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
	skipping to change at page 1, line 42 ¶		skipping to change at page 1, line 42 ¶
	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
	skipping to change at page 2, line 21 ¶		skipping to change at page 2, line 21 ¶
	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,
	skipping to change at page 5, line 31 ¶		skipping to change at page 6, line 17 ¶
	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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