draft-ietf-httpbis-p6-cache-05.txt		draft-ietf-httpbis-p6-cache-06.txt
	Network Working Group R. Fielding, Ed.		HTTPbis Working Group R. Fielding, Ed.
	Internet-Draft Day Software		Internet-Draft Day Software
	Obsoletes: 2616 (if approved) J. Gettys		Obsoletes: 2616 (if approved) J. Gettys
	Intended status: Standards Track One Laptop per Child		Intended status: Standards Track One Laptop per Child
	Expires: May 20, 2009 J. Mogul		Expires: September 10, 2009 J. Mogul
	HP		HP
	H. Frystyk		H. Frystyk
	Microsoft		Microsoft
	L. Masinter		L. Masinter
	Adobe Systems		Adobe Systems
	P. Leach		P. Leach
	Microsoft		Microsoft
	T. Berners-Lee		T. Berners-Lee
	W3C/MIT		W3C/MIT
	Y. Lafon, Ed.		Y. Lafon, Ed.
	W3C		W3C
	J. Reschke, Ed.		J. Reschke, Ed.
	greenbytes		greenbytes
	November 16, 2008		March 9, 2009
	HTTP/1.1, part 6: Caching		HTTP/1.1, part 6: Caching
	draft-ietf-httpbis-p6-cache-05		draft-ietf-httpbis-p6-cache-06
	Status of this Memo		Status of this Memo
	By submitting this Internet-Draft, each author represents that any		This Internet-Draft is submitted to IETF in full conformance with the
	applicable patent or other IPR claims of which he or she is aware		provisions of BCP 78 and BCP 79. This document may contain material
	have been or will be disclosed, and any of which he or she becomes		from IETF Documents or IETF Contributions published or made publicly
	aware will be disclosed, in accordance with Section 6 of BCP 79.		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.
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF), its areas, and its working groups. Note that		Task Force (IETF), its areas, and its working groups. Note that
	other groups may also distribute working documents as Internet-		other groups may also distribute working documents as Internet-
	Drafts.		Drafts.
	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."
	skipping to change at page 1, line 42		skipping to change at page 2, line 4
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF), its areas, and its working groups. Note that		Task Force (IETF), its areas, and its working groups. Note that
	other groups may also distribute working documents as Internet-		other groups may also distribute working documents as Internet-
	Drafts.		Drafts.
	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."
	The list of current Internet-Drafts can be accessed at		The list of current Internet-Drafts can be accessed at
	http://www.ietf.org/ietf/1id-abstracts.txt.		http://www.ietf.org/ietf/1id-abstracts.txt.
	The list of Internet-Draft Shadow Directories can be accessed at		The list of Internet-Draft Shadow Directories can be accessed at
	http://www.ietf.org/shadow.html.		http://www.ietf.org/shadow.html.
	This Internet-Draft will expire on May 20, 2009.		This Internet-Draft will expire on September 10, 2009.
			Copyright Notice
			Copyright (c) 2009 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 in effect on the date of
			publication of this document (http://trustee.ietf.org/license-info).
			Please review these documents carefully, as they describe your rights
			and restrictions with respect to this document.
	Abstract		Abstract
	The Hypertext Transfer Protocol (HTTP) is an application-level		The Hypertext Transfer Protocol (HTTP) is an application-level
	protocol for distributed, collaborative, hypermedia information		protocol for distributed, collaborative, hypermedia information
	systems. HTTP has been in use by the World Wide Web global		systems. This document is Part 6 of the seven-part specification
	information initiative since 1990. This document is Part 6 of the		that defines the protocol referred to as "HTTP/1.1" and, taken
	seven-part specification that defines the protocol referred to as		together, obsoletes RFC 2616. Part 6 defines requirements on HTTP
	"HTTP/1.1" and, taken together, obsoletes RFC 2616. Part 6 defines		caches and the associated header fields that control cache behavior
	requirements on HTTP caches and the associated header fields that		or indicate cacheable response messages.
	control cache behavior or indicate cacheable response messages.
	Editorial Note (To be removed by RFC Editor)		Editorial Note (To be removed by RFC Editor)
	Discussion of this draft should take place on the HTTPBIS working		Discussion of this draft should take place on the HTTPBIS working
	group mailing list (ietf-http-wg@w3.org). The current issues list is		group mailing list (ietf-http-wg@w3.org). The current issues list is
	at <http://tools.ietf.org/wg/httpbis/trac/report/11> and related		at <http://tools.ietf.org/wg/httpbis/trac/report/11> 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 B.6.		The changes in this draft are summarized in Appendix C.7.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
	1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 5		1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 5
	1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6		1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
	1.3. Requirements . . . . . . . . . . . . . . . . . . . . . . . 7		1.3. Requirements . . . . . . . . . . . . . . . . . . . . . . . 6
	2. Notational Conventions and Generic Grammar . . . . . . . . . . 8		1.4. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 7
	3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 8		1.4.1. Core Rules . . . . . . . . . . . . . . . . . . . . . . 7
	3.1. Cache Correctness . . . . . . . . . . . . . . . . . . . . 8		1.4.2. ABNF Rules defined in other Parts of the
	3.2. Warnings . . . . . . . . . . . . . . . . . . . . . . . . . 9		Specification . . . . . . . . . . . . . . . . . . . . 7
	3.3. Cache-control Mechanisms . . . . . . . . . . . . . . . . . 10		2. Cache Operation . . . . . . . . . . . . . . . . . . . . . . . 7
	3.4. Explicit User Agent Warnings . . . . . . . . . . . . . . . 10		2.1. Response Cacheability . . . . . . . . . . . . . . . . . . 7
	3.5. Exceptions to the Rules and Warnings . . . . . . . . . . . 11		2.1.1. Storing Partial and Incomplete Responses . . . . . . . 8
	3.6. Client-controlled Behavior . . . . . . . . . . . . . . . . 11		2.2. Constructing Responses from Caches . . . . . . . . . . . . 8
	4. Expiration Model . . . . . . . . . . . . . . . . . . . . . . . 12		2.3. Freshness Model . . . . . . . . . . . . . . . . . . . . . 9
	4.1. Server-Specified Expiration . . . . . . . . . . . . . . . 12		2.3.1. Calculating Freshness Lifetime . . . . . . . . . . . . 10
	4.2. Heuristic Expiration . . . . . . . . . . . . . . . . . . . 13		2.3.2. Calculating Age . . . . . . . . . . . . . . . . . . . 11
	4.3. Age Calculations . . . . . . . . . . . . . . . . . . . . . 13		2.3.3. Serving Stale Responses . . . . . . . . . . . . . . . 13
	4.4. Expiration Calculations . . . . . . . . . . . . . . . . . 15		2.4. Validation Model . . . . . . . . . . . . . . . . . . . . . 13
	4.5. Disambiguating Expiration Values . . . . . . . . . . . . . 16		2.5. Request Methods that Invalidate . . . . . . . . . . . . . 14
	4.6. Disambiguating Multiple Responses . . . . . . . . . . . . 17		2.6. Caching Negotiated Responses . . . . . . . . . . . . . . . 15
	5. Validation Model . . . . . . . . . . . . . . . . . . . . . . . 17		2.7. Combining Responses . . . . . . . . . . . . . . . . . . . 16
	6. Response Cacheability . . . . . . . . . . . . . . . . . . . . 18		3. Header Field Definitions . . . . . . . . . . . . . . . . . . . 16
	7. Constructing Responses From Caches . . . . . . . . . . . . . . 19		3.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
	7.1. End-to-end and Hop-by-hop Headers . . . . . . . . . . . . 19		3.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 17
	7.2. Non-modifiable Headers . . . . . . . . . . . . . . . . . . 20		3.2.1. Request Cache-Control Directives . . . . . . . . . . . 18
	7.3. Combining Headers . . . . . . . . . . . . . . . . . . . . 21		3.2.2. Response Cache-Control Directives . . . . . . . . . . 20
	8. Caching Negotiated Responses . . . . . . . . . . . . . . . . . 22		3.2.3. Cache Control Extensions . . . . . . . . . . . . . . . 22
	9. Shared and Non-Shared Caches . . . . . . . . . . . . . . . . . 23		3.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 23
	10. Errors or Incomplete Response Cache Behavior . . . . . . . . . 24		3.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 23
	11. Side Effects of GET and HEAD . . . . . . . . . . . . . . . . . 24		3.5. Vary . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
	12. Invalidation After Updates or Deletions . . . . . . . . . . . 24		3.6. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 25
	13. Write-Through Mandatory . . . . . . . . . . . . . . . . . . . 25		4. History Lists . . . . . . . . . . . . . . . . . . . . . . . . 28
	14. Cache Replacement . . . . . . . . . . . . . . . . . . . . . . 26		5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
	15. History Lists . . . . . . . . . . . . . . . . . . . . . . . . 26		5.1. Message Header Registration . . . . . . . . . . . . . . . 28
	16. Header Field Definitions . . . . . . . . . . . . . . . . . . . 27		6. Security Considerations . . . . . . . . . . . . . . . . . . . 29
	16.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 27		7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 29
	16.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 27		8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
	16.2.1. What is Cacheable . . . . . . . . . . . . . . . . . . 29		8.1. Normative References . . . . . . . . . . . . . . . . . . . 29
	16.2.2. What May be Stored by Caches . . . . . . . . . . . . 30		8.2. Informative References . . . . . . . . . . . . . . . . . . 30
	16.2.3. Modifications of the Basic Expiration Mechanism . . . 31		Appendix A. Compatibility with Previous Versions . . . . . . . . 31
	16.2.4. Cache Revalidation and Reload Controls . . . . . . . 33		A.1. Changes from RFC 2068 . . . . . . . . . . . . . . . . . . 31
	16.2.5. No-Transform Directive . . . . . . . . . . . . . . . 35		A.2. Changes from RFC 2616 . . . . . . . . . . . . . . . . . . 31
	16.2.6. Cache Control Extensions . . . . . . . . . . . . . . 36		Appendix B. Collected ABNF . . . . . . . . . . . . . . . . . . . 31
	16.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 37		Appendix C. Change Log (to be removed by RFC Editor before
	16.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 38		publication) . . . . . . . . . . . . . . . . . . . . 33
	16.5. Vary . . . . . . . . . . . . . . . . . . . . . . . . . . . 38		C.1. Since RFC2616 . . . . . . . . . . . . . . . . . . . . . . 33
	16.6. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 39		C.2. Since draft-ietf-httpbis-p6-cache-00 . . . . . . . . . . . 33
	17. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 42		C.3. Since draft-ietf-httpbis-p6-cache-01 . . . . . . . . . . . 34
	17.1. Message Header Registration . . . . . . . . . . . . . . . 42		C.4. Since draft-ietf-httpbis-p6-cache-02 . . . . . . . . . . . 34
	18. Security Considerations . . . . . . . . . . . . . . . . . . . 42		C.5. Since draft-ietf-httpbis-p6-cache-03 . . . . . . . . . . . 34
	19. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 43		C.6. Since draft-ietf-httpbis-p6-cache-04 . . . . . . . . . . . 35
	20. References . . . . . . . . . . . . . . . . . . . . . . . . . . 43		C.7. Since draft-ietf-httpbis-p6-cache-05 . . . . . . . . . . . 35
	20.1. Normative References . . . . . . . . . . . . . . . . . . . 43		Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
	20.2. Informative References . . . . . . . . . . . . . . . . . . 44		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 38
	Appendix A. Compatibility with Previous Versions . . . . . . . . 44
	A.1. Changes from RFC 2068 . . . . . . . . . . . . . . . . . . 44
	A.2. Changes from RFC 2616 . . . . . . . . . . . . . . . . . . 45
	Appendix B. Change Log (to be removed by RFC Editor before
	publication) . . . . . . . . . . . . . . . . . . . . 45
	B.1. Since RFC2616 . . . . . . . . . . . . . . . . . . . . . . 45
	B.2. Since draft-ietf-httpbis-p6-cache-00 . . . . . . . . . . . 45
	B.3. Since draft-ietf-httpbis-p6-cache-01 . . . . . . . . . . . 46
	B.4. Since draft-ietf-httpbis-p6-cache-02 . . . . . . . . . . . 46
	B.5. Since draft-ietf-httpbis-p6-cache-03 . . . . . . . . . . . 46
	B.6. Since draft-ietf-httpbis-p6-cache-04 . . . . . . . . . . . 47
	Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 49
	Intellectual Property and Copyright Statements . . . . . . . . . . 52
	1. Introduction		1. Introduction
	HTTP is typically used for distributed information systems, where		HTTP is typically used for distributed information systems, where
	performance can be improved by the use of response caches, and		performance can be improved by the use of response caches. This
	includes a number of elements intended to make caching work as well
	as possible. Because these elements interact with each other, it is
	useful to describe the caching design of HTTP separately. This
	document defines aspects of HTTP/1.1 related to caching and reusing		document defines aspects of HTTP/1.1 related to caching and reusing
	response messages.		response messages.
	1.1. Purpose		1.1. Purpose
	An HTTP cache is a local store of response messages and the subsystem		An HTTP cache is a local store of response messages and the subsystem
	that controls its message storage, retrieval, and deletion. A cache		that controls its message storage, retrieval, and deletion. A cache
	stores cacheable responses in order to reduce the response time and		stores cacheable responses in order to reduce the response time and
	network bandwidth consumption on future, equivalent requests. Any		network bandwidth consumption on future, equivalent requests. Any
	client or server may include a cache, though a cache cannot be used		client or server may include a cache, though a cache cannot be used
	by a server that is acting as a tunnel.		by a server that is acting as a tunnel.
	Caching would be useless if it did not significantly improve		Caching would be useless if it did not significantly improve
	performance. The goal of caching in HTTP/1.1 is to reuse a prior		performance. The goal of caching in HTTP/1.1 is to reuse a prior
	response message to satisfy a current request. In some cases, the		response message to satisfy a current request. In some cases, a
	existing response can be reused without the need for a network		stored response can be reused without the need for a network request,
	request, reducing latency and network round-trips; we use an		reducing latency and network round-trips; a "freshness" mechanism is
	"expiration" mechanism for this purpose (see Section 4). Even when a		used for this purpose (see Section 2.3). Even when a new request is
	new request is required, it is often possible to reuse all or parts		required, it is often possible to reuse all or parts of the payload
	of the payload of a prior response to satisfy the request, thereby		of a prior response to satisfy the request, thereby reducing network
	reducing network bandwidth usage; we use a "validation" mechanism for		bandwidth usage; a "validation" mechanism is used for this purpose
	this purpose (see Section 5).		(see Section 2.4).
	A cache behaves in a "semantically transparent" manner, with respect
	to a particular response, when its use affects neither the requesting
	client nor the origin server, except to improve performance. When a
	cache is semantically transparent, the client receives exactly the
	same response status and payload that it would have received had its
	request been handled directly by the origin server.
	In an ideal world, all interactions with an HTTP cache would be
	semantically transparent. However, for some resources, semantic
	transparency is not always necessary and can be effectively traded
	for the sake of bandwidth scaling, disconnected operation, and high
	availability. HTTP/1.1 allows origin servers, caches, and clients to
	explicitly reduce transparency when necessary. However, because non-
	transparent operation may confuse non-expert users and might be
	incompatible with certain server applications (such as those for
	ordering merchandise), the protocol requires that transparency be
	relaxed
	o only by an explicit protocol-level request when relaxed by client
	or origin server
	o only with an explicit warning to the end user when relaxed by
	cache or client
	Therefore, HTTP/1.1 provides these important elements:
	1. Protocol features that provide full semantic transparency when
	this is required by all parties.
	2. Protocol features that allow an origin server or user agent to
	explicitly request and control non-transparent operation.
	3. Protocol features that allow a cache to attach warnings to
	responses that do not preserve the requested approximation of
	semantic transparency.
	A basic principle is that it must be possible for the clients to
	detect any potential relaxation of semantic transparency.
	Note: The server, cache, or client implementor might be faced with
	design decisions not explicitly discussed in this specification.
	If a decision might affect semantic transparency, the implementor
	ought to err on the side of maintaining transparency unless a
	careful and complete analysis shows significant benefits in
	breaking transparency.
	1.2. Terminology		1.2. Terminology
	This specification uses a number of terms to refer to the roles		This specification uses a number of terms to refer to the roles
	played by participants in, and objects of, HTTP caching.		played by participants in, and objects of, HTTP caching.
	cacheable		cacheable
	A response is cacheable if a cache is allowed to store a copy of		A response is cacheable if a cache is allowed to store a copy of
	the response message for use in answering subsequent requests.		the response message for use in answering subsequent requests.
