draft-ietf-quic-qpack-03.txt		draft-ietf-quic-qpack-04.txt
	QUIC C. Krasic		QUIC C. Krasic
	Internet-Draft Netflix		Internet-Draft Netflix
	Intended status: Standards Track M. Bishop		Intended status: Standards Track M. Bishop
	Expires: April 6, 2019 Akamai Technologies		Expires: June 7, 2019 Akamai Technologies
	A. Frindell, Ed.		A. Frindell, Ed.
	Facebook		Facebook
	October 03, 2018		December 04, 2018
	QPACK: Header Compression for HTTP over QUIC		QPACK: Header Compression for HTTP over QUIC
	draft-ietf-quic-qpack-03		draft-ietf-quic-qpack-04
	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/QUIC.		efficiently representing HTTP header fields, to be used in HTTP/3.
	This is a variation of HPACK header compression that seeks to reduce		This is a variation of HPACK header compression that seeks to reduce
	head-of-line blocking.		head-of-line blocking.
	Note to Readers		Note to Readers
	Discussion of this draft takes place on the QUIC working group		Discussion of this draft takes place on the QUIC working group
	mailing list (quic@ietf.org), which is archived at		mailing list (quic@ietf.org), which is archived at
	https://mailarchive.ietf.org/arch/search/?email_list=quic [1].		https://mailarchive.ietf.org/arch/search/?email_list=quic [1].
	Working Group information can be found at https://github.com/quicwg		Working Group information can be found at https://github.com/quicwg
	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 April 6, 2019.		This Internet-Draft will expire on June 7, 2019.
	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
	publication of this document. Please review these documents		publication of this document. Please review these documents
	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
	2. Header Tables . . . . . . . . . . . . . . . . . . . . . . . . 4		1.1. Conventions and Definitions . . . . . . . . . . . . . . . 4
	2.1. Static Table . . . . . . . . . . . . . . . . . . . . . . 4		1.2. Notational Conventions . . . . . . . . . . . . . . . . . 4
	2.2. Dynamic Table . . . . . . . . . . . . . . . . . . . . . . 4		2. Compression Process Overview . . . . . . . . . . . . . . . . 5
	2.2.1. Maximum Table Size . . . . . . . . . . . . . . . . . 5		2.1. Encoder . . . . . . . . . . . . . . . . . . . . . . . . . 5
	2.2.2. Calculating Table Size . . . . . . . . . . . . . . . 6		2.1.1. Reference Tracking . . . . . . . . . . . . . . . . . 6
	2.2.3. Absolute Indexing . . . . . . . . . . . . . . . . . . 6		2.1.2. Blocked Dynamic Table Insertions . . . . . . . . . . 6
	2.2.4. Relative Indexing . . . . . . . . . . . . . . . . . . 6		2.1.3. Avoiding Head-of-Line Blocking . . . . . . . . . . . 7
	2.2.5. Post-Base Indexing . . . . . . . . . . . . . . . . . 7		2.1.4. Largest Known Received . . . . . . . . . . . . . . . 7
	2.3. Avoiding Head-of-Line Blocking in HTTP/QUIC . . . . . . . 8		2.2. Decoder . . . . . . . . . . . . . . . . . . . . . . . . . 8
	2.3.1. State Synchronization . . . . . . . . . . . . . . . . 9		2.2.1. State Synchronization . . . . . . . . . . . . . . . . 8
	3. Conventions and Definitions . . . . . . . . . . . . . . . . . 10		2.2.2. Blocked Decoding . . . . . . . . . . . . . . . . . . 9
	3.1. Notational Conventions . . . . . . . . . . . . . . . . . 10		3. Header Tables . . . . . . . . . . . . . . . . . . . . . . . . 9
	4. Configuration . . . . . . . . . . . . . . . . . . . . . . . . 10		3.1. Static Table . . . . . . . . . . . . . . . . . . . . . . 9
	5. Wire Format . . . . . . . . . . . . . . . . . . . . . . . . . 11		3.2. Dynamic Table . . . . . . . . . . . . . . . . . . . . . . 9
	5.1. Primitives . . . . . . . . . . . . . . . . . . . . . . . 11		3.2.1. Maximum Table Size . . . . . . . . . . . . . . . . . 10
	5.1.1. Prefixed Integers . . . . . . . . . . . . . . . . . . 11		3.2.2. Calculating Table Size . . . . . . . . . . . . . . . 10
	5.1.2. String Literals . . . . . . . . . . . . . . . . . . . 11		3.2.3. Absolute Indexing . . . . . . . . . . . . . . . . . . 11
	5.2. QPACK Encoder Stream . . . . . . . . . . . . . . . . . . 12		3.2.4. Relative Indexing . . . . . . . . . . . . . . . . . . 11
	5.2.1. Insert With Name Reference . . . . . . . . . . . . . 12		3.2.5. Post-Base Indexing . . . . . . . . . . . . . . . . . 12
	5.2.2. Insert Without Name Reference . . . . . . . . . . . . 13		3.2.6. Invalid References . . . . . . . . . . . . . . . . . 12
	5.2.3. Duplicate . . . . . . . . . . . . . . . . . . . . . . 13		4. Wire Format . . . . . . . . . . . . . . . . . . . . . . . . . 13
	5.2.4. Dynamic Table Size Update . . . . . . . . . . . . . . 14		4.1. Primitives . . . . . . . . . . . . . . . . . . . . . . . 13
	5.3. QPACK Decoder Stream . . . . . . . . . . . . . . . . . . 14		4.1.1. Prefixed Integers . . . . . . . . . . . . . . . . . . 13
	5.3.1. Table State Synchronize . . . . . . . . . . . . . . . 14		4.1.2. String Literals . . . . . . . . . . . . . . . . . . . 13
	5.3.2. Header Acknowledgement . . . . . . . . . . . . . . . 15		4.2. Stream Types . . . . . . . . . . . . . . . . . . . . . . 13
	5.3.3. Stream Cancellation . . . . . . . . . . . . . . . . . 16		4.3. Encoder Stream . . . . . . . . . . . . . . . . . . . . . 14
	5.4. Request and Push Streams . . . . . . . . . . . . . . . . 17		4.3.1. Insert With Name Reference . . . . . . . . . . . . . 14
	5.4.1. Header Data Prefix . . . . . . . . . . . . . . . . . 17		4.3.2. Insert Without Name Reference . . . . . . . . . . . . 15
	5.4.2. Instructions . . . . . . . . . . . . . . . . . . . . 18		4.3.3. Duplicate . . . . . . . . . . . . . . . . . . . . . . 15
	6. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 21		4.3.4. Dynamic Table Size Update . . . . . . . . . . . . . . 16
	7. Encoding Strategies . . . . . . . . . . . . . . . . . . . . . 22
	7.1. Single Pass Encoding . . . . . . . . . . . . . . . . . . 22		4.4. Decoder Stream . . . . . . . . . . . . . . . . . . . . . 16
	7.2. Preventing Eviction Races . . . . . . . . . . . . . . . . 22		4.4.1. Table State Synchronize . . . . . . . . . . . . . . . 16
	7.3. Reference Tracking . . . . . . . . . . . . . . . . . . . 22		4.4.2. Header Acknowledgement . . . . . . . . . . . . . . . 17
	7.3.1. Blocked Dynamic Table Insertions . . . . . . . . . . 22		4.4.3. Stream Cancellation . . . . . . . . . . . . . . . . . 18
	7.3.2. Blocked Decoding . . . . . . . . . . . . . . . . . . 23		4.5. Request and Push Streams . . . . . . . . . . . . . . . . 18
	7.4. Speculative table updates . . . . . . . . . . . . . . . . 23		4.5.1. Header Data Prefix . . . . . . . . . . . . . . . . . 18
	7.5. Sample One Pass Encoding Algorithm . . . . . . . . . . . 24		4.5.2. Indexed Header Field . . . . . . . . . . . . . . . . 20
	8. Security Considerations . . . . . . . . . . . . . . . . . . . 26		4.5.3. Indexed Header Field With Post-Base Index . . . . . . 21
	9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26		4.5.4. Literal Header Field With Name Reference . . . . . . 21
	9.1. Settings Registration . . . . . . . . . . . . . . . . . . 26		4.5.5. Literal Header Field With Post-Base Name Reference . 22
	9.2. Stream Type Registration . . . . . . . . . . . . . . . . 26		4.5.6. Literal Header Field Without Name Reference . . . . . 22
	9.3. Error Code Registration . . . . . . . . . . . . . . . . . 26		5. Configuration . . . . . . . . . . . . . . . . . . . . . . . . 23
	10. References . . . . . . . . . . . . . . . . . . . . . . . . . 27		6. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 23
	10.1. Normative References . . . . . . . . . . . . . . . . . . 27		7. Security Considerations . . . . . . . . . . . . . . . . . . . 24
	10.2. Informative References . . . . . . . . . . . . . . . . . 28		8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
	10.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 28		8.1. Settings Registration . . . . . . . . . . . . . . . . . . 24
	Appendix A. Static Table . . . . . . . . . . . . . . . . . . . . 28		8.2. Stream Type Registration . . . . . . . . . . . . . . . . 24
	Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 33		8.3. Error Code Registration . . . . . . . . . . . . . . . . . 24
	B.1. Since draft-ietf-quic-qpack-02 . . . . . . . . . . . . . 33		9. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
	B.2. Since draft-ietf-quic-qpack-01 . . . . . . . . . . . . . 33		9.1. Normative References . . . . . . . . . . . . . . . . . . 25
	B.3. Since draft-ietf-quic-qpack-00 . . . . . . . . . . . . . 33		9.2. Informative References . . . . . . . . . . . . . . . . . 26
	B.4. Since draft-ietf-quic-qcram-00 . . . . . . . . . . . . . 34		9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 26
			Appendix A. Static Table . . . . . . . . . . . . . . . . . . . . 26
			Appendix B. Sample One Pass Encoding Algorithm . . . . . . . . . 31
			Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 33
			C.1. Since draft-ietf-quic-qpack-03 . . . . . . . . . . . . . 33
			C.2. Since draft-ietf-quic-qpack-02 . . . . . . . . . . . . . 33
			C.3. Since draft-ietf-quic-qpack-01 . . . . . . . . . . . . . 33
			C.4. Since draft-ietf-quic-qpack-00 . . . . . . . . . . . . . 33
			C.5. Since draft-ietf-quic-qcram-00 . . . . . . . . . . . . . 34
	Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 34		Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 34
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34
	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. HTTP/2 uses HPACK ([RFC7541]) for header compression, but		HTTP/2. HTTP/2 uses HPACK ([RFC7541]) for header compression, but
	QUIC's stream multiplexing comes into some conflict with HPACK. A		QUIC's stream multiplexing comes into some conflict with HPACK. A
	key goal of the design of QUIC is to improve stream multiplexing		key goal of the design of QUIC is to improve stream multiplexing
	relative to HTTP/2 by reducing head-of-line blocking. If HPACK were		relative to HTTP/2 by reducing head-of-line blocking. If HPACK were
	used for HTTP/QUIC, it would induce head-of-line blocking due to		used for HTTP/3, it would induce head-of-line blocking due to built-
	built-in assumptions of a total ordering across frames on all		in assumptions of a total ordering across frames on all streams.
