
	the requirements associated with its role(s). Note that SHOULD-level		the requirements associated with the roles it partakes in HTTP. Note
	requirements are relevant here, unless one of the documented		that SHOULD-level requirements are relevant here, unless one of the
	exceptions is applicable.		documented exceptions is applicable.
	This document also uses ABNF to define valid protocol elements		This document also uses ABNF to define valid protocol elements
	(Section 1.2). In addition to the prose requirements placed upon		(Section 1.2). In addition to the prose requirements placed upon
	them, Senders MUST NOT generate protocol elements that are invalid.		them, senders MUST NOT generate protocol elements that do not match
			the grammar defined by the ABNF rules for those protocol elements
			that are applicable to the sender's role. If a received protocol
			element is processed, the recipient MUST be able to parse any value
			that would match the ABNF rules for that protocol element, excluding
			only those rules not applicable to the recipient's role.
	Unless noted otherwise, Recipients MAY take steps to recover a usable		Unless noted otherwise, a recipient MAY attempt to recover a usable
	protocol element from an invalid construct. However, HTTP does not		protocol element from an invalid construct. HTTP does not define
	define specific error handling mechanisms, except in cases where it		specific error handling mechanisms except when they have a direct
	has direct impact on security. This is because different uses of the		impact on security, since different applications of the protocol
	protocol require different error handling strategies; for example, a		require different error handling strategies. For example, a Web
	Web browser may wish to transparently recover from a response where		browser might wish to transparently recover from a response where the
	the Location header field doesn't parse according to the ABNF,		Location header field doesn't parse according to the ABNF, whereas a
	whereby in a systems control protocol using HTTP, this type of error		systems control client might consider any form of error recovery to
	recovery could lead to dangerous consequences.		be dangerous.
	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 the list rule extension defined in Section		notation of [RFC5234] with the list rule extension defined in Section
	1.2 of [Part1]. Appendix B shows the collected ABNF with the list		1.2 of [Part1]. Appendix B describes rules imported from other
	rule expanded.		documents. Appendix C 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), and VCHAR (any visible US-ASCII
	character).
	The ABNF rules below are defined in [Part1] and [Part2]:
	OWS = <OWS, defined in [Part1], Section 3.2.1>
	obs-text = <obs-text, defined in [Part1], Section 3.2.4>
	HTTP-date = <HTTP-date, defined in [Part2], Section 8>
	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 and opaque entity tags. Additional metadata that		modification dates (Section 2.2) and opaque entity tags
	reflects resource state has been defined by various extensions of		(Section 2.3). Additional metadata that reflects resource state has
	HTTP, such as WebDAV [RFC4918], that are beyond the scope of this		been defined by various extensions of HTTP, such as WebDAV [RFC4918],
	specification. A resource metadata value is referred to as a		that are beyond the scope of this specification. A resource metadata
	"validator" when it is used within a precondition.		value is referred 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.
	A "strong validator" is a representation metadata value that MUST be		A "strong validator" is a representation metadata value that MUST be
	changed to a new, previously unused or guaranteed unique, value		changed to a new, previously unused or guaranteed unique, value
	whenever a change occurs to the representation data such that a		whenever a change occurs to the representation data such that a
	change would be observable in the payload body of a 200 response to		change would be observable in the payload body of a 200 (OK) response
	GET. A strong validator MAY be changed for other reasons, such as		to GET.
	when a semantically significant part of the representation metadata
	is changed (e.g., Content-Type), but it is in the best interests of		A strong validator MAY be changed for other reasons, such as when a
	the origin server to only change the value when it is necessary to		semantically significant part of the representation metadata is
			changed (e.g., Content-Type), but it is in the best interests of the
			origin server to only change the value when it is necessary to
	invalidate the stored responses held by remote caches and authoring		invalidate the stored responses held by remote caches and authoring
	tools. A strong validator MUST be unique across all representations		tools. A strong validator MUST be unique across all representations
	of a given resource, such that no two representations of that		of a given resource, such that no two representations of that
	resource share the same validator unless their payload body would be		resource share the same validator unless their payload body would be
	identical.		identical.
	Cache entries might persist for arbitrarily long periods, regardless		Cache entries might persist for arbitrarily long periods, regardless
	of expiration times. Thus, a cache might attempt to validate an		of expiration times. Thus, a cache might attempt to validate an
	entry using a validator that it obtained in the distant past. A		entry using a validator that it obtained in the distant past. A
	strong validator MUST be unique across all versions of all		strong validator MUST be unique across all versions of all
	representations associated with a particular resource over time.		representations associated with a particular resource over time.
	However, there is no implication of uniqueness across representations		However, there is no implication of uniqueness across representations
	of different resources (i.e., the same strong validator might be in		of different resources (i.e., the same strong validator might be in
	use for representations of multiple resources at the same time and		use for representations of multiple resources at the same time and
	does not imply that those representations are equivalent).		does not imply that those representations are equivalent).
	There are a variety of strong validators used in practice. The best		There are a variety of strong validators used in practice. The best
	are based on strict revision control, wherein each change to a		are based on strict revision control, wherein each change to a
	representation always results in a unique node name and revision		representation always results in a unique node name and revision
	identifier being assigned before the representation is made		identifier being assigned before the representation is made
	accessible to GET. A cryptographic hash function applied to the		accessible to GET. A collision-resistant hash function applied to
	representation data is also sufficient if the data is available prior		the representation data is also sufficient if the data is available
	to the response header fields being sent and the digest does not need		prior to the response header fields being sent and the digest does
	to be recalculated every time a validation request is received.		not need to be recalculated every time a validation request is
	However, if a resource has distinct representations that differ only		received. However, if a resource has distinct representations that
	in their metadata, such as might occur with content negotiation over		differ only in their metadata, such as might occur with content
	media types that happen to share the same data format, then a server		negotiation over media types that happen to share the same data
	SHOULD incorporate additional information in the validator to		format, then the origin server SHOULD incorporate additional
	distinguish those representations and avoid confusing cache behavior.		information in the validator to distinguish those representations and
			avoid confusing cache behavior.
