Independent Submission M. Nottingham

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: [email protected]
URI: http://www.mnot.net/

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