	Even when a response is cacheable, there may be additional		Even when a response is cacheable, there may be additional
	constraints on whether a cache can use the cached copy for a		constraints on whether a cache can use the cached copy to satisfy
	particular request.		a particular request.
	first-hand
	A response is first-hand if it comes directly and without
	unnecessary delay from the origin server, perhaps via one or more
	proxies. A response is also first-hand if its validity has just
	been checked directly with the origin server.
	explicit expiration time		explicit expiration time
	The time at which the origin server intends that an entity should		The time at which the origin server intends that an entity should
	no longer be returned by a cache without further validation.		no longer be returned by a cache without further validation.
	heuristic expiration time		heuristic expiration time
	An expiration time assigned by a cache when no explicit expiration		An expiration time assigned by a cache when no explicit expiration
	time is available.		time is available.
	age		age
	The age of a response is the time since it was sent by, or		The age of a response is the time since it was sent by, or
	successfully validated with, the origin server.		successfully validated with, the origin server.
			first-hand
			A response is first-hand if the freshness model is not in use;
			i.e., its age is 0.
	freshness lifetime		freshness lifetime
	The length of time between the generation of a response and its		The length of time between the generation of a response and its
	expiration time.		expiration time.
	fresh		fresh
	A response is fresh if its age has not yet exceeded its freshness		A response is fresh if its age has not yet exceeded its freshness
	lifetime.		lifetime.
	stale		stale
	A response is stale if its age has passed its freshness lifetime.		A response is stale if its age has passed its freshness lifetime
			(either explicit or heuristic).
	validator		validator
	A protocol element (e.g., an entity tag or a Last-Modified time)		A protocol element (e.g., an entity tag or a Last-Modified time)
	that is used to find out whether a cache entry is an equivalent		that is used to find out whether a stored response is an
	copy of an entity.		equivalent copy of an entity.
			shared cache
			A cache that is accessible to more than one user. A non-shared
			cache is dedicated to a single user.
	1.3. Requirements		1.3. Requirements
	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].
	An implementation is not compliant if it fails to satisfy one or more		An implementation is not compliant if it fails to satisfy one or more
	of the MUST or REQUIRED level requirements for the protocols it		of the MUST or REQUIRED level requirements for the protocols it
	implements. An implementation that satisfies all the MUST or		implements. An implementation that satisfies all the MUST or
	REQUIRED level and all the SHOULD level requirements for its		REQUIRED level and all the SHOULD level requirements for its
	protocols is said to be "unconditionally compliant"; one that		protocols is said to be "unconditionally compliant"; one that
	satisfies all the MUST level requirements but not all the SHOULD		satisfies all the MUST level requirements but not all the SHOULD
	level requirements for its protocols is said to be "conditionally		level requirements for its protocols is said to be "conditionally
	compliant."		compliant."
	2. Notational Conventions and Generic Grammar		1.4. Syntax Notation
	This specification uses the ABNF syntax defined in Section 2.1 of		This specification uses the ABNF syntax defined in Section 1.2 of
	[Part1] and the core rules defined in Section 2.2 of [Part1]:		[Part1] (which extends the syntax defined in [RFC5234] with a list
			rule). Appendix B shows the collected ABNF, with the list rule
			expanded.
	DIGIT = <DIGIT, defined in [Part1], Section 2.2>		The following core rules are included by reference, as defined in
	DQUOTE = <DQUOTE, defined in [Part1], Section 2.2>		[RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF
	SP = <SP, defined in [Part1], Section 2.2>		(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).
	quoted-string = <quoted-string, defined in [Part1], Section 2.2>		1.4.1. Core Rules
	token = <token, defined in [Part1], Section 2.2>
	OWS = <OWS, defined in [Part1], Section 2.2>		The core rules below are defined in Section 1.2.2 of [Part1]:
			quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
			token = <token, defined in [Part1], Section 1.2.2>
			OWS = <OWS, defined in [Part1], Section 1.2.2>
			1.4.2. ABNF Rules defined in other Parts of the Specification
	The ABNF rules below are defined in other parts:		The ABNF rules below are defined in other parts:
	field-name = <field-name, defined in [Part1], Section 4.2>		field-name = <field-name, defined in [Part1], Section 4.2>
	HTTP-date = <HTTP-date, defined in [Part1], Section 3.3.1>		HTTP-date = <HTTP-date, defined in [Part1], Section 3.2.1>
	port = <port, defined in [Part1], Section 3.2>		port = <port, defined in [Part1], Section 2.1>
	pseudonym = <pseudonym, defined in [Part1], Section 8.9>		pseudonym = <pseudonym, defined in [Part1], Section 8.9>
	uri-host = <uri-host, defined in [Part1], Section 3.2>		uri-host = <uri-host, defined in [Part1], Section 2.1>
	3. Overview		2. Cache Operation
	3.1. Cache Correctness		2.1. Response Cacheability
	A correct cache MUST respond to a request with the most up-to-date		A cache MUST NOT store a response to any request, unless:
	response held by the cache that is appropriate to the request (see
	Sections 4.5, 4.6, and 14) which meets one of the following
	conditions:
	1. It has been checked for equivalence with what the origin server		o The request method is defined as being cacheable, and
	would have returned by revalidating the response with the origin
	server (Section 5);
	2. It is "fresh enough" (see Section 4). In the default case, this		o the "no-store" cache directive (see Section 3.2) does not appear
	means it meets the least restrictive freshness requirement of the		in request or response headers, and
	client, origin server, and cache (see Section 16.2); if the
	origin server so specifies, it is the freshness requirement of
	the origin server alone. If a stored response is not "fresh
	enough" by the most restrictive freshness requirement of both the
	client and the origin server, in carefully considered
	circumstances the cache MAY still return the response with the
	appropriate Warning header (see Sections 3.5 and 16.6), unless
	such a response is prohibited (e.g., by a "no-store" cache-
	directive, or by a "no-cache" cache-request-directive; see
	Section 16.2).
	3. It is an appropriate 304 (Not Modified), 305 (Use Proxy), or		o the "private" cache response directive (see Section 3.2 does not
	error (4xx or 5xx) response message.		appear in the response, if the cache is shared, and
	If the cache can not communicate with the origin server, then a		o the "Authorization" header (see Section 3.1 of [Part7]) does not
	correct cache SHOULD respond as above if the response can be		appear in the request, if the cache is shared (unless the "public"
	correctly served from the cache; if not it MUST return an error or		directive is present; see Section 3.2), and
	warning indicating that there was a communication failure.
	If a cache receives a response (either an entire response, or a 304		o the cache understands partial responses, if the response is
	(Not Modified) response) that it would normally forward to the		partial or incomplete (see Section 2.1.1).
	requesting client, and the received response is no longer fresh, the
	cache SHOULD forward it to the requesting client without adding a new
	Warning (but without removing any existing Warning headers). A cache
	SHOULD NOT attempt to revalidate a response simply because that
	response became stale in transit; this might lead to an infinite
	loop. A user agent that receives a stale response without a Warning
	MAY display a warning indication to the user.
	3.2. Warnings		Note that in normal operation, most caches will not store a response
			that has neither a cache validator nor an explicit expiration time,
			as such responses are not usually useful to store. However, caches
			are not prohibited from storing such responses.
	Whenever a cache returns a response that is neither first-hand nor		2.1.1. Storing Partial and Incomplete Responses
	"fresh enough" (in the sense of condition 2 in Section 3.1), it MUST
	attach a warning to that effect, using a Warning general-header. The
	Warning header and the currently defined warnings are described in
	Section 16.6. The warning allows clients to take appropriate action.
	Warnings MAY be used for other purposes, both cache-related and		A cache that receives an incomplete response (for example, with fewer
	otherwise. The use of a warning, rather than an error status code,		bytes of data than specified in a Content-Length header) can store
	distinguish these responses from true failures.		the response, but MUST treat it as a partial response [Part5].
			Partial responses can be combined as described in Section 4 of
			[Part5]; the result might be a full response or might still be
			partial. A cache MUST NOT return a partial response to a client
			without explicitly marking it as such using the 206 (Partial Content)
			status code.
	Warnings are assigned three digit warn-codes. The first digit		A cache that does not support the Range and Content-Range headers
	indicates whether the Warning MUST or MUST NOT be deleted from a		MUST NOT store incomplete or partial responses.
	stored cache entry after a successful revalidation:
	1xx Warnings that describe the freshness or revalidation status of		2.2. Constructing Responses from Caches
	the response, and so MUST be deleted after a successful
	revalidation. 1xx warn-codes MAY be generated by a cache only when
	validating a cached entry. It MUST NOT be generated by clients.
	2xx Warnings that describe some aspect of the entity body or entity		For a presented request, a cache MUST NOT return a stored response,
	headers that is not rectified by a revalidation (for example, a		unless:
	lossy compression of the entity bodies) and which MUST NOT be
	deleted after a successful revalidation.
	See Section 16.6 for the definitions of the codes themselves.		o The presented Request-URI and that of the stored response match
			(see [[anchor1: TBD]]), and
	HTTP/1.0 caches will cache all Warnings in responses, without		o the request method associated with the stored response allows it
	deleting the ones in the first category. Warnings in responses that		to be used for the presented request, and
	are passed to HTTP/1.0 caches carry an extra warning-date field,
	which prevents a future HTTP/1.1 recipient from believing an
	erroneously cached Warning.
	Warnings also carry a warning text. The text MAY be in any		o selecting request-headers nominated by the stored response (if
	appropriate natural language (perhaps based on the client's Accept		any) match those presented (see Section 2.6), and
	headers), and include an OPTIONAL indication of what character set is		o the presented request and stored response are free from directives
	used.		that would prevent its use (see Section 3.2 and Section 3.4), and
	Multiple warnings MAY be attached to a response (either by the origin		o the stored response is either:
	server or by a cache), including multiple warnings with the same code
	number. For example, a server might provide the same warning with
	texts in both English and Basque.
	When multiple warnings are attached to a response, it might not be		* fresh (see Section 2.3), or
	practical or reasonable to display all of them to the user. This
	version of HTTP does not specify strict priority rules for deciding
	which warnings to display and in what order, but does suggest some
	heuristics.
	3.3. Cache-control Mechanisms		* allowed to be served stale (see Section 2.3.3), or
	The basic cache mechanisms in HTTP/1.1 (server-specified expiration		* successfully validated (see Section 2.4).
	times and validators) are implicit directives to caches. In some
	cases, a server or client might need to provide explicit directives
	to the HTTP caches. We use the Cache-Control header for this
	purpose.
	The Cache-Control header allows a client or server to transmit a		[[anchor2: TODO: define method cacheability for GET, HEAD and POST in
	variety of directives in either requests or responses. These		p2-semantics.]]
	directives typically override the default caching algorithms. As a
	general rule, if there is any apparent conflict between header
	values, the most restrictive interpretation is applied (that is, the
	one that is most likely to preserve semantic transparency). However,
	in some cases, cache-control directives are explicitly specified as
	weakening the approximation of semantic transparency (for example,
	"max-stale" or "public").
	The cache-control directives are described in detail in Section 16.2.		When a stored response is used to satisfy a request, caches MUST
			include a single Age header field Section 3.1 in the response with a
			value equal to the stored response's current_age; see Section 2.3.2.
			[[anchor3: DISCUSS: this currently includes successfully validated
			responses.]]
	3.4. Explicit User Agent Warnings		Requests with methods that are unsafe (Section 7.1.1 of [Part2]) MUST
			be written through the cache to the origin server; i.e., A cache must
			not reply to such a request before having forwarded the request and
			having received a corresponding response.
	Many user agents make it possible for users to override the basic		Also, note that unsafe requests might invalidate already stored
	caching mechanisms. For example, the user agent might allow the user		responses; see Section 2.5.
	to specify that cached entities (even explicitly stale ones) are
	never validated. Or the user agent might habitually add "Cache-
	Control: max-stale=3600" to every request. The user agent SHOULD NOT
	default to either non-transparent behavior, or behavior that results
	in abnormally ineffective caching, but MAY be explicitly configured
	to do so by an explicit action of the user.
	If the user has overridden the basic caching mechanisms, the user		Caches MUST use the most recent response (as determined by the Date
	agent SHOULD explicitly indicate to the user whenever this results in		header) when more than one suitable response is stored. They can
	the display of information that might not meet the server's		also forward a request with "Cache-Control: max-age=0" or "Cache-
	transparency requirements (in particular, if the displayed entity is		Control: no-cache" to disambiguate which response to use.
	known to be stale). Since the protocol normally allows the user
	agent to determine if responses are stale or not, this indication
	need only be displayed when this actually happens. The indication
	need not be a dialog box; it could be an icon (for example, a picture
	of a rotting fish) or some other indicator.
	If the user has overridden the caching mechanisms in a way that would		[[anchor4: TODO: end-to-end and hop-by-hop headers, non-modifiable
	abnormally reduce the effectiveness of caches, the user agent SHOULD		headers removed; re-spec in p1]]
	continually indicate this state to the user (for example, by a
	display of a picture of currency in flames) so that the user does not
	inadvertently consume excess resources or suffer from excessive
	latency.
	3.5. Exceptions to the Rules and Warnings		2.3. Freshness Model
	In some cases, the operator of a cache MAY choose to configure it to		When a response is "fresh" in the cache, it can be used to satisfy
	return stale responses even when not requested by clients. This		subsequent requests without contacting the origin server, thereby
	decision ought not be made lightly, but may be necessary for reasons		improving efficiency.
	of availability or performance, especially when the cache is poorly
	connected to the origin server. Whenever a cache returns a stale
	response, it MUST mark it as such (using a Warning header) enabling
	the client software to alert the user that there might be a potential
	problem.
	It also allows the user agent to take steps to obtain a first-hand or		The primary mechanism for determining freshness is for an origin
	fresh response. For this reason, a cache SHOULD NOT return a stale		server to provide an explicit expiration time in the future, using
	response if the client explicitly requests a first-hand or fresh one,		either the Expires header (Section 3.3) or the max-age response cache
	unless it is impossible to comply for technical or policy reasons.		directive (Section 3.2.2). Generally, origin servers will assign
			future explicit expiration times to responses in the belief that the
			entity is not likely to change in a semantically significant way
			before the expiration time is reached.
	3.6. Client-controlled Behavior		If an origin server wishes to force a cache to validate every
			request, it can assign an explicit expiration time in the past. This
			means that the response is always stale, so that caches should
			validate it before using it for subsequent requests. [[anchor5: This
			wording may cause confusion, because the response may still be served
			stale.]]
	While the origin server (and to a lesser extent, intermediate caches,		Since origin servers do not always provide explicit expiration times,
	by their contribution to the age of a response) are the primary		HTTP caches may also assign heuristic expiration times when they are
	source of expiration information, in some cases the client might need		not specified, employing algorithms that use other header values
	to control a cache's decision about whether to return a cached		(such as the Last-Modified time) to estimate a plausible expiration
	response without validating it. Clients do this using several		time. The HTTP/1.1 specification does not provide specific
	directives of the Cache-Control header.		algorithms, but does impose worst-case constraints on their results.
	A client's request MAY specify the maximum age it is willing to		The calculation to determine if a response is fresh is:
	accept of an unvalidated response; specifying a value of zero forces
	the cache(s) to revalidate all responses. A client MAY also specify
	the minimum time remaining before a response expires. Both of these
	options increase constraints on the behavior of caches, and so cannot
	further relax the cache's approximation of semantic transparency.
	A client MAY also specify that it will accept stale responses, up to		response_is_fresh = (freshness_lifetime > current_age)
	some maximum amount of staleness. This loosens the constraints on
	the caches, and so might violate the origin server's specified
	constraints on semantic transparency, but might be necessary to
	support disconnected operation, or high availability in the face of
	poor connectivity.
	4. Expiration Model		The freshness_lifetime is defined in Section 2.3.1; the current_age
			is defined in Section 2.3.2.
	4.1. Server-Specified Expiration		Additionally, clients may need to influence freshness calculation.
			They can do this using several request cache directives, with the
			effect of either increasing or loosening constraints on freshness.
			See Section 3.2.1.
	HTTP caching works best when caches can entirely avoid making		[[anchor6: ISSUE: there are not requirements directly applying to
	requests to the origin server. The primary mechanism for avoiding		cache-request-directives and freshness.]]
	requests is for an origin server to provide an explicit expiration
	time in the future, indicating that a response MAY be used to satisfy
	subsequent requests. In other words, a cache can return a fresh
	response without first contacting the server.
