HTTP Cache-Control Extensions for Stale Content
RFC 5861

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Independent Submission                                     M. Nottingham
Request for Comments: 5861                                   Yahoo! Inc.
Category: Informational                                         May 2010
ISSN: 2070-1721

            HTTP Cache-Control Extensions for Stale Content

Abstract

   This document defines two independent HTTP Cache-Control extensions
   that allow control over the use of stale responses by caches.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5861.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . . . 2
   3.  The stale-while-revalidate Cache-Control Extension  . . . . . . 2
     3.1.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   4.  The stale-if-error Cache-Control Extension  . . . . . . . . . . 3
     4.1.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   6.  Normative References  . . . . . . . . . . . . . . . . . . . . . 5
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . . . 6

1.  Introduction

   HTTP [RFC2616] requires that caches "respond to a request with the
   most up-to-date response held... that is appropriate to the request,"
   although "in carefully considered circumstances" a stale response is
   allowed to be returned.  This document defines two independent Cache-
   Control extensions that allow for such control, stale-if-error and
   stale-while-revalidate.

   The stale-if-error HTTP Cache-Control extension allows a cache to
   return a stale response when an error -- e.g., a 500 Internal Server
   Error, a network segment, or DNS failure -- is encountered, rather
   than returning a "hard" error.  This improves availability.

   The stale-while-revalidate HTTP Cache-Control extension allows a
   cache to immediately return a stale response while it revalidates it
   in the background, thereby hiding latency (both in the network and on
   the server) from clients.

2.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   This specification uses the augmented Backus-Naur Form of RFC 2616
   [RFC2616], and it includes the delta-seconds rule from that
   specification.

3.  The stale-while-revalidate Cache-Control Extension

   When present in an HTTP response, the stale-while-revalidate Cache-
   Control extension indicates that caches MAY serve the response in
   which it appears after it becomes stale, up to the indicated number
   of seconds.

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     stale-while-revalidate = "stale-while-revalidate" "=" delta-seconds

   If a cached response is served stale due to the presence of this
   extension, the cache SHOULD attempt to revalidate it while still
   serving stale responses (i.e., without blocking).

   Note that "stale" implies that the response will have a non-zero Age
   header and a warning header, as per HTTP's requirements.

   If delta-seconds passes without the cached entity being revalidated,
   it SHOULD NOT continue to be served stale, absent other information.

3.1.  Example

   A response containing:

     Cache-Control: max-age=600, stale-while-revalidate=30

   indicates that it is fresh for 600 seconds, and it may continue to be
   served stale for up to an additional 30 seconds while an asynchronous
   validation is attempted.  If validation is inconclusive, or if there
   is not traffic that triggers it, after 30 seconds the stale-while-
   revalidate function will cease to operate, and the cached response
   will be "truly" stale (i.e., the next request will block and be
   handled normally).

   Generally, servers will want to set the combination of max-age and
   stale-while-revalidate to the longest total potential freshness
   lifetime that they can tolerate.  For example, with both set to 600,
   the server must be able to tolerate the response being served from
   cache for up to 20 minutes.

   Since asynchronous validation will only happen if a request occurs
   after the response has become stale, but before the end of the stale-
   while-revalidate window, the size of that window and the likelihood
   of a request during it determines how likely it is that all requests
   will be served without delay.  If the window is too small, or traffic
   is too sparse, some requests will fall outside of it, and block until
   the server can validate the cached response.

4.  The stale-if-error Cache-Control Extension

   The stale-if-error Cache-Control extension indicates that when an
   error is encountered, a cached stale response MAY be used to satisfy
   the request, regardless of other freshness information.

     stale-if-error = "stale-if-error" "=" delta-seconds

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   When used as a request Cache-Control extension, its scope of
   application is the request it appears in; when used as a response
   Cache-Control extension, its scope is any request applicable to the
   cached response in which it occurs.

   Its value indicates the upper limit to staleness; when the cached
   response is more stale than the indicated amount, the cached response
   SHOULD NOT be used to satisfy the request, absent other information.

   In this context, an error is any situation that would result in a
   500, 502, 503, or 504 HTTP response status code being returned.

   Note that this directive does not affect freshness; stale cached
   responses that are used SHOULD still be visibly stale when sent
   (i.e., have a non-zero Age header and a warning header, as per HTTP's
   requirements).

4.1.  Example

   A response containing:

     HTTP/1.1 200 OK
     Cache-Control: max-age=600, stale-if-error=1200
     Content-Type: text/plain

     success

   indicates that it is fresh for 600 seconds, and that it may be used
   if an error is encountered after becoming stale for an additional
   1200 seconds.

   Thus, if the cache attempts to validate 900 seconds afterwards and
   encounters:

     HTTP/1.1 500 Internal Server Error
     Content-Type: text/plain

     failure

   the successful response can be returned instead:

     HTTP/1.1 200 OK
     Cache-Control: max-age=600, stale-if-error=1200
     Age: 900
     Content-Type: text/plain

     success

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   After the age is greater than 1800 seconds (i.e., it has been stale
   for 1200 seconds), the cache must write the error message through.

     HTTP/1.1 500 Internal Server Error
     Content-Type: text/plain

     failure

5.  Security Considerations

   The stale-while-revalidate extension provides origin servers with a
   mechanism for dictating that stale content should be served from
   caches under certain circumstances, with the expectation that the
   cached response will be revalidated in the background.  It is
   suggested that such validation be predicated upon an incoming
   request, to avoid the possibility of an amplification attack (as can
   be seen in some other pre-fetching and automatic refresh mechanisms).
   Cache implementers should keep this in mind when deciding the
   circumstances under which they will generate a request that is not
   directly initiated by a user or client.

   The stale-if-error provides origin servers and clients a mechanism
   for dictating that stale content should be served from caches under
   certain circumstances, and does not pose additional security
   considerations over those of RFC 2616, which also allows stale
   content to be served.

6.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

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Appendix A.  Acknowledgements

   Thanks to Ben Drees, John Nienart, Henrik Nordstrom, Evan Torrie, and
   Chris Westin for their suggestions.  The author takes all
   responsibility for errors and omissions.

Author's Address

   Mark Nottingham
   Yahoo! Inc.

   EMail: mnot@yahoo-inc.com
   URI:   http://www.mnot.net/

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