draft-ietf-quic-qpack-05.txt   draft-ietf-quic-qpack-06.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: June 21, 2019 Akamai Technologies Expires: July 27, 2019 Akamai Technologies
A. Frindell, Ed. A. Frindell, Ed.
Facebook Facebook
December 18, 2018 January 23, 2019
QPACK: Header Compression for HTTP over QUIC QPACK: Header Compression for HTTP over QUIC
draft-ietf-quic-qpack-05 draft-ietf-quic-qpack-06
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/3. 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
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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 June 21, 2019. This Internet-Draft will expire on July 27, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
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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
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions and Definitions . . . . . . . . . . . . . . . 4 1.1. Conventions and Definitions . . . . . . . . . . . . . . . 4
1.2. Notational Conventions . . . . . . . . . . . . . . . . . 5 1.2. Notational Conventions . . . . . . . . . . . . . . . . . 5
2. Compression Process Overview . . . . . . . . . . . . . . . . 5 2. Compression Process Overview . . . . . . . . . . . . . . . . 5
2.1. Encoder . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1. Encoder . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.1. Reference Tracking . . . . . . . . . . . . . . . . . 6 2.1.1. Reference Tracking . . . . . . . . . . . . . . . . . 6
2.1.2. Blocked Dynamic Table Insertions . . . . . . . . . . 6 2.1.2. Blocked Dynamic Table Insertions . . . . . . . . . . 6
2.1.3. Avoiding Head-of-Line Blocking . . . . . . . . . . . 7 2.1.3. Avoiding Head-of-Line Blocking . . . . . . . . . . . 7
2.1.4. Largest Known Received . . . . . . . . . . . . . . . 8 2.1.4. Known Received Count . . . . . . . . . . . . . . . . 8
2.2. Decoder . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.2. Decoder . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2.1. State Synchronization . . . . . . . . . . . . . . . . 8 2.2.1. State Synchronization . . . . . . . . . . . . . . . . 8
2.2.2. Blocked Decoding . . . . . . . . . . . . . . . . . . 9 2.2.2. Blocked Decoding . . . . . . . . . . . . . . . . . . 9
3. Header Tables . . . . . . . . . . . . . . . . . . . . . . . . 9 3. Header Tables . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1. Static Table . . . . . . . . . . . . . . . . . . . . . . 9 3.1. Static Table . . . . . . . . . . . . . . . . . . . . . . 9
3.2. Dynamic Table . . . . . . . . . . . . . . . . . . . . . . 9 3.2. Dynamic Table . . . . . . . . . . . . . . . . . . . . . . 10
3.2.1. Calculating Table Size . . . . . . . . . . . . . . . 10 3.2.1. Dynamic Table Size . . . . . . . . . . . . . . . . . 10
3.2.2. Eviction . . . . . . . . . . . . . . . . . . . . . . 10 3.2.2. Dynamic Table Capacity and Eviction . . . . . . . . . 10
3.2.3. Maximum Table Size . . . . . . . . . . . . . . . . . 10 3.2.3. Maximum Dynamic Table Capacity . . . . . . . . . . . 11
3.2.4. Absolute Indexing . . . . . . . . . . . . . . . . . . 11 3.2.4. Initial Dynamic Table Capacity . . . . . . . . . . . 11
3.2.5. Relative Indexing . . . . . . . . . . . . . . . . . . 11 3.2.5. Absolute Indexing . . . . . . . . . . . . . . . . . . 11
3.2.6. Post-Base Indexing . . . . . . . . . . . . . . . . . 12 3.2.6. Relative Indexing . . . . . . . . . . . . . . . . . . 11
3.2.7. Invalid References . . . . . . . . . . . . . . . . . 12 3.2.7. Post-Base Indexing . . . . . . . . . . . . . . . . . 12
3.2.8. Invalid References . . . . . . . . . . . . . . . . . 13
4. Wire Format . . . . . . . . . . . . . . . . . . . . . . . . . 13 4. Wire Format . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.1. Primitives . . . . . . . . . . . . . . . . . . . . . . . 13 4.1. Primitives . . . . . . . . . . . . . . . . . . . . . . . 13
4.1.1. Prefixed Integers . . . . . . . . . . . . . . . . . . 13 4.1.1. Prefixed Integers . . . . . . . . . . . . . . . . . . 13
4.1.2. String Literals . . . . . . . . . . . . . . . . . . . 13 4.1.2. String Literals . . . . . . . . . . . . . . . . . . . 13
4.2. Stream Types . . . . . . . . . . . . . . . . . . . . . . 13 4.2. Stream Types . . . . . . . . . . . . . . . . . . . . . . 14
4.3. Encoder Stream . . . . . . . . . . . . . . . . . . . . . 14 4.3. Encoder Stream . . . . . . . . . . . . . . . . . . . . . 14
4.3.1. Insert With Name Reference . . . . . . . . . . . . . 14 4.3.1. Insert With Name Reference . . . . . . . . . . . . . 15
4.3.2. Insert Without Name Reference . . . . . . . . . . . . 15 4.3.2. Insert Without Name Reference . . . . . . . . . . . . 15
4.3.3. Duplicate . . . . . . . . . . . . . . . . . . . . . . 15 4.3.3. Duplicate . . . . . . . . . . . . . . . . . . . . . . 16
4.3.4. Dynamic Table Size Update . . . . . . . . . . . . . . 15 4.3.4. Set Dynamic Table Capacity . . . . . . . . . . . . . 16
4.4. Decoder Stream . . . . . . . . . . . . . . . . . . . . . 16 4.4. Decoder Stream . . . . . . . . . . . . . . . . . . . . . 17
4.4.1. Table State Synchronize . . . . . . . . . . . . . . . 16 4.4.1. Insert Count Increment . . . . . . . . . . . . . . . 17
4.4.2. Header Acknowledgement . . . . . . . . . . . . . . . 17 4.4.2. Header Acknowledgement . . . . . . . . . . . . . . . 18
4.4.3. Stream Cancellation . . . . . . . . . . . . . . . . . 18 4.4.3. Stream Cancellation . . . . . . . . . . . . . . . . . 18
4.5. Request and Push Streams . . . . . . . . . . . . . . . . 18 4.5. Request and Push Streams . . . . . . . . . . . . . . . . 19
4.5.1. Header Data Prefix . . . . . . . . . . . . . . . . . 18 4.5.1. Header Block Prefix . . . . . . . . . . . . . . . . . 19
4.5.2. Indexed Header Field . . . . . . . . . . . . . . . . 20 4.5.2. Indexed Header Field . . . . . . . . . . . . . . . . 21
4.5.3. Indexed Header Field With Post-Base Index . . . . . . 21 4.5.3. Indexed Header Field With Post-Base Index . . . . . . 22
4.5.4. Literal Header Field With Name Reference . . . . . . 21 4.5.4. Literal Header Field With Name Reference . . . . . . 22
