wdiff draft-ietf-httpbis-immutable-00.txt draft-ietf-httpbis-immutable-01.txt

Network
HTTP Working Group P. McManus
Internet-Draft Mozilla
Intended status: Standards Track January 12, March 13, 2017
Expires: July 16, September 14, 2017

HTTP Immutable Responses
draft-ietf-httpbis-immutable-00
draft-ietf-httpbis-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.

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 .

Status of This Memo

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1. Introduction

The HTTP

HTTP's freshness lifetime mechanism [RFC7234] caching attribute specifies
that allows a client may to
safely reuse a stored response to satisfy future requests
over for a specific
specified period of time. It does not specify However, it is still possible 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 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
current cached resources
responses typically invoke use the reload (or refresh) mechanism in the their
user agent. agents. This in turn generates a conditional request [RFC7232]
and either a new representation or, if unmodified, a 304 (Not
Modified) response [RFC7231] [RFC7232] is returned. A user agent that manages
understands HTML and fetches its dependent sub-resources may might issue
hundreds of conditional requests to refresh all portions of a common HTML
page [REQPERPAGE].

Through the use of the versioned URL design pattern

However some content providers never create more than one variant of
a sub-resource. sub-resource, because they use "versioned" URLs. When these
resources need an update they are simply published under a new URL,
typically embedding a variant 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 "https://www.example.com/101016/main.css" might be
updated and republished as https://www.example.com/102026/main.css and the
html "https://www.example.com/102026/main.css",
with any links that references it is being changed at the same time.
This design pattern allows a very large freshness lifetime to be applied to used
for the sub-resource without guessing when it will be updated in the
future.

Unfortunately, the user-agent is user agent does not aware of the know when this versioned URL
design pattern. User driven refresh events 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 "immutable" HTTP response Cache-Control extension allows servers
to identify resources responses that will not be updated during their freshness
lifetime.
lifetimes.

This effectively instructs the client informs clients that any conditional request for a previously served variant of
that resource may response can be safely skipped without worrying that it has been
updated.

2. The immutable Cache-Control extension

When present in an HTTP response, the immutable "immutable" Cache-Control
extension indicates that the origin server MUST NOT 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 and if arguments. If any arguments are
present
present, they have no meaning. 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 for whe received by
proxy clients as it does for when received by User-Agent based clients and they therefore MAY also
presume clients.
Therefore proxies SHOULD skip conditionally revalidating fresh
responses containing the immutable extension unless there is a conditional revalidation for signal
from the client that a response marked immutable
would return 304. validation is necessary (e.g. a no-cache
Cache-Control request directive).

A proxy client who that uses immutable to anticipate bypass a
304 response conditional revalidation may
choose whether to reply with a 304 or 200 to its requesting client. 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 SHOULD ignore immutable for 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 mechanismss: 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 SHOULD ignore
immutable as well.

o Clients should 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.
interactions and to Mark Nottingham for help with the documentation.

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>.

Author's Address

Patrick McManus
Mozilla

Email: pmcmanus@mozilla.com