draft-ietf-httpbis-cache-00.txt   draft-ietf-httpbis-cache-01.txt 
HTTP Working Group R. Fielding, Ed. HTTP Working Group R. Fielding, Ed.
Internet-Draft Adobe Internet-Draft Adobe
Obsoletes: 7234 (if approved) M. Nottingham, Ed. Obsoletes: 7234 (if approved) M. Nottingham, Ed.
Intended status: Standards Track Fastly Intended status: Standards Track Fastly
Expires: October 5, 2018 J. Reschke, Ed. Expires: December 2, 2018 J. Reschke, Ed.
greenbytes greenbytes
April 3, 2018 May 31, 2018
Hypertext Transfer Protocol (HTTP): Caching HTTP Caching
draft-ietf-httpbis-cache-00 draft-ietf-httpbis-cache-01
Abstract Abstract
The Hypertext Transfer Protocol (HTTP) is a stateless application- The Hypertext Transfer Protocol (HTTP) is a stateless application-
level protocol for distributed, collaborative, hypertext information level protocol for distributed, collaborative, hypertext information
systems. This document defines HTTP caches and the associated header systems. This document defines HTTP caches and the associated header
fields that control cache behavior or indicate cacheable response fields that control cache behavior or indicate cacheable response
messages. messages.
This document obsoletes RFC 7234. This document obsoletes RFC 7234.
Editorial Note Editorial Note
This note is to be removed before publishing as an RFC. This note is to be removed before publishing as an RFC.
Discussion of this draft takes place on the HTTP working group Discussion of this draft takes place on the HTTP working group
mailing list (ietf-http-wg@w3.org), which is archived at mailing list (ietf-http-wg@w3.org), which is archived at
<http://lists.w3.org/Archives/Public/ietf-http-wg/>. <https://lists.w3.org/Archives/Public/ietf-http-wg/>.
Working Group information can be found at <http://httpwg.github.io/>; Working Group information can be found at <https://httpwg.org/>;
source code and issues list for this draft can be found at source code and issues list for this draft can be found at
<https://github.com/httpwg/http-core>. <https://github.com/httpwg/http-core>.
The changes in this draft are summarized in Appendix D.1. The changes in this draft are summarized in Appendix C.2.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 5, 2018. This Internet-Draft will expire on December 2, 2018.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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Without obtaining an adequate license from the person(s) controlling Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other it for publication as an RFC or to translate it into languages other
than English. than English.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Conformance and Error Handling . . . . . . . . . . . . . 4 1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 5
1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 4 1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 5
1.2.1. Delta Seconds . . . . . . . . . . . . . . . . . . . . 5 1.3. Delta Seconds . . . . . . . . . . . . . . . . . . . . . . 5
2. Overview of Cache Operation . . . . . . . . . . . . . . . . . 5 2. Overview of Cache Operation . . . . . . . . . . . . . . . . . 6
3. Storing Responses in Caches . . . . . . . . . . . . . . . . . 6 3. Storing Responses in Caches . . . . . . . . . . . . . . . . . 7
3.1. Storing Incomplete Responses . . . . . . . . . . . . . . 7 3.1. Storing Incomplete Responses . . . . . . . . . . . . . . 8
3.2. Storing Responses to Authenticated Requests . . . . . . . 7 3.2. Storing Responses to Authenticated Requests . . . . . . . 8
3.3. Combining Partial Content . . . . . . . . . . . . . . . . 8 3.3. Combining Partial Content . . . . . . . . . . . . . . . . 8
4. Constructing Responses from Caches . . . . . . . . . . . . . 8 4. Constructing Responses from Caches . . . . . . . . . . . . . 9
4.1. Calculating Secondary Keys with Vary . . . . . . . . . . 9 4.1. Calculating Secondary Keys with Vary . . . . . . . . . . 10
4.2. Freshness . . . . . . . . . . . . . . . . . . . . . . . . 10 4.2. Freshness . . . . . . . . . . . . . . . . . . . . . . . . 11
4.2.1. Calculating Freshness Lifetime . . . . . . . . . . . 12 4.2.1. Calculating Freshness Lifetime . . . . . . . . . . . 12
4.2.2. Calculating Heuristic Freshness . . . . . . . . . . . 12 4.2.2. Calculating Heuristic Freshness . . . . . . . . . . . 13
4.2.3. Calculating Age . . . . . . . . . . . . . . . . . . . 13 4.2.3. Calculating Age . . . . . . . . . . . . . . . . . . . 14
4.2.4. Serving Stale Responses . . . . . . . . . . . . . . . 15 4.2.4. Serving Stale Responses . . . . . . . . . . . . . . . 15
4.3. Validation . . . . . . . . . . . . . . . . . . . . . . . 15 4.3. Validation . . . . . . . . . . . . . . . . . . . . . . . 16
4.3.1. Sending a Validation Request . . . . . . . . . . . . 15 4.3.1. Sending a Validation Request . . . . . . . . . . . . 16
4.3.2. Handling a Received Validation Request . . . . . . . 16 4.3.2. Handling a Received Validation Request . . . . . . . 17
4.3.3. Handling a Validation Response . . . . . . . . . . . 17 4.3.3. Handling a Validation Response . . . . . . . . . . . 18
4.3.4. Freshening Stored Responses upon Validation . . . . . 18 4.3.4. Freshening Stored Responses upon Validation . . . . . 18
4.3.5. Freshening Responses via HEAD . . . . . . . . . . . . 19 4.3.5. Freshening Responses via HEAD . . . . . . . . . . . . 19
4.4. Invalidation . . . . . . . . . . . . . . . . . . . . . . 19 4.4. Invalidation . . . . . . . . . . . . . . . . . . . . . . 20
5. Header Field Definitions . . . . . . . . . . . . . . . . . . 20 5. Header Field Definitions . . . . . . . . . . . . . . . . . . 21
5.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 5.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 21 5.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 22
5.2.1. Request Cache-Control Directives . . . . . . . . . . 21 5.2.1. Request Cache-Control Directives . . . . . . . . . . 23
5.2.2. Response Cache-Control Directives . . . . . . . . . . 23 5.2.2. Response Cache-Control Directives . . . . . . . . . . 25
5.2.3. Cache Control Extensions . . . . . . . . . . . . . . 26 5.2.3. Cache Control Extensions . . . . . . . . . . . . . . 28
5.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 27 5.2.4. Cache Directive Registry . . . . . . . . . . . . . . 29
5.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . 28 5.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 29
5.5. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 29 5.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . 30
5.5.1. Warning: 110 - "Response is Stale" . . . . . . . . . 31 5.5. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 31
5.5.2. Warning: 111 - "Revalidation Failed" . . . . . . . . 31 5.5.1. Warning: 110 - "Response is Stale" . . . . . . . . . 33
5.5.3. Warning: 112 - "Disconnected Operation" . . . . . . . 31 5.5.2. Warning: 111 - "Revalidation Failed" . . . . . . . . 33
5.5.4. Warning: 113 - "Heuristic Expiration" . . . . . . . . 31 5.5.3. Warning: 112 - "Disconnected Operation" . . . . . . . 33
5.5.5. Warning: 199 - "Miscellaneous Warning" . . . . . . . 31 5.5.4. Warning: 113 - "Heuristic Expiration" . . . . . . . . 34
5.5.6. Warning: 214 - "Transformation Applied" . . . . . . . 31 5.5.5. Warning: 199 - "Miscellaneous Warning" . . . . . . . 34
5.5.7. Warning: 299 - "Miscellaneous Persistent Warning" . . 31 5.5.6. Warning: 214 - "Transformation Applied" . . . . . . . 34
6. History Lists . . . . . . . . . . . . . . . . . . . . . . . . 32 5.5.7. Warning: 299 - "Miscellaneous Persistent Warning" . . 34
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32 5.5.8. Warn Code Registry . . . . . . . . . . . . . . . . . 34
7.1. Cache Directive Registry . . . . . . . . . . . . . . . . 32 6. History Lists . . . . . . . . . . . . . . . . . . . . . . . . 34
7.1.1. Procedure . . . . . . . . . . . . . . . . . . . . . . 32 7. Security Considerations . . . . . . . . . . . . . . . . . . . 35
7.1.2. Considerations for New Cache Control Directives . . . 32 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36
7.1.3. Registrations . . . . . . . . . . . . . . . . . . . . 33 8.1. Header Field Registration . . . . . . . . . . . . . . . . 36
7.2. Warn Code Registry . . . . . . . . . . . . . . . . . . . 33 8.2. Cache Directive Registration . . . . . . . . . . . . . . 36
7.2.1. Procedure . . . . . . . . . . . . . . . . . . . . . . 33 8.3. Warn Code Registration . . . . . . . . . . . . . . . . . 36
7.2.2. Registrations . . . . . . . . . . . . . . . . . . . . 34 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 36
7.3. Header Field Registration . . . . . . . . . . . . . . . . 34 9.1. Normative References . . . . . . . . . . . . . . . . . . 36
8. Security Considerations . . . . . . . . . . . . . . . . . . . 34 9.2. Informative References . . . . . . . . . . . . . . . . . 37
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 35 Appendix A. Collected ABNF . . . . . . . . . . . . . . . . . . . 38
9.1. Normative References . . . . . . . . . . . . . . . . . . 35 Appendix B. Changes from RFC 7234 . . . . . . . . . . . . . . . 39
9.2. Informative References . . . . . . . . . . . . . . . . . 36 Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 39
Appendix A. Changes from RFC 7234 . . . . . . . . . . . . . . . 38 C.1. Between RFC7234 and draft 00 . . . . . . . . . . . . . . 39
Appendix B. Imported ABNF . . . . . . . . . . . . . . . . . . . 38 C.2. Since draft-ietf-httpbis-cache-00 . . . . . . . . . . . . 39
Appendix C. Collected ABNF . . . . . . . . . . . . . . . . . . . 38 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Appendix D. Change Log . . . . . . . . . . . . . . . . . . . . . 39
D.1. Since RFC 7234 . . . . . . . . . . . . . . . . . . . . . 40
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 42 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 42
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 42 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 42
1. Introduction 1. Introduction
The Hypertext Transfer Protocol (HTTP) is a stateless application-
level request/response protocol that uses extensible semantics and
self-descriptive messages for flexible interaction with network-based
hypertext information systems. HTTP is defined by a series of
documents that collectively form the HTTP/1.1 specification:
o "HTTP Semantics" [Semantics]
o "HTTP Caching" (this document)
o "HTTP/1.1 Messaging" [Messaging]
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. This performance can be improved by the use of response caches. This
document defines aspects of HTTP/1.1 related to caching and reusing document defines aspects of HTTP related to caching and reusing
response messages. response messages.
