HTTP Immutable Responses
RFC 8246

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Internet Engineering Task Force (IETF)                        P. McManus
Request for Comments: 8246                                       Mozilla
Category: Standards Track                                 September 2017
ISSN: 2070-1721

                        HTTP Immutable Responses

Abstract

   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 in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://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 to BCP 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.

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RFC 8246                 HTTP Immutable Response          September 2017

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  The Immutable Cache-Control Extension . . . . . . . . . . . .   3
     2.1.  About Intermediaries  . . . . . . . . . . . . . . . . . .   4
     2.2.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

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, "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.

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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.