Network Working Group M. Nottingham Internet-Draft Yahoo! Inc. Intended status: Informational February 19, 2007 Expires: August 23, 2007 The stale-if-error HTTP Cache-Control Extension draft-nottingham-http-stale-if-error-00 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on August 23, 2007. Copyright Notice Copyright (C) The IETF Trust (2007). 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. Nottingham Expires August 23, 2007 [Page 1] Internet-Draft stale-if-error February 2007 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. Normative References . . . . . . . . . . . . . . . . . . . . . 5 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 5 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 5 Intellectual Property and Copyright Statements . . . . . . . . . . 6 Nottingham Expires August 23, 2007 [Page 2] Internet-Draft stale-if-error February 2007 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 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-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. Nottingham Expires August 23, 2007 [Page 3] Internet-Draft stale-if-error February 2007 In this context, an error is any situation which would result in a 500, 501, 502, 503 or 504 HTTP response status code to be returned. Future extensions may define additional error conditions. 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 Nottingham Expires August 23, 2007 [Page 4] Internet-Draft stale-if-error February 2007 5. Security Considerations This mechanism does not have any security-specific concerns. 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. 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/ Nottingham Expires August 23, 2007 [Page 5] Internet-Draft stale-if-error February 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). 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The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Nottingham Expires August 23, 2007 [Page 6]