Internet DRAFT - draft-nottingham-cache-trailers
draft-nottingham-cache-trailers
Network Working Group M. Nottingham
Internet-Draft
Intended status: Standards Track J. Snell
Expires: 9 September 2021 8 March 2021
Updating HTTP Caching Policy in Trailers
draft-nottingham-cache-trailers-00
Abstract
This specification defines how to update caching policy for a
response in HTTP trailer fields, after the content has been sent.
Note to Readers
_RFC EDITOR: please remove this section before publication_
The issues list for this draft can be found at
https://github.com/mnot/I-D/labels/cache-trailers
(https://github.com/mnot/I-D/labels/cache-trailers).
The most recent (often, unpublished) draft is at
https://mnot.github.io/I-D/cache-trailers/ (https://mnot.github.io/I-
D/cache-trailers/).
Recent changes are listed at https://github.com/mnot/I-D/commits/gh-
pages/cache-trailers (https://github.com/mnot/I-D/commits/gh-pages/
cache-trailers).
See also the draft's current status in the IETF datatracker, at
https://datatracker.ietf.org/doc/draft-nottingham-cache-trailers/
(https://datatracker.ietf.org/doc/draft-nottingham-cache-trailers/).
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 https://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."
Nottingham & Snell Expires 9 September 2021 [Page 1]
Internet-Draft Updating HTTP Caching Policy in Trailers March 2021
This Internet-Draft will expire on 9 September 2021.
Copyright Notice
Copyright (c) 2021 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 (https://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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. The "trailer-update" HTTP Cache Directive . . . . . . . . . . 3
2.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . 4
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 5
5. Normative References . . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
Web content that is "dynamically" generated -- i.e., with the
response body streamed by the server to the client as it is created
-- is often assumed to be uncacheable. In practice, though, there
are some scenarios where caching is beneficial; for example, when a
private cache might be able to reuse a personalised, dynamic response
for a period, or when such a response can be shared by a number of
clients.
A server choosing a caching policy for such a response faces a
conundrum: if an error or other unforeseen condition happens during
the generation of the response, that caching policy might be too
liberal. Currently, the only available solutions are to:
1. prevent or severely curtail downstream caching, or
2. buffer the response until a caching policy can be confidently
assigned.
Nottingham & Snell Expires 9 September 2021 [Page 2]
Internet-Draft Updating HTTP Caching Policy in Trailers March 2021
In both cases, performance suffers; in the former, caching efficiency
is less than it could be in the common case, In the latter, the
server consumes additional resources and delays the response.
This specification provides a third solution: updating the caching
policy in HTTP trailer fields, after the content has been sent.
Doing so allows content to be streamed, while caching policy can be
determined after the content is actually generated.
1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. The "trailer-update" HTTP Cache Directive
The "trailer-update" cache response directive indicates that the
caching policy for that response (as indicated by the header field
that contains the directive) might be updated by a corresponding
trailer field.
When it is present as a cache directive in a header field and a
trailer field with the same field name is received, a cache that
implements this specification MUST completely replace the stored
header field value for that response with the trailer field's value,
MUST update its handling of that response to account for the new
field value (after any outstanding requests are satisfied), and MUST
use that value for the header field in responses to future requests
satisfied from that cache entry (i.e., the trailer field is
"promoted" to a header field).
In responses where the trailer field value has replaced the header
field value, the "trailer-update" directive will have been removed as
part of that process. Note that the presence of "trailer-update"
does not guarantee that a trailer field will follow.
Caches MAY temporarily store a response that has a caching policy
with both the "no-store" and "trailer-update" directives, but MUST
NOT reuse that response until the caching policy is updated in a
manner that allows it. If the caching policy is not updated or the
"no-store" directive is still present in the updated response, the
cache MUST immediately and permanently discard the temporarily stored
response.
Nottingham & Snell Expires 9 September 2021 [Page 3]
Internet-Draft Updating HTTP Caching Policy in Trailers March 2021
For purposes of calculating a stored response's age
([I-D.ietf-httpbis-cache], Section 4.2.3), caches receiving such a
trailer SHOULD consider the response_time to be when the trailer is
received, but only when calculating resident_time (not
response_delay, as that would be counterproductive for the purpose of
estimating network delay).
2.1. Examples
Given a resource that supports this specification but encounters no
errors in the generation of a response's content, that response might
look like this:
HTTP/1.1 200 OK
Content-Type: text/html
Cache-Control: max-age=3600, trailer-update
[body]
Caches that do not implement this specification will cache it by the
header policy; caches that do implement will see no updates in the
trailer and do the same.
If a change in caching policy is encountered during the generation of
the response content, the resource can prevent reuse by caches that
implement this specification by sending:
HTTP/1.1 200 OK
Content-Type: text/html
Cache-Control: max-age=3600, trailer-update
[body]
Cache-Control: no-store
In this case, caches that do not implement this specification will
again act as instructed by the header policy, but caches that do
implement will see the update in the trailers and prevent reuse of
the response after the trailer is received (although it might have
been used to satisfy requests that were received in the meantime).
If a resource wishes to prevent non-implementing caches from storing
the response, they can send:
Nottingham & Snell Expires 9 September 2021 [Page 4]
Internet-Draft Updating HTTP Caching Policy in Trailers March 2021
HTTP/1.1 200 OK
Content-Type: text/html
Cache-Control: no-store; trailer-update
[body]
Cache-Control: max-age=3600
Here, a non-implementing cache will only see "no-store", and so will
not store the response. An implementing cache can optimistically
store the response based upon "trailer-update", but only allow its
reuse after the caching policy is updated to something which permits
that in trailers.
Note that when a downstream cache does not implement this
specification, and also does not forward a message's trailer section
(as allowed by HTTP), any updates will effectively be lost, even if
further downstream caches do implement.
3. IANA Considerations
_TBD_
4. Security Considerations
_TBD_
5. Normative References
[I-D.ietf-httpbis-cache]
Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
Caching", Work in Progress, Internet-Draft, draft-ietf-
httpbis-cache-14, 12 January 2021,
<https://www.ietf.org/archive/id/draft-ietf-httpbis-cache-
14.txt>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
Authors' Addresses
Nottingham & Snell Expires 9 September 2021 [Page 5]
Internet-Draft Updating HTTP Caching Policy in Trailers March 2021
Mark Nottingham
Prahran VIC
Australia
Email: mnot@mnot.net
URI: https://www.mnot.net/
James Snell
Email: jasnell@gmail.com
Nottingham & Snell Expires 9 September 2021 [Page 6]
