draft-ietf-quic-qpack-00.txt		draft-ietf-quic-qpack-01.txt
	QUIC C. Krasic		QUIC C. Krasic
	Internet-Draft Google, Inc		Internet-Draft Netflix
	Intended status: Standards Track M. Bishop		Intended status: Standards Track M. Bishop
	Expires: November 24, 2018 Akamai Technologies		Expires: December 30, 2018 Akamai Technologies
	A. Frindell, Ed.		A. Frindell, Ed.
	Facebook		Facebook
	May 23, 2018		June 28, 2018
	QPACK: Header Compression for HTTP over QUIC		QPACK: Header Compression for HTTP over QUIC
	draft-ietf-quic-qpack-00		draft-ietf-quic-qpack-01
	Abstract		Abstract
	This specification defines QPACK, a compression format for		This specification defines QPACK, a compression format for
	efficiently representing HTTP header fields, to be used in HTTP over		efficiently representing HTTP header fields, to be used in HTTP over
	QUIC. This is a variation of HPACK header compression that seeks to		QUIC. This is a variation of HPACK header compression that seeks to
	reduce head-of-line blocking.		reduce head-of-line blocking.
	Note to Readers		Note to Readers
	skipping to change at page 1, line 46 ¶		skipping to change at page 1, line 46 ¶
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at https://datatracker.ietf.org/drafts/current/.		Drafts is at https://datatracker.ietf.org/drafts/current/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on November 24, 2018.		This Internet-Draft will expire on December 30, 2018.
	Copyright Notice		Copyright Notice
	Copyright (c) 2018 IETF Trust and the persons identified as the		Copyright (c) 2018 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(https://trustee.ietf.org/license-info) in effect on the date of		(https://trustee.ietf.org/license-info) in effect on the date of
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	carefully, as they describe your rights and restrictions with respect		carefully, as they describe your rights and restrictions with respect
	to this document. Code Components extracted from this document must		to this document. Code Components extracted from this document must
	include Simplified BSD License text as described in Section 4.e of		include Simplified BSD License text as described in Section 4.e of
	the Trust Legal Provisions and are provided without warranty as		the Trust Legal Provisions and are provided without warranty as
	described in the Simplified BSD License.		described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
	1.1. Head-of-Line Blocking in HPACK . . . . . . . . . . . . . 3		2. Header Tables . . . . . . . . . . . . . . . . . . . . . . . . 3
	1.2. Avoiding Head-of-Line Blocking in HTTP/QUIC . . . . . . . 4		2.1. Static Table . . . . . . . . . . . . . . . . . . . . . . 4
	2. Conventions and Definitions . . . . . . . . . . . . . . . . . 5		2.2. Dynamic Table . . . . . . . . . . . . . . . . . . . . . . 4
	2.1. Notational Conventions . . . . . . . . . . . . . . . . . 5		2.2.1. Absolute and Relative Indexing . . . . . . . . . . . 5
	3. Wire Format . . . . . . . . . . . . . . . . . . . . . . . . . 5		2.2.2. Post-Base Indexing . . . . . . . . . . . . . . . . . 6
	3.1. Primitives . . . . . . . . . . . . . . . . . . . . . . . 6		2.3. Avoiding Head-of-Line Blocking in HTTP/QUIC . . . . . . . 7
	3.2. Indexing . . . . . . . . . . . . . . . . . . . . . . . . 6		2.3.1. State Synchronization . . . . . . . . . . . . . . . . 8
	3.3. QPACK Encoder Stream . . . . . . . . . . . . . . . . . . 8		3. Conventions and Definitions . . . . . . . . . . . . . . . . . 8
	3.3.1. Insert With Name Reference . . . . . . . . . . . . . 8		3.1. Notational Conventions . . . . . . . . . . . . . . . . . 9
	3.3.2. Insert Without Name Reference . . . . . . . . . . . . 9		4. Configuration . . . . . . . . . . . . . . . . . . . . . . . . 9
	3.3.3. Duplicate . . . . . . . . . . . . . . . . . . . . . . 9		5. Wire Format . . . . . . . . . . . . . . . . . . . . . . . . . 9
	3.3.4. Dynamic Table Size Update . . . . . . . . . . . . . . 10		5.1. Primitives . . . . . . . . . . . . . . . . . . . . . . . 10
	3.4. QPACK Decoder Stream . . . . . . . . . . . . . . . . . . 10		5.1.1. Prefixed Integers . . . . . . . . . . . . . . . . . . 10
	3.4.1. Table State Synchronize . . . . . . . . . . . . . . . 11		5.1.2. String Literals . . . . . . . . . . . . . . . . . . . 10
	3.4.2. Header Acknowledgement . . . . . . . . . . . . . . . 11		5.2. QPACK Encoder Stream . . . . . . . . . . . . . . . . . . 11
	3.5. Request and Push Streams . . . . . . . . . . . . . . . . 11		5.2.1. Insert With Name Reference . . . . . . . . . . . . . 11
	3.5.1. Header Data Prefix . . . . . . . . . . . . . . . . . 12		5.2.2. Insert Without Name Reference . . . . . . . . . . . . 11
	3.5.2. Instructions . . . . . . . . . . . . . . . . . . . . 12		5.2.3. Duplicate . . . . . . . . . . . . . . . . . . . . . . 12
	4. Encoding Strategies . . . . . . . . . . . . . . . . . . . . . 15		5.2.4. Dynamic Table Size Update . . . . . . . . . . . . . . 12
	4.1. Single pass encoding . . . . . . . . . . . . . . . . . . 15		5.3. QPACK Decoder Stream . . . . . . . . . . . . . . . . . . 13
	4.2. Preventing Eviction Races . . . . . . . . . . . . . . . . 15		5.3.1. Table State Synchronize . . . . . . . . . . . . . . . 13
	4.3. Reference Tracking . . . . . . . . . . . . . . . . . . . 15		5.3.2. Header Acknowledgement . . . . . . . . . . . . . . . 14
	4.3.1. Blocked Eviction . . . . . . . . . . . . . . . . . . 16		5.3.3. Stream Cancellation . . . . . . . . . . . . . . . . . 14
	4.3.2. Blocked Decoding . . . . . . . . . . . . . . . . . . 16		5.4. Request and Push Streams . . . . . . . . . . . . . . . . 15
	4.4. Speculative table updates . . . . . . . . . . . . . . . . 16		5.4.1. Header Data Prefix . . . . . . . . . . . . . . . . . 15
	4.5. Sample One Pass Encoding Algorithm . . . . . . . . . . . 17		5.4.2. Instructions . . . . . . . . . . . . . . . . . . . . 16
	5. Security Considerations . . . . . . . . . . . . . . . . . . . 18		6. Encoding Strategies . . . . . . . . . . . . . . . . . . . . . 19
	6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18		6.1. Single Pass Encoding . . . . . . . . . . . . . . . . . . 19
	7. References . . . . . . . . . . . . . . . . . . . . . . . . . 18		6.2. Preventing Eviction Races . . . . . . . . . . . . . . . . 19
	7.1. Normative References . . . . . . . . . . . . . . . . . . 18		6.3. Reference Tracking . . . . . . . . . . . . . . . . . . . 19
	7.2. Informative References . . . . . . . . . . . . . . . . . 18		6.3.1. Blocked Eviction . . . . . . . . . . . . . . . . . . 20
	7.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 19		6.3.2. Blocked Decoding . . . . . . . . . . . . . . . . . . 20
	Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 19		6.4. Speculative table updates . . . . . . . . . . . . . . . . 20
	Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . . 19		6.5. Sample One Pass Encoding Algorithm . . . . . . . . . . . 20
	B.1. Since draft-ietf-quic-qcram-00 . . . . . . . . . . . . . 19		7. Security Considerations . . . . . . . . . . . . . . . . . . . 22
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20		8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
			8.1. Settings Registration . . . . . . . . . . . . . . . . . . 22
			8.2. Stream Type Registration . . . . . . . . . . . . . . . . 22
			9. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
			9.1. Normative References . . . . . . . . . . . . . . . . . . 22
			9.2. Informative References . . . . . . . . . . . . . . . . . 23
			9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 23
			Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 23
			A.1. Since draft-ietf-quic-qpack-00 . . . . . . . . . . . . . 23
			A.2. Since draft-ietf-quic-qcram-00 . . . . . . . . . . . . . 24
			Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 24
			Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
	1. Introduction		1. Introduction
	The QUIC transport protocol was designed from the outset to support		The QUIC transport protocol was designed from the outset to support
	HTTP semantics, and its design subsumes many of the features of		HTTP semantics, and its design subsumes many of the features of
	HTTP/2. QUIC's stream multiplexing comes into some conflict with		HTTP/2. HTTP/2 used HPACK ([RFC7541]) for header compression, but
	header compression. A key goal of the design of QUIC is to improve		QUIC's stream multiplexing comes into some conflict with HPACK. A
	stream multiplexing relative to HTTP/2 by eliminating HoL (head of		key goal of the design of QUIC is to improve stream multiplexing
	line) blocking, which can occur in HTTP/2. HoL blocking can happen		relative to HTTP/2 by reducing head-of-line blocking. If HPACK were
	because all HTTP/2 streams are multiplexed onto a single TCP		used for HTTP/QUIC, it would induce head-of-line blocking due to
	connection with its in-order semantics. QUIC can maintain		built-in assumptions of a total ordering across frames on all
	independence between streams because it implements core transport		streams.
	functionality in a fully stream-aware manner. However, the HTTP/QUIC
	mapping is still subject to HoL blocking if HPACK is used directly.
