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4	HTTPbis Working Group M. Belshe
5	Internet-Draft Twist
6	Expires: July 26, 2013 R. Peon
7	Google, Inc
8	M. Thomson, Ed.
9	Microsoft
10	A. Melnikov, Ed.
11	Isode Ltd
12	January 22, 2013
13
14
15	Hypertext Transfer Protocol version 2.0
16	draft-ietf-httpbis-http2-01
17
18	Abstract
19
20	This document describes an optimised expression of the semantics of
21	the HTTP protocol. The HTTP/2.0 encapsulation enables more efficient
22	transfer of resources over HTTP by providing compressed headers,
23	simultaneous requests, and unsolicited push of resources from server
24	to client.
25
26	This document is an alternative to, but does not obsolete
27	RFC{http-p1}. The HTTP protocol semantics described in RFC{http-
28	p2..p7} are unmodified.
29
30	Editorial Note (To be removed by RFC Editor)
31
32	This draft is a work-in-progress, and does not yet reflect Working
33	Group consensus.
34
35	This draft contains features from the SPDY Protocol as a starting
36	point, as per the Working Group's charter. Future drafts will add,
37	remove and change text, based upon the Working Group's decisions.
38
39	Discussion of this draft takes place on the HTTPBIS working group
40	mailing list (ietf-http-wg@w3.org), which is archived at
41	<http://lists.w3.org/Archives/Public/ietf-http-wg/>.
42
43	The current issues list is at
44	<http://tools.ietf.org/wg/httpbis/trac/report/21> and related
45	documents (including fancy diffs) can be found at
46	<http://tools.ietf.org/wg/httpbis/>.
47
48	The changes in this draft are summarized in Appendix A.1.
49
50	Status of This Memo
51
52
53
54
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59
60	This Internet-Draft is submitted in full conformance with the
61	provisions of BCP 78 and BCP 79.
62
63	Internet-Drafts are working documents of the Internet Engineering
64	Task Force (IETF). Note that other groups may also distribute
65	working documents as Internet-Drafts. The list of current Internet-
66	Drafts is at http://datatracker.ietf.org/drafts/current/.
67
68	Internet-Drafts are draft documents valid for a maximum of six months
69	and may be updated, replaced, or obsoleted by other documents at any
70	time. It is inappropriate to use Internet-Drafts as reference
71	material or to cite them other than as "work in progress."
72
73	This Internet-Draft will expire on July 26, 2013.
74
75	Copyright Notice
76
77	Copyright (c) 2013 IETF Trust and the persons identified as the
78	document authors. All rights reserved.
79
80	This document is subject to BCP 78 and the IETF Trust's Legal
81	Provisions Relating to IETF Documents
82	(http://trustee.ietf.org/license-info) in effect on the date of
83	publication of this document. Please review these documents
84	carefully, as they describe your rights and restrictions with respect
85	to this document. Code Components extracted from this document must
86	include Simplified BSD License text as described in Section 4.e of
87	the Trust Legal Provisions and are provided without warranty as
88	described in the Simplified BSD License.
89
90	Table of Contents
91
92	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
93	1.1. Document Organization . . . . . . . . . . . . . . . . . . 5
94	1.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . 6
95	2. Starting HTTP/2.0 . . . . . . . . . . . . . . . . . . . . . . 6
96	2.1. HTTP/2.0 Version Identification . . . . . . . . . . . . . 6
97	2.2. Starting HTTP/2.0 for "http:" URIs . . . . . . . . . . . . 7
98	2.3. Starting HTTP/2.0 for "https:" URIs . . . . . . . . . . . 8
99	3. HTTP/2.0 Framing Layer . . . . . . . . . . . . . . . . . . . . 8
100	3.1. Session (Connections) . . . . . . . . . . . . . . . . . . 8
101	3.2. Framing . . . . . . . . . . . . . . . . . . . . . . . . . 8
102	3.2.1. Control frames . . . . . . . . . . . . . . . . . . . . 9
103	3.2.2. Data frames . . . . . . . . . . . . . . . . . . . . . 10
104	3.3. Streams . . . . . . . . . . . . . . . . . . . . . . . . . 11
105	3.3.1. Stream frames . . . . . . . . . . . . . . . . . . . . 11
106	3.3.2. Stream creation . . . . . . . . . . . . . . . . . . . 11
107	3.3.3. Stream priority . . . . . . . . . . . . . . . . . . . 12
108
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116	3.3.4. Stream headers . . . . . . . . . . . . . . . . . . . . 12
117	3.3.5. Stream data exchange . . . . . . . . . . . . . . . . . 13
118	3.3.6. Stream half-close . . . . . . . . . . . . . . . . . . 13
119	3.3.7. Stream close . . . . . . . . . . . . . . . . . . . . . 13
120	3.4. Error Handling . . . . . . . . . . . . . . . . . . . . . . 14
121	3.4.1. Session Error Handling . . . . . . . . . . . . . . . . 14
122	3.4.2. Stream Error Handling . . . . . . . . . . . . . . . . 14
123	3.5. Stream Flow Control . . . . . . . . . . . . . . . . . . . 15
124	3.5.1. Flow Control Principles . . . . . . . . . . . . . . . 15
125	3.5.2. Basic Flow Control Algorithm . . . . . . . . . . . . . 16
126	3.6. Control frame types . . . . . . . . . . . . . . . . . . . 16
127	3.6.1. SYN_STREAM . . . . . . . . . . . . . . . . . . . . . . 16
128	3.6.2. SYN_REPLY . . . . . . . . . . . . . . . . . . . . . . 18
129	3.6.3. RST_STREAM . . . . . . . . . . . . . . . . . . . . . . 19
130	3.6.4. SETTINGS . . . . . . . . . . . . . . . . . . . . . . . 20
131	3.6.5. PING . . . . . . . . . . . . . . . . . . . . . . . . . 23
132	3.6.6. GOAWAY . . . . . . . . . . . . . . . . . . . . . . . . 24
133	3.6.7. HEADERS . . . . . . . . . . . . . . . . . . . . . . . 25
134	3.6.8. WINDOW_UPDATE . . . . . . . . . . . . . . . . . . . . 26
135	3.6.9. CREDENTIAL . . . . . . . . . . . . . . . . . . . . . . 28
136	3.6.10. Name/Value Header Block . . . . . . . . . . . . . . . 30
137	4. HTTP Layering over HTTP/2.0 . . . . . . . . . . . . . . . . . 36
138	4.1. Connection Management . . . . . . . . . . . . . . . . . . 36
139	4.1.1. Use of GOAWAY . . . . . . . . . . . . . . . . . . . . 36
140	4.2. HTTP Request/Response . . . . . . . . . . . . . . . . . . 37
141	4.2.1. Request . . . . . . . . . . . . . . . . . . . . . . . 37
142	4.2.2. Response . . . . . . . . . . . . . . . . . . . . . . . 39
143	4.2.3. Authentication . . . . . . . . . . . . . . . . . . . . 39
144	4.3. Server Push Transactions . . . . . . . . . . . . . . . . . 40
145	4.3.1. Server implementation . . . . . . . . . . . . . . . . 41
146	4.3.2. Client implementation . . . . . . . . . . . . . . . . 42
147	5. Design Rationale and Notes . . . . . . . . . . . . . . . . . . 43
148	5.1. Separation of Framing Layer and Application Layer . . . . 43
149	5.2. Error handling - Framing Layer . . . . . . . . . . . . . . 43
150	5.3. One Connection Per Domain . . . . . . . . . . . . . . . . 44
151	5.4. Fixed vs Variable Length Fields . . . . . . . . . . . . . 44
152	5.5. Compression Context(s) . . . . . . . . . . . . . . . . . . 45
153	5.6. Unidirectional streams . . . . . . . . . . . . . . . . . . 45
154	5.7. Data Compression . . . . . . . . . . . . . . . . . . . . . 45
155	5.8. Server Push . . . . . . . . . . . . . . . . . . . . . . . 46
156	6. Security Considerations . . . . . . . . . . . . . . . . . . . 46
157	6.1. Use of Same-origin constraints . . . . . . . . . . . . . . 46
158	6.2. HTTP Headers and HTTP/2.0 Headers . . . . . . . . . . . . 46
159	6.3. Cross-Protocol Attacks . . . . . . . . . . . . . . . . . . 46
160	6.4. Server Push Implicit Headers . . . . . . . . . . . . . . . 46
161	7. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 47
162	7.1. Long Lived Connections . . . . . . . . . . . . . . . . . . 47
163	7.2. SETTINGS frame . . . . . . . . . . . . . . . . . . . . . . 47
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172	8. Requirements Notation . . . . . . . . . . . . . . . . . . . . 47
173	9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 47
174	10. Normative References . . . . . . . . . . . . . . . . . . . . . 48
175	Appendix A. Change Log (to be removed by RFC Editor before
176	publication) . . . . . . . . . . . . . . . . . . . . 49
177	A.1. Since draft-ietf-httpbis-http2-00 . . . . . . . . . . . . 49
178	A.2. Since draft-mbelshe-httpbis-spdy-00 . . . . . . . . . . . 49
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228	1. Introduction
229
230	HTTP is a wildly successful protocol. HTTP/1.1 message encapsulation
231	[HTTP-p1] is optimized for implementation simplicity and
232	accessibility, not application performance. As such it has several
233	characteristics that have a negative overall effect on application
234	performance.
235
236	The HTTP/1.1 encapsulation ensures that only one request can be
237	delivered at a time on a given connection. HTTP/1.1 pipelining,
238	which is not widely deployed, only partially addresses these
239	concerns. Clients that need to make multiple requests therefore use
240	commonly multiple connections to a server or servers in order to
241	reduce the overall latency of those requests.
242
243	Furthermore, HTTP/1.1 headers are represented in an inefficient
244	fashion, which, in addition to generating more or larger network
245	packets, can cause the small initial TCP window to fill more quickly
246	than is ideal. This results in excessive latency where multiple
247	requests are made on a new TCP connection.
248
249	This document defines an optimized mapping of the HTTP semantics to a
250	TCP connection. This optimization reduces the latency costs of HTTP
251	by allowing parallel requests on the same connection and by using an
252	efficient coding for HTTP headers. Prioritization of requests lets
253	more important requests complete faster, further improving
254	application performance.
255
256	HTTP/2.0 applications have an improved impact on network congestion
257	due to the use of fewer TCP connections to achieve the same effect.
258	Fewer TCP connections compete more fairly with other flows. Long-
259	lived connections are also more able to take better advantage of the
260	available network capacity, rather than operating in the slow start
261	phase of TCP.
262
263	The HTTP/2.0 encapsulation also enables more efficient processing of
264	messages by providing efficient message framing. Processing of
265	headers in HTTP/2.0 messages is more efficient (for entities that
266	process many messages).
267
268	1.1. Document Organization
269
270	The HTTP/2.0 Specification is split into three parts: starting
271	HTTP/2.0 (Section 2), which covers how a HTTP/2.0 is started; a
272	framing layer (Section 3), which multiplexes a TCP connection into
273	independent, length-prefixed frames; and an HTTP layer (Section 4),
274	which specifies the mechanism for overlaying HTTP request/response
275	pairs on top of the framing layer. While some of the framing layer
276
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283
284	concepts are isolated from the HTTP layer, building a generic framing
285	layer has not been a goal. The framing layer is tailored to the
286	needs of the HTTP protocol and server push.
287
288	1.2. Definitions
289
290	client: The endpoint initiating the HTTP/2.0 session.
291
292	connection: A transport-level connection between two endpoints.
293
294	endpoint: Either the client or server of a connection.
295
296	frame: A header-prefixed sequence of bytes sent over a HTTP/2.0
297	session.
298
299	server: The endpoint which did not initiate the HTTP/2.0 session.
300
301	session: A synonym for a connection.
302
303	session error: An error on the HTTP/2.0 session.
304
305	stream: A bi-directional flow of bytes across a virtual channel
306	within a HTTP/2.0 session.
307
308	stream error: An error on an individual HTTP/2.0 stream.
309
310	2. Starting HTTP/2.0
311
312	Just as HTTP/1.1 does, HTTP/2.0 uses the "http:" and "https:" URI
313	schemes. An HTTP/2.0-capable client is therefore required to
314	discover whether a server (or intermediary) supports HTTP/2.0.
315
316	Different discovery mechanisms are defined for "http:" and "https:"
317	URIs. Discovery for "http:" URIs is described in Section 2.2;
318	discovery for "https:" URIs is described in Section 2.3.
319
320	2.1. HTTP/2.0 Version Identification
321
322	HTTP/2.0 is identified in using the string "HTTP/2.0". This
323	identification is used in the HTTP/1.1 Upgrade header, in the TLS-NPN
324	[TLSNPN] [[TBD]] field and other places where protocol identification
325	is required.
326
327	[[Editor's Note: please remove the following text prior to the
328	publication of a final version of this document.]]
329
330	Only implementations of the final, published RFC can identify
331	themselves as "HTTP/2.0". Until such an RFC exists, implementations
332
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340	MUST NOT identify themselves using "HTTP/2.0".
341
342	Examples and text throughout the rest of this document use "HTTP/2.0"
343	as a matter of editorial convenience only. Implementations of draft
344	versions MUST NOT identify using this string.
345
346	Implementations of draft versions of the protocol MUST add the
347	corresponding draft number to the identifier before the separator
348	('/'). For example, draft-ietf-httpbis-http2-03 is identified using
349	the string "HTTP-03/2.0".
350
351	Non-compatible experiments that are based on these draft versions
352	MUST include a further identifier. For example, an experimental
353	implementation of packet mood-based encoding based on
354	draft-ietf-httpbis-http2-07 might identify itself as "HTTP-07-
355	emo/2.0". Note that any label MUST conform with the "token" syntax
356	defined in Section 3.2.4 of [HTTP-p1]. Experimenters are encouraged
357	to coordinate their experiments on the ietf-http-wg@w3.org mailing
358	list.
359
360	2.2. Starting HTTP/2.0 for "http:" URIs
361
362	A client that makes a request to an "http:" URI without prior
363	knowledge about support for HTTP/2.0 uses the HTTP Upgrade mechanism
364	[HTTP-p2]. The client makes an HTTP/1.1 request that includes an
365	Upgrade header field identifying HTTP/2.0.
366
367	For example:
368
369	GET /default.htm HTTP/1.1
370	Host: server.example.com
371	Connection: Upgrade
372	Upgrade: HTTP/2.0
373
374	A server that does not support HTTP/2.0 can respond to the request as
375	though the Upgrade header field were absent:
376
377	HTTP/1.1 200 OK
378	Content-length: 243
379	Content-type: text/html
380	...
381
382	A server that supports HTTP/2.0 can accept the upgrade with a 101
383	(Switching Protocols) status code. After the empty line that
384	terminates the 101 response, the server can begin sending HTTP/2.0
385	frames. These frames MUST include a response to the request that
386	initiated the Upgrade.
387
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395
396	HTTP/1.1 101 Switching Protocols
397	Connection: Upgrade
398	Upgrade: HTTP/2.0
399
400	[ HTTP/2.0 frames ...
