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Internet Engineering Task Force (IETF)                        P. McManusRequest for Comments: 8246                                       MozillaCategory: Standards Track                                 September 2017ISSN: 2070-1721HTTP Immutable ResponsesAbstract   The immutable HTTP response Cache-Control extension allows servers to   identify resources that will not be updated during their freshness   lifetime.  This ensures that a client never needs to revalidate a   cached fresh resource to be certain it has not been modified.Status of This Memo   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   Internet Standards is available inSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttps://www.rfc-editor.org/info/rfc8246.Copyright Notice   Copyright (c) 2017 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (https://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.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.McManus                      Standards Track                    [Page 1]

RFC 8246                 HTTP Immutable Response          September 2017Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .21.1.  Notational Conventions  . . . . . . . . . . . . . . . . .32.  The Immutable Cache-Control Extension . . . . . . . . . . . .32.1.  About Intermediaries  . . . . . . . . . . . . . . . . . .42.2.  Example . . . . . . . . . . . . . . . . . . . . . . . . .43.  Security Considerations . . . . . . . . . . . . . . . . . . .44.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .55.  References  . . . . . . . . . . . . . . . . . . . . . . . . .55.1.  Normative References  . . . . . . . . . . . . . . . . . .55.2.  Informative References  . . . . . . . . . . . . . . . . .5   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .6   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .61.  Introduction   HTTP's freshness lifetime mechanism [RFC7234] allows a client to   safely reuse a stored response to satisfy future requests for a   specified period of time.  However, it is still possible that the   resource will be modified during that period.   For instance, a front-page newspaper photo with a freshness lifetime   of one hour would mean that no user would see a cached photo more   than one hour old.  However, the photo could be updated at any time,   resulting in different users seeing different photos depending on the   contents of their caches for up to one hour.  This is compliant with   the caching mechanism defined in [RFC7234].   Users that need to confirm there have been no updates to their cached   responses typically use the reload (or refresh) mechanism in their   user agents.  This in turn generates a conditional request [RFC7232],   and either a new representation or, if unmodified, a 304 (Not   Modified) response [RFC7232] is returned.  A user agent that   understands HTML and fetches its dependent sub-resources might issue   hundreds of conditional requests to refresh all portions of a common   page [REQPERPAGE].   However, some content providers never create more than one variant of   a sub-resource, because they use "versioned" URLs.  When these   resources need an update, they are simply published under a new URL,   typically embedding an identifier unique to that version of the   resource in the path, and references to the sub-resource are updated   with the new path information.   For example, "https://www.example.com/101016/main.css" might be   updated and republished as "https://www.example.com/102026/main.css",   with any links that reference it being changed at the same time.McManus                      Standards Track                    [Page 2]

RFC 8246                 HTTP Immutable Response          September 2017   This design pattern allows a very large freshness lifetime to be used   for the sub-resource without guessing when it will be updated in the   future.   Unfortunately, the user agent does not know when this versioned URL   design pattern is used.  As a result, user-driven refreshes still   translate into wasted conditional requests for each sub-resource as   each will return 304 responses.   The immutable HTTP response Cache-Control extension allows servers to   identify responses that will not be updated during their freshness   lifetimes.   This effectively informs clients that any conditional request for   that response can be safely skipped without worrying that it has been   updated.1.1.  Notational Conventions   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and   "OPTIONAL" in this document are to be interpreted as described inBCP14 [RFC2119] [RFC8174] when, and only when, they appear in all   capitals, as shown here.2.  The Immutable Cache-Control Extension   When present in an HTTP response, the immutable Cache-Control   extension indicates that the origin server will not update the   representation of that resource during the freshness lifetime of the   response.   Clients SHOULD NOT issue a conditional request during the response's   freshness lifetime (e.g., upon a reload) unless explicitly overridden   by the user (e.g., a force reload).   The immutable extension only applies during the freshness lifetime of   the stored response.  Stale responses SHOULD be revalidated as they   normally would be in the absence of the immutable extension.   The immutable extension takes no arguments.  If any arguments are   present, they have no meaning and MUST be ignored.  Multiple   instances of the immutable extension are equivalent to one instance.   The presence of an immutable Cache-Control extension in a request has   no effect.McManus                      Standards Track                    [Page 3]

RFC 8246                 HTTP Immutable Response          September 20172.1.  About Intermediaries   An immutable response has the same semantic meaning when received by   proxy clients as it does when received by user-agent-based clients.   Therefore, proxies SHOULD skip conditionally revalidating fresh   responses containing the immutable extension unless there is a signal   from the client that a validation is necessary (e.g., a no-cache   Cache-Control request directive defined inSection 5.2.1.4 of   [RFC7234]).   A proxy that uses the immutable extension to bypass a conditional   revalidation can choose whether to reply with a 304 or 200 response   to its requesting client based on the request headers the proxy   received.2.2.  Example   Cache-Control: max-age=31536000, immutable3.  Security Considerations   The immutable mechanism acts as form of soft pinning and, as with all   pinning mechanisms, creates a vector for amplification of cache   corruption incidents.  These incidents include cache-poisoning   attacks.  Three mechanisms are suggested for mitigation of this risk:   o  Clients SHOULD ignore the immutable extension from resources that      are not part of an authenticated context such as HTTPS.      Authenticated resources are less vulnerable to cache poisoning.   o  User agents often provide two different refresh mechanisms: reload      and some form of force-reload.  The latter is used to rectify      interrupted loads and other corruption.  These reloads, typically      indicated through no-cache request attributes, SHOULD ignore the      immutable extension as well.   o  Clients SHOULD ignore the immutable extension for resources that      do not provide a strong indication that the stored response size      is the correct response size such as responses delimited by      connection close.McManus                      Standards Track                    [Page 4]

RFC 8246                 HTTP Immutable Response          September 20174.  IANA Considerations   The immutable extension has been registered in the "Hypertext   Transfer Protocol (HTTP) Cache Directive Registry" per the guidelines   described inSection 7.1 of [RFC7234].   o  Cache Directive: immutable   o  Reference:RFC 82465.  References5.1.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119,              DOI 10.17487/RFC2119, March 1997,              <https://www.rfc-editor.org/info/rfc2119>.   [RFC7232]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer              Protocol (HTTP/1.1): Conditional Requests",RFC 7232,              DOI 10.17487/RFC7232, June 2014,              <https://www.rfc-editor.org/info/rfc7232>.   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",RFC 7234, DOI 10.17487/RFC7234, June 2014,              <https://www.rfc-editor.org/info/rfc7234>.   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase inRFC2119 Key Words",BCP 14,RFC 8174, DOI 10.17487/RFC8174,              May 2017, <https://www.rfc-editor.org/info/rfc8174>.5.2.  Informative References   [REQPERPAGE]              HTTP Archive, "Total Requests per Page",              <http://httparchive.org/interesting.php#reqTotal>.McManus                      Standards Track                    [Page 5]

RFC 8246                 HTTP Immutable Response          September 2017Acknowledgments   Thank you to Ben Maurer for partnership in developing and testing   this idea.  Thank you to Amos Jeffries for help with proxy   interactions and to Mark Nottingham for help with the documentation.Author's Address   Patrick McManus   Mozilla   Email: mcmanus@ducksong.comMcManus                      Standards Track                    [Page 6]