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 storage, retrieval, and deletion of messages in it. A that controls storage, retrieval, and deletion of messages in it. A
cache stores cacheable responses in order to reduce the response time cache stores cacheable responses in order to reduce the response time
and network bandwidth consumption on future, equivalent requests. and network bandwidth consumption on future, equivalent requests.
Any client or server MAY employ a cache, though a cache cannot be Any client or server MAY employ a cache, though a cache cannot be
used by a server that is acting as a tunnel. used by a server that is acting as a tunnel.
A shared cache is a cache that stores responses to be reused by more A shared cache is a cache that stores responses to be reused by more
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performance by reusing a prior response message to satisfy a current performance by reusing a prior response message to satisfy a current
request. A stored response is considered "fresh", as defined in request. A stored response is considered "fresh", as defined in
Section 4.2, if the response can be reused without "validation" Section 4.2, if the response can be reused without "validation"
(checking with the origin server to see if the cached response (checking with the origin server to see if the cached response
remains valid for this request). A fresh response can therefore remains valid for this request). A fresh response can therefore
reduce both latency and network overhead each time it is reused. reduce both latency and network overhead each time it is reused.
When a cached response is not fresh, it might still be reusable if it When a cached response is not fresh, it might still be reusable if it
can be freshened by validation (Section 4.3) or if the origin is can be freshened by validation (Section 4.3) or if the origin is
unavailable (Section 4.2.4). unavailable (Section 4.2.4).
This specification obsoletes RFC 7234, with the changes being This document obsoletes RFC 7234, with the changes being summarized
summarized in Appendix A. in Appendix B.
1.1. Conformance and Error Handling 1.1. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
Conformance criteria and considerations regarding error handling are Conformance criteria and considerations regarding error handling are
defined in Section 2.5 of [MESSGNG]. defined in Section 3 of [Semantics].
1.2. Syntax Notation 1.2. Syntax Notation
This specification uses the Augmented Backus-Naur Form (ABNF) This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234] with a list extension, defined in Section 7 of notation of [RFC5234] with a list extension, defined in Section 11 of
[MESSGNG], that allows for compact definition of comma-separated [Semantics], that allows for compact definition of comma-separated
lists using a '#' operator (similar to how the '*' operator indicates lists using a '#' operator (similar to how the '*' operator indicates
repetition). Appendix B describes rules imported from other repetition). Appendix A shows the collected grammar with all list
documents. Appendix C shows the collected grammar with all list
operators expanded to standard ABNF notation. operators expanded to standard ABNF notation.
1.2.1. Delta Seconds The following core rules are included by reference, as defined in
[RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF
(CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote),
HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line
feed), OCTET (any 8-bit sequence of data), SP (space), and VCHAR (any
visible [USASCII] character).
The rules below are defined in [Semantics]:
HTTP-date = <HTTP-date, see [Semantics], Section 10.1.1.1>
OWS = <OWS, see [Semantics], Section 4.3>
field-name = <field-name, see [Semantics], Section 4.2>
quoted-string = <quoted-string, see [Semantics], Section 4.2.3>
token = <token, see [Semantics], Section 4.2.3>
uri-host = <host, see [RFC3986], Section 3.2.2>
port = <port, see [RFC3986], Section 3.2.3>
pseudonym = <pseudonym, see [Semantics], Section 5.6.1>
1.3. Delta Seconds
The delta-seconds rule specifies a non-negative integer, representing The delta-seconds rule specifies a non-negative integer, representing
time in seconds. time in seconds.
delta-seconds = 1*DIGIT delta-seconds = 1*DIGIT
A recipient parsing a delta-seconds value and converting it to binary A recipient parsing a delta-seconds value and converting it to binary
form ought to use an arithmetic type of at least 31 bits of non- form ought to use an arithmetic type of at least 31 bits of non-
negative integer range. If a cache receives a delta-seconds value negative integer range. If a cache receives a delta-seconds value
greater than the greatest integer it can represent, or if any of its greater than the greatest integer it can represent, or if any of its
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to be stored in binary form; an implementation could produce it as to be stored in binary form; an implementation could produce it as
a canned string if any overflow occurs, even if the calculations a canned string if any overflow occurs, even if the calculations
are performed with an arithmetic type incapable of directly are performed with an arithmetic type incapable of directly
representing that number. What matters here is that an overflow representing that number. What matters here is that an overflow
be detected and not treated as a negative value in later be detected and not treated as a negative value in later
calculations. calculations.
2. Overview of Cache Operation 2. Overview of Cache Operation
Proper cache operation preserves the semantics of HTTP transfers Proper cache operation preserves the semantics of HTTP transfers
([SEMNTCS]) while eliminating the transfer of information already ([Semantics]) while eliminating the transfer of information already
held in the cache. Although caching is an entirely OPTIONAL feature held in the cache. Although caching is an entirely OPTIONAL feature
of HTTP, it can be assumed that reusing a cached response is of HTTP, it can be assumed that reusing a cached response is
desirable and that such reuse is the default behavior when no desirable and that such reuse is the default behavior when no
requirement or local configuration prevents it. Therefore, HTTP requirement or local configuration prevents it. Therefore, HTTP
cache requirements are focused on preventing a cache from either cache requirements are focused on preventing a cache from either
storing a non-reusable response or reusing a stored response storing a non-reusable response or reusing a stored response
inappropriately, rather than mandating that caches always store and inappropriately, rather than mandating that caches always store and
reuse particular responses. reuse particular responses.
Each cache entry consists of a cache key and one or more HTTP Each cache entry consists of a cache key and one or more HTTP
responses corresponding to prior requests that used the same key. responses corresponding to prior requests that used the same key.
The most common form of cache entry is a successful result of a The most common form of cache entry is a successful result of a
retrieval request: i.e., a 200 (OK) response to a GET request, which retrieval request: i.e., a 200 (OK) response to a GET request, which
contains a representation of the resource identified by the request contains a representation of the resource identified by the request
target (Section 4.3.1 of [SEMNTCS]). However, it is also possible to target (Section 7.3.1 of [Semantics]). However, it is also possible
cache permanent redirects, negative results (e.g., 404 (Not Found)), to cache permanent redirects, negative results (e.g., 404 (Not
incomplete results (e.g., 206 (Partial Content)), and responses to Found)), incomplete results (e.g., 206 (Partial Content)), and
methods other than GET if the method's definition allows such caching responses to methods other than GET if the method's definition allows
and defines something suitable for use as a cache key. such caching and defines something suitable for use as a cache key.
The primary cache key consists of the request method and target URI. The primary cache key consists of the request method and target URI.
However, since HTTP caches in common use today are typically limited However, since HTTP caches in common use today are typically limited
to caching responses to GET, many caches simply decline other methods to caching responses to GET, many caches simply decline other methods
and use only the URI as the primary cache key. and use only the URI as the primary cache key.
If a request target is subject to content negotiation, its cache If a request target is subject to content negotiation, its cache
entry might consist of multiple stored responses, each differentiated entry might consist of multiple stored responses, each differentiated
by a secondary key for the values of the original request's selecting by a secondary key for the values of the original request's selecting
header fields (Section 4.1). header fields (Section 4.1).
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cacheable, and cacheable, and
o the response status code is understood by the cache, and o the response status code is understood by the cache, and
o the "no-store" cache directive (see Section 5.2) does not appear o the "no-store" cache directive (see Section 5.2) does not appear
in request or response header fields, and in request or response header fields, and
o the "private" response directive (see Section 5.2.2.6) does not o the "private" response directive (see Section 5.2.2.6) does not
appear in the response, if the cache is shared, and appear in the response, if the cache is shared, and
o the Authorization header field (see Section 4.2 of [AUTHFRM]) does o the Authorization header field (see Section 8.5.3 of [Semantics])
not appear in the request, if the cache is shared, unless the does not appear in the request, if the cache is shared, unless the
response explicitly allows it (see Section 3.2), and response explicitly allows it (see Section 3.2), and
o the response either: o the response either:
* contains an Expires header field (see Section 5.3), or * contains an Expires header field (see Section 5.3), or
* contains a max-age response directive (see Section 5.2.2.8), or * contains a max-age response directive (see Section 5.2.2.8), or
* contains a s-maxage response directive (see Section 5.2.2.9) * contains a s-maxage response directive (see Section 5.2.2.9)
and the cache is shared, or and the cache is shared, or
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caching-related behavior. caching-related behavior.
Note that, in normal operation, some caches will not store a response Note that, in normal operation, some caches will not store a response
that has neither a cache validator nor an explicit expiration time, that has neither a cache validator nor an explicit expiration time,
as such responses are not usually useful to store. However, caches as such responses are not usually useful to store. However, caches
are not prohibited from storing such responses. are not prohibited from storing such responses.
3.1. Storing Incomplete Responses 3.1. Storing Incomplete Responses
A response message is considered complete when all of the octets A response message is considered complete when all of the octets
indicated by the message framing ([MESSGNG]) are received prior to indicated by the message framing ([Messaging]) are received prior to
the connection being closed. If the request method is GET, the the connection being closed. If the request method is GET, the
response status code is 200 (OK), and the entire response header response status code is 200 (OK), and the entire response header
section has been received, a cache MAY store an incomplete response section has been received, a cache MAY store an incomplete response
message body if the cache entry is recorded as incomplete. Likewise, message body if the cache entry is recorded as incomplete. Likewise,
a 206 (Partial Content) response MAY be stored as if it were an a 206 (Partial Content) response MAY be stored as if it were an
incomplete 200 (OK) cache entry. However, a cache MUST NOT store incomplete 200 (OK) cache entry. However, a cache MUST NOT store
incomplete or partial-content responses if it does not support the incomplete or partial-content responses if it does not support the
Range and Content-Range header fields or if it does not understand Range and Content-Range header fields or if it does not understand
the range units used in those fields. the range units used in those fields.