401
402	A client can learn that a particular server supports HTTP/2.0 by
403	other means. A client MAY immediately send HTTP/2.0 frames to a
404	server that is known to support HTTP/2.0. [[Open Issue: This is not
405	definite. We may yet choose to perform negotiation for every
406	connection. Reasons include intermediaries; phased upgrade of load-
407	balanced server farms; etc...]] [[Open Issue: We need to enumerate
408	the ways that clients can learn of HTTP/2.0 support.]]
409
410	2.3. Starting HTTP/2.0 for "https:" URIs
411
412	[[TBD, maybe NPN]]
413
414	3. HTTP/2.0 Framing Layer
415
416	3.1. Session (Connections)
417
418	The HTTP/2.0 framing layer (or "session") runs atop a reliable
419	transport layer such as TCP [RFC0793]. The client is the TCP
420	connection initiator. HTTP/2.0 connections are persistent
421	connections.
422
423	For best performance, it is expected that clients will not close open
424	connections until the user navigates away from all web pages
425	referencing a connection, or until the server closes the connection.
426	Servers are encouraged to leave connections open for as long as
427	possible, but can terminate idle connections if necessary. When
428	either endpoint closes the transport-level connection, it MUST first
429	send a GOAWAY (Section 3.6.6) frame so that the endpoints can
430	reliably determine if requests finished before the close.
431
432	3.2. Framing
433
434	Once the connection is established, clients and servers exchange
435	framed messages. There are two types of frames: control frames
436	(Section 3.2.1) and data frames (Section 3.2.2). Frames always have
437	a common header which is 8 bytes in length.
438
439	The first bit is a control bit indicating whether a frame is a
440	control frame or data frame. Control frames carry a version number,
441	a frame type, flags, and a length. Data frames contain the stream
442	ID, flags, and the length for the payload carried after the common
443	header. The simple header is designed to make reading and writing of
444
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452	frames easy.
453
454	All integer values, including length, version, and type, are in
455	network byte order. HTTP/2.0 does not enforce alignment of types in
456	dynamically sized frames.
457
458	3.2.1. Control frames
459
460	+----------------------------------+
461	\|C\| Version(15bits) \| Type(16bits) \|
462	+----------------------------------+
463	\| Flags (8) \| Length (24 bits) \|
464	+----------------------------------+
465	\| Data \|
466	+----------------------------------+
467
468	Control bit: The 'C' bit is a single bit indicating if this is a
469	control message. For control frames this value is always 1.
470
471	Version: The version number of the HTTP/2.0 protocol. This document
472	describes HTTP/2.0 version 3.
473
474	Type: The type of control frame. See Control Frames for the complete
475	list of control frames.
476
477	Flags: Flags related to this frame. Flags for control frames and
478	data frames are different.
479
480	Length: An unsigned 24-bit value representing the number of bytes
481	after the length field.
482
483	Data: data associated with this control frame. The format and length
484	of this data is controlled by the control frame type.
485
486	Control frame processing requirements:
487
488	Note that full length control frames (16MB) can be large for
489	implementations running on resource-limited hardware. In such
490	cases, implementations MAY limit the maximum length frame
491	supported. However, all implementations MUST be able to receive
492	control frames of at least 8192 octets in length.
493
494
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508	3.2.2. Data frames
509
510	+----------------------------------+
511	\|C\| Stream-ID (31bits) \|
512	+----------------------------------+
513	\| Flags (8) \| Length (24 bits) \|
514	+----------------------------------+
515	\| Data \|
516	+----------------------------------+
517
518	Control bit: For data frames this value is always 0.
519
520	Stream-ID: A 31-bit value identifying the stream.
521
522	Flags: Flags related to this frame. Valid flags are:
523
524	0x01 = FLAG_FIN - signifies that this frame represents the last
525	frame to be transmitted on this stream. See Stream Close
526	(Section 3.3.7) below.
527
528	0x02 = FLAG_COMPRESS - indicates that the data in this frame has
529	been compressed.
530
531	Length: An unsigned 24-bit value representing the number of bytes
532	after the length field. The total size of a data frame is 8 bytes +
533	length. It is valid to have a zero-length data frame.
534
535	Data: The variable-length data payload; the length was defined in the
536	length field.
537
538	Data frame processing requirements:
539
540	If an endpoint receives a data frame for a stream-id which is not
541	open and the endpoint has not sent a GOAWAY (Section 3.6.6) frame,
542	it MUST send issue a stream error (Section 3.4.2) with the error
543	code INVALID_STREAM for the stream-id.
544
545	If the endpoint which created the stream receives a data frame
546	before receiving a SYN_REPLY on that stream, it is a protocol
547	error, and the recipient MUST issue a stream error (Section 3.4.2)
548	with the status code PROTOCOL_ERROR for the stream-id.
549
550	Implementors note: If an endpoint receives multiple data frames
551	for invalid stream-ids, it MAY close the session.
552
553	All HTTP/2.0 endpoints MUST accept compressed data frames.
554	Compression of data frames is always done using zlib compression.
555	Each stream initializes and uses its own compression context
556
557
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564	dedicated to use within that stream. Endpoints are encouraged to
565	use application level compression rather than HTTP/2.0 stream
566	level compression.
567
568	Each HTTP/2.0 stream sending compressed frames creates its own
569	zlib context for that stream, and these compression contexts MUST
570	be distinct from the compression contexts used with SYN_STREAM/
571	SYN_REPLY/HEADER compression. (Thus, if both endpoints of a
572	stream are compressing data on the stream, there will be two zlib
573	contexts, one for sending and one for receiving).
574
575	3.3. Streams
576
577	Streams are independent sequences of bi-directional data divided into
578	frames with several properties:
579
580	Streams may be created by either the client or server.
581
582	Streams optionally carry a set of name/value header pairs.
583
584	Streams can concurrently send data interleaved with other streams.
585
586	Streams may be cancelled.
587
588	3.3.1. Stream frames
589
590	HTTP/2.0 defines 3 control frames to manage the lifecycle of a
591	stream:
592
593	SYN_STREAM - Open a new stream
594
595	SYN_REPLY - Remote acknowledgement of a new, open stream
596
597	RST_STREAM - Close a stream
598
599	3.3.2. Stream creation
600
601	A stream is created by sending a control frame with the type set to
602	SYN_STREAM (Section 3.6.1). If the server is initiating the stream,
603	the Stream-ID must be even. If the client is initiating the stream,
604	the Stream-ID must be odd. 0 is not a valid Stream-ID. Stream-IDs
605	from each side of the connection must increase monotonically as new
606	streams are created. E.g. Stream 2 may be created after stream 3,
607	but stream 7 must not be created after stream 9. Stream IDs do not
608	wrap: when a client or server cannot create a new stream id without
609	exceeding a 31 bit value, it MUST NOT create a new stream.
610
611	The stream-id MUST increase with each new stream. If an endpoint
612
613
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619
620	receives a SYN_STREAM with a stream id which is less than any
621	previously received SYN_STREAM, it MUST issue a session error
622	(Section 3.4.1) with the status PROTOCOL_ERROR.
623
624	It is a protocol error to send two SYN_STREAMs with the same
625	stream-id. If a recipient receives a second SYN_STREAM for the same
626	stream, it MUST issue a stream error (Section 3.4.2) with the status
627	code PROTOCOL_ERROR.
628
629	Upon receipt of a SYN_STREAM, the recipient can reject the stream by
630	sending a stream error (Section 3.4.2) with the error code
631	REFUSED_STREAM. Note, however, that the creating endpoint may have
632	already sent additional frames for that stream which cannot be
633	immediately stopped.
634
635	Once the stream is created, the creator may immediately send HEADERS
636	or DATA frames for that stream, without needing to wait for the
637	recipient to acknowledge.
638
639	3.3.2.1. Unidirectional streams
640
641	When an endpoint creates a stream with the FLAG_UNIDIRECTIONAL flag
642	set, it creates a unidirectional stream which the creating endpoint
643	can use to send frames, but the receiving endpoint cannot. The
644	receiving endpoint is implicitly already in the half-closed
645	(Section 3.3.6) state.
646
647	3.3.2.2. Bidirectional streams
648
649	SYN_STREAM frames which do not use the FLAG_UNIDIRECTIONAL flag are
650	bidirectional streams. Both endpoints can send data on a bi-
651	directional stream.
652
653	3.3.3. Stream priority
654
655	The creator of a stream assigns a priority for that stream. Priority
656	is represented as an integer from 0 to 7. 0 represents the highest
657	priority and 7 represents the lowest priority.
658
659	The sender and recipient SHOULD use best-effort to process streams in
660	the order of highest priority to lowest priority.
661
662	3.3.4. Stream headers
663
664	Streams carry optional sets of name/value pair headers which carry
665	metadata about the stream. After the stream has been created, and as
666	long as the sender is not closed (Section 3.3.7) or half-closed
667	(Section 3.3.6), each side may send HEADERS frame(s) containing the
668
669
670
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675
676	header data. Header data can be sent in multiple HEADERS frames, and
677	HEADERS frames may be interleaved with data frames.
678
679	3.3.5. Stream data exchange
680
681	Once a stream is created, it can be used to send arbitrary amounts of
682	data. Generally this means that a series of data frames will be sent
683	on the stream until a frame containing the FLAG_FIN flag is set. The
684	FLAG_FIN can be set on a SYN_STREAM (Section 3.6.1), SYN_REPLY
685	(Section 3.6.2), HEADERS (Section 3.6.7) or a DATA (Section 3.2.2)
686	frame. Once the FLAG_FIN has been sent, the stream is considered to
687	be half-closed.
688
689	3.3.6. Stream half-close
690
691	When one side of the stream sends a frame with the FLAG_FIN flag set,
692	the stream is half-closed from that endpoint. The sender of the
693	FLAG_FIN MUST NOT send further frames on that stream. When both
694	sides have half-closed, the stream is closed.
695
696	If an endpoint receives a data frame after the stream is half-closed
697	from the sender (e.g. the endpoint has already received a prior frame
698	for the stream with the FIN flag set), it MUST send a RST_STREAM to
699	the sender with the status STREAM_ALREADY_CLOSED.
700
701	3.3.7. Stream close
702
703	There are 3 ways that streams can be terminated:
704
705	Normal termination: Normal stream termination occurs when both
706	sender and recipient have half-closed the stream by sending a
707	FLAG_FIN.
708
709	Abrupt termination: Either the client or server can send a
710	RST_STREAM control frame at any time. A RST_STREAM contains an
711	error code to indicate the reason for failure. When a RST_STREAM
712	is sent from the stream originator, it indicates a failure to
713	complete the stream and that no further data will be sent on the
714	stream. When a RST_STREAM is sent from the stream recipient, the
715	sender, upon receipt, should stop sending any data on the stream.
716	The stream recipient should be aware that there is a race between
717	data already in transit from the sender and the time the
718	RST_STREAM is received. See Stream Error Handling (Section 3.4.2)
719
720	TCP connection teardown: If the TCP connection is torn down while
721	un-closed streams exist, then the endpoint must assume that the
722	stream was abnormally interrupted and may be incomplete.
723
724
725
726
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731
732	If an endpoint receives a data frame after the stream is closed, it
733	must send a RST_STREAM to the sender with the status PROTOCOL_ERROR.
734
735	3.4. Error Handling
736
737	The HTTP/2.0 framing layer has only two types of errors, and they are
738	always handled consistently. Any reference in this specification to
739	"issue a session error" refers to Section 3.4.1. Any reference to
740	"issue a stream error" refers to Section 3.4.2.
741
742	3.4.1. Session Error Handling
743
744	A session error is any error which prevents further processing of the
745	framing layer or which corrupts the session compression state. When
746	a session error occurs, the endpoint encountering the error MUST
747	first send a GOAWAY (Section 3.6.6) frame with the stream id of most
748	recently received stream from the remote endpoint, and the error code
749	for why the session is terminating. After sending the GOAWAY frame,
750	the endpoint MUST close the TCP connection.
751
752	Note that the session compression state is dependent upon both
753	endpoints always processing all compressed data. If an endpoint
754	partially processes a frame containing compressed data without
755	updating compression state properly, future control frames which use
756	compression will be always be errored. Implementations SHOULD always
757	try to process compressed data so that errors which could be handled
758	as stream errors do not become session errors.
759
760	Note that because this GOAWAY is sent during a session error case, it
761	is possible that the GOAWAY will not be reliably received by the
762	receiving endpoint. It is a best-effort attempt to communicate with
763	the remote about why the session is going down.
764
765	3.4.2. Stream Error Handling
766
767	A stream error is an error related to a specific stream-id which does
768	not affect processing of other streams at the framing layer. Upon a
769	stream error, the endpoint MUST send a RST_STREAM (Section 3.6.3)
770	frame which contains the stream id of the stream where the error
771	occurred and the error status which caused the error. After sending
772	the RST_STREAM, the stream is closed to the sending endpoint. After
773	sending the RST_STREAM, if the sender receives any frames other than
774	a RST_STREAM for that stream id, it will result in sending additional
775	RST_STREAM frames. An endpoint MUST NOT send a RST_STREAM in
776	response to an RST_STREAM, as doing so would lead to RST_STREAM
777	loops. Sending a RST_STREAM does not cause the HTTP/2.0 session to
778	be closed.
779
780
781
782
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787
788	If an endpoint has multiple RST_STREAM frames to send in succession
789	for the same stream-id and the same error code, it MAY coalesce them
790	into a single RST_STREAM frame. (This can happen if a stream is
791	closed, but the remote sends multiple data frames. There is no
792	reason to send a RST_STREAM for each frame in succession).
793
794	3.5. Stream Flow Control
795
796	Multiplexing streams introduces contention for access to the shared
797	TCP connection. Stream contention can result in streams being
798	blocked by other streams. A flow control scheme ensures that streams
799	do not destructively interfere with other streams on the same TCP
800	connection.
801
802	3.5.1. Flow Control Principles
803
804	Experience with TCP congestion control has shown that algorithms can
805	evolve over time to become more sophisticated without requiring
806	protocol changes. TCP congestion control and its evolution is
807	clearly different from HTTP/2.0 flow control, though the evolution of
808	TCP congestion control algorithms shows that a similar approach could
809	be feasible for HTTP/2.0 flow control.
810
811	HTTP/2.0 stream flow control aims to allow for future improvements to
812	flow control algorithms without requiring protocol changes. The
813	following principles guide the HTTP/2.0 design:
814
815	1. Flow control is hop-by-hop, not end-to-end.
816
817	2. Flow control is based on window update messages. Receivers
818	advertise how many octets they are prepared to receive on a
819	stream. This is a credit-based scheme.
820
821	3. Flow control is directional with overall control provided by the
822	receiver. A receiver MAY choose to set any window size that it
823	desires for each stream [[TBD: ... and for the overall
824	connection]]. A sender MUST respect flow control limits imposed
825	by a receiver. Clients, servers and intermediaries all
826	independently advertise their flow control preferences as a
827	receiver and abide by the flow control limits set by their peer
828	when sending.
