draft-ietf-httpbis-immutable-00.txt   draft-ietf-httpbis-immutable-01.txt 
Network Working Group P. McManus HTTP Working Group P. McManus
Internet-Draft Mozilla Internet-Draft Mozilla
Intended status: Standards Track January 12, 2017 Intended status: Standards Track March 13, 2017
Expires: July 16, 2017 Expires: September 14, 2017
HTTP Immutable Responses HTTP Immutable Responses
draft-ietf-httpbis-immutable-00 draft-ietf-httpbis-immutable-01
Abstract Abstract
The immutable HTTP response Cache-Control extension allows servers to The immutable HTTP response Cache-Control extension allows servers to
identify resources that will not be updated during their freshness identify resources that will not be updated during their freshness
lifetime. This assures that a client never needs to revalidate a lifetime. This assures that a client never needs to revalidate a
cached fresh resource to be certain it has not been modified. 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 Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on July 16, 2017. This Internet-Draft will expire on September 14, 2017.
Copyright Notice Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
1. Introduction 1. Introduction
The HTTP freshness lifetime [RFC7234] caching attribute specifies HTTP's freshness lifetime mechanism [RFC7234] allows a client to
that a client may safely reuse a response to satisfy future requests safely reuse a stored response to satisfy future requests for a
over a specific period of time. It does not specify that the specified period of time. However, it is still possible that the
resource will be not be modified during that period. resource will be modified during that period.
For instance, a front page newspaper photo with a freshness lifetime 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 of one hour would mean that no user would see a cached photo more
hour old. However, the photo could be updated at any time resulting than one hour old. However, the photo could be updated at any time
in different users seeing different photos depending on the contents resulting in different users seeing different photos depending on the
of their caches for up to one hour. This is compliant with the contents of their caches for up to one hour. This is compliant with
caching mechanism defined in [RFC7234]. the caching mechanism defined in [RFC7234].
Users that need to confirm there have been no updates to their Users that need to confirm there have been no updates to their cached
current cached resources typically invoke the reload (or refresh) responses typically use the reload (or refresh) mechanism in their
mechanism in the user agent. This in turn generates a conditional user agents. This in turn generates a conditional request [RFC7232]
request [RFC7232] and either a new representation or, if unmodified, and either a new representation or, if unmodified, a 304 (Not
a 304 response [RFC7231] is returned. A user agent that manages HTML Modified) response [RFC7232] is returned. A user agent that
and its dependent sub-resources may issue hundreds of conditional understands HTML and fetches its dependent sub-resources might issue
requests to refresh all portions of a common HTML page [REQPERPAGE]. hundreds of conditional requests to refresh all portions of a common
page [REQPERPAGE].
Through the use of the versioned URL design pattern some content However some content providers never create more than one variant of
providers never create more than one variant of a sub-resource. When a sub-resource, because they use "versioned" URLs. When these
these resources need an update they are simply published under a new resources need an update they are simply published under a new URL,
URL, typically embedding a variant identifier in the path, and typically embedding an identifier unique to that version of the
references to the sub-resource are updated with the new path resource in the path, and references to the sub-resource are updated
information. with the new path information.
For example, https://www.example.com/101016/main.css might be updated For example, "https://www.example.com/101016/main.css" might be
and republished as https://www.example.com/102026/main.css and the updated and republished as "https://www.example.com/102026/main.css",
html that references it is changed at the same time. This design with any links that references it being changed at the same time.
pattern allows a very large freshness lifetime to be applied to the This design pattern allows a very large freshness lifetime to be used
sub-resource without guessing when it will be updated in the future. for the sub-resource without guessing when it will be updated in the
future.
Unfortunately, the user-agent is not aware of the versioned URL Unfortunately, the user agent does not know when this versioned URL
design pattern. User driven refresh events still translate into design pattern is used. As a result, user-driven refreshes still
wasted conditional requests for each sub-resource as each will return translate into wasted conditional requests for each sub-resource as
304 responses. each will return 304 responses.
The immutable HTTP response Cache-Control extension allows servers to The "immutable" HTTP response Cache-Control extension allows servers
identify resources that will not be updated during their freshness to identify responses that will not be updated during their freshness
lifetime. This effectively instructs the client that any conditional lifetimes.
request for a previously served variant of that resource may be
safely skipped without worrying that it has been updated. This effectively informs clients that any conditional request for
that response can be safely skipped without worrying that it has been
updated.
