draft-ietf-httpbis-early-hints-02.txt   draft-ietf-httpbis-early-hints-03.txt 
HTTP Working Group K. Oku HTTP Working Group K. Oku
Internet-Draft DeNA Co., Ltd. Internet-Draft Fastly
Intended status: Experimental May 16, 2017 Intended status: Experimental June 20, 2017
Expires: November 17, 2017 Expires: December 22, 2017
An HTTP Status Code for Indicating Hints An HTTP Status Code for Indicating Hints
draft-ietf-httpbis-early-hints-02 draft-ietf-httpbis-early-hints-03
Abstract Abstract
This memo introduces an informational HTTP status code that can be This memo introduces an informational HTTP status code that can be
used to convey hints that help a client make preparations for used to convey hints that help a client make preparations for
processing the final response. processing the final response.
Note to Readers Note to Readers
Discussion of this draft takes place on the HTTP working group Discussion of this draft takes place on the HTTP working group
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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 November 17, 2017. This Internet-Draft will expire on December 22, 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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include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3 1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. 103 Early Hints . . . . . . . . . . . . . . . . . . . . . . . 3 2. 103 Early Hints . . . . . . . . . . . . . . . . . . . . . . . 3
3. Security Considerations . . . . . . . . . . . . . . . . . . . 4 3. Security Considerations . . . . . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
6. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 6. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Since draft-ietf-httpbis-early-hints-01 . . . . . . . . . 4 6.1. Since draft-ietf-httpbis-early-hints-02 . . . . . . . . . 5
6.2. Since draft-ietf-httpbis-early-hints-00 . . . . . . . . . 4 6.2. Since draft-ietf-httpbis-early-hints-01 . . . . . . . . . 5
6.3. Since draft-ietf-httpbis-early-hints-00 . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5 7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 5 7.2. Informative References . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction 1. Introduction
It is common for HTTP responses to contain links to external It is common for HTTP responses to contain links to external
resources that need to be fetched prior to their use; for example, resources that need to be fetched prior to their use; for example,
rendering HTML by a Web browser. Having such links available to the rendering HTML by a Web browser. Having such links available to the
client as early as possible helps to minimize perceived latency. client as early as possible helps to minimize perceived latency.
The "preload" ([Preload]) link relation can be used to convey such The "preload" ([Preload]) link relation can be used to convey such
links in the Link header field of an HTTP response. However, it is links in the Link header field of an HTTP response. However, it is
not always possible for an origin server to generate a response not always possible for an origin server to generate the header block
header block immediately after receiving a request. For example, the of a final response immediately after receiving a request. For
origin server might need to query a database before generating a example, the origin server might delegate a request to an upstream
response, or it might delegate a request to an upstream HTTP server HTTP server running at a distant location, or the status code might
running at a distant location. depend on the result of a database query.
The dilemma here is that even though it is preferable for an origin 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 server to send some header fields as soon as it receives a request,
cannot do so until the status code and the full headers of the final it cannot do so until the status code and the full header fields of
HTTP response are determined. the final HTTP response are determined.
HTTP/2 ([RFC7540]) server push can be used as a solution to this 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 issue, but has its own limitations. The responses that can be pushed
using HTTP/2 are limited to those belonging to the same origin. using HTTP/2 are limited to those belonging to the same origin.
Also, it is impossible to send only the links using server push. Also, it is impossible to send only the links using server push.
Finally, sending HTTP responses for every resource is an inefficient Finally, sending HTTP responses for every resource is an inefficient
way of using bandwidth, especially when a caching server exists as an way of using bandwidth, especially when a caching server exists as an
intermediary. intermediary.
This memo defines a status code for sending an informational response This memo defines a status code for sending an informational response
([RFC7231], section 6.2) that contains headers that are likely to be ([RFC7231], Section 6.2) that contains header fields that are likely
included in the final response. A server can send the informational to be included in the final response. A server can send the
response containing some of the headers to help the client start informational response containing some of the header fields to help
making preparations for processing the final response, and then run the client start making preparations for processing the final
time-consuming operations to generate the final response. The response, and then run time-consuming operations to generate the
informational response can also be used by an origin server to final response. The informational response can also be used by an
trigger HTTP/2 server push at a caching intermediary. origin server to trigger HTTP/2 server push at a caching
intermediary.
1.1. Notational Conventions 1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
2. 103 Early Hints 2. 103 Early Hints
The 103 (Early Hints) informational status code indicates the client The 103 (Early Hints) informational status code indicates to the
that the server is likely to send a final response with the headers client that the server is likely to send a final response with the
included in the informational response. header fields included in the informational response.
A server MUST NOT include Content-Length, Transfer-Encoding, or any A server MUST NOT include Content-Length, Transfer-Encoding, or any
hop-by-hop header fields ([RFC7230], section 6.1) in a 103 (Early hop-by-hop header fields ([RFC7230], Section 6.1) in a 103 (Early
Hints) response. Hints) response.