4.5.5. Literal Header Field With Post-Base Name Reference . 22 4.5.5. Literal Header Field With Post-Base Name Reference . 23
4.5.6. Literal Header Field Without Name Reference . . . . . 22 4.5.6. Literal Header Field Without Name Reference . . . . . 23
5. Configuration . . . . . . . . . . . . . . . . . . . . . . . . 23 5. Configuration . . . . . . . . . . . . . . . . . . . . . . . . 24
6. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 23 6. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 24
7. Security Considerations . . . . . . . . . . . . . . . . . . . 24 7. Security Considerations . . . . . . . . . . . . . . . . . . . 25
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
8.1. Settings Registration . . . . . . . . . . . . . . . . . . 24 8.1. Settings Registration . . . . . . . . . . . . . . . . . . 25
8.2. Stream Type Registration . . . . . . . . . . . . . . . . 24 8.2. Stream Type Registration . . . . . . . . . . . . . . . . 25
8.3. Error Code Registration . . . . . . . . . . . . . . . . . 24 8.3. Error Code Registration . . . . . . . . . . . . . . . . . 25
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 25 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
9.1. Normative References . . . . . . . . . . . . . . . . . . 25 9.1. Normative References . . . . . . . . . . . . . . . . . . 26
9.2. Informative References . . . . . . . . . . . . . . . . . 26 9.2. Informative References . . . . . . . . . . . . . . . . . 27
9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 26 9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Appendix A. Static Table . . . . . . . . . . . . . . . . . . . . 26 Appendix A. Static Table . . . . . . . . . . . . . . . . . . . . 27
Appendix B. Sample One Pass Encoding Algorithm . . . . . . . . . 31 Appendix B. Sample One Pass Encoding Algorithm . . . . . . . . . 32
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 33 Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 34
C.1. Since draft-ietf-quic-qpack-04 . . . . . . . . . . . . . 33 C.1. Since draft-ietf-quic-qpack-05 . . . . . . . . . . . . . 34
C.2. Since draft-ietf-quic-qpack-03 . . . . . . . . . . . . . 33 C.2. Since draft-ietf-quic-qpack-04 . . . . . . . . . . . . . 34
C.3. Since draft-ietf-quic-qpack-02 . . . . . . . . . . . . . 33 C.3. Since draft-ietf-quic-qpack-03 . . . . . . . . . . . . . 34
C.4. Since draft-ietf-quic-qpack-01 . . . . . . . . . . . . . 33 C.4. Since draft-ietf-quic-qpack-02 . . . . . . . . . . . . . 34
C.5. Since draft-ietf-quic-qpack-00 . . . . . . . . . . . . . 33 C.5. Since draft-ietf-quic-qpack-01 . . . . . . . . . . . . . 35
C.6. Since draft-ietf-quic-qcram-00 . . . . . . . . . . . . . 34 C.6. Since draft-ietf-quic-qpack-00 . . . . . . . . . . . . . 35
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 34 C.7. Since draft-ietf-quic-qcram-00 . . . . . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 36
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/3, it would induce head-of-line blocking due to built- used for HTTP/3, it would induce head-of-line blocking due to built-
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Header block: The compressed representation of a header list. Header block: The compressed representation of a header list.
Encoder: An implementation which transforms a header list into a Encoder: An implementation which transforms a header list into a
header block. header block.
Decoder: An implementation which transforms a header block into a Decoder: An implementation which transforms a header block into a
header list. header list.
Absolute Index: A unique index for each entry in the dynamic table. Absolute Index: A unique index for each entry in the dynamic table.
Base Index: An absolute index in a header block from which relative Base: A reference point for relative indicies. Dynamic references
indices are made. are made relative to a Base in header blocks.
Largest Reference: The largest absolute index of an entry referenced Insert Count: The total number of entries inserted in the dynamic
in a header block. table.
QPACK is a name, not an acronym. QPACK is a name, not an acronym.
1.2. Notational Conventions 1.2. Notational Conventions
Diagrams use the format described in Section 3.1 of [RFC2360], with Diagrams use the format described in Section 3.1 of [RFC2360], with
the following additional conventions: the following additional conventions:
x (A) Indicates that x is A bits long x (A) Indicates that x is A bits long
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table entry is not received by the decoder after the referenced entry table entry is not received by the decoder after the referenced entry
has been evicted. Hence the encoder needs to track information about has been evicted. Hence the encoder needs to track information about
each compressed header block that references the dynamic table until each compressed header block that references the dynamic table until
that header block is acknowledged by the decoder. that header block is acknowledged by the decoder.
2.1.2. Blocked Dynamic Table Insertions 2.1.2. Blocked Dynamic Table Insertions
An encoder MUST NOT insert an entry into the dynamic table (or An encoder MUST NOT insert an entry into the dynamic table (or
duplicate an existing entry) if doing so would evict an entry with duplicate an existing entry) if doing so would evict an entry with
unacknowledged references. For header blocks that might rely on the unacknowledged references. For header blocks that might rely on the
newly added entry, the encoder can use a literal representation and newly added entry, the encoder can use a literal representation.
maybe insert the entry later.
To ensure that the encoder is not prevented from adding new entries, To ensure that the encoder is not prevented from adding new entries,
the encoder can avoid referencing entries that are close to eviction. the encoder can avoid referencing entries that are close to eviction.
Rather than reference such an entry, the encoder can emit a Duplicate Rather than reference such an entry, the encoder can emit a Duplicate
instruction (see Section 4.3.3), and reference the duplicate instead. instruction (see Section 4.3.3), and reference the duplicate instead.
Determining which entries are too close to eviction to reference is Determining which entries are too close to eviction to reference is
an encoder preference. One heuristic is to target a fixed amount of an encoder preference. One heuristic is to target a fixed amount of
available space in the dynamic table: either unused space or space available space in the dynamic table: either unused space or space
that can be reclaimed by evicting unreferenced entries. To achieve that can be reclaimed by evicting unreferenced entries. To achieve
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entries with an absolute index lower than the draining index, the entries with an absolute index lower than the draining index, the
number of unacknowledged references to those entries will eventually number of unacknowledged references to those entries will eventually
become zero, allowing them to be evicted. become zero, allowing them to be evicted.
+----------+---------------------------------+--------+ +----------+---------------------------------+--------+
| Draining | Referenceable | Unused | | Draining | Referenceable | Unused |
| Entries | Entries | Space | | Entries | Entries | Space |
+----------+---------------------------------+--------+ +----------+---------------------------------+--------+
^ ^ ^ ^ ^ ^
| | | | | |
Dropping Draining Index Base Index / Dropping Draining Index Insertion Point
Point Insertion Point Point
Figure 1: Draining Dynamic Table Entries Figure 1: Draining Dynamic Table Entries
2.1.3. Avoiding Head-of-Line Blocking 2.1.3. Avoiding Head-of-Line Blocking
Because QUIC does not guarantee order between data on different Because QUIC does not guarantee order between data on different
streams, a header block might reference an entry in the dynamic table streams, a header block might reference an entry in the dynamic table
that has not yet been received. that has not yet been received.
Each header block contains a Largest Reference (Section 4.5.1) which Each header block contains a Required Insert Count, the lowest
identifies the table state necessary for decoding. If the greatest possible value for the Insert Count with which the header block can
absolute index in the dynamic table is less than the value of the be decoded. For a header block with no references to the dynamic
Largest Reference, the stream is considered "blocked." While table, the Required Insert Count is zero.
When the Required Insert Count is zero, the frame contains no
references to the dynamic table and can always be processed
immediately.
If the Required Insert Count is greater than the number of dynamic
table entries received, the stream is considered "blocked." While
blocked, header field data SHOULD remain in the blocked stream's flow blocked, header field data SHOULD remain in the blocked stream's flow
control window. When the Largest Reference is zero, the frame control window. A stream becomes unblocked when the Insert Count
contains no references to the dynamic table and can always be becomes greater than or equal to the Required Insert Count for all
processed immediately. A stream becomes unblocked when the greatest header blocks the decoder has started reading from the stream.
absolute index in the dynamic table becomes greater than or equal to
the Largest Reference for all header blocks the decoder has started If the decoder encounters a header block where the largest Absolute
reading from the stream. If the decoder encounters a header block Index used is not equal to the largest value permitted by the
where the actual largest reference is not equal to the Largest Required Insert Count, it MAY treat this as a stream error of type
Reference declared in the prefix, it MAY treat this as a stream error HTTP_QPACK_DECOMPRESSION_FAILED.
of type HTTP_QPACK_DECOMPRESSION_FAILED.
The SETTINGS_QPACK_BLOCKED_STREAMS setting (see Section 5) specifies The SETTINGS_QPACK_BLOCKED_STREAMS setting (see Section 5) specifies
an upper bound on the number of streams which can be blocked. An an upper bound on the number of streams which can be blocked. An
encoder MUST limit the number of streams which could become blocked encoder MUST limit the number of streams which could become blocked
to the value of SETTINGS_QPACK_BLOCKED_STREAMS at all times. Note to the value of SETTINGS_QPACK_BLOCKED_STREAMS at all times. Note
that the decoder might not actually become blocked on every stream that the decoder might not actually become blocked on every stream
which risks becoming blocked. If the decoder encounters more blocked which risks becoming blocked. If the decoder encounters more blocked
streams than it promised to support, it MUST treat this as a stream streams than it promised to support, it MUST treat this as a stream
error of type HTTP_QPACK_DECOMPRESSION_FAILED. error of type HTTP_QPACK_DECOMPRESSION_FAILED.
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streams than it promised to support, it MUST treat this as a stream streams than it promised to support, it MUST treat this as a stream
error of type HTTP_QPACK_DECOMPRESSION_FAILED. error of type HTTP_QPACK_DECOMPRESSION_FAILED.
An encoder can decide whether to risk having a stream become blocked. An encoder can decide whether to risk having a stream become blocked.
If permitted by the value of SETTINGS_QPACK_BLOCKED_STREAMS, If permitted by the value of SETTINGS_QPACK_BLOCKED_STREAMS,
compression efficiency can often be improved by referencing dynamic compression efficiency can often be improved by referencing dynamic
table entries that are still in transit, but if there is loss or table entries that are still in transit, but if there is loss or
reordering the stream can become blocked at the decoder. An encoder reordering the stream can become blocked at the decoder. An encoder
avoids the risk of blocking by only referencing dynamic table entries avoids the risk of blocking by only referencing dynamic table entries
which have been acknowledged, but this could mean using literals. which have been acknowledged, but this could mean using literals.
Since literals make the header block larger, this can result in the Since literals make the header block larger, this can result in the
encoder becoming blocked on congestion or flow control limits. encoder becoming blocked on congestion or flow control limits.
2.1.4. Largest Known Received 2.1.4. Known Received Count
In order to identify which dynamic table entries can be safely used In order to identify which dynamic table entries can be safely used
without a stream becoming blocked, the encoder tracks the absolute without a stream becoming blocked, the encoder tracks the number of
index of the decoder's Largest Known Received entry. entries received by the decoder. The Known Received Count tracks the
total number of acknowledged insertions.
When blocking references are permitted, the encoder uses header block When blocking references are permitted, the encoder uses header block
acknowledgement to identify the Largest Known Received index, as acknowledgement to maintain the Known Received Count, as described in
described in Section 4.4.2. Section 4.4.2.