829
830	4. Flow control can be disabled by a receiver. A receiver can
831	choose to either disable flow control, or to declare an infinite
832	flow control limit. [[TBD: determine whether just one mechanism
833	is sufficient, and then which alternative]]
834
835
836
837
838
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843
844	5. HTTP/2.0 standardizes only the format of the window update
845	message (Section 3.6.8). This does not stipulate how a receiver
846	decides when to send this message or the value that it sends.
847	Nor does it specify how a sender chooses to send packets.
848	Implementations are able to select any algorithm that suits their
849	needs. An example flow control algorithm is described in
850	Section 3.5.2.
851
852	Implementations are also responsible for managing how requests and
853	responses are sent based on priority; choosing how to avoid head of
854	line blocking for requests; and managing the creation of new streams.
855	Algorithm choices for these could interact with any flow control
856	algorithm.
857
858	3.5.2. Basic Flow Control Algorithm
859
860	This section describes a basic flow control algorithm. This
861	algorithm is provided as an example, implementations can use any
862	algorithm that complies with flow control requirements.
863
864	[[Algorithm TBD]]
865
866	3.6. Control frame types
867
868	3.6.1. SYN_STREAM
869
870	The SYN_STREAM control frame allows the sender to asynchronously
871	create a stream between the endpoints. See Stream Creation
872	(Section 3.3.2)
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
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899
900	+------------------------------------+
901	\|1\| version \| 1 \|
902	+------------------------------------+
903	\| Flags (8) \| Length (24 bits) \|
904	+------------------------------------+
905	\|X\| Stream-ID (31bits) \|
906	+------------------------------------+
907	\|X\| Associated-To-Stream-ID (31bits) \|
908	+------------------------------------+
909	\| Pri\|Unused \| Slot \| \|
910	+-------------------+ \|
911	\| Number of Name/Value pairs (int32) \| <+
912	+------------------------------------+ \|
913	\| Length of name (int32) \| \| This section is the
914	+------------------------------------+ \| "Name/Value Header
915	\| Name (string) \| \| Block", and is
916	+------------------------------------+ \| compressed.
917	\| Length of value (int32) \| \|
918	+------------------------------------+ \|
919	\| Value (string) \| \|
920	+------------------------------------+ \|
921	\| (repeats) \| <+
922
923	Flags: Flags related to this frame. Valid flags are:
924
925	0x01 = FLAG_FIN - marks this frame as the last frame to be
926	transmitted on this stream and puts the sender in the half-closed
927	(Section 3.3.6) state.
928
929	0x02 = FLAG_UNIDIRECTIONAL - a stream created with this flag puts
930	the recipient in the half-closed (Section 3.3.6) state.
931
932	Length: The length is the number of bytes which follow the length
933	field in the frame. For SYN_STREAM frames, this is 10 bytes plus the
934	length of the compressed Name/Value block.
935
936	Stream-ID: The 31-bit identifier for this stream. This stream-id
937	will be used in frames which are part of this stream.
938
939	Associated-To-Stream-ID: The 31-bit identifier for a stream which
940	this stream is associated to. If this stream is independent of all
941	other streams, it should be 0.
942
943	Priority: A 3-bit priority (Section 3.3.3) field.
944
945	Unused: 5 bits of unused space, reserved for future use.
946
947	Slot: An 8 bit unsigned integer specifying the index in the server's
948
949
950
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954
955
956	CREDENTIAL vector of the client certificate to be used for this
957	request. see CREDENTIAL frame (Section 3.6.9). The value 0 means no
958	client certificate should be associated with this stream.
959
960	Name/Value Header Block: A set of name/value pairs carried as part of
961	the SYN_STREAM. see Name/Value Header Block (Section 3.6.10).
962
963	If an endpoint receives a SYN_STREAM which is larger than the
964	implementation supports, it MAY send a RST_STREAM with error code
965	FRAME_TOO_LARGE. All implementations MUST support the minimum size
966	limits defined in the Control Frames section (Section 3.2.1).
967
968	3.6.2. SYN_REPLY
969
970	SYN_REPLY indicates the acceptance of a stream creation by the
971	recipient of a SYN_STREAM frame.
972
973	+------------------------------------+
974	\|1\| version \| 2 \|
975	+------------------------------------+
976	\| Flags (8) \| Length (24 bits) \|
977	+------------------------------------+
978	\|X\| Stream-ID (31bits) \|
979	+------------------------------------+
980	\| Number of Name/Value pairs (int32) \| <+
981	+------------------------------------+ \|
982	\| Length of name (int32) \| \| This section is the
983	+------------------------------------+ \| "Name/Value Header
984	\| Name (string) \| \| Block", and is
985	+------------------------------------+ \| compressed.
986	\| Length of value (int32) \| \|
987	+------------------------------------+ \|
988	\| Value (string) \| \|
989	+------------------------------------+ \|
990	\| (repeats) \| <+
991
992	Flags: Flags related to this frame. Valid flags are:
993
994	0x01 = FLAG_FIN - marks this frame as the last frame to be
995	transmitted on this stream and puts the sender in the half-closed
996	(Section 3.3.6) state.
997
998	Length: The length is the number of bytes which follow the length
999	field in the frame. For SYN_REPLY frames, this is 4 bytes plus the
1000	length of the compressed Name/Value block.
1001
1002	Stream-ID: The 31-bit identifier for this stream.
1003
1004
1005
1006
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1011
1012	If an endpoint receives multiple SYN_REPLY frames for the same active
1013	stream ID, it MUST issue a stream error (Section 3.4.2) with the
1014	error code STREAM_IN_USE.
1015
1016	Name/Value Header Block: A set of name/value pairs carried as part of
1017	the SYN_STREAM. see Name/Value Header Block (Section 3.6.10).
1018
1019	If an endpoint receives a SYN_REPLY which is larger than the
1020	implementation supports, it MAY send a RST_STREAM with error code
1021	FRAME_TOO_LARGE. All implementations MUST support the minimum size
1022	limits defined in the Control Frames section (Section 3.2.1).
1023
1024	3.6.3. RST_STREAM
1025
1026	The RST_STREAM frame allows for abnormal termination of a stream.
1027	When sent by the creator of a stream, it indicates the creator wishes
1028	to cancel the stream. When sent by the recipient of a stream, it
1029	indicates an error or that the recipient did not want to accept the
1030	stream, so the stream should be closed.
1031
1032	+----------------------------------+
1033	\|1\| version \| 3 \|
1034	+----------------------------------+
1035	\| Flags (8) \| 8 \|
1036	+----------------------------------+
1037	\|X\| Stream-ID (31bits) \|
1038	+----------------------------------+
1039	\| Status code \|
1040	+----------------------------------+
1041
1042	Flags: Flags related to this frame. RST_STREAM does not define any
1043	flags. This value must be 0.
1044
1045	Length: An unsigned 24-bit value representing the number of bytes
1046	after the length field. For RST_STREAM control frames, this value is
1047	always 8.
1048
1049	Stream-ID: The 31-bit identifier for this stream.
1050
1051	Status code: (32 bits) An indicator for why the stream is being
1052	terminated.The following status codes are defined:
1053
1054	1 - PROTOCOL_ERROR. This is a generic error, and should only be
1055	used if a more specific error is not available.
1056
1057	2 - INVALID_STREAM. This is returned when a frame is received for
1058	a stream which is not active.
1059
1060
1061
1062
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1066
1067
1068	3 - REFUSED_STREAM. Indicates that the stream was refused before
1069	any processing has been done on the stream.
1070
1071	4 - UNSUPPORTED_VERSION. Indicates that the recipient of a stream
1072	does not support the HTTP/2.0 version requested.
1073
1074	5 - CANCEL. Used by the creator of a stream to indicate that the
1075	stream is no longer needed.
1076
1077	6 - INTERNAL_ERROR. This is a generic error which can be used
1078	when the implementation has internally failed, not due to anything
1079	in the protocol.
1080
1081	7 - FLOW_CONTROL_ERROR. The endpoint detected that its peer
1082	violated the flow control protocol.
1083
1084	8 - STREAM_IN_USE. The endpoint received a SYN_REPLY for a stream
1085	already open.
1086
1087	9 - STREAM_ALREADY_CLOSED. The endpoint received a data or
1088	SYN_REPLY frame for a stream which is half closed.
1089
1090	10 - INVALID_CREDENTIALS. The server received a request for a
1091	resource whose origin does not have valid credentials in the
1092	client certificate vector.
1093
1094	11 - FRAME_TOO_LARGE. The endpoint received a frame which this
1095	implementation could not support. If FRAME_TOO_LARGE is sent for
1096	a SYN_STREAM, HEADERS, or SYN_REPLY frame without fully processing
1097	the compressed portion of those frames, then the compression state
1098	will be out-of-sync with the other endpoint. In this case,
1099	senders of FRAME_TOO_LARGE MUST close the session.
1100
1101	Note: 0 is not a valid status code for a RST_STREAM.
1102
1103	After receiving a RST_STREAM on a stream, the recipient must not send
1104	additional frames for that stream, and the stream moves into the
1105	closed state.
1106
1107	3.6.4. SETTINGS
1108
1109	A SETTINGS frame contains a set of id/value pairs for communicating
1110	configuration data about how the two endpoints may communicate.
1111	SETTINGS frames can be sent at any time by either endpoint, are
1112	optionally sent, and are fully asynchronous. When the server is the
1113	sender, the sender can request that configuration data be persisted
1114	by the client across HTTP/2.0 sessions and returned to the server in
1115	future communications.
1116
1117
1118
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1123
1124	Persistence of SETTINGS ID/Value pairs is done on a per origin/IP
1125	pair (the "origin" is the set of scheme, host, and port from the URI.
1126	See [RFC6454]). That is, when a client connects to a server, and the
1127	server persists settings within the client, the client SHOULD return
1128	the persisted settings on future connections to the same origin AND
1129	IP address and TCP port. Clients MUST NOT request servers to use the
1130	persistence features of the SETTINGS frames, and servers MUST ignore
1131	persistence related flags sent by a client.
1132
1133	+----------------------------------+
1134	\|1\| version \| 4 \|
1135	+----------------------------------+
1136	\| Flags (8) \| Length (24 bits) \|
1137	+----------------------------------+
1138	\| Number of entries \|
1139	+----------------------------------+
1140	\| ID/Value Pairs \|
1141	\| ... \|
1142
1143	Control bit: The control bit is always 1 for this message.
1144
1145	Version: The HTTP/2.0 version number.
1146
1147	Type: The message type for a SETTINGS message is 4.
1148
1149	Flags: FLAG_SETTINGS_CLEAR_SETTINGS (0x1): When set, the client
1150	should clear any previously persisted SETTINGS ID/Value pairs. If
1151	this frame contains ID/Value pairs with the
1152	FLAG_SETTINGS_PERSIST_VALUE set, then the client will first clear its
1153	existing, persisted settings, and then persist the values with the
1154	flag set which are contained within this frame. Because persistence
1155	is only implemented on the client, this flag can only be used when
1156	the sender is the server.
1157
1158	Length: An unsigned 24-bit value representing the number of bytes
1159	after the length field. The total size of a SETTINGS frame is 8
1160	bytes + length.
1161
1162	Number of entries: A 32-bit value representing the number of ID/value
1163	pairs in this message.
1164
1165	ID: A 32-bit ID number, comprised of 8 bits of flags and 24 bits of
1166	unique ID.
1167
1168	ID.flags:
1169
1170	FLAG_SETTINGS_PERSIST_VALUE (0x1): When set, the sender of this
1171	SETTINGS frame is requesting that the recipient persist the ID/
1172
1173
1174
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1178
1179
1180	Value and return it in future SETTINGS frames sent from the
1181	sender to this recipient. Because persistence is only
1182	implemented on the client, this flag is only sent by the
1183	server.
1184
1185	FLAG_SETTINGS_PERSISTED (0x2): When set, the sender is
1186	notifying the recipient that this ID/Value pair was previously
1187	sent to the sender by the recipient with the
1188	FLAG_SETTINGS_PERSIST_VALUE, and the sender is returning it.
1189	Because persistence is only implemented on the client, this
1190	flag is only sent by the client.
1191
1192	Defined IDs:
1193
1194	1 - SETTINGS_UPLOAD_BANDWIDTH allows the sender to send its
1195	expected upload bandwidth on this channel. This number is an
1196	estimate. The value should be the integral number of kilobytes
1197	per second that the sender predicts as an expected maximum
1198	upload channel capacity.
1199
1200	2 - SETTINGS_DOWNLOAD_BANDWIDTH allows the sender to send its
1201	expected download bandwidth on this channel. This number is an
1202	estimate. The value should be the integral number of kilobytes
1203	per second that the sender predicts as an expected maximum
1204	download channel capacity.
1205
1206	3 - SETTINGS_ROUND_TRIP_TIME allows the sender to send its
1207	expected round-trip-time on this channel. The round trip time
1208	is defined as the minimum amount of time to send a control
1209	frame from this client to the remote and receive a response.
1210	The value is represented in milliseconds.
1211
1212	4 - SETTINGS_MAX_CONCURRENT_STREAMS allows the sender to inform
1213	the remote endpoint the maximum number of concurrent streams
1214	which it will allow. By default there is no limit. For
1215	implementors it is recommended that this value be no smaller
1216	than 100.
1217
1218	5 - SETTINGS_CURRENT_CWND allows the sender to inform the
1219	remote endpoint of the current TCP CWND value.
1220
1221	6 - SETTINGS_DOWNLOAD_RETRANS_RATE allows the sender to inform
1222	the remote endpoint the retransmission rate (bytes
1223	retransmitted / total bytes transmitted).
1224
1225	7 - SETTINGS_INITIAL_WINDOW_SIZE allows the sender to inform
1226	the remote endpoint the initial window size (in bytes) for new
1227	streams.
1228
1229
1230
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1234
1235
1236	8 - SETTINGS_CLIENT_CERTIFICATE_VECTOR_SIZE allows the server
1237	to inform the client of the new size of the client certificate
1238	vector.
1239
1240	Value: A 32-bit value.
1241
1242	The message is intentionally extensible for future information which
1243	may improve client-server communications. The sender does not need
1244	to send every type of ID/value. It must only send those for which it
1245	has accurate values to convey. When multiple ID/value pairs are
1246	sent, they should be sent in order of lowest id to highest id. A
1247	single SETTINGS frame MUST not contain multiple values for the same
1248	ID. If the recipient of a SETTINGS frame discovers multiple values
1249	for the same ID, it MUST ignore all values except the first one.
1250
1251	A server may send multiple SETTINGS frames containing different ID/
1252	Value pairs. When the same ID/Value is sent twice, the most recent
1253	value overrides any previously sent values. If the server sends IDs
1254	1, 2, and 3 with the FLAG_SETTINGS_PERSIST_VALUE in a first SETTINGS
1255	frame, and then sends IDs 4 and 5 with the
1256	FLAG_SETTINGS_PERSIST_VALUE, when the client returns the persisted
1257	state on its next SETTINGS frame, it SHOULD send all 5 settings (1,
1258	2, 3, 4, and 5 in this example) to the server.
