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draft-mcmanus-immutable-01.txt
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Network Working Group P. McManus
Internet-Draft Mozilla
Intended status: Standards Track December 19, 2016
Expires: June 22, 2017
HTTP Immutable Responses
draft-mcmanus-immutable-01
Abstract
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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 22, 2017.
Copyright Notice
Copyright (c) 2016 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
(http://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.
McManus Expires June 22, 2017 [Page 1]
Internet-Draft I-D December 2016
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
information.
For example, https://www.example.com/101016/main.css might be updated
and republished as https://www.example.com/102026/main.css 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 June 22, 2017 [Page 2]
Internet-Draft I-D December 2016
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
response.
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.
McManus Expires June 22, 2017 [Page 3]
Internet-Draft I-D December 2016
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
interactions.
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,
<http://www.rfc-editor.org/info/rfc7231>.
[RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
DOI 10.17487/RFC7232, June 2014,
<http://www.rfc-editor.org/info/rfc7232>.
[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,
<http://www.rfc-editor.org/info/rfc7234>.
6.2. Informative References
[REQPERPAGE]
"HTTP Archive", n.d.,
<http://httparchive.org/interesting.php#reqTotal>.
McManus Expires June 22, 2017 [Page 4]
Internet-Draft I-D December 2016
Author's Address
Patrick McManus
Mozilla
Email: [email protected]
McManus Expires June 22, 2017 [Page 5]