HTTP Working Group K. Oku
Internet-Draft DeNA Co., Ltd.
Intended status: Experimental March 29, 2017
Expires: September 30, 2017
An HTTP Status Code for Indicating Hints
draft-ietf-httpbis-early-hints-01
Abstract
This memo introduces an informational status code for HTTP that can
be used for indicating hints to help a client start making
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.
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This Internet-Draft will expire on September 30, 2017.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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-00 . . . . . . . . . 4
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
Most if not all of the web pages processed by a web browser contain
links to external resources that need to be fetched prior to
rendering the documents. Therefore, it is beneficial to send such
links as early as possible in order to minimize the time spent until
the browser becomes possible to render the document. Link header of
type "preload" ([Preload]) can be used to indicate such links within
the response headers of an HTTP response.
However, it is not always possible for an origin server to send a
response immediately after receiving a request. In fact, it is often
the contrary. There are many deployments in which an origin server
needs to query a database before generating a response. It is also
not unusual for an origin server to 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 headers of the final HTTP
response is determined.
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HTTP/2 ([RFC7540]) push can be used as a solution to the issue, but
has its own limitations. The resources that can be pushed using
HTTP/2 are limited to those belonging to the same origin. Also, it
is impossible to send only the links of the resources using HTTP/2
push. 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 push at an 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
This 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 headers ([RFC7230], section 6.1) in the informational
response using the status code.
A client MAY speculatively evaluate the headers included in the
informational response while waiting for the final response. For
example, a client may recognize the link header of type preload and
start fetching the resource. However, the evaluation MUST NOT affect
how the final response is processed; the client must behave as if it
had not seen the informational response. A client MUST NOT process
the headers included in the response as if they belonged to the
informational response.
An intermediary MAY drop the informational response. It MAY send
HTTP/2 ([RFC7540]) push responses using the information found in the
informational response.
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3. Security Considerations
Clients may have issues handling Early Hints, since informational
response is rarely used for requests not including an Expect header
([RFC7231], section 5.1.1).
An HTTP/1.1 client that mishandles the informational response as a
final response is likely to consider all the 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
If Early Hints is standardized, the HTTP Status Codes Registry should
be updated with the following entries:
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-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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