1259
1260	3.6.5. PING
1261
1262	The PING control frame is a mechanism for measuring a minimal round-
1263	trip time from the sender. It can be sent from the client or the
1264	server. Recipients of a PING frame should send an identical frame to
1265	the sender as soon as possible (if there is other pending data
1266	waiting to be sent, PING should take highest priority). Each ping
1267	sent by a sender should use a unique ID.
1268
1269	+----------------------------------+
1270	\|1\| version \| 6 \|
1271	+----------------------------------+
1272	\| 0 (flags) \| 4 (length) \|
1273	+----------------------------------\|
1274	\| 32-bit ID \|
1275	+----------------------------------+
1276
1277	Control bit: The control bit is always 1 for this message.
1278
1279	Version: The HTTP/2.0 version number.
1280
1281	Type: The message type for a PING message is 6.
1282
1283	Length: This frame is always 4 bytes long.
1284
1285
1286
1287	Belshe, et al. Expires July 26, 2013 [Page 23]
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1289	Internet-Draft HTTP/2.0 January 2013
1290
1291
1292	ID: A unique ID for this ping, represented as an unsigned 32 bit
1293	value. When the client initiates a ping, it must use an odd numbered
1294	ID. When the server initiates a ping, it must use an even numbered
1295	ping. Use of odd/even IDs is required in order to avoid accidental
1296	looping on PINGs (where each side initiates an identical PING at the
1297	same time).
1298
1299	Note: If a sender uses all possible PING ids (e.g. has sent all 2^31
1300	possible IDs), it can wrap and start re-using IDs.
1301
1302	If a server receives an even numbered PING which it did not initiate,
1303	it must ignore the PING. If a client receives an odd numbered PING
1304	which it did not initiate, it must ignore the PING.
1305
1306	3.6.6. GOAWAY
1307
1308	The GOAWAY control frame is a mechanism to tell the remote side of
1309	the connection to stop creating streams on this session. It can be
1310	sent from the client or the server. Once sent, the sender will not
1311	respond to any new SYN_STREAMs on this session. Recipients of a
1312	GOAWAY frame must not send additional streams on this session,
1313	although a new session can be established for new streams. The
1314	purpose of this message is to allow an endpoint to gracefully stop
1315	accepting new streams (perhaps for a reboot or maintenance), while
1316	still finishing processing of previously established streams.
1317
1318	There is an inherent race condition between an endpoint sending
1319	SYN_STREAMs and the remote sending a GOAWAY message. To deal with
1320	this case, the GOAWAY contains a last-stream-id indicating the
1321	stream-id of the last stream which was created on the sending
1322	endpoint in this session. If the receiver of the GOAWAY sent new
1323	SYN_STREAMs for sessions after this last-stream-id, they were not
1324	processed by the server and the receiver may treat the stream as
1325	though it had never been created at all (hence the receiver may want
1326	to re-create the stream later on a new session).
1327
1328	Endpoints should always send a GOAWAY message before closing a
1329	connection so that the remote can know whether a stream has been
1330	partially processed or not. (For example, if an HTTP client sends a
1331	POST at the same time that a server closes a connection, the client
1332	cannot know if the server started to process that POST request if the
1333	server does not send a GOAWAY frame to indicate where it stopped
1334	working).
1335
1336	After sending a GOAWAY message, the sender must ignore all SYN_STREAM
1337	frames for new streams.
1338
1339
1340
1341
1342
1343	Belshe, et al. Expires July 26, 2013 [Page 24]
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1345	Internet-Draft HTTP/2.0 January 2013
1346
1347
1348	+----------------------------------+
1349	\|1\| version \| 7 \|
1350	+----------------------------------+
1351	\| 0 (flags) \| 8 (length) \|
1352	+----------------------------------\|
1353	\|X\| Last-good-stream-ID (31 bits) \|
1354	+----------------------------------+
1355	\| Status code \|
1356	+----------------------------------+
1357
1358	Control bit: The control bit is always 1 for this message.
1359
1360	Version: The HTTP/2.0 version number.
1361
1362	Type: The message type for a GOAWAY message is 7.
1363
1364	Length: This frame is always 8 bytes long.
1365
1366	Last-good-stream-Id: The last stream id which was replied to (with
1367	either a SYN_REPLY or RST_STREAM) by the sender of the GOAWAY
1368	message. If no streams were replied to, this value MUST be 0.
1369
1370	Status: The reason for closing the session.
1371
1372	0 - OK. This is a normal session teardown.
1373
1374	1 - PROTOCOL_ERROR. This is a generic error, and should only be
1375	used if a more specific error is not available.
1376
1377	2 - INTERNAL_ERROR. This is a generic error which can be used
1378	when the implementation has internally failed, not due to anything
1379	in the protocol.
1380
1381	3.6.7. HEADERS
1382
1383	The HEADERS frame augments a stream with additional headers. It may
1384	be optionally sent on an existing stream at any time. Specific
1385	application of the headers in this frame is application-dependent.
1386	The name/value header block within this frame is compressed.
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399	Belshe, et al. Expires July 26, 2013 [Page 25]
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1401	Internet-Draft HTTP/2.0 January 2013
1402
1403
1404	+------------------------------------+
1405	\|1\| version \| 8 \|
1406	+------------------------------------+
1407	\| Flags (8) \| Length (24 bits) \|
1408	+------------------------------------+
1409	\|X\| Stream-ID (31bits) \|
1410	+------------------------------------+
1411	\| Number of Name/Value pairs (int32) \| <+
1412	+------------------------------------+ \|
1413	\| Length of name (int32) \| \| This section is the
1414	+------------------------------------+ \| "Name/Value Header
1415	\| Name (string) \| \| Block", and is
1416	+------------------------------------+ \| compressed.
1417	\| Length of value (int32) \| \|
1418	+------------------------------------+ \|
1419	\| Value (string) \| \|
1420	+------------------------------------+ \|
1421	\| (repeats) \| <+
1422
1423	Flags: Flags related to this frame. Valid flags are:
1424
1425	0x01 = FLAG_FIN - marks this frame as the last frame to be
1426	transmitted on this stream and puts the sender in the half-closed
1427	(Section 3.3.6) state.
1428
1429	Length: An unsigned 24 bit value representing the number of bytes
1430	after the length field. The minimum length of the length field is 4
1431	(when the number of name value pairs is 0).
1432
1433	Stream-ID: The stream this HEADERS block is associated with.
1434
1435	Name/Value Header Block: A set of name/value pairs carried as part of
1436	the SYN_STREAM. see Name/Value Header Block (Section 3.6.10).
1437
1438	3.6.8. WINDOW_UPDATE
1439
1440	The WINDOW_UPDATE control frame is used to implement per stream flow
1441	control in HTTP/2.0. Flow control in HTTP/2.0 is per hop, that is,
1442	only between the two endpoints of a HTTP/2.0 connection. If there
1443	are one or more intermediaries between the client and the origin
1444	server, flow control signals are not explicitly forwarded by the
1445	intermediaries. (However, throttling of data transfer by any
1446	recipient may have the effect of indirectly propagating flow control
1447	information upstream back to the original sender.) Flow control only
1448	applies to the data portion of data frames. Recipients must buffer
1449	all control frames. If a recipient fails to buffer an entire control
1450	frame, it MUST issue a stream error (Section 3.4.2) with the status
1451	code FLOW_CONTROL_ERROR for the stream.
1452
1453
1454
1455	Belshe, et al. Expires July 26, 2013 [Page 26]
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1457	Internet-Draft HTTP/2.0 January 2013
1458
1459
1460	Flow control in HTTP/2.0 is implemented by a data transfer window
1461	kept by the sender of each stream. The data transfer window is a
1462	simple uint32 that indicates how many bytes of data the sender can
1463	transmit. After a stream is created, but before any data frames have
1464	been transmitted, the sender begins with the initial window size.
1465	This window size is a measure of the buffering capability of the
1466	recipient. The sender must not send a data frame with data length
1467	greater than the transfer window size. After sending each data
1468	frame, the sender decrements its transfer window size by the amount
1469	of data transmitted. When the window size becomes less than or equal
1470	to 0, the sender must pause transmitting data frames. At the other
1471	end of the stream, the recipient sends a WINDOW_UPDATE control back
1472	to notify the sender that it has consumed some data and freed up
1473	buffer space to receive more data.
1474
1475	+----------------------------------+
1476	\|1\| version \| 9 \|
1477	+----------------------------------+
1478	\| 0 (flags) \| 8 (length) \|
1479	+----------------------------------+
1480	\|X\| Stream-ID (31-bits) \|
1481	+----------------------------------+
1482	\|X\| Delta-Window-Size (31-bits) \|
1483	+----------------------------------+
1484
1485	Control bit: The control bit is always 1 for this message.
1486
1487	Version: The HTTP/2.0 version number.
1488
1489	Type: The message type for a WINDOW_UPDATE message is 9.
1490
1491	Length: The length field is always 8 for this frame (there are 8
1492	bytes after the length field).
1493
1494	Stream-ID: The stream ID that this WINDOW_UPDATE control frame is
1495	for.
1496
1497	Delta-Window-Size: The additional number of bytes that the sender can
1498	transmit in addition to existing remaining window size. The legal
1499	range for this field is 1 to 2^31 - 1 (0x7fffffff) bytes.
1500
1501	The window size as kept by the sender must never exceed 2^31
1502	(although it can become negative in one special case). If a sender
1503	receives a WINDOW_UPDATE that causes the its window size to exceed
1504	this limit, it must send RST_STREAM with status code
1505	FLOW_CONTROL_ERROR to terminate the stream.
1506
1507	When a HTTP/2.0 connection is first established, the default initial
1508
1509
1510
1511	Belshe, et al. Expires July 26, 2013 [Page 27]
1512
1513	Internet-Draft HTTP/2.0 January 2013
1514
1515
1516	window size for all streams is 64KB. An endpoint can use the
1517	SETTINGS control frame to adjust the initial window size for the
1518	connection. That is, its peer can start out using the 64KB default
1519	initial window size when sending data frames before receiving the
1520	SETTINGS. Because SETTINGS is asynchronous, there may be a race
1521	condition if the recipient wants to decrease the initial window size,
1522	but its peer immediately sends 64KB on the creation of a new
1523	connection, before waiting for the SETTINGS to arrive. This is one
1524	case where the window size kept by the sender will become negative.
1525	Once the sender detects this condition, it must stop sending data
1526	frames and wait for the recipient to catch up. The recipient has two
1527	choices:
1528
1529	immediately send RST_STREAM with FLOW_CONTROL_ERROR status code.
1530
1531	allow the head of line blocking (as there is only one stream for
1532	the session and the amount of data in flight is bounded by the
1533	default initial window size), and send WINDOW_UPDATE as it
1534	consumes data.
1535
1536	In the case of option 2, both sides must compute the window size
1537	based on the initial window size in the SETTINGS. For example, if
1538	the recipient sets the initial window size to be 16KB, and the sender
1539	sends 64KB immediately on connection establishment, the sender will
1540	discover its window size is -48KB on receipt of the SETTINGS. As the
1541	recipient consumes the first 16KB, it must send a WINDOW_UPDATE of
1542	16KB back to the sender. This interaction continues until the
1543	sender's window size becomes positive again, and it can resume
1544	transmitting data frames.
1545
1546	After the recipient reads in a data frame with FLAG_FIN that marks
1547	the end of the data stream, it should not send WINDOW_UPDATE frames
1548	as it consumes the last data frame. A sender should ignore all the
1549	WINDOW_UPDATE frames associated with the stream after it send the
1550	last frame for the stream.
1551
1552	The data frames from the sender and the WINDOW_UPDATE frames from the
1553	recipient are completely asynchronous with respect to each other.
1554	This property allows a recipient to aggressively update the window
1555	size kept by the sender to prevent the stream from stalling.
1556
1557	3.6.9. CREDENTIAL
1558
1559	The CREDENTIAL control frame is used by the client to send additional
1560	client certificates to the server. A HTTP/2.0 client may decide to
1561	send requests for resources from different origins on the same
1562	HTTP/2.0 session if it decides that that server handles both origins.
1563	For example if the IP address associated with both hostnames matches
1564
1565
1566
1567	Belshe, et al. Expires July 26, 2013 [Page 28]
1568
1569	Internet-Draft HTTP/2.0 January 2013
1570
1571
1572	and the SSL server certificate presented in the initial handshake is
1573	valid for both hostnames. However, because the SSL connection can
1574	contain at most one client certificate, the client needs a mechanism
1575	to send additional client certificates to the server.
1576
1577	The server is required to maintain a vector of client certificates
1578	associated with a HTTP/2.0 session. When the client needs to send a
1579	client certificate to the server, it will send a CREDENTIAL frame
1580	that specifies the index of the slot in which to store the
1581	certificate as well as proof that the client posesses the
1582	corresponding private key. The initial size of this vector must be
1583	8. If the client provides a client certificate during the first TLS
1584	handshake, the contents of this certificate must be copied into the
1585	first slot (index 1) in the CREDENTIAL vector, though it may be
1586	overwritten by subsequent CREDENTIAL frames. The server must
1587	exclusively use the CREDENTIAL vector when evaluating the client
1588	certificates associated with an origin. The server may change the
1589	size of this vector by sending a SETTINGS frame with the setting
1590	SETTINGS_CLIENT_CERTIFICATE_VECTOR_SIZE value specified. In the
1591	event that the new size is smaller than the current size, truncation
1592	occurs preserving lower-index slots as possible.
1593
1594	TLS renegotiation with client authentication is incompatible with
1595	HTTP/2.0 given the multiplexed nature of HTTP/2.0. Specifically,
1596	imagine that the client has 2 requests outstanding to the server for
1597	two different pages (in different tabs). When the renegotiation +
1598	client certificate request comes in, the browser is unable to
1599	determine which resource triggered the client certificate request, in
1600	order to prompt the user accordingly.
1601
1602	+----------------------------------+
1603	\|1\|000000000000001\|0000000000001011\|
1604	+----------------------------------+
1605	\| flags (8) \| Length (24 bits) \|
1606	+----------------------------------+
1607	\| Slot (16 bits) \| \|
1608	+-----------------+ \|
1609	\| Proof Length (32 bits) \|
1610	+----------------------------------+
1611	\| Proof \|
1612	+----------------------------------+ <+
1613	\| Certificate Length (32 bits) \| \|
1614	+----------------------------------+ \| Repeated until end of frame
1615	\| Certificate \| \|
1616	+----------------------------------+ <+
1617
1618	Slot: The index in the server's client certificate vector where this
1619	certificate should be stored. If there is already a certificate
1620
1621
1622
1623	Belshe, et al. Expires July 26, 2013 [Page 29]
1624
1625	Internet-Draft HTTP/2.0 January 2013
1626
1627
1628	stored at this index, it will be overwritten. The index is one
1629	based, not zero based; zero is an invalid slot index.
1630
1631	Proof: Cryptographic proof that the client has possession of the
1632	private key associated with the certificate. The format is a TLS
1633	digitally-signed element ([RFC5246], Section 4.7). The signature
1634	algorithm must be the same as that used in the CertificateVerify
1635	message. However, since the MD5+SHA1 signature type used in TLS 1.0
1636	connections can not be correctly encoded in a digitally-signed
1637	element, SHA1 must be used when MD5+SHA1 was used in the SSL
1638	connection. The signature is calculated over a 32 byte TLS extractor
1639	value (http://tools.ietf.org/html/rfc5705) with a label of "EXPORTER
1640	HTTP/2.0 certificate proof" using the empty string as context.
1641	ForRSA certificates the signature would be a PKCS#1 v1.5 signature.
1642	For ECDSA, it would be an ECDSA-Sig-Value
1643	(http://tools.ietf.org/html/rfc5480#appendix-A). For a 1024-bit RSA
1644	key, the CREDENTIAL message would be ~500 bytes.
1645
1646	Certificate: The certificate chain, starting with the leaf
1647	certificate. Each certificate must be encoded as a 32 bit length,
1648	followed by a DER encoded certificate. The certificate must be of
1649	the same type (RSA, ECDSA, etc) as the client certificate associated
1650	with the SSL connection.
1651
1652	If the server receives a request for a resource with unacceptable
1653	credential (either missing or invalid), it must reply with a
1654	RST_STREAM frame with the status code INVALID_CREDENTIALS. Upon
1655	receipt of a RST_STREAM frame with INVALID_CREDENTIALS, the client
1656	should initiate a new stream directly to the requested origin and
1657	resend the request. Note, HTTP/2.0 does not allow the server to
1658	request different client authentication for different resources in
1659	the same origin.
1660
1661	If the server receives an invalid CREDENTIAL frame, it MUST respond
1662	with a GOAWAY frame and shutdown the session.
1663
1664	3.6.10. Name/Value Header Block
1665
1666	The Name/Value Header Block is found in the SYN_STREAM, SYN_REPLY and
1667	HEADERS control frames, and shares a common format:
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679	Belshe, et al. Expires July 26, 2013 [Page 30]
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1681	Internet-Draft HTTP/2.0 January 2013
1682
1683
1684	+------------------------------------+
1685	\| Number of Name/Value pairs (int32) \|
1686	+------------------------------------+
1687	\| Length of name (int32) \|
1688	+------------------------------------+
1689	\| Name (string) \|
1690	+------------------------------------+
1691	\| Length of value (int32) \|
1692	+------------------------------------+
1693	\| Value (string) \|
1694	+------------------------------------+
1695	\| (repeats) \|
1696
1697	Number of Name/Value pairs: The number of repeating name/value pairs
1698	following this field.
1699
1700	List of Name/Value pairs:
1701
1702	Length of Name: a 32-bit value containing the number of octets in
1703	the name field. Note that in practice, this length must not
1704	exceed 2^24, as that is the maximum size of a HTTP/2.0 frame.
1705
1706	Name: 0 or more octets, 8-bit sequences of data, excluding 0.
1707
1708	Length of Value: a 32-bit value containing the number of octets in
1709	the value field. Note that in practice, this length must not
1710	exceed 2^24, as that is the maximum size of a HTTP/2.0 frame.
1711
1712	Value: 0 or more octets, 8-bit sequences of data, excluding 0.
1713
1714	Each header name must have at least one value. Header names are
1715	encoded using the US-ASCII character set [ASCII] and must be all
1716	lower case. The length of each name must be greater than zero. A
1717	recipient of a zero-length name MUST issue a stream error
1718	(Section 3.4.2) with the status code PROTOCOL_ERROR for the
1719	stream-id.
1720
1721	Duplicate header names are not allowed. To send two identically
1722	named headers, send a header with two values, where the values are
1723	separated by a single NUL (0) byte. A header value can either be
1724	empty (e.g. the length is zero) or it can contain multiple, NUL-
1725	separated values, each with length greater than zero. The value
1726	never starts nor ends with a NUL character. Recipients of illegal
1727	value fields MUST issue a stream error (Section 3.4.2) with the
1728	status code PROTOCOL_ERROR for the stream-id.
1729
1730
1731
1732
1733
1734
1735	Belshe, et al. Expires July 26, 2013 [Page 31]
1736
1737	Internet-Draft HTTP/2.0 January 2013
1738
1739
1740	3.6.10.1. Compression
1741
1742	The Name/Value Header Block is a section of the SYN_STREAM,
1743	SYN_REPLY, and HEADERS frames used to carry header meta-data. This
1744	block is always compressed using zlib compression. Within this
1745	specification, any reference to 'zlib' is referring to the ZLIB
1746	Compressed Data Format Specification Version 3.3 as part of RFC1950.
1747	[RFC1950]
1748
1749	For each HEADERS compression instance, the initial state is
1750	initialized using the following dictionary [UDELCOMPRESSION]:
1751
1752	<CODE BEGINS>
1753
1754	const unsigned char http2_dictionary_txt[] = {
1755	0x00, 0x00, 0x00, 0x07, 0x6f, 0x70, 0x74, 0x69, \\ - - - - o p t i
1756	0x6f, 0x6e, 0x73, 0x00, 0x00, 0x00, 0x04, 0x68, \\ o n s - - - - h
1757	0x65, 0x61, 0x64, 0x00, 0x00, 0x00, 0x04, 0x70, \\ e a d - - - - p
1758	0x6f, 0x73, 0x74, 0x00, 0x00, 0x00, 0x03, 0x70, \\ o s t - - - - p
1759	0x75, 0x74, 0x00, 0x00, 0x00, 0x06, 0x64, 0x65, \\ u t - - - - d e
1760	0x6c, 0x65, 0x74, 0x65, 0x00, 0x00, 0x00, 0x05, \\ l e t e - - - -
1761	0x74, 0x72, 0x61, 0x63, 0x65, 0x00, 0x00, 0x00, \\ t r a c e - - -
1762	0x06, 0x61, 0x63, 0x63, 0x65, 0x70, 0x74, 0x00, \\ - a c c e p t -
1763	0x00, 0x00, 0x0e, 0x61, 0x63, 0x63, 0x65, 0x70, \\ - - - a c c e p
1764	0x74, 0x2d, 0x63, 0x68, 0x61, 0x72, 0x73, 0x65, \\ t - c h a r s e
1765	0x74, 0x00, 0x00, 0x00, 0x0f, 0x61, 0x63, 0x63, \\ t - - - - a c c
1766	0x65, 0x70, 0x74, 0x2d, 0x65, 0x6e, 0x63, 0x6f, \\ e p t - e n c o
1767	0x64, 0x69, 0x6e, 0x67, 0x00, 0x00, 0x00, 0x0f, \\ d i n g - - - -
1768	0x61, 0x63, 0x63, 0x65, 0x70, 0x74, 0x2d, 0x6c, \\ a c c e p t - l
1769	0x61, 0x6e, 0x67, 0x75, 0x61, 0x67, 0x65, 0x00, \\ a n g u a g e -
1770	0x00, 0x00, 0x0d, 0x61, 0x63, 0x63, 0x65, 0x70, \\ - - - a c c e p
1771	0x74, 0x2d, 0x72, 0x61, 0x6e, 0x67, 0x65, 0x73, \\ t - r a n g e s
1772	0x00, 0x00, 0x00, 0x03, 0x61, 0x67, 0x65, 0x00, \\ - - - - a g e -
1773	0x00, 0x00, 0x05, 0x61, 0x6c, 0x6c, 0x6f, 0x77, \\ - - - a l l o w
1774	0x00, 0x00, 0x00, 0x0d, 0x61, 0x75, 0x74, 0x68, \\ - - - - a u t h
1775	0x6f, 0x72, 0x69, 0x7a, 0x61, 0x74, 0x69, 0x6f, \\ o r i z a t i o
1776	0x6e, 0x00, 0x00, 0x00, 0x0d, 0x63, 0x61, 0x63, \\ n - - - - c a c
1777	0x68, 0x65, 0x2d, 0x63, 0x6f, 0x6e, 0x74, 0x72, \\ h e - c o n t r
1778	0x6f, 0x6c, 0x00, 0x00, 0x00, 0x0a, 0x63, 0x6f, \\ o l - - - - c o
1779	0x6e, 0x6e, 0x65, 0x63, 0x74, 0x69, 0x6f, 0x6e, \\ n n e c t i o n
1780	0x00, 0x00, 0x00, 0x0c, 0x63, 0x6f, 0x6e, 0x74, \\ - - - - c o n t
1781	0x65, 0x6e, 0x74, 0x2d, 0x62, 0x61, 0x73, 0x65, \\ e n t - b a s e
1782	0x00, 0x00, 0x00, 0x10, 0x63, 0x6f, 0x6e, 0x74, \\ - - - - c o n t
1783	0x65, 0x6e, 0x74, 0x2d, 0x65, 0x6e, 0x63, 0x6f, \\ e n t - e n c o
1784	0x64, 0x69, 0x6e, 0x67, 0x00, 0x00, 0x00, 0x10, \\ d i n g - - - -
1785	0x63, 0x6f, 0x6e, 0x74, 0x65, 0x6e, 0x74, 0x2d, \\ c o n t e n t -
1786	0x6c, 0x61, 0x6e, 0x67, 0x75, 0x61, 0x67, 0x65, \\ l a n g u a g e
1787	0x00, 0x00, 0x00, 0x0e, 0x63, 0x6f, 0x6e, 0x74, \\ - - - - c o n t
1788
1789
1790
1791	Belshe, et al. Expires July 26, 2013 [Page 32]
1792
1793	Internet-Draft HTTP/2.0 January 2013
1794
1795
1796	0x65, 0x6e, 0x74, 0x2d, 0x6c, 0x65, 0x6e, 0x67, \\ e n t - l e n g
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1799	0x63, 0x61, 0x74, 0x69, 0x6f, 0x6e, 0x00, 0x00, \\ c a t i o n - -
1800	0x00, 0x0b, 0x63, 0x6f, 0x6e, 0x74, 0x65, 0x6e, \\ - - c o n t e n
1801	0x74, 0x2d, 0x6d, 0x64, 0x35, 0x00, 0x00, 0x00, \\ t - m d 5 - - -
1802	0x0d, 0x63, 0x6f, 0x6e, 0x74, 0x65, 0x6e, 0x74, \\ - c o n t e n t
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1804	0x00, 0x0c, 0x63, 0x6f, 0x6e, 0x74, 0x65, 0x6e, \\ - - c o n t e n
1805	0x74, 0x2d, 0x74, 0x79, 0x70, 0x65, 0x00, 0x00, \\ t - t y p e - -
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1809	0x00, 0x00, 0x00, 0x07, 0x65, 0x78, 0x70, 0x69, \\ - - - - e x p i
1810	0x72, 0x65, 0x73, 0x00, 0x00, 0x00, 0x04, 0x66, \\ r e s - - - - f
1811	0x72, 0x6f, 0x6d, 0x00, 0x00, 0x00, 0x04, 0x68, \\ r o m - - - - h
1812	0x6f, 0x73, 0x74, 0x00, 0x00, 0x00, 0x08, 0x69, \\ o s t - - - - i
1813	0x66, 0x2d, 0x6d, 0x61, 0x74, 0x63, 0x68, 0x00, \\ f - m a t c h -
1814	0x00, 0x00, 0x11, 0x69, 0x66, 0x2d, 0x6d, 0x6f, \\ - - - i f - m o
1815	0x64, 0x69, 0x66, 0x69, 0x65, 0x64, 0x2d, 0x73, \\ d i f i e d - s
1816	0x69, 0x6e, 0x63, 0x65, 0x00, 0x00, 0x00, 0x0d, \\ i n c e - - - -
1817	0x69, 0x66, 0x2d, 0x6e, 0x6f, 0x6e, 0x65, 0x2d, \\ i f - n o n e -
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1820	0x65, 0x00, 0x00, 0x00, 0x13, 0x69, 0x66, 0x2d, \\ e - - - - i f -
1821	0x75, 0x6e, 0x6d, 0x6f, 0x64, 0x69, 0x66, 0x69, \\ u n m o d i f i
1822	0x65, 0x64, 0x2d, 0x73, 0x69, 0x6e, 0x63, 0x65, \\ e d - s i n c e
1823	0x00, 0x00, 0x00, 0x0d, 0x6c, 0x61, 0x73, 0x74, \\ - - - - l a s t
1824	0x2d, 0x6d, 0x6f, 0x64, 0x69, 0x66, 0x69, 0x65, \\ - m o d i f i e
1825	0x64, 0x00, 0x00, 0x00, 0x08, 0x6c, 0x6f, 0x63, \\ d - - - - l o c
1826	0x61, 0x74, 0x69, 0x6f, 0x6e, 0x00, 0x00, 0x00, \\ a t i o n - - -
1827	0x0c, 0x6d, 0x61, 0x78, 0x2d, 0x66, 0x6f, 0x72, \\ - m a x - f o r
1828	0x77, 0x61, 0x72, 0x64, 0x73, 0x00, 0x00, 0x00, \\ w a r d s - - -
1829	0x06, 0x70, 0x72, 0x61, 0x67, 0x6d, 0x61, 0x00, \\ - p r a g m a -
1830	0x00, 0x00, 0x12, 0x70, 0x72, 0x6f, 0x78, 0x79, \\ - - - p r o x y