To acknowledge dynamic table entries which are not referenced by To acknowledge dynamic table entries which are not referenced by
header blocks, for example because the encoder or the decoder have header blocks, for example because the encoder or the decoder have
chosen not to risk blocked streams, the decoder sends a Table State chosen not to risk blocked streams, the decoder sends an Insert Count
Synchronize instruction (see Section 4.4.1). Increment instruction (see Section 4.4.1).
2.2. Decoder 2.2. Decoder
As in HPACK, the decoder processes header blocks and emits the As in HPACK, the decoder processes header blocks and emits the
corresponding header lists. It also processes dynamic table corresponding header lists. It also processes dynamic table
modifications from instructions on the encoder stream. modifications from instructions on the encoder stream.
The decoder MUST emit header fields in the order their The decoder MUST emit header fields in the order their
representations appear in the input header block. representations appear in the input header block.
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The decoder stream (Section 4.4) signals key events at the decoder The decoder stream (Section 4.4) signals key events at the decoder
that permit the encoder to track the decoder's state. These events that permit the encoder to track the decoder's state. These events
are: are:
o Complete processing of a header block o Complete processing of a header block
o Abandonment of a stream which might have remaining header blocks o Abandonment of a stream which might have remaining header blocks
o Receipt of new dynamic table entries o Receipt of new dynamic table entries
Knowledge that a header block with references to the dynamic table Knowledge that a header block with references to the dynamic table
has been processed permits the encoder to evict entries to which no has been processed permits the encoder to evict entries to which no
unacknowledged references remain, regardless of whether those unacknowledged references remain, regardless of whether those
references were potentially blocking (see Section 2.1.2). When a references were potentially blocking (see Section 2.1.2). When a
stream is reset or abandoned, the indication that these header blocks stream is reset or abandoned, the indication that these header blocks
will never be processed serves a similar function; see Section 4.4.3. will never be processed serves a similar function; see Section 4.4.3.
The decoder chooses when to emit Table State Synchronize instructions The decoder chooses when to emit Insert Count Increment instructions
(see Section 4.4.1). Emitting an instruction after adding each new (see Section 4.4.1). Emitting an instruction after adding each new
dynamic table entry will provide the most timely feedback to the dynamic table entry will provide the most timely feedback to the
encoder, but could be redundant with other decoder feedback. By encoder, but could be redundant with other decoder feedback. By
delaying a Table State Synchronize instruction, the decoder might be delaying an Insert Count Increment instruction, the decoder might be
able to coalesce multiple Table State Synchronize instructions, or able to coalesce multiple Insert Count Increment instructions, or
replace them entirely with Header Acknowledgements (see replace them entirely with Header Acknowledgements (see
Section 4.4.2). However, delaying too long may lead to compression Section 4.4.2). However, delaying too long may lead to compression
inefficiencies if the encoder waits for an entry to be acknowledged inefficiencies if the encoder waits for an entry to be acknowledged
before using it. before using it.
2.2.2. Blocked Decoding 2.2.2. Blocked Decoding
To track blocked streams, the necessary Largest Reference value for To track blocked streams, the Required Insert Count value for each
each stream can be used. Whenever the decoder processes a table stream can be used. Whenever the decoder processes a table update,
update, it can begin decoding any blocked streams that now have their it can begin decoding any blocked streams that now have their
dependencies satisfied. dependencies satisfied.
3. Header Tables 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.
3.1. Static Table 3.1. Static Table
skipping to change at page 10, line 5 skipping to change at page 10, line 16
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 4.3). Section 4.3).
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 the decoder. be treated as an error by the decoder.
3.2.1. Calculating Table Size 3.2.1. Dynamic 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 bytes (as The size of an entry is the sum of its name's length in bytes (as
defined in Section 4.1.2), its value's length in bytes, 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 Huffman encoding applied.
3.2.2. Eviction 3.2.2. Dynamic Table Capacity and Eviction
The encoder sets the capacity of the dynamic table, which serves as
the upper limit on its size.
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 (table capacity - size of new entry) or
table is empty. The encoder MUST NOT evict a dynamic table entry until the table is empty. The encoder MUST NOT evict a dynamic table
unless it has first been acknowledged by the decoder. entry 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 dynamic
size, that entry is added to the table. It is an error to attempt to table capacity, then that entry is added to the table. It is an
add an entry that is larger than the maximum size; this MUST be error if the encoder attempts to add an entry that is larger than the
treated as a connection error of type dynamic table capacity; the decoder MUST treat this as a connection
"HTTP_QPACK_ENCODER_STREAM_ERROR". error of type "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 or value if the referenced entry is evicted from the dynamic table
inserting the new entry. prior to inserting the new entry.
Whenever the maximum size for the dynamic table is reduced by the Whenever the dynamic table capacity is reduced by the encoder,
encoder, entries are evicted from the end of the dynamic table until entries are evicted from the end of the dynamic table until the size
the size of the dynamic table is less than or equal to the new of the dynamic table is less than or equal to the new table capacity.
maximum size. This mechanism can be used to completely clear entries This mechanism can be used to completely clear entries from the
from the dynamic table by setting a maxiumum size of 0, which can dynamic table by setting a capacity of 0, which can subsequently be
subsequently be restored. restored.
3.2.3. Maximum Table Size 3.2.3. Maximum Dynamic Table Capacity
The encoder decides how to update the dynamic table size and as such To bound the memory requirements of the decoder, the decoder limits
can control how much memory is used by the dynamic table. To limit the maximum value the encoder is permitted to set for the dynamic
the memory requirements of the decoder, the dynamic table size is table capacity. In HTTP/3, this limit is determined by the value of
strictly bounded. The decoder determines the maximum size that the SETTINGS_QPACK_MAX_TABLE_CAPACITY sent by the decoder (see
encoder is permitted to set for the dynamic table. In HTTP/3, this Section 5). The encoder MUST not set a dynamic table capacity that
value is determined by the SETTINGS_HEADER_TABLE_SIZE setting (see
Section 5). The encoder MUST not set a dynamic table size that
exceeds this maximum, but it can choose to use a lower dynamic table exceeds this maximum, but it can choose to use a lower dynamic table
size (see Section 4.3.4). capacity (see Section 4.3.4).
