HTTP Immutable Responses

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Document Type Active Internet-Draft (httpbis WG)
Last updated 2017-01-12
Replaces draft-mcmanus-immutable
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Network Working Group                                         P. McManus
Internet-Draft                                                   Mozilla
Intended status: Standards Track                        January 12, 2017
Expires: July 16, 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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McManus                   Expires July 16, 2017                 [Page 1]
Internet-Draft                     I-D                      January 2017

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.

McManus                   Expires July 16, 2017                 [Page 2]
Internet-Draft                     I-D                      January 2017

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
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