1831	0x2d, 0x61, 0x75, 0x74, 0x68, 0x65, 0x6e, 0x74, \\ - a u t h e n t
1832	0x69, 0x63, 0x61, 0x74, 0x65, 0x00, 0x00, 0x00, \\ i c a t e - - -
1833	0x13, 0x70, 0x72, 0x6f, 0x78, 0x79, 0x2d, 0x61, \\ - p r o x y - a
1834	0x75, 0x74, 0x68, 0x6f, 0x72, 0x69, 0x7a, 0x61, \\ u t h o r i z a
1835	0x74, 0x69, 0x6f, 0x6e, 0x00, 0x00, 0x00, 0x05, \\ t i o n - - - -
1836	0x72, 0x61, 0x6e, 0x67, 0x65, 0x00, 0x00, 0x00, \\ r a n g e - - -
1837	0x07, 0x72, 0x65, 0x66, 0x65, 0x72, 0x65, 0x72, \\ - r e f e r e r
1838	0x00, 0x00, 0x00, 0x0b, 0x72, 0x65, 0x74, 0x72, \\ - - - - r e t r
1839	0x79, 0x2d, 0x61, 0x66, 0x74, 0x65, 0x72, 0x00, \\ y - a f t e r -
1840	0x00, 0x00, 0x06, 0x73, 0x65, 0x72, 0x76, 0x65, \\ - - - s e r v e
1841	0x72, 0x00, 0x00, 0x00, 0x02, 0x74, 0x65, 0x00, \\ r - - - - t e -
1842	0x00, 0x00, 0x07, 0x74, 0x72, 0x61, 0x69, 0x6c, \\ - - - t r a i l
1843	0x65, 0x72, 0x00, 0x00, 0x00, 0x11, 0x74, 0x72, \\ e r - - - - t r
1844
1845
1846
1847	Belshe, et al. Expires July 26, 2013 [Page 33]
1848
1849	Internet-Draft HTTP/2.0 January 2013
1850
1851
1852	0x61, 0x6e, 0x73, 0x66, 0x65, 0x72, 0x2d, 0x65, \\ a n s f e r - e
1853	0x6e, 0x63, 0x6f, 0x64, 0x69, 0x6e, 0x67, 0x00, \\ n c o d i n g -
1854	0x00, 0x00, 0x07, 0x75, 0x70, 0x67, 0x72, 0x61, \\ - - - u p g r a
1855	0x64, 0x65, 0x00, 0x00, 0x00, 0x0a, 0x75, 0x73, \\ d e - - - - u s
1856	0x65, 0x72, 0x2d, 0x61, 0x67, 0x65, 0x6e, 0x74, \\ e r - a g e n t
1857	0x00, 0x00, 0x00, 0x04, 0x76, 0x61, 0x72, 0x79, \\ - - - - v a r y
1858	0x00, 0x00, 0x00, 0x03, 0x76, 0x69, 0x61, 0x00, \\ - - - - v i a -
1859	0x00, 0x00, 0x07, 0x77, 0x61, 0x72, 0x6e, 0x69, \\ - - - w a r n i
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1861	0x77, 0x2d, 0x61, 0x75, 0x74, 0x68, 0x65, 0x6e, \\ w - a u t h e n
1862	0x74, 0x69, 0x63, 0x61, 0x74, 0x65, 0x00, 0x00, \\ t i c a t e - -
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1864	0x00, 0x00, 0x00, 0x03, 0x67, 0x65, 0x74, 0x00, \\ - - - - g e t -
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1866	0x73, 0x00, 0x00, 0x00, 0x06, 0x32, 0x30, 0x30, \\ s - - - - 2 0 0
1867	0x20, 0x4f, 0x4b, 0x00, 0x00, 0x00, 0x07, 0x76, \\ - O K - - - - v
1868	0x65, 0x72, 0x73, 0x69, 0x6f, 0x6e, 0x00, 0x00, \\ e r s i o n - -
1869	0x00, 0x08, 0x48, 0x54, 0x54, 0x50, 0x2f, 0x31, \\ - - H T T P - 1
1870	0x2e, 0x31, 0x00, 0x00, 0x00, 0x03, 0x75, 0x72, \\ - 1 - - - - u r
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1872	0x6c, 0x69, 0x63, 0x00, 0x00, 0x00, 0x0a, 0x73, \\ l i c - - - - s
1873	0x65, 0x74, 0x2d, 0x63, 0x6f, 0x6f, 0x6b, 0x69, \\ e t - c o o k i
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1875	0x70, 0x2d, 0x61, 0x6c, 0x69, 0x76, 0x65, 0x00, \\ p - a l i v e -
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1877	0x6e, 0x31, 0x30, 0x30, 0x31, 0x30, 0x31, 0x32, \\ n 1 0 0 1 0 1 2
1878	0x30, 0x31, 0x32, 0x30, 0x32, 0x32, 0x30, 0x35, \\ 0 1 2 0 2 2 0 5
1879	0x32, 0x30, 0x36, 0x33, 0x30, 0x30, 0x33, 0x30, \\ 2 0 6 3 0 0 3 0
1880	0x32, 0x33, 0x30, 0x33, 0x33, 0x30, 0x34, 0x33, \\ 2 3 0 3 3 0 4 3
1881	0x30, 0x35, 0x33, 0x30, 0x36, 0x33, 0x30, 0x37, \\ 0 5 3 0 6 3 0 7
1882	0x34, 0x30, 0x32, 0x34, 0x30, 0x35, 0x34, 0x30, \\ 4 0 2 4 0 5 4 0
1883	0x36, 0x34, 0x30, 0x37, 0x34, 0x30, 0x38, 0x34, \\ 6 4 0 7 4 0 8 4
1884	0x30, 0x39, 0x34, 0x31, 0x30, 0x34, 0x31, 0x31, \\ 0 9 4 1 0 4 1 1
1885	0x34, 0x31, 0x32, 0x34, 0x31, 0x33, 0x34, 0x31, \\ 4 1 2 4 1 3 4 1
1886	0x34, 0x34, 0x31, 0x35, 0x34, 0x31, 0x36, 0x34, \\ 4 4 1 5 4 1 6 4
1887	0x31, 0x37, 0x35, 0x30, 0x32, 0x35, 0x30, 0x34, \\ 1 7 5 0 2 5 0 4
1888	0x35, 0x30, 0x35, 0x32, 0x30, 0x33, 0x20, 0x4e, \\ 5 0 5 2 0 3 - N
1889	0x6f, 0x6e, 0x2d, 0x41, 0x75, 0x74, 0x68, 0x6f, \\ o n - A u t h o
1890	0x72, 0x69, 0x74, 0x61, 0x74, 0x69, 0x76, 0x65, \\ r i t a t i v e
1891	0x20, 0x49, 0x6e, 0x66, 0x6f, 0x72, 0x6d, 0x61, \\ - I n f o r m a
1892	0x74, 0x69, 0x6f, 0x6e, 0x32, 0x30, 0x34, 0x20, \\ t i o n 2 0 4 -
1893	0x4e, 0x6f, 0x20, 0x43, 0x6f, 0x6e, 0x74, 0x65, \\ N o - C o n t e
1894	0x6e, 0x74, 0x33, 0x30, 0x31, 0x20, 0x4d, 0x6f, \\ n t 3 0 1 - M o
1895	0x76, 0x65, 0x64, 0x20, 0x50, 0x65, 0x72, 0x6d, \\ v e d - P e r m
1896	0x61, 0x6e, 0x65, 0x6e, 0x74, 0x6c, 0x79, 0x34, \\ a n e n t l y 4
1897	0x30, 0x30, 0x20, 0x42, 0x61, 0x64, 0x20, 0x52, \\ 0 0 - B a d - R
1898	0x65, 0x71, 0x75, 0x65, 0x73, 0x74, 0x34, 0x30, \\ e q u e s t 4 0
1899	0x31, 0x20, 0x55, 0x6e, 0x61, 0x75, 0x74, 0x68, \\ 1 - U n a u t h
1900
1901
1902
1903	Belshe, et al. Expires July 26, 2013 [Page 34]
1904
1905	Internet-Draft HTTP/2.0 January 2013
1906
1907
1908	0x6f, 0x72, 0x69, 0x7a, 0x65, 0x64, 0x34, 0x30, \\ o r i z e d 4 0
1909	0x33, 0x20, 0x46, 0x6f, 0x72, 0x62, 0x69, 0x64, \\ 3 - F o r b i d
1910	0x64, 0x65, 0x6e, 0x34, 0x30, 0x34, 0x20, 0x4e, \\ d e n 4 0 4 - N
1911	0x6f, 0x74, 0x20, 0x46, 0x6f, 0x75, 0x6e, 0x64, \\ o t - F o u n d
1912	0x35, 0x30, 0x30, 0x20, 0x49, 0x6e, 0x74, 0x65, \\ 5 0 0 - I n t e
1913	0x72, 0x6e, 0x61, 0x6c, 0x20, 0x53, 0x65, 0x72, \\ r n a l - S e r
1914	0x76, 0x65, 0x72, 0x20, 0x45, 0x72, 0x72, 0x6f, \\ v e r - E r r o
1915	0x72, 0x35, 0x30, 0x31, 0x20, 0x4e, 0x6f, 0x74, \\ r 5 0 1 - N o t
1916	0x20, 0x49, 0x6d, 0x70, 0x6c, 0x65, 0x6d, 0x65, \\ - I m p l e m e
1917	0x6e, 0x74, 0x65, 0x64, 0x35, 0x30, 0x33, 0x20, \\ n t e d 5 0 3 -
1918	0x53, 0x65, 0x72, 0x76, 0x69, 0x63, 0x65, 0x20, \\ S e r v i c e -
1919	0x55, 0x6e, 0x61, 0x76, 0x61, 0x69, 0x6c, 0x61, \\ U n a v a i l a
1920	0x62, 0x6c, 0x65, 0x4a, 0x61, 0x6e, 0x20, 0x46, \\ b l e J a n - F
1921	0x65, 0x62, 0x20, 0x4d, 0x61, 0x72, 0x20, 0x41, \\ e b - M a r - A
1922	0x70, 0x72, 0x20, 0x4d, 0x61, 0x79, 0x20, 0x4a, \\ p r - M a y - J
1923	0x75, 0x6e, 0x20, 0x4a, 0x75, 0x6c, 0x20, 0x41, \\ u n - J u l - A
1924	0x75, 0x67, 0x20, 0x53, 0x65, 0x70, 0x74, 0x20, \\ u g - S e p t -
1925	0x4f, 0x63, 0x74, 0x20, 0x4e, 0x6f, 0x76, 0x20, \\ O c t - N o v -
1926	0x44, 0x65, 0x63, 0x20, 0x30, 0x30, 0x3a, 0x30, \\ D e c - 0 0 - 0
1927	0x30, 0x3a, 0x30, 0x30, 0x20, 0x4d, 0x6f, 0x6e, \\ 0 - 0 0 - M o n
1928	0x2c, 0x20, 0x54, 0x75, 0x65, 0x2c, 0x20, 0x57, \\ - - T u e - - W
1929	0x65, 0x64, 0x2c, 0x20, 0x54, 0x68, 0x75, 0x2c, \\ e d - - T h u -
1930	0x20, 0x46, 0x72, 0x69, 0x2c, 0x20, 0x53, 0x61, \\ - F r i - - S a
1931	0x74, 0x2c, 0x20, 0x53, 0x75, 0x6e, 0x2c, 0x20, \\ t - - S u n - -
1932	0x47, 0x4d, 0x54, 0x63, 0x68, 0x75, 0x6e, 0x6b, \\ G M T c h u n k
1933	0x65, 0x64, 0x2c, 0x74, 0x65, 0x78, 0x74, 0x2f, \\ e d - t e x t -
1934	0x68, 0x74, 0x6d, 0x6c, 0x2c, 0x69, 0x6d, 0x61, \\ h t m l - i m a
1935	0x67, 0x65, 0x2f, 0x70, 0x6e, 0x67, 0x2c, 0x69, \\ g e - p n g - i
1936	0x6d, 0x61, 0x67, 0x65, 0x2f, 0x6a, 0x70, 0x67, \\ m a g e - j p g
1937	0x2c, 0x69, 0x6d, 0x61, 0x67, 0x65, 0x2f, 0x67, \\ - i m a g e - g
1938	0x69, 0x66, 0x2c, 0x61, 0x70, 0x70, 0x6c, 0x69, \\ i f - a p p l i
1939	0x63, 0x61, 0x74, 0x69, 0x6f, 0x6e, 0x2f, 0x78, \\ c a t i o n - x
1940	0x6d, 0x6c, 0x2c, 0x61, 0x70, 0x70, 0x6c, 0x69, \\ m l - a p p l i
1941	0x63, 0x61, 0x74, 0x69, 0x6f, 0x6e, 0x2f, 0x78, \\ c a t i o n - x
1942	0x68, 0x74, 0x6d, 0x6c, 0x2b, 0x78, 0x6d, 0x6c, \\ h t m l - x m l
1943	0x2c, 0x74, 0x65, 0x78, 0x74, 0x2f, 0x70, 0x6c, \\ - t e x t - p l
1944	0x61, 0x69, 0x6e, 0x2c, 0x74, 0x65, 0x78, 0x74, \\ a i n - t e x t
1945	0x2f, 0x6a, 0x61, 0x76, 0x61, 0x73, 0x63, 0x72, \\ - j a v a s c r
1946	0x69, 0x70, 0x74, 0x2c, 0x70, 0x75, 0x62, 0x6c, \\ i p t - p u b l
1947	0x69, 0x63, 0x70, 0x72, 0x69, 0x76, 0x61, 0x74, \\ i c p r i v a t
1948	0x65, 0x6d, 0x61, 0x78, 0x2d, 0x61, 0x67, 0x65, \\ e m a x - a g e
1949	0x3d, 0x67, 0x7a, 0x69, 0x70, 0x2c, 0x64, 0x65, \\ - g z i p - d e
1950	0x66, 0x6c, 0x61, 0x74, 0x65, 0x2c, 0x73, 0x64, \\ f l a t e - s d
1951	0x63, 0x68, 0x63, 0x68, 0x61, 0x72, 0x73, 0x65, \\ c h c h a r s e
1952	0x74, 0x3d, 0x75, 0x74, 0x66, 0x2d, 0x38, 0x63, \\ t - u t f - 8 c
1953	0x68, 0x61, 0x72, 0x73, 0x65, 0x74, 0x3d, 0x69, \\ h a r s e t - i
1954	0x73, 0x6f, 0x2d, 0x38, 0x38, 0x35, 0x39, 0x2d, \\ s o - 8 8 5 9 -
1955	0x31, 0x2c, 0x75, 0x74, 0x66, 0x2d, 0x2c, 0x2a, \\ 1 - u t f - - -
1956
1957
1958
1959	Belshe, et al. Expires July 26, 2013 [Page 35]
1960
1961	Internet-Draft HTTP/2.0 January 2013
1962
1963
1964	0x2c, 0x65, 0x6e, 0x71, 0x3d, 0x30, 0x2e \\ - e n q - 0 -
1965	};
1966
1967	<CODE ENDS>
1968
1969	The entire contents of the name/value header block is compressed
1970	using zlib. There is a single zlib stream for all name value pairs
1971	in one direction on a connection. HTTP/2.0 uses a SYNC_FLUSH between
1972	each compressed frame.
1973
1974	Implementation notes: the compression engine can be tuned to favor
1975	speed or size. Optimizing for size increases memory use and CPU
1976	consumption. Because header blocks are generally small, implementors
1977	may want to reduce the window-size of the compression engine from the
1978	default 15bits (a 32KB window) to more like 11bits (a 2KB window).
1979	The exact setting is chosen by the compressor, the decompressor will
1980	work with any setting.
1981
1982	4. HTTP Layering over HTTP/2.0
1983
1984	HTTP/2.0 is intended to be as compatible as possible with current
1985	web-based applications. This means that, from the perspective of the
1986	server business logic or application API, the features of HTTP are
1987	unchanged. To achieve this, all of the application request and
1988	response header semantics are preserved, although the syntax of
1989	conveying those semantics has changed. Thus, the rules from the
1990	HTTP/1.1 specification in RFC2616 [RFC2616] apply with the changes in
1991	the sections below.
1992
1993	4.1. Connection Management
1994
1995	Clients SHOULD NOT open more than one HTTP/2.0 session to a given
1996	origin [RFC6454] concurrently.
1997
1998	Note that it is possible for one HTTP/2.0 session to be finishing
1999	(e.g. a GOAWAY message has been sent, but not all streams have
2000	finished), while another HTTP/2.0 session is starting.
2001
2002	4.1.1. Use of GOAWAY
2003
2004	HTTP/2.0 provides a GOAWAY message which can be used when closing a
2005	connection from either the client or server. Without a server GOAWAY
2006	message, HTTP has a race condition where the client sends a request
2007	(a new SYN_STREAM) just as the server is closing the connection, and
2008	the client cannot know if the server received the stream or not. By
2009	using the last-stream-id in the GOAWAY, servers can indicate to the
2010	client if a request was processed or not.
2011
2012
2013
2014
2015	Belshe, et al. Expires July 26, 2013 [Page 36]
2016
2017	Internet-Draft HTTP/2.0 January 2013
2018
2019
2020	Note that some servers will choose to send the GOAWAY and immediately
2021	terminate the connection without waiting for active streams to
2022	finish. The client will be able to determine this because HTTP/2.0
2023	streams are determinstically closed. This abrupt termination will
2024	force the client to heuristically decide whether to retry the pending
2025	requests. Clients always need to be capable of dealing with this
2026	case because they must deal with accidental connection termination
2027	cases, which are the same as the server never having sent a GOAWAY.
2028
2029	More sophisticated servers will use GOAWAY to implement a graceful
2030	teardown. They will send the GOAWAY and provide some time for the
2031	active streams to finish before terminating the connection.
2032
2033	If a HTTP/2.0 client closes the connection, it should also send a
2034	GOAWAY message. This allows the server to know if any server-push
2035	streams were received by the client.
2036
2037	If the endpoint closing the connection has not received any
2038	SYN_STREAMs from the remote, the GOAWAY will contain a last-stream-id
2039	of 0.
2040
2041	4.2. HTTP Request/Response
2042
2043	4.2.1. Request
2044
2045	The client initiates a request by sending a SYN_STREAM frame. For
2046	requests which do not contain a body, the SYN_STREAM frame MUST set
2047	the FLAG_FIN, indicating that the client intends to send no further
2048	data on this stream. For requests which do contain a body, the
2049	SYN_STREAM will not contain the FLAG_FIN, and the body will follow
2050	the SYN_STREAM in a series of DATA frames. The last DATA frame will
2051	set the FLAG_FIN to indicate the end of the body.
2052
2053	The SYN_STREAM Name/Value section will contain all of the HTTP
2054	headers which are associated with an HTTP request. The header block
2055	in HTTP/2.0 is mostly unchanged from today's HTTP header block, with
2056	the following differences:
2057
2058	The first line of the request is unfolded into name/value pairs
2059	like other HTTP headers and MUST be present:
2060
2061	":method" - the HTTP method for this request (e.g. "GET",
2062	"POST", "HEAD", etc)
2063
2064	":path" - the url-path for this url with "/" prefixed. (See
2065	RFC1738 [RFC1738]). For example, for
2066	"http://www.google.com/search?q=dogs" the path would be
2067	"/search?q=dogs".
2068
2069
2070
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2074
2075
2076	":version" - the HTTP version of this request (e.g.
2077	"HTTP/1.1")
2078
2079	In addition, the following two name/value pairs must also be
2080	present in every request:
2081
2082	":host" - the hostport (See RFC1738 [RFC1738]) portion of the
2083	URL for this request (e.g. "www.google.com:1234"). This header
2084	is the same as the HTTP 'Host' header.
2085
2086	":scheme" - the scheme portion of the URL for this request
2087	(e.g. "https"))
2088
2089	Header names are all lowercase.
2090
2091	The Connection, Host, Keep-Alive, Proxy-Connection, and Transfer-
2092	Encoding headers are not valid and MUST not be sent.
2093
2094	User-agents MUST support gzip compression. Regardless of the
2095	Accept-Encoding sent by the user-agent, the server may always send
2096	content encoded with gzip or deflate encoding.
2097
2098	If a server receives a request where the sum of the data frame
2099	payload lengths does not equal the size of the Content-Length
2100	header, the server MUST return a 400 (Bad Request) error.
2101
2102	POST-specific changes:
2103
2104	Although POSTs are inherently chunked, POST requests SHOULD
2105	also be accompanied by a Content-Length header. There are two
2106	reasons for this: First, it assists with upload progress meters
2107	for an improved user experience. But second, we know from
2108	early versions of HTTP/2.0 that failure to send a content
2109	length header is incompatible with many existing HTTP server
2110	implementations. Existing user-agents do not omit the Content-
2111	Length header, and server implementations have come to depend
2112	upon this.
2113
2114	The user-agent is free to prioritize requests as it sees fit. If the
2115	user-agent cannot make progress without receiving a resource, it
2116	should attempt to raise the priority of that resource. Resources
2117	such as images, SHOULD generally use the lowest priority.
2118
2119	If a client sends a SYN_STREAM without all of the method, host, path,
2120	scheme, and version headers, the server MUST reply with a HTTP 400
2121	Bad Request reply.
2122
2123
2124
2125
2126
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2130
2131
2132	4.2.2. Response
2133
2134	The server responds to a client request with a SYN_REPLY frame.
2135	Symmetric to the client's upload stream, server will send data after
2136	the SYN_REPLY frame via a series of DATA frames, and the last data
2137	frame will contain the FLAG_FIN to indicate successful end-of-stream.
2138	If a response (like a 202 or 204 response) contains no body, the
2139	SYN_REPLY frame may contain the FLAG_FIN flag to indicate no further
2140	data will be sent on the stream.
2141
2142	The response status line is unfolded into name/value pairs like
2143	other HTTP headers and must be present:
2144
2145	":status" - The HTTP response status code (e.g. "200" or "200
2146	OK")
2147
2148	":version" - The HTTP response version (e.g. "HTTP/1.1")
2149
2150	All header names must be lowercase.
2151
2152	The Connection, Keep-Alive, Proxy-Connection, and Transfer-
2153	Encoding headers are not valid and MUST not be sent.
2154
2155	Responses MAY be accompanied by a Content-Length header for
2156	advisory purposes. (e.g. for UI progress meters)
2157
2158	If a client receives a response where the sum of the data frame
2159	payload lengths does not equal the size of the Content-Length
2160	header, the client MUST ignore the content length header.
2161
2162	If a client receives a SYN_REPLY without a status or without a
2163	version header, the client must reply with a RST_STREAM frame
2164	indicating a PROTOCOL ERROR.
2165
2166	4.2.3. Authentication
2167
2168	When a client sends a request to an origin server that requires
2169	authentication, the server can reply with a "401 Unauthorized"
2170	response, and include a WWW-Authenticate challenge header that
2171	defines the authentication scheme to be used. The client then
2172	retries the request with an Authorization header appropriate to the
2173	specified authentication scheme.
2174
2175	There are four options for proxy authentication, Basic, Digest, NTLM
2176	and Negotiate (SPNEGO). The first two options were defined in
2177	RFC2617 [RFC2617], and are stateless. The second two options were
2178	developed by Microsoft and specified in RFC4559 [RFC4559], and are
2179	stateful; otherwise known as multi-round authentication, or
2180
2181
2182
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2186
2187
2188	connection authentication.
2189
2190	4.2.3.1. Stateless Authentication
2191
2192	Stateless Authentication over HTTP/2.0 is identical to how it is
2193	performed over HTTP. If multiple HTTP/2.0 streams are concurrently
2194	sent to a single server, each will authenticate independently,
2195	similar to how two HTTP connections would independently authenticate
2196	to a proxy server.
2197
2198	4.2.3.2. Stateful Authentication
2199
2200	Unfortunately, the stateful authentication mechanisms were
2201	implemented and defined in a such a way that directly violates
2202	RFC2617 - they do not include a "realm" as part of the request. This
2203	is problematic in HTTP/2.0 because it makes it impossible for a
2204	client to disambiguate two concurrent server authentication
2205	challenges.
2206
2207	To deal with this case, HTTP/2.0 servers using Stateful
2208	Authentication MUST implement one of two changes:
2209
2210	Servers can add a "realm=<desired realm>" header so that the two
2211	authentication requests can be disambiguated and run concurrently.
2212	Unfortunately, given how these mechanisms work, this is probably
2213	not practical.
2214
2215	Upon sending the first stateful challenge response, the server
2216	MUST buffer and defer all further frames which are not part of
2217	completing the challenge until the challenge has completed.
2218	Completing the authentication challenge may take multiple round
2219	trips. Once the client receives a "401 Authenticate" response for
2220	a stateful authentication type, it MUST stop sending new requests
2221	to the server until the authentication has completed by receiving
2222	a non-401 response on at least one stream.
2223
2224	4.3. Server Push Transactions
2225
2226	HTTP/2.0 enables a server to send multiple replies to a client for a
2227	single request. The rationale for this feature is that sometimes a
2228	server knows that it will need to send multiple resources in response
2229	to a single request. Without server push features, the client must
2230	first download the primary resource, then discover the secondary
2231	resource(s), and request them. Pushing of resources avoids the
2232	round-trip delay, but also creates a potential race where a server
2233	can be pushing content which a user-agent is in the process of
2234	requesting. The following mechanics attempt to prevent the race
2235	condition while enabling the performance benefit.
2236
2237
2238
2239	Belshe, et al. Expires July 26, 2013 [Page 40]
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2242
2243
2244	Browsers receiving a pushed response MUST validate that the server is
2245	authorized to push the URL using the browser same-origin [RFC6454]
2246	policy. For example, a HTTP/2.0 connection to www.foo.com is
2247	generally not permitted to push a response for www.evil.com.
2248
2249	If the browser accepts a pushed response (e.g. it does not send a
2250	RST_STREAM), the browser MUST attempt to cache the pushed response in
2251	same way that it would cache any other response. This means
2252	validating the response headers and inserting into the disk cache.
2253
2254	Because pushed responses have no request, they have no request
2255	headers associated with them. At the framing layer, HTTP/2.0 pushed
2256	streams contain an "associated-stream-id" which indicates the
2257	requested stream for which the pushed stream is related. The pushed
2258	stream inherits all of the headers from the associated-stream-id with
2259	the exception of ":host", ":scheme", and ":path", which are provided
2260	as part of the pushed response stream headers. The browser MUST
2261	store these inherited and implied request headers with the cached
2262	resource.
2263
2264	Implementation note: With server push, it is theoretically possible
2265	for servers to push unreasonable amounts of content or resources to
2266	the user-agent. Browsers MUST implement throttles to protect against
2267	unreasonable push attacks.
2268
2269	4.3.1. Server implementation
2270
2271	When the server intends to push a resource to the user-agent, it
2272	opens a new stream by sending a unidirectional SYN_STREAM. The
2273	SYN_STREAM MUST include an Associated-To-Stream-ID, and MUST set the
2274	FLAG_UNIDIRECTIONAL flag. The SYN_STREAM MUST include headers for
2275	":scheme", ":host", ":path", which represent the URL for the resource
2276	being pushed. Subsequent headers may follow in HEADERS frames. The
2277	purpose of the association is so that the user-agent can
2278	differentiate which request induced the pushed stream; without it, if
2279	the user-agent had two tabs open to the same page, each pushing
2280	unique content under a fixed URL, the user-agent would not be able to
2281	differentiate the requests.
2282
2283	The Associated-To-Stream-ID must be the ID of an existing, open
2284	stream. The reason for this restriction is to have a clear endpoint
2285	for pushed content. If the user-agent requested a resource on stream
2286	11, the server replies on stream 11. It can push any number of
2287	additional streams to the client before sending a FLAG_FIN on stream
2288	11. However, once the originating stream is closed no further push
2289	streams may be associated with it. The pushed streams do not need to
2290	be closed (FIN set) before the originating stream is closed, they
2291	only need to be created before the originating stream closes.
2292
2293
2294
2295	Belshe, et al. Expires July 26, 2013 [Page 41]
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2298
2299
2300	It is illegal for a server to push a resource with the Associated-To-
2301	Stream-ID of 0.
2302
2303	To minimize race conditions with the client, the SYN_STREAM for the
2304	pushed resources MUST be sent prior to sending any content which
2305	could allow the client to discover the pushed resource and request
2306	it.
2307
2308	The server MUST only push resources which would have been returned
2309	from a GET request.
2310
2311	Note: If the server does not have all of the Name/Value Response
2312	headers available at the time it issues the HEADERS frame for the
2313	pushed resource, it may later use an additional HEADERS frame to
2314	augment the name/value pairs to be associated with the pushed stream.
2315	The subsequent HEADERS frame(s) must not contain a header for
2316	':host', ':scheme', or ':path' (e.g. the server can't change the
2317	identity of the resource to be pushed). The HEADERS frame must not
2318	contain duplicate headers with a previously sent HEADERS frame. The
2319	server must send a HEADERS frame including the scheme/host/port
2320	headers before sending any data frames on the stream.
2321
2322	4.3.2. Client implementation
2323
2324	When fetching a resource the client has 3 possibilities:
2325
2326	the resource is not being pushed
2327
2328	the resource is being pushed, but the data has not yet arrived
2329
2330	the resource is being pushed, and the data has started to arrive
2331
2332	When a SYN_STREAM and HEADERS frame which contains an Associated-To-
2333	Stream-ID is received, the client must not issue GET requests for the
2334	resource in the pushed stream, and instead wait for the pushed stream
2335	to arrive.
2336
2337	If a client receives a server push stream with stream-id 0, it MUST
2338	issue a session error (Section 3.4.1) with the status code
2339	PROTOCOL_ERROR.
2340
2341	When a client receives a SYN_STREAM from the server without a the
2342	':host', ':scheme', and ':path' headers in the Name/Value section, it
2343	MUST reply with a RST_STREAM with error code HTTP_PROTOCOL_ERROR.
2344
2345	To cancel individual server push streams, the client can issue a
2346	stream error (Section 3.4.2) with error code CANCEL. Upon receipt,
2347	the server MUST stop sending on this stream immediately (this is an
2348
2349
2350
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2354
2355
2356	Abrupt termination).
2357
2358	To cancel all server push streams related to a request, the client
2359	may issue a stream error (Section 3.4.2) with error code CANCEL on
2360	the associated-stream-id. By cancelling that stream, the server MUST
2361	immediately stop sending frames for any streams with
2362	in-association-to for the original stream.
2363
2364	If the server sends a HEADER frame containing duplicate headers with
2365	a previous HEADERS frame for the same stream, the client must issue a
2366	stream error (Section 3.4.2) with error code PROTOCOL ERROR.
2367
2368	If the server sends a HEADERS frame after sending a data frame for
2369	the same stream, the client MAY ignore the HEADERS frame. Ignoring
2370	the HEADERS frame after a data frame prevents handling of HTTP's
2371	trailing headers
2372	(http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.40).
2373
2374	5. Design Rationale and Notes
2375
2376	Authors' notes: The notes in this section have no bearing on the
2377	HTTP/2.0 protocol as specified within this document, and none of
2378	these notes should be considered authoritative about how the protocol
2379	works. However, these notes may prove useful in future debates about
2380	how to resolve protocol ambiguities or how to evolve the protocol
2381	going forward. They may be removed before the final draft.
2382
2383	5.1. Separation of Framing Layer and Application Layer
2384
2385	Readers may note that this specification sometimes blends the framing
2386	layer (Section 3) with requirements of a specific application - HTTP
2387	(Section 4). This is reflected in the request/response nature of the
2388	streams, the definition of the HEADERS and compression contexts which
2389	are very similar to HTTP, and other areas as well.