A client MAY speculatively evaluate the headers included in a 103 A client can speculatively evaluate the header fields included in a
(Early Hints) response while waiting for the final response. For 103 (Early Hints) response while waiting for the final response. For
example, a client might recognize a Link header field value example, a client might recognize a Link header field value
containing the relation type "preload" and start fetching the target containing the relation type "preload" and start fetching the target
resource. resource.
However, this MUST NOT affect how the final response is processed; However, these header fields only provide hints to the client; they
when handling it, the client MUST behave as if it had not seen the do not replace the header fields on the final response. Aside from
informational response. In particular, a client MUST NOT process the performance optimizations, such evaluation of the 103 (Early Hints)
headers included in the final response as if they belonged to the response's header fields MUST NOT affect how the final response is
informational response, or vice versa. processed. A client MUST NOT interpret the 103 (Early Hints)
response header fields as if they applied to the informational
response itself (e.g., as metadata about the 103 (Early Hints)
response).
An intermediary MAY drop the informational response. It MAY send An intermediary MAY drop the informational response. It MAY send
HTTP/2 ([RFC7540]) server pushes using the information found in the HTTP/2 ([RFC7540]) server pushes using the information found in the
103 (Early Hints) response. 103 (Early Hints) response.
The following example illustrates a typical message exchange that
involves a 103 (Early Hints) response.
Client request:
GET / HTTP/1.1
Host: example.com
Server response:
HTTP/1.1 103 Early Hints
Link: </style.css>; rel=preload; as=style
Link: </script.js>; rel=preload; as=script
HTTP/1.1 200 OK
Date: Fri, 26 May 2017 10:02:11 GMT
Content-Length: 1234
Content-Type: text/html; charset=utf-8
Link: </style.css>; rel=preload; as=style
Link: </script.js>; rel=preload; as=script
<!doctype html>
[... rest of the response body is ommitted from the example ...]
3. Security Considerations 3. Security Considerations
Some clients may have issues handling 103 (Early Hints), since Some clients might have issues handling 103 (Early Hints), since
informational responses are rarely used in reply to requests not informational responses are rarely used in reply to requests not
including an Expect header ([RFC7231], section 5.1.1). including an Expect header ([RFC7231], Section 5.1.1).
In particular, an HTTP/1.1 client that mishandles an informational In particular, an HTTP/1.1 client that mishandles an informational
response as a final response is likely to consider all responses to 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 succeeding requests sent over the same connection to be part of
the final response. Such behavior may constitute a cross-origin the final response. Such behavior may constitute a cross-origin
information disclosure vulnerability in case the client multiplexes information disclosure vulnerability in case the client multiplexes
requests to different origins onto a single persistent connection. requests to different origins onto a single persistent connection.
Therefore, a server might refrain from sending Early Hints over Therefore, a server might refrain from sending Early Hints over
HTTP/1.1 unless when the client is known to handle informational HTTP/1.1 unless when the client is known to handle informational
responses correctly. responses correctly.
HTTP/2 clients are less likely to suffer from incorrect framing since 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 handling of the response header fields does not affect how the end of
response body is determined. the response body is determined.
4. IANA Considerations 4. IANA Considerations
The HTTP Status Codes Registry will be updated with the following The HTTP Status Codes Registry will be updated with the following
entry: entry:
o Code: 103 o Code: 103
o Description: Early Hints o Description: Early Hints
o Specification: [this document] o Specification: [this document]
5. Acknowledgements 5. Acknowledgements
Thanks to Tatsuhiro Tsujikawa for coming up with the idea of sending Thanks to Tatsuhiro Tsujikawa for coming up with the idea of sending
the link headers using an informational response. the Link header fields using an informational response.
6. Changes 6. Changes
6.1. Since draft-ietf-httpbis-early-hints-01 6.1. Since draft-ietf-httpbis-early-hints-02
o Editorial changes. o Editorial changes.
6.2. Since draft-ietf-httpbis-early-hints-00 o Added an example.
6.2. Since draft-ietf-httpbis-early-hints-01
o Editorial changes.
6.3. Since draft-ietf-httpbis-early-hints-00
o Forbid processing the headers of a 103 response as part of the o Forbid processing the headers of a 103 response as part of the
informational response. informational response.
7. References 7. References
7.1. Normative References 7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
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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>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext [RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540, Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015, DOI 10.17487/RFC7540, May 2015,
<http://www.rfc-editor.org/info/rfc7540>. <http://www.rfc-editor.org/info/rfc7540>.
7.2. Informative References 7.2. Informative References
[Preload] Grigorik, I., "Preload", September 2016, [Preload] Grigorik, I., "Preload", n.d., <https://w3c.github.io/
<https://w3c.github.io/preload/>. preload/>.
Author's Address Author's Address
Kazuho Oku Kazuho Oku
DeNA Co., Ltd. Fastly
Email: kazuhooku@gmail.com Email: kazuhooku@gmail.com
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