	In contrast, a "weak validator" is a representation metadata value		In contrast, a "weak validator" is a representation metadata value
	that might not be changed for every change to the representation		that might not be changed for every change to the representation
	data. This weakness might be due to limitations in how the value is		data. This weakness might be due to limitations in how the value is
	calculated, such as clock resolution or an inability to ensure		calculated, such as clock resolution or an inability to ensure
	uniqueness for all possible representations of the resource, or due		uniqueness for all possible representations of the resource, or due
	to a desire by the resource owner to group representations by some		to a desire by the resource owner to group representations by some
	self-determined set of equivalency rather than unique sequences of		self-determined set of equivalency rather than unique sequences of
	data. A weak entity-tag SHOULD change whenever the origin server		data. An origin server SHOULD change a weak entity-tag whenever it
	considers prior representations to be unacceptable as a substitute		considers prior representations to be unacceptable as a substitute
	for the current representation. In other words, a weak entity-tag		for the current representation. In other words, a weak entity-tag
	SHOULD change whenever the origin server wants caches to invalidate		ought to change whenever the origin server wants caches to invalidate
	old responses.		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).
	Likewise, a representation's modification time, if defined with only		Likewise, a representation's modification time, if defined with only
	one-second resolution, might be a weak validator if it is possible		one-second resolution, might be a weak validator if it is possible
	skipping to change at page 8, line 16		skipping to change at page 8, line 25
	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
	responses.		responses.
	An origin server SHOULD obtain the Last-Modified value of the		An origin server SHOULD obtain the Last-Modified value of the
	representation as close as possible to the time that it generates the		representation as close as possible to the time that it generates the
	Date field-value for its response. This allows a recipient to make		Date field value for its response. This allows a recipient to make
	an accurate assessment of the representation's modification time,		an accurate assessment of the representation's modification time,
	especially if the representation changes near the time that the		especially if the representation changes near the time that the
	response is generated.		response is generated.
	An origin server with a clock MUST NOT send a Last-Modified date that		An origin server with a clock MUST NOT send a Last-Modified date that
	is later than the server's time of message origination (Date). If		is later than the server's time of message origination (Date). If
	the last modification time is derived from implementation-specific		the last modification time is derived from implementation-specific
	metadata that evaluates to some time in the future, according to the		metadata that evaluates to some time in the future, according to the
	origin server's clock, then the origin server MUST replace that value		origin server's clock, then the origin server MUST replace that value
	with the message origination date. This prevents a future		with the message origination date. This prevents a future
	skipping to change at page 9, line 34		skipping to change at page 9, line 41
	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 provides the current entity-tag for the		The "ETag" header field provides the current entity-tag for the
	selected representation. An entity-tag is an opaque validator for		selected representation. An entity-tag is an opaque validator for
	differentiating between multiple representations of the same		differentiating between multiple representations of the same
	resource, regardless of whether those multiple representations are		resource, regardless of whether those multiple representations are
	due to resource state changes over time, content negotiation		due to resource state changes over time, content negotiation
	resulting in multiple representations being valid at the same time,		resulting in multiple representations being valid at the same time,
	or both. An entity-tag consists of an opaque quoted string, possibly		or both. An entity-tag consists of an opaque quoted string, possibly
	prefixed by a weakness indicator.		prefixed by a weakness indicator.
	ETag = entity-tag		ETag = entity-tag
	skipping to change at page 10, line 42		skipping to change at page 10, line 50
	knows the implementation of a resource well enough to select the most		knows the implementation of a resource well enough to select the most
	accurate and efficient validation mechanism for that resource, and		accurate and efficient validation mechanism for that resource, and
	that any such mechanism can be mapped to a simple sequence of octets		that any such mechanism can be mapped to a simple sequence of octets
	for easy comparison. Since the value is opaque, there is no need for		for easy comparison. Since the value is opaque, there is no need for
	the client to be aware of how each entity-tag is constructed.		the client to be aware of how each entity-tag is constructed.
	For example, a resource that has implementation-specific versioning		For example, a resource that has implementation-specific versioning
	applied to all changes might use an internal revision number, perhaps		applied to all changes might use an internal revision number, perhaps
	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 stored hash of representation content, a		implementations might use a collision-resistant hash of
	combination of various filesystem attributes, or a modification		representation content, a combination of various filesystem
	timestamp that has sub-second resolution.		attributes, or a modification timestamp that has sub-second
			resolution.
	Origin servers SHOULD send ETag for any selected representation for		Origin servers SHOULD send ETag for any selected representation for
	which detection of changes can be reasonably and consistently		which detection of changes can be reasonably and consistently
	determined, since the entity-tag's use in conditional requests and		determined, since the entity-tag's use in conditional requests and
	evaluating cache freshness ([Part6]) can result in a substantial		evaluating cache freshness ([Part6]) can result in a substantial
	reduction of HTTP network traffic and can be a significant factor in		reduction of HTTP network traffic and can be a significant factor in
	improving service scalability and reliability.		improving service scalability and reliability.