2390
2391	This blending is intentional - the primary goal of this protocol is
2392	to create a low-latency protocol for use with HTTP. Isolating the
2393	two layers is convenient for description of the protocol and how it
2394	relates to existing HTTP implementations. However, the ability to
2395	reuse the HTTP/2.0 framing layer is a non goal.
2396
2397	5.2. Error handling - Framing Layer
2398
2399	Error handling at the HTTP/2.0 layer splits errors into two groups:
2400	Those that affect an individual HTTP/2.0 stream, and those that do
2401	not.
2402
2403	When an error is confined to a single stream, but general framing is
2404
2405
2406
2407	Belshe, et al. Expires July 26, 2013 [Page 43]
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2410
2411
2412	in tact, HTTP/2.0 attempts to use the RST_STREAM as a mechanism to
2413	invalidate the stream but move forward without aborting the
2414	connection altogether.
2415
2416	For errors occuring outside of a single stream context, HTTP/2.0
2417	assumes the entire session is hosed. In this case, the endpoint
2418	detecting the error should initiate a connection close.
2419
2420	5.3. One Connection Per Domain
2421
2422	HTTP/2.0 attempts to use fewer connections than other protocols have
2423	traditionally used. The rationale for this behavior is because it is
2424	very difficult to provide a consistent level of service (e.g. TCP
2425	slow-start), prioritization, or optimal compression when the client
2426	is connecting to the server through multiple channels.
2427
2428	Through lab measurements, we have seen consistent latency benefits by
2429	using fewer connections from the client. The overall number of
2430	packets sent by HTTP/2.0 can be as much as 40% less than HTTP.
2431	Handling large numbers of concurrent connections on the server also
2432	does become a scalability problem, and HTTP/2.0 reduces this load.
2433
2434	The use of multiple connections is not without benefit, however.
2435	Because HTTP/2.0 multiplexes multiple, independent streams onto a
2436	single stream, it creates a potential for head-of-line blocking
2437	problems at the transport level. In tests so far, the negative
2438	effects of head-of-line blocking (especially in the presence of
2439	packet loss) is outweighed by the benefits of compression and
2440	prioritization.
2441
2442	5.4. Fixed vs Variable Length Fields
2443
2444	HTTP/2.0 favors use of fixed length 32bit fields in cases where
2445	smaller, variable length encodings could have been used. To some,
2446	this seems like a tragic waste of bandwidth. HTTP/2.0 choses the
2447	simple encoding for speed and simplicity.
2448
2449	The goal of HTTP/2.0 is to reduce latency on the network. The
2450	overhead of HTTP/2.0 frames is generally quite low. Each data frame
2451	is only an 8 byte overhead for a 1452 byte payload (~0.6%). At the
2452	time of this writing, bandwidth is already plentiful, and there is a
2453	strong trend indicating that bandwidth will continue to increase.
2454	With an average worldwide bandwidth of 1Mbps, and assuming that a
2455	variable length encoding could reduce the overhead by 50%, the
2456	latency saved by using a variable length encoding would be less than
2457	100 nanoseconds. More interesting are the effects when the larger
2458	encodings force a packet boundary, in which case a round-trip could
2459	be induced. However, by addressing other aspects of HTTP/2.0 and TCP
2460
2461
2462
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2466
2467
2468	interactions, we believe this is completely mitigated.
2469
2470	5.5. Compression Context(s)
2471
2472	When isolating the compression contexts used for communicating with
2473	multiple origins, we had a few choices to make. We could have
2474	maintained a map (or list) of compression contexts usable for each
2475	origin. The basic case is easy - each HEADERS frame would need to
2476	identify the context to use for that frame. However, compression
2477	contexts are not cheap, so the lifecycle of each context would need
2478	to be bounded. For proxy servers, where we could churn through many
2479	contexts, this would be a concern. We considered using a static set
2480	of contexts, say 16 of them, which would bound the memory use. We
2481	also considered dynamic contexts, which could be created on the fly,
2482	and would need to be subsequently destroyed. All of these are
2483	complicated, and ultimately we decided that such a mechanism creates
2484	too many problems to solve.
2485
2486	Alternatively, we've chosen the simple approach, which is to simply
2487	provide a flag for resetting the compression context. For the common
2488	case (no proxy), this fine because most requests are to the same
2489	origin and we never need to reset the context. For cases where we
2490	are using two different origins over a single HTTP/2.0 session, we
2491	simply reset the compression state between each transition.
2492
2493	5.6. Unidirectional streams
2494
2495	Many readers notice that unidirectional streams are both a bit
2496	confusing in concept and also somewhat redundant. If the recipient
2497	of a stream doesn't wish to send data on a stream, it could simply
2498	send a SYN_REPLY with the FLAG_FIN bit set. The FLAG_UNIDIRECTIONAL
2499	is, therefore, not necessary.
2500
2501	It is true that we don't need the UNIDIRECTIONAL markings. It is
2502	added because it avoids the recipient of pushed streams from needing
2503	to send a set of empty frames (e.g. the SYN_STREAM w/ FLAG_FIN) which
2504	otherwise serve no purpose.
2505
2506	5.7. Data Compression
2507
2508	Generic compression of data portion of the streams (as opposed to
2509	compression of the headers) without knowing the content of the stream
2510	is redundant. There is no value in compressing a stream which is
2511	already compressed. Because of this, HTTP/2.0 does allow data
2512	compression to be optional. We included it because study of existing
2513	websites shows that many sites are not using compression as they
2514	should, and users suffer because of it. We wanted a mechanism where,
2515	at the HTTP/2.0 layer, site administrators could simply force
2516
2517
2518
2519	Belshe, et al. Expires July 26, 2013 [Page 45]
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2522
2523
2524	compression - it is better to compress twice than to not compress.
2525
2526	Overall, however, with this feature being optional and sometimes
2527	redundant, it is unclear if it is useful at all. We will likely
2528	remove it from the specification.
2529
2530	5.8. Server Push
2531
2532	A subtle but important point is that server push streams must be
2533	declared before the associated stream is closed. The reason for this
2534	is so that proxies have a lifetime for which they can discard
2535	information about previous streams. If a pushed stream could
2536	associate itself with an already-closed stream, then endpoints would
2537	not have a specific lifecycle for when they could disavow knowledge
2538	of the streams which went before.
2539
2540	6. Security Considerations
2541
2542	6.1. Use of Same-origin constraints
2543
2544	This specification uses the same-origin policy [RFC6454] in all cases
2545	where verification of content is required.
2546
2547	6.2. HTTP Headers and HTTP/2.0 Headers
2548
2549	At the application level, HTTP uses name/value pairs in its headers.
2550	Because HTTP/2.0 merges the existing HTTP headers with HTTP/2.0
2551	headers, there is a possibility that some HTTP applications already
2552	use a particular header name. To avoid any conflicts, all headers
2553	introduced for layering HTTP over HTTP/2.0 are prefixed with ":". ":"
2554	is not a valid sequence in HTTP header naming, preventing any
2555	possible conflict.
2556
2557	6.3. Cross-Protocol Attacks
2558
2559	By utilizing TLS, we believe that HTTP/2.0 introduces no new cross-
2560	protocol attacks. TLS encrypts the contents of all transmission
2561	(except the handshake itself), making it difficult for attackers to
2562	control the data which could be used in a cross-protocol attack.
2563
2564	6.4. Server Push Implicit Headers
2565
2566	Pushed resources do not have an associated request. In order for
2567	existing HTTP cache control validations (such as the Vary header) to
2568	work, however, all cached resources must have a set of request
2569	headers. For this reason, browsers MUST be careful to inherit
2570	request headers from the associated stream for the push. This
2571	includes the 'Cookie' header.
2572
2573
2574
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2578
2579
2580	7. Privacy Considerations
2581
2582	7.1. Long Lived Connections
2583
2584	HTTP/2.0 aims to keep connections open longer between clients and
2585	servers in order to reduce the latency when a user makes a request.
2586	The maintenance of these connections over time could be used to
2587	expose private information. For example, a user using a browser
2588	hours after the previous user stopped using that browser may be able
2589	to learn about what the previous user was doing. This is a problem
2590	with HTTP in its current form as well, however the short lived
2591	connections make it less of a risk.
2592
2593	7.2. SETTINGS frame
2594
2595	The HTTP/2.0 SETTINGS frame allows servers to store out-of-band
2596	transmitted information about the communication between client and
2597	server on the client. Although this is intended only to be used to
2598	reduce latency, renegade servers could use it as a mechanism to store
2599	identifying information about the client in future requests.
2600
2601	Clients implementing privacy modes, such as Google Chrome's
2602	"incognito mode", may wish to disable client-persisted SETTINGS
2603	storage.
2604
2605	Clients MUST clear persisted SETTINGS information when clearing the
2606	cookies.
2607
2608	TODO: Put range maximums on each type of setting to limit
2609	inappropriate uses.
2610
2611	8. Requirements Notation
2612
2613	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
2614	"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
2615	document are to be interpreted as described in RFC 2119 [RFC2119].
2616
2617	9. Acknowledgements
2618
2619	This document includes substantial input from the following
2620	individuals:
2621
2622	o Adam Langley, Wan-Teh Chang, Jim Morrison, Mark Nottingham, Alyssa
2623	Wilk, Costin Manolache, William Chan, Vitaliy Lvin, Joe Chan, Adam
2624	Barth, Ryan Hamilton, Gavin Peters, Kent Alstad, Kevin Lindsay,
2625	Paul Amer, Fan Yang, Jonathan Leighton (SPDY contributors).
2626
2627
2628
2629
2630
2631	Belshe, et al. Expires July 26, 2013 [Page 47]
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2634
2635
2636	o Gabriel Montenegro and Willy Tarreau (Upgrade mechanism)
2637
2638	o William Chan, Salvatore Loreto, Osama Mazahir, Gabriel Montenegro,
2639	Jitu Padhye, Roberto Peon, Rob Trace (Flow control principles)
2640
2641	o Mark Nottingham and Julian Reschke
2642
2643	10. Normative References
2644
2645	[ASCII] "US-ASCII. Coded Character Set - 7-Bit American
2646	Standard Code for Information Interchange.
2647	Standard ANSI X3.4-1986, ANSI, 1986.".
2648
2649	[HTTP-p1] Fielding, R. and J. Reschke, "Hypertext Transfer
2650	Protocol (HTTP/1.1): Message Syntax and Routing",
2651	draft-ietf-httpbis-p1-messaging-21 (work in
2652	progress), October 2012.
2653
2654	[HTTP-p2] Fielding, R. and J. Reschke, "Hypertext Transfer
2655	Protocol (HTTP/1.1): Semantics and Content",
2656	draft-ietf-httpbis-p2-semantics-21 (work in
2657	progress), October 2012.
2658
2659	[RFC0793] Postel, J., "Transmission Control Protocol",
2660	STD 7, RFC 793, September 1981.
2661
2662	[RFC1738] Berners-Lee, T., Masinter, L., and M. McCahill,
2663	"Uniform Resource Locators (URL)", RFC 1738,
2664	December 1994.
2665
2666	[RFC1950] Deutsch, L. and J. Gailly, "ZLIB Compressed Data
2667	Format Specification version 3.3", RFC 1950,
2668	May 1996.
2669
2670	[RFC2119] Bradner, S., "Key words for use in RFCs to
2671	Indicate Requirement Levels", BCP 14, RFC 2119,
2672	March 1997.
2673
2674	[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
2675	Masinter, L., Leach, P., and T. Berners-Lee,
2676	"Hypertext Transfer Protocol -- HTTP/1.1",
2677	RFC 2616, June 1999.
2678
2679	[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J.,
2680	Lawrence, S., Leach, P., Luotonen, A., and L.
2681	Stewart, "HTTP Authentication: Basic and Digest
2682	Access Authentication", RFC 2617, June 1999.
2683
2684
2685
2686
2687	Belshe, et al. Expires July 26, 2013 [Page 48]
2688
2689	Internet-Draft HTTP/2.0 January 2013
2690
2691
2692	[RFC4559] Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO-
2693	based Kerberos and NTLM HTTP Authentication in
2694	Microsoft Windows", RFC 4559, June 2006.
2695
2696	[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer
2697	Security (TLS) Protocol Version 1.2", RFC 5246,
2698	August 2008.
2699
2700	[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
2701	December 2011.
2702
2703	[TLSNPN] Langley, A., "TLS Next Protocol Negotiation",
2704	draft-agl-tls-nextprotoneg-01 (work in progress),
2705	August 2010.
2706
2707	[UDELCOMPRESSION] Yang, F., Amer, P., and J. Leighton, "A
2708	Methodology to Derive SPDY's Initial Dictionary
2709	for Zlib Compression", <http://www.eecis.udel.edu/
2710	~amer/PEL/poc/pdf/SPDY-Fan.pdf>.
2711
2712	Appendix A. Change Log (to be removed by RFC Editor before publication)
2713
2714	A.1. Since draft-ietf-httpbis-http2-00
2715
2716	Changed title throughout.
2717
2718	Removed section on Incompatibilities with SPDY draft#2.
2719
2720	Changed INTERNAL_ERROR on GOAWAY to have a value of 2 <https://
2721	groups.google.com/forum/?fromgroups#!topic/spdy-dev/cfUef2gL3iU>.
2722
2723	Replaced abstract and introduction.
2724
2725	Added section on starting HTTP/2.0, including upgrade mechanism.
2726
2727	Removed unused references.
2728
2729	Added flow control principles (Section 3.5.1) based on <http://
2730	tools.ietf.org/html/draft-montenegro-httpbis-http2-fc-principles-01>.
2731
2732	A.2. Since draft-mbelshe-httpbis-spdy-00
2733
2734	Adopted as base for draft-ietf-httpbis-http2.
2735
2736	Updated authors/editors list.
2737
2738	Added status note.
2739
2740
2741
2742
2743	Belshe, et al. Expires July 26, 2013 [Page 49]
2744
2745	Internet-Draft HTTP/2.0 January 2013
2746
2747
2748	Authors' Addresses
2749
2750	Mike Belshe
2751	Twist
2752
2753	EMail: mbelshe@chromium.org
2754
2755
2756	Roberto Peon
2757	Google, Inc
2758
2759	EMail: fenix@google.com
2760
2761
2762	Martin Thomson (editor)
2763	Microsoft
2764	3210 Porter Drive
2765	Palo Alto 94043
2766	US
2767
2768	EMail: martin.thomson@skype.net
2769
2770
2771	Alexey Melnikov (editor)
2772	Isode Ltd
2773	5 Castle Business Village
2774	36 Station Road
2775	Hampton, Middlesex TW12 2BX
2776	UK
2777
2778	EMail: Alexey.Melnikov@isode.com
2779
2780
2781
2782
2783
2784
2785
2786
2787
2788
2789
2790
2791
2792
2793
2794
2795
2796
2797
2798
2799	Belshe, et al. Expires July 26, 2013 [Page 50]
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