	streams.
	QUIC is described in [QUIC-TRANSPORT]. The HTTP/QUIC mapping is		QUIC is described in [QUIC-TRANSPORT]. The HTTP/3 mapping is
	described in [QUIC-HTTP]. For a full description of HTTP/2, see		described in [HTTP3]. For a full description of HTTP/2, see
	[RFC7540]. The description of HPACK is [RFC7541].		[RFC7540]. The description of HPACK is [RFC7541].
	QPACK reuses core concepts from HPACK, but is redesigned to allow		QPACK reuses core concepts from HPACK, but is redesigned to allow
	correctness in the presence of out-of-order delivery, with		correctness in the presence of out-of-order delivery, with
	flexibility for implementations to balance between resilience against		flexibility for implementations to balance between resilience against
	head-of-line blocking and optimal compression ratio. The design		head-of-line blocking and optimal compression ratio. The design
	goals are to closely approach the compression ratio of HPACK with		goals are to closely approach the compression ratio of HPACK with
	substantially less head-of-line blocking under the same loss		substantially less head-of-line blocking under the same loss
	conditions.		conditions.
	QPACK preserves the ordering of header fields within each header		1.1. Conventions and Definitions
	list. An encoder MUST emit header field representations in the order
	they appear in the input header list. A decoder MUST must emit
	header fields in the order their representations appear in the input
	header block.
	2. Header Tables		The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
			"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
			"OPTIONAL" in this document are to be interpreted as described in BCP
			14 [RFC2119] [RFC8174] when, and only when, they appear in all
			capitals, as shown here.
			Definitions of terms that are used in this document:
			Header field: A name-value pair sent as part of an HTTP message.
			Header list: An ordered collection of header fields associated with
			an HTTP message. A header list can contain multiple header fields
			with the same name. It can also contain duplicate header fields.
			Header block: The compressed representation of a header list.
			Encoder: An implementation which transforms a header list into a
			header block.
			Decoder: An implementation which transforms a header block into a
			header list.
			Absolute Index: A unique index for each entry in the dynamic table.
			Base Index: An absolute index in a header block from which relative
			indices are made.
			Largest Reference: The largest absolute index of an entry referenced
			in a header block.
			QPACK is a name, not an acronym.
			1.2. Notational Conventions
			Diagrams use the format described in Section 3.1 of [RFC2360], with
			the following additional conventions:
			x (A) Indicates that x is A bits long
			x (A+) Indicates that x uses the prefixed integer encoding defined
			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
			the region.
			2. Compression Process Overview
	Like HPACK, QPACK uses two tables for associating header fields to		Like HPACK, QPACK uses two tables for associating header fields to
	indices. The static table (see Section 2.1) is predefined and		indices. The static table (see Section 3.1) is predefined and
	contains common header fields (some of them with an empty value).		contains common header fields (some of them with an empty value).
	The dynamic table (see Section 2.2) is built up over the course of		The dynamic table (see Section 3.2) is built up over the course of
	the connection and can be used by the encoder to index header fields		the connection and can be used by the encoder to index header fields
	repeated in the encoded header lists.		in the encoded header lists.
			QPACK instructions appear in three different types of streams:
			o The encoder uses a unidirectional stream to modify the state of
			the dynamic table without emitting header fields associated with
			any particular request.
			o HEADERS and PUSH_PROMISE frames on request and push streams
			reference the table state without modifying it.
			o The decoder sends feedback to the encoder on a unidirectional
			stream. This feedback enables the encoder to manage dynamic table
			state.
			2.1. Encoder
			An encoder compresses a header list by emitting either an indexed or
			a literal representation for each header field in the list.
			References to the static table and literal representations do not
			require any dynamic state and never risk head-of-line blocking.
			References to the dynamic table risk head-of-line blocking if the
			encoder has not received an acknowledgement indicating the entry is
			available at the decoder.
			An encoder MAY insert any entry in the dynamic table it chooses; it
			is not limited to header fields it is compressing.
			QPACK preserves the ordering of header fields within each header
			list. An encoder MUST emit header field representations in the order
			they appear in the input header list.
			QPACK is designed to contain the more complex state tracking to the
			encoder, while the decoder is relatively simple.
			2.1.1. Reference Tracking
			An encoder MUST ensure that a header block which references a dynamic
			table entry is not received by the decoder after the referenced entry
			has been evicted. Hence the encoder needs to track information about
			each compressed header block that references the dynamic table until
			that header block is acknowledged by the decoder.
			2.1.2. Blocked Dynamic Table Insertions
			An encoder MUST NOT insert an entry into the dynamic table (or
			duplicate an existing entry) if doing so would evict an entry with
			unacknowledged references. For header blocks that might rely on the
			newly added entry, the encoder can use a literal representation and
			maybe insert the entry later.
			To ensure that the encoder is not prevented from adding new entries,
			the encoder can avoid referencing entries that are close to eviction.
			Rather than reference such an entry, the encoder can emit a Duplicate
			instruction (see Section 4.3.3), and reference the duplicate instead.
			Determining which entries are too close to eviction to reference is
			an encoder preference. One heuristic is to target a fixed amount of
			available space in the dynamic table: either unused space or space
			that can be reclaimed by evicting unreferenced entries. To achieve
			this, the encoder can maintain a draining index, which is the
			smallest absolute index in the dynamic table that it will emit a
			reference for. As new entries are inserted, the encoder increases
			the draining index to maintain the section of the table that it will
			not reference. If the encoder does not create new references to
			entries with an absolute index lower than the draining index, the
			number of unacknowledged references to those entries will eventually
			become zero, allowing them to be evicted.
			+----------+---------------------------------+--------+
			| Draining | Referenceable | Unused |
			| Entries | Entries | Space |
			+----------+---------------------------------+--------+
			^ ^ ^
			| | |
			Dropping Draining Index Base Index /
			Point Insertion Point
			Figure 1: Draining Dynamic Table Entries
			2.1.3. Avoiding Head-of-Line Blocking
			Because QUIC does not guarantee order between data on different
			streams, a header block might reference an entry in the dynamic table
			that has not yet been received.
			Each header block contains a Largest Reference (Section 4.5.1) which
			identifies the table state necessary for decoding. If the greatest
			absolute index in the dynamic table is less than the value of the
			Largest Reference, the stream is considered "blocked." While
			blocked, header field data SHOULD remain in the blocked stream's flow
			control window. When the Largest Reference is zero, the frame
			contains no references to the dynamic table and can always be
			processed immediately. A stream becomes unblocked when the greatest
			absolute index in the dynamic table becomes greater than or equal to
			the Largest Reference for all header blocks the decoder has started
			reading from the stream. If the 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.
			The SETTINGS_QPACK_BLOCKED_STREAMS setting (see Section 5) specifies
			an upper bound on the number of streams which can be blocked. An
			encoder MUST limit the number of streams which could become blocked
			to the value of SETTINGS_QPACK_BLOCKED_STREAMS at all times. Note
			that the decoder might not actually become blocked on every stream
			which risks becoming blocked. If the decoder encounters more blocked
			streams than it promised to support, it MUST treat this as a stream
			error of type HTTP_QPACK_DECOMPRESSION_FAILED.