	2.3.2. Comparison		2.3.2. Comparison
	skipping to change at page 11, line 33		skipping to change at page 11, line 41
	| 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 5		Consider a resource that is subject to content negotiation (Section 8
	of [Part3]), and where the representations returned upon a GET		of [Part2]), and where the representations returned upon a GET
	request vary based on the Accept-Encoding request header field		request vary based on the Accept-Encoding request header field
	(Section 6.3 of [Part3]):		(Section 9.3 of [Part2]):
	>> 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:
	skipping to change at page 12, line 36		skipping to change at page 12, line 39
	Date: Thu, 26 Mar 2010 00:05:00 GMT		Date: Thu, 26 Mar 2010 00:05:00 GMT
	ETag: "123-b"		ETag: "123-b"
	Content-Length: 43		Content-Length: 43
	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, so		Note: Content codings are a property of the representation, so
	therefore an entity-tag of an encoded representation must be		therefore an entity-tag of an encoded representation has to be
	distinct from an unencoded representation to prevent conflicts		distinct from an unencoded representation to prevent conflicts
	during cache updates and range requests. In contrast, transfer		during cache updates and range requests. In contrast, transfer
	codings (Section 4 of [Part1]) apply only during message transfer		codings (Section 4 of [Part1]) apply only during message transfer
	and do not require distinct entity-tags.		and do not require distinct entity-tags.
	2.4. Rules for When to Use Entity-tags and Last-Modified Dates		2.4. Rules for When to Use Entity-tags and Last-Modified Dates
	We adopt a set of rules and recommendations for origin servers,		We adopt a set of rules and recommendations for origin servers,
	clients, and caches regarding when various validator types ought to		clients, and caches regarding when various validator types ought to
	be used, and for what purposes.		be used, and for what purposes.
	skipping to change at page 14, line 8		skipping to change at page 14, line 11
	Note: The general principle behind these rules is that HTTP/1.1		Note: The general principle behind these rules is that HTTP/1.1
	servers and clients ought to transmit as much non-redundant		servers and clients ought to transmit as much non-redundant
	information as is available in their responses and requests.		information as is available in their responses and requests.
	HTTP/1.1 systems receiving this information will make the most		HTTP/1.1 systems receiving this information will make the most
	conservative assumptions about the validators they receive.		conservative assumptions about the validators they receive.
	HTTP/1.0 clients and caches might ignore entity-tags. Generally,		HTTP/1.0 clients and caches might ignore entity-tags. Generally,
	last-modified values received or used by these systems will		last-modified values received or used by these systems will
	support transparent and efficient caching, and so HTTP/1.1 origin		support transparent and efficient caching, and so HTTP/1.1 origin
	servers should provide Last-Modified values. In those rare cases		servers still ought to provide Last-Modified values.
	where the use of a Last-Modified value as a validator by an
	HTTP/1.0 system could result in a serious problem, then HTTP/1.1
	origin servers should not provide one.
	3. Precondition Header Fields		3. Precondition Header Fields
	This section defines the syntax and semantics of HTTP/1.1 header		This section defines the syntax and semantics of HTTP/1.1 header
	fields for applying preconditions on requests.		fields for applying preconditions on requests. Section 5 defines the
			order of evaluation when more than one precondition is present in a
			request.
	3.1. If-Match		3.1. If-Match
	The "If-Match" header field MAY be used to make a request method		The "If-Match" header field can be used to make a request method
	conditional on the current existence or value of an entity-tag for		conditional on the current existence or value of an entity-tag for
	one or more representations of the target resource. If-Match is		one or more representations of the target resource.
	generally useful for resource update requests, such as PUT requests,
	as a means for protecting against accidental overwrites when multiple		If-Match is generally useful for resource update requests, such as
	clients are acting in parallel on the same resource (i.e., the "lost		PUT requests, as a means for protecting against accidental overwrites
	update" problem). An If-Match field-value of "*" places the		when multiple clients are acting in parallel on the same resource
	precondition on the existence of any current representation for the		(i.e., the "lost update" problem). An If-Match field-value of "*"
	target resource.		places the precondition on the existence of any current
			representation for the target resource.
	If-Match = "*" / 1#entity-tag		If-Match = "*" / 1#entity-tag
	If any of the entity-tags listed in the If-Match field value match		The If-Match condition is met if and only if any of the entity-tags
	(as per Section 2.3.2) the entity-tag of the selected representation		listed in the If-Match field value match the entity-tag of the
	for the target resource, or if "*" is given and any current		selected representation for the target resource (as per
	representation exists for the target resource, then the server MAY		Section 2.3.2), or if "*" is given and any current representation
	perform the request method as if the If-Match header field was not		exists for the target resource.
	present.
	If none of the entity-tags match, or if "*" is given and no current		If the condition is met, the server MAY perform the request method as
	representation exists, the server MUST NOT perform the requested		if the If-Match header field was not present.
	method. Instead, the server MUST respond with the 412 (Precondition
			Origin servers MUST NOT perform the requested method if the condition
			is not met; instead they MUST respond with the 412 (Precondition
	Failed) status code.		Failed) status code.
			Proxy servers using a cached response as the selected representation
			MUST NOT perform the requested method if the condition is not met;
			instead, they MUST forward the request towards the origin server.
	If the request would, without the If-Match header field, result in		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		anything other than a 2xx (Successful) or 412 (Precondition Failed)
	header field MUST be ignored.		status code, then the If-Match header field MUST be ignored.
	Examples:		Examples:
	If-Match: "xyzzy"		If-Match: "xyzzy"
	If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"		If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
	If-Match: *		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 field is
	undefined by this specification.