A cache MAY complete a stored incomplete response by making a A cache MAY complete a stored incomplete response by making a
subsequent range request ([RANGERQ]) and combining the successful subsequent range request (Section 8.3 of [Semantics]) and combining
response with the stored entry, as defined in Section 3.3. A cache the successful response with the stored entry, as defined in
MUST NOT use an incomplete response to answer requests unless the Section 3.3. A cache MUST NOT use an incomplete response to answer
response has been made complete or the request is partial and requests unless the response has been made complete or the request is
specifies a range that is wholly within the incomplete response. A partial and specifies a range that is wholly within the incomplete
cache MUST NOT send a partial response to a client without explicitly response. A cache MUST NOT send a partial response to a client
marking it as such using the 206 (Partial Content) status code. without explicitly marking it as such using the 206 (Partial Content)
status code.
3.2. Storing Responses to Authenticated Requests 3.2. Storing Responses to Authenticated Requests
A shared cache MUST NOT use a cached response to a request with an A shared cache MUST NOT use a cached response to a request with an
Authorization header field (Section 4.2 of [AUTHFRM]) to satisfy any Authorization header field (Section 8.5.3 of [Semantics]) to satisfy
subsequent request unless a cache directive that allows such any subsequent request unless a cache directive that allows such
responses to be stored is present in the response. responses to be stored is present in the response.
In this specification, the following Cache-Control response In this specification, the following Cache-Control response
directives (Section 5.2.2) have such an effect: must-revalidate, directives (Section 5.2.2) have such an effect: must-revalidate,
public, and s-maxage. public, and s-maxage.
Note that cached responses that contain the "must-revalidate" and/or Note that cached responses that contain the "must-revalidate" and/or
"s-maxage" response directives are not allowed to be served stale "s-maxage" response directives are not allowed to be served stale
(Section 4.2.4) by shared caches. In particular, a response with (Section 4.2.4) by shared caches. In particular, a response with
either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to
satisfy a subsequent request without revalidating it on the origin satisfy a subsequent request without revalidating it on the origin
server. server.
3.3. Combining Partial Content 3.3. Combining Partial Content
A response might transfer only a partial representation if the A response might transfer only a partial representation if the
connection closed prematurely or if the request used one or more connection closed prematurely or if the request used one or more
Range specifiers ([RANGERQ]). After several such transfers, a cache Range specifiers (Section 8.3 of [Semantics]). After several such
might have received several ranges of the same representation. A transfers, a cache might have received several ranges of the same
cache MAY combine these ranges into a single stored response, and representation. A cache MAY combine these ranges into a single
reuse that response to satisfy later requests, if they all share the stored response, and reuse that response to satisfy later requests,
same strong validator and the cache complies with the client if they all share the same strong validator and the cache complies
requirements in Section 4.3 of [RANGERQ]. with the client requirements in Section 9.3.7.3 of [Semantics].
When combining the new response with one or more stored responses, a When combining the new response with one or more stored responses, a
cache MUST: cache MUST:
o delete any Warning header fields in the stored response with warn- o delete any Warning header fields in the stored response with warn-
code 1xx (see Section 5.5); code 1xx (see Section 5.5);
o retain any Warning header fields in the stored response with warn- o retain any Warning header fields in the stored response with warn-
code 2xx; and, code 2xx; and,
o use other header fields provided in the new response, aside from o use other header fields provided in the new response, aside from
Content-Range, to replace all instances of the corresponding Content-Range, to replace all instances of the corresponding
header fields in the stored response. header fields in the stored response.
4. Constructing Responses from Caches 4. Constructing Responses from Caches
When presented with a request, a cache MUST NOT reuse a stored When presented with a request, a cache MUST NOT reuse a stored
response, unless: response, unless:
o The presented effective request URI (Section 5.5 of [MESSGNG]) and o The presented effective request URI (Section 5.3 of [Semantics])
that of the stored response match, and and that of the stored response match, and
o the request method associated with the stored response allows it o the request method associated with the stored response allows it
to be used for the presented request, and to be used for the presented request, and
o selecting header fields nominated by the stored response (if any) o selecting header fields nominated by the stored response (if any)
match those presented (see Section 4.1), and match those presented (see Section 4.1), and
o the presented request does not contain the no-cache pragma o the presented request does not contain the no-cache pragma
(Section 5.4), nor the no-cache cache directive (Section 5.2.1), (Section 5.4), nor the no-cache cache directive (Section 5.2.1),
unless the stored response is successfully validated unless the stored response is successfully validated
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Note that any of the requirements listed above can be overridden by a Note that any of the requirements listed above can be overridden by a
cache-control extension; see Section 5.2.3. cache-control extension; see Section 5.2.3.
When a stored response is used to satisfy a request without When a stored response is used to satisfy a request without
validation, a cache MUST generate an Age header field (Section 5.1), validation, a cache MUST generate an Age header field (Section 5.1),
replacing any present in the response with a value equal to the replacing any present in the response with a value equal to the
stored response's current_age; see Section 4.2.3. stored response's current_age; see Section 4.2.3.
A cache MUST write through requests with methods that are unsafe A cache MUST write through requests with methods that are unsafe
(Section 4.2.1 of [SEMNTCS]) to the origin server; i.e., a cache is (Section 7.2.1 of [Semantics]) to the origin server; i.e., a cache is
not allowed to generate a reply to such a request before having not allowed to generate a reply to such a request before having
forwarded the request and having received a corresponding response. forwarded the request and having received a corresponding response.
Also, note that unsafe requests might invalidate already-stored Also, note that unsafe requests might invalidate already-stored
responses; see Section 4.4. responses; see Section 4.4.
When more than one suitable response is stored, a cache MUST use the When more than one suitable response is stored, a cache MUST use the
most recent response (as determined by the Date header field). It most recent response (as determined by the Date header field). It
can also forward the request with "Cache-Control: max-age=0" or can also forward the request with "Cache-Control: max-age=0" or
"Cache-Control: no-cache" to disambiguate which response to use. "Cache-Control: no-cache" to disambiguate which response to use.
A cache that does not have a clock available MUST NOT use stored A cache that does not have a clock available MUST NOT use stored
responses without revalidating them upon every use. responses without revalidating them upon every use.
4.1. Calculating Secondary Keys with Vary 4.1. Calculating Secondary Keys with Vary
When a cache receives a request that can be satisfied by a stored When a cache receives a request that can be satisfied by a stored
response that has a Vary header field (Section 7.1.4 of [SEMNTCS]), response that has a Vary header field (Section 10.1.4 of
it MUST NOT use that response unless all of the selecting header [Semantics]), it MUST NOT use that response unless all of the
fields nominated by the Vary header field match in both the original selecting header fields nominated by the Vary header field match in
request (i.e., that associated with the stored response), and the both the original request (i.e., that associated with the stored
presented request. response), and the presented request.
The selecting header fields from two requests are defined to match if The selecting header fields from two requests are defined to match if
and only if those in the first request can be transformed to those in and only if those in the first request can be transformed to those in
the second request by applying any of the following: the second request by applying any of the following:
o adding or removing whitespace, where allowed in the header field's o adding or removing whitespace, where allowed in the header field's
syntax syntax
o combining multiple header fields with the same field name (see o combining multiple header fields with the same field name (see
Section 3.2 of [MESSGNG]) Section 4.2 of [Semantics])
o normalizing both header field values in a way that is known to o normalizing both header field values in a way that is known to
have identical semantics, according to the header field's have identical semantics, according to the header field's
specification (e.g., reordering field values when order is not specification (e.g., reordering field values when order is not
significant; case-normalization, where values are defined to be significant; case-normalization, where values are defined to be
case-insensitive) case-insensitive)
If (after any normalization that might take place) a header field is If (after any normalization that might take place) a header field is
absent from a request, it can only match another request if it is absent from a request, it can only match another request if it is
also absent there. also absent there.
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is not specified, employing algorithms that use other header field is not specified, employing algorithms that use other header field
values (such as the Last-Modified time) to estimate a plausible values (such as the Last-Modified time) to estimate a plausible
expiration time. This specification does not provide specific expiration time. This specification does not provide specific
algorithms, but does impose worst-case constraints on their results. algorithms, but does impose worst-case constraints on their results.
A cache MUST NOT use heuristics to determine freshness when an A cache MUST NOT use heuristics to determine freshness when an
explicit expiration time is present in the stored response. Because explicit expiration time is present in the stored response. Because
of the requirements in Section 3, this means that, effectively, of the requirements in Section 3, this means that, effectively,
heuristics can only be used on responses without explicit freshness heuristics can only be used on responses without explicit freshness
whose status codes are defined as cacheable by default (see whose status codes are defined as cacheable by default (see
Section 6.1 of [SEMNTCS]), and those responses without explicit Section 9.1 of [Semantics]), and those responses without explicit
freshness that have been marked as explicitly cacheable (e.g., with a freshness that have been marked as explicitly cacheable (e.g., with a
"public" response directive). "public" response directive).
If the response has a Last-Modified header field (Section 2.2 of If the response has a Last-Modified header field (Section 10.2.2 of
[CONDTNL]), caches are encouraged to use a heuristic expiration value [Semantics]), caches are encouraged to use a heuristic expiration
that is no more than some fraction of the interval since that time. value that is no more than some fraction of the interval since that
A typical setting of this fraction might be 10%. time. A typical setting of this fraction might be 10%.
When a heuristic is used to calculate freshness lifetime, a cache When a heuristic is used to calculate freshness lifetime, a cache
SHOULD generate a Warning header field with a 113 warn-code (see SHOULD generate a Warning header field with a 113 warn-code (see
Section 5.5.4) in the response if its current_age is more than 24 Section 5.5.4) in the response if its current_age is more than 24
hours and such a warning is not already present. hours and such a warning is not already present.