			An encoder can decide whether to risk having a stream become blocked.
			If permitted by the value of SETTINGS_QPACK_BLOCKED_STREAMS,
			compression efficiency can often be improved by referencing dynamic
			table 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 risk of blocking by only referencing dynamic table entries
			which have been acknowledged, but this could mean using literals.
			Since literals make the header block larger, this can result in the
			encoder becoming blocked on congestion or flow control limits.
			2.1.4. Largest Known Received
			In order 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 header block
			acknowledgement to identify the Largest Known Received index, as
			described in Section 4.4.2.
			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 4.4.1).
			2.2. Decoder
			As in HPACK, the decoder processes header blocks and emits the
			corresponding header lists. It also processes dynamic table
			modifications from instructions on the encoder stream.
			The decoder MUST emit header fields in the order their
			representations appear in the input header block.
			2.2.1. State Synchronization
			The decoder stream (Section 4.4) signals key events at the decoder
			that permit the encoder to track the decoder's state. These events
			are:
			o Complete processing of a header block
			o Abandonment of a stream which might have remaining header blocks
			o Receipt of new dynamic table entries
			Knowledge that a header block with references to the dynamic table
			has been processed permits the encoder to evict entries to which no
			unacknowledged references remain, regardless of whether those
			references were potentially blocking (see Section 2.1.2). When a
			stream is reset or abandoned, the indication that these header blocks
			will never be processed serves a similar function; see Section 4.4.3.
			The decoder chooses when to emit Table State Synchronize instructions
			(see Section 4.4.1). Emitting an instruction 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 instruction, the decoder might be
			able to coalesce multiple Table State Synchronize instructions, or
			replace them entirely with Header Acknowledgements (see
			Section 4.4.2). However, delaying too long may lead to compression
			inefficiencies if the encoder waits for an entry to be acknowledged
			before using it.
			2.2.2. Blocked Decoding
			To track blocked streams, the necessary Largest Reference value for
			each stream can be used. Whenever the decoder processes a table
			update, it can begin decoding any blocked streams that now have their
			dependencies satisfied.
			3. Header Tables
	Unlike in HPACK, entries in the QPACK static and dynamic tables are		Unlike in HPACK, entries in the QPACK static and dynamic tables are
	addressed separately. The following sections describe how entries in		addressed separately. The following sections describe how entries in
	each table are addressed.		each table are addressed.
	2.1. Static Table		3.1. Static Table
	The static table consists of a predefined static list of header		The static table consists of a predefined static list of header
	fields, each of which has a fixed index over time. Its entries are		fields, each of which has a fixed index over time. Its entries are
	defined in Appendix A.		defined in Appendix A.
	A decoder that encounters an invalid static table index on a request		Note the QPACK static table is indexed from 0, whereas the HPACK
	stream or push stream MUST treat this as a stream error of type		static table is indexed from 1.
	"HTTP_QPACK_DECOMPRESSION_FAILED". If this index is received on the
	encoder stream, this MUST be treated as a connection error of type
	"HTTP_QPACK_ENCODER_STREAM_ERROR".
	2.2. Dynamic Table		When the decoder encounters an invalid static table index on a
			request stream or push stream it MUST treat this as a stream error of
			type "HTTP_QPACK_DECOMPRESSION_FAILED". If this index is received on
			the encoder stream, this MUST be treated as a connection error of
			type "HTTP_QPACK_ENCODER_STREAM_ERROR".
			3.2. Dynamic Table
	The dynamic table consists of a list of header fields maintained in		The dynamic table consists of a list of header fields maintained in
	first-in, first-out order. The dynamic table is initially empty.		first-in, first-out order. The dynamic table is initially empty.
	Entries are added by instructions on the encoder stream (see		Entries are added by instructions on the encoder stream (see
	Section 5.2).		Section 4.3).
	The maximum size of the dynamic table can be modified by the encoder,		The maximum size of the dynamic table can be modified by the encoder,
	subject to a decoder-controlled limit (see Section 4 and		subject to a decoder-controlled limit (see Section 5 and
	Section 5.2.4). The initial maximum size is determined by the		Section 4.3.4). The initial maximum size is determined by the
	corresponding setting when HTTP requests or responses are first		corresponding setting when HTTP requests or responses are first
	permitted to be sent. For clients using 0-RTT data in HTTP/QUIC, the		permitted to be sent. For clients using 0-RTT data in HTTP/3, the
	table size is the remembered value of the setting, even if the server		table size is the remembered value of the setting, even if the server
	later specifies a larger maximum in its SETTINGS frame. For HTTP/		later specifies a larger maximum in its SETTINGS frame. For HTTP/3
	QUIC servers and HTTP/QUIC clients when 0-RTT is not attempted or is		servers and HTTP/3 clients when 0-RTT is not attempted or is
	rejected, the initial maximum table size is the value of the setting		rejected, the initial maximum table size is the value of the setting
	in the peer's SETTINGS frame.		in the peer's SETTINGS frame.
	Before a new entry is added to the dynamic table, entries are evicted		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		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		is less than or equal to (maximum size - new entry size) or until the
	table is empty. The encoder MUST NOT evict a dynamic table entry		table is empty. The encoder MUST NOT evict a dynamic table entry
	unless it has first been acknowledged by the decoder.		unless it has first been acknowledged by the decoder.
	If the size of the new entry is less than or equal to the maximum		If the size of the new entry is less than or equal to the maximum
	skipping to change at page 5, line 28 ¶		skipping to change at page 10, line 22 ¶
	"HTTP_QPACK_ENCODER_STREAM_ERROR".		"HTTP_QPACK_ENCODER_STREAM_ERROR".
	A new entry can reference an entry in the dynamic table that will be		A new entry can reference an entry in the dynamic table that will be
	evicted when adding this new entry into the dynamic table.		evicted when adding this new entry into the dynamic table.
	Implementations are cautioned to avoid deleting the referenced name		Implementations are cautioned to avoid deleting the referenced name
	if the referenced entry is evicted from the dynamic table prior to		if the referenced entry is evicted from the dynamic table prior to
	inserting the new entry.		inserting the new entry.
	The dynamic table can contain duplicate entries (i.e., entries with		The dynamic table can contain duplicate entries (i.e., entries with
	the same name and same value). Therefore, duplicate entries MUST NOT		the same name and same value). Therefore, duplicate entries MUST NOT
	be treated as an error by a decoder.		be treated as an error by the decoder.
	2.2.1. Maximum Table Size		3.2.1. Maximum Table Size
	The encoder decides how to update the dynamic table and as such can		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		control how much memory is used by the dynamic table. To limit the
	memory requirements of the decoder, the dynamic table size is		memory requirements of the decoder, the dynamic table size is
	strictly bounded.		strictly bounded.
	The decoder determines the maximum size that the encoder is permitted		The decoder determines the maximum size that the encoder is permitted
	to use for the dynamic table. In HTTP/QUIC, this value is determined		to use for the dynamic table. In HTTP/3, this value is determined by
	by the SETTINGS_HEADER_TABLE_SIZE setting (see Section 4).		the SETTINGS_HEADER_TABLE_SIZE setting (see Section 5).
	An encoder can choose to use less capacity than this maximum size		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		(see Section 4.3.4), but the chosen size MUST stay lower than or
	equal to the maximum set by the decoder. Whenever the maximum size		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		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		the dynamic table until the size of the dynamic table is less than or
	equal to the maximum size.		equal to the maximum size.
	This mechanism can be used to completely clear entries from the		This mechanism can be used to completely clear entries from the
	dynamic table by setting a maximum size of 0, which can subsequently		dynamic table by setting a maximum size of 0, which can subsequently
	be restored.		be restored.
	2.2.2. Calculating Table Size		3.2.2. Calculating Table Size
	The size of the dynamic table is the sum of the size of its entries.		The size of the dynamic table is the sum of the size of its entries.
	The size of an entry is the sum of its name's length in octets (as		The size of an entry is the sum of its name's length in bytes (as
	defined in Section 5.1.2), its value's length in octets, and 32.		defined in Section 4.1.2), its value's length in bytes, and 32.
	The size of an entry is calculated using the length of its name and		The size of an entry is calculated using the length of its name and
	value without any Huffman encoding applied.		value without any Huffman encoding applied.
	"MaxEntries" is the maximum number of entries that the dynamic table		"MaxEntries" is the maximum number of entries that the dynamic table
	can have. The smallest entry has empty name and value strings and		can have. The smallest entry has empty name and value strings and
	has the size of 32. The MaxEntries is calculated as		has the size of 32. The MaxEntries is calculated as
	MaxEntries = floor( MaxTableSize / 32 )		MaxEntries = floor( MaxTableSize / 32 )
	MaxTableSize is the maximum size of the dynamic table as specified by		MaxTableSize is the maximum size of the dynamic table as specified by
	the decoder (see Section 2.2.1).		the decoder (see Section 3.2.1).