	Our expectation is that servers will assign future explicit		Note that freshness applies only to cache operation; it cannot be
	expiration times to responses in the belief that the entity is not		used to force a user agent to refresh its display or reload a
	likely to change, in a semantically significant way, before the		resource. See Section 4 for an explanation of the difference between
	expiration time is reached. This normally preserves semantic		caches and history mechanisms.
	transparency, as long as the server's expiration times are carefully
	chosen.
	The expiration mechanism applies only to responses taken from a cache		2.3.1. Calculating Freshness Lifetime
	and not to first-hand responses forwarded immediately to the
	requesting client.
	If an origin server wishes to force a semantically transparent cache		A cache can calculate the freshness lifetime (denoted as
	to validate every request, it MAY assign an explicit expiration time		freshness_lifetime) of a response by using the first match of:
	in the past. This means that the response is always stale, and so
	the cache SHOULD validate it before using it for subsequent requests.
	See Section 16.2.4 for a more restrictive way to force revalidation.
	If an origin server wishes to force any HTTP/1.1 cache, no matter how		o If the cache is shared and the s-maxage response cache directive
	it is configured, to validate every request, it SHOULD use the "must-		(Section 3.2.2) is present, use its value, or
	revalidate" cache-control directive (see Section 16.2).
	Servers specify explicit expiration times using either the Expires		o If the max-age response cache directive (Section 3.2.2) is
	header, or the max-age directive of the Cache-Control header.		present, use its value, or
			o If the Expires response header (Section 3.3) is present, use its
			value minus the value of the Date response header, or
	An expiration time cannot be used to force a user agent to refresh		o Otherwise, no explicit expiration time is present in the response,
	its display or reload a resource; its semantics apply only to caching		but a heuristic may be used; see Section 2.3.1.1.
	mechanisms, and such mechanisms need only check a resource's
	expiration status when a new request for that resource is initiated.
	See Section 15 for an explanation of the difference between caches
	and history mechanisms.
	4.2. Heuristic Expiration		Note that this calculation is not vulnerable to clock skew, since all
			of the information comes from the origin server.
	Since origin servers do not always provide explicit expiration times,		2.3.1.1. Calculating Heuristic Freshness
	HTTP caches typically assign heuristic expiration times, employing
	algorithms that use other header values (such as the Last-Modified
	time) to estimate a plausible expiration time. The HTTP/1.1
	specification does not provide specific algorithms, but does impose
	worst-case constraints on their results. Since heuristic expiration
	times might compromise semantic transparency, they ought to be used
	cautiously, and we encourage origin servers to provide explicit
	expiration times as much as possible.
	4.3. Age Calculations		If no explicit expiration time is present in a stored response that
			has a status code of 200, 203, 206, 300, 301 or 410, a heuristic
			expiration time can be calculated. Heuristics MUST NOT be used for
			other response status codes.
	In order to know if a cached entry is fresh, a cache needs to know if		When a heuristic is used to calculate freshness lifetime, the cache
	its age exceeds its freshness lifetime. We discuss how to calculate		SHOULD attach a Warning header with a 113 warn-code to the response
	the latter in Section 4.4; this section describes how to calculate		if its current_age is more than 24 hours and such a warning is not
	the age of a response or cache entry.		already present.
	In this discussion, we use the term "now" to mean "the current value		Also, if the response has a Last-Modified header (Section 6.6 of
	of the clock at the host performing the calculation." Hosts that use		[Part4]), the heuristic expiration value SHOULD be no more than some
	HTTP, but especially hosts running origin servers and caches, SHOULD		fraction of the interval since that time. A typical setting of this
	use NTP [RFC1305] or some similar protocol to synchronize their		fraction might be 10%.
	clocks to a globally accurate time standard.
	HTTP/1.1 requires origin servers to send a Date header, if possible,		[[anchor7: REVIEW: took away HTTP/1.0 query string heuristic
	with every response, giving the time at which the response was		uncacheability.]]
	generated (see Section 8.3 of [Part1]). We use the term "date_value"
	to denote the value of the Date header, in a form appropriate for		2.3.2. Calculating Age
	arithmetic operations.
	HTTP/1.1 uses the Age response-header to convey the estimated age of		HTTP/1.1 uses the Age response-header to convey the estimated age of
	the response message when obtained from a cache. The Age field value		the response message when obtained from a cache. The Age field value
	is the cache's estimate of the amount of time since the response was		is the cache's estimate of the amount of time since the response was
	generated or revalidated by the origin server.		generated or validated by the origin server. In essence, the Age
			value is the sum of the time that the response has been resident in
			each of the caches along the path from the origin server, plus the
			amount of time it has been in transit along network paths.
	In essence, the Age value is the sum of the time that the response		The term "age_value" denotes the value of the Age header, in a form
	has been resident in each of the caches along the path from the		appropriate for arithmetic operations.
	origin server, plus the amount of time it has been in transit along
	network paths.
	We use the term "age_value" to denote the value of the Age header, in		HTTP/1.1 requires origin servers to send a Date header, if possible,
	a form appropriate for arithmetic operations.		with every response, giving the time at which the response was
			generated (see Section 8.3 of [Part1]). The term "date_value"
			denotes the value of the Date header, in a form appropriate for
			arithmetic operations.
			The term "now" means "the current value of the clock at the host
			performing the calculation." Hosts that use HTTP, but especially
			hosts running origin servers and caches, SHOULD use NTP [RFC1305] or
			some similar protocol to synchronize their clocks to a globally
			accurate time standard.
	A response's age can be calculated in two entirely independent ways:		A response's age can be calculated in two entirely independent ways:
	1. now minus date_value, if the local clock is reasonably well		1. now minus date_value, if the local clock is reasonably well
	synchronized to the origin server's clock. If the result is		synchronized to the origin server's clock. If the result is
	negative, the result is replaced by zero.		negative, the result is replaced by zero.
	2. age_value, if all of the caches along the response path implement		2. age_value, if all of the caches along the response path implement
	HTTP/1.1.		HTTP/1.1.
	Given that we have two independent ways to compute the age of a		These are combined as
	response when it is received, we can combine these as
	corrected_received_age = max(now - date_value, age_value)		corrected_received_age = max(now - date_value, age_value)
	and as long as we have either nearly synchronized clocks or all-		When an Age value is received, it MUST be interpreted relative to the
	HTTP/1.1 paths, one gets a reliable (conservative) result.		time the request was initiated, not the time that the response was
			received.
	Because of network-imposed delays, some significant interval might
	pass between the time that a server generates a response and the time
	it is received at the next outbound cache or client. If uncorrected,
	this delay could result in improperly low ages.
	Because the request that resulted in the returned Age value must have
	been initiated prior to that Age value's generation, we can correct
	for delays imposed by the network by recording the time at which the
	request was initiated. Then, when an Age value is received, it MUST
	be interpreted relative to the time the request was initiated, not
	the time that the response was received. This algorithm results in
	conservative behavior no matter how much delay is experienced. So,
	we compute:
	corrected_initial_age = corrected_received_age		corrected_initial_age = corrected_received_age
	+ (now - request_time)		+ (now - request_time)
	where "request_time" is the time (according to the local clock) when		where "request_time" is the time (according to the local clock) when
	the request that elicited this response was sent.		the request that elicited this response was sent.
	Summary of age calculation algorithm, when a cache receives a		The current_age of a stored response can then be calculated by adding
	response:		the amount of time (in seconds) since the stored response was last
			validated by the origin server to the corrected_initial_age.
	/*		In summary:
	* age_value
	* is the value of Age: header received by the cache with		age_value - Age header field-value received with the response
	* this response.		date_value - Date header field-value received with the response
	* date_value		request_time - local time when the cache made the request
	* is the value of the origin server's Date: header		resulting in the stored response
	* request_time		response_time - local time when the cache received the response
	* is the (local) time when the cache made the request		now - current local time
	* that resulted in this cached response
	* response_time
	* is the (local) time when the cache received the
	* response
	* now
	* is the current (local) time
	*/
	apparent_age = max(0, response_time - date_value);		apparent_age = max(0, response_time - date_value);
	corrected_received_age = max(apparent_age, age_value);		corrected_received_age = max(apparent_age, age_value);
	response_delay = response_time - request_time;		response_delay = response_time - request_time;
	corrected_initial_age = corrected_received_age + response_delay;		corrected_initial_age = corrected_received_age + response_delay;
	resident_time = now - response_time;		resident_time = now - response_time;
	current_age = corrected_initial_age + resident_time;		current_age = corrected_initial_age + resident_time;
	The current_age of a cache entry is calculated by adding the amount		2.3.3. Serving Stale Responses
	of time (in seconds) since the cache entry was last validated by the
	origin server to the corrected_initial_age. When a response is
	generated from a cache entry, the cache MUST include a single Age
	header field in the response with a value equal to the cache entry's
	current_age.
	The presence of an Age header field in a response implies that a
	response is not first-hand. However, the converse is not true, since
	the lack of an Age header field in a response does not imply that the
	response is first-hand unless all caches along the request path are
	compliant with HTTP/1.1 (i.e., older HTTP caches did not implement
	the Age header field).
	4.4. Expiration Calculations
	In order to decide whether a response is fresh or stale, we need to
	compare its freshness lifetime to its age. The age is calculated as
	described in Section 4.3; this section describes how to calculate the
	freshness lifetime, and to determine if a response has expired. In
	the discussion below, the values can be represented in any form
	appropriate for arithmetic operations.
	We use the term "expires_value" to denote the value of the Expires
	header. We use the term "max_age_value" to denote an appropriate
	value of the number of seconds carried by the "max-age" directive of
	the Cache-Control header in a response (see Section 16.2.3).
	The max-age directive takes priority over Expires, so if max-age is
	present in a response, the calculation is simply:
	freshness_lifetime = max_age_value
	Otherwise, if Expires is present in the response, the calculation is:
	freshness_lifetime = expires_value - date_value
	Note that neither of these calculations is vulnerable to clock skew,
	since all of the information comes from the origin server.
	If none of Expires, Cache-Control: max-age, or Cache-Control:
	s-maxage (see Section 16.2.3) appears in the response, and the
	response does not include other restrictions on caching, the cache
	MAY compute a freshness lifetime using a heuristic. The cache MUST
	attach Warning 113 to any response whose age is more than 24 hours if
	such warning has not already been added.
	Also, if the response does have a Last-Modified time, the heuristic
	expiration value SHOULD be no more than some fraction of the interval
	since that time. A typical setting of this fraction might be 10%.
	The calculation to determine if a response has expired is quite
	simple:
	response_is_fresh = (freshness_lifetime > current_age)
	4.5. Disambiguating Expiration Values
	Because expiration values are assigned optimistically, it is possible
	for two caches to contain fresh values for the same resource that are
	different.
	If a client performing a retrieval receives a non-first-hand response
	for a request that was already fresh in its own cache, and the Date
	header in its existing cache entry is newer than the Date on the new
	response, then the client MAY ignore the response. If so, it MAY
	retry the request with a "Cache-Control: max-age=0" directive (see
	Section 16.2), to force a check with the origin server.
	If a cache has two fresh responses for the same representation with
	different validators, it MUST use the one with the more recent Date
	header. This situation might arise because the cache is pooling
	responses from other caches, or because a client has asked for a
	reload or a revalidation of an apparently fresh cache entry.
	4.6. Disambiguating Multiple Responses
	Because a client might be receiving responses via multiple paths, so
	that some responses flow through one set of caches and other
	responses flow through a different set of caches, a client might
	receive responses in an order different from that in which the origin
	server sent them. We would like the client to use the most recently
	generated response, even if older responses are still apparently
	fresh.
	Neither the entity tag nor the expiration value can impose an
	ordering on responses, since it is possible that a later response
	intentionally carries an earlier expiration time. The Date values
	are ordered to a granularity of one second.
	When a client tries to revalidate a cache entry, and the response it
	receives contains a Date header that appears to be older than the one
	for the existing entry, then the client SHOULD repeat the request
	unconditionally, and include
	Cache-Control: max-age=0
	to force any intermediate caches to validate their copies directly
	with the origin server, or
	Cache-Control: no-cache
	to force any intermediate caches to obtain a new copy from the origin
	server.
	If the Date values are equal, then the client MAY use either response
	(or MAY, if it is being extremely prudent, request a new response).
	Servers MUST NOT depend on clients being able to choose
	deterministically between responses generated during the same second,
	if their expiration times overlap.
	5. Validation Model
	When a cache has a stale entry that it would like to use as a
	response to a client's request, it first has to check with the origin
	server (or possibly an intermediate cache with a fresh response) to
	see if its cached entry is still usable. We call this "validating"
	the cache entry.
	HTTP's conditional request mechanism, defined in [Part4], is used to
	avoid retransmitting the response payload when the cached entry is
	valid. When a cached response includes one or more "cache
	validators," such as the field values of an ETag or Last-Modified
	header field, then a validating GET request SHOULD be made
	conditional to those field values. The server checks the conditional
	request's validator against the current state of the requested
	resource and, if they match, the server responds with a 304 (Not
	Modified) status code to indicate that the cached response can be
	refreshed and reused without retransmitting the response payload. If
	the validator does not match the current state of the requested
	resource, then the server returns a full response, including payload,
	so that the request can be satisfied and the cache entry supplanted
	without the need for an additional network round-trip.
	6. Response Cacheability
	Unless specifically constrained by a cache-control (Section 16.2)
	directive, a caching system MAY always store a successful response
	(see Section 10) as a cache entry, MAY return it without validation
	if it is fresh, and MAY return it after successful validation. If
	there is neither a cache validator nor an explicit expiration time
	associated with a response, we do not expect it to be cached, but
	certain caches MAY violate this expectation (for example, when little
	or no network connectivity is available). A client can usually
	detect that such a response was taken from a cache by comparing the
	Date header to the current time.
	Note: some HTTP/1.0 caches are known to violate this expectation
	without providing any Warning.
	However, in some cases it might be inappropriate for a cache to
	retain an entity, or to return it in response to a subsequent
	request. This might be because absolute semantic transparency is
	deemed necessary by the service author, or because of security or
	privacy considerations. Certain cache-control directives are
	therefore provided so that the server can indicate that certain
	resource entities, or portions thereof, are not to be cached
	regardless of other considerations.
	Note that Section 4.1 of [Part7] normally prevents a shared cache
	from saving and returning a response to a previous request if that
	request included an Authorization header.
	A response received with a status code of 200, 203, 206, 300, 301 or
	410 MAY be stored by a cache and used in reply to a subsequent
	request, subject to the expiration mechanism, unless a cache-control
	directive prohibits caching. However, a cache that does not support
	the Range and Content-Range headers MUST NOT cache 206 (Partial
	Content) responses.
	A response received with any other status code (e.g. status codes 302
	and 307) MUST NOT be returned in a reply to a subsequent request
	unless there are cache-control directives or another header(s) that
	explicitly allow it. For example, these include the following: an
	Expires header (Section 16.3); a "max-age", "s-maxage", "must-
	revalidate", "proxy-revalidate", "public" or "private" cache-control
	directive (Section 16.2).
	7. Constructing Responses From Caches
	The purpose of an HTTP cache is to store information received in
	response to requests for use in responding to future requests. In
	many cases, a cache simply returns the appropriate parts of a
	response to the requester. However, if the cache holds a cache entry
	based on a previous response, it might have to combine parts of a new
	response with what is held in the cache entry.
	7.1. End-to-end and Hop-by-hop Headers
	For the purpose of defining the behavior of caches and non-caching
	proxies, we divide HTTP headers into two categories:
	o End-to-end headers, which are transmitted to the ultimate
	recipient of a request or response. End-to-end headers in
	responses MUST be stored as part of a cache entry and MUST be
	transmitted in any response formed from a cache entry.
	o Hop-by-hop headers, which are meaningful only for a single
	transport-level connection, and are not stored by caches or
	forwarded by proxies.
	The following HTTP/1.1 headers are hop-by-hop headers:
	o Connection
	o Keep-Alive
	o Proxy-Authenticate
	o Proxy-Authorization
	o TE
	o Trailer
	o Transfer-Encoding
	o Upgrade
	All other headers defined by HTTP/1.1 are end-to-end headers.
	Other hop-by-hop headers MUST be listed in a Connection header
	(Section 8.1 of [Part1]).
	7.2. Non-modifiable Headers
	Some features of HTTP/1.1, such as Digest Authentication, depend on
	the value of certain end-to-end headers. A transparent proxy SHOULD
	NOT modify an end-to-end header unless the definition of that header
	requires or specifically allows that.