2. The immutable Cache-Control extension 2. The immutable Cache-Control extension
When present in an HTTP response, the immutable Cache-Control When present in an HTTP response, the "immutable" Cache-Control
extension indicates that the origin server MUST NOT update the extension indicates that the origin server will not update the
representation of that resource during the freshness lifetime of the representation of that resource during the freshness lifetime of the
response. 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 immutable extension only applies during the freshness lifetime of
the response. Stale responses SHOULD be revalidated as they normally the stored response. Stale responses SHOULD be revalidated as they
would be in the absence of immutable. normally would be in the absence of immutable.
The immutable extension takes no arguments and if any arguments are The immutable extension takes no arguments. If any arguments are
present they have no meaning. Multiple instances of the immutable present, they have no meaning, and MUST be ignored. Multiple
extension are equivalent to one instance. The presence of an instances of the immutable extension are equivalent to one instance.
immutable Cache-Control extension in a request has no effect. The presence of an immutable Cache-Control extension in a request has
no effect.
2.1. About Intermediaries 2.1. About Intermediaries
An immutable response has the same semantic meaning for proxy clients An immutable response has the same semantic meaning whe received by
as it does for User-Agent based clients and they therefore MAY also proxy clients as it does when received by User-Agent based clients.
presume a conditional revalidation for a response marked immutable Therefore proxies SHOULD skip conditionally revalidating fresh
would return 304. A proxy client who uses immutable to anticipate a responses containing the immutable extension unless there is a signal
304 response may choose whether to reply with a 304 or 200 to its from the client that a validation is necessary (e.g. a no-cache
requesting client. Cache-Control request directive).
A proxy that uses immutable to bypass a conditional revalidation may
choose whether to reply with a 304 or 200 to its requesting client
based on the request headers the proxy received.
2.2. Example 2.2. Example
Cache-Control: max-age=31536000, immutable Cache-Control: max-age=31536000, immutable
3. Security Considerations 3. Security Considerations
The immutable mechanism acts as form of soft pinning and, as with all The immutable mechanism acts as form of soft pinning and, as with all
pinning mechanisms, creates a vector for amplification of cache pinning mechanisms, creates a vector for amplification of cache
corruption incidents. These incidents include cache poisoning corruption incidents. These incidents include cache poisoning
attacks. Three mechanisms are suggested for mitigation of this risk: attacks. Three mechanisms are suggested for mitigation of this risk:
o Clients should ignore immutable for resources that are not part of o Clients SHOULD ignore immutable from resources that are not part
an authenticated context such as HTTPS. Authenticated resources of an authenticated context such as HTTPS. Authenticated
are less vulnerable to cache poisoning. resources are less vulnerable to cache poisoning.
o User-Agents often provide two different refresh mechanismss: o User-Agents often provide two different refresh mechanisms: reload
reload and some form of force-reload. The latter is used to and some form of force-reload. The latter is used to rectify
rectify interrupted loads and other corruption. These reloads, interrupted loads and other corruption. These reloads, typically
typically indicated through no-cache request attributes, should indicated through no-cache request attributes, SHOULD ignore
ignore immutable as well. immutable as well.
o Clients should ignore immutable for resources that do not provide o Clients SHOULD ignore immutable for resources that do not provide
a strong indication that the stored response size is the correct a strong indication that the stored response size is the correct
response size such as responses delimited by connection close. response size such as responses delimited by connection close.
4. IANA Considerations 4. IANA Considerations
[RFC7234] sections 7.1 and 7.1.2 require registration of the [RFC7234] sections 7.1 and 7.1.2 require registration of the
immutable extension in the "Hypertext Transfer Protocol (HTTP) Cache immutable extension in the "Hypertext Transfer Protocol (HTTP) Cache
Directive Registry" with IETF Review. Directive Registry" with IETF Review.
o Cache-Directive: immutable o Cache-Directive: immutable
o Pointer to specification text: [this document] o Pointer to specification text: [this document]
5. Acknowledgments 5. Acknowledgments
Thank you to Ben Maurer for partnership in developing and testing Thank you to Ben Maurer for partnership in developing and testing
this idea. Thank you to Amos Jeffries for help with proxy 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. References
6.1. Normative References 6.1. Normative References
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231, Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014, DOI 10.17487/RFC7231, June 2014,
<http://www.rfc-editor.org/info/rfc7231>. <http://www.rfc-editor.org/info/rfc7231>.
[RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Conditional Requests", RFC 7232, Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
DOI 10.17487/RFC7232, June 2014, DOI 10.17487/RFC7232, June 2014,
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