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QUIC C. Krasic
Internet-Draft Netflix
Intended status: Standards Track M. Bishop
Expires: 11 December 2020 Akamai Technologies
A. Frindell, Ed.
Facebook
9 June 2020
QPACK: Header Compression for HTTP/3
draft-ietf-quic-qpack-16
Abstract
This specification defines QPACK, a compression format for
efficiently representing HTTP fields, to be used in HTTP/3. This is
a variation of HPACK compression that seeks to reduce head-of-line
blocking.
Note to Readers
Discussion of this draft takes place on the QUIC working group
mailing list (quic@ietf.org (mailto:quic@ietf.org)), which is
archived at https://mailarchive.ietf.org/arch/
search/?email_list=quic.
Working Group information can be found at https://github.com/quicwg;
source code and issues list for this draft can be found at
https://github.com/quicwg/base-drafts/labels/-qpack.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 11 December 2020.
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Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Conventions and Definitions . . . . . . . . . . . . . . . 4
1.2. Notational Conventions . . . . . . . . . . . . . . . . . 5
2. Compression Process Overview . . . . . . . . . . . . . . . . 5
2.1. Encoder . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1.1. Limits on Dynamic Table Insertions . . . . . . . . . 6
2.1.2. Blocked Streams . . . . . . . . . . . . . . . . . . . 7
2.1.3. Avoiding Flow Control Deadlocks . . . . . . . . . . . 8
2.1.4. Known Received Count . . . . . . . . . . . . . . . . 9
2.2. Decoder . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2.1. Blocked Decoding . . . . . . . . . . . . . . . . . . 9
2.2.2. State Synchronization . . . . . . . . . . . . . . . . 10
2.2.3. Invalid References . . . . . . . . . . . . . . . . . 11
3. Reference Tables . . . . . . . . . . . . . . . . . . . . . . 11
3.1. Static Table . . . . . . . . . . . . . . . . . . . . . . 11
3.2. Dynamic Table . . . . . . . . . . . . . . . . . . . . . . 12
3.2.1. Dynamic Table Size . . . . . . . . . . . . . . . . . 12
3.2.2. Dynamic Table Capacity and Eviction . . . . . . . . . 12
3.2.3. Maximum Dynamic Table Capacity . . . . . . . . . . . 13
3.2.4. Absolute Indexing . . . . . . . . . . . . . . . . . . 13
3.2.5. Relative Indexing . . . . . . . . . . . . . . . . . . 14
3.2.6. Post-Base Indexing . . . . . . . . . . . . . . . . . 15
4. Wire Format . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.1. Primitives . . . . . . . . . . . . . . . . . . . . . . . 15
4.1.1. Prefixed Integers . . . . . . . . . . . . . . . . . . 15
4.1.2. String Literals . . . . . . . . . . . . . . . . . . . 15
4.2. Encoder and Decoder Streams . . . . . . . . . . . . . . . 16
4.3. Encoder Instructions . . . . . . . . . . . . . . . . . . 17
4.3.1. Set Dynamic Table Capacity . . . . . . . . . . . . . 17
4.3.2. Insert With Name Reference . . . . . . . . . . . . . 17
4.3.3. Insert Without Name Reference . . . . . . . . . . . . 18
4.3.4. Duplicate . . . . . . . . . . . . . . . . . . . . . . 18
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4.4. Decoder Instructions . . . . . . