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Versions: 00 01

Network Working Group                                      M. Nottingham
Internet-Draft                                               Yahoo! Inc.
Intended status: Informational                               May 9, 2008
Expires: November 10, 2008


            The stale-if-error HTTP Cache-Control Extension
                draft-nottingham-http-stale-if-error-01

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Abstract

   The stale-if-error HTTP Cache-Control extension improves availability
   of some kinds of cached content by allowing servers and clients to
   instruct caches to use stale responses when certain error conditions
   are encountered.











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Table of Contents

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







































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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 controlled circumstances" a stale response is
   allowed to be returned.

   Those circumstances are not well-defined.  Often, it is useful to
   return a stale response when an error -- e.g., a 500 Internal Server
   Error, a network segment, or DNS failure -- is encountered, but
   caches are understandably reluctant to act without explicit
   instructions about the appropriate behaviour.

   The stale-if-error HTTP Cache-Control extension addresses this by
   allowing origin servers as well as clients to instruct caches to use
   a stale response under certain conditions, rather than returning a
   "hard" error, thus improving availability.


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 RFC2616
   [RFC2616], and includes the delta-seconds rule from that
   specification.


3.  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

   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 it occurs in.

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

   In this context, an error is any situation which would result in a



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   500, 502, 503 or 504 HTTP response status code being returned.

   Note that this directive only affects the freshness of a response in
   an implementation that recognises it; stale cached responses that are
   used SHOULD still be visibly stale when sent.


4.  Response stale-if-error 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

     succcess

   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






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5.  Security Considerations

   This document 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 RFC2616, which also allows stale content
   to be served.


6.  IANA Considerations

   This document has no actions for IANA.


7.  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.


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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