	HPACK exploits multiplexing for greater compression, shrinking the
	representation of headers that have appeared earlier on the same
	connection. In the context of QUIC, this imposes a vulnerability to
	HoL blocking (see Section 1.1).
	QUIC is described in [QUIC-TRANSPORT]. The HTTP/QUIC mapping is		QUIC is described in [QUIC-TRANSPORT]. The HTTP/QUIC mapping is
	described in [QUIC-HTTP]. For a full description of HTTP/2, see		described in [QUIC-HTTP]. For a full description of HTTP/2, see
	[RFC7540]. The description of HPACK is [RFC7541], with important		[RFC7540]. The description of HPACK is [RFC7541], with important
	terminology in Section 1.3.		terminology in Section 1.3.
	QPACK modifies HPACK to allow correctness in the presence of out-of-		QPACK reuses core concepts from HPACK, but is redesigned to allow
	order delivery, with flexibility for implementations to balance		correctness in the presence of out-of-order delivery, with
	between resilience against HoL blocking and optimal compression		flexibility for implementations to balance between resilience against
	ratio. The design goals are to closely approach the compression		head-of-line blocking and optimal compression ratio. The design
	ratio of HPACK with substantially less head-of-line blocking under		goals are to closely approach the compression ratio of HPACK with
	the same loss conditions.		substantially less head-of-line blocking under the same loss
			conditions.
	QPACK is intended to be a relatively non-intrusive extension to		2. Header Tables
	HPACK; an implementation should be easily shared within stacks
	supporting both HTTP/2 over (TLS+)TCP and HTTP/QUIC.
	1.1. Head-of-Line Blocking in HPACK		Like HPACK, QPACK uses two tables for associating header fields to
			indexes. The static table (see Section 2.1) is predefined and
			contains common header fields (some of them with an empty value).
	HPACK enables several types of header representations, one of which		The dynamic table (see Section 2.2) built up over the course of the
	also adds the header to a dynamic table of header values. These		connection and can be used by the encoder to index header fields
	values are then available for reuse in subsequent header blocks		repeated in the encoded header lists.
	simply by referencing the entry number in the table.
	If the packet containing a header is lost, that stream cannot		Unlike in HPACK, entries in the QPACK static and dynamic tables are
	complete header processing until the packet is retransmitted. This		addressed separately. The following sections describe how entries in
	is unavoidable. However, other streams which rely on the state		each table is addressed.
	created by that packet _also_ cannot make progress. This is the
	problem which QUIC solves in general, but which is reintroduced by
	HPACK when the loss includes a HEADERS frame.
	1.2. Avoiding Head-of-Line Blocking in HTTP/QUIC		2.1. Static Table
			The static table consists of a predefined static list of header
			fields, each of which has a fixed index over time. Its entries are
			defined in Appendix A of [RFC7541]. Note that because HPACK did not
			use zero-based references, there is no value at index zero of the
			static table.
			2.2. Dynamic Table
			The dynamic table consists of a list of header fields maintained in
			first-in, first-out order. The dynamic table is initially empty.
			Entries are added by instructions on the Encoder Stream (see
			Section 5.2).
			Before a new entry is added to the dynamic table, entries are evicted
			from the end of the dynamic table until the size of the dynamic table
			is less than or equal to (maximum size - new entry size) or until the
			table is empty.
			If the size of the new entry is less than or equal to the maximum
			size, that entry is added to the table. It is an error to attempt to
			add an entry that is larger than the maximum size; this MUST be
			treated as a connection error of type
			"HTTP_QPACK_DECOMPRESSION_FAILED".
			A new entry can reference an entry in the dynamic table that will be
			evicted when adding this new entry into the dynamic table.
			Implementations are cautioned to avoid deleting the referenced name
			if the referenced entry is evicted from the dynamic table prior to
			inserting the new entry.
			The dynamic table can contain duplicate entries (i.e., entries with
			the same name and same value). Therefore, duplicate entries MUST NOT
			be treated as an error by a decoder.
			The encoder decides how to update the dynamic table and as such can
			control how much memory is used by the dynamic table. To limit the
			memory requirements of the decoder, the dynamic table size is
			strictly bounded.
			The decoder determines the maximum size that the encoder is permitted
			to use for the dynamic table. In HTTP/QUIC, this value is determined
			by the SETTINGS_HEADER_TABLE_SIZE setting (see Section 4.2.5.2 of
			[QUIC-HTTP]).
			An encoder can choose to use less capacity than this maximum size
			(see Section 5.2.4), but the chosen size MUST stay lower than or
			equal to the maximum set by the decoder. Whenever the maximum size
			for the dynamic table is reduced, entries are evicted from the end of
			the dynamic table until the size of the dynamic table is less than or
			equal to the maximum size.
			This mechanism can be used to completely clear entries from the
			dynamic table by setting a maximum size of 0, which can subsequently
			be restored.
			2.2.1. Absolute and Relative Indexing
			Each entry possesses both an absolute index which is fixed for the
			lifetime of that entry and a relative index which changes over time
			based on the context of the reference. The first entry inserted has
			an absolute index of "1"; indices increase sequentially with each
			insertion.
			The relative index begins at zero and increases in the opposite
			direction from the absolute index. Determining which entry has a
			relative index of "0" depends on the context of the reference.
			On the control stream, a relative index of "0" always refers to the
			most recently inserted value in the dynamic table. Note that this
			means the entry referenced by a given relative index will change
			while interpreting instructions on the encoder stream.
			+---+---------------+-----------+
			| n | ... | d + 1 | Absolute Index
			+ - +---------------+ - - - - - +
			| 0 | ... | n - d - 1 | Relative Index
			+---+---------------+-----------+
			^ |
			| V
			Insertion Point Dropping Point
			n = count of entries inserted
			d = count of entries dropped
			Example Dynamic Table Indexing - Control Stream
			Because frames from request streams can be delivered out of order
			with instructions on the control stream, relative indices are
			relative to the Base Index at the beginning of the header block (see
			Section 5.4.1). The Base Index is an absolute index. When
			interpreting the rest of the frame, the entry identified by Base
			Index has a relative index of zero. The relative indices of entries
			do not change while interpreting headers on a request or push stream.
			Base Index
			|
			V
			+---+-----+-----+-----+-------+
			| n | n-1 | n-2 | ... | d+1 | Absolute Index
			+---+-----+ - +-----+ - +
			| 0 | ... | n-d-3 | Relative Index
			+-----+-----+-------+
			n = count of entries inserted
			d = count of entries dropped
			Example Dynamic Table Indexing - Request Stream
			2.2.2. Post-Base Indexing
			A header block on the request stream can reference entries added
			after the entry identified by the Base Index. This allows an encoder
			to process a header block in a single pass and include references to
			entries added while processing this (or other) header blocks. Newly
			added entries are referenced using Post-Base instructions. Indices
			for Post-Base instructions increase in the same direction as absolute
			indices, but the zero value is one higher than the Base Index.
			Base Index
			|
			V
			+---+-----+-----+-----+-----+
			| n | n-1 | n-2 | ... | d+1 | Absolute Index
			+---+-----+-----+-----+-----+
			| 1 | 0 | Post-Base Index
			+---+-----+
			n = count of entries inserted
			d = count of entries dropped
			Dynamic Table Indexing - Post-Base References
			If the decoder encounters a reference to an entry which has already
			been dropped from the table or which is greater than the declared
			Largest Reference (see Section 5.4.1), this MUST be treated as a
			stream error of type "HTTP_QPACK_DECOMPRESSION_FAILED" error code.
			If this reference occurs on the control stream, this MUST be treated
			as a session error.
			2.3. Avoiding Head-of-Line Blocking in HTTP/QUIC
	Because QUIC does not guarantee order between data on different		Because QUIC does not guarantee order between data on different
	streams, a header block might reference an entry in the dynamic table		streams, a header block might reference an entry in the dynamic table
	that has not yet been received.		that has not yet been received.
