An HTTP Status Code for Indicating Hints
draft-ietf-httpbis-early-hints-02
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Kazuho Oku
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HTTP Working Group K. Oku
Internet-Draft DeNA Co., Ltd.
Intended status: Experimental May 16, 2017
Expires: November 17, 2017
An HTTP Status Code for Indicating Hints
draft-ietf-httpbis-early-hints-02
Abstract
This memo introduces an informational HTTP status code that can be
used to convey hints that help a client make preparations for
processing the final response.
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 https://httpwg.github.io/ ;
source code and issues list for this draft can be found at
https://github.com/httpwg/http-extensions/labels/early-hints .
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 http://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."
This Internet-Draft will expire on November 17, 2017.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://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. 103 Early Hints . . . . . . . . . . . . . . . . . . . . . . . 3
3. Security Considerations . . . . . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4
6. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
6.1. Since draft-ietf-httpbis-early-hints-01 . . . . . . . . . 4
6.2. Since draft-ietf-httpbis-early-hints-00 . . . . . . . . . 4
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
It is common for HTTP responses to contain links to external
resources that need to be fetched prior to their use; for example,
rendering HTML by a Web browser. Having such links available to the
client as early as possible helps to minimize perceived latency.
The "preload" ([Preload]) link relation can be used to convey such
links in the Link header field of an HTTP response. However, it is
not always possible for an origin server to generate a response
header block immediately after receiving a request. For example, the
origin server might need to query a database before generating a
response, or it might delegate a request to an upstream HTTP server
running at a distant location.
The dilemma here is that even though it is preferable for an origin
server to send some headers as soon as it receives a request, it
cannot do so until the status code and the full headers of the final
HTTP response are determined.
HTTP/2 ([RFC7540]) server push can be used as a solution to this
issue, but has its own limitations. The responses that can be pushed
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using HTTP/2 are limited to those belonging to the same origin.
Also, it is impossible to send only the links using server push.
Finally, sending HTTP responses for every resource is an inefficient
way of using bandwidth, especially when a caching server exists as an
intermediary.
This memo defines a status code for sending an informational response
([RFC7231], section 6.2) that contains headers that are likely to be
included in the final response. A server can send the informational
response containing some of the headers to help the client start
making preparations for processing the final response, and then run
time-consuming operations to generate the final response. The
informational response can also be used by an origin server to
trigger HTTP/2 server push at a caching intermediary.
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. 103 Early Hints
The 103 (Early Hints) informational status code indicates the client
that the server is likely to send a final response with the headers
included in the informational response.
A server MUST NOT include Content-Length, Transfer-Encoding, or any
hop-by-hop header fields ([RFC7230], section 6.1) in a 103 (Early
Hints) response.
A client MAY speculatively evaluate the headers included in a 103
(Early Hints) response while waiting for the final response. For
example, a client might recognize a Link header field value
containing the relation type "preload" and start fetching the target
resource.
However, this MUST NOT affect how the final response is processed;
when handling it, the client MUST behave as if it had not seen the
informational response. In particular, a client MUST NOT process the
headers included in the final response as if they belonged to the
informational response, or vice versa.
An intermediary MAY drop the informational response. It MAY send
HTTP/2 ([RFC7540]) server pushes using the information found in the
103 (Early Hints) response.
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3. Security Considerations
Some clients may have issues handling 103 (Early Hints), since
informational responses are rarely used in reply to requests not
including an Expect header ([RFC7231], section 5.1.1).
In particular, an HTTP/1.1 client that mishandles an informational
response as a final response is likely to consider all responses to
the succeeding requests sent over the same connection to be part of
the final response. Such behavior may constitute a cross-origin
information disclosure vulnerability in case the client multiplexes
requests to different origins onto a single persistent connection.
Therefore, a server might refrain from sending Early Hints over
HTTP/1.1 unless when the client is known to handle informational
responses correctly.
HTTP/2 clients are less likely to suffer from incorrect framing since
handling of the response headers does not affect how the end of the
response body is determined.
4. IANA Considerations
The HTTP Status Codes Registry will be updated with the following
entry:
o Code: 103
o Description: Early Hints
o Specification: [this document]
5. Acknowledgements
Thanks to Tatsuhiro Tsujikawa for coming up with the idea of sending
the link headers using an informational response.
6. Changes
6.1. Since draft-ietf-httpbis-early-hints-01
o Editorial changes.
6.2. Since draft-ietf-httpbis-early-hints-00
o Forbid processing the headers of a 103 response as part of the
informational response.
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7. References
7.1. 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>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<http://www.rfc-editor.org/info/rfc7230>.
[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>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<http://www.rfc-editor.org/info/rfc7540>.
7.2. Informative References
[Preload] Grigorik, I., "Preload", September 2016,
<https://w3c.github.io/preload/>.
Author's Address
Kazuho Oku
DeNA Co., Ltd.
Email: kazuhooku@gmail.com
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