	3.2. If-None-Match		3.2. If-None-Match
	The "If-None-Match" header field MAY be used to make a request method		The "If-None-Match" header field can be used to make a request method
	conditional on not matching any of the current entity-tag values for		conditional on not matching any of the current entity-tag values for
	representations of the target resource. If-None-Match is primarily		representations of the target resource.
	used in conditional GET requests to enable efficient updates of
	cached information with a minimum amount of transaction overhead. A
	client that has one or more representations previously obtained from
	the target resource can send If-None-Match with a list of the
	associated entity-tags in the hope of receiving a 304 response if at
	least one of those representations matches the selected
	representation.
	If-None-Match MAY also be used with a value of "*" to prevent an		If-None-Match is primarily used in conditional GET requests to enable
			efficient updates of cached information with a minimum amount of
			transaction overhead. A client that has one or more representations
			previously obtained from the target resource can send If-None-Match
			with a list of the associated entity-tags in the hope of receiving a
			304 (Not Modified) response if at least one of those representations
			matches the selected representation.
			If-None-Match can also be used with a value of "*" to prevent an
	unsafe request method (e.g., PUT) from inadvertently modifying an		unsafe request method (e.g., PUT) from inadvertently modifying an
	existing representation of the target resource when the client		existing representation of the target resource when the client
	believes that the resource does not have a current representation.		believes that the resource does not have a current representation.
	This is a variation on the "lost update" problem that might arise if		This is a variation on the "lost update" problem that might arise if
	more than one client attempts to create an initial representation for		more than one client attempts to create an initial representation for
	the target resource.		the target resource.
	If-None-Match = "*" / 1#entity-tag		If-None-Match = "*" / 1#entity-tag
	If any of the entity-tags listed in the If-None-Match field-value		The If-None-Match condition is met if and only if none of the entity-
	match (as per Section 2.3.2) the entity-tag of the selected		tags listed in the If-None-Match field value match the entity-tag of
	representation, or if "*" is given and any current representation		the selected representation for the target resource (as per
	exists for that resource, then the server MUST NOT perform the		Section 2.3.2), or if "*" is given and no current representation
			exists for that resource.
			If the condition is not met, the server MUST NOT perform the
	requested method. Instead, if the request method was GET or HEAD,		requested method. Instead, if the request method was GET or HEAD,
	the server SHOULD respond with a 304 (Not Modified) status code,		the server SHOULD respond with a 304 (Not Modified) status code,
	including the cache-related header fields (particularly ETag) of the		including the cache-related header fields (particularly ETag) of the
	selected representation that has a matching entity-tag. For all		selected representation that has a matching entity-tag. For all
	other request methods, the server MUST respond with a 412		other request methods, the server MUST respond with a 412
	(Precondition Failed) status code.		(Precondition Failed) status code.
	If none of the entity-tags match, then the server MAY perform the		If the condition is met, the server MAY perform the requested method
	requested method as if the If-None-Match header field did not exist,		as if the If-None-Match header field did not exist, but MUST also
	but MUST also ignore any If-Modified-Since header field(s) in the		ignore any If-Modified-Since header field(s) in the request. That
	request. That is, if no entity-tags match, then the server MUST NOT		is, if no entity-tags match, then the server MUST NOT return a 304
	return a 304 (Not Modified) response.		(Not Modified) response.
	If the request would, without the If-None-Match header field, result		If the request would, without the If-None-Match header field, result
	in anything other than a 2xx or 304 status code, then the If-None-		in anything other than a 2xx (Successful) or 304 (Not Modified)
	Match header field MUST be ignored. (See Section 2.4 for a		status code, then the If-None-Match header field MUST be ignored.
	discussion of server behavior when both If-Modified-Since and If-		(See Section 2.4 for a discussion of server behavior when both If-
	None-Match appear in the same request.)		Modified-Since and If-None-Match appear in the same request.)
	Examples:		Examples:
	If-None-Match: "xyzzy"		If-None-Match: "xyzzy"
	If-None-Match: W/"xyzzy"		If-None-Match: W/"xyzzy"
	If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"		If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
	If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"		If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
	If-None-Match: *		If-None-Match: *
	The result of a request having both an If-None-Match header field and
	either an If-Match or an If-Unmodified-Since header field is
	undefined by this specification.
	3.3. If-Modified-Since		3.3. If-Modified-Since
	The "If-Modified-Since" header field MAY be used to make a request		The "If-Modified-Since" header field can be used with GET or HEAD to
	method conditional by modification date: if the selected		make the method conditional by modification date: if the selected
	representation has not been modified since the time specified in this		representation has not been modified since the time specified in this
	field, then do not perform the request method; instead, respond as		field, then do not perform the request method; instead, respond as
	detailed below.		detailed below.
	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
	skipping to change at page 17, line 34		skipping to change at page 17, line 40
	Unsynchronized clocks and rounding problems, due to the different		Unsynchronized clocks and rounding problems, due to the different
	encodings of time between the client and server, are concerns.		encodings of time between the client and server, are concerns.
	This includes the possibility of race conditions if the document		This includes the possibility of race conditions if the document
	has changed between the time it was first requested and the If-		has changed between the time it was first requested and the If-
	Modified-Since date of a subsequent request, and the possibility		Modified-Since date of a subsequent request, and the possibility
	of clock-skew-related problems if the If-Modified-Since date is		of clock-skew-related problems if the If-Modified-Since date is
	derived from the client's clock without correction to the server's		derived from the client's clock without correction to the server's
	clock. Corrections for different time bases between client and		clock. Corrections for different time bases between client and
	server are at best approximate due to network latency.		server are at best approximate due to network latency.
	The result of a request having both an If-Modified-Since header field
	and either an If-Match or an If-Unmodified-Since header field is
	undefined by this specification.