	A transparent proxy MUST NOT modify any of the following fields in a
	request or response, and it MUST NOT add any of these fields if not
	already present:
	o Content-Location
	o Content-MD5
	o ETag		A "stale" response is one that either has explicit expiry
			information, or is allowed to have heuristic expiry calculated, but
			is not fresh according to the calculations in Section 2.3.
	o Last-Modified		Caches MUST NOT return a stale response if it is prohibited by an
			explicit in-protocol directive (e.g., by a "no-store" or "no-cache"
			cache directive, a "must-revalidate" cache-response-directive, or an
			applicable "s-maxage" or "proxy-revalidate" cache-response-directive;
			see Section 3.2.2).
	A transparent proxy MUST NOT modify any of the following fields in a		Caches SHOULD NOT return stale responses unless they are disconnected
	response:		(i.e., it cannot contact the origin server or otherwise find a
			forward path) or otherwise explicitly allowed (e.g., the max-stale
			request directive; see Section 3.2.1).
	o Expires		Stale responses SHOULD have a Warning header with the 110 warn-code
			(see Section 3.6). Likewise, the 112 warn-code SHOULD be sent on
			stale responses if the cache is disconnected.
	but it MAY add any of these fields if not already present. If an		If a cache receives a first-hand response (either an entire response,
	Expires header is added, it MUST be given a field-value identical to		or a 304 (Not Modified) response) that it would normally forward to
	that of the Date header in that response.		the requesting client, and the received response is no longer fresh,
			the cache SHOULD forward it to the requesting client without adding a
			new Warning (but without removing any existing Warning headers). A
			cache SHOULD NOT attempt to validate a response simply because that
			response became stale in transit.
	A proxy MUST NOT modify or add any of the following fields in a		2.4. Validation Model
	message that contains the no-transform cache-control directive, or in
	any request:
	o Content-Encoding		Checking with the origin server to see if a stale or otherwise
			unusable cached response can be reused is called "validating" or
			"revalidating." Doing so potentially avoids the overhead of
			retransmitting the response body when the stored response is valid.
	o Content-Range		HTTP's conditional request mechanism [Part4] is used for this
	o Content-Type		purpose. When a stored response includes one or more validators,
			such as the field values of an ETag or Last-Modified header field,
			then a validating request SHOULD be made conditional to those field
			values.
	A non-transparent proxy MAY modify or add these fields to a message		A 304 (Not Modified) response status code indicates that the stored
	that does not include no-transform, but if it does so, it MUST add a		response can be updated and reused; see Section 2.7.
	Warning 214 (Transformation applied) if one does not already appear
	in the message (see Section 16.6).
	Warning: unnecessary modification of end-to-end headers might		If instead the cache receives a full response (i.e., one with a
	cause authentication failures if stronger authentication		response body), it is used to satisfy the request and replace the
	mechanisms are introduced in later versions of HTTP. Such		stored response. [[anchor8: Should there be a requirement here?]]
	authentication mechanisms MAY rely on the values of header fields		If a cache receives a 5xx response while attempting to validate a
	not listed here.		response, it MAY either forward this response to the requesting
			client, or act as if the server failed to respond. In the latter
			case, it MAY return a previously stored response (which SHOULD
			include the 111 warn-code; see Section 3.6) unless the stored
			response includes the "must-revalidate" cache directive (see
			Section 2.3.3).
	The Content-Length field of a request or response is added or deleted		2.5. Request Methods that Invalidate
	according to the rules in Section 4.4 of [Part1]. A transparent
	proxy MUST preserve the entity-length (Section 4.2.2 of [Part3]) of
	the entity-body, although it MAY change the transfer-length (Section
	4.4 of [Part1]).
	7.3. Combining Headers		Because unsafe methods (Section 7.1.1 of [Part2]) have the potential
			for changing state on the origin server, intervening caches can use
			them to keep their contents up-to-date.
	When a cache makes a validating request to a server, and the server		The following HTTP methods MUST cause a cache to invalidate the
	provides a 304 (Not Modified) response or a 206 (Partial Content)		Request-URI as well as the Location and Content-Location headers (if
	response, the cache then constructs a response to send to the		present):
	requesting client.
	If the status code is 304 (Not Modified), the cache uses the entity-		o PUT
	body stored in the cache entry as the entity-body of this outgoing
	response. If the status code is 206 (Partial Content) and the ETag
	or Last-Modified headers match exactly, the cache MAY combine the
	contents stored in the cache entry with the new contents received in
	the response and use the result as the entity-body of this outgoing
	response, (see Section 5 of [Part5]).
	The end-to-end headers stored in the cache entry are used for the		o DELETE
	constructed response, except that
	o any stored Warning headers with warn-code 1xx (see Section 16.6)		o POST
	MUST be deleted from the cache entry and the forwarded response.
	o any stored Warning headers with warn-code 2xx MUST be retained in		An invalidation based on the URI in a Location or Content-Location
	the cache entry and the forwarded response.		header MUST NOT be performed if the host part of that URI differs
			from the host part in the Request-URI. This helps prevent denial of
			service attacks.
	o any end-to-end headers provided in the 304 or 206 response MUST		[[anchor9: TODO: "host part" needs to be specified better.]]
	replace the corresponding headers from the cache entry.
	Unless the cache decides to remove the cache entry, it MUST also		A cache that passes through requests for methods it does not
	replace the end-to-end headers stored with the cache entry with		understand SHOULD invalidate the Request-URI.
	corresponding headers received in the incoming response, except for
	Warning headers as described immediately above. If a header field-
	name in the incoming response matches more than one header in the
	cache entry, all such old headers MUST be replaced.
	In other words, the set of end-to-end headers received in the		Here, "invalidate" means that the cache will either remove all stored
	incoming response overrides all corresponding end-to-end headers		responses related to the Request-URI, or will mark these as "invalid"
	stored with the cache entry (except for stored Warning headers with		and in need of a mandatory validation before they can be returned in
	warn-code 1xx, which are deleted even if not overridden).		response to a subsequent request.
	Note: this rule allows an origin server to use a 304 (Not		Note that this does not guarantee that all appropriate responses are
	Modified) or a 206 (Partial Content) response to update any header		invalidated. For example, the request that caused the change at the
	associated with a previous response for the same entity or sub-		origin server might not have gone through the cache where a response
	ranges thereof, although it might not always be meaningful or		is stored.
	correct to do so. This rule does not allow an origin server to
	use a 304 (Not Modified) or a 206 (Partial Content) response to
	entirely delete a header that it had provided with a previous
	response.
	8. Caching Negotiated Responses		[[anchor10: TODO: specify that only successful (2xx, 3xx?) responses
			invalidate.]]
	Use of server-driven content negotiation (Section 5.1 of [Part3]), as		2.6. Caching Negotiated Responses
	indicated by the presence of a Vary header field in a response,
	alters the conditions and procedure by which a cache can use the
	response for subsequent requests. See Section 16.5 for use of the
	Vary header field by servers.
	A server SHOULD use the Vary header field to inform a cache of what		Use of server-driven content negotiation (Section 4.1 of [Part3])
	request-header fields were used to select among multiple		alters the conditions under which a cache can use the response for
	representations of a cacheable response subject to server-driven		subsequent requests.
	negotiation. The set of header fields named by the Vary field value
	is known as the "selecting" request-headers.
	When the cache receives a subsequent request whose Request-URI		When a cache receives a request that can be satisfied by a stored
	specifies one or more cache entries including a Vary header field,		response that includes a Vary header field (Section 3.5), it MUST NOT
	the cache MUST NOT use such a cache entry to construct a response to		use that response unless all of the selecting request-headers in the
	the new request unless all of the selecting request-headers present		presented request match the corresponding stored request-headers from
	in the new request match the corresponding stored request-headers in
	the original request.		the original request.
	The selecting request-headers from two requests are defined to match		The selecting request-headers from two requests are defined to match
	if and only if the selecting request-headers in the first request can		if and only if the selecting request-headers in the first request can
	be transformed to the selecting request-headers in the second request		be transformed to the selecting request-headers in the second request
	by adding or removing linear white space (LWS) at places where this		by adding or removing linear white space [[anchor11: [ref]]] at
	is allowed by the corresponding BNF, and/or combining multiple		places where this is allowed by the corresponding ABNF, and/or
	message-header fields with the same field name following the rules		combining multiple message-header fields with the same field name
	about message headers in Section 4.2 of [Part1].		following the rules about message headers in Section 4.2 of [Part1].
			[[anchor12: DISCUSS: header-specific canonicalisation]]
	A Vary header field-value of "*" always fails to match and subsequent
	requests on that resource can only be properly interpreted by the
	origin server.
	If the selecting request header fields for the cached entry do not		A Vary header field-value of "*" always fails to match, and
	match the selecting request header fields of the new request, then		subsequent requests to that resource can only be properly interpreted
	the cache MUST NOT use a cached entry to satisfy the request unless		by the origin server.
	it first relays the new request to the origin server in a conditional
	request and the server responds with 304 (Not Modified), including an
	entity tag or Content-Location that indicates the entity to be used.
	If an entity tag was assigned to a cached representation, the		If no stored response matches, the cache MAY forward the presented
	forwarded request SHOULD be conditional and include the entity tags		request to the origin server in a conditional request, and SHOULD
	in an If-None-Match header field from all its cache entries for the		include all ETags stored with potentially suitable responses in an
	resource. This conveys to the server the set of entities currently		If-None-Match request header. If the server responds with 304 (Not
	held by the cache, so that if any one of these entities matches the		Modified) and includes an entity tag or Content-Location that
	requested entity, the server can use the ETag header field in its 304		indicates the entity to be used, that cached response MUST be used to
	(Not Modified) response to tell the cache which entry is appropriate.		satisfy the presented request, and SHOULD be used to update the
	If the entity-tag of the new response matches that of an existing		corresponding stored response; see Section 2.7.
	entry, the new response SHOULD be used to update the header fields of
	the existing entry, and the result MUST be returned to the client.
	If any of the existing cache entries contains only partial content		If any of the stored responses contains only partial content, its
	for the associated entity, its entity-tag SHOULD NOT be included in		entity-tag SHOULD NOT be included in the If-None-Match header field
	the If-None-Match header field unless the request is for a range that		unless the request is for a range that would be fully satisfied by
	would be fully satisfied by that entry.		that stored response.
	If a cache receives a successful response whose Content-Location		If a cache receives a successful response whose Content-Location
	field matches that of an existing cache entry for the same Request-		field matches that of an existing stored response for the same
	URI, whose entity-tag differs from that of the existing entry, and		Request-URI, whose entity-tag differs from that of the existing
	whose Date is more recent than that of the existing entry, the		stored response, and whose Date is more recent than that of the
	existing entry SHOULD NOT be returned in response to future requests		existing response, the existing response SHOULD NOT be returned in
	and SHOULD be deleted from the cache.		response to future requests and SHOULD be deleted from the
			cache.[[anchor13: DISCUSS: Not sure if this is necessary.]]
	9. Shared and Non-Shared Caches
	For reasons of security and privacy, it is necessary to make a
	distinction between "shared" and "non-shared" caches. A non-shared
	cache is one that is accessible only to a single user. Accessibility
	in this case SHOULD be enforced by appropriate security mechanisms.
	All other caches are considered to be "shared." Other sections of
	this specification place certain constraints on the operation of
	shared caches in order to prevent loss of privacy or failure of
	access controls.
	10. Errors or Incomplete Response Cache Behavior
	A cache that receives an incomplete response (for example, with fewer
	bytes of data than specified in a Content-Length header) MAY store
	the response. However, the cache MUST treat this as a partial
	response. Partial responses MAY be combined as described in Section
	5 of [Part5]; the result might be a full response or might still be
	partial. A cache MUST NOT return a partial response to a client
	without explicitly marking it as such, using the 206 (Partial
	Content) status code. A cache MUST NOT return a partial response
	using a status code of 200 (OK).
	If a cache receives a 5xx response while attempting to revalidate an
	entry, it MAY either forward this response to the requesting client,
	or act as if the server failed to respond. In the latter case, it
	MAY return a previously received response unless the cached entry
	includes the "must-revalidate" cache-control directive (see
	Section 16.2).
	11. Side Effects of GET and HEAD
	Unless the origin server explicitly prohibits the caching of their
	responses, the application of GET and HEAD methods to any resources
	SHOULD NOT have side effects that would lead to erroneous behavior if
	these responses are taken from a cache. They MAY still have side
	effects, but a cache is not required to consider such side effects in
	its caching decisions. Caches are always expected to observe an
	origin server's explicit restrictions on caching.
	We note one exception to this rule: since some applications have
	traditionally used GET and HEAD requests with URLs containing a query
	part to perform operations with significant side effects, caches MUST
	NOT treat responses to such URIs as fresh unless the server provides
	an explicit expiration time. This specifically means that responses
	from HTTP/1.0 servers for such URIs SHOULD NOT be taken from a cache.
	See Section 8.1.1 of [Part2] for related information.
	12. Invalidation After Updates or Deletions
	The effect of certain methods performed on a resource at the origin
	server might cause one or more existing cache entries to become non-
	transparently invalid. That is, although they might continue to be
	"fresh," they do not accurately reflect what the origin server would
	return for a new request on that resource.
	There is no way for HTTP to guarantee that all such cache entries are
	marked invalid. For example, the request that caused the change at
	the origin server might not have gone through the proxy where a cache
	entry is stored. However, several rules help reduce the likelihood
	of erroneous behavior.
	In this section, the phrase "invalidate an entity" means that the
	cache will either remove all instances of that entity from its
	storage, or will mark these as "invalid" and in need of a mandatory
	revalidation before they can be returned in response to a subsequent
	request.
	Some HTTP methods MUST cause a cache to invalidate an entity. This
	is either the entity referred to by the Request-URI, or by the
	Location or Content-Location headers (if present). These methods
	are:
	o PUT
	o DELETE
	o POST
	An invalidation based on the URI in a Location or Content-Location
	header MUST NOT be performed if the host part of that URI differs
	from the host part in the Request-URI. This helps prevent denial of
	service attacks.
	A cache that passes through requests for methods it does not
	understand SHOULD invalidate any entities referred to by the Request-
	URI.
	13. Write-Through Mandatory
	All methods that might be expected to cause modifications to the
	origin server's resources MUST be written through to the origin
	server. This currently includes all methods except for GET and HEAD.
	A cache MUST NOT reply to such a request from a client before having
	transmitted the request to the inbound server, and having received a
	corresponding response from the inbound server. This does not
	prevent a proxy cache from sending a 100 (Continue) response before
	the inbound server has sent its final reply.
	The alternative (known as "write-back" or "copy-back" caching) is not
	allowed in HTTP/1.1, due to the difficulty of providing consistent
	updates and the problems arising from server, cache, or network
	failure prior to write-back.
	14. Cache Replacement		2.7. Combining Responses
	If a new cacheable (see Sections 16.2.2, 4.5, 4.6 and 10) response is		When a cache receives a 304 (Not Modified) response or a 206 (Partial
	received from a resource while any existing responses for the same		Content) response, it needs to update the stored response with the
	resource are cached, the cache SHOULD use the new response to reply		new one, so that the updated response can be sent to the client.
	to the current request. It MAY insert it into cache storage and MAY,
	if it meets all other requirements, use it to respond to any future
	requests that would previously have caused the old response to be
	returned. If it inserts the new response into cache storage the
	rules in Section 7.3 apply.
	Note: a new response that has an older Date header value than		If the status code is 304 (Not Modified), the cache SHOULD use the
	existing cached responses is not cacheable.		stored entity-body as the updated entity-body. If the status code is
			206 (Partial Content) and the ETag or Last-Modified headers match
			exactly, the cache MAY combine the stored entity-body in the stored
			response with the updated entity-body received in the response and
			use the result as the updated entity-body (see Section 4 of [Part5]).
	15. History Lists		The stored response headers are used for the updated response, except
			that
	User agents often have history mechanisms, such as "Back" buttons and		o any stored Warning headers with warn-code 1xx (see Section 3.6)
	history lists, which can be used to redisplay an entity retrieved		MUST be deleted from the stored response and the forwarded
	earlier in a session.		response.
	History mechanisms and caches are different. In particular history		o any stored Warning headers with warn-code 2xx MUST be retained in
	mechanisms SHOULD NOT try to show a semantically transparent view of		the stored response and the forwarded response.
	the current state of a resource. Rather, a history mechanism is
	meant to show exactly what the user saw at the time when the resource
	was retrieved.