	2.2.3. Absolute Indexing		3.2.3. Absolute Indexing
	Each entry possesses both an absolute index which is fixed for the		Each entry possesses both an absolute index which is fixed for the
	lifetime of that entry and a relative index which changes over time		lifetime of that entry and a relative index which changes based on
	based on the context of the reference. The first entry inserted has		the context of the reference. The first entry inserted has an
	an absolute index of "1"; indices increase sequentially with each		absolute index of "1"; indices increase sequentially with each
	insertion.		insertion.
	2.2.4. Relative Indexing		3.2.4. Relative Indexing
	The relative index begins at zero and increases in the opposite		The relative index begins at zero and increases in the opposite
	direction from the absolute index. Determining which entry has a		direction from the absolute index. Determining which entry has a
	relative index of "0" depends on the context of the reference.		relative index of "0" depends on the context of the reference.
	On the encoder stream, a relative index of "0" always refers to the		On the encoder stream, a relative index of "0" always refers to the
	most recently inserted value in the dynamic table. Note that this		most recently inserted value in the dynamic table. Note that this
	means the entry referenced by a given relative index will change		means the entry referenced by a given relative index will change
	while interpreting instructions on the encoder stream.		while interpreting instructions on the encoder stream.
	skipping to change at page 7, line 22 ¶		skipping to change at page 12, line 4 ¶
	Insertion Point Dropping Point		Insertion Point Dropping Point
	n = count of entries inserted		n = count of entries inserted
	d = count of entries dropped		d = count of entries dropped
	Example Dynamic Table Indexing - Control Stream		Example Dynamic Table Indexing - Control Stream
	Because frames from request streams can be delivered out of order		Because frames from request streams can be delivered out of order
	with instructions on the encoder stream, relative indices are		with instructions on the encoder stream, relative indices are
	relative to the Base Index at the beginning of the header block (see		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		Section 4.5.1). The Base Index is an absolute index. When
	interpreting the rest of the frame, the entry identified by Base		interpreting the rest of the frame, the entry identified by Base
	Index has a relative index of zero. The relative indices of entries		Index has a relative index of zero. The relative indices of entries
	do not change while interpreting headers on a request or push stream.		do not change while interpreting headers on a request or push stream.
	Base Index		Base Index
	|		|
	V		V
	+---+-----+-----+-----+-------+		+---+-----+-----+-----+-------+
	| n | n-1 | n-2 | ... | d+1 | Absolute Index		| n | n-1 | n-2 | ... | d+1 | Absolute Index
	+---+-----+ - +-----+ - +		+---+-----+ - +-----+ - +
	| 0 | ... | n-d-3 | Relative Index		| 0 | ... | n-d-3 | Relative Index
	+-----+-----+-------+		+-----+-----+-------+
	n = count of entries inserted		n = count of entries inserted
	d = count of entries dropped		d = count of entries dropped
	Example Dynamic Table Indexing - Relative Index on Request Stream		Example Dynamic Table Indexing - Relative Index on Request Stream
	2.2.5. Post-Base Indexing		3.2.5. Post-Base Indexing
	A header block on the request stream can reference entries added		A header block on the request stream can reference entries added
	after the entry identified by the Base Index. This allows an encoder		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		to process a header block in a single pass and include references to
	entries added while processing this (or other) header blocks. Newly		entries added while processing this (or other) header blocks. Newly
	added entries are referenced using Post-Base instructions. Indices		added entries are referenced using Post-Base instructions. Indices
	for Post-Base instructions increase in the same direction as absolute		for Post-Base instructions increase in the same direction as absolute
	indices, but the zero value is one higher than the Base Index.		indices, but the zero value is one higher than the Base Index.
	Base Index		Base Index
	skipping to change at page 8, line 19 ¶		skipping to change at page 12, line 47 ¶
	| n | n-1 | n-2 | ... | d+1 | Absolute Index		| n | n-1 | n-2 | ... | d+1 | Absolute Index
	+---+-----+-----+-----+-----+		+---+-----+-----+-----+-----+
	| 1 | 0 | Post-Base Index		| 1 | 0 | Post-Base Index
	+---+-----+		+---+-----+
	n = count of entries inserted		n = count of entries inserted
	d = count of entries dropped		d = count of entries dropped
	Example Dynamic Table Indexing - Post-Base Index on Request Stream		Example Dynamic Table Indexing - Post-Base Index on Request Stream
			3.2.6. Invalid References
	If the decoder encounters a reference on a request or push stream to		If the decoder encounters a reference on a request or push stream to
	a dynamic table entry which has already been dropped or which has an		a dynamic table entry which has already been evicted or which has an
	absolute index greater than the declared Largest Reference (see		absolute index greater than the declared Largest Reference (see
	Section 5.4.1), it MUST treat this as a stream error of type		Section 4.5.1), it MUST treat this as a stream error of type
	"HTTP_QPACK_DECOMPRESSION_FAILED".		"HTTP_QPACK_DECOMPRESSION_FAILED".
	If the decoder encounters a reference on the encoder stream to a		If the decoder encounters a reference on the encoder stream to a
	dynamic table entry which has already been dropped, it MUST treat		dynamic table entry which has already been dropped, it MUST treat
	this as a connection error of type "HTTP_QPACK_ENCODER_STREAM_ERROR".		this as a connection error of type "HTTP_QPACK_ENCODER_STREAM_ERROR".
	2.3. Avoiding Head-of-Line Blocking in HTTP/QUIC		4. Wire Format
	Because QUIC does not guarantee order between data on different
	streams, a header block might reference an entry in the dynamic table
	that has not yet been received.
	Each header block contains a Largest Reference which identifies the
	table state necessary for decoding. If the greatest absolute index
	in the dynamic table is less than the value of the Largest Reference,
	the stream is considered "blocked." While blocked, header field data
	should remain in the blocked stream's flow control window. When the
	Largest Reference is zero, the frame contains no references to the
	dynamic table and can always be processed immediately. A stream
	becomes unblocked when the greatest absolute index in the dynamic
	table becomes greater than or equal to the Largest Reference for all
	header blocks the decoder has started reading from the stream. If a
	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
	SETTINGS_QPACK_BLOCKED_STREAMS to a non-zero value (see Section 4).
	This setting specifies an upper bound on the number of streams which
	can be blocked.
	An encoder can decide whether to risk having a stream become blocked.
	If permitted by the value of SETTINGS_QPACK_BLOCKED_STREAMS,
	compression efficiency can be improved by referencing dynamic table
	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
	risk of blocking by only referencing dynamic table entries which have
	been acknowledged, but this means using literals. Since literals
	make the header block larger, this can result in the encoder becoming
	blocked on congestion or flow control limits.
	An encoder MUST limit the number of streams which could become
	blocked to the value of SETTINGS_QPACK_BLOCKED_STREAMS at all times.
	Note that the decoder might not actually become blocked on every
	stream which risks becoming blocked. If the decoder encounters more
	blocked streams than it promised to support, it MUST treat this as a
	stream error of type HTTP_QPACK_DECOMPRESSION_FAILED.
	2.3.1. State Synchronization
	The decoder stream (Section 5.3) signals key events at the decoder
	that permit the encoder to track the decoder's state. These events
	are:
	o Complete 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 has been processed permits the encoder to evict
	entries to which no unacknowledged references remain; see
	Section 7.3.1. 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",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
	"OPTIONAL" in this document are to be interpreted as described in BCP
	14 [RFC2119] [RFC8174] when, and only when, they appear in all
	capitals, as shown here.
	Definitions of terms that are used in this document:
	Header field: A name-value pair sent as part of an HTTP message.
	Header list: The ordered collection of header fields associated with
	an HTTP message. A header list can contain multiple header fields
	with the same name. It can also contain duplicate header fields.
	Header block: The compressed representation of a header list.
	Encoder: An implementation which transforms a header list into a
	header block.
	Decoder: An implementation which transforms a header block into a
	header list.
	QPACK is a name, not an acronym.
	3.1. Notational Conventions
	Diagrams use the format described in Section 3.1 of [RFC2360], with		4.1. Primitives
	the following additional conventions:
	x (A) Indicates that x is A bits long		4.1.1. Prefixed Integers
	x (A+) Indicates that x uses the prefixed integer encoding defined		The prefixed integer from Section 5.1 of [RFC7541] is used heavily
	in Section 5.1 of [RFC7541], beginning with an A-bit prefix.		throughout this document. The format from [RFC7541] is used
			unmodified. QPACK implementations MUST be able to decode integers up
			to 62 bits long.
	x ... Indicates that x is variable-length and extends to the end of		4.1.2. String Literals
	the region.
	4. Configuration		The string literal defined by Section 5.2 of [RFC7541] is also used
			throughout. This string format includes optional Huffman encoding.
	QPACK defines two settings which are included in the HTTP/QUIC		HPACK defines string literals to begin on a byte boundary. They
	SETTINGS frame.		begin with a single flag (indicating whether the string is Huffman-
			coded), followed by the Length encoded as a 7-bit prefix integer, and
			finally Length bytes of data. When Huffman encoding is enabled, the
			Huffman table from Appendix B of [RFC7541] is used without
			modification.