Note: Section 13.9 of [RFC2616] prohibited caches from calculating Note: Section 13.9 of [RFC2616] prohibited caches from calculating
heuristic freshness for URIs with query components (i.e., those heuristic freshness for URIs with query components (i.e., those
containing '?'). In practice, this has not been widely containing '?'). In practice, this has not been widely
implemented. Therefore, origin servers are encouraged to send implemented. Therefore, origin servers are encouraged to send
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The following data is used for the age calculation: The following data is used for the age calculation:
age_value age_value
The term "age_value" denotes the value of the Age header field The term "age_value" denotes the value of the Age header field
(Section 5.1), in a form appropriate for arithmetic operation; or (Section 5.1), in a form appropriate for arithmetic operation; or
0, if not available. 0, if not available.
date_value date_value
The term "date_value" denotes the value of the Date header field, The term "date_value" denotes the value of the Date header field,
in a form appropriate for arithmetic operations. See in a form appropriate for arithmetic operations. See
Section 7.1.1.2 of [SEMNTCS] for the definition of the Date header Section 10.1.1.2 of [Semantics] for the definition of the Date
field, and for requirements regarding responses without it. header field, and for requirements regarding responses without it.
now now
The term "now" means "the current value of the clock at the host The term "now" means "the current value of the clock at the host
performing the calculation". A host ought to use NTP ([RFC5905]) performing the calculation". A host ought to use NTP ([RFC5905])
or some similar protocol to synchronize its clocks to Coordinated or some similar protocol to synchronize its clocks to Coordinated
Universal Time. Universal Time.
request_time request_time
The current value of the clock at the host at the time the request The current value of the clock at the host at the time the request
resulting in the stored response was made. resulting in the stored response was made.
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A cache SHOULD NOT generate a new Warning header field when A cache SHOULD NOT generate a new Warning header field when
forwarding a response that does not have an Age header field, even if forwarding a response that does not have an Age header field, even if
the response is already stale. A cache need not validate a response the response is already stale. A cache need not validate a response
that merely became stale in transit. that merely became stale in transit.
4.3. Validation 4.3. Validation
When a cache has one or more stored responses for a requested URI, When a cache has one or more stored responses for a requested URI,
but cannot serve any of them (e.g., because they are not fresh, or but cannot serve any of them (e.g., because they are not fresh, or
one cannot be selected; see Section 4.1), it can use the conditional one cannot be selected; see Section 4.1), it can use the conditional
request mechanism [CONDTNL] in the forwarded request to give the next request mechanism Section 8.2 of [Semantics] in the forwarded request
inbound server an opportunity to select a valid stored response to to give the next inbound server an opportunity to select a valid
use, updating the stored metadata in the process, or to replace the stored response to use, updating the stored metadata in the process,
stored response(s) with a new response. This process is known as or to replace the stored response(s) with a new response. This
"validating" or "revalidating" the stored response. process is known as "validating" or "revalidating" the stored
response.
4.3.1. Sending a Validation Request 4.3.1. Sending a Validation Request
When sending a conditional request for cache validation, a cache When sending a conditional request for cache validation, a cache
sends one or more precondition header fields containing validator sends one or more precondition header fields containing validator
metadata from its stored response(s), which is then compared by metadata from its stored response(s), which is then compared by
recipients to determine whether a stored response is equivalent to a recipients to determine whether a stored response is equivalent to a
current representation of the resource. current representation of the resource.
One such validator is the timestamp given in a Last-Modified header One such validator is the timestamp given in a Last-Modified header
field (Section 2.2 of [CONDTNL]), which can be used in an If- field (Section 10.2.2 of [Semantics]), which can be used in an If-
Modified-Since header field for response validation, or in an If- Modified-Since header field for response validation, or in an If-
Unmodified-Since or If-Range header field for representation Unmodified-Since or If-Range header field for representation
selection (i.e., the client is referring specifically to a previously selection (i.e., the client is referring specifically to a previously
obtained representation with that timestamp). obtained representation with that timestamp).
Another validator is the entity-tag given in an ETag header field Another validator is the entity-tag given in an ETag header field
(Section 2.3 of [CONDTNL]). One or more entity-tags, indicating one (Section 10.2.3 of [Semantics]). One or more entity-tags, indicating
or more stored responses, can be used in an If-None-Match header one or more stored responses, can be used in an If-None-Match header
field for response validation, or in an If-Match or If-Range header field for response validation, or in an If-Match or If-Range header
field for representation selection (i.e., the client is referring field for representation selection (i.e., the client is referring
specifically to one or more previously obtained representations with specifically to one or more previously obtained representations with
the listed entity-tags). the listed entity-tags).
4.3.2. Handling a Received Validation Request 4.3.2. Handling a Received Validation Request
Each client in the request chain may have its own cache, so it is Each client in the request chain may have its own cache, so it is
common for a cache at an intermediary to receive conditional requests common for a cache at an intermediary to receive conditional requests
from other (outbound) caches. Likewise, some user agents make use of from other (outbound) caches. Likewise, some user agents make use of
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received in that request with respect to the corresponding validators received in that request with respect to the corresponding validators
contained within the selected response. A cache MUST NOT evaluate contained within the selected response. A cache MUST NOT evaluate
conditional header fields that are only applicable to an origin conditional header fields that are only applicable to an origin
server, found in a request with semantics that cannot be satisfied server, found in a request with semantics that cannot be satisfied
with a cached response, or applied to a target resource for which it with a cached response, or applied to a target resource for which it
has no stored responses; such preconditions are likely intended for has no stored responses; such preconditions are likely intended for
some other (inbound) server. some other (inbound) server.
The proper evaluation of conditional requests by a cache depends on The proper evaluation of conditional requests by a cache depends on
the received precondition header fields and their precedence, as the received precondition header fields and their precedence, as
defined in Section 6 of [CONDTNL]. The If-Match and If-Unmodified- defined in Section 8.2.2 of [Semantics]. The If-Match and If-
Since conditional header fields are not applicable to a cache. Unmodified-Since conditional header fields are not applicable to a
cache.
A request containing an If-None-Match header field (Section 3.2 of A request containing an If-None-Match header field (Section 8.2.4 of
[CONDTNL]) indicates that the client wants to validate one or more of [Semantics]) indicates that the client wants to validate one or more
its own stored responses in comparison to whichever stored response of its own stored responses in comparison to whichever stored
is selected by the cache. If the field-value is "*", or if the response is selected by the cache. If the field-value is "*", or if
field-value is a list of entity-tags and at least one of them matches the field-value is a list of entity-tags and at least one of them
the entity-tag of the selected stored response, a cache recipient matches the entity-tag of the selected stored response, a cache
SHOULD generate a 304 (Not Modified) response (using the metadata of recipient SHOULD generate a 304 (Not Modified) response (using the
the selected stored response) instead of sending that stored metadata of the selected stored response) instead of sending that
response. stored response.
When a cache decides to revalidate its own stored responses for a When a cache decides to revalidate its own stored responses for a
request that contains an If-None-Match list of entity-tags, the cache request that contains an If-None-Match list of entity-tags, the cache
MAY combine the received list with a list of entity-tags from its own MAY combine the received list with a list of entity-tags from its own
stored set of responses (fresh or stale) and send the union of the stored set of responses (fresh or stale) and send the union of the
two lists as a replacement If-None-Match header field value in the two lists as a replacement If-None-Match header field value in the
forwarded request. If a stored response contains only partial forwarded request. If a stored response contains only partial
content, the cache MUST NOT include its entity-tag in the union content, the cache MUST NOT include its entity-tag in the union
unless the request is for a range that would be fully satisfied by unless the request is for a range that would be fully satisfied by
that partial stored response. If the response to the forwarded that partial stored response. If the response to the forwarded
request is 304 (Not Modified) and has an ETag header field value with request is 304 (Not Modified) and has an ETag header field value with
an entity-tag that is not in the client's list, the cache MUST an entity-tag that is not in the client's list, the cache MUST
generate a 200 (OK) response for the client by reusing its generate a 200 (OK) response for the client by reusing its
corresponding stored response, as updated by the 304 response corresponding stored response, as updated by the 304 response
metadata (Section 4.3.4). metadata (Section 4.3.4).
If an If-None-Match header field is not present, a request containing If an If-None-Match header field is not present, a request containing
an If-Modified-Since header field (Section 3.3 of [CONDTNL]) an If-Modified-Since header field (Section 8.2.5 of [Semantics])
indicates that the client wants to validate one or more of its own indicates that the client wants to validate one or more of its own
stored responses by modification date. A cache recipient SHOULD stored responses by modification date. A cache recipient SHOULD
generate a 304 (Not Modified) response (using the metadata of the generate a 304 (Not Modified) response (using the metadata of the
selected stored response) if one of the following cases is true: 1) selected stored response) if one of the following cases is true: 1)
the selected stored response has a Last-Modified field-value that is the selected stored response has a Last-Modified field-value that is
earlier than or equal to the conditional timestamp; 2) no Last- earlier than or equal to the conditional timestamp; 2) no Last-
Modified field is present in the selected stored response, but it has Modified field is present in the selected stored response, but it has
a Date field-value that is earlier than or equal to the conditional a Date field-value that is earlier than or equal to the conditional
timestamp; or, 3) neither Last-Modified nor Date is present in the timestamp; or, 3) neither Last-Modified nor Date is present in the
selected stored response, but the cache recorded it as having been selected stored response, but the cache recorded it as having been
received at a time earlier than or equal to the conditional received at a time earlier than or equal to the conditional
timestamp. timestamp.
A cache that implements partial responses to range requests, as A cache that implements partial responses to range requests, as
defined in [RANGERQ], also needs to evaluate a received If-Range defined in Section 8.3 of [Semantics], also needs to evaluate a
header field (Section 3.2 of [RANGERQ]) with respect to its selected received If-Range header field (Section 8.2.7 of [Semantics]) with
stored response. respect to its selected stored response.
4.3.3. Handling a Validation Response 4.3.3. Handling a Validation Response
Cache handling of a response to a conditional request is dependent Cache handling of a response to a conditional request is dependent
upon its status code: upon its status code:
o A 304 (Not Modified) response status code indicates that the o A 304 (Not Modified) response status code indicates that the
stored response can be updated and reused; see Section 4.3.4. stored response can be updated and reused; see Section 4.3.4.
o A full response (i.e., one with a payload body) indicates that o A full response (i.e., one with a payload body) indicates that
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When a cache receives a 304 (Not Modified) response and already has When a cache receives a 304 (Not Modified) response and already has
one or more stored 200 (OK) responses for the same cache key, the one or more stored 200 (OK) responses for the same cache key, the
cache needs to identify which of the stored responses are updated by cache needs to identify which of the stored responses are updated by
this new response and then update the stored response(s) with the new this new response and then update the stored response(s) with the new
information provided in the 304 response. information provided in the 304 response.
