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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