The initial maximum size is determined by the corresponding setting 3.2.4. Initial Dynamic Table Capacity
when HTTP requests or responses are first 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 later specifies a larger
maximum in its SETTINGS frame. For HTTP/3 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 in the peer's SETTINGS frame.
3.2.4. Absolute Indexing The initial dynamic table capacity is determined by the corresponding
setting when HTTP requests or responses are first permitted to be
sent. For clients using 0-RTT data in HTTP/3, the initial table
capacity is the remembered value of the setting, even if the server
later specifies a larger maximum dynamic table capacity in its
SETTINGS frame. For HTTP/3 servers and HTTP/3 clients when 0-RTT is
not attempted or is rejected, the initial table capacity is the value
of the setting in the peer's SETTINGS frame.
3.2.5. 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 based on lifetime of that entry and a relative index which changes based on
the context of the reference. The first entry inserted has an the context of the reference. The first entry inserted has an
absolute index of "1"; indices increase sequentially with each absolute index of "0"; indices increase by one with each insertion.
insertion.
3.2.5. Relative Indexing 3.2.6. 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.
+---+---------------+-----------+ +-----+---------------+-------+
| n | ... | d + 1 | Absolute Index | n-1 | ... | d | Absolute Index
+ - +---------------+ - - - - - + + - - +---------------+ - - - +
| 0 | ... | n - d - 1 | Relative Index | 0 | ... | n-d-1 | Relative Index
+---+---------------+-----------+ +-----+---------------+-------+
^ | ^ |
| V | V
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 Unlike on the encoder stream, relative indices on push and request
with instructions on the encoder stream, relative indices are streams are relative to the Base at the beginning of the header block
relative to the Base Index at the beginning of the header block (see (see Section 4.5.1). This ensures that references are stable even if
Section 4.5.1). The Base Index is an absolute index. When the dynamic table is updated while decoding a header block.
interpreting the rest of the frame, the entry identified by Base
Index has a relative index of zero. The relative indices of entries
do not change while interpreting headers on a request or push stream.
Base Index The Base is encoded as a value relative to the Required Insert Count.
| The Base identifies which dynamic table entries can be referenced
V using relative indexing, starting with 0 at the last entry added.
+---+-----+-----+-----+-------+
| n | n-1 | n-2 | ... | d+1 | Absolute Index Post-Base references are used for entries inserted after base,
+---+-----+ - +-----+ - + starting at 0 for the first entry added after the Base, see
| 0 | ... | n-d-3 | Relative Index Section 3.2.7.
+-----+-----+-------+
Required
Insert
Count Base
| |
V V
+-----+-----+-----+-----+-------+
| n-1 | n-2 | n-3 | ... | d | Absolute 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
3.2.6. Post-Base Indexing 3.2.7. 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. This allows an encoder to
to process a header block in a single pass and include references 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, with the zero value being the first entry inserted after the
Base.
Base Index Base
| |
V V
+---+-----+-----+-----+-----+ +-----+-----+-----+-----+-----+
| n | n-1 | n-2 | ... | d+1 | Absolute Index | n-1 | n-2 | n-3 | ... | d | 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
3.2.7. Invalid References 3.2.8. 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 evicted 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 or equal to the declared Required Insert
Section 4.5.1), it MUST treat this as a stream error of type Count (see Section 4.5.1), it MUST treat this as a stream error of
"HTTP_QPACK_DECOMPRESSION_FAILED". type "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".
4. Wire Format 4. Wire Format
4.1. Primitives 4.1. Primitives
4.1.1. Prefixed Integers 4.1.1. Prefixed Integers
skipping to change at page 14, line 10 skipping to change at page 14, line 39
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. These
of either unidirectional stream MUST be treated as a connection error streams MUST NOT be closed. Closure of either unidirectional stream
of type HTTP_CLOSED_CRITICAL_STREAM. MUST be treated as a connection error of type
HTTP_CLOSED_CRITICAL_STREAM.
This section describes the instructions which are possible on each This section describes the instructions which are possible on each
stream type. stream type.
4.3. Encoder Stream 4.3. 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
skipping to change at page 15, line 46 skipping to change at page 16, line 37
+---+---+---+-------------------+ +---+---+---+-------------------+
Figure 2: 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.
4.3.4. Dynamic Table Size Update 4.3.4. Set Dynamic Table Capacity
An encoder informs the decoder of a change to the size of the dynamic An encoder informs the decoder of a change to the dynamic table
table using an instruction which begins with the '001' three-bit capacity using an instruction which begins with the '001' three-bit
pattern. The new maximum table size is represented as an integer pattern. The new dynamic table capacity 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 | Capacity (5+) |
+---+---+---+-------------------+ +---+---+---+-------------------+
Figure 3: Maximum Dynamic Table Size Change Figure 3: Set Dynamic Table Capacity
The new maximum size MUST be lower than or equal to the limit The new capacity MUST be lower than or equal to the limit described
described in Section 3.2.3. In HTTP/3, this limit is the value of in Section 3.2.3. In HTTP/3, this limit is the value of the
the SETTINGS_HEADER_TABLE_SIZE parameter (see Section 5) received SETTINGS_QPACK_MAX_TABLE_CAPACITY parameter (see Section 5) received
from the decoder. The decoder MUST treat a value that exceeds this from the decoder. The decoder MUST treat a new dynamic table
limit as a connection error of type capacity value that exceeds this limit as a connection error of type
"HTTP_QPACK_ENCODER_STREAM_ERROR". "HTTP_QPACK_ENCODER_STREAM_ERROR".
Reducing the maximum size of the dynamic table can cause entries to Reducing the dynamic table capacity can cause entries to be evicted
be evicted (see Section 4.3 of [RFC7541]). This MUST NOT cause the (see Section 3.2.2). This MUST NOT cause the eviction of entries
eviction of entries with outstanding references (see Section 2.1.1). with outstanding references (see Section 2.1.1). Changing the
Changing the size of the dynamic table is not acknowledged as this capacity of the dynamic table is not acknowledged as this instruction
instruction does not insert an entry. does not insert an entry.