	SETTINGS_HEADER_TABLE_SIZE (0x1): An integer with a maximum value of		This document expands the definition of string literals and permits
	2^30 - 1. The default value is 4,096 bytes. See Section 2.2 for		them to begin other than on a byte boundary. An "N-bit prefix string
	usage.		literal" begins with the same Huffman flag, followed by the length
			encoded as an (N-1)-bit prefix integer. The remainder of the string
			literal is unmodified.
	SETTINGS_QPACK_BLOCKED_STREAMS (0x7): An integer with a maximum		A string literal without a prefix length noted is an 8-bit prefix
	value of 2^16 - 1. The default value is 100. See Section 2.3.		string literal and follows the definitions in [RFC7541] without
			modification.
	5. Wire Format		4.2. Stream Types
	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 The encoder stream is a unidirectional stream of type "0x48"		o The encoder stream is a unidirectional stream of type "0x48"
	(ASCII 'H') which carries table updates from encoder to decoder.		(ASCII 'H') which carries table updates from encoder to decoder.
	Instructions on this stream modify the dynamic table state without
	generating output to any particular request.
	o The decoder stream is a unidirectional stream of type "0x68"		o The decoder stream is a unidirectional stream of type "0x68"
	(ASCII 'h') which carries acknowledgements of table modifications		(ASCII 'h') which carries acknowledgements of table modifications
	and header processing from decoder to encoder.		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 and push 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		There MUST be exactly one of each unidirectional stream type in each
	direction. Receipt of a second instance of either stream type MUST		direction. Receipt of a second instance of either stream type MUST
	be treated as a connection error of HTTP_WRONG_STREAM_COUNT. Closure		be treated as a connection error of HTTP_WRONG_STREAM_COUNT. Closure
	of either unidirectional stream MUST be treated as a connection error		of either unidirectional stream MUST be treated as a connection error
	of type HTTP_CLOSED_CRITICAL_STREAM.		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 encoder stream. Request streams and		4.3. Encoder Stream
	push streams only carry header blocks that do not modify the state of
	the table.
	5.1. Primitives
	5.1.1. Prefixed Integers
	The prefixed integer from Section 5.1 of [RFC7541] is used heavily
	throughout this document. The format from [RFC7541] is used
	unmodified. QPACK implementations MUST be able to decode integers up
	to 62 bits long.
	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
	begin with a single flag (indicating whether the string is Huffman-
	coded), followed by the Length encoded as a 7-bit prefix integer, and
	finally Length octets of data. When Huffman encoding is enabled, the
	Huffman table from Appendix B of [RFC7541] is used without
	modification.
	This document expands the definition of string literals and permits
	them to begin other than on a byte boundary. An "N-bit prefix string
	literal" begins with the same Huffman flag, followed by the length
	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
	string literal and follows the definitions in [RFC7541] without
	modification.
	5.2. 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.
	The contents of the encoder stream are an unframed sequence of the		The contents of the encoder stream are an unframed sequence of the
	following instructions.		following instructions.
	5.2.1. Insert With Name Reference		4.3.1. 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 6-bit prefix integer (see Section 5.1 of [RFC7541])		(S=0) table. The 6-bit prefix integer (see Section 5.1 of [RFC7541])
	that follows is used to locate the table entry for the header name.		that follows is used to locate the table entry for the header name.
	When S=1, the number represents the static table index; when S=0, the		When S=1, the number represents the static table index; when S=0, the
	number is the relative index of the entry in the dynamic table.		number is the relative index of the entry in the dynamic table.
	The header name reference is followed by the header field value		The header name reference is followed by the header field value
	represented as a string literal (see Section 5.2 of [RFC7541]).		represented as a string literal (see Section 5.2 of [RFC7541]).
	0 1 2 3 4 5 6 7		0 1 2 3 4 5 6 7
	+---+---+---+---+---+---+---+---+		+---+---+---+---+---+---+---+---+
	| 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 bytes) |
	+-------------------------------+		+-------------------------------+
	Insert Header Field -- Indexed Name		Insert Header Field -- Indexed Name
	5.2.2. Insert Without Name Reference		4.3.2. 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 5.1) starts with the '01' two-bit pattern.		Section 4.1) 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 bytes) |
	+---+---------------------------+		+---+---------------------------+
	| H | Value Length (7+) |		| H | Value Length (7+) |
	+---+---------------------------+		+---+---------------------------+
	| Value String (Length octets) |		| Value String (Length bytes) |
	+-------------------------------+		+-------------------------------+
	Insert Header Field -- New Name		Insert Header Field -- New Name
	5.2.3. Duplicate		4.3.3. 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 2: 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.
	5.2.4. Dynamic Table Size Update		4.3.4. 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 3: 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 connection error of type		limit MUST be treated as a connection error of type
	"HTTP_QPACK_ENCODER_STREAM_ERROR". In HTTP/QUIC, this limit is the		"HTTP_QPACK_ENCODER_STREAM_ERROR". In HTTP/3, this limit is the
	value of the SETTINGS_HEADER_TABLE_SIZE parameter (see Section 4)		value of the SETTINGS_HEADER_TABLE_SIZE parameter (see Section 5)
	received from the decoder.		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 7.3).		eviction of entries with outstanding references (see Section 2.1.1).
	Changing the size of the dynamic table is not acknowledged as this		Changing the size of the dynamic table is not acknowledged as this
	instruction does not insert an entry.		instruction does not insert an entry.
	5.3. QPACK Decoder Stream		4.4. 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 decoder to the
	QPACK encoder; that is, the server informs the client about the		encoder; that is, the server informs the client about the processing
	processing of the client's header blocks and table updates, and the		of the client's header blocks and table updates, and the client
	client informs the server about the processing of the server's header		informs the server about the processing of the server's header blocks
	blocks and table updates.		and table updates.
	The contents of the decoder stream are an unframed sequence of the		The contents of the decoder stream are an unframed sequence of the
	following instructions.		following instructions.
	5.3.1. Table State Synchronize		4.4.1. Table State Synchronize
	The Table State Synchronize instruction begins with the '00' two-bit		The Table State Synchronize instruction begins with the '00' two-bit
	pattern. The instruction specifies the total number of dynamic table		pattern. The instruction specifies the total number of dynamic table
	inserts and duplications since the last Table State Synchronize or		inserts and duplications since the last Table State Synchronize or
	Header Acknowledgement that increased the Largest Known Received		Header Acknowledgement that increased the Largest Known Received
	dynamic table entry. This is encoded as a 6-bit prefix integer. The		dynamic table entry (see Section 2.1.4). This is encoded as a 6-bit
	encoder uses this value to determine which table entries might cause		prefix integer. The encoder uses this value to determine which table
	a stream to become blocked, as described in Section 2.3.1.		entries might cause a stream to become blocked, as described in
			Section 2.2.1.
	0 1 2 3 4 5 6 7		0 1 2 3 4 5 6 7
	+---+---+---+---+---+---+---+---+		+---+---+---+---+---+---+---+---+
	| 0 | 0 | Insert Count (6+) |		| 0 | 0 | Insert Count (6+) |
	+---+---+-----------------------+		+---+---+-----------------------+
	Figure 3: Table State Synchronize		Figure 4: Table State Synchronize
	An encoder that receives an Insert Count equal to zero or one that		An encoder that receives an Insert Count equal to zero or one that
	increases Largest Known Received beyond what the encoder has sent		increases Largest Known Received beyond what the encoder has sent
	MUST treat this as a connection error of type		MUST treat this as a connection error of type
	"HTTP_QPACK_DECODER_STREAM_ERROR".		"HTTP_QPACK_DECODER_STREAM_ERROR".
	A decoder chooses when to emit Table State Synchronize instructions.		4.4.2. Header Acknowledgement
	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 whose declared Largest Reference is		After processing a header block whose declared Largest Reference is
	not zero, the decoder emits a Header Acknowledgement instruction on		not zero, the decoder emits a Header Acknowledgement instruction on
	the decoder stream. The instruction begins with the '1' one-bit		the decoder stream. The instruction begins with the '1' one-bit
	pattern and includes the request stream's stream ID, encoded as a		pattern and 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		7-bit prefix integer. It is used by the peer's encoder to know when
	when it is safe to evict an entry.		it is safe to evict an entry, and possibly update Largest Known
			Received.
	0 1 2 3 4 5 6 7		0 1 2 3 4 5 6 7
	+---+---+---+---+---+---+---+---+		+---+---+---+---+---+---+---+---+
	| 1 | Stream ID (7+) |		| 1 | Stream ID (7+) |
	+---+---------------------------+		+---+---------------------------+
	Figure 4: Header Acknowledgement		Figure 5: Header Acknowledgement
	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.
