draft-ietf-httpbis-early-hints-01.txt		draft-ietf-httpbis-early-hints-02.txt
	HTTP Working Group K. Oku		HTTP Working Group K. Oku
	Internet-Draft DeNA Co., Ltd.		Internet-Draft DeNA Co., Ltd.
	Intended status: Experimental March 29, 2017		Intended status: Experimental May 16, 2017
	Expires: September 30, 2017		Expires: November 17, 2017
	An HTTP Status Code for Indicating Hints		An HTTP Status Code for Indicating Hints
	draft-ietf-httpbis-early-hints-01		draft-ietf-httpbis-early-hints-02
	Abstract		Abstract
	This memo introduces an informational status code for HTTP that can		This memo introduces an informational HTTP status code that can be
	be used for indicating hints to help a client start making		used to convey hints that help a client make preparations for
	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
	mailing list (ietf-http-wg@w3.org), which is archived at		mailing list (ietf-http-wg@w3.org), which is archived at
	https://lists.w3.org/Archives/Public/ietf-http-wg/ .		https://lists.w3.org/Archives/Public/ietf-http-wg/ .
	Working Group information can be found at https://httpwg.github.io/ ;		Working Group information can be found at https://httpwg.github.io/ ;
	source code and issues list for this draft can be found at		source code and issues list for this draft can be found at
	https://github.com/httpwg/http-extensions/labels/early-hints .		https://github.com/httpwg/http-extensions/labels/early-hints .
	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 September 30, 2017.		This Internet-Draft will expire on November 17, 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 24 ¶		skipping to change at page 2, line 24 ¶
	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 . . . . . . . . . . . . . . . . . . . . . 4
	5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4		5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 4
	6. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 4		6. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
	6.1. Since draft-ietf-httpbis-early-hints-00 . . . . . . . . . 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. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
	7.1. Normative References . . . . . . . . . . . . . . . . . . 5		7.1. Normative References . . . . . . . . . . . . . . . . . . 5
	7.2. Informative References . . . . . . . . . . . . . . . . . 5		7.2. Informative References . . . . . . . . . . . . . . . . . 5
	Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5		Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
	1. Introduction		1. Introduction
	Most if not all of the web pages processed by a web browser contain		It is common for HTTP responses to contain links to external
	links to external resources that need to be fetched prior to		resources that need to be fetched prior to their use; for example,
	rendering the documents. Therefore, it is beneficial to send such		rendering HTML by a Web browser. Having such links available to the
	links as early as possible in order to minimize the time spent until		client as early as possible helps to minimize perceived latency.
	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		The "preload" ([Preload]) link relation can be used to convey such
	response immediately after receiving a request. In fact, it is often		links in the Link header field of an HTTP response. However, it is
	the contrary. There are many deployments in which an origin server		not always possible for an origin server to generate a response
	needs to query a database before generating a response. It is also		header block immediately after receiving a request. For example, the
	not unusual for an origin server to delegate a request to an upstream		origin server might need to query a database before generating a
	HTTP server running at a distant location.		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		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 headers as soon as it receives a request, it
	cannot do so until the status code and the headers of the final HTTP		cannot do so until the status code and the full headers of the final
	response is determined.		HTTP response are determined.
	HTTP/2 ([RFC7540]) push can be used as a solution to the issue, but		HTTP/2 ([RFC7540]) server push can be used as a solution to this
	has its own limitations. The resources that can be pushed using		issue, but has its own limitations. The responses that can be pushed
	HTTP/2 are limited to those belonging to the same origin. Also, it		using HTTP/2 are limited to those belonging to the same origin.
	is impossible to send only the links of the resources using HTTP/2		Also, it is impossible to send only the links using server push.
	push. 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 headers that are likely to be
	included in the final response. A server can send the informational		included in the final response. A server can send the informational
	response containing some of the headers to help the client start		response containing some of the headers to help the client start
	making preparations for processing the final response, and then run		making preparations for processing the final response, and then run
	time-consuming operations to generate the final response. The		time-consuming operations to generate the final response. The
	informational response can also be used by an origin server to		informational response can also be used by an origin server to
	trigger HTTP/2 push at an caching intermediary.		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
	This informational status code indicates the client that the server		The 103 (Early Hints) informational status code indicates the client
	is likely to send a final response with the headers included in the		that the server is likely to send a final response with the headers
	informational response.		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 headers ([RFC7230], section 6.1) in the informational		hop-by-hop header fields ([RFC7230], section 6.1) in a 103 (Early
	response using the status code.		Hints) response.
	A client MAY speculatively evaluate the headers included in the		A client MAY speculatively evaluate the headers included in a 103
	informational response while waiting for the final response. For		(Early Hints) response while waiting for the final response. For
	example, a client may recognize the link header of type preload and		example, a client might recognize a Link header field value
	start fetching the resource. However, the evaluation MUST NOT affect		containing the relation type "preload" and start fetching the target
	how the final response is processed; the client must behave as if it		resource.
	had not seen the informational response. A client MUST NOT process
	the headers included in the response as if they belonged to the		However, this MUST NOT affect how the final response is processed;
	informational response.		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		An intermediary MAY drop the informational response. It MAY send
	HTTP/2 ([RFC7540]) push responses using the information found in the		HTTP/2 ([RFC7540]) server pushes using the information found in the
	informational response.		103 (Early Hints) response.
	3. Security Considerations		3. Security Considerations
	Clients may have issues handling Early Hints, since informational		Some clients may have issues handling 103 (Early Hints), since
	response is rarely used for requests not including an Expect header		informational responses are rarely used in reply to requests not
	([RFC7231], section 5.1.1).		including an Expect header ([RFC7231], section 5.1.1).
	An HTTP/1.1 client that mishandles the informational response as a		In particular, an HTTP/1.1 client that mishandles an informational
	final response is likely to consider all the responses to the		response as a final response is likely to consider all responses to
	succeeding requests sent over the same connection to be part of the		the succeeding requests sent over the same connection to be part of
	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 headers does not affect how the end of the
	response body is determined.		response body is determined.
	4. IANA Considerations		4. IANA Considerations
	If Early Hints is standardized, the HTTP Status Codes Registry should		The HTTP Status Codes Registry will be updated with the following
	be updated with the following entries:		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 headers using an informational response.
	6. Changes		6. Changes
	6.1. Since draft-ietf-httpbis-early-hints-00		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		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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