4.4. 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 decoder to the the dynamic table. Information is sent from the decoder to the
encoder; that is, the server informs the client about the processing encoder; that is, the server informs the client about the processing
of the client's header blocks and table updates, and the client of the client's header blocks and table updates, and the client
informs the server about the processing of the server's header blocks informs the server about the processing of the server's header 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.
4.4.1. Table State Synchronize 4.4.1. Insert Count Increment
The Table State Synchronize instruction begins with the '00' two-bit The Insert Count Increment 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 Insert Count Increment or
Header Acknowledgement that increased the Largest Known Received Header Acknowledgement that increased the Known Received Count for
dynamic table entry (see Section 2.1.4). This is encoded as a 6-bit the dynamic table (see Section 2.1.4). The Increment field is
prefix integer. The encoder uses this value to determine which table encoded as a 6-bit prefix integer. The encoder uses this value to
entries might cause a stream to become blocked, as described in determine which table entries might cause a stream to become blocked,
Section 2.2.1. 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 | Increment (6+) |
+---+---+-----------------------+ +---+---+-----------------------+
Figure 4: Table State Synchronize Figure 4: Insert Count Increment
An encoder that receives an Insert Count equal to zero or one that An encoder that receives an Increment field equal to zero or one that
increases Largest Known Received beyond what the encoder has sent increases the Known Received Count 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".
4.4.2. Header Acknowledgement 4.4.2. Header Acknowledgement
After processing a header block whose declared Largest Reference is After processing a header block whose declared Required Insert Count
not zero, the decoder emits a Header Acknowledgement instruction on is not zero, the decoder emits a Header Acknowledgement instruction
the decoder stream. The instruction begins with the '1' one-bit on 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 encoder to know when 7-bit prefix integer. It is used by the peer's encoder to know when
it is safe to evict an entry, and possibly update Largest Known it is safe to evict an entry, and possibly update the Known Received
Received. Count.
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 5: 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 Required
Reference has already been acknowledged, that MUST be treated as a Insert Count has already been acknowledged, that MUST be treated as a
connection error of type "HTTP_QPACK_DECODER_STREAM_ERROR". connection error of type "HTTP_QPACK_DECODER_STREAM_ERROR".
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 Known Received Count.
index. If a header block was potentially blocking, the If a header block was potentially blocking, the acknowledgement
acknowledgement implies that the decoder has received all dynamic implies that the decoder has received all dynamic table state
table state necessary to process the header block. If the Largest necessary to process the header block. If the Required Insert Count
Reference of an acknowledged header block was greater than the of an acknowledged header block was greater than the encoder's
encoder's current Largest Known Received index, the block's Largest current Known Received Count, the block's Required Insert Count
Reference becomes the new Largest Known Received. becomes the new Known Received Count.
4.4.3. Stream Cancellation 4.4.3. Stream Cancellation
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+) |
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Figure 6: Stream Cancellation Figure 6: Stream Cancellation
A stream that is reset might have multiple outstanding header blocks A stream that is reset might have multiple outstanding header blocks
with dynamic table references. When an endpoint receives a stream with dynamic table references. When an endpoint receives a stream
reset before the end of a stream, it generates a Stream Cancellation reset before the end of a stream, it generates a Stream Cancellation
instruction on the decoder stream. Similarly, when an endpoint instruction on the decoder stream. Similarly, when an endpoint
abandons reading of a stream it needs to signal this using the Stream abandons reading of a stream it needs to signal this using the Stream
Cancellation instruction. This signals to the encoder that all Cancellation instruction. This signals to the encoder that all
references to the dynamic table on that stream are no longer references to the dynamic table on that stream are no longer
outstanding. A decoder with a maximum dynamic table size equal to outstanding. A decoder with a maximum dynamic table capacity equal
zero (see Section 3.2.3) MAY omit sending Stream Cancellations, to zero (see Section 3.2.3) MAY omit sending Stream Cancellations,
because the encoder cannot have any dynamic table references. because the encoder cannot have any dynamic table references.
An encoder cannot infer from this instruction that any updates to the An encoder cannot infer from this instruction that any updates to the
dynamic table have been received. dynamic table have been received.
4.5. Request and Push Streams 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.
4.5.1. Header Data Prefix 4.5.1. Header Block Prefix
Header data is prefixed with two integers, "Largest Reference" and Each header block is prefixed with two integers. The Required Insert
"Base Index". Count is encoded as an integer with an 8-bit prefix after the
encoding described in Section 4.5.1.1). The Base is encoded as sign-
and-modulus integer, using a single sign bit and a value with a 7-bit
prefix (see Section 4.5.1.2).
These two values are followed by instructions for compressed headers.
The entire block is expected to be framed by the using protocol.
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
| Largest Reference (8+) | | Required Insert Count (8+) |
+---+---------------------------+ +---+---------------------------+
| S | Delta Base Index (7+) | | S | Delta Base (7+) |
+---+---------------------------+ +---+---------------------------+
| Compressed Headers ... | Compressed Headers ...
+-------------------------------+ +-------------------------------+
Figure 7: Frame Payload Figure 7: Frame Payload
4.5.1.1. Largest Reference 4.5.1.1. Required Insert Count
"Largest Reference" identifies the largest absolute dynamic index Required Insert Count identifies the state of the dynamic table
referenced in the block. Blocking decoders use the Largest Reference needed to process the header block. Blocking decoders use the
to determine when it is safe to process the rest of the block. If Required Insert Count to determine when it is safe to process the
Largest Reference is greater than zero, the encoder transforms it as rest of the block.
follows before encoding:
LargestReference = (LargestReference mod (2 * MaxEntries)) + 1 If no references are made to the dynamic table, a value of 0 is
encoded. Alternatively, where the Required Insert Count is greater
than zero, the encoder transforms it as follows before encoding:
EncodedInsertCount = (ReqInsertCount mod (2 * MaxEntries)) + 1
Here "MaxEntries" is the maximum number of entries that the dynamic Here "MaxEntries" is the maximum number of entries that the dynamic
table can have. The smallest entry has empty name and value strings table can have. The smallest entry has empty name and value strings
and has the size of 32. Hence "MaxEntries" is calculated as and has the size of 32. Hence "MaxEntries" is calculated as
MaxEntries = floor( MaxTableSize / 32 ) MaxEntries = floor( MaxTableCapacity / 32 )
"MaxTableSize" is the maximum size of the dynamic table as specified "MaxTableCapacity" is the maximum capacity of the dynamic table as
by the decoder (see Section 3.2.3). specified by the decoder (see Section 3.2.3).