	By default, an expiration time does not apply to history mechanisms.		o any headers provided in the 304 or 206 response MUST replace the
	If the entity is still in storage, a history mechanism SHOULD display		corresponding headers from the stored response.
	it even if the entity has expired, unless the user has specifically
	configured the agent to refresh expired history documents.
	This is not to be construed to prohibit the history mechanism from		A cache MUST also replace any stored headers with corresponding
	telling the user that a view might be stale.		headers received in the incoming response, except for Warning headers
			as described immediately above. If a header field-name in the
			incoming response matches more than one header in the stored
			response, all such old headers MUST be replaced. It MAY store the
			combined entity-body.
	Note: if history list mechanisms unnecessarily prevent users from		[[anchor14: ISSUE: discuss how to handle HEAD updates]]
	viewing stale resources, this will tend to force service authors
	to avoid using HTTP expiration controls and cache controls when
	they would otherwise like to. Service authors may consider it
	important that users not be presented with error messages or
	warning messages when they use navigation controls (such as BACK)
	to view previously fetched resources. Even though sometimes such
	resources ought not be cached, or ought to expire quickly, user
	interface considerations may force service authors to resort to
	other means of preventing caching (e.g. "once-only" URLs) in order
	not to suffer the effects of improperly functioning history
	mechanisms.
	16. Header Field Definitions		3. Header Field Definitions
	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 related to caching.		fields related to caching.
	For entity-header fields, both sender and recipient refer to either		For entity-header fields, both sender and recipient refer to either
	the client or the server, depending on who sends and who receives the		the client or the server, depending on who sends and who receives the
	entity.		entity.
	16.1. Age		3.1. Age
	The response-header field "Age" conveys the sender's estimate of the		The response-header field "Age" conveys the sender's estimate of the
	amount of time since the response (or its revalidation) was generated		amount of time since the response (or its validation) was generated
	at the origin server. A cached response is "fresh" if its age does		at the origin server. Age values are calculated as specified in
	not exceed its freshness lifetime. Age values are calculated as		Section 2.3.2.
	specified in Section 4.3.
	Age = "Age" ":" OWS Age-v		Age = "Age" ":" OWS Age-v
	Age-v = delta-seconds		Age-v = delta-seconds
	Age values are non-negative decimal integers, representing time in		Age field-values are non-negative decimal integers, representing time
	seconds.		in seconds.
	delta-seconds = 1*DIGIT		delta-seconds = 1*DIGIT
	If a cache receives a value larger than the largest positive integer		If a cache receives a value larger than the largest positive integer
	it can represent, or if any of its age calculations overflows, it		it can represent, or if any of its age calculations overflows, it
	MUST transmit an Age header with a value of 2147483648 (2^31). An		MUST transmit an Age header with a field-value of 2147483648 (2^31).
	HTTP/1.1 server that includes a cache MUST include an Age header		Caches SHOULD use an arithmetic type of at least 31 bits of range.
	field in every response generated from its own cache. Caches SHOULD
	use an arithmetic type of at least 31 bits of range.
	16.2. Cache-Control		The presence of an Age header field in a response implies that a
			response is not first-hand. However, the converse is not true, since
			HTTP/1.0 caches may not implement the Age header field.
			3.2. Cache-Control
	The general-header field "Cache-Control" is used to specify		The general-header field "Cache-Control" is used to specify
	directives that MUST be obeyed by all caching mechanisms along the		directives that MUST be obeyed by all caches along the request/
	request/response chain. The directives specify behavior intended to		response chain. The directives specify behavior intended to prevent
	prevent caches from adversely interfering with the request or		caches from adversely interfering with the request or response.
	response. These directives typically override the default caching		Cache directives are unidirectional in that the presence of a
	algorithms. Cache directives are unidirectional in that the presence		directive in a request does not imply that the same directive is to
	of a directive in a request does not imply that the same directive is		be given in the response.
	to be given in the response.
	Note that HTTP/1.0 caches might not implement Cache-Control and		Note that HTTP/1.0 caches might not implement Cache-Control and
	might only implement Pragma: no-cache (see Section 16.4).		might only implement Pragma: no-cache (see Section 3.4).
	Cache directives MUST be passed through by a proxy or gateway		Cache directives MUST be passed through by a proxy or gateway
	application, regardless of their significance to that application,		application, regardless of their significance to that application,
	since the directives might be applicable to all recipients along the		since the directives might be applicable to all recipients along the
	request/response chain. It is not possible to specify a cache-		request/response chain. It is not possible to target a directive to
	directive for a specific cache.		a specific cache.
	Cache-Control = "Cache-Control" ":" OWS Cache-Control-v		Cache-Control = "Cache-Control" ":" OWS Cache-Control-v
	Cache-Control-v = 1#cache-directive		Cache-Control-v = 1#cache-directive
	cache-directive = cache-request-directive		cache-directive = cache-request-directive
	/ cache-response-directive		/ cache-response-directive
	cache-request-directive =
	"no-cache" ; Section 16.2.1
	/ "no-store" ; Section 16.2.2
	/ "max-age" "=" delta-seconds ; Section 16.2.3, 16.2.4
	/ "max-stale" [ "=" delta-seconds ] ; Section 16.2.3
	/ "min-fresh" "=" delta-seconds ; Section 16.2.3
	/ "no-transform" ; Section 16.2.5
	/ "only-if-cached" ; Section 16.2.4
	/ cache-extension ; Section 16.2.6
	cache-response-directive =
	"public" ; Section 16.2.1
	/ "private" [ "=" DQUOTE 1#field-name DQUOTE ] ; Section 16.2.1
	/ "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ] ; Section 16.2.1
	/ "no-store" ; Section 16.2.2
	/ "no-transform" ; Section 16.2.5
	/ "must-revalidate" ; Section 16.2.4
	/ "proxy-revalidate" ; Section 16.2.4
	/ "max-age" "=" delta-seconds ; Section 16.2.3
	/ "s-maxage" "=" delta-seconds ; Section 16.2.3
	/ cache-extension ; Section 16.2.6
	cache-extension = token [ "=" ( token / quoted-string ) ]		cache-extension = token [ "=" ( token / quoted-string ) ]
	When a directive appears without any 1#field-name parameter, the		3.2.1. Request Cache-Control Directives
	directive applies to the entire request or response. When such a
	directive appears with a 1#field-name parameter, it applies only to
	the named field or fields, and not to the rest of the request or
	response. This mechanism supports extensibility; implementations of
	future versions of HTTP might apply these directives to header fields
	not defined in HTTP/1.1.
	The cache-control directives can be broken down into these general
	categories:
	o Restrictions on what are cacheable; these may only be imposed by
	the origin server.
	o Restrictions on what may be stored by a cache; these may be
	imposed by either the origin server or the user agent.
	o Modifications of the basic expiration mechanism; these may be
	imposed by either the origin server or the user agent.
	o Controls over cache revalidation and reload; these may only be
	imposed by a user agent.
	o Control over transformation of entities.
	o Extensions to the caching system.
	16.2.1. What is Cacheable
	By default, a response is cacheable if the requirements of the
	request method, request header fields, and the response status
	indicate that it is cacheable. Section 6 summarizes these defaults
	for cacheability. The following Cache-Control response directives
	allow an origin server to override the default cacheability of a
	response:
	public
	Indicates that the response MAY be cached by any cache, even if it
	would normally be non-cacheable or cacheable only within a non-
	shared cache. (See also Authorization, Section 4.1 of [Part7],
	for additional details.)
	private
	Indicates that all or part of the response message is intended for
	a single user and MUST NOT be cached by a shared cache. This
	allows an origin server to state that the specified parts of the
	response are intended for only one user and are not a valid
	response for requests by other users. A private (non-shared)
	cache MAY cache the response.
	Note: This usage of the word private only controls where the		cache-request-directive =
	response may be cached, and cannot ensure the privacy of the		"no-cache"
	message content.		/ "no-store"
			/ "max-age" "=" delta-seconds
			/ "max-stale" [ "=" delta-seconds ]
			/ "min-fresh" "=" delta-seconds
			/ "no-transform"
			/ "only-if-cached"
			/ cache-extension
	no-cache		no-cache
	If the no-cache directive does not specify a field-name, then a		The no-cache request directive indicates that a stored response
	cache MUST NOT use the response to satisfy a subsequent request		MUST NOT be used to satisfy the request without successful
	without successful revalidation with the origin server. This		validation on the origin server.
	allows an origin server to prevent caching even by caches that
	have been configured to return stale responses to client requests.
	If the no-cache directive does specify one or more field-names,
	then a cache MAY use the response to satisfy a subsequent request,
	subject to any other restrictions on caching. However, the
	specified field-name(s) MUST NOT be sent in the response to a
	subsequent request without successful revalidation with the origin
	server. This allows an origin server to prevent the re-use of
	certain header fields in a response, while still allowing caching
	of the rest of the response.
	Note: Most HTTP/1.0 caches will not recognize or obey this
	directive.
	16.2.2. What May be Stored by Caches
	no-store		no-store
	The purpose of the no-store directive is to prevent the		The no-store request directive indicates that a cache MUST NOT
	inadvertent release or retention of sensitive information (for		store any part of either this request or any response to it. This
	example, on backup tapes). The no-store directive applies to the		directive applies to both non-shared and shared caches. "MUST NOT
	entire message, and MAY be sent either in a response or in a		store" in this context means that the cache MUST NOT intentionally
	request. If sent in a request, a cache MUST NOT store any part of		store the information in non-volatile storage, and MUST make a
	either this request or any response to it. If sent in a response,		best-effort attempt to remove the information from volatile
	a cache MUST NOT store any part of either this response or the		storage as promptly as possible after forwarding it.
	request that elicited it. This directive applies to both non-
	shared and shared caches. "MUST NOT store" in this context means
	that the cache MUST NOT intentionally store the information in
	non-volatile storage, and MUST make a best-effort attempt to
	remove the information from volatile storage as promptly as
	possible after forwarding it.
	Even when this directive is associated with a response, users
	might explicitly store such a response outside of the caching
	system (e.g., with a "Save As" dialog). History buffers MAY store
	such responses as part of their normal operation.
	The purpose of this directive is to meet the stated requirements
	of certain users and service authors who are concerned about
	accidental releases of information via unanticipated accesses to
	cache data structures. While the use of this directive might
	improve privacy in some cases, we caution that it is NOT in any
	way a reliable or sufficient mechanism for ensuring privacy. In
	particular, malicious or compromised caches might not recognize or
	obey this directive, and communications networks might be
	vulnerable to eavesdropping.
	16.2.3. Modifications of the Basic Expiration Mechanism
	The expiration time of an entity MAY be specified by the origin
	server using the Expires header (see Section 16.3). Alternatively,
	it MAY be specified using the max-age directive in a response. When
	the max-age cache-control directive is present in a cached response,
	the response is stale if its current age is greater than the age
	value given (in seconds) at the time of a new request for that
	resource. The max-age directive on a response implies that the
	response is cacheable (i.e., "public") unless some other, more
	restrictive cache directive is also present.
	If a response includes both an Expires header and a max-age
	directive, the max-age directive overrides the Expires header, even
	if the Expires header is more restrictive. This rule allows an
	origin server to provide, for a given response, a longer expiration
	time to an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This
	might be useful if certain HTTP/1.0 caches improperly calculate ages
	or expiration times, perhaps due to desynchronized clocks.
	Many HTTP/1.0 cache implementations will treat an Expires value that
	is less than or equal to the response Date value as being equivalent
	to the Cache-Control response directive "no-cache". If an HTTP/1.1
	cache receives such a response, and the response does not include a
	Cache-Control header field, it SHOULD consider the response to be
	non-cacheable in order to retain compatibility with HTTP/1.0 servers.
	Note: An origin server might wish to use a relatively new HTTP
	cache control feature, such as the "private" directive, on a
	network including older caches that do not understand that
	feature. The origin server will need to combine the new feature
	with an Expires field whose value is less than or equal to the
	Date value. This will prevent older caches from improperly
	caching the response.
	s-maxage
	If a response includes an s-maxage directive, then for a shared		This directive is NOT a reliable or sufficient mechanism for
	cache (but not for a private cache), the maximum age specified by		ensuring privacy. In particular, malicious or compromised caches
	this directive overrides the maximum age specified by either the		might not recognize or obey this directive, and communications
	max-age directive or the Expires header. The s-maxage directive		networks may be vulnerable to eavesdropping.
	also implies the semantics of the proxy-revalidate directive (see
	Section 16.2.4), i.e., that the shared cache must not use the
	entry after it becomes stale to respond to a subsequent request
	without first revalidating it with the origin server. The
	s-maxage directive is always ignored by a private cache.
	Note that most older caches, not compliant with this specification,		max-age
	do not implement any cache-control directives. An origin server
	wishing to use a cache-control directive that restricts, but does not
	prevent, caching by an HTTP/1.1-compliant cache MAY exploit the
	requirement that the max-age directive overrides the Expires header,
	and the fact that pre-HTTP/1.1-compliant caches do not observe the
	max-age directive.
	Other directives allow a user agent to modify the basic expiration		The max-age request directive indicates that the client is willing
	mechanism. These directives MAY be specified on a request:		to accept a response whose age is no greater than the specified
			time in seconds. Unless max-stale directive is also included, the
			client is not willing to accept a stale response.
	max-age		max-stale
	Indicates that the client is willing to accept a response whose		The max-stale request directive indicates that the client is
	age is no greater than the specified time in seconds. Unless max-		willing to accept a response that has exceeded its expiration
	stale directive is also included, the client is not willing to		time. If max-stale is assigned a value, then the client is
	accept a stale response.		willing to accept a response that has exceeded its expiration time
			by no more than the specified number of seconds. If no value is
			assigned to max-stale, then the client is willing to accept a
			stale response of any age. [[anchor15: of any staleness? --mnot]]
	min-fresh		min-fresh
	Indicates that the client is willing to accept a response whose		The min-fresh request directive indicates that the client is
	freshness lifetime is no less than its current age plus the		willing to accept a response whose freshness lifetime is no less
	specified time in seconds. That is, the client wants a response		than its current age plus the specified time in seconds. That is,
	that will still be fresh for at least the specified number of		the client wants a response that will still be fresh for at least
	seconds.		the specified number of seconds.
	max-stale
	Indicates that the client is willing to accept a response that has		no-transform
	exceeded its expiration time. If max-stale is assigned a value,
	then the client is willing to accept a response that has exceeded
	its expiration time by no more than the specified number of
	seconds. If no value is assigned to max-stale, then the client is
	willing to accept a stale response of any age.
	If a cache returns a stale response, either because of a max-stale		The no-transform request directive indicates that an intermediate
	directive on a request, or because the cache is configured to		cache or proxy MUST NOT change the Content-Encoding, Content-Range
	override the expiration time of a response, the cache MUST attach a		or Content-Type request headers, nor the request entity-body.
	Warning header to the stale response, using Warning 110 (Response is
	stale).
	A cache MAY be configured to return stale responses without		only-if-cached
	validation, but only if this does not conflict with any "MUST"-level
	requirements concerning cache validation (e.g., a "must-revalidate"
	cache-control directive).
	If both the new request and the cached entry include "max-age"		The only-if-cached request directive indicates that the client
	directives, then the lesser of the two values is used for determining		only wishes to return a stored response. If it receives this
	the freshness of the cached entry for that request.		directive, a cache SHOULD either respond using a stored response
			that is consistent with the other constraints of the request, or
			respond with a 504 (Gateway Timeout) status. If a group of caches
			is being operated as a unified system with good internal
			connectivity, such a request MAY be forwarded within that group of
			caches.
	16.2.4. Cache Revalidation and Reload Controls		3.2.2. Response Cache-Control Directives
	Sometimes a user agent might want or need to insist that a cache		cache-response-directive =
	revalidate its cache entry with the origin server (and not just with		"public"
	the next cache along the path to the origin server), or to reload its		/ "private" [ "=" DQUOTE 1#field-name DQUOTE ]
	cache entry from the origin server. End-to-end revalidation might be		/ "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]
	necessary if either the cache or the origin server has overestimated		/ "no-store"
	the expiration time of the cached response. End-to-end reload may be		/ "no-transform"
	necessary if the cache entry has become corrupted for some reason.		/ "must-revalidate"
			/ "proxy-revalidate"
			/ "max-age" "=" delta-seconds
			/ "s-maxage" "=" delta-seconds
			/ cache-extension
	End-to-end revalidation may be requested either when the client does		public
	not have its own local cached copy, in which case we call it
	"unspecified end-to-end revalidation", or when the client does have a
	local cached copy, in which case we call it "specific end-to-end
	revalidation."