	3.4. If-Unmodified-Since		3.4. If-Unmodified-Since
	The "If-Unmodified-Since" header field MAY be used to make a request		The "If-Unmodified-Since" header field can be used to make a request
	method conditional by modification date: if the selected		method conditional by modification date: if the selected
	representation has been modified since the time specified in this		representation has been modified since the time specified in this
	field, then the server MUST NOT perform the requested operation and		field, then the server MUST NOT perform the requested operation and
	MUST instead respond with the 412 (Precondition Failed) status code.		MUST instead respond with the 412 (Precondition Failed) status code.
	If the selected representation has not been modified since the time		If the selected representation has not been modified since the time
	specified in this field, the server SHOULD perform the request method		specified in this field, the server SHOULD perform the request method
	as if the If-Unmodified-Since header field were not present.		as if the If-Unmodified-Since header field were not present.
	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
	If the request normally (i.e., without the If-Unmodified-Since header		If a request normally (i.e., in absence of the If-Unmodified-Since
	field) would result in anything other than a 2xx or 412 status code,		header field) would result in anything other than a 2xx (Successful)
	the If-Unmodified-Since header field SHOULD be ignored.		or 412 (Precondition Failed) status code, the If-Unmodified-Since
			header field SHOULD be ignored.
	If the specified date is invalid, the header field MUST be ignored.		If the specified date is invalid, the header field MUST be 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
	field is undefined by this specification.
	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 If-Match and If-Unmodified-Since but		mechanism that is similar to If-Match and If-Unmodified-Since but
	specific to HTTP range requests. If-Range is defined in Section 5.3		specific to HTTP range requests. If-Range is defined in Section 5.3
	of [Part5].		of [Part5].
	4. Status Code Definitions		4. Status Code Definitions
	4.1. 304 Not Modified		4.1. 304 Not Modified
	The 304 status code indicates that a conditional GET request has been		The 304 status code indicates that a conditional GET request has been
	received and would have resulted in a 200 (OK) response if it were		received and would have resulted in a 200 (OK) response if it were
	not for the fact that the condition has evaluated to false. In other		not for the fact that the condition has evaluated to false. In other
	words, there is no need for the server to transfer a representation		words, there is no need for the server to transfer a representation
	of the target resource because the client's request indicates that it		of the target resource because the client's request indicates that it
	already has a valid representation, as indicated by the 304 response		already has a valid representation, as indicated by the 304 response
	header fields, and is therefore redirecting the client to make use of		header fields, and is therefore redirecting the client to make use of
	that stored representation as if it were the payload of a 200		that stored representation as if it were the payload of a 200
	response. The 304 response MUST NOT contain a message-body, and thus		response. The 304 response MUST NOT contain a message-body, and thus
	is always terminated by the first empty line after the header fields.		is always terminated by the first empty line after the header fields.
	A 304 response MUST include a Date header field (Section 10.2 of		A 304 response MUST include a Date header field (Section 9.10 of
	[Part2]) unless the origin server does not have a clock that can		[Part2]) unless the origin server does not have a clock that can
	provide a reasonable approximation of the current time. If a 200		provide a reasonable approximation of the current time. If a 200
	response to the same request would have included any of the header		(OK) response to the same request would have included any of the
	fields Cache-Control, Content-Location, ETag, Expires, or Vary, then		header fields Cache-Control, Content-Location, ETag, Expires, or
	those same header fields MUST be sent in a 304 response.		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		Since the goal of a 304 response is to minimize information transfer
	when the recipient already has one or more cached representations,		when the recipient already has one or more cached representations,
	the response SHOULD NOT include representation metadata other than		the response SHOULD NOT include representation metadata other than
	the above listed fields unless said metadata exists for the purpose		the above listed fields unless said metadata exists for the purpose
	of guiding cache updates (e.g., future HTTP extensions).		of guiding cache updates (e.g., future HTTP extensions).
	If the recipient of a 304 response does not have a cached		If the recipient of a 304 response does not have a cached
	representation corresponding to the entity-tag indicated by the 304		representation corresponding to the entity-tag indicated by the 304
	response, then the recipient MUST NOT use the 304 to update its own		response, then the recipient MUST NOT use the 304 to update its own
	cache. If this conditional request originated with an outbound		cache. If this conditional request originated with an outbound
	client, such as a user agent with its own cache sending a conditional		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		GET to a shared proxy, then the 304 response MAY be forwarded to that
	outbound client. Otherwise, the recipient MUST disregard the 304		client. Otherwise, the recipient MUST disregard the 304 response and
	response and repeat the request without any preconditions.		repeat the request without any preconditions.
	If a cache uses a received 304 response to update a cache entry, the		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		cache MUST update the entry to reflect any new field values given in
	the response.		the response.
	4.2. 412 Precondition Failed		4.2. 412 Precondition Failed
	The 412 status code indicates that one or more preconditions given in		The 412 status code indicates that one or more preconditions given in
	the request header fields evaluated to false when tested on the		the request header fields evaluated to false when tested on the
	server. This response code allows the client to place preconditions		server. This response code allows the client to place preconditions
	on the current resource state (its current representations and		on the current resource state (its current representations and
	metadata) and thus prevent the request method from being applied if		metadata) and thus prevent the request method from being applied if
	the target resource is in an unexpected state.		the target resource is in an unexpected state.
	5. IANA Considerations		5. Precedence
	5.1. Status Code Registration		When more than one conditional request header field is present in a
			request, the order in which the fields are evaluated becomes
			important. In practice, the fields defined in this document are
			consistently implemented in a single, logical order, due to the fact
			that entity tags are presumed to be more accurate than date
			validators. For example, the only reason to send both If-Modified-
			Since and If-None-Match in the same GET request is to support
			intermediary caches that might not have implemented If-None-Match, so
			it makes sense to ignore the If-Modified-Since when entity tags are
			understood and available for the selected representation.
