Internet DRAFT - draft-ietf-httpbis-immutable
draft-ietf-httpbis-immutable
HTTP Working Group P. McManus
Internet-Draft Mozilla
Intended status: Standards Track July 3, 2017
Expires: January 4, 2018
HTTP Immutable Responses
draft-ietf-httpbis-immutable-03
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.
Note to Readers
Discussion of this draft takes place on the HTTP working group
mailing list (ietf-http-wg@w3.org), which is archived at
https://lists.w3.org/Archives/Public/ietf-http-wg/ .
Working Group information can be found at http://httpwg.github.io/ ;
source code and issues list for this draft can be found at
https://github.com/httpwg/http-extensions/labels/immutable .
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This Internet-Draft will expire on January 4, 2018.
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, "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.
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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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
to identify responses that will not be updated during their freshness
lifetimes.
This effectively informs clients that any conditional request for
that response can be safely skipped without worrying that it has been
updated.
1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. The immutable Cache-Control extension
When present in an HTTP response, the "immutable" Cache-Control
extension indicates that the origin server will not update the
representation of that resource during the freshness lifetime of the
response.
Clients SHOULD NOT issue a conditional request during the response's
freshness lifetime (e.g. upon a reload) unless explicitly overridden
by the user (e.g. a force reload).
The immutable extension only applies during the freshness lifetime of
the stored response. Stale responses SHOULD be revalidated as they
normally would be in the absence of immutable.
The immutable extension takes no arguments. If any arguments are
present, they have no meaning, and MUST be ignored. 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 when received by
proxy clients as it does when received by User-Agent based clients.
Therefore proxies SHOULD skip conditionally revalidating fresh
responses containing the immutable extension unless there is a signal
from the client that a validation is necessary (e.g. a no-cache
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Cache-Control request directive defined by Section 5.2.1.4 of
[RFC7234]).
A proxy that uses immutable to bypass a conditional revalidation can
choose whether to reply with a 304 or 200 to its requesting client
based on the request headers the proxy received.
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 from 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 mechanisms: 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.
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
Section 7.1 of [RFC7234] requires 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 and to Mark Nottingham for help with the documentation.
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6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[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>.
Author's Address
Patrick McManus
Mozilla
Email: pmcmanus@mozilla.com
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