	The client can specify these three kinds of action using Cache-		The public response directive indicates that the response MAY be
	Control request directives:		cached, even if it would normally be non-cacheable or cacheable
			only within a non-shared cache. (See also Authorization, Section
			3.1 of [Part7], for additional details.)
	End-to-end reload		private
	The request includes a "no-cache" cache-control directive or, for		The private response directive indicates that the response message
	compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field		is intended for a single user and MUST NOT be stored by a shared
	names MUST NOT be included with the no-cache directive in a		cache. A private (non-shared) cache MAY store the response.
	request. The server MUST NOT use a cached copy when responding to
	such a request.
	Specific end-to-end revalidation		If the private response directive specifies one or more field-
			names, this requirement is limited to the field-values associated
			with the listed response headers. That is, the specified field-
			names(s) MUST NOT be stored by a shared cache, whereas the
			remainder of the response message MAY be.
	The request includes a "max-age=0" cache-control directive, which		Note: This usage of the word private only controls where the
	forces each cache along the path to the origin server to		response may be stored, and cannot ensure the privacy of the
	revalidate its own entry, if any, with the next cache or server.		message content.
	The initial request includes a cache-validating conditional with
	the client's current validator.
	Unspecified end-to-end revalidation		no-cache
	The request includes "max-age=0" cache-control directive, which		The no-cache response directive indicates that the response MUST
	forces each cache along the path to the origin server to		NOT be used to satisfy a subsequent request without successful
	revalidate its own entry, if any, with the next cache or server.		validation on the origin server. This allows an origin server to
	The initial request does not include a cache-validating		prevent caching even by caches that have been configured to return
	conditional; the first cache along the path (if any) that holds a		stale responses.
	cache entry for this resource includes a cache-validating
	conditional with its current validator.
	max-age		If the no-cache response directive specifies one or more field-
	When an intermediate cache is forced, by means of a max-age=0		names, this requirement is limited to the field-values assosicated
	directive, to revalidate its own cache entry, and the client has		with the listed response headers. That is, the specified field-
	supplied its own validator in the request, the supplied validator		name(s) MUST NOT be sent in the response to a subsequent request
	might differ from the validator currently stored with the cache		without successful validation on the origin server. This allows
	entry. In this case, the cache MAY use either validator in making		an origin server to prevent the re-use of certain header fields in
	its own request without affecting semantic transparency.		a response, while still allowing caching of the rest of the
			response.
	However, the choice of validator might affect performance. The		Note: Most HTTP/1.0 caches will not recognize or obey this
	best approach is for the intermediate cache to use its own		directive.
	validator when making its request. If the server replies with 304
	(Not Modified), then the cache can return its now validated copy
	to the client with a 200 (OK) response. If the server replies
	with a new entity and cache validator, however, the intermediate
	cache can compare the returned validator with the one provided in
	the client's request, using the strong comparison function. If
	the client's validator is equal to the origin server's, then the
	intermediate cache simply returns 304 (Not Modified). Otherwise,
	it returns the new entity with a 200 (OK) response.
	If a request includes the no-cache directive, it SHOULD NOT		no-store
	include min-fresh, max-stale, or max-age.
	only-if-cached		The no-store response directive indicates that a cache MUST NOT
			store any part of either the immediate request or response. This
			directive applies to both non-shared and shared caches. "MUST NOT
			store" in this context means that the cache MUST NOT intentionally
			store the information in non-volatile storage, and MUST make a
			best-effort attempt to remove the information from volatile
			storage as promptly as possible after forwarding it.
	In some cases, such as times of extremely poor network		This directive is NOT a reliable or sufficient mechanism for
	connectivity, a client may want a cache to return only those		ensuring privacy. In particular, malicious or compromised caches
	responses that it currently has stored, and not to reload or		might not recognize or obey this directive, and communications
	revalidate with the origin server. To do this, the client may		networks may be vulnerable to eavesdropping.
	include the only-if-cached directive in a request. If it receives
	this directive, a cache SHOULD either respond using a cached entry
	that is consistent with the other constraints of the request, or
	respond with a 504 (Gateway Timeout) status. However, if a group
	of caches is being operated as a unified system with good internal
	connectivity, such a request MAY be forwarded within that group of
	caches.
	must-revalidate		must-revalidate
	Because a cache MAY be configured to ignore a server's specified		The must-revalidate response directive indicates that once it has
	expiration time, and because a client request MAY include a max-		become stale, the response MUST NOT be used to satisfy subsequent
	stale directive (which has a similar effect), the protocol also		requests without successful validation on the origin server.
	includes a mechanism for the origin server to require revalidation
	of a cache entry on any subsequent use. When the must-revalidate
	directive is present in a response received by a cache, that cache
	MUST NOT use the entry after it becomes stale to respond to a
	subsequent request without first revalidating it with the origin
	server. (I.e., the cache MUST do an end-to-end revalidation every
	time, if, based solely on the origin server's Expires or max-age
	value, the cached response is stale.)
	The must-revalidate directive is necessary to support reliable		The must-revalidate directive is necessary to support reliable
	operation for certain protocol features. In all circumstances an		operation for certain protocol features. In all circumstances an
	HTTP/1.1 cache MUST obey the must-revalidate directive; in		HTTP/1.1 cache MUST obey the must-revalidate directive; in
	particular, if the cache cannot reach the origin server for any		particular, if the cache cannot reach the origin server for any
	reason, it MUST generate a 504 (Gateway Timeout) response.		reason, it MUST generate a 504 (Gateway Timeout) response.
	Servers SHOULD send the must-revalidate directive if and only if		Servers SHOULD send the must-revalidate directive if and only if
	failure to revalidate a request on the entity could result in		failure to validate a request on the entity could result in
	incorrect operation, such as a silently unexecuted financial		incorrect operation, such as a silently unexecuted financial
	transaction. Recipients MUST NOT take any automated action that		transaction.
	violates this directive, and MUST NOT automatically provide an
	unvalidated copy of the entity if revalidation fails.
	Although this is not recommended, user agents operating under
	severe connectivity constraints MAY violate this directive but, if
	so, MUST explicitly warn the user that an unvalidated response has
	been provided. The warning MUST be provided on each unvalidated
	access, and SHOULD require explicit user confirmation.
	proxy-revalidate		proxy-revalidate
	The proxy-revalidate directive has the same meaning as the must-		The proxy-revalidate response directive has the same meaning as
	revalidate directive, except that it does not apply to non-shared		the must-revalidate response directive, except that it does not
	user agent caches. It can be used on a response to an		apply to non-shared caches.
	authenticated request to permit the user's cache to store and
	later return the response without needing to revalidate it (since
	it has already been authenticated once by that user), while still
	requiring proxies that service many users to revalidate each time
	(in order to make sure that each user has been authenticated).
	Note that such authenticated responses also need the public cache
	control directive in order to allow them to be cached at all.
	16.2.5. No-Transform Directive		max-age
			The max-age response directive indicates that response is to be
			considered stale after its age is greater than the specified
			number of seconds.
	no-transform		s-maxage
	Implementors of intermediate caches (proxies) have found it useful		The s-maxage response directive indicates that, in shared caches,
	to convert the media type of certain entity bodies. A non-		the maximum age specified by this directive overrides the maximum
	transparent proxy might, for example, convert between image		age specified by either the max-age directive or the Expires
	formats in order to save cache space or to reduce the amount of		header. The s-maxage directive also implies the semantics of the
	traffic on a slow link.		proxy-revalidate response directive.
	Serious operational problems occur, however, when these		no-transform
	transformations are applied to entity bodies intended for certain
	kinds of applications. For example, applications for medical
	imaging, scientific data analysis and those using end-to-end
	authentication, all depend on receiving an entity body that is bit
	for bit identical to the original entity-body.
	Therefore, if a message includes the no-transform directive, an		The no-transform response directive indicates that an intermediate
	intermediate cache or proxy MUST NOT change those headers that are		cache or proxy MUST NOT change the Content-Encoding, Content-Range
	listed in Section 7.2 as being subject to the no-transform		or Content-Type response headers, nor the response entity-body.
	directive. This implies that the cache or proxy MUST NOT change
	any aspect of the entity-body that is specified by these headers,
	including the value of the entity-body itself.
	16.2.6. Cache Control Extensions		3.2.3. Cache Control Extensions
	The Cache-Control header field can be extended through the use of one		The Cache-Control header field can be extended through the use of one
	or more cache-extension tokens, each with an optional assigned value.		or more cache-extension tokens, each with an optional value.
	Informational extensions (those which do not require a change in		Informational extensions (those that do not require a change in cache
	cache behavior) MAY be added without changing the semantics of other		behavior) can be added without changing the semantics of other
	directives. Behavioral extensions are designed to work by acting as		directives. Behavioral extensions are designed to work by acting as
	modifiers to the existing base of cache directives. Both the new		modifiers to the existing base of cache directives. Both the new
	directive and the standard directive are supplied, such that		directive and the standard directive are supplied, such that
	applications which do not understand the new directive will default		applications that do not understand the new directive will default to
	to the behavior specified by the standard directive, and those that		the behavior specified by the standard directive, and those that
	understand the new directive will recognize it as modifying the		understand the new directive will recognize it as modifying the
	requirements associated with the standard directive. In this way,		requirements associated with the standard directive. In this way,
	extensions to the cache-control directives can be made without		extensions to the cache-control directives can be made without
	requiring changes to the base protocol.		requiring changes to the base protocol.
	This extension mechanism depends on an HTTP cache obeying all of the		This extension mechanism depends on an HTTP cache obeying all of the
	cache-control directives defined for its native HTTP-version, obeying		cache-control directives defined for its native HTTP-version, obeying
	certain extensions, and ignoring all directives that it does not		certain extensions, and ignoring all directives that it does not
	understand.		understand.
	For example, consider a hypothetical new response directive called		For example, consider a hypothetical new response directive called
	community which acts as a modifier to the private directive. We		"community" that acts as a modifier to the private directive. We
	define this new directive to mean that, in addition to any non-shared		define this new directive to mean that, in addition to any non-shared
	cache, any cache which is shared only by members of the community		cache, any cache that is shared only by members of the community
	named within its value may cache the response. An origin server		named within its value may cache the response. An origin server
	wishing to allow the UCI community to use an otherwise private		wishing to allow the UCI community to use an otherwise private
	response in their shared cache(s) could do so by including		response in their shared cache(s) could do so by including
	Cache-Control: private, community="UCI"		Cache-Control: private, community="UCI"
	A cache seeing this header field will act correctly even if the cache		A cache seeing this header field will act correctly even if the cache
	does not understand the community cache-extension, since it will also		does not understand the community cache-extension, since it will also
	see and understand the private directive and thus default to the safe		see and understand the private directive and thus default to the safe
	behavior.		behavior.
	Unrecognized cache-directives MUST be ignored; it is assumed that any		Unrecognized cache directives MUST be ignored; it is assumed that any
	cache-directive likely to be unrecognized by an HTTP/1.1 cache will		cache directive likely to be unrecognized by an HTTP/1.1 cache will
	be combined with standard directives (or the response's default		be combined with standard directives (or the response's default
	cacheability) such that the cache behavior will remain minimally		cacheability) such that the cache behavior will remain minimally
	correct even if the cache does not understand the extension(s).		correct even if the cache does not understand the extension(s).
	16.3. Expires		3.3. Expires
	The Expires entity-header field gives the date/time after which the		The entity-header field "Expires" gives the date/time after which the
	response is considered stale. A stale cache entry may not normally		response is considered stale. See Section 2.3 for further discussion
	be returned by a cache (either a proxy cache or a user agent cache)		of the freshness model.
	unless it is first validated with the origin server (or with an
	intermediate cache that has a fresh copy of the entity). See
	Section 4 for further discussion of the expiration model.
	The presence of an Expires field does not imply that the original		The presence of an Expires field does not imply that the original
	resource will change or cease to exist at, before, or after that		resource will change or cease to exist at, before, or after that
	time.		time.
	The format is an absolute date and time as defined by HTTP-date in		The field-value is an absolute date and time as defined by HTTP-date
	Section 3.3.1 of [Part1]; it MUST be sent in rfc1123-date format.		in Section 3.2.1 of [Part1]; it MUST be sent in rfc1123-date format.
	Expires = "Expires" ":" OWS Expires-v		Expires = "Expires" ":" OWS Expires-v
	Expires-v = HTTP-date		Expires-v = HTTP-date
	An example of its use is		For example
	Expires: Thu, 01 Dec 1994 16:00:00 GMT		Expires: Thu, 01 Dec 1994 16:00:00 GMT
	Note: if a response includes a Cache-Control field with the max-		Note: if a response includes a Cache-Control field with the max-
	age directive (see Section 16.2.3), that directive overrides the		age directive (see Section 3.2.2), that directive overrides the
	Expires field.		Expires field. Likewise, the s-maxage directive overrides Expires
			in shared caches.
			HTTP/1.1 servers SHOULD NOT send Expires dates more than one year in
			the future.
	HTTP/1.1 clients and caches MUST treat other invalid date formats,		HTTP/1.1 clients and caches MUST treat other invalid date formats,
	especially including the value "0", as in the past (i.e., "already		especially including the value "0", as in the past (i.e., "already
	expired").		expired").
	To mark a response as "already expired," an origin server sends an		3.4. Pragma
	Expires date that is equal to the Date header value. (See the rules
	for expiration calculations in Section 4.4.)
	To mark a response as "never expires," an origin server sends an
	Expires date approximately one year from the time the response is
	sent. HTTP/1.1 servers SHOULD NOT send Expires dates more than one
	year in the future.
	The presence of an Expires header field with a date value of some
	time in the future on a response that otherwise would by default be
	non-cacheable indicates that the response is cacheable, unless
	indicated otherwise by a Cache-Control header field (Section 16.2).
	16.4. Pragma
	The general-header field "Pragma" is used to include implementation-		The general-header field "Pragma" is used to include implementation-
	specific directives that might apply to any recipient along the		specific directives that might apply to any recipient along the
	request/response chain. All pragma directives specify optional		request/response chain. All pragma directives specify optional
	behavior from the viewpoint of the protocol; however, some systems		behavior from the viewpoint of the protocol; however, some systems
	MAY require that behavior be consistent with the directives.		MAY require that behavior be consistent with the directives.
	Pragma = "Pragma" ":" OWS Pragma-v		Pragma = "Pragma" ":" OWS Pragma-v
	Pragma-v = 1#pragma-directive		Pragma-v = 1#pragma-directive
	pragma-directive = "no-cache" / extension-pragma		pragma-directive = "no-cache" / extension-pragma
	extension-pragma = token [ "=" ( token / quoted-string ) ]		extension-pragma = token [ "=" ( token / quoted-string ) ]
	When the no-cache directive is present in a request message, an		When the no-cache directive is present in a request message, an
	application SHOULD forward the request toward the origin server even		application SHOULD forward the request toward the origin server even
	if it has a cached copy of what is being requested. This pragma		if it has a cached copy of what is being requested. This pragma
	directive has the same semantics as the no-cache cache-directive (see		directive has the same semantics as the no-cache response directive
	Section 16.2) and is defined here for backward compatibility with		(see Section 3.2.2) and is defined here for backward compatibility
	HTTP/1.0. Clients SHOULD include both header fields when a no-cache		with HTTP/1.0. Clients SHOULD include both header fields when a no-
	request is sent to a server not known to be HTTP/1.1 compliant.		cache request is sent to a server not known to be HTTP/1.1 compliant.
	Pragma directives MUST be passed through by a proxy or gateway
	application, regardless of their significance to that application,
	since the directives might be applicable to all recipients along the
	request/response chain. It is not possible to specify a pragma for a
	specific recipient; however, any pragma directive not relevant to a
	recipient SHOULD be ignored by that recipient.
	HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had		HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had
	sent "Cache-Control: no-cache". No new Pragma directives will be		sent "Cache-Control: no-cache".
	defined in HTTP.
	Note: because the meaning of "Pragma: no-cache" as a response-		Note: because the meaning of "Pragma: no-cache" as a response-
	header field is not actually specified, it does not provide a		header field is not actually specified, it does not provide a
	reliable replacement for "Cache-Control: no-cache" in a response.		reliable replacement for "Cache-Control: no-cache" in a response.
	16.5. Vary		This mechanism is deprecated; no new Pragma directives will be
			defined in HTTP.
