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.
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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
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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.
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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.
skipping to change at page 21, line 20 skipping to change at page 23, line 15
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. 
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