The stored response to update is identified by using the first match The stored response to update is identified by using the first match
(if any) of the following: (if any) of the following:
o If the new response contains a strong validator (see Section 2.1 o If the new response contains a strong validator (see
of [CONDTNL]), then that strong validator identifies the selected Section 10.2.1 of [Semantics]), then that strong validator
representation for update. All of the stored responses with the identifies the selected representation for update. All of the
same strong validator are selected. If none of the stored stored responses with the same strong validator are selected. If
responses contain the same strong validator, then the cache MUST none of the stored responses contain the same strong validator,
NOT use the new response to update any stored responses. then the cache MUST NOT use the new response to update any stored
responses.
o If the new response contains a weak validator and that validator o If the new response contains a weak validator and that validator
corresponds to one of the cache's stored responses, then the most corresponds to one of the cache's stored responses, then the most
recent of those matching stored responses is selected for update. recent of those matching stored responses is selected for update.
o If the new response does not include any form of validator (such o If the new response does not include any form of validator (such
as in the case where a client generates an If-Modified-Since as in the case where a client generates an If-Modified-Since
request from a source other than the Last-Modified response header request from a source other than the Last-Modified response header
field), and there is only one stored response, and that stored field), and there is only one stored response, and that stored
response also lacks a validator, then that stored response is response also lacks a validator, then that stored response is
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code 2xx; and, code 2xx; and,
o use other header fields provided in the HEAD response to replace o use other header fields provided in the HEAD response to replace
all instances of the corresponding header fields in the stored all instances of the corresponding header fields in the stored
response and append new header fields to the stored response's response and append new header fields to the stored response's
header section unless otherwise restricted by the Cache-Control header section unless otherwise restricted by the Cache-Control
header field. header field.
4.4. Invalidation 4.4. Invalidation
Because unsafe request methods (Section 4.2.1 of [SEMNTCS]) such as Because unsafe request methods (Section 7.2.1 of [Semantics]) such as
PUT, POST or DELETE have the potential for changing state on the PUT, POST or DELETE have the potential for changing state on the
origin server, intervening caches can use them to keep their contents origin server, intervening caches can use them to keep their contents
up to date. up to date.
A cache MUST invalidate the effective Request URI (Section 5.5 of A cache MUST invalidate the effective Request URI (Section 5.3 of
[MESSGNG]) as well as the URI(s) in the Location and Content-Location [Semantics]) as well as the URI(s) in the Location and Content-
response header fields (if present) when a non-error status code is Location response header fields (if present) when a non-error status
received in response to an unsafe request method. code is received in response to an unsafe request method.
However, a cache MUST NOT invalidate a URI from a Location or However, a cache MUST NOT invalidate a URI from a Location or
Content-Location response header field if the host part of that URI Content-Location response header field if the host part of that URI
differs from the host part in the effective request URI (Section 5.5 differs from the host part in the effective request URI (Section 5.3
of [MESSGNG]). This helps prevent denial-of-service attacks. of [Semantics]). This helps prevent denial-of-service attacks.
A cache MUST invalidate the effective request URI (Section 5.5 of A cache MUST invalidate the effective request URI (Section 5.3 of
[MESSGNG]) when it receives a non-error response to a request with a [Semantics]) when it receives a non-error response to a request with
method whose safety is unknown. a method whose safety is unknown.
Here, a "non-error response" is one with a 2xx (Successful) or 3xx Here, a "non-error response" is one with a 2xx (Successful) or 3xx
(Redirection) status code. "Invalidate" means that the cache will (Redirection) status code. "Invalidate" means that the cache will
either remove all stored responses related to the effective request either remove all stored responses related to the effective request
URI or will mark these as "invalid" and in need of a mandatory URI or will mark these as "invalid" and in need of a mandatory
validation before they can be sent in response to a subsequent validation before they can be sent in response to a subsequent
request. request.
Note that this does not guarantee that all appropriate responses are Note that this does not guarantee that all appropriate responses are
invalidated. For example, a state-changing request might invalidate invalidated. For example, a state-changing request might invalidate
responses in the caches it travels through, but relevant responses responses in the caches it travels through, but relevant responses
still might be stored in other caches that it has not. still might be stored in other caches that it has not.
5. Header Field Definitions 5. 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.
+-------------------+----------+----------+--------------+
| Header Field Name | Protocol | Status | Reference |
+-------------------+----------+----------+--------------+
| Age | http | standard | Section 5.1 |
| Cache-Control | http | standard | Section 5.2 |
| Expires | http | standard | Section 5.3 |
| Pragma | http | standard | Section 5.4 |
| Warning | http | standard | Section 5.5 |
+-------------------+----------+----------+--------------+
5.1. Age 5.1. Age
The "Age" header field conveys the sender's estimate of the amount of The "Age" header field conveys the sender's estimate of the amount of
time since the response was generated or successfully validated at time since the response was generated or successfully validated at
the origin server. Age values are calculated as specified in the origin server. Age values are calculated as specified in
Section 4.2.3. Section 4.2.3.
Age = delta-seconds Age = delta-seconds
The Age field-value is a non-negative integer, representing time in The Age field-value is a non-negative integer, representing time in
seconds (see Section 1.2.1). seconds (see Section 1.3).
The presence of an Age header field implies that the response was not The presence of an Age header field implies that the response was not
generated or validated by the origin server for this request. generated or validated by the origin server for this request.
However, lack of an Age header field does not imply the origin was However, lack of an Age header field does not imply the origin was
contacted, since the response might have been received from an contacted, since the response might have been received from an
HTTP/1.0 cache that does not implement Age. HTTP/1.0 cache that does not implement Age.
5.2. Cache-Control 5.2. Cache-Control
The "Cache-Control" header field is used to specify directives for The "Cache-Control" header field is used to specify directives for
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is documented to be preferred. For any directive not defined by this is documented to be preferred. For any directive not defined by this
specification, a recipient MUST accept both forms. specification, a recipient MUST accept both forms.
Cache-Control = 1#cache-directive Cache-Control = 1#cache-directive
cache-directive = token [ "=" ( token / quoted-string ) ] cache-directive = token [ "=" ( token / quoted-string ) ]
For the cache directives defined below, no argument is defined (nor For the cache directives defined below, no argument is defined (nor
allowed) unless stated otherwise. allowed) unless stated otherwise.
+------------------------+-----------------------------------+
| Cache Directive | Reference |
+------------------------+-----------------------------------+
| max-age | Section 5.2.1.1, Section 5.2.2.8 |
| max-stale | Section 5.2.1.2 |
| min-fresh | Section 5.2.1.3 |
| must-revalidate | Section 5.2.2.1 |
| no-cache | Section 5.2.1.4, Section 5.2.2.2 |
| no-store | Section 5.2.1.5, Section 5.2.2.3 |
| no-transform | Section 5.2.1.6, Section 5.2.2.4 |
| only-if-cached | Section 5.2.1.7 |
| private | Section 5.2.2.6 |
| proxy-revalidate | Section 5.2.2.7 |
| public | Section 5.2.2.5 |
| s-maxage | Section 5.2.2.9 |
| stale-if-error | [RFC5861], Section 4 |
| stale-while-revalidate | [RFC5861], Section 3 |
+------------------------+-----------------------------------+
5.2.1. Request Cache-Control Directives 5.2.1. Request Cache-Control Directives
5.2.1.1. max-age 5.2.1.1. max-age
Argument syntax: Argument syntax:
delta-seconds (see Section 1.2.1) delta-seconds (see Section 1.3)
The "max-age" request directive indicates that the client is The "max-age" request directive indicates that the client is
unwilling to accept a response whose age is greater than the unwilling to accept a response whose age is greater than the
specified number of seconds. Unless the max-stale request directive specified number of seconds. Unless the max-stale request directive
is also present, the client is not willing to accept a stale is also present, the client is not willing to accept a stale
response. response.
This directive uses the token form of the argument syntax: e.g., This directive uses the token form of the argument syntax: e.g.,
'max-age=5' not 'max-age="5"'. A sender SHOULD NOT generate the 'max-age=5' not 'max-age="5"'. A sender SHOULD NOT generate the
quoted-string form. quoted-string form.
5.2.1.2. max-stale 5.2.1.2. max-stale
Argument syntax: Argument syntax:
delta-seconds (see Section 1.2.1) delta-seconds (see Section 1.3)
The "max-stale" request directive indicates that the client is The "max-stale" request directive indicates that the client is
willing to accept a response that has exceeded its freshness willing to accept a response that has exceeded its freshness
lifetime. If max-stale is assigned a value, then the client is lifetime. If max-stale is assigned a value, then the client is
willing to accept a response that has exceeded its freshness lifetime willing to accept a response that has exceeded its freshness lifetime
by no more than the specified number of seconds. If no value is 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 assigned to max-stale, then the client is willing to accept a stale
response of any age. response of any age.
This directive uses the token form of the argument syntax: e.g., This directive uses the token form of the argument syntax: e.g.,
'max-stale=10' not 'max-stale="10"'. A sender SHOULD NOT generate 'max-stale=10' not 'max-stale="10"'. A sender SHOULD NOT generate
the quoted-string form. the quoted-string form.
5.2.1.3. min-fresh 5.2.1.3. min-fresh
Argument syntax: Argument syntax:
delta-seconds (see Section 1.2.1) delta-seconds (see Section 1.3)
The "min-fresh" request directive indicates that the client is The "min-fresh" request directive indicates that the client is
willing to accept a response whose freshness lifetime is no less than willing to accept a response whose freshness lifetime is no less than
its current age plus the specified time in seconds. That is, the its current age plus the specified time in seconds. That is, the
client wants a response that will still be fresh for at least the client wants a response that will still be fresh for at least the
specified number of seconds. specified number of seconds.
This directive uses the token form of the argument syntax: e.g., This directive uses the token form of the argument syntax: e.g.,
'min-fresh=20' not 'min-fresh="20"'. A sender SHOULD NOT generate 'min-fresh=20' not 'min-fresh="20"'. A sender SHOULD NOT generate
the quoted-string form. the quoted-string form.
skipping to change at page 23, line 28 skipping to change at page 25, line 9
be vulnerable to eavesdropping. be vulnerable to eavesdropping.
Note that if a request containing this directive is satisfied from a Note that if a request containing this directive is satisfied from a
cache, the no-store request directive does not apply to the already cache, the no-store request directive does not apply to the already
stored response. stored response.
5.2.1.6. no-transform 5.2.1.6. no-transform
The "no-transform" request directive indicates that an intermediary The "no-transform" request directive indicates that an intermediary
(whether or not it implements a cache) MUST NOT transform the (whether or not it implements a cache) MUST NOT transform the
payload, as defined in Section 5.7.2 of [MESSGNG]. payload, as defined in Section 5.6.2 of [Semantics].
5.2.1.7. only-if-cached 5.2.1.7. only-if-cached
The "only-if-cached" request directive indicates that the client only The "only-if-cached" request directive indicates that the client only
wishes to obtain a stored response. If it receives this directive, a wishes to obtain a stored response. If it receives this directive, a
cache SHOULD either respond using a stored response that is cache SHOULD either respond using a stored response that is
consistent with the other constraints of the request, or respond with consistent with the other constraints of the request, or respond with
a 504 (Gateway Timeout) status code. If a group of caches is being a 504 (Gateway Timeout) status code. If a group of caches is being
operated as a unified system with good internal connectivity, a operated as a unified system with good internal connectivity, a
member cache MAY forward such a request within that group of caches. member cache MAY forward such a request within that group of caches.
skipping to change at page 25, line 16 skipping to change at page 26, line 47
This directive is NOT a reliable or sufficient mechanism for ensuring This directive is NOT a reliable or sufficient mechanism for ensuring
privacy. In particular, malicious or compromised caches might not privacy. In particular, malicious or compromised caches might not
recognize or obey this directive, and communications networks might recognize or obey this directive, and communications networks might
be vulnerable to eavesdropping. be vulnerable to eavesdropping.
5.2.2.4. no-transform 5.2.2.4. no-transform
The "no-transform" response directive indicates that an intermediary The "no-transform" response directive indicates that an intermediary
(regardless of whether it implements a cache) MUST NOT transform the (regardless of whether it implements a cache) MUST NOT transform the
payload, as defined in Section 5.7.2 of [MESSGNG]. payload, as defined in Section 5.6.2 of [Semantics].
5.2.2.5. public 5.2.2.5. public
The "public" response directive indicates that any cache MAY store The "public" response directive indicates that any cache MAY store
the response, even if the response would normally be non-cacheable or the response, even if the response would normally be non-cacheable or
cacheable only within a private cache. (See Section 3.2 for cacheable only within a private cache. (See Section 3.2 for
additional details related to the use of public in response to a additional details related to the use of public in response to a
request containing Authorization, and Section 3 for details of how request containing Authorization, and Section 3 for details of how
public affects responses that would normally not be stored, due to public affects responses that would normally not be stored, due to
their status codes not being defined as cacheable by default; see their status codes not being defined as cacheable by default; see
skipping to change at page 26, line 22 skipping to change at page 28, line 9
5.2.2.7. proxy-revalidate 5.2.2.7. proxy-revalidate
The "proxy-revalidate" response directive has the same meaning as the The "proxy-revalidate" response directive has the same meaning as the
must-revalidate response directive, except that it does not apply to must-revalidate response directive, except that it does not apply to
private caches. private caches.
5.2.2.8. max-age 5.2.2.8. max-age
Argument syntax: Argument syntax:
delta-seconds (see Section 1.2.1) delta-seconds (see Section 1.3)
The "max-age" response directive indicates that the response is to be The "max-age" response directive indicates that the response is to be
considered stale after its age is greater than the specified number considered stale after its age is greater than the specified number
of seconds. of seconds.
This directive uses the token form of the argument syntax: e.g., This directive uses the token form of the argument syntax: e.g.,
'max-age=5' not 'max-age="5"'. A sender SHOULD NOT generate the 'max-age=5' not 'max-age="5"'. A sender SHOULD NOT generate the
quoted-string form. quoted-string form.
5.2.2.9. s-maxage 5.2.2.9. s-maxage
Argument syntax: Argument syntax:
delta-seconds (see Section 1.2.1) delta-seconds (see Section 1.3)
The "s-maxage" response directive indicates that, in shared caches, The "s-maxage" response directive indicates that, in shared caches,
the maximum age specified by this directive overrides the maximum age the maximum age specified by this directive overrides the maximum age
specified by either the max-age directive or the Expires header specified by either the max-age directive or the Expires header
field. The s-maxage directive also implies the semantics of the field. The s-maxage directive also implies the semantics of the
proxy-revalidate response directive. proxy-revalidate response directive.
This directive uses the token form of the argument syntax: e.g., This directive uses the token form of the argument syntax: e.g.,
's-maxage=10' not 's-maxage="10"'. A sender SHOULD NOT generate the 's-maxage=10' not 's-maxage="10"'. A sender SHOULD NOT generate the
quoted-string form. quoted-string form.
skipping to change at page 27, line 32 skipping to change at page 29, line 19
server wishing to allow the UCI community to use an otherwise private server 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 that recognizes such a community cache-extension could A cache that recognizes such a community cache-extension could
broaden its behavior in accordance with that extension. A cache that broaden its behavior in accordance with that extension. A cache that
does not recognize the community cache-extension would ignore it and does not recognize the community cache-extension would ignore it and
adhere to the private directive. adhere to the private directive.
New extension directives ought to consider defining:
o What it means for a directive to be specified multiple times,
o When the directive does not take an argument, what it means when
an argument is present,
o When the directive requires an argument, what it means when it is
missing,
o Whether the directive is specific to requests, responses, or able
to be used in either.
5.2.4. Cache Directive Registry
The "Hypertext Transfer Protocol (HTTP) Cache Directive Registry"
defines the namespace for the cache directives. It has been created
and is now maintained at <https://www.iana.org/assignments/http-
cache-directives>.
A registration MUST include the following fields:
o Cache Directive Name
o Pointer to specification text
Values to be added to this namespace require IETF Review (see
[RFC5226], Section 4.1).
5.3. Expires 5.3. Expires
The "Expires" header field gives the date/time after which the The "Expires" header field gives the date/time after which the
response is considered stale. See Section 4.2 for further discussion response is considered stale. See Section 4.2 for further discussion
of the freshness model. of the freshness 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 Expires value is an HTTP-date timestamp, as defined in The Expires value is an HTTP-date timestamp, as defined in
Section 7.1.1.1 of [SEMNTCS]. Section 10.1.1.1 of [Semantics].
Expires = HTTP-date Expires = HTTP-date
For example For example
Expires: Thu, 01 Dec 1994 16:00:00 GMT Expires: Thu, 01 Dec 1994 16:00:00 GMT
A cache recipient MUST interpret invalid date formats, especially the A cache recipient MUST interpret invalid date formats, especially the
value "0", as representing a time in the past (i.e., "already value "0", as representing a time in the past (i.e., "already
expired"). expired").
skipping to change at page 31, line 4 skipping to change at page 33, line 20
fields receives a warn-date that is different from the Date value in fields receives a warn-date that is different from the Date value in
the same message, the recipient MUST exclude the warning-value the same message, the recipient MUST exclude the warning-value
containing that warn-date before storing, forwarding, or using the containing that warn-date before storing, forwarding, or using the
message. This allows recipients to exclude warning-values that were message. This allows recipients to exclude warning-values that were
improperly retained after a cache validation. If all of the warning- improperly retained after a cache validation. If all of the warning-
values are excluded, the recipient MUST exclude the Warning header values are excluded, the recipient MUST exclude the Warning header
field as well. field as well.
The following warn-codes are defined by this specification, each with The following warn-codes are defined by this specification, each with
a recommended warn-text in English, and a description of its meaning. a recommended warn-text in English, and a description of its meaning.
The procedure for defining additional warn codes is described in The procedure for defining additional warn codes is described in
Section 7.2.1. Section 5.5.8.
+-----------+----------------------------------+----------------+
| Warn Code | Short Description | Reference |
+-----------+----------------------------------+----------------+
| 110 | Response is Stale | Section 5.5.1 |
| 111 | Revalidation Failed | Section 5.5.2 |
| 112 | Disconnected Operation | Section 5.5.3 |
| 113 | Heuristic Expiration | Section 5.5.4 |
| 199 | Miscellaneous Warning | Section 5.5.5 |
| 214 | Transformation Applied | Section 5.5.6 |
| 299 | Miscellaneous Persistent Warning | Section 5.5.7 |
+-----------+----------------------------------+----------------+
5.5.1. Warning: 110 - "Response is Stale" 5.5.1. Warning: 110 - "Response is Stale"
A cache SHOULD generate this whenever the sent response is stale. A cache SHOULD generate this whenever the sent response is stale.
5.5.2. Warning: 111 - "Revalidation Failed" 5.5.2. Warning: 111 - "Revalidation Failed"
A cache SHOULD generate this when sending a stale response because an A cache SHOULD generate this when sending a stale response because an
attempt to validate the response failed, due to an inability to reach attempt to validate the response failed, due to an inability to reach
the server. the server.
skipping to change at page 32, line 5 skipping to change at page 34, line 31
transformation to the representation, such as changing the content- transformation to the representation, such as changing the content-
coding, media-type, or modifying the representation data, unless this coding, media-type, or modifying the representation data, unless this
Warning code already appears in the response. Warning code already appears in the response.
5.5.7. Warning: 299 - "Miscellaneous Persistent Warning" 5.5.7. Warning: 299 - "Miscellaneous Persistent Warning"
The warning text can include arbitrary information to be presented to The warning text can include arbitrary information to be presented to
a human user or logged. A system receiving this warning MUST NOT a human user or logged. A system receiving this warning MUST NOT
take any automated action. take any automated action.
6. History Lists 5.5.8. Warn Code Registry
User agents often have history mechanisms, such as "Back" buttons and
history lists, that can be used to redisplay a representation
retrieved earlier in a session.