	If an encoder receives a Header Acknowledgement instruction referring		If an encoder receives a Header Acknowledgement instruction referring
	to a stream on which every header block with a non-zero Largest		to a stream on which every header block with a non-zero Largest
	skipping to change at page 16, line 21 ¶		skipping to change at page 18, line 14 ¶
	When blocking references are permitted, the encoder uses		When blocking references are permitted, the encoder uses
	acknowledgement of header blocks to update the Largest Known Received		acknowledgement of header blocks to update the Largest Known Received
	index. If a header block was potentially blocking, the		index. If a header block was potentially blocking, the
	acknowledgement implies that the decoder has received all dynamic		acknowledgement implies that the decoder has received all dynamic
	table state necessary to process the header block. If the Largest		table state necessary to process the header block. If the Largest
	Reference of an acknowledged header block was greater than the		Reference of an acknowledged header block was greater than the
	encoder's current Largest Known Received index, the block's Largest		encoder's current Largest Known Received index, the block's Largest
	Reference becomes the new Largest Known Received.		Reference becomes the new Largest Known Received.
	5.3.3. Stream Cancellation		4.4.3. Stream Cancellation
	A stream that is reset might have multiple outstanding header blocks
	with dynamic table references. 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. A decoder with a maximum dynamic table size equal to
	zero MAY omit sending Stream Cancellations, because the encoder
	cannot have any dynamic table references.
	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		The instruction begins with the '01' two-bit pattern. The
	instruction includes the stream ID of the affected stream - a request		instruction includes the stream ID of the affected stream - a request
	or push stream - encoded as a 6-bit prefix integer.		or push stream - encoded as a 6-bit prefix integer.
	0 1 2 3 4 5 6 7		0 1 2 3 4 5 6 7
	+---+---+---+---+---+---+---+---+		+---+---+---+---+---+---+---+---+
	| 0 | 1 | Stream ID (6+) |		| 0 | 1 | Stream ID (6+) |
	+---+---+-----------------------+		+---+---+-----------------------+
	Figure 5: Stream Cancellation		Figure 6: Stream Cancellation
	5.4. Request and Push Streams		A stream that is reset might have multiple outstanding header blocks
			with dynamic table references. When an endpoint receives a stream
			reset before the end of a stream, it generates a Stream Cancellation
			instruction on the decoder stream. Similarly, when an endpoint
			abandons reading of a stream it 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. A decoder with a maximum dynamic table size equal to
			zero MAY omit sending Stream Cancellations, because the encoder
			cannot have any dynamic table references.
			An encoder cannot infer from this instruction that any updates to the
			dynamic table have been received.
			4.5. 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.
	5.4.1. Header Data Prefix		4.5.1. 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 6: Frame Payload		Figure 7: Frame Payload
			4.5.1.1. Largest Reference
	"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. If		to determine when it is safe to process the rest of the block. If
	Largest Reference is greater than zero, the encoder transforms it as		Largest Reference is greater than zero, the encoder transforms it as
	follows before encoding:		follows before encoding:
	LargestReference = LargestReference mod 2*MaxEntries + 1		LargestReference = (LargestReference mod (2 * MaxEntries)) + 1
	The decoder reconstructs the Largest Reference using the following		The decoder reconstructs the Largest Reference using the following
	algorithm:		algorithm:
	if LargestReference > 0:		if LargestReference > 0:
	LargestReference -= 1		LargestReference -= 1
	CurrentWrapped = TableLargestAbsoluteIndex mod 2*MaxEntries		CurrentWrapped = TotalNumberOfInserts mod (2 * MaxEntries)
	if CurrentWrapped >= LargestReference + MaxEntries:		if CurrentWrapped >= LargestReference + MaxEntries:
	# Largest Reference wrapped around 1 extra time		# Largest Reference wrapped around 1 extra time
	LargestReference += 2*MaxEntries		LargestReference += 2 * MaxEntries
	else if CurrentWrapped + MaxEntries < LargestReference		else if CurrentWrapped + MaxEntries < LargestReference
	# Decoder wrapped around 1 extra time		# Decoder wrapped around 1 extra time
	CurrentWrapped += 2*MaxEntries		CurrentWrapped += 2 * MaxEntries
	LargestReference +=		LargestReference += TotalNumberOfInserts - CurrentWrapped
	(TableLargestAbsoluteIndex - CurrentWrapped)
	TableLargestAbsoluteIndex is the Absolute Index of the most recently		TotalNumberOfInserts is the total number of inserts into the
	inserted item in the decoder's dynamic table. This encoding limits		decoder's dynamic table. This encoding limits the length of the
	the length of the prefix on long-lived connections.		prefix on long-lived connections.
			4.5.1.2. Base Index
	"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 2.2.4.		described in Section 3.2.4.
	To save space, Base Index is encoded relative to Largest Reference		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		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		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		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		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		the Base Index. A sign bit of 1 indicates that the Base Index is
	less than the Largest Reference. That is:		less than the Largest Reference. That is:
	if sign == 0:		if sign == 0:
	baseIndex = largestReference + deltaBaseIndex		baseIndex = largestReference + deltaBaseIndex
	else:		else:
	baseIndex = largestReference - deltaBaseIndex		baseIndex = largestReference - deltaBaseIndex
	A single-pass encoder is expected to determine the absolute value of		A single-pass encoder determines the absolute value of Base Index
	Base Index before encoding a header block. If the encoder inserted		before encoding a header block. If the encoder inserted entries in
	entries in the dynamic table while encoding the header block, Largest		the dynamic table while encoding the header block, Largest Reference
	Reference will be greater than Base Index, so the encoded difference		will be greater than Base Index, so the encoded difference is
	is negative and the sign bit is set to 1. If the header block did		negative and the sign bit is set to 1. If the header block did not
	not reference the most recent entry in the table and did not insert		reference the most recent entry in the table and did not insert any
	any new entries, Base Index will be greater than the Largest		new entries, Base Index will be greater than the Largest Reference,
	Reference, so the delta will be positive and the sign bit is set to		so the delta will be positive and the sign bit is set to 0.
	0.
	An encoder that produces table updates before encoding a header block		An encoder that produces table updates before encoding a header block
	might set Largest Reference and Base Index to the same value. When		might set Largest Reference and Base Index to the same value. When
	Largest Reference and Base Index are equal, the Delta Base Index is		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		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.		Base Index is 0 MUST be treated as a decoder error.
	A header block that does not reference the dynamic table can use any		A header block that does not reference the dynamic table can use any
	value for Base Index; setting both Largest Reference and Base Index		value for Base Index; setting both Largest Reference and Base Index
	to zero is the most efficient encoding.		to zero is the most efficient encoding.
	5.4.2. Instructions		4.5.2. Indexed Header Field
	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 19, line 20 ¶		skipping to change at page 21, line 8 ¶
	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]).
	5.4.2.2. Indexed Header Field With Post-Base Index		4.5.3. Indexed Header Field With Post-Base Index
	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 '0001' 4-bit		than Base Index, the representation starts with the '0001' 4-bit
	pattern, followed by the post-base index (see Section 2.2.5) of the		pattern, followed by the post-base index (see Section 3.2.5) 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]).
	0 1 2 3 4 5 6 7		0 1 2 3 4 5 6 7
	+---+---+---+---+---+---+---+---+		+---+---+---+---+---+---+---+---+
	| 0 | 0 | 0 | 1 | Index (4+) |		| 0 | 0 | 0 | 1 | Index (4+) |
	+---+---+---+---+---------------+		+---+---+---+---+---------------+
	Indexed Header Field with Post-Base Index		Indexed Header Field with Post-Base Index
	5.4.2.3. Literal Header Field With Name Reference		4.5.4. 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 '01' 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 '0000' four-bit pattern.		representation starts with the '0000' four-bit pattern.
	skipping to change at page 20, line 14 ¶		skipping to change at page 22, line 11 ¶
	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 | 1 | 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 bytes) |
	+-------------------------------+		+-------------------------------+
	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		4.5.5. Literal Header Field With Post-Base Name Reference
	For entries in the dynamic table with an absolute index greater than		For entries in the dynamic table with an absolute index greater than
	Base Index, the header field name is represented using the post-base		Base Index, the header field name is represented using the post-base
	index of that entry (see Section 2.2.5) encoded as an integer with a		index of that entry (see Section 3.2.5) encoded as an integer with a
	3-bit prefix.		3-bit prefix.
	0 1 2 3 4 5 6 7		0 1 2 3 4 5 6 7
	+---+---+---+---+---+---+---+---+		+---+---+---+---+---+---+---+---+
	| 0 | 0 | 0 | 0 | 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 bytes) |
	+-------------------------------+		+-------------------------------+
	Literal Header Field With Post-Base Name Reference		Literal Header Field With Post-Base Name Reference
	5.4.2.5. Literal Header Field Without Name Reference		4.5.6. 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 5.1) starts with the '001' three-bit pattern.		Section 4.1) 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 | 0 | 1 | N | H |NameLen(3+)|		| 0 | 0 | 1 | N | H |NameLen(3+)|
	+---+---+---+---+---+-----------+		+---+---+---+---+---+-----------+
	| Name String (Length octets) |		| Name String (Length bytes) |
	+---+---------------------------+		+---+---------------------------+
	| H | Value Length (7+) |		| H | Value Length (7+) |
	+---+---------------------------+		+---+---------------------------+
	| Value String (Length octets) |		| Value String (Length bytes) |
	+-------------------------------+		+-------------------------------+
	Literal Header Field Without Name Reference		Literal Header Field Without Name Reference
			5. Configuration
			QPACK defines two settings which are included in the HTTP/3 SETTINGS
			frame.
