HTTP Immutable Responses

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Document Type Active Internet-Draft (httpbis WG)
Last updated 2017-05-22 (latest revision 2017-05-01)
Replaces draft-mcmanus-immutable
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Intended RFC status Proposed Standard
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Document shepherd Mark Nottingham
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HTTP                                                          P. McManus
Internet-Draft                                                   Mozilla
Intended status: Standards Track                          April 30, 2017
Expires: November 1, 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.

Note to Readers

   Discussion of this draft takes place on the HTTP working group
   mailing list (, which is archived at .

   Working Group information can be found at ;
   source code and issues list for this draft can be found at .

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   This Internet-Draft will expire on November 1, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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1.  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, "" might be
   updated and republished as "",
   with any links that reference it being changed at the same time.
   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

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   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
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