HTTP Cache-Control Extensions for Stale Content
RFC 5861

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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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RFC 5861                   HTTP stale controls                  May 2010

     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