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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.

Nottingham Informational [Page 1] RFC 5861 HTTP stale controls May 2010

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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