			The general rule of conditional precedence is that exact match
			conditions are evaluated before cache-validating conditions and,
			within that order, last-modified conditions are only evaluated if the
			corresponding entity tag condition is not present (or not applicable
			because the selected representation does not have an entity tag).
			Specifically, the fields defined by this specification are evaluated
			as follows:
			1. When If-Match is present, evaluate it:
			* if true, continue to step 3
			* if false, respond 412 (Precondition Failed)
			2. When If-Match is not present and If-Unmodified-Since is present,
			evaluate it:
			* if true, continue to step 3
			* if false, respond 412 (Precondition Failed)
			3. When the method is GET and both Range and If-Range are present,
			evaluate it:
			* if the validator matches, respond 206 (Partial Content)
			* if the validator does not match, respond 200 (OK)
			4. When If-None-Match is present, evaluate it:
			* if true, all conditions are met
			* if false for GET/HEAD, respond 304 (Not Modified)
			* if false for other methods, respond 412 (Precondition Failed)
			5. When the method is GET or HEAD, If-None-Match is not present, and
			If-Modified-Since is present, evaluate it:
			* if true, all conditions are met
			* if false, respond 304 (Not Modified)
			Any extension to HTTP/1.1 that defines additional conditional request
			header fields ought to define its own expectations regarding the
			order for evaluating such fields in relation to those defined in this
			document and other conditionals that might be found in practice.
			6. IANA Considerations
			6.1. Status Code Registration
	The HTTP Status Code Registry located at		The HTTP Status Code Registry located at
	<http://www.iana.org/assignments/http-status-codes> shall be updated		<http://www.iana.org/assignments/http-status-codes> shall be updated
	with the registrations below:		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 |
	+-------+---------------------+-------------+		+-------+---------------------+-------------+
	5.2. Header Field Registration		6.2. Header Field Registration
	The Message Header Field Registry located at <http://www.iana.org/		The Message Header Field Registry located at <http://www.iana.org/
	assignments/message-headers/message-header-index.html> shall be		assignments/message-headers/message-header-index.html> shall be
	updated with the permanent registrations below (see [RFC3864]):		updated with the permanent registrations below (see [RFC3864]):
	+---------------------+----------+----------+-------------+		+---------------------+----------+----------+-------------+
	| Header Field Name | Protocol | Status | Reference |		| Header Field Name | Protocol | Status | Reference |
	+---------------------+----------+----------+-------------+		+---------------------+----------+----------+-------------+
	| ETag | http | standard | Section 2.3 |		| 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 |
	| Last-Modified | http | standard | Section 2.2 |		| Last-Modified | http | standard | Section 2.2 |
	+---------------------+----------+----------+-------------+		+---------------------+----------+----------+-------------+
	The change controller is: "IETF (iesg@ietf.org) - Internet		The change controller is: "IETF (iesg@ietf.org) - Internet
	Engineering Task Force".		Engineering Task Force".
	6. Security Considerations		7. Security Considerations
	No additional security considerations have been identified beyond		No additional security considerations have been identified beyond
	those applicable to HTTP in general [Part1].		those applicable to HTTP in general [Part1].
	7. Acknowledgments		The validators defined by this specification are not intended to
			ensure the validity of a representation, guard against malicious
			changes, or detect man-in-the-middle attacks. At best, they enable
			more efficient cache updates and optimistic concurrent writes when
			all participants are behaving nicely. At worst, the conditions will
			fail and the client will receive a response that is no more harmful
			than an HTTP exchange without conditional requests.
	See Section 9 of [Part1].		8. Acknowledgments
	8. References		See Section 9 of [Part1].
	8.1. Normative References		9. References
			9.1. Normative References
	[Part1] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,		[Part1] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
	"HTTP/1.1, part 1: URIs, Connections, and Message		"HTTP/1.1, part 1: Message Routing and Syntax"",
	Parsing", draft-ietf-httpbis-p1-messaging-19 (work in		draft-ietf-httpbis-p1-messaging-20 (work in progress),
	progress), March 2012.		July 2012.
	[Part2] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,		[Part2] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
	"HTTP/1.1, part 2: Message Semantics",		"HTTP/1.1, part 2: Semantics and Payloads",
	draft-ietf-httpbis-p2-semantics-19 (work in progress),		draft-ietf-httpbis-p2-semantics-20 (work in progress),
	March 2012.		July 2012.
	[Part3] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
	"HTTP/1.1, part 3: Message Payload and Content
	Negotiation", draft-ietf-httpbis-p3-payload-19 (work in
	progress), March 2012.
	[Part5] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,		[Part5] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
	"HTTP/1.1, part 5: Range Requests and Partial Responses",		"HTTP/1.1, part 5: Range Requests",
	draft-ietf-httpbis-p5-range-19 (work in progress),		draft-ietf-httpbis-p5-range-20 (work in progress),
	March 2012.		July 2012.
	[Part6] Fielding, R., Ed., Lafon, Y., Ed., Nottingham, M., Ed.,		[Part6] Fielding, R., Ed., Lafon, Y., Ed., Nottingham, M., Ed.,
	and J. Reschke, Ed., "HTTP/1.1, part 6: Caching",		and J. Reschke, Ed., "HTTP/1.1, part 6: Caching",
	draft-ietf-httpbis-p6-cache-19 (work in progress),		draft-ietf-httpbis-p6-cache-20 (work in progress),
	March 2012.		July 2012.
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119, March 1997.		Requirement Levels", BCP 14, RFC 2119, March 1997.