			3.5. Vary
	The "Vary" response-header field's value indicates the set of		The "Vary" response-header field's value indicates the set of
	request-header fields that fully determines, while the response is		request-header fields that determines, while the response is fresh,
	fresh, whether a cache is permitted to use the response to reply to a		whether a cache is permitted to use the response to reply to a
	subsequent request without revalidation. For uncacheable or stale		subsequent request without validation; see Section 2.6.
	responses, the Vary field value advises the user agent about the
	criteria that were used to select the representation. A Vary field		In uncacheable or stale responses, the Vary field value advises the
	value of "*" implies that a cache cannot determine from the request		user agent about the criteria that were used to select the
	headers of a subsequent request whether this response is the		representation.
	appropriate representation. See Section 8 for use of the Vary header
	field by caches.
	Vary = "Vary" ":" OWS Vary-v		Vary = "Vary" ":" OWS Vary-v
	Vary-v = "*" / 1#field-name		Vary-v = "*" / 1#field-name
	An HTTP/1.1 server SHOULD include a Vary header field with any		The set of header fields named by the Vary field value is known as
	cacheable response that is subject to server-driven negotiation.		the selecting request-headers.
	Doing so allows a cache to properly interpret future requests on that
	resource and informs the user agent about the presence of negotiation		Servers SHOULD include a Vary header field with any cacheable
	on that resource. A server MAY include a Vary header field with a		response that is subject to server-driven negotiation. Doing so
	non-cacheable response that is subject to server-driven negotiation,		allows a cache to properly interpret future requests on that resource
			and informs the user agent about the presence of negotiation on that
			resource. A server MAY include a Vary header field with a non-
			cacheable response that is subject to server-driven negotiation,
	since this might provide the user agent with useful information about		since this might provide the user agent with useful information about
	the dimensions over which the response varies at the time of the		the dimensions over which the response varies at the time of the
	response.		response.
	A Vary field value consisting of a list of field-names signals that		A Vary field value of "*" signals that unspecified parameters not
	the representation selected for the response is based on a selection		limited to the request-headers (e.g., the network address of the
	algorithm which considers ONLY the listed request-header field values		client), play a role in the selection of the response representation;
	in selecting the most appropriate representation. A cache MAY assume		therefore, a cache cannot determine whether this response is
	that the same selection will be made for future requests with the		appropriate. The "*" value MUST NOT be generated by a proxy server;
	same values for the listed field names, for the duration of time for		it may only be generated by an origin server.
	which the response is fresh.
	The field-names given are not limited to the set of standard request-		The field-names given are not limited to the set of standard request-
	header fields defined by this specification. Field names are case-		header fields defined by this specification. Field names are case-
	insensitive.		insensitive.
	A Vary field value of "*" signals that unspecified parameters not		3.6. Warning
	limited to the request-headers (e.g., the network address of the
	client), play a role in the selection of the response representation.
	The "*" value MUST NOT be generated by a proxy server; it may only be
	generated by an origin server.
	16.6. Warning
	The general-header field "Warning" is used to carry additional		The general-header field "Warning" is used to carry additional
	information about the status or transformation of a message which		information about the status or transformation of a message that
	might not be reflected in the message. This information is typically		might not be reflected in the message. This information is typically
	used to warn about a possible lack of semantic transparency from		used to warn about possible incorrectness introduced by caching
	caching operations or transformations applied to the entity body of		operations or transformations applied to the entity body of the
	the message.		message.
	Warning headers are sent with responses using:		Warnings can be used for other purposes, both cache-related and
			otherwise. The use of a warning, rather than an error status code,
			distinguish these responses from true failures.
			Warning headers can in general be applied to any message, however
			some warn-codes are specific to caches and can only be applied to
			response messages.
	Warning = "Warning" ":" OWS Warning-v		Warning = "Warning" ":" OWS Warning-v
	Warning-v = 1#warning-value		Warning-v = 1#warning-value
	warning-value = warn-code SP warn-agent SP warn-text		warning-value = warn-code SP warn-agent SP warn-text
	[SP warn-date]		[SP warn-date]
	warn-code = 3DIGIT		warn-code = 3DIGIT
	warn-agent = ( uri-host [ ":" port ] ) / pseudonym		warn-agent = ( uri-host [ ":" port ] ) / pseudonym
	; the name or pseudonym of the server adding		; the name or pseudonym of the server adding
	; the Warning header, for use in debugging		; the Warning header, for use in debugging
	warn-text = quoted-string		warn-text = quoted-string
	warn-date = DQUOTE HTTP-date DQUOTE		warn-date = DQUOTE HTTP-date DQUOTE
	A response MAY carry more than one Warning header.		Multiple warnings can be attached to a response (either by the origin
			server or by a cache), including multiple warnings with the same code
			number. For example, a server might provide the same warning with
			texts in both English and Basque.
			When this occurs, the user agent SHOULD inform the user of as many of
			them as possible, in the order that they appear in the response. If
			it is not possible to inform the user of all of the warnings, the
			user agent SHOULD follow these heuristics:
			o Warnings that appear early in the response take priority over
			those appearing later in the response.
			o Warnings in the user's preferred character set take priority over
			warnings in other character sets but with identical warn-codes and
			warn-agents.
			Systems that generate multiple Warning headers SHOULD order them with
			this user agent behavior in mind. New Warning headers SHOULD be
			added after any existing Warning headers.
			Warnings are assigned three digit warn-codes. The first digit
			indicates whether the Warning is required to be deleted from a stored
			response after validation:
			o 1xx Warnings that describe the freshness or validation status of
			the response, and so MUST be deleted by caches after validation.
			They MUST NOT be generated by a cache except when validating a
			cached entry, and MUST NOT be generated by clients.
			o 2xx Warnings that describe some aspect of the entity body or
			entity headers that is not rectified by a validation (for example,
			a lossy compression of the entity bodies) and MUST NOT be deleted
			by caches after validation, unless a full response is returned, in
			which case they MUST be.
	The warn-text SHOULD be in a natural language and character set that		The warn-text SHOULD be in a natural language and character set that
	is most likely to be intelligible to the human user receiving the		is most likely to be intelligible to the human user receiving the
	response. This decision MAY be based on any available knowledge,		response. This decision can be based on any available knowledge,
	such as the location of the cache or user, the Accept-Language field		such as the location of the cache or user, the Accept-Language field
	in a request, the Content-Language field in a response, etc. The		in a request, the Content-Language field in a response, etc. The
	default language is English and the default character set is ISO-		default language is English and the default character set is ISO-
	8859-1 ([ISO-8859-1]).		8859-1 ([ISO-8859-1]).
	If a character set other than ISO-8859-1 is used, it MUST be encoded		If a character set other than ISO-8859-1 is used, it MUST be encoded
	in the warn-text using the method described in [RFC2047].		in the warn-text using the method described in [RFC2047].
	Warning headers can in general be applied to any message, however		If an implementation sends a message with one or more Warning headers
	some specific warn-codes are specific to caches and can only be		to a receiver whose version is HTTP/1.0 or lower, then the sender
	applied to response messages. New Warning headers SHOULD be added		MUST include in each warning-value a warn-date that matches the Date
	after any existing Warning headers. A cache MUST NOT delete any		header in the message.
	Warning header that it received with a message. However, if a cache
	successfully validates a cache entry, it SHOULD remove any Warning
	headers previously attached to that entry except as specified for
	specific Warning codes. It MUST then add any Warning headers
	received in the validating response. In other words, Warning headers
	are those that would be attached to the most recent relevant
	response.
	When multiple Warning headers are attached to a response, the user
	agent ought to inform the user of as many of them as possible, in the
	order that they appear in the response. If it is not possible to
	inform the user of all of the warnings, the user agent SHOULD follow
	these heuristics:
	o Warnings that appear early in the response take priority over
	those appearing later in the response.
	o Warnings in the user's preferred character set take priority over
	warnings in other character sets but with identical warn-codes and
	warn-agents.
	Systems that generate multiple Warning headers SHOULD order them with
	this user agent behavior in mind.
	Requirements for the behavior of caches with respect to Warnings are		If an implementation receives a message with a warning-value that
	stated in Section 3.2.		includes a warn-date, and that warn-date is different from the Date
			value in the response, then that warning-value MUST be deleted from
			the message before storing, forwarding, or using it. (preventing the
			consequences of naive caching of Warning header fields.) If all of
			the warning-values are deleted for this reason, the Warning header
			MUST be deleted as well.
	This is a list of the currently-defined warn-codes, each with a		The following warn-codes are defined by this specification, each with
	recommended warn-text in English, and a description of its meaning.		a recommended warn-text in English, and a description of its meaning.
	110 Response is stale		110 Response is stale
	MUST be included whenever the returned response is stale.		SHOULD be included whenever the returned response is stale.
	111 Revalidation failed		111 Revalidation failed
	MUST be included if a cache returns a stale response because an		SHOULD be included if a cache returns a stale response because an
	attempt to revalidate the response failed, due to an inability to		attempt to validate the response failed, due to an inability to
	reach the server.		reach the server.
	112 Disconnected operation		112 Disconnected operation
	SHOULD be included if the cache is intentionally disconnected from		SHOULD be included if the cache is intentionally disconnected from
	the rest of the network for a period of time.		the rest of the network for a period of time.
	113 Heuristic expiration		113 Heuristic expiration
	MUST be included if the cache heuristically chose a freshness		SHOULD be included if the cache heuristically chose a freshness
	lifetime greater than 24 hours and the response's age is greater		lifetime greater than 24 hours and the response's age is greater
	than 24 hours.		than 24 hours.
	199 Miscellaneous warning		199 Miscellaneous warning
	The warning text MAY include arbitrary information to be presented		The warning text can include arbitrary information to be presented
	to a human user, or logged. A system receiving this warning MUST		to a human user, or logged. A system receiving this warning MUST
	NOT take any automated action, besides presenting the warning to		NOT take any automated action, besides presenting the warning to
	the user.		the user.
	214 Transformation applied		214 Transformation applied
	MUST be added by an intermediate cache or proxy if it applies any		MUST be added by an intermediate cache or proxy if it applies any
	transformation changing the content-coding (as specified in the		transformation changing the content-coding (as specified in the
	Content-Encoding header) or media-type (as specified in the		Content-Encoding header) or media-type (as specified in the
	Content-Type header) of the response, or the entity-body of the		Content-Type header) of the response, or the entity-body of the
	response, unless this Warning code already appears in the		response, unless this Warning code already appears in the
	response.		response.
	299 Miscellaneous persistent warning		299 Miscellaneous persistent warning
	The warning text MAY include arbitrary information to be presented		The warning text can include arbitrary information to be presented
	to a human user, or logged. A system receiving this warning MUST		to a human user, or logged. A system receiving this warning MUST
	NOT take any automated action.		NOT take any automated action.
	If an implementation sends a message with one or more Warning headers		4. History Lists
	whose version is HTTP/1.0 or lower, then the sender MUST include in
	each warning-value a warn-date that matches the date in the response.
	If an implementation receives a message with a warning-value that		User agents often have history mechanisms, such as "Back" buttons and
	includes a warn-date, and that warn-date is different from the Date		history lists, that can be used to redisplay an entity retrieved
	value in the response, then that warning-value MUST be deleted from		earlier in a session.
	the message before storing, forwarding, or using it. (This prevents
	bad consequences of naive caching of Warning header fields.) If all
	of the warning-values are deleted for this reason, the Warning header
	MUST be deleted as well.
	17. IANA Considerations		History mechanisms and caches are different. In particular history
			mechanisms SHOULD NOT try to show a correct view of the current state
			of a resource. Rather, a history mechanism is meant to show exactly
			what the user saw at the time when the resource was retrieved.
	17.1. Message Header Registration		By default, an expiration time does not apply to history mechanisms.
			If the entity is still in storage, a history mechanism SHOULD display
			it even if the entity has expired, unless the user has specifically
			configured the agent to refresh expired history documents.
			This is not to be construed to prohibit the history mechanism from
			telling the user that a view might be stale.
			Note: if history list mechanisms unnecessarily prevent users from
			viewing stale resources, this will tend to force service authors
			to avoid using HTTP expiration controls and cache controls when
			they would otherwise like to. Service authors may consider it
			important that users not be presented with error messages or
			warning messages when they use navigation controls (such as BACK)
			to view previously fetched resources. Even though sometimes such
			resources ought not be cached, or ought to expire quickly, user
			interface considerations may force service authors to resort to
			other means of preventing caching (e.g. "once-only" URLs) in order
			not to suffer the effects of improperly functioning history
			mechanisms.
			5. IANA Considerations
			5.1. Message Header Registration
	The Message Header Registry located at <http://www.iana.org/		The Message Header Registry located at <http://www.iana.org/
	assignments/message-headers/message-header-index.html> should be		assignments/message-headers/message-header-index.html> should 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 |
	+-------------------+----------+----------+--------------+		+-------------------+----------+----------+-------------+
	| Age | http | standard | Section 16.1 |		| Age | http | standard | Section 3.1 |
	| Cache-Control | http | standard | Section 16.2 |		| Cache-Control | http | standard | Section 3.2 |
	| Expires | http | standard | Section 16.3 |		| Expires | http | standard | Section 3.3 |
	| Pragma | http | standard | Section 16.4 |		| Pragma | http | standard | Section 3.4 |
	| Vary | http | standard | Section 16.5 |		| Vary | http | standard | Section 3.5 |
	| Warning | http | standard | Section 16.6 |		| Warning | http | standard | Section 3.6 |
	+-------------------+----------+----------+--------------+		+-------------------+----------+----------+-------------+
	The change controller is: "IETF (iesg@ietf.org) - Internet		The change controller is: "IETF (iesg@ietf.org) - Internet
	Engineering Task Force".		Engineering Task Force".
	18. Security Considerations		6. Security Considerations
	Caching proxies provide additional potential vulnerabilities, since		Caches expose additional potential vulnerabilities, since the
	the contents of the cache represent an attractive target for		contents of the cache represent an attractive target for malicious
	malicious exploitation. Because cache contents persist after an HTTP		exploitation. Because cache contents persist after an HTTP request
	request is complete, an attack on the cache can reveal information		is complete, an attack on the cache can reveal information long after
	long after a user believes that the information has been removed from		a user believes that the information has been removed from the
	the network. Therefore, cache contents should be protected as		network. Therefore, cache contents should be protected as sensitive
	sensitive information.		information.
	19. Acknowledgments		7. Acknowledgments
	Much of the content and presentation of the caching design is due to		Much of the content and presentation of the caching design is due to
	suggestions and comments from individuals including: Shel Kaphan,		suggestions and comments from individuals including: Shel Kaphan,
	Paul Leach, Koen Holtman, David Morris, and Larry Masinter.		Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
	20. References		8. References
	20.1. Normative References		8.1. Normative References
	[ISO-8859-1]		[ISO-8859-1]
	International Organization for Standardization,		International Organization for Standardization,
	"Information technology -- 8-bit single-byte coded graphic		"Information technology -- 8-bit single-byte coded graphic
	character sets -- Part 1: Latin alphabet No. 1", ISO/		character sets -- Part 1: Latin alphabet No. 1", ISO/
	IEC 8859-1:1998, 1998.		IEC 8859-1:1998, 1998.
	[Part1] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,		[Part1] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
	Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,		Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
	and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections,		and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections,
	and Message Parsing", draft-ietf-httpbis-p1-messaging-05		and Message Parsing", draft-ietf-httpbis-p1-messaging-06
	(work in progress), November 2008.		(work in progress), March 2009.
	[Part2] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,		[Part2] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
	Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,		Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
	and J. Reschke, Ed., "HTTP/1.1, part 2: Message		and J. Reschke, Ed., "HTTP/1.1, part 2: Message
	Semantics", draft-ietf-httpbis-p2-semantics-05 (work in		Semantics", draft-ietf-httpbis-p2-semantics-06 (work in
	progress), November 2008.		progress), March 2009.
	[Part3] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,		[Part3] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
	Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,		Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
	and J. Reschke, Ed., "HTTP/1.1, part 3: Message Payload		and J. Reschke, Ed., "HTTP/1.1, part 3: Message Payload
	and Content Negotiation", draft-ietf-httpbis-p3-payload-05		and Content Negotiation", draft-ietf-httpbis-p3-payload-06
	(work in progress), November 2008.		(work in progress), March 2009.
