Network Working Group                                         P. McManus
Internet-Draft                                                   Mozilla
Intended status: Standards Track                       December 19, 2016
Expires: June 22, 2017

                        HTTP Immutable Responses


   The immutable HTTP response Cache-Control extension allows servers to
   identify resources that will not be updated during their freshness
   lifetime.  This assures that a client never needs to revalidate a
   cached fresh resource to be certain it has not been modified.

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1.  Introduction

   The HTTP freshness lifetime [RFC7234] caching attribute specifies
   that a client may safely reuse a response to satisfy future requests
   over a specific period of time.  It does not specify that the
   resource will be not be modified during that period.

   For instance, a front page newspaper photo with a freshness lifetime
   of one hour would mean that no user should see a 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
   current cached resources typically invoke the reload (or refresh)
   mechanism in the user agent.  This in turn generates a conditional
   request [RFC7232] and either a new representation or, if unmodified,
   a 304 response [RFC7231] is returned.  A user agent that manages HTML
   and its dependent sub-resources may issue hundreds of conditional
   requests to refresh all portions of a common HTML page [REQPERPAGE].

   Through the use of the versioned URL design pattern some content
   providers never create more than one variant of a sub-resource.  When
   these resources need an update they are simply published under a new
   URL, typically embedding a variant identifier in the path, and
   references to the sub-resource are updated with the new path

   For example, might be updated
   and republished as and the
   html that references it is changed at the same time.  This design
   pattern allows a very large freshness lifetime to be applied to the
   sub-resource without guessing when it will be updated in the future.

   Unfortunately, the user-agent is not aware of the versioned URL
   design pattern.  User driven refresh events 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 resources that will not be updated during their freshness
   lifetime.  This effectively instructs the client that any conditional
   request for a previously served variant of that resource may be
   safely skipped without worrying that it has been updated.

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2.  The immutable Cache-Control extension

   When present in an HTTP response, the immutable Cache-Control
   extension indicates that the origin server MUST NOT update the
   representation of that resource during the freshness lifetime of the

   The immutable extension only applies during the freshness lifetime of
   the response.  Stale responses SHOULD be revalidated as they normally
   would be in the absence of immutable.

   The immutable extension takes no arguments and if any arguments are
   present they have no meaning.  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.

2.1.  About Intermediaries

   An immutable response has the same semantic meaning for proxy clients
   as it does for User-Agent based clients and they therefore MAY also
   presume a conditional revalidation for a response marked immutable
   would return 304.  A proxy client who uses immutable to anticipate a
   304 response may choose whether to reply with a 304 or 200 to its
   requesting client.

2.2.  Example

   Cache-Control: max-age=31536000, immutable

3.  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 immutable for 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 mechanismss:
      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 immutable as well.

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   o  Clients should ignore immutable 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.

4.  IANA Considerations

   [RFC7234] sections 7.1 and 7.1.2 require registration of the
   immutable extension in the "Hypertext Transfer Protocol (HTTP) Cache
   Directive Registry" with IETF Review.

   o  Cache-Directive: immutable

   o  Pointer to specification text: [this document]

5.  Acknowledgments

   Thank you to Ben Maurer for partnership in developing and testing
   this idea.  Thank you to Amos Jeffries for help with proxy

6.  References

6.1.  Normative References

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,

   [RFC7232]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
              DOI 10.17487/RFC7232, June 2014,

   [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,

6.2.  Informative References

              "HTTP Archive", n.d.,

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Author's Address

   Patrick McManus


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