HTTP Immutable Responses
RFC 8246
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
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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 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.
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.
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