The decoder reconstructs the Largest Reference using the following The decoder reconstructs the Required Insert Count using the
algorithm: following algorithm, where TotalNumberOfInserts is the total number
of inserts into the decoder's dynamic table:
if LargestReference > 0: if EncodedInsertCount == 0:
LargestReference -= 1 ReqInsertCount = 0
else:
InsertCount = EncodedInsertCount - 1
CurrentWrapped = TotalNumberOfInserts mod (2 * MaxEntries) CurrentWrapped = TotalNumberOfInserts mod (2 * MaxEntries)
if CurrentWrapped >= LargestReference + MaxEntries: if CurrentWrapped >= InsertCount + MaxEntries:
# Largest Reference wrapped around 1 extra time # Insert Count wrapped around 1 extra time
LargestReference += 2 * MaxEntries ReqInsertCount += 2 * MaxEntries
else if CurrentWrapped + MaxEntries < LargestReference else if CurrentWrapped + MaxEntries < InsertCount:
# Decoder wrapped around 1 extra time # Decoder wrapped around 1 extra time
CurrentWrapped += 2 * MaxEntries CurrentWrapped += 2 * MaxEntries
LargestReference += TotalNumberOfInserts - CurrentWrapped ReqInsertCount += TotalNumberOfInserts - CurrentWrapped
TotalNumberOfInserts is the total number of inserts into the This encoding limits the length of the prefix on long-lived
decoder's dynamic table. This encoding limits the length of the connections.
prefix on long-lived connections.
4.5.1.2. Base Index For example, if the dynamic table is 100 bytes, then the Required
Insert Count will be encoded modulo 6. If a decoder has received 10
inserts, then an encoded value of 3 indicates that the Required
Insert Count is 9 for the header block.
"Base Index" is used to resolve references in the dynamic table as 4.5.1.2. Base
described in Section 3.2.5.
To save space, Base Index is encoded relative to Largest Reference The "Base" is used to resolve references in the dynamic table as
using a one-bit sign and the "Delta Base Index" value. A sign bit of described in Section 3.2.6.
0 indicates that the Base Index has an absolute index that is greater
than or equal to the Largest Reference; the value of Delta Base Index
is added to the Largest Reference to determine the absolute value of
the Base Index. A sign bit of 1 indicates that the Base Index is
less than the Largest Reference. That is:
if sign == 0: To save space, the Base is encoded relative to the Insert Count using
baseIndex = largestReference + deltaBaseIndex a one-bit sign and the "Delta Base" value. A sign bit of 0 indicates
that the Base is greater than or equal to the value of the Insert
Count; the value of Delta Base is added to the Insert Count to
determine the value of the Base. A sign bit of 1 indicates that the
Base is less than the Insert Count. That is:
if S == 0:
Base = ReqInsertCount + DeltaBase
else: else:
baseIndex = largestReference - deltaBaseIndex - 1 Base = ReqInsertCount - DeltaBase - 1
A single-pass encoder determines the absolute value of Base Index A single-pass encoder determines the Base before encoding a header
before encoding a header block. If the encoder inserted entries in block. If the encoder inserted entries in the dynamic table while
the dynamic table while encoding the header block, Largest Reference encoding the header block, Required Insert Count will be greater than
will be greater than Base Index, so the encoded difference is the Base, so the encoded difference is negative and the sign bit is
negative and the sign bit is set to 1. If the header block did not set to 1. If the header block did not reference the most recent
reference the most recent entry in the table and did not insert any entry in the table and did not insert any new entries, the Base will
new entries, Base Index will be greater than the Largest Reference, be greater than the Required Insert Count, so the delta will be
so the delta will be positive and the sign bit is set to 0. positive and the sign bit is set to 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. In might set Required Insert Count and the Base to the same value. In
such case, both the sign bit and the Delta Base Index will be set to such case, both the sign bit and the Delta Base will be set to zero.
zero.
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 the Base; setting Delta Base to zero is the most efficient
to zero is the most efficient encoding. encoding.
For example, with an Required Insert Count of 9, a decoder receives a
S bit of 1 and a Delta Base of 2. This sets the Base to 6 and
enables post-base indexing for three entries. In this example, a
regular index of 1 refers to the 5th entry that was added to the
table; a post-base index of 1 refers to the 8th entry.
4.5.2. Indexed Header Field 4.5.2. 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+) |
+---+---+-----------------------+ +---+---+-----------------------+
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 the Base, this representation starts with
starts with the '1' 1-bit pattern, followed by the "S" bit indicating the '1' 1-bit pattern, followed by the "S" bit indicating whether the
whether the reference is into the static (S=1) or dynamic (S=0) reference is into the static (S=1) or dynamic (S=0) table. Finally,
table. Finally, the relative index of the matching header field is the relative index of the matching header field is represented as an
represented as an integer with a 6-bit prefix (see Section 5.1 of integer with a 6-bit prefix (see Section 5.1 of [RFC7541]).
[RFC7541]).
4.5.3. 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 or equal to the Base, the representation starts with the '0001'
pattern, followed by the post-base index (see Section 3.2.6) of the 4-bit pattern, followed by the post-base index (see Section 3.2.7) of
matching header field, represented as an integer with a 4-bit prefix the matching header field, represented as an integer with a 4-bit
(see Section 5.1 of [RFC7541]). 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 | 0 | 1 | Index (4+) | | 0 | 0 | 0 | 1 | Index (4+) |
+---+---+---+---+---------------+ +---+---+---+---+---------------+
Indexed Header Field with Post-Base Index Indexed Header Field with Post-Base Index
4.5.4. 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 the Base, this representation starts with
starts with the '01' two-bit pattern. If the entry is in the dynamic the '01' two-bit pattern. If the entry is in the dynamic table with
table with an absolute index greater than Base Index, the an absolute index greater than or equal to the Base, the
representation starts with the '0000' four-bit pattern. representation starts with the '0000' four-bit pattern.