The freshness model (Section 4.2) does not necessarily apply to
history mechanisms. That is, a history mechanism can display a
previous representation even if it has expired.
This does not prohibit the history mechanism from telling the user
that a view might be stale or from honoring cache directives (e.g.,
Cache-Control: no-store).
7. IANA Considerations
7.1. Cache Directive Registry
The "Hypertext Transfer Protocol (HTTP) Cache Directive Registry"
defines the namespace for the cache directives. It has been created
and is now maintained at <http://www.iana.org/assignments/http-cache-
directives>.
7.1.1. Procedure
A registration MUST include the following fields:
o Cache Directive Name
o Pointer to specification text
Values to be added to this namespace require IETF Review (see
[RFC5226], Section 4.1).
7.1.2. Considerations for New Cache Control Directives
New extension directives ought to consider defining:
o What it means for a directive to be specified multiple times,
o When the directive does not take an argument, what it means when
an argument is present,
o When the directive requires an argument, what it means when it is
missing,
o Whether the directive is specific to requests, responses, or able
to be used in either.
See also Section 5.2.3.
7.1.3. Registrations
The registry has been populated with the registrations below:
+------------------------+-----------------------------------+
| Cache Directive | Reference |
+------------------------+-----------------------------------+
| max-age | Section 5.2.1.1, Section 5.2.2.8 |
| max-stale | Section 5.2.1.2 |
| min-fresh | Section 5.2.1.3 |
| must-revalidate | Section 5.2.2.1 |
| no-cache | Section 5.2.1.4, Section 5.2.2.2 |
| no-store | Section 5.2.1.5, Section 5.2.2.3 |
| no-transform | Section 5.2.1.6, Section 5.2.2.4 |
| only-if-cached | Section 5.2.1.7 |
| private | Section 5.2.2.6 |
| proxy-revalidate | Section 5.2.2.7 |
| public | Section 5.2.2.5 |
| s-maxage | Section 5.2.2.9 |
| stale-if-error | [RFC5861], Section 4 |
| stale-while-revalidate | [RFC5861], Section 3 |
+------------------------+-----------------------------------+
7.2. Warn Code Registry
The "Hypertext Transfer Protocol (HTTP) Warn Codes" registry defines The "Hypertext Transfer Protocol (HTTP) Warn Codes" registry defines
the namespace for warn codes. It has been created and is now the namespace for warn codes. It has been created and is now
maintained at <http://www.iana.org/assignments/http-warn-codes>. maintained at <https://www.iana.org/assignments/http-warn-codes>.
7.2.1. Procedure
A registration MUST include the following fields: A registration MUST include the following fields:
o Warn Code (3 digits) o Warn Code (3 digits)
o Short Description o Short Description
o Pointer to specification text o Pointer to specification text
Values to be added to this namespace require IETF Review (see Values to be added to this namespace require IETF Review (see
[RFC5226], Section 4.1). [RFC5226], Section 4.1).
7.2.2. Registrations 6. History Lists
The registry has been populated with the registrations below:
+-----------+----------------------------------+----------------+
| Warn Code | Short Description | Reference |
+-----------+----------------------------------+----------------+
| 110 | Response is Stale | Section 5.5.1 |
| 111 | Revalidation Failed | Section 5.5.2 |
| 112 | Disconnected Operation | Section 5.5.3 |
| 113 | Heuristic Expiration | Section 5.5.4 |
| 199 | Miscellaneous Warning | Section 5.5.5 |
| 214 | Transformation Applied | Section 5.5.6 |
| 299 | Miscellaneous Persistent Warning | Section 5.5.7 |
+-----------+----------------------------------+----------------+
7.3. Header Field Registration
HTTP header fields are registered within the "Message Headers"
registry maintained at <http://www.iana.org/assignments/message-
headers/>.
This document defines the following HTTP header fields, so the User agents often have history mechanisms, such as "Back" buttons and
"Permanent Message Header Field Names" registry has been updated history lists, that can be used to redisplay a representation
accordingly (see [BCP90]). retrieved earlier in a session.
+-------------------+----------+----------+--------------+ The freshness model (Section 4.2) does not necessarily apply to
| Header Field Name | Protocol | Status | Reference | history mechanisms. That is, a history mechanism can display a
+-------------------+----------+----------+--------------+ previous representation even if it has expired.
| Age | http | standard | Section 5.1 |
| Cache-Control | http | standard | Section 5.2 |
| Expires | http | standard | Section 5.3 |
| Pragma | http | standard | Section 5.4 |
| Warning | http | standard | Section 5.5 |
+-------------------+----------+----------+--------------+
The change controller is: "IETF (iesg@ietf.org) - Internet This does not prohibit the history mechanism from telling the user
Engineering Task Force". that a view might be stale or from honoring cache directives (e.g.,
Cache-Control: no-store).
8. Security Considerations 7. Security Considerations
This section is meant to inform developers, information providers, This section is meant to inform developers, information providers,
and users of known security concerns specific to HTTP caching. More and users of known security concerns specific to HTTP caching. More
general security considerations are addressed in HTTP messaging general security considerations are addressed in HTTP messaging
[MESSGNG] and semantics [SEMNTCS]. [Messaging] and semantics [Semantics].
Caches expose additional potential vulnerabilities, since the Caches expose additional potential vulnerabilities, since the
contents of the cache represent an attractive target for malicious contents of the cache represent an attractive target for malicious
exploitation. Because cache contents persist after an HTTP request exploitation. Because cache contents persist after an HTTP request
is complete, an attack on the cache can reveal information long after is complete, an attack on the cache can reveal information long after
a user believes that the information has been removed from the a user believes that the information has been removed from the
network. Therefore, cache contents need to be protected as sensitive network. Therefore, cache contents need to be protected as sensitive
information. information.
In particular, various attacks might be amplified by being stored in In particular, various attacks might be amplified by being stored in
a shared cache; such "cache poisoning" attacks use the cache to a shared cache; such "cache poisoning" attacks use the cache to
distribute a malicious payload to many clients, and are especially distribute a malicious payload to many clients, and are especially
effective when an attacker can use implementation flaws, elevated effective when an attacker can use implementation flaws, elevated
privileges, or other techniques to insert such a response into a privileges, or other techniques to insert such a response into a
cache. One common attack vector for cache poisoning is to exploit cache. One common attack vector for cache poisoning is to exploit
differences in message parsing on proxies and in user agents; see differences in message parsing on proxies and in user agents; see
Section 3.3.3 of [MESSGNG] for the relevant requirements. Section 6.3 of [Messaging] for the relevant requirements.
Likewise, implementation flaws (as well as misunderstanding of cache Likewise, implementation flaws (as well as misunderstanding of cache
operation) might lead to caching of sensitive information (e.g., operation) might lead to caching of sensitive information (e.g.,
authentication credentials) that is thought to be private, exposing authentication credentials) that is thought to be private, exposing
it to unauthorized parties. it to unauthorized parties.
Furthermore, the very use of a cache can bring about privacy Furthermore, the very use of a cache can bring about privacy
concerns. For example, if two users share a cache, and the first one concerns. For example, if two users share a cache, and the first one
browses to a site, the second may be able to detect that the other browses to a site, the second may be able to detect that the other
has been to that site, because the resources from it load more has been to that site, because the resources from it load more
quickly, thanks to the cache. quickly, thanks to the cache.
Note that the Set-Cookie response header field [RFC6265] does not Note that the Set-Cookie response header field [RFC6265] does not
inhibit caching; a cacheable response with a Set-Cookie header field inhibit caching; a cacheable response with a Set-Cookie header field
can be (and often is) used to satisfy subsequent requests to caches. can be (and often is) used to satisfy subsequent requests to caches.
Servers who wish to control caching of these responses are encouraged Servers who wish to control caching of these responses are encouraged
to emit appropriate Cache-Control response header fields. to emit appropriate Cache-Control response header fields.
9. References 8. IANA Considerations
9.1. Normative References This section is to be removed before publishing as an RFC.
[AUTHFRM] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, The change controller for the following registrations is: "IETF
Ed., "Hypertext Transfer Protocol (HTTP): Authentication", (iesg@ietf.org) - Internet Engineering Task Force".
draft-ietf-httpbis-auth-00 (work in progress), April 2018.
[CONDTNL] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, 8.1. Header Field Registration
Ed., "Hypertext Transfer Protocol (HTTP): Conditional
Requests", draft-ietf-httpbis-conditional-00 (work in
progress), April 2018.
[MESSGNG] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Please update the "Message Headers" registry of "Permanent Message
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Message Header Field Names" at <https://www.iana.org/assignments/message-
Syntax and Routing", draft-ietf-httpbis-messaging-00 (work headers> with the header field names listed in the table of
in progress), April 2018. Section 5.
[RANGERQ] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, 8.2. Cache Directive Registration
Ed., "Hypertext Transfer Protocol (HTTP): Range Requests",
draft-ietf-httpbis-range-00 (work in progress), April Please update the "Hypertext Transfer Protocol (HTTP) Cache Directive
2018. Registry" at <https://www.iana.org/assignments/http-cache-directives>
with the registration procedure of Section 5.2.4 and the cache
directive names summarized in the table of Section 5.2.
8.3. Warn Code Registration
Please update the "Hypertext Transfer Protocol (HTTP) Warn Codes"
registry at <https://www.iana.org/assignments/http-warn-codes> with
the registration procedure of Section 5.5.8 and the warn code values
summarized in the table of Section 5.5.
9. References
9.1. Normative References
[Messaging]
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP/1.1 Messaging", draft-ietf-httpbis-messaging-01
(work in progress), May 2018.
[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, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008, DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>. <https://www.rfc-editor.org/info/rfc5234>.
[SEMNTCS] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, [Semantics]
Ed., "Hypertext Transfer Protocol (HTTP): Semantics and Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Content", draft-ietf-httpbis-semantics-00 (work in Ed., "HTTP Semantics", draft-ietf-httpbis-semantics-01
progress), April 2018. (work in progress), May 2018.
9.2. Informative References [USASCII] American National Standards Institute, "Coded Character
Set -- 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
[BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration 9.2. Informative References
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004, <https://www.rfc-editor.org/info/bcp90>.