	Each header block contains a Largest Reference (see Section 3.5.1)		Each header block contains a Largest Reference which identifies the
	which identifies the table state necessary for decoding. If the		table state necessary for decoding. If the greatest absolute index
	greatest absolute index in the dynamic table is less than the value		in the dynamic table is less than the value of the Largest Reference,
	of the Largest Reference, the stream is considered "blocked." While		the stream is considered "blocked." While blocked, header field data
	blocked, header field data should remain in the blocked stream's flow		should remain in the blocked stream's flow control window. When the
	control window. When the Largest Reference is zero, the frame		Largest Reference is zero, the frame contains no references to the
	contains no references to the dynamic table and can always be		dynamic table and can always be processed immediately. A stream
	processed immediately. A stream becomes unblocked when the greatest		becomes unblocked when the greatest absolute index in the dynamic
	absolute index in the dynamic table becomes greater than or equal to		table becomes greater than or equal to the Largest Reference for all
	the Largest Reference for all header blocks the decoder has started		header blocks the decoder has started reading from the stream. If a
	reading from the stream.		decoder encounters a header block where the actual largest reference
			is not equal to the largest reference declared in the prefix, it MAY
			treat this as a stream error of type HTTP_QPACK_DECOMPRESSION_FAILED.
	A decoder can permit the possibility of blocked streams by setting		A decoder can permit the possibility of blocked streams by setting
	SETTINGS_QPACK_BLOCKED_STREAMS to a non-zero value. This setting		SETTINGS_QPACK_BLOCKED_STREAMS to a non-zero value (see Section 4).
	specifies an upper bound on the number of streams which can be		This setting specifies an upper bound on the number of streams which
	blocked.		can be blocked.
	An encoder can decide whether to risk having a stream become blocked.		An encoder can decide whether to risk having a stream become blocked.
	If permitted by the value of SETTINGS_QPACK_BLOCKED_STREAMS,		If permitted by the value of SETTINGS_QPACK_BLOCKED_STREAMS,
	compression efficiency can be improved by referencing dynamic table		compression efficiency can be improved by referencing dynamic table
	entries that are still in transit, but if there is loss or reordering		entries that are still in transit, but if there is loss or reordering
	the stream can become blocked at the decoder. An encoder avoids the		the stream can become blocked at the decoder. An encoder avoids the
	risk of blocking by only referencing dynamic table entries which have		risk of blocking by only referencing dynamic table entries which have
	been acknowledged, but this means using literals. Since literals		been acknowledged, but this means using literals. Since literals
	make the header block larger, this can result in the encoder becoming		make the header block larger, this can result in the encoder becoming
	blocked on congestion or flow control limits.		blocked on congestion or flow control limits.
	An encoder MUST limit the number of streams which could become		An encoder MUST limit the number of streams which could become
	blocked to the value of SETTINGS_QPACK_BLOCKED_STREAMS at all times.		blocked to the value of SETTINGS_QPACK_BLOCKED_STREAMS at all times.
	Note that the decoder might not actually become blocked on every		Note that the decoder might not actually become blocked on every
	stream which risks becoming blocked. If the decoder encounters more		stream which risks becoming blocked. If the decoder encounters more
	blocked streams than it promised to support, it SHOULD treat this as		blocked streams than it promised to support, it SHOULD treat this as
	a stream error of type HTTP_QPACK_DECOMPRESSION_FAILED.		a stream error of type HTTP_QPACK_DECOMPRESSION_FAILED.
	2. Conventions and Definitions		2.3.1. State Synchronization
			The decoder stream signals key events at the decoder that permit the
			encoder to track the decoder's state. These events are:
			o Successful processing of a header block
			o Abandonment of a stream which might have remaining header blocks
			o Receipt of new dynamic table entries
			Regardless of whether a header block contained blocking references,
			the knowledge that it was processed successfully permits the encoder
			to avoid evicting entries while references remain outstanding; see
			Section 6.3.1. When a stream is reset or abandoned, the indication
			that these header blocks will never be processed serves a similar
			function; see Section 5.3.3.
			For the encoder to identify which dynamic table entries can be safely
			used without a stream becoming blocked, the encoder tracks the
			absolute index of the decoder's Largest Known Received entry.
			When blocking references are permitted, the encoder uses
			acknowledgement of header blocks to identify the Largest Known
			Received index, as described in Section 5.3.2.
			To acknowledge dynamic table entries which are not referenced by
			header blocks, for example because the encoder or the decoder have
			chosen not to risk blocked streams, the decoder sends a Table State
			Synchronize instruction (see Section 5.3.1).
			3. Conventions and Definitions
	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",		The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and		"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
	"OPTIONAL" in this document are to be interpreted as described in BCP		"OPTIONAL" in this document are to be interpreted as described in BCP
	14 [RFC2119] [RFC8174] when, and only when, they appear in all		14 [RFC2119] [RFC8174] when, and only when, they appear in all
	capitals, as shown here.		capitals, as shown here.
	Definitions of terms that are used in this document:		Definitions of terms that are used in this document:
	Header: A name-value pair sent as part of an HTTP message.		Header: A name-value pair sent as part of an HTTP message.
	skipping to change at page 5, line 30 ¶		skipping to change at page 9, line 13 ¶
	Header block: The compressed representation of a header set.		Header block: The compressed representation of a header set.
	Encoder: An implementation which transforms a header set into a		Encoder: An implementation which transforms a header set into a
	header block.		header block.
	Decoder: An implementation which transforms a header block into a		Decoder: An implementation which transforms a header block into a
	header set.		header set.
	QPACK is a name, not an acronym.		QPACK is a name, not an acronym.
	2.1. Notational Conventions		3.1. Notational Conventions
	Diagrams use the format described in Section 3.1 of [RFC2360], with		Diagrams use the format described in Section 3.1 of [RFC2360], with
	the following additional conventions:		the following additional conventions:
	x (A) Indicates that x is A bits long		x (A) Indicates that x is A bits long
	x (A+) Indicates that x uses the prefixed integer encoding defined		x (A+) Indicates that x uses the prefixed integer encoding defined
	in Section 5.1 of [RFC7541], beginning with an A-bit prefix.		in Section 5.1 of [RFC7541], beginning with an A-bit prefix.
	x ... Indicates that x is variable-length and extends to the end of		x ... Indicates that x is variable-length and extends to the end of
	the region.		the region.
	3. Wire Format		4. Configuration
			QPACK defines two settings which are included in the HTTP/QUIC
			SETTINGS frame.
			SETTINGS_HEADER_TABLE_SIZE (0x1): An integer with a maximum value of
			2^30 - 1. The default value is 4,096 bytes. See (TODO: reference
			PR#1357) for usage.
			SETTINGS_QPACK_BLOCKED_STREAMS (0x7): An integer with a maximum
			value of 2^16 - 1. The default value is 100. See Section 2.3.
			5. Wire Format
	QPACK instructions occur in three locations, each of which uses a		QPACK instructions occur in three locations, each of which uses a
	separate instruction space:		separate instruction space:
	o Table updates are carried by a unidirectional stream from encoder		o The encoder stream is a unidirectional stream of type "0x48"
	to decoder. Instructions on this stream modify the dynamic table		(ASCII 'H') which carries table updates from encoder to decoder.
	state without generating output to any particular request.		Instructions on this stream modify the dynamic table state without
			generating output to any particular request.
	o Acknowledgements of table modifications and header processing are		o The decoder stream is a unidirectional stream of type "0x68"
	carried by a unidirectional stream from decoder to encoder.		(ASCII 'h') which carries acknowledgements of table modifications
			and header processing from decoder to encoder.
	o Finally, the contents of HEADERS and PUSH_PROMISE frames on		o Finally, the contents of HEADERS and PUSH_PROMISE frames on
	request streams reference the QPACK table state.		request streams and push streams reference the QPACK table state.
			There MUST be exactly one of each unidirectional stream type in each
			direction. Receipt of a second instance of either stream type MUST
			be treated as a connection error of HTTP_WRONG_STREAM_COUNT. Closure
			of either unidirectional stream MUST be treated as a connection error
			of type HTTP_CLOSED_CRITICAL_STREAM.
	This section describes the instructions which are possible on each		This section describes the instructions which are possible on each
	stream type.		stream type.
	All table updates occur on the control stream. Request streams only		All table updates occur on the encoder stream. Request streams and
	carry header blocks that do not modify the state of the table.		push streams only carry header blocks that do not modify the state of
			the table.
	3.1. Primitives		5.1. Primitives
			5.1.1. Prefixed Integers
	The prefixed integer from Section 5.1 of [RFC7541] is used heavily		The prefixed integer from Section 5.1 of [RFC7541] is used heavily
	throughout this document. The string literal, defined by Section 5.2		throughout this document. The format from [RFC7541] is used
	of [RFC7541], is used with the following modification.		unmodified.
			5.1.2. String Literals
			The string literal defined by Section 5.2 of [RFC7541] is also used
			throughout. This string format includes optional Huffman encoding.