			SETTINGS_HEADER_TABLE_SIZE (0x1): An integer with a maximum value of
			2^30 - 1. The default value is zero bytes. See Section 3.2 for
			usage.
			SETTINGS_QPACK_BLOCKED_STREAMS (0x7): An integer with a maximum
			value of 2^16 - 1. The default value is zero. See Section 2.1.3.
	6. Error Handling		6. Error Handling
	The following error codes are defined for HTTP/QUIC to indicate		The following error codes are defined for HTTP/3 to indicate failures
	failures of QPACK which prevent the stream or connection from		of QPACK which prevent the stream or connection from continuing:
	continuing:
	HTTP_QPACK_DECOMPRESSION_FAILED (TBD): The decoder failed to		HTTP_QPACK_DECOMPRESSION_FAILED (TBD): The decoder failed to
	interpret an instruction on a request or push stream and is not		interpret an instruction on a request or push stream and is not
	able to continue decoding that header block.		able to continue decoding that header block.
	HTTP_QPACK_ENCODER_STREAM_ERROR (TBD):
	The decoder failed to interpret an instruction on the encoder		HTTP_QPACK_ENCODER_STREAM_ERROR (TBD): The decoder failed to
	stream. HTTP_QPACK_DECODER_STREAM_ERROR (TBD):		interpret an instruction on the encoder stream.
	The encoder failed to interpret an instruction on the decoder		HTTP_QPACK_DECODER_STREAM_ERROR (TBD): The encoder failed to
	stream.		interpret an instruction on the decoder stream.
	Upon encountering an error, an implementation MAY elect to treat it		Upon encountering an error, an implementation MAY elect to treat it
	as a connection error even if this document prescribes that it MUST		as a connection error even if this document prescribes that it MUST
	be treated as a stream error.		be treated as a stream error.
	7. Encoding Strategies		7. Security Considerations
	7.1. Single Pass Encoding
	An encoder making a single pass over a list of headers must choose
	Base Index before knowing Largest Reference. When trying to
	reference a header inserted to the table after encoding has begun,
	the entry is encoded with different instructions that tell the
	decoder to use an absolute index greater than the Base Index.
	7.2. Preventing Eviction Races
	Due to out-of-order arrival, QPACK's eviction algorithm requires
	changes (relative to HPACK) to avoid the possibility that an indexed
	representation is decoded after the referenced entry has already been
	evicted. QPACK employs a two-phase eviction algorithm, in which the
	encoder will not evict entries that have outstanding (unacknowledged)
	references.
	7.3. Reference Tracking
	An encoder MUST ensure that a header block which references a dynamic
	table entry is not received by the decoder after the referenced entry
	has already been evicted. An encoder also respects the limit set by
	the decoder on the number of streams that are allowed to become
	blocked. Even if the decoder is willing to tolerate blocked streams,
	the encoder might choose to avoid them in certain cases.
	In order to enable this, the encoder will need to track outstanding
	(unacknowledged) header blocks and table updates using feedback
	received from the decoder.
	7.3.1. Blocked Dynamic Table Insertions
	An encoder MUST NOT insert an entry into the dynamic table (or
	duplicate an existing entry) if doing so would evict an entry with
	unacknowledged references. For header blocks that might rely on the
	newly added entry, the encoder can use a literal representation and
	maybe insert the entry later.
	To ensure that the encoder is not prevented from adding new entries,
	the encoder can avoid referencing entries that will be evicted
	soonest. Rather than reference such an entry, the encoder SHOULD
	emit a Duplicate instruction (see Section 5.2.3), and reference the
	duplicate instead.
	Determining which entries are too close to eviction to reference is
	an encoder preference. One heuristic is to target a fixed amount of
	available space in the dynamic table: either unused space or space
	that can be reclaimed by evicting unreferenced entries. To achieve
	this, the encoder can maintain a draining index, which is the
	smallest absolute index in the dynamic table that it will emit a
	reference for. As new entries are inserted, the encoder increases
	the draining index to maintain the section of the table that it will
	not reference. Draining entries - entries with an absolute index
	lower than the draining index - will not accumulate new references.
	The number of unacknowledged references to draining entries will
	eventually become zero, making the entry available for eviction.
	+----------+---------------------------------+--------+
	| Draining | Referenceable | Unused |
	| Entries | Entries | Space |
	+----------+---------------------------------+--------+
	^ ^ ^
	| | |
	Dropping Draining Index Base Index /
	Point Insertion Point
	Figure 7: Draining Dynamic Table Entries
	7.3.2. Blocked Decoding
	For header blocks encoded in non-blocking mode, the encoder needs to
	forego indexed representations that refer to table updates which have
	not yet been acknowledged (see Section 5.3). Since all table updates
	are processed in sequence on the control stream, an index into the
	dynamic table is sufficient to track which entries have been
	acknowledged.
	To track blocked streams, the necessary Base Index value for each
	stream can be used. Whenever the decoder processes a table update,
	it can begin decoding any blocked streams that now have their
	dependencies satisfied.
	7.4. Speculative table updates
	Implementations can _speculatively_ send instructions on the encoder
	stream which are not needed for any current HTTP request or response.
	Such headers could be used strategically to improve performance. For
	instance, the encoder might decide to _refresh_ by sending Duplicate
	representations for popular header fields (Section 5.2.3), ensuring
	they have small indices and hence minimal size on the wire.
	7.5. Sample One Pass Encoding Algorithm
	Pseudo-code for single pass encoding, excluding handling of
	duplicates, non-blocking mode, and reference tracking.
	baseIndex = dynamicTable.baseIndex
	largestReference = 0
	for header in headers:
	staticIdx = staticTable.getIndex(header)
	if staticIdx:
	encodeIndexReference(streamBuffer, staticIdx)
	continue
	dynamicIdx = dynamicTable.getIndex(header)
	if !dynamicIdx:
	# No matching entry. Either insert+index or encode literal
	nameIdx = getNameIndex(header)
	if shouldIndex(header) and dynamicTable.canIndex(header):
	encodeLiteralWithIncrementalIndex(controlBuffer, nameIdx,
	header)
	dynamicTable.add(header)
	dynamicIdx = dynamicTable.baseIndex
	if !dynamicIdx:
	# Couldn't index it, literal
	if nameIdx <= staticTable.size:
	encodeLiteral(streamBuffer, nameIndex, header)
	else:
	# encode literal, possibly with nameIdx above baseIndex
	encodeDynamicLiteral(streamBuffer, nameIndex, baseIndex,
	header)
	largestReference = max(largestReference,
	dynamicTable.toAbsolute(nameIdx))
	else:
	# Dynamic index reference
	assert(dynamicIdx)
	largestReference = max(largestReference, dynamicIdx)
	# Encode dynamicIdx, possibly with dynamicIdx above baseIndex
	encodeDynamicIndexReference(streamBuffer, dynamicIdx,
	baseIndex)
	# encode the prefix
	encodeInteger(prefixBuffer, 0x00, largestReference, 8)
	if baseIndex >= largestReference:
	encodeInteger(prefixBuffer, 0, baseIndex - largestReference, 7)
	else:
	encodeInteger(prefixBuffer, 0x80,
	largestReference - baseIndex, 7)
	return controlBuffer, prefixBuffer + streamBuffer
	8. Security Considerations
	TBD.		TBD.
	9. IANA Considerations		8. IANA Considerations
	9.1. Settings Registration		8.1. Settings Registration
	This document creates two new settings in the "HTTP/QUIC Settings"		This document creates two new settings in the "HTTP/3 Settings"
	registry established in [QUIC-HTTP].		registry established in [HTTP3].
	The entries in the following table are registered by this document.		The entries in the following table are registered by this document.
	+-----------------------+------+---------------+		+-----------------------+------+---------------+
	| Setting Name | Code | Specification |		| Setting Name | Code | Specification |
	+-----------------------+------+---------------+		+-----------------------+------+---------------+
	| HEADER_TABLE_SIZE | 0x1 | Section 4 |		| HEADER_TABLE_SIZE | 0x1 | Section 5 |
	| | | |		| | | |
	| QPACK_BLOCKED_STREAMS | 0x7 | Section 4 |		| QPACK_BLOCKED_STREAMS | 0x7 | Section 5 |
	+-----------------------+------+---------------+		+-----------------------+------+---------------+
	9.2. Stream Type Registration		8.2. Stream Type Registration
	This document creates two new settings in the "HTTP/QUIC Stream Type"		This document creates two new settings in the "HTTP/3 Stream Type"
	registry established in [QUIC-HTTP].		registry established in [HTTP3].
	The entries in the following table are registered by this document.		The entries in the following table are registered by this document.
