


Network Working Group                                      M. Nottingham
Internet-Draft                                               Yahoo! Inc.
Intended status: Informational                         February 19, 2007
Expires: August 23, 2007


        The stale-while-revalidate HTTP Cache-Control Extension
            draft-nottingham-http-stale-while-revalidate-00

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   The stale-while-revalidate HTTP response Cache-Control extension
   allows servers to instruct shared caches to serve stale responses
   while validating them, to avoid latency in some situations.








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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . . . 3
   3.  The stale-while-revalidate Cache-Control Extension  . . . . . . 3
   4.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 4
   6.  Normative References  . . . . . . . . . . . . . . . . . . . . . 4
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . . . 5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 5
   Intellectual Property and Copyright Statements  . . . . . . . . . . 6








































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1.  Introduction

   The potential for latency (due to the network as well as server
   processing) introduced by cache validation in HTTP [RFC2616] is often
   undesirable; while subsequent requests can be served from the cache
   quickly, the request that triggers validation sees degraded service.

   In some situations, it may be useful to avoid this latency, at the
   cost of serving slightly stale responses. the stale-while-revalidate
   HTTP response Cache-Control extension allows origin servers to
   instruct caches to do this.


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 BCP 14 [RFC2119], as
   scoped to those conformance targets.

   This specification uses the augmented Backus-Naur Form of RFC2616
   [RFC2616], and 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 shared caches MAY serve the response
   it appears in after it becomes stale, up to the indicated number of
   seconds.

     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 MUST NOT continue to be served stale, absent other information.

   Private caches MUST ignore this Cache-Control extension.






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4.  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 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
   too sparse, some requests will fall outside of it, and block until
   the server can validate the cached response.


5.  Security Considerations

   This mechanism 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.


6.  Normative References

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




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   [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.


Appendix A.  Acknowledgements

   Thanks to 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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Full Copyright Statement

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