[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, Transfer Protocol -- HTTP/1.1", RFC 2616,
DOI 10.17487/RFC2616, June 1999, DOI 10.17487/RFC2616, June 1999,
<https://www.rfc-editor.org/info/rfc2616>. <https://www.rfc-editor.org/info/rfc2616>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226, IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008, DOI 10.17487/RFC5226, May 2008,
skipping to change at page 38, line 5 skipping to change at page 38, line 5
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011, DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>. <https://www.rfc-editor.org/info/rfc6265>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP): Caching", Ed., "Hypertext Transfer Protocol (HTTP): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014, RFC 7234, DOI 10.17487/RFC7234, June 2014,
<https://www.rfc-editor.org/info/rfc7234>. <https://www.rfc-editor.org/info/rfc7234>.
Appendix A. Changes from RFC 7234 Appendix A. Collected ABNF
None yet.
Appendix B. Imported ABNF
The following core rules are included by reference, as defined in
Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return),
CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any
8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII
character).
The rules below are defined in [MESSGNG]:
OWS = <OWS, see [MESSGNG], Section 3.2.3>
field-name = <field-name, see [MESSGNG], Section 3.2>
quoted-string = <quoted-string, see [MESSGNG], Section 3.2.6>
token = <token, see [MESSGNG], Section 3.2.6>
port = <port, see [MESSGNG], Section 2.7>
pseudonym = <pseudonym, see [MESSGNG], Section 5.7.1>
uri-host = <uri-host, see [MESSGNG], Section 2.7>
The rules below are defined in other parts:
HTTP-date = <HTTP-date, see [SEMNTCS], Section 7.1.1.1>
Appendix C. Collected ABNF
In the collected ABNF below, list rules are expanded as per In the collected ABNF below, list rules are expanded as per
Section 1.2 of [MESSGNG]. Section 11 of [Semantics].
Age = delta-seconds Age = delta-seconds
Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS
cache-directive ] ) cache-directive ] )
Expires = HTTP-date Expires = HTTP-date
HTTP-date = <HTTP-date, see [SEMNTCS], Section 7.1.1.1> HTTP-date = <HTTP-date, see [Semantics], Section 10.1.1.1>
OWS = <OWS, see [MESSGNG], Section 3.2.3> OWS = <OWS, see [Semantics], Section 4.3>
Pragma = *( "," OWS ) pragma-directive *( OWS "," [ OWS Pragma = *( "," OWS ) pragma-directive *( OWS "," [ OWS
pragma-directive ] ) pragma-directive ] )
Warning = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ] Warning = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
) )
cache-directive = token [ "=" ( token / quoted-string ) ] cache-directive = token [ "=" ( token / quoted-string ) ]
delta-seconds = 1*DIGIT delta-seconds = 1*DIGIT
extension-pragma = token [ "=" ( token / quoted-string ) ] extension-pragma = token [ "=" ( token / quoted-string ) ]
field-name = <field-name, see [MESSGNG], Section 3.2> field-name = <field-name, see [Semantics], Section 4.2>
port = <port, see [MESSGNG], Section 2.7> port = <port, see [RFC3986], Section 3.2.3>
pragma-directive = "no-cache" / extension-pragma pragma-directive = "no-cache" / extension-pragma
pseudonym = <pseudonym, see [MESSGNG], Section 5.7.1> pseudonym = <pseudonym, see [Semantics], Section 5.6.1>
quoted-string = <quoted-string, see [MESSGNG], Section 3.2.6> quoted-string = <quoted-string, see [Semantics], Section 4.2.3>
token = <token, see [MESSGNG], Section 3.2.6> token = <token, see [Semantics], Section 4.2.3>
uri-host = <uri-host, see [MESSGNG], Section 2.7> uri-host = <host, see [RFC3986], Section 3.2.2>
warn-agent = ( uri-host [ ":" port ] ) / pseudonym warn-agent = ( uri-host [ ":" port ] ) / pseudonym
warn-code = 3DIGIT warn-code = 3DIGIT
warn-date = DQUOTE HTTP-date DQUOTE warn-date = DQUOTE HTTP-date DQUOTE
warn-text = quoted-string warn-text = quoted-string
warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
] ]
Appendix D. Change Log Appendix B. Changes from RFC 7234
None yet.
Appendix C. Change Log
This section is to be removed before publishing as an RFC. This section is to be removed before publishing as an RFC.
D.1. Since RFC 7234 C.1. Between RFC7234 and draft 00
The changes in this draft are purely editorial: The changes were purely editorial:
o Change boilerplate and abstract to indicate the "draft" status, o Change boilerplate and abstract to indicate the "draft" status,
and update references to ancestor specifications. and update references to ancestor specifications.
o Remove version "1.1" from document title, indicating that this o Remove version "1.1" from document title, indicating that this
specification applies to all HTTP versions. specification applies to all HTTP versions.
o Adjust historical notes. o Adjust historical notes.
o Update links to sibling specifications. o Update links to sibling specifications.
o Replace sections listing changes from RFC 2616 by new empty o Replace sections listing changes from RFC 2616 by new empty
sections referring to RFC 723x. sections referring to RFC 723x.
o Remove acknowledgements specific to RFC 723x. o Remove acknowledgements specific to RFC 723x.
o Move "Acknowledgements" to the very end and make them unnumbered. o Move "Acknowledgements" to the very end and make them unnumbered.
C.2. Since draft-ietf-httpbis-cache-00
The changes are purely editorial:
o Moved all extensibility tips, registration procedures, and
registry tables from the IANA considerations to normative
sections, reducing the IANA considerations to just instructions
that will be removed prior to publication as an RFC.
Index Index
1 1
110 (warn-code) 31 110 (warn-code) 33
111 (warn-code) 31 111 (warn-code) 33
112 (warn-code) 31 112 (warn-code) 33
113 (warn-code) 31 113 (warn-code) 34
199 (warn-code) 31 199 (warn-code) 34
2 2
214 (warn-code) 31 214 (warn-code) 34
299 (warn-code) 31 299 (warn-code) 34
A A
Age header field 20 Age header field 21
age 11 age 11
C C
Cache-Control header field 21 Cache-Control header field 22
cache 4 cache 4
cache entry 5 cache entry 6
cache key 5-6 cache key 6
D D
Disconnected Operation (warn-text) 31 Disconnected Operation (warn-text) 33
E E
Expires header field 27 Expires header field 29
explicit expiration time 10 explicit expiration time 11
F F
fresh 10 fresh 11
freshness lifetime 10 freshness lifetime 11
G G
Grammar Grammar
Age 20 Age 21
Cache-Control 21 ALPHA 5
cache-directive 21 Cache-Control 22
cache-directive 22
CR 5
CRLF 5
CTL 5
delta-seconds 5 delta-seconds 5
Expires 27 DIGIT 5
extension-pragma 28 DQUOTE 5
Pragma 28 Expires 30
pragma-directive 28 extension-pragma 31
warn-agent 29 HEXDIG 5
warn-code 29 HTAB 5
warn-date 29 LF 5
warn-text 29 OCTET 5
Warning 29 Pragma 31
warning-value 29 pragma-directive 31
SP 5
VCHAR 5
warn-agent 32
warn-code 32
warn-date 32
warn-text 32
Warning 32
warning-value 32
H H
Heuristic Expiration (warn-text) 31 Heuristic Expiration (warn-text) 34
heuristic expiration time 10 heuristic expiration time 11
M M
Miscellaneous Persistent Warning (warn-text) 31 Miscellaneous Persistent Warning (warn-text) 34
Miscellaneous Warning (warn-text) 31 Miscellaneous Warning (warn-text) 34
max-age (cache directive) 21, 26 max-age (cache directive) 23, 28
max-stale (cache directive) 22 max-stale (cache directive) 23
min-fresh (cache directive) 22 min-fresh (cache directive) 24
must-revalidate (cache directive) 23 must-revalidate (cache directive) 25
N N
no-cache (cache directive) 22, 24 no-cache (cache directive) 24-25
no-store (cache directive) 23-24 no-store (cache directive) 24, 26
no-transform (cache directive) 23, 25 no-transform (cache directive) 25-26
O O
only-if-cached (cache directive) 23 only-if-cached (cache directive) 25
P P
Pragma header field 28 Pragma header field 30
private (cache directive) 25 private (cache directive) 27
private cache 4 private cache 4
proxy-revalidate (cache directive) 26 proxy-revalidate (cache directive) 27
public (cache directive) 25 public (cache directive) 27
R R
Response is Stale (warn-text) 31 Response is Stale (warn-text) 33
Revalidation Failed (warn-text) 31 Revalidation Failed (warn-text) 33
S S
s-maxage (cache directive) 26 s-maxage (cache directive) 28
shared cache 4 shared cache 4
stale 10 stale 11
strong validator 18 strong validator 19
T T
Transformation Applied (warn-text) 31 Transformation Applied (warn-text) 34
V V
validator 15 validator 16
W W
Warning header field 29 Warning header field 31
Acknowledgments Acknowledgments
See Appendix "Acknowledgments" of [MESSGNG]. See Appendix "Acknowledgments" of [Semantics].
Authors' Addresses Authors' Addresses
Roy T. Fielding (editor) Roy T. Fielding (editor)
Adobe Adobe
345 Park Ave 345 Park Ave
San Jose, CA 95110 San Jose, CA 95110
USA USA
EMail: fielding@gbiv.com EMail: fielding@gbiv.com
URI: http://roy.gbiv.com/ URI: https://roy.gbiv.com/
Mark Nottingham (editor) Mark Nottingham (editor)
Fastly Fastly
EMail: mnot@mnot.net EMail: mnot@mnot.net
URI: https://www.mnot.net/ URI: https://www.mnot.net/
Julian F. Reschke (editor) Julian F. Reschke (editor)
greenbytes GmbH greenbytes GmbH
Hafenweg 16 Hafenweg 16
Muenster, NW 48155 Muenster, NW 48155
Germany Germany
EMail: julian.reschke@greenbytes.de EMail: julian.reschke@greenbytes.de
URI: http://greenbytes.de/tech/webdav/ URI: https://greenbytes.de/tech/webdav/
 End of changes. 119 change blocks. 
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