	+----------------------+------+---------------+--------+		+----------------------+------+---------------+--------+
	| Stream Type | Code | Specification | Sender |		| Stream Type | Code | Specification | Sender |
	+----------------------+------+---------------+--------+		+----------------------+------+---------------+--------+
	| QPACK Encoder Stream | 0x48 | Section 5 | Both |		| QPACK Encoder Stream | 0x48 | Section 4 | Both |
	| | | | |		| | | | |
	| QPACK Decoder Stream | 0x68 | Section 5 | Both |		| QPACK Decoder Stream | 0x68 | Section 4 | Both |
	+----------------------+------+---------------+--------+		+----------------------+------+---------------+--------+
	9.3. Error Code Registration		8.3. Error Code Registration
	This document establishes the following new error codes in the "HTTP/		This document establishes the following new error codes in the
	QUIC Error Code" registry established in [QUIC-HTTP].		"HTTP/3 Error Code" registry established in [HTTP3].
	+------------------------------+------+--------------+--------------+		+------------------------------+------+--------------+--------------+
	| Name | Code | Description | Specificatio |		| Name | Code | Description | Specificatio |
	| | | | n |		| | | | n |
	+------------------------------+------+--------------+--------------+		+------------------------------+------+--------------+--------------+
	| HTTP_QPACK_DECOMPRESSION_FAI | TBD | Decompressio | Section 6 |		| HTTP_QPACK_DECOMPRESSION_FAI | TBD | Decompressio | Section 6 |
	| LED | | n of a | |		| LED | | n of a | |
	| | | header block | |		| | | header block | |
	| | | failed | |		| | | failed | |
	| | | | |		| | | | |
	| HTTP_QPACK_ENCODER_STREAM_ER | TBD | Error on the | Section 6 |		| HTTP_QPACK_ENCODER_STREAM_ER | TBD | Error on the | Section 6 |
	| ROR | | encoder | |		| ROR | | encoder | |
	| | | stream | |		| | | stream | |
	| | | | |		| | | | |
	| HTTP_QPACK_DECODER_STREAM_ER | TBD | Error on the | Section 6 |		| HTTP_QPACK_DECODER_STREAM_ER | TBD | Error on the | Section 6 |
	| ROR | | decoder | |		| ROR | | decoder | |
	| | | stream | |		| | | stream | |
	+------------------------------+------+--------------+--------------+		+------------------------------+------+--------------+--------------+
	10. References		9. References
	10.1. Normative References		9.1. Normative References
	[QUIC-HTTP]		[HTTP3] Bishop, M., Ed., "Hypertext Transfer Protocol Version 3
	Bishop, M., Ed., "Hypertext Transfer Protocol (HTTP) over		(HTTP/3)", draft-ietf-quic-http-16 (work in progress),
	QUIC", draft-ietf-quic-http-15 (work in progress), October		December 2018.
	2018.
	[QUIC-TRANSPORT]		[QUIC-TRANSPORT]
	Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based		Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
	Multiplexed and Secure Transport", draft-ietf-quic-		Multiplexed and Secure Transport", draft-ietf-quic-
	transport-14 (work in progress), October 2018.		transport-16 (work in progress), December 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>.
	10.2. Informative References		9.2. Informative References
	[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>.
	10.3. 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
	Appendix A. Static Table		Appendix A. Static Table
	+------+-----------------------------+------------------------------+		+------+-----------------------------+------------------------------+
	skipping to change at page 33, line 5 ¶		skipping to change at page 31, line 5 ¶
	| | | |		| | | |
	| 95 | user-agent | |		| 95 | user-agent | |
	| | | |		| | | |
	| 96 | x-forwarded-for | |		| 96 | x-forwarded-for | |
	| | | |		| | | |
	| 97 | x-frame-options | deny |		| 97 | x-frame-options | deny |
	| | | |		| | | |
	| 98 | x-frame-options | sameorigin |		| 98 | x-frame-options | sameorigin |
	+------+-----------------------------+------------------------------+		+------+-----------------------------+------------------------------+
	Appendix B. Change Log		Appendix B. Sample One Pass Encoding Algorithm
			Pseudo-code for single pass encoding, excluding handling of
			duplicates, non-blocking mode, and reference tracking.
			baseIndex = dynamicTable.baseIndex
			largestReference = 0
			for header in headers:
			staticIdx = staticTable.getIndex(header)
			if staticIdx:
			encodeIndexReference(streamBuffer, staticIdx)
			continue
			dynamicIdx = dynamicTable.getIndex(header)
			if !dynamicIdx:
			# No matching entry. Either insert+index or encode literal
			nameIdx = getNameIndex(header)
			if shouldIndex(header) and dynamicTable.canIndex(header):
			encodeLiteralWithIncrementalIndex(controlBuffer, nameIdx,
			header)
			dynamicTable.add(header)
			dynamicIdx = dynamicTable.baseIndex
			if !dynamicIdx:
			# Couldn't index it, literal
			if nameIdx <= staticTable.size:
			encodeLiteral(streamBuffer, nameIndex, header)
			else:
			# encode literal, possibly with nameIdx above baseIndex
			encodeDynamicLiteral(streamBuffer, nameIndex, baseIndex,
			header)
			largestReference = max(largestReference,
			dynamicTable.toAbsolute(nameIdx))
			else:
			# Dynamic index reference
			assert(dynamicIdx)
			largestReference = max(largestReference, dynamicIdx)
			# Encode dynamicIdx, possibly with dynamicIdx above baseIndex
			encodeDynamicIndexReference(streamBuffer, dynamicIdx,
			baseIndex)
			# encode the prefix
			encodeInteger(prefixBuffer, 0x00, largestReference, 8)
			if baseIndex >= largestReference:
			encodeInteger(prefixBuffer, 0, baseIndex - largestReference, 7)
			else:
			encodeInteger(prefixBuffer, 0x80,
			largestReference - baseIndex, 7)
			return controlBuffer, prefixBuffer + streamBuffer
			Appendix C. Change Log
	*RFC Editor's Note:* Please remove this section prior to		*RFC Editor's Note:* Please remove this section prior to
	publication of a final version of this document.		publication of a final version of this document.
	B.1. Since draft-ietf-quic-qpack-02		C.1. Since draft-ietf-quic-qpack-03
			Substantial editorial reorganization; no technical changes.
			C.2. Since draft-ietf-quic-qpack-02
	o Largest Reference encoded modulo MaxEntries (#1763)		o Largest Reference encoded modulo MaxEntries (#1763)
	o New Static Table (#1355)		o New Static Table (#1355)
	o Table Size Update with Insert Count=0 is a connection error		o Table Size Update with Insert Count=0 is a connection error
	(#1762)		(#1762)
	o Stream Cancellations are optional when		o Stream Cancellations are optional when
	SETTINGS_HEADER_TABLE_SIZE=0 (#1761)		SETTINGS_HEADER_TABLE_SIZE=0 (#1761)
	skipping to change at page 33, line 32 ¶		skipping to change at page 33, line 36 ¶
	o Implementations must handle 62 bit integers (#1760)		o Implementations must handle 62 bit integers (#1760)
	o Different error types for each QPACK stream, other changes to		o Different error types for each QPACK stream, other changes to
	error handling (#1726)		error handling (#1726)
	o Preserve header field order (#1725)		o Preserve header field order (#1725)
	o Initial table size is the maximum permitted when table is first		o Initial table size is the maximum permitted when table is first
	usable (#1642)		usable (#1642)
	B.2. Since draft-ietf-quic-qpack-01		C.3. Since draft-ietf-quic-qpack-01
	o Only header blocks that reference the dynamic table are		o Only header blocks that reference the dynamic table are
	acknowledged (#1603, #1605)		acknowledged (#1603, #1605)
	B.3. Since draft-ietf-quic-qpack-00		C.4. Since draft-ietf-quic-qpack-00
	o Renumbered instructions for consistency (#1471, #1472)		o Renumbered instructions for consistency (#1471, #1472)
	o Decoder is allowed to validate largest reference (#1404, #1469)		o Decoder is allowed to validate largest reference (#1404, #1469)
	o Header block acknowledgments also acknowledge the associated		o Header block acknowledgments also acknowledge the associated
	largest reference (#1370, #1400)		largest reference (#1370, #1400)
	o Added an acknowledgment for unread streams (#1371, #1400)		o Added an acknowledgment for unread streams (#1371, #1400)
	skipping to change at page 33, line 49 ¶		skipping to change at page 34, line 4 ¶
	o Renumbered instructions for consistency (#1471, #1472)		o Renumbered instructions for consistency (#1471, #1472)
	o Decoder is allowed to validate largest reference (#1404, #1469)		o Decoder is allowed to validate largest reference (#1404, #1469)
	o Header block acknowledgments also acknowledge the associated		o Header block acknowledgments also acknowledge the associated
	largest reference (#1370, #1400)		largest reference (#1370, #1400)
	o Added an acknowledgment for unread streams (#1371, #1400)		o Added an acknowledgment for unread streams (#1371, #1400)
	o Removed framing from encoder stream (#1361,#1467)		o Removed framing from encoder stream (#1361,#1467)
	o Control streams use typed unidirectional streams rather than fixed		o Control streams use typed unidirectional streams rather than fixed
	stream IDs (#910,#1359)		stream IDs (#910,#1359)
	B.4. Since draft-ietf-quic-qcram-00		C.5. 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,
End of changes. 122 change blocks.
	531 lines changed or deleted		545 lines changed or added
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