	[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax		[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
	Specifications: ABNF", STD 68, RFC 5234, January 2008.		Specifications: ABNF", STD 68, RFC 5234, January 2008.
	8.2. Informative References		9.2. Informative References
	[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.
	[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration		[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
	Procedures for Message Header Fields", BCP 90, RFC 3864,		Procedures for Message Header Fields", BCP 90, RFC 3864,
	September 2004.		September 2004.
	[RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed		[RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed
	skipping to change at page 21, line 34		skipping to change at page 23, line 4
	[RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed		[RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed
	Authoring and Versioning (WebDAV)", RFC 4918, June 2007.		Authoring and Versioning (WebDAV)", RFC 4918, June 2007.
	Appendix A. Changes from RFC 2616		Appendix A. Changes from RFC 2616
	Allow weak entity-tags in all requests except range requests		Allow weak entity-tags in all requests except range requests
	(Sections 2.1 and 3.2).		(Sections 2.1 and 3.2).
	Change ETag header field ABNF not to use quoted-string, thus avoiding		Change ETag header field ABNF not to use quoted-string, thus avoiding
	escaping issues. (Section 2.3)		escaping issues. (Section 2.3)
	Change ABNF productions for header fields to only define the field		Change ABNF productions for header fields to only define the field
	value. (Section 3)		value. (Section 3)
	Appendix B. Collected ABNF		Appendix B. Imported ABNF
			The following core rules are included by reference, as defined in
			Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return),
			CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
			quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any
			8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII
			character).
			The rules below are defined in [Part1]:
			OWS = <OWS, defined in [Part1], Section 3.2.1>
			obs-text = <obs-text, defined in [Part1], Section 3.2.4>
			The rules below are defined in other parts:
			HTTP-date = <HTTP-date, defined in [Part2], Section 5.1>
			Appendix C. Collected ABNF
	ETag = entity-tag		ETag = entity-tag
	HTTP-date = <HTTP-date, defined in [Part2], Section 8>		HTTP-date = <HTTP-date, defined in [Part2], Section 5.1>
	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
	skipping to change at page 22, line 31		skipping to change at page 24, line 5
	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 3.2.4>		obs-text = <obs-text, defined in [Part1], Section 3.2.4>
	opaque-tag = DQUOTE *etagc DQUOTE		opaque-tag = DQUOTE *etagc DQUOTE
	weak = %x57.2F ; W/		weak = %x57.2F ; W/
	ABNF diagnostics:		Appendix D. Change Log (to be removed by RFC Editor before publication)
	; 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
	Appendix 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:
	o <http://tools.ietf.org/wg/httpbis/trac/ticket/35>: "Normative and
	Informative references"
	Other changes:
	o Move definitions of 304 and 412 condition codes from Part2.
	C.3. Since draft-ietf-httpbis-p4-conditional-01
	Ongoing work on ABNF conversion
	(<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
	o Add explicit references to BNF syntax and rules imported from
	other parts of the specification.
	C.4. Since draft-ietf-httpbis-p4-conditional-02
	Closed issues:
	o <http://tools.ietf.org/wg/httpbis/trac/ticket/116>: "Weak ETags on
	non-GET requests"
	Ongoing work on IANA Message Header Field Registration
	(<http://tools.ietf.org/wg/httpbis/trac/ticket/40>):
	o Reference RFC 3984, and update header field registrations for
	header fields defined in this document.
	C.5. Since draft-ietf-httpbis-p4-conditional-03
	Closed issues:
	o <http://tools.ietf.org/wg/httpbis/trac/ticket/71>: "Examples for
	ETag matching"
	o <http://tools.ietf.org/wg/httpbis/trac/ticket/124>: "'entity
	value' undefined"
	o <http://tools.ietf.org/wg/httpbis/trac/ticket/126>: "bogus 2068
	Date header reference"
	C.6. Since draft-ietf-httpbis-p4-conditional-04
	Ongoing work on ABNF conversion
	(<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
	o Use "/" instead of "|" for alternatives.
	o Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
	whitespace ("OWS") and required whitespace ("RWS").
	o Rewrite ABNFs to spell out whitespace rules, factor out header
	field value format definitions.
	C.7. Since draft-ietf-httpbis-p4-conditional-05
	Final work on ABNF conversion
	(<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
	o Add appendix containing collected and expanded ABNF, reorganize
	ABNF introduction.
	C.8. Since draft-ietf-httpbis-p4-conditional-06
	Closed issues:
	o <http://tools.ietf.org/wg/httpbis/trac/ticket/153>: "case-
	sensitivity of etag weakness indicator"
	C.9. Since draft-ietf-httpbis-p4-conditional-07
	Closed issues:
	o <http://tools.ietf.org/wg/httpbis/trac/ticket/116>: "Weak ETags on
	non-GET requests" (If-Match still was defined to require strong
	matching)
	o <http://tools.ietf.org/wg/httpbis/trac/ticket/198>: "move IANA
	registrations for optional status codes"
	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:
	o <http://tools.ietf.org/wg/httpbis/trac/ticket/69>: "Clarify
	'Requested Variant'"
	o <http://tools.ietf.org/wg/httpbis/trac/ticket/109>: "Clarify
	entity / representation / variant terminology"
	o <http://tools.ietf.org/wg/httpbis/trac/ticket/220>: "consider
	removing the 'changes from 2068' sections"
	C.13. Since draft-ietf-httpbis-p4-conditional-11
	None.
