draft-ietf-httpbis-immutable-01.txt   draft-ietf-httpbis-immutable-latest.txt 
HTTP Working Group P. McManus HTTP Working Group P. McManus
Internet-Draft Mozilla Internet-Draft Mozilla
Intended status: Standards Track March 13, 2017 Intended status: Standards Track April 26, 2017
Expires: September 14, 2017 Expires: October 28, 2017
HTTP Immutable Responses HTTP Immutable Responses
draft-ietf-httpbis-immutable-01 draft-ietf-httpbis-immutable-latest
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 Note to Readers
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 14, 2017. This Internet-Draft will expire on October 28, 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.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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However some content providers never create more than one variant of However some content providers never create more than one variant of
a sub-resource, because they use "versioned" URLs. When these a sub-resource, because they use "versioned" URLs. When these
resources need an update they are simply published under a new URL, resources need an update they are simply published under a new URL,
typically embedding an identifier unique to that version of the typically embedding an identifier unique to that version of the
resource in the path, and references to the sub-resource are updated resource in the path, and references to the sub-resource are updated
with the new path information. with the new path information.
For example, "https://www.example.com/101016/main.css" might be For example, "https://www.example.com/101016/main.css" might be
updated and republished as "https://www.example.com/102026/main.css", updated and republished as "https://www.example.com/102026/main.css",
with any links that references it being changed at the same time. with any links that reference it being changed at the same time.
This design pattern allows a very large freshness lifetime to be used 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 for the sub-resource without guessing when it will be updated in the
future. future.
Unfortunately, the user agent does not know when this versioned URL Unfortunately, the user agent does not know when this versioned URL
design pattern is used. As a result, user-driven refreshes still design pattern is used. As a result, user-driven refreshes still
translate into wasted conditional requests for each sub-resource as translate into wasted conditional requests for each sub-resource as
each will return 304 responses. each will return 304 responses.
The "immutable" HTTP response Cache-Control extension allows servers The "immutable" HTTP response Cache-Control extension allows servers
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normally would be in the absence of immutable. normally would be in the absence of immutable.
The immutable extension takes no arguments. If any arguments are The immutable extension takes no arguments. If any arguments are
present, they have no meaning, and MUST be ignored. Multiple present, they have no meaning, and MUST be ignored. Multiple
instances of the immutable extension are equivalent to one instance. instances of the immutable extension are equivalent to one instance.
The presence of an immutable Cache-Control extension in a request has The presence of an immutable Cache-Control extension in a request has
no effect. no effect.
2.1. About Intermediaries 2.1. About Intermediaries
An immutable response has the same semantic meaning whe received by An immutable response has the same semantic meaning when received by
proxy clients as it does when received by User-Agent based clients. proxy clients as it does when received by User-Agent based clients.
Therefore proxies SHOULD skip conditionally revalidating fresh Therefore proxies SHOULD skip conditionally revalidating fresh
responses containing the immutable extension unless there is a signal responses containing the immutable extension unless there is a signal
from the client that a validation is necessary (e.g. a no-cache from the client that a validation is necessary (e.g. a no-cache
Cache-Control request directive). Cache-Control request directive).
A proxy that uses immutable to bypass a conditional revalidation may A proxy that uses immutable to bypass a conditional revalidation may
choose whether to reply with a 304 or 200 to its requesting client choose whether to reply with a 304 or 200 to its requesting client
based on the request headers the proxy received. based on the request headers the proxy received.
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