	[Part4] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,		[Part4] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
	Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,		Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
	and J. Reschke, Ed., "HTTP/1.1, part 4: Conditional		and J. Reschke, Ed., "HTTP/1.1, part 4: Conditional
	Requests", draft-ietf-httpbis-p4-conditional-05 (work in		Requests", draft-ietf-httpbis-p4-conditional-06 (work in
	progress), November 2008.		progress), March 2009.
	[Part5] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,		[Part5] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
	Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,		Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
	and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and		and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and
	Partial Responses", draft-ietf-httpbis-p5-range-05 (work		Partial Responses", draft-ietf-httpbis-p5-range-06 (work
	in progress), November 2008.		in progress), March 2009.
	[Part7] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,		[Part7] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
	Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,		Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
	and J. Reschke, Ed., "HTTP/1.1, part 7: Authentication",		and J. Reschke, Ed., "HTTP/1.1, part 7: Authentication",
	draft-ietf-httpbis-p7-auth-05 (work in progress),		draft-ietf-httpbis-p7-auth-06 (work in progress),
	November 2008.		March 2009.
	[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)		[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
	Part Three: Message Header Extensions for Non-ASCII Text",		Part Three: Message Header Extensions for Non-ASCII Text",
	RFC 2047, November 1996.		RFC 2047, November 1996.
	[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.
	20.2. Informative References		[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
			Specifications: ABNF", STD 68, RFC 5234, January 2008.
			8.2. Informative References
	[RFC1305] Mills, D., "Network Time Protocol (Version 3)		[RFC1305] Mills, D., "Network Time Protocol (Version 3)
	Specification, Implementation", RFC 1305, March 1992.		Specification, Implementation", RFC 1305, March 1992.
	[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.
	Appendix A. Compatibility with Previous Versions		Appendix A. Compatibility with Previous Versions
	A.1. Changes from RFC 2068		A.1. Changes from RFC 2068
	A case was missed in the Cache-Control model of HTTP/1.1; s-maxage		A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
	was introduced to add this missing case. (Sections 6, 16.2, 16.2.3)		was introduced to add this missing case. (Sections 2.1, 3.2).
	Transfer-coding and message lengths all interact in ways that		Transfer-coding and message lengths all interact in ways that
	required fixing exactly when chunked encoding is used (to allow for		required fixing exactly when chunked encoding is used (to allow for
	transfer encoding that may not be self delimiting); it was important		transfer encoding that may not be self delimiting); it was important
	to straighten out exactly how message lengths are computed.		to straighten out exactly how message lengths are computed. (see also
	(Section 7.2, see also [Part1], [Part3] and [Part5])		[Part1], [Part3] and [Part5]) [[anchor18: This used to refer to the
			text about non-modifiable headers, and will have to be updated later
			on. --jre]]
	Proxies should be able to add Content-Length when appropriate.		Proxies should be able to add Content-Length when appropriate.
	(Section 7.2)		[[anchor19: This used to refer to the text about non-modifiable
			headers, and will have to be updated later on. --jre]]
	Range request responses would become very verbose if all meta-data		Range request responses would become very verbose if all meta-data
	were always returned; by allowing the server to only send needed		were always returned; by allowing the server to only send needed
	headers in a 206 response, this problem can be avoided.		headers in a 206 response, this problem can be avoided.
	(Section 7.3)		(Section 2.7)
	The Cache-Control: max-age directive was not properly defined for		The Cache-Control: max-age directive was not properly defined for
	responses. (Section 16.2.3)		responses. (Section 3.2.2)
	Warnings could be cached incorrectly, or not updated appropriately.		Warnings could be cached incorrectly, or not updated appropriately.
	(Section 3.2, 4.4, 7.2, 7.3, 16.2.3, and 16.6) Warning also needed to		(Section 2.3, 2.7, 3.2, and 3.6) Warning also needed to be a general
	be a general header, as PUT or other methods may have need for it in		header, as PUT or other methods may have need for it in requests.
	requests.
	A.2. Changes from RFC 2616		A.2. Changes from RFC 2616
	Clarify denial of service attack avoidance requirement. (Section 12)		Clarify denial of service attack avoidance requirement.
			(Section 2.5)
	Appendix B. Change Log (to be removed by RFC Editor before publication)		Appendix B. Collected ABNF
	B.1. Since RFC2616		Age = "Age:" OWS Age-v
			Age-v = delta-seconds
			Cache-Control = "Cache-Control:" OWS Cache-Control-v
			Cache-Control-v = *( "," OWS ) cache-directive *( OWS "," [ OWS
			cache-directive ] )
			Expires = "Expires:" OWS Expires-v
			Expires-v = HTTP-date
			HTTP-date = <HTTP-date, defined in [Part1], Section 3.2.1>
			OWS = <OWS, defined in [Part1], Section 1.2.2>
			Pragma = "Pragma:" OWS Pragma-v
			Pragma-v = *( "," OWS ) pragma-directive *( OWS "," [ OWS
			pragma-directive ] )
			Vary = "Vary:" OWS Vary-v
			Vary-v = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
			] ) )
			Warning = "Warning:" OWS Warning-v
			Warning-v = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
			] )
			cache-directive = cache-request-directive / cache-response-directive
			cache-extension = token [ "=" ( token / quoted-string ) ]
			cache-request-directive = "no-cache" / "no-store" / ( "max-age="
			delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
			"min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
			cache-extension
			cache-response-directive = "public" / ( "private" [ "=" DQUOTE *( ","
			OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
			"no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
			field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
			"must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
			) / ( "s-maxage=" delta-seconds ) / cache-extension
			delta-seconds = 1*DIGIT
			extension-pragma = token [ "=" ( token / quoted-string ) ]
			field-name = <field-name, defined in [Part1], Section 4.2>
			port = <port, defined in [Part1], Section 2.1>
			pragma-directive = "no-cache" / extension-pragma
			pseudonym = <pseudonym, defined in [Part1], Section 8.9>
			quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
			token = <token, defined in [Part1], Section 1.2.2>
			uri-host = <uri-host, defined in [Part1], Section 2.1>
			warn-agent = ( uri-host [ ":" port ] ) / pseudonym
			warn-code = 3DIGIT
			warn-date = DQUOTE HTTP-date DQUOTE
			warn-text = quoted-string
			warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
			]
			ABNF diagnostics:
			; Age defined but not used
			; Cache-Control defined but not used
			; Expires defined but not used
			; Pragma defined but not used
			; Vary defined but not used
			; Warning defined but not used
			Appendix C. Change Log (to be removed by RFC Editor before publication)
			C.1. Since RFC2616
	Extracted relevant partitions from [RFC2616].		Extracted relevant partitions from [RFC2616].
	B.2. Since draft-ietf-httpbis-p6-cache-00		C.2. Since draft-ietf-httpbis-p6-cache-00
	Closed issues:		Closed issues:
	o <http://tools.ietf.org/wg/httpbis/trac/ticket/9>: "Trailer"		o <http://tools.ietf.org/wg/httpbis/trac/ticket/9>: "Trailer"
	(<http://purl.org/NET/http-errata#trailer-hop>)		(<http://purl.org/NET/http-errata#trailer-hop>)
	o <http://tools.ietf.org/wg/httpbis/trac/ticket/12>: "Invalidation		o <http://tools.ietf.org/wg/httpbis/trac/ticket/12>: "Invalidation
	after Update or Delete"		after Update or Delete"
	(<http://purl.org/NET/http-errata#invalidupd>)		(<http://purl.org/NET/http-errata#invalidupd>)
	skipping to change at page 46, line 18		skipping to change at page 34, line 21
	to-date references"		to-date references"
	o <http://tools.ietf.org/wg/httpbis/trac/ticket/87>: "typo in		o <http://tools.ietf.org/wg/httpbis/trac/ticket/87>: "typo in
	13.2.2"		13.2.2"
	Other changes:		Other changes:
	o Use names of RFC4234 core rules DQUOTE and HTAB (work in progress		o Use names of RFC4234 core rules DQUOTE and HTAB (work in progress
	on <http://tools.ietf.org/wg/httpbis/trac/ticket/36>)		on <http://tools.ietf.org/wg/httpbis/trac/ticket/36>)
	B.3. Since draft-ietf-httpbis-p6-cache-01		C.3. Since draft-ietf-httpbis-p6-cache-01
	Closed issues:		Closed issues:
	o <http://tools.ietf.org/wg/httpbis/trac/ticket/82>: "rel_path not		o <http://tools.ietf.org/wg/httpbis/trac/ticket/82>: "rel_path not
	used"		used"
	Other changes:		Other changes:
	o Get rid of duplicate BNF rule names ("host" -> "uri-host") (work		o Get rid of duplicate BNF rule names ("host" -> "uri-host") (work
	in progress on <http://tools.ietf.org/wg/httpbis/trac/ticket/36>)		in progress on <http://tools.ietf.org/wg/httpbis/trac/ticket/36>)
	o Add explicit references to BNF syntax and rules imported from		o Add explicit references to BNF syntax and rules imported from
	other parts of the specification.		other parts of the specification.
	B.4. Since draft-ietf-httpbis-p6-cache-02		C.4. Since draft-ietf-httpbis-p6-cache-02
	Ongoing work on IANA Message Header Registration		Ongoing work on IANA Message Header Registration
	(<http://tools.ietf.org/wg/httpbis/trac/ticket/40>):		(<http://tools.ietf.org/wg/httpbis/trac/ticket/40>):
	o Reference RFC 3984, and update header registrations for headers		o Reference RFC 3984, and update header registrations for headers
	defined in this document.		defined in this document.
	B.5. Since draft-ietf-httpbis-p6-cache-03		C.5. Since draft-ietf-httpbis-p6-cache-03
	Closed issues:		Closed issues:
	o <http://tools.ietf.org/wg/httpbis/trac/ticket/106>: "Vary header		o <http://tools.ietf.org/wg/httpbis/trac/ticket/106>: "Vary header
	classification"		classification"
	B.6. Since draft-ietf-httpbis-p6-cache-04		C.6. Since draft-ietf-httpbis-p6-cache-04
	Ongoing work on ABNF conversion		Ongoing work on ABNF conversion
	(<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):		(<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):
	o Use "/" instead of "|" for alternatives.		o Use "/" instead of "|" for alternatives.
	o Introduce new ABNF rules for "bad" whitespace ("BWS"), optional		o Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
	whitespace ("OWS") and required whitespace ("RWS").		whitespace ("OWS") and required whitespace ("RWS").
	o Rewrite ABNFs to spell out whitespace rules, factor out header		o Rewrite ABNFs to spell out whitespace rules, factor out header
	value format definitions.		value format definitions.
			C.7. Since draft-ietf-httpbis-p6-cache-05
			This is a total rewrite of this part of the specification.
			Affected issues:
			o <http://tools.ietf.org/wg/httpbis/trac/ticket/54>: "Definition of
			1xx Warn-Codes"
			o <http://trac.tools.ietf.org/wg/httpbis/trac/ticket/60>: "Placement
			of 13.5.1 and 13.5.2"
			o <http://trac.tools.ietf.org/wg/httpbis/trac/ticket/138>: "The role
			of Warning and Semantic Transparency in Caching"
			o <http://trac.tools.ietf.org/wg/httpbis/trac/ticket/139>: "Methods
			and Caching"
			In addition: 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.
	Index		Index
	A		A
	age 7		age 6
	Age header 27		Age header 17
	C		C
	cache 5		cache 5
	Cache Directives		Cache Directives
	max-age 32-33		max-age 18, 21
	max-stale 32		max-stale 19
	min-fresh 32		min-fresh 19
	must-revalidate 34		must-revalidate 21
	no-cache 29		no-cache 18, 20
	no-store 30		no-store 18, 21
	no-transform 35		no-transform 19, 22
	only-if-cached 34		only-if-cached 19
	private 29		private 20
	proxy-revalidate 35		proxy-revalidate 21
	public 29		public 20
	s-maxage 31		s-maxage 22
	Cache-Control header 27		Cache-Control header 17
	cacheable 6		cacheable 5
	E		E
	Expires header 37		Expires header 23
	explicit expiration time 7		explicit expiration time 5
	F		F
	first-hand 6		first-hand 6
	fresh 7		fresh 6
	freshness lifetime 7		freshness lifetime 6
	G		G
	Grammar		Grammar
	Age 27		Age 17
	Age-v 27		Age-v 17
	Cache-Control 28		Cache-Control 18
	Cache-Control-v 28		Cache-Control-v 18
	cache-directive 28		cache-extension 18
	cache-extension 28		cache-request-directive 18
	cache-request-directive 28		cache-response-directive 20
	cache-response-directive 28		delta-seconds 17
	delta-seconds 27		Expires 23
	Expires 37		Expires-v 23
	Expires-v 37		extension-pragma 24
	extension-pragma 38		Pragma 24
	Pragma 38		pragma-directive 24
	pragma-directive 38		Pragma-v 24
	Pragma-v 38		Vary 24
	Vary 39		Vary-v 24
	Vary-v 39		warn-agent 25
	warn-agent 40		warn-code 25
	warn-code 40		warn-date 25
	warn-date 40		warn-text 25
	warn-text 40		Warning 25
	Warning 40		Warning-v 25
	Warning-v 40		warning-value 25
	warning-value 40
	H		H
	Headers		Headers
	Age 27		Age 17
	Cache-Control 27		Cache-Control 17
	Expires 37		Expires 23
	Pragma 38		Pragma 23
	Vary 38		Vary 24
	Warning 39		Warning 25
	heuristic expiration time 7		heuristic expiration time 5
	M		M
	max-age		max-age
	Cache Directive 32-33		Cache Directive 18, 21
	max-stale		max-stale
	Cache Directive 32		Cache Directive 19
	min-fresh		min-fresh
	Cache Directive 32		Cache Directive 19
	must-revalidate		must-revalidate
	Cache Directive 34		Cache Directive 21
	N		N
	no-cache		no-cache
	Cache Directive 29		Cache Directive 18, 20
	no-store		no-store
	Cache Directive 30		Cache Directive 18, 21
	no-transform		no-transform
	Cache Directive 35		Cache Directive 19, 22
	O		O
	only-if-cached		only-if-cached
	Cache Directive 34		Cache Directive 19
	P		P
	Pragma header 38		Pragma header 23
	private		private
	Cache Directive 29		Cache Directive 20
	proxy-revalidate		proxy-revalidate
	Cache Directive 35		Cache Directive 21
	public		public
	Cache Directive 29		Cache Directive 20
	S		S
	s-maxage		s-maxage
	Cache Directive 31		Cache Directive 22
	semantically transparent 5		stale 6
	stale 7
	V		V
	validator 7		validator 6
	Vary header 38		Vary header 24
	W		W
	Warning header 39		Warning header 25
	Authors' Addresses		Authors' Addresses
	Roy T. Fielding (editor)		Roy T. Fielding (editor)
	Day Software		Day Software
	23 Corporate Plaza DR, Suite 280		23 Corporate Plaza DR, Suite 280
	Newport Beach, CA 92660		Newport Beach, CA 92660
	USA		USA
	Phone: +1-949-706-5300		Phone: +1-949-706-5300
	skipping to change at page 52, line 4		skipping to change at line 1775
	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		Phone: +49 251 2807760
	Fax: +49 251 2807761		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/
	Full Copyright Statement
	Copyright (C) The IETF Trust (2008).
	This document is subject to the rights, licenses and restrictions
	contained in BCP 78, and except as set forth therein, the authors
	retain all their rights.
	This document and the information contained herein are provided on an
	"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
	OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
	THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
	OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
	THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
	WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
	Intellectual Property
	The IETF takes no position regarding the validity or scope of any
	Intellectual Property Rights or other rights that might be claimed to
	pertain to the implementation or use of the technology described in
	this document or the extent to which any license under such rights
	might or might not be available; nor does it represent that it has
	made any independent effort to identify any such rights. Information
	on the procedures with respect to rights in RFC documents can be
	found in BCP 78 and BCP 79.
	Copies of IPR disclosures made to the IETF Secretariat and any
	assurances of licenses to be made available, or the result of an
	attempt made to obtain a general license or permission for the use of
	such proprietary rights by implementers or users of this
	specification can be obtained from the IETF on-line IPR repository at
	http://www.ietf.org/ipr.
	The IETF invites any interested party to bring to its attention any
	copyrights, patents or patent applications, or other proprietary
	rights that may cover technology that may be required to implement
	this standard. Please address the information to the IETF at
	ietf-ipr@ietf.org.
End of changes. 254 change blocks.
	1510 lines changed or deleted		957 lines changed or added
This html diff was produced by rfcdiff 1.35. The latest version is available from http://tools.ietf.org/tools/rfcdiff/