	C.14. Since draft-ietf-httpbis-p4-conditional-12
	Closed issues:
	o <http://tools.ietf.org/wg/httpbis/trac/ticket/224>: "Header
	Classification"
	C.15. Since draft-ietf-httpbis-p4-conditional-13
	Closed issues:
	o <http://tools.ietf.org/wg/httpbis/trac/ticket/89>: "If-* and
	entities"
	o <http://tools.ietf.org/wg/httpbis/trac/ticket/101>: "Definition of
	validator weakness"
	o <http://tools.ietf.org/wg/httpbis/trac/ticket/276>: "untangle
	ABNFs for header fields"
	o <http://tools.ietf.org/wg/httpbis/trac/ticket/269>: "ETags and
	Quotes"
	C.16. Since draft-ietf-httpbis-p4-conditional-14
	None.
	C.17. Since draft-ietf-httpbis-p4-conditional-15
	Closed issues:
	o <http://tools.ietf.org/wg/httpbis/trac/ticket/304>: "If-Range		Changes up to the first Working Group Last Call draft are summarized
	should be listed when dicussing contexts where L-M can be		in <http://tools.ietf.org/html/
	considered strong"		draft-ietf-httpbis-p4-conditional-19#appendix-C>.
	C.18. Since draft-ietf-httpbis-p4-conditional-16		D.1. Since draft-ietf-httpbis-p4-conditional-19
	Closed issues:		Closed issues:
	o <http://tools.ietf.org/wg/httpbis/trac/ticket/186>: "Document		o <http://tools.ietf.org/wg/httpbis/trac/ticket/241>: "Need to
	HTTP's error-handling philosophy"		clarify eval order/interaction of conditional headers"
	C.19. Since draft-ietf-httpbis-p4-conditional-17		o <http://tools.ietf.org/wg/httpbis/trac/ticket/354>: "ETags and
			Conditional Requests"
	Closed issues:		o <http://tools.ietf.org/wg/httpbis/trac/ticket/361>: "ABNF
			requirements for recipients"
	o <http://tools.ietf.org/wg/httpbis/trac/ticket/306>: "does etag		o <http://tools.ietf.org/wg/httpbis/trac/ticket/363>: "Rare cases"
	value really use quoted-string"
	C.20. Since draft-ietf-httpbis-p4-conditional-18		o <http://tools.ietf.org/wg/httpbis/trac/ticket/365>: "Conditional
			Request Security Considerations"
	Closed issues:		o <http://tools.ietf.org/wg/httpbis/trac/ticket/371>: "If-Modified-
			Since lacks definition for method != GET"
	o <http://tools.ietf.org/wg/httpbis/trac/ticket/345>: "Required		o <http://tools.ietf.org/wg/httpbis/trac/ticket/372>: "refactor
	headers on 304 and 206"		conditional header field descriptions"
	Index		Index
	3		3
	304 Not Modified (status code) 18		304 Not Modified (status code) 18
	4		4
	412 Precondition Failed (status code) 19		412 Precondition Failed (status code) 19
	E		E
	ETag header field 9		ETag header field 9
	G		G
	Grammar		Grammar
	entity-tag 9		entity-tag 10
	ETag 9		ETag 10
	etagc 9		etagc 10
	If-Match 14		If-Match 14
	If-Modified-Since 16		If-Modified-Since 16
	If-None-Match 15		If-None-Match 15
	If-Unmodified-Since 17		If-Unmodified-Since 17
	Last-Modified 7		Last-Modified 7
	opaque-tag 9		opaque-tag 10
	weak 9		weak 10
	H		H
	Header Fields		Header Fields
	ETag 9		ETag 9
	If-Match 14		If-Match 14
	If-Modified-Since 16		If-Modified-Since 16
	If-None-Match 15		If-None-Match 15
	If-Unmodified-Since 17		If-Unmodified-Since 17
	Last-Modified 7		Last-Modified 7
	skipping to change at page 27, line 23		skipping to change at page 25, line 39
	metadata 5		metadata 5
	S		S
	selected representation 4		selected representation 4
	Status Codes		Status Codes
	304 Not Modified 18		304 Not Modified 18
	412 Precondition Failed 19		412 Precondition Failed 19
	V		V
	validator 5		validator 5
	strong 5		strong 6
	weak 5		weak 6
	Authors' Addresses		Authors' Addresses
	Roy T. Fielding (editor)		Roy T. Fielding (editor)
	Adobe Systems Incorporated		Adobe Systems Incorporated
	345 Park Ave		345 Park Ave
	San Jose, CA 95110		San Jose, CA 95110
	USA		USA
	EMail: fielding@gbiv.com		EMail: fielding@gbiv.com
	skipping to change at page 28, line 4		skipping to change at page 26, line 25
	Yves Lafon (editor)		Yves Lafon (editor)
	World Wide Web Consortium		World Wide Web Consortium
	W3C / ERCIM		W3C / ERCIM
	2004, rte des Lucioles		2004, rte des Lucioles
	Sophia-Antipolis, AM 06902		Sophia-Antipolis, AM 06902
	France		France
	EMail: ylafon@w3.org		EMail: ylafon@w3.org
	URI: http://www.raubacapeu.net/people/yves/		URI: http://www.raubacapeu.net/people/yves/
	Julian F. Reschke (editor)		Julian F. Reschke (editor)
	greenbytes GmbH		greenbytes GmbH
	Hafenweg 16		Hafenweg 16
	Muenster, NW 48155		Muenster, NW 48155
	Germany		Germany
	Phone: +49 251 2807760
	Fax: +49 251 2807761
	EMail: julian.reschke@greenbytes.de		EMail: julian.reschke@greenbytes.de
	URI: http://greenbytes.de/tech/webdav/		URI: http://greenbytes.de/tech/webdav/
End of changes. 84 change blocks.
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