The following bit, 'N', indicates whether an intermediary is The following bit, 'N', indicates whether an intermediary is
permitted to add this header to the dynamic header table on permitted to add this header to the dynamic header table on
subsequent hops. When the 'N' bit is set, the encoded header MUST subsequent hops. When the 'N' bit is set, the encoded header MUST
always be encoded with a literal representation. In particular, when always be encoded with a literal representation. In particular, when
a peer sends a header field that it received represented as a literal a peer sends a header field that it received represented as a literal
header field with the 'N' bit set, it MUST use a literal header field with the 'N' bit set, it MUST use a literal
representation to forward this header field. This bit is intended representation to forward this header field. This bit is intended
for protecting header field values that are not to be put at risk by for protecting header field values that are not to be put at risk by
skipping to change at page 22, line 20 skipping to change at page 23, line 19
| 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 bytes) | | 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 the Base, the header field name is
name is represented using the relative index of that entry, which 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.
4.5.5. 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 or equal to the Base, the header field name is represented using the
index of that entry (see Section 3.2.6) encoded as an integer with a post-base index of that entry (see Section 3.2.7) encoded as an
3-bit prefix. integer with a 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 bytes) | | Value String (Length bytes) |
+-------------------------------+ +-------------------------------+
skipping to change at page 23, line 34 skipping to change at page 24, line 32
| Value String (Length bytes) | | Value String (Length bytes) |
+-------------------------------+ +-------------------------------+
Literal Header Field Without Name Reference Literal Header Field Without Name Reference
5. Configuration 5. Configuration
QPACK defines two settings which are included in the HTTP/3 SETTINGS QPACK defines two settings which are included in the HTTP/3 SETTINGS
frame. frame.
SETTINGS_HEADER_TABLE_SIZE (0x1): An integer with a maximum value of SETTINGS_QPACK_MAX_TABLE_CAPACITY (0x1): An integer with a maximum
2^30 - 1. The default value is zero bytes. See Section 3.2 for value of 2^30 - 1. The default value is zero bytes. See
usage. Section 3.2 for usage. This is the equivalent of the
SETTINGS_HEADER_TABLE_SIZE from HTTP/2.
SETTINGS_QPACK_BLOCKED_STREAMS (0x7): An integer with a maximum 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. 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/3 to indicate failures The following error codes are defined for HTTP/3 to indicate failures
of QPACK which prevent the stream or connection from continuing: of QPACK which prevent the stream or connection from continuing:
HTTP_QPACK_DECOMPRESSION_FAILED (TBD): The decoder failed to HTTP_QPACK_DECOMPRESSION_FAILED (TBD): The decoder failed to
skipping to change at page 24, line 25 skipping to change at page 25, line 25
8. IANA Considerations 8. IANA Considerations
8.1. Settings Registration 8.1. Settings Registration
This document creates two new settings in the "HTTP/3 Settings" This document creates two new settings in the "HTTP/3 Settings"
registry established in [HTTP3]. 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 5 | | QPACK_MAX_TABLE_CAPACITY | 0x1 | Section 5 |
| | | | | | | |
| QPACK_BLOCKED_STREAMS | 0x7 | Section 5 | | QPACK_BLOCKED_STREAMS | 0x7 | Section 5 |
+-----------------------+------+---------------+ +--------------------------+------+---------------+
8.2. Stream Type Registration 8.2. Stream Type Registration
This document creates two new settings in the "HTTP/3 Stream Type" This document creates two new settings in the "HTTP/3 Stream Type"
registry established in [HTTP3]. 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 |
skipping to change at page 25, line 28 skipping to change at page 26, line 28
| 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 | |
+------------------------------+------+--------------+--------------+ +------------------------------+------+--------------+--------------+
9. References 9. References
9.1. Normative References 9.1. Normative References
[HTTP3] Bishop, M., Ed., "Hypertext Transfer Protocol Version 3 [HTTP3] Bishop, M., Ed., "Hypertext Transfer Protocol Version 3
(HTTP/3)", draft-ietf-quic-http-17 (work in progress), (HTTP/3)", draft-ietf-quic-http-18 (work in progress),
December 2018. January 2019.
[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-16 (work in progress), December 2018. transport-17 (work in progress), January 2019.
[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>.
skipping to change at page 33, line 10 skipping to change at page 34, line 10
encodeInteger(prefixBuffer, 0x80, encodeInteger(prefixBuffer, 0x80,
largestReference - baseIndex, 7) largestReference - baseIndex, 7)
return controlBuffer, prefixBuffer + streamBuffer return controlBuffer, prefixBuffer + streamBuffer
Appendix C. Change Log 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.
C.1. Since draft-ietf-quic-qpack-04 C.1. Since draft-ietf-quic-qpack-05
o Introduced the terms dynamic table capacity and maximum dynamic
table capacity.
o Renamed SETTINGS_HEADER_TABLE_SIZE to
SETTINGS_QPACK_MAX_TABLE_CAPACITY.
C.2. Since draft-ietf-quic-qpack-04
o Changed calculation of Delta Base Index to avoid an illegal value o Changed calculation of Delta Base Index to avoid an illegal value
(#2002, #2005) (#2002, #2005)
C.2. Since draft-ietf-quic-qpack-03 C.3. Since draft-ietf-quic-qpack-03
o Change HTTP settings defaults (#2038) o Change HTTP settings defaults (#2038)
o Substantial editorial reorganization o Substantial editorial reorganization
C.3. Since draft-ietf-quic-qpack-02 C.4. 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 43 skipping to change at page 35, line 5
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)
C.4. Since draft-ietf-quic-qpack-01 C.5. 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)
C.5. Since draft-ietf-quic-qpack-00 C.6. 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)
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)
C.6. Since draft-ietf-quic-qcram-00 C.7. 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. 105 change blocks. 
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