	HPACK defines string literals to begin on a byte boundary. They		HPACK defines string literals to begin on a byte boundary. They
	begin with a single flag (indicating whether the string is Huffman-		begin with a single flag (indicating whether the string is Huffman-
	coded), followed by the Length encoded as a 7-bit prefix integer, and		coded), followed by the Length encoded as a 7-bit prefix integer, and
	finally Length octets of data.		finally Length octets of data. When Huffman encoding is enabled, the
			Huffman table from Appendix B of [RFC7541] is used without
			modification.
	QPACK permits strings to begin other than on a byte boundary. An		This document expands the definition of string literals and permits
	"N-bit prefix string literal" begins with the same Huffman flag,		them to begin other than on a byte boundary. An "N-bit prefix string
	followed by the length encoded as an (N-1)-bit prefix integer. The		literal" begins with the same Huffman flag, followed by the length
	remainder of the string literal is unmodified.		encoded as an (N-1)-bit prefix integer. The remainder of the string
			literal is unmodified.
	A string literal without a prefix length noted is an 8-bit prefix		A string literal without a prefix length noted is an 8-bit prefix
	string literal and follows the definitions in [RFC7541] without		string literal and follows the definitions in [RFC7541] without
	modification.		modification.
	3.2. Indexing		5.2. QPACK Encoder Stream
	Entries in the QPACK static and dynamic tables are addressed
	separately.
	Entries in the static table have the same indices at all times. The
	static table is defined in Appendix A of [RFC7541]. Note that
	because HPACK did not use zero-based references, there is no value at
	index zero of the static table.
	Entries are inserted into the dynamic table over time. Each entry
	possesses both an absolute index which is fixed for the lifetime of
	that entry and a relative index which changes over time based on the
	context of the reference. The first entry inserted has an absolute
	index of "1"; indices increase sequentially with each insertion.
	On the control stream, a relative index of "0" always refers to the
	most recently inserted value in the dynamic table. Note that this
	means the entry referenced by a given relative index can change while
	interpreting a HEADERS frame as new entries are inserted.
	+---+---------------+-------+
	| n | ... | d + 1 | Absolute Index
	+ - +---------------+ - +
	| 0 | ... | n-d-1 | Relative Index
	+---+---------------+-------+
	^ |
	| V
	Insertion Point Dropping Point
	n = count of entries inserted
	d = count of entries dropped
	Example Dynamic Table Indexing - Control Stream
	Because frames from request streams can be delivered out of order
	with instructions on the control stream, relative indices are
	relative to the Base Index at the beginning of the header block (see
	Section 3.5.1). The Base Index is the absolute index of the entry
	which has the relative index of zero when interpreting the frame.
	The relative indices of entries do not change while interpreting
	headers on a request or push stream.
	Base Index
	|
	V
	+---+-----+-----+-----+-------+
	| n | n-1 | n-2 | ... | d+1 | Absolute Index
	+---+-----+ - +-----+ - +
	| 0 | ... | n-d-3 | Relative Index
	+-----+-----+-------+
	n = count of entries inserted
	d = count of entries dropped
	Example Dynamic Table Indexing - Request Stream
	Entries with an absolute index greater than a frame's Base Index can
	be referenced using specific Post-Base instructions. The relative
	indices of Post-Base references count up from Base Index.
	Base Index
	|
	V
	+---+-----+-----+-----+-----+
	| n | n-1 | n-2 | ... | d+1 | Absolute Index
	+---+-----+-----+-----+-----+
	| 1 | 0 | Post-Base Index
	+---+-----+
	n = count of entries inserted
	d = count of entries dropped
	Dynamic Table Indexing - Post-Base References
	If the decoder encounters a reference to an entry which has already
	been dropped from the table or which is greater than the declared
	Largest Reference, this MUST be treated as a stream error of type
	"HTTP_QPACK_DECOMPRESSION_FAILED" error code. If this reference
	occurs on the control stream, this MUST be treated as a session
	error.
	3.3. QPACK Encoder Stream
	Table updates can add a table entry, possibly using existing entries		Table updates can add a table entry, possibly using existing entries
	to avoid transmitting redundant information. The name can be		to avoid transmitting redundant information. The name can be
	transmitted as a reference to an existing entry in the static or the		transmitted as a reference to an existing entry in the static or the
	dynamic table or as a string literal. For entries which already		dynamic table or as a string literal. For entries which already
	exist in the dynamic table, the full entry can also be used by		exist in the dynamic table, the full entry can also be used by
	reference, creating a duplicate entry.		reference, creating a duplicate entry.
	Each set of encoder instructions is prefaced by its length, encoded		The contents of the encoder stream are an unframed sequence of the
	as a variable length integer with an 8-bit prefix. Instructions MUST		following instructions.
	NOT span more than one block.
	0 1 2 3 4 5 6 7
	+---+---+---+---+---+---+---+---+
	| Block Length (8+) |
	+-------------------------------+
	| Instruction Block (*) ...
	+-------------------------------+
	Encoder instruction block
	3.3.1. Insert With Name Reference		5.2.1. Insert With Name Reference
	An addition to the header table where the header field name matches		An addition to the header table where the header field name matches
	the header field name of an entry stored in the static table or the		the header field name of an entry stored in the static table or the
	dynamic table starts with the '1' one-bit pattern. The "S" bit		dynamic table starts with the '1' one-bit pattern. The "S" bit
	indicates whether the reference is to the static (S=1) or dynamic		indicates whether the reference is to the static (S=1) or dynamic
	(S=0) table. The header field name is represented using the relative		(S=0) table. The header field name is represented using the relative
	index of that entry, which is represented as an integer with a 6-bit		index of that entry, which is represented as an integer with a 6-bit
	prefix (see Section 5.1 of [RFC7541]).		prefix (see Section 5.1 of [RFC7541]).
	The header name reference is followed by the header field value		The header name reference is followed by the header field value
	skipping to change at page 9, line 23 ¶		skipping to change at page 11, line 41 ¶
	+---+---+---+---+---+---+---+---+		+---+---+---+---+---+---+---+---+
	| 1 | S | Name Index (6+) |		| 1 | S | Name Index (6+) |
	+---+---+-----------------------+		+---+---+-----------------------+
	| H | Value Length (7+) |		| H | Value Length (7+) |
	+---+---------------------------+		+---+---------------------------+
	| Value String (Length octets) |		| Value String (Length octets) |
	+-------------------------------+		+-------------------------------+
	Insert Header Field -- Indexed Name		Insert Header Field -- Indexed Name
	3.3.2. Insert Without Name Reference		5.2.2. Insert Without Name Reference
	An addition to the header table where both the header field name and		An addition to the header table where both the header field name and
	the header field value are represented as string literals (see		the header field value are represented as string literals (see
	Section 3.1) starts with the '01' two-bit pattern.		Section 5.1) starts with the '01' two-bit pattern.
	The name is represented as a 6-bit prefix string literal, while the		The name is represented as a 6-bit prefix string literal, while the
	value is represented as an 8-bit prefix string literal.		value is represented as an 8-bit prefix string literal.
	0 1 2 3 4 5 6 7		0 1 2 3 4 5 6 7
	+---+---+---+---+---+---+---+---+		+---+---+---+---+---+---+---+---+
	| 0 | 1 | H | Name Length (5+) |		| 0 | 1 | H | Name Length (5+) |
	+---+---+---+-------------------+		+---+---+---+-------------------+
	| Name String (Length octets) |		| Name String (Length octets) |
	+---+---------------------------+		+---+---------------------------+
	| H | Value Length (7+) |		| H | Value Length (7+) |
	+---+---------------------------+		+---+---------------------------+
	| Value String (Length octets) |		| Value String (Length octets) |
	+-------------------------------+		+-------------------------------+
	Insert Header Field -- New Name		Insert Header Field -- New Name
	3.3.3. Duplicate		5.2.3. Duplicate
	Duplication of an existing entry in the dynamic table starts with the		Duplication of an existing entry in the dynamic table starts with the
	'000' three-bit pattern. The relative index of the existing entry is		'000' three-bit pattern. The relative index of the existing entry is
	represented as an integer with a 5-bit prefix.		represented as an integer with a 5-bit prefix.
	0 1 2 3 4 5 6 7		0 1 2 3 4 5 6 7
	+---+---+---+---+---+---+---+---+		+---+---+---+---+---+---+---+---+
	| 0 | 0 | 0 | Index (5+) |		| 0 | 0 | 0 | Index (5+) |
	+---+---+---+-------------------+		+---+---+---+-------------------+
	Figure 1: Duplicate		Figure 1: Duplicate
	The existing entry is re-inserted into the dynamic table without		The existing entry is re-inserted into the dynamic table without
	resending either the name or the value. This is useful to mitigate		resending either the name or the value. This is useful to mitigate
	the eviction of older entries which are frequently referenced, both		the eviction of older entries which are frequently referenced, both
	to avoid the need to resend the header and to avoid the entry in the		to avoid the need to resend the header and to avoid the entry in the
	table blocking the ability to insert new headers.		table blocking the ability to insert new headers.
	3.3.4. Dynamic Table Size Update		5.2.4. Dynamic Table Size Update
	An encoder informs the decoder of a change to the size of the dynamic		An encoder informs the decoder of a change to the size of the dynamic
	table using an instruction which begins with the '001' three-bit		table using an instruction which begins with the '001' three-bit
	pattern. The new maximum table size is represented as an integer		pattern. The new maximum table size is represented as an integer
	with a 5-bit prefix (see Section 5.1 of [RFC7541]).		with a 5-bit prefix (see Section 5.1 of [RFC7541]).
	0 1 2 3 4 5 6 7		0 1 2 3 4 5 6 7
	+---+---+---+---+---+---+---+---+		+---+---+---+---+---+---+---+---+
	| 0 | 0 | 1 | Max size (5+) |		| 0 | 0 | 1 | Max size (5+) |
	+---+---+---+-------------------+		+---+---+---+-------------------+
	Figure 2: Maximum Dynamic Table Size Change		Figure 2: Maximum Dynamic Table Size Change
	The new maximum size MUST be lower than or equal to the limit		The new maximum size MUST be lower than or equal to the limit
	determined by the protocol using QPACK. A value that exceeds this		determined by the protocol using QPACK. A value that exceeds this
	limit MUST be treated as a decoding error. In HTTP/QUIC, this limit		limit MUST be treated as a decoding error. In HTTP/QUIC, this limit
	is the value of the SETTINGS_HEADER_TABLE_SIZE parameter (see		is the value of the SETTINGS_HEADER_TABLE_SIZE parameter (see
	[QUIC-HTTP]) received from the decoder.		Section 4) received from the decoder.
	Reducing the maximum size of the dynamic table can cause entries to		Reducing the maximum size of the dynamic table can cause entries to
	be evicted (see Section 4.3 of [RFC7541]). This MUST NOT cause the		be evicted (see Section 4.3 of [RFC7541]). This MUST NOT cause the
	eviction of entries with outstanding references (see Section 4.3).		eviction of entries with outstanding references (see Section 6.3).
			Changing the size of the dynamic table is not acknowledged as this
			instruction does not insert an entry.
	3.4. QPACK Decoder Stream		5.3. QPACK Decoder Stream
	The decoder stream carries information used to ensure consistency of		The decoder stream carries information used to ensure consistency of
	the dynamic table. Information is sent from the QPACK decoder to the		the dynamic table. Information is sent from the QPACK decoder to the
	QPACK encoder; that is, the server informs the client about the		QPACK encoder; that is, the server informs the client about the
	processing of the client's header blocks and table updates, and the		processing of the client's header blocks and table updates, and the
	client informs the server about the processing of the server's header		client informs the server about the processing of the server's header
	blocks and table updates.		blocks and table updates.
	3.4.1. Table State Synchronize		The contents of the decoder stream are an unframed sequence of the
			following instructions.
	After processing a set of instructions on the encoder stream, the		5.3.1. Table State Synchronize
	decoder will emit a Table State Synchronize instruction on the
	decoder stream. The instruction begins with the '1' one-bit pattern.		The Table State Synchronize instruction begins with the '00' two-bit
	The instruction specifies the total number of dynamic table inserts		pattern. The instruction specifies the total number of dynamic table
	and duplications since the last Table State Synchronize, encoded as a		inserts and duplications since the last Table State Synchronize or
	7-bit prefix integer. The encoder uses this value to determine which		Header Acknowledgement that increased the Largest Known Received
	table entries are vulnerable to head-of-line blocking. A decoder MAY		dynamic table entry. This is encoded as a 6-bit prefix integer. The
	coalesce multiple synchronization updates into a single update.		encoder uses this value to determine which table entries might cause
			a stream to become blocked, as described in Section 2.3.1.
	0 1 2 3 4 5 6 7		0 1 2 3 4 5 6 7
	+---+---+---+---+---+---+---+---+		+---+---+---+---+---+---+---+---+
	| 1 | Insert Count (7+) |		| 0 | 0 | Insert Count (6+) |
	+---+---------------------------+		+---+---+-----------------------+
	Figure 3: Table Size Synchronize		Figure 3: Table State Synchronize
	3.4.2. Header Acknowledgement		A decoder chooses when to emit Table State Synchronize instructions.
			Emitting a Table State Synchronize after adding each new dynamic
			table entry will provide the most timely feedback to the encoder, but
			could be redundant with other decoder feedback. By delaying a
			Table State Synchronize, a decoder might be able to coalesce multiple
			Table State Synchronize instructions, or replace them entirely with
			Header Acknowledgements. However, delaying too long may lead to
			compression inefficiencies if the encoder waits for an entry to be
			acknowledged before using it.
			5.3.2. Header Acknowledgement
	After processing a header block on a request or push stream, the		After processing a header block on a request or push stream, the
	decoder emits a Header Acknowledgement instruction on the decoder		decoder emits a Header Acknowledgement instruction on the decoder
	stream. The instruction begins with the '0' one-bit pattern and		stream. The instruction begins with the '1' one-bit pattern and
	includes the request stream's stream ID, encoded as a 7-bit prefix		includes the request stream's stream ID, encoded as a 7-bit prefix
	integer. It is used by the peer's QPACK encoder to know when it is		integer. It is used by the peer's QPACK encoder to know when it is
	safe to evict an entry.		safe to evict an entry.
	The same Stream ID can be identified multiple times, as multiple		The same Stream ID can be identified multiple times, as multiple
	header blocks can be sent on a single stream in the case of		header blocks can be sent on a single stream in the case of
	intermediate responses, trailers, and pushed requests. Since header		intermediate responses, trailers, and pushed requests. Since header
	frames on each stream are received and processed in order, this gives		frames on each stream are received and processed in order, this gives
	the encoder precise feedback on which header blocks within a stream		the encoder precise feedback on which header blocks within a stream
	have been fully processed.		have been fully processed.
	0 1 2 3 4 5 6 7		0 1 2 3 4 5 6 7
	+---+---+---+---+---+---+---+---+		+---+---+---+---+---+---+---+---+
	| 0 | Stream ID (7+) |		| 1 | Stream ID (7+) |
	+---+---------------------------+		+---+---------------------------+
	Figure 4: Header Acknowledgement		Figure 4: Header Acknowledgement
	3.5. Request and Push Streams		When blocking references are permitted, the encoder uses
			acknowledgement of header blocks to update the Largest Known Received
			index. If a header block was potentially blocking, the
			acknowledgement implies that the decoder has received all dynamic
			table state necessary to process the header block. If the Largest
			Reference of an acknowledged header block was greater than the
			encoder's current Largest Known Received index, the block's Largest
			Reference becomes the new Largest Known Received.
			5.3.3. Stream Cancellation
			A stream that is reset might have multiple outstanding header blocks.
			A decoder that receives a stream reset before the end of a stream
			generates a Stream Cancellation instruction on the decoder stream.
			Similarly, a decoder that abandons reading of a stream needs to
			signal this using the Stream Cancellation instruction. This signals
			to the encoder that all references to the dynamic table on that
			stream are no longer outstanding.
			An encoder cannot infer from this instruction that any updates to the
			dynamic table have been received.
			The instruction begins with the '01' two-bit pattern. The
			instruction includes the stream ID of the affected stream - a request
			or push stream - encoded as a 6-bit prefix integer.
			0 1 2 3 4 5 6 7
			+---+---+---+---+---+---+---+---+
			| 0 | 1 | Stream ID (6+) |
			+---+---+-----------------------+
			Figure 5: Stream Cancellation
			5.4. Request and Push Streams
	HEADERS and PUSH_PROMISE frames on request and push streams reference		HEADERS and PUSH_PROMISE frames on request and push streams reference
	the dynamic table in a particular state without modifying it. Frames		the dynamic table in a particular state without modifying it. Frames
	on these streams emit the headers for an HTTP request or response.		on these streams emit the headers for an HTTP request or response.
	3.5.1. Header Data Prefix		5.4.1. Header Data Prefix
	Header data is prefixed with two integers, "Largest Reference" and		Header data is prefixed with two integers, "Largest Reference" and
	"Base Index".		"Base Index".
	0 1 2 3 4 5 6 7		0 1 2 3 4 5 6 7
	+---+---+---+---+---+---+---+---+		+---+---+---+---+---+---+---+---+
	| Largest Reference (8+) |		| Largest Reference (8+) |
	+---+---------------------------+		+---+---------------------------+
	| S | Delta Base Index (7+) |		| S | Delta Base Index (7+) |
	+---+---------------------------+		+---+---------------------------+
	| Compressed Headers ...		| Compressed Headers ...
	+-------------------------------+		+-------------------------------+
	Figure 5: Frame Payload		Figure 6: Frame Payload
	"Largest Reference" identifies the largest absolute dynamic index		"Largest Reference" identifies the largest absolute dynamic index
	referenced in the block. Blocking decoders use the Largest Reference		referenced in the block. Blocking decoders use the Largest Reference
	to determine when it is safe to process the rest of the block.		to determine when it is safe to process the rest of the block.
	"Base Index" is used to resolve references in the dynamic table as		"Base Index" is used to resolve references in the dynamic table as
	described in Section 3.2. To save space, Base Index is encoded		described in Section 2.2.1.
	relative to Largest Reference using a one-bit sign flag.
	baseIndex = largestReference + deltaBaseIndex		To save space, Base Index is encoded relative to Largest Reference
			using a one-bit sign and the "Delta Base Index" value. A sign bit of
			0 indicates that the Base Index has an absolute index that is greater
			than or equal to the Largest Reference; the value of Delta Base Index
			is added to the Largest Reference to determine the absolute value of
			the Base Index. A sign bit of 1 indicates that the Base Index is
			less than the Largest Reference. That is:
	If the encoder inserted entries to the table while the encoding the		if sign == 0:
	block, Largest Reference will be greater than Base Index, so		baseIndex = largestReference + deltaBaseIndex
	deltaBaseIndex will be negative and encoded with S=1. If the block		else:
	did not reference the most recent entry in the table and did not		baseIndex = largestReference - deltaBaseIndex
	insert any new entries, Largest Reference will be less than Base
	Index, so deltaBaseIndex will be positive and encoded with S=0. When
	Largest Reference and Base Index are equal, deltaBaseIndex is 0 and
	encoded with S=0.
	3.5.2. Instructions		A single-pass encoder is expected to determine the absolute value of
			Base Index before encoding a header block. If the encoder inserted
			entries in the dynamic table while encoding the header block, Largest
			Reference will be greater than Base Index, so the encoded difference
			is negative and the sign bit is set to 1. If the header block did
			not reference the most recent entry in the table and did not insert
			any new entries, Base Index will be greater than the Largest
			Reference, so the delta will be positive and the sign bit is set to
			0.
	3.5.2.1. Indexed Header Field		An encoder that produces table updates before encoding a header block
			might set Largest Reference and Base Index to the same value. When
			Largest Reference and Base Index are equal, the Delta Base Index is
			encoded with a zero sign bit. A sign bit set to 1 when the Delta
			Base Index is 0 MUST be treated as a decoder error.
			A header block that does not reference the dynamic table can use any
			value for Base Index; setting both Largest Reference and Base Index
			to zero is the most efficient encoding.
			5.4.2. Instructions
			5.4.2.1. Indexed Header Field
	An indexed header field representation identifies an entry in either		An indexed header field representation identifies an entry in either
	the static table or the dynamic table and causes that header field to		the static table or the dynamic table and causes that header field to
	be added to the decoded header list, as described in Section 3.2 of		be added to the decoded header list, as described in Section 3.2 of
	[RFC7541].		[RFC7541].
	0 1 2 3 4 5 6 7		0 1 2 3 4 5 6 7
	+---+---+---+---+---+---+---+---+		+---+---+---+---+---+---+---+---+
	| 1 | S | Index (6+) |		| 1 | S | Index (6+) |
	+---+---+-----------------------+		+---+---+-----------------------+
	skipping to change at page 13, line 20 ¶		skipping to change at page 17, line 5 ¶
	Indexed Header Field		Indexed Header Field
	If the entry is in the static table, or in the dynamic table with an		If the entry is in the static table, or in the dynamic table with an
	absolute index less than or equal to Base Index, this representation		absolute index less than or equal to Base Index, this representation
	starts with the '1' 1-bit pattern, followed by the "S" bit indicating		starts with the '1' 1-bit pattern, followed by the "S" bit indicating
	whether the reference is into the static (S=1) or dynamic (S=0)		whether the reference is into the static (S=1) or dynamic (S=0)
	table. Finally, the relative index of the matching header field is		table. Finally, the relative index of the matching header field is
	represented as an integer with a 6-bit prefix (see Section 5.1 of		represented as an integer with a 6-bit prefix (see Section 5.1 of
	[RFC7541]).		[RFC7541]).
	0 1 2 3 4 5 6 7		5.4.2.2. Indexed Header Field With Post-Base Index
	+---+---+---+---+---+---+---+---+
	| 0 | 1 | 0 | 0 | Index (4+) |
	+---+---+-----------------------+
	Indexed Header Field
	If the entry is in the dynamic table with an absolute index greater		If the entry is in the dynamic table with an absolute index greater
	than Base Index, the representation starts with the '0100' 4-bit		than Base Index, the representation starts with the '0001' 4-bit
	pattern, followed by the post-base index (see Section 3.2) of the		pattern, followed by the post-base index (see Section 2.2.1) of the
	matching header field, represented as an integer with a 4-bit prefix		matching header field, represented as an integer with a 4-bit prefix
	(see Section 5.1 of [RFC7541]).		(see Section 5.1 of [RFC7541]).
	3.5.2.2. Literal Header Field With Name Reference		0 1 2 3 4 5 6 7
			+---+---+---+---+---+---+---+---+
			| 0 | 0 | 0 | 1 | Index (4+) |
			+---+---+---+---+---------------+
			Indexed Header Field with Post-Base Index
			5.4.2.3. Literal Header Field With Name Reference
	A literal header field with a name reference represents a header		A literal header field with a name reference represents a header
	where the header field name matches the header field name of an entry		where the header field name matches the header field name of an entry
	stored in the static table or the dynamic table.		stored in the static table or the dynamic table.
	If the entry is in the static table, or in the dynamic table with an		If the entry is in the static table, or in the dynamic table with an
	absolute index less than or equal to Base Index, this representation		absolute index less than or equal to Base Index, this representation
	starts with the '00' two-bit pattern. If the entry is in the dynamic		starts with the '01' two-bit pattern. If the entry is in the dynamic
	table with an absolute index greater than Base Index, the		table with an absolute index greater than Base Index, the
	representation starts with the '0101' four-bit pattern.		representation starts with the '0000' four-bit pattern.
	The following bit, 'N', indicates whether an intermediary is		The following bit, 'N', indicates whether an intermediary is
	permitted to add this header to the dynamic header table on		permitted to add this header to the dynamic header table on
	subsequent hops. When the 'N' bit is set, the encoded header MUST		subsequent hops. When the 'N' bit is set, the encoded header MUST
	always be encoded with a literal representation. In particular, when		always be encoded with a literal representation. In particular, when
	a peer sends a header field that it received represented as a literal		a peer sends a header field that it received represented as a literal
	header field with the 'N' bit set, it MUST use a literal		header field with the 'N' bit set, it MUST use a literal
	representation to forward this header field. This bit is intended		representation to forward this header field. This bit is intended
	for protecting header field values that are not to be put at risk by		for protecting header field values that are not to be put at risk by
	compressing them (see Section 7.1 of [RFC7541] for more details).		compressing them (see Section 7.1 of [RFC7541] for more details).
	0 1 2 3 4 5 6 7		0 1 2 3 4 5 6 7
	+---+---+---+---+---+---+---+---+		+---+---+---+---+---+---+---+---+
	| 0 | 0 | N | S |Name Index (4+)|		| 0 | 1 | N | S |Name Index (4+)|
	+---+---+-----------------------+		+---+---+---+---+---------------+
	| H | Value Length (7+) |		| H | Value Length (7+) |
	+---+---------------------------+		+---+---------------------------+
	| Value String (Length octets) |		| Value String (Length octets) |
	+-------------------------------+		+-------------------------------+
	Literal Header Field With Name Reference		Literal Header Field With Name Reference
	For entries in the static table or in the dynamic table with an		For entries in the static table or in the dynamic table with an
	absolute index less than or equal to Base Index, the header field		absolute index less than or equal to Base Index, the header field
	name is represented using the relative index of that entry, which is		name is represented using the relative index of that entry, which is
	represented as an integer with a 4-bit prefix (see Section 5.1 of		represented as an integer with a 4-bit prefix (see Section 5.1 of
	[RFC7541]). The "S" bit indicates whether the reference is to the		[RFC7541]). The "S" bit indicates whether the reference is to the
	static (S=1) or dynamic (S=0) table.		static (S=1) or dynamic (S=0) table.
			5.4.2.4. Literal Header Field With Post-Base Name Reference
			For entries in the dynamic table with an absolute index greater than
			Base Index, the header field name is represented using the post-base
			index of that entry (see Section 2.2.1) encoded as an integer with a
			3-bit prefix.
	0 1 2 3 4 5 6 7		0 1 2 3 4 5 6 7
	+---+---+---+---+---+---+---+---+		+---+---+---+---+---+---+---+---+
	| 0 | 1 | 0 | 1 | N |NameIdx(3+)|		| 0 | 0 | 0 | 0 | N |NameIdx(3+)|
	+---+---+-----------------------+		+---+---+---+---+---+-----------+
	| H | Value Length (7+) |		| H | Value Length (7+) |
	+---+---------------------------+		+---+---------------------------+
	| Value String (Length octets) |		| Value String (Length octets) |
	+-------------------------------+		+-------------------------------+
	Literal Header Field With Post-Base Name Reference		Literal Header Field With Post-Base Name Reference
	For entries in the dynamic table with an absolute index greater than		5.4.2.5. Literal Header Field Without Name Reference
	Base Index, the header field name is represented using the post-base
	index of that entry (see Section 3.2) encoded as an integer with a
	3-bit prefix.
	3.5.2.3. Literal Header Field Without Name Reference
	An addition to the header table where both the header field name and		An addition to the header table where both the header field name and
	the header field value are represented as string literals (see		the header field value are represented as string literals (see
	Section 3.1) starts with the '011' three-bit pattern.		Section 5.1) starts with the '001' three-bit pattern.
	The fourth bit, 'N', indicates whether an intermediary is permitted		The fourth bit, 'N', indicates whether an intermediary is permitted
	to add this header to the dynamic header table on subsequent hops.		to add this header to the dynamic header table on subsequent hops.
	When the 'N' bit is set, the encoded header MUST always be encoded		When the 'N' bit is set, the encoded header MUST always be encoded
	with a literal representation. In particular, when a peer sends a		with a literal representation. In particular, when a peer sends a
	header field that it received represented as a literal header field		header field that it received represented as a literal header field
	with the 'N' bit set, it MUST use a literal representation to forward		with the 'N' bit set, it MUST use a literal representation to forward
	this header field. This bit is intended for protecting header field		this header field. This bit is intended for protecting header field
	values that are not to be put at risk by compressing them (see		values that are not to be put at risk by compressing them (see
	Section 7.1 of [RFC7541] for more details).		Section 7.1 of [RFC7541] for more details).
	The name is represented as a 4-bit prefix string literal, while the		The name is represented as a 4-bit prefix string literal, while the
	value is represented as an 8-bit prefix string literal.		value is represented as an 8-bit prefix string literal.
	0 1 2 3 4 5 6 7		0 1 2 3 4 5 6 7
	+---+---+---+---+---+---+---+---+		+---+---+---+---+---+---+---+---+
	| 0 | 1 | 1 | N | H |NameLen(3+)|		| 0 | 0 | 1 | N | H |NameLen(3+)|
	+---+---+---+-------------------+		+---+---+---+---+---+-----------+
	| Name String (Length octets) |		| Name String (Length octets) |
	+---+---------------------------+		+---+---------------------------+
	| H | Value Length (7+) |		| H | Value Length (7+) |
	+---+---------------------------+		+---+---------------------------+
	| Value String (Length octets) |		| Value String (Length octets) |
	+-------------------------------+		+-------------------------------+
	Literal Header Field Without Name Reference		Literal Header Field Without Name Reference
	4. Encoding Strategies		6. Encoding Strategies
	4.1. Single pass encoding		6.1. Single Pass Encoding
	An encoder making a single pass over a list of headers must choose		An encoder making a single pass over a list of headers must choose
	Base Index before knowing Largest Reference. When trying to		Base Index before knowing Largest Reference. When trying to
	reference a header inserted to the table after encoding has begun,		reference a header inserted to the table after encoding has begun,
	the entry is encoded with different instructions that tell the		the entry is encoded with different instructions that tell the
	decoder to use an absolute index greater than the Base Index.		decoder to use an absolute index greater than the Base Index.
	4.2. Preventing Eviction Races		6.2. Preventing Eviction Races
	Due to out-of-order arrival, QPACK's eviction algorithm requires		Due to out-of-order arrival, QPACK's eviction algorithm requires
	changes (relative to HPACK) to avoid the possibility that an indexed		changes (relative to HPACK) to avoid the possibility that an indexed
	representation is decoded after the referenced entry has already been		representation is decoded after the referenced entry has already been
	evicted. QPACK employs a two-phase eviction algorithm, in which the		evicted. QPACK employs a two-phase eviction algorithm, in which the
	encoder will not evict entries that have outstanding (unacknowledged)		encoder will not evict entries that have outstanding (unacknowledged)
	references.		references.
	4.3. Reference Tracking		6.3. Reference Tracking
	An encoder MUST ensure that a header block which references a dynamic		An encoder MUST ensure that a header block which references a dynamic
	table entry is not received by the decoder after the referenced entry		table entry is not received by the decoder after the referenced entry
	has already been evicted. An encoder also respects the limit set by		has already been evicted. An encoder also respects the limit set by
	the decoder on the number of streams that are allowed to become		the decoder on the number of streams that are allowed to become
	blocked. Even if the decoder is willing to tolerate blocked streams,		blocked. Even if the decoder is willing to tolerate blocked streams,
	the encoder might choose to avoid them in certain cases.		the encoder might choose to avoid them in certain cases.
	In order to enable this, the encoder will need to track outstanding		In order to enable this, the encoder will need to track outstanding
	(unacknowledged) header blocks and table updates using feedback		(unacknowledged) header blocks and table updates using feedback
	received from the decoder.		received from the decoder.
	4.3.1. Blocked Eviction		6.3.1. Blocked Eviction
	The encoder MUST NOT permit an entry to be evicted while a reference		The encoder MUST NOT permit an entry to be evicted while a reference
	to that entry remains unacknowledged. If a new header to be inserted		to that entry remains unacknowledged. If a new header to be inserted
	into the dynamic table would cause the eviction of such an entry, the		into the dynamic table would cause the eviction of such an entry, the
	encoder MUST NOT emit the insert instruction until the reference has		encoder MUST NOT emit the insert instruction until the reference has
	been processed by the decoder and acknowledged.		been processed by the decoder and acknowledged.
	The encoder can emit a literal representation for the new header in		The encoder can emit a literal representation for the new header in
	order to avoid encoding delays, and MAY insert the header into the		order to avoid encoding delays, and MAY insert the header into the
	table later if desired.		table later if desired.
	To ensure that the blocked eviction case is rare, references to the		To ensure that the blocked eviction case is rare, references to the
	oldest entries in the dynamic table SHOULD be avoided. When one of		oldest entries in the dynamic table SHOULD be avoided. When one of
	the oldest entries in the table is still actively used for		the oldest entries in the table is still actively used for
	references, the encoder SHOULD emit an Duplicate representation		references, the encoder SHOULD emit an Duplicate representation
	instead (see Section 3.3.3).		instead (see Section 5.2.3).
	4.3.2. Blocked Decoding		6.3.2. Blocked Decoding
	For header blocks encoded in non-blocking mode, the encoder needs to		For header blocks encoded in non-blocking mode, the encoder needs to
	forego indexed representations that refer to table updates which have		forego indexed representations that refer to table updates which have
	not yet been acknowledged with Section 3.4. Since all table updates		not yet been acknowledged with Section 5.3. Since all table updates
	are processed in sequence on the control stream, an index into the		are processed in sequence on the control stream, an index into the
	dynamic table is sufficient to track which entries have been		dynamic table is sufficient to track which entries have been
	acknowledged.		acknowledged.
	To track blocked streams, the necessary Base Index value for each		To track blocked streams, the necessary Base Index value for each
	stream can be used. Whenever the decoder processes a table update,		stream can be used. Whenever the decoder processes a table update,
	it can begin decoding any blocked streams that now have their		it can begin decoding any blocked streams that now have their
	dependencies satisfied.		dependencies satisfied.
	4.4. Speculative table updates		6.4. Speculative table updates
	Implementations can _speculatively_ send header frames on the HTTP		Implementations can _speculatively_ send header frames on the HTTP
	Control Streams which are not needed for any current HTTP request or		Control Streams which are not needed for any current HTTP request or
	response. Such headers could be used strategically to improve		response. Such headers could be used strategically to improve
	performance. For instance, the encoder might decide to _refresh_ by		performance. For instance, the encoder might decide to _refresh_ by
	sending Duplicate representations for popular header fields		sending Duplicate representations for popular header fields
	(Section 3.3.3), ensuring they have small indices and hence minimal		(Section 5.2.3), ensuring they have small indices and hence minimal
	size on the wire.		size on the wire.
	4.5. Sample One Pass Encoding Algorithm		6.5. Sample One Pass Encoding Algorithm
	Pseudo-code for single pass encoding, excluding handling of		Pseudo-code for single pass encoding, excluding handling of
	duplicates, non-blocking mode, and reference tracking.		duplicates, non-blocking mode, and reference tracking.
	baseIndex = dynamicTable.baseIndex		baseIndex = dynamicTable.baseIndex
	largestReference = 0		largestReference = 0
	for header in headers:		for header in headers:
	staticIdx = staticTable.getIndex(header)		staticIdx = staticTable.getIndex(header)
	if staticIdx:		if staticIdx:
	encodeIndexReference(streamBuffer, staticIdx)		encodeIndexReference(streamBuffer, staticIdx)
	skipping to change at page 17, line 48 ¶		skipping to change at page 21, line 43 ¶
	else:		else:
	# Dynamic index reference		# Dynamic index reference
	assert(dynamicIdx)		assert(dynamicIdx)
	largestReference = max(largestReference, dynamicIdx)		largestReference = max(largestReference, dynamicIdx)
	# Encode dynamicIdx, possibly with dynamicIdx above baseIndex		# Encode dynamicIdx, possibly with dynamicIdx above baseIndex
	encodeDynamicIndexReference(streamBuffer, dynamicIdx,		encodeDynamicIndexReference(streamBuffer, dynamicIdx,
	baseIndex)		baseIndex)
	# encode the prefix		# encode the prefix
	encodeInteger(prefixBuffer, 0x00, largestReference, 8)		encodeInteger(prefixBuffer, 0x00, largestReference, 8)
	delta = largestReference - baseIndex		if baseIndex >= largestReference:
	sign = delta > 0 ? 0x80 : 0		encodeInteger(prefixBuffer, 0, baseIndex - largestReference, 7)
	encodeInteger(prefixBuffer, sign, delta, 7)		else:
			encodeInteger(prefixBuffer, 0x80,
			largestReference - baseIndex, 7)
	return controlBuffer, prefixBuffer + streamBuffer		return controlBuffer, prefixBuffer + streamBuffer
	5. Security Considerations		7. Security Considerations
	TBD.		TBD.
	6. IANA Considerations		8. IANA Considerations
	None.		8.1. Settings Registration
	7. References		This document creates two new settings in the "HTTP/QUIC Settings"
			registry established in [QUIC-HTTP].
	7.1. Normative References		The entries in the following table are registered by this document.
			+-----------------------+------+---------------+
			| Setting Name | Code | Specification |
			+-----------------------+------+---------------+
			| HEADER_TABLE_SIZE | 0x1 | Section 4 |
			| | | |
			| QPACK_BLOCKED_STREAMS | 0x7 | Section 4 |
			+-----------------------+------+---------------+
			8.2. Stream Type Registration
			This document creates two new settings in the "HTTP/QUIC Stream Type"
			registry established in [QUIC-HTTP].
			The entries in the following table are registered by this document.
			+----------------------+------+---------------+--------+
			| Stream Type | Code | Specification | Sender |
			+----------------------+------+---------------+--------+
			| QPACK Encoder Stream | 0x48 | Section 5 | Both |
			| | | | |
			| QPACK Decoder Stream | 0x68 | Section 5 | Both |
			+----------------------+------+---------------+--------+
			9. References
			9.1. Normative References
	[QUIC-HTTP]		[QUIC-HTTP]
	Bishop, M., "Hypertext Transfer Protocol (HTTP) over		Bishop, M., Ed., "Hypertext Transfer Protocol (HTTP) over
	QUIC", draft-ietf-quic-http-12 (work in progress), April		QUIC", draft-ietf-quic-http-13 (work in progress), June
	2018.		2018.
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119,		Requirement Levels", BCP 14, RFC 2119,
	DOI 10.17487/RFC2119, March 1997,		DOI 10.17487/RFC2119, March 1997,
	<https://www.rfc-editor.org/info/rfc2119>.		<https://www.rfc-editor.org/info/rfc2119>.
	[RFC7541] Peon, R. and H. Ruellan, "HPACK: Header Compression for		[RFC7541] Peon, R. and H. Ruellan, "HPACK: Header Compression for
	HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,		HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
	<https://www.rfc-editor.org/info/rfc7541>.		<https://www.rfc-editor.org/info/rfc7541>.
	[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC		[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
	2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,		2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
	May 2017, <https://www.rfc-editor.org/info/rfc8174>.		May 2017, <https://www.rfc-editor.org/info/rfc8174>.
	7.2. Informative References		9.2. Informative References
	[QUIC-TRANSPORT]		[QUIC-TRANSPORT]
	Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed		Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
	and Secure Transport", draft-ietf-quic-transport-11 (work		and Secure Transport", draft-ietf-quic-transport-12 (work
	in progress), April 2018.		in progress), May 2018.
	[RFC2360] Scott, G., "Guide for Internet Standards Writers", BCP 22,		[RFC2360] Scott, G., "Guide for Internet Standards Writers", BCP 22,
	RFC 2360, DOI 10.17487/RFC2360, June 1998,		RFC 2360, DOI 10.17487/RFC2360, June 1998,
	<https://www.rfc-editor.org/info/rfc2360>.		<https://www.rfc-editor.org/info/rfc2360>.
	[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext		[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
	Transfer Protocol Version 2 (HTTP/2)", RFC 7540,		Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
	DOI 10.17487/RFC7540, May 2015,		DOI 10.17487/RFC7540, May 2015,
	<https://www.rfc-editor.org/info/rfc7540>.		<https://www.rfc-editor.org/info/rfc7540>.
	7.3. URIs		9.3. URIs
	[1] https://mailarchive.ietf.org/arch/search/?email_list=quic		[1] https://mailarchive.ietf.org/arch/search/?email_list=quic
	[2] https://github.com/quicwg		[2] https://github.com/quicwg
	[3] https://github.com/quicwg/base-drafts/labels/-qpack		[3] https://github.com/quicwg/base-drafts/labels/-qpack
	Acknowledgments		Appendix A. Change Log
	This draft draws heavily on the text of [RFC7541]. The indirect
	input of those authors is gratefully acknowledged, as well as ideas
	from:
	o Ryan Hamilton
	o Patrick McManus		*RFC Editor's Note:* Please remove this section prior to
			publication of a final version of this document.
	o Kazuho Oku		A.1. Since draft-ietf-quic-qpack-00
	o Biren Roy		o Renumbered instructions for consistency (#1471, #1472)
	o Ian Swett		o Decoder is allowed to validate largest reference (#1404, #1469)
			o Header block acknowledgments also acknowledge the associated
			largest reference (#1370, #1400)
	o Dmitri Tikhonov		o Added an acknowledgment for unread streams (#1371, #1400)
	Change Log		o Removed framing from encoder stream (#1361,#1467)
	*RFC Editor's Note:* Please remove this section prior to		o Control streams use typed unidirectional streams rather than fixed
	publication of a final version of this document.		stream IDs (#910,#1359)
	B.1. Since draft-ietf-quic-qcram-00		A.2. Since draft-ietf-quic-qcram-00
	o Separate instruction sets for table updates and header blocks		o Separate instruction sets for table updates and header blocks
	(#1235, #1142, #1141)		(#1235, #1142, #1141)
	o Reworked indexing scheme (#1176, #1145, #1136, #1130, #1125,		o Reworked indexing scheme (#1176, #1145, #1136, #1130, #1125,
	#1314)		#1314)
	o Added mechanisms that support one-pass encoding (#1138, #1320)		o Added mechanisms that support one-pass encoding (#1138, #1320)
	o Added a setting to control the number of blocked decoders (#238,		o Added a setting to control the number of blocked decoders (#238,
	#1140, #1143)		#1140, #1143)
	o Moved table updates and acknowledgments to dedicated streams		o Moved table updates and acknowledgments to dedicated streams
	(#1121, #1122, #1238)		(#1121, #1122, #1238)
			Acknowledgments
			This draft draws heavily on the text of [RFC7541]. The indirect
			input of those authors is gratefully acknowledged, as well as ideas
			from:
			o Ryan Hamilton
			o Patrick McManus
			o Kazuho Oku
			o Biren Roy
			o Ian Swett
			o Dmitri Tikhonov
			Buck's contribution was supported by Google during his employment
			there.
			A substantial portion of Mike's contribution was supported by
			Microsoft during his employment there.
	Authors' Addresses		Authors' Addresses
	Charles 'Buck' Krasic		Charles 'Buck' Krasic
	Google, Inc		Netflix
	Email: ckrasic@google.com		Email: ckrasic@netflix.com
	Mike Bishop		Mike Bishop
	Akamai Technologies		Akamai Technologies
	Email: mbishop@evequefou.be		Email: mbishop@evequefou.be
	Alan Frindell (editor)		Alan Frindell (editor)
	Facebook		Facebook
	Email: afrind@fb.com		Email: afrind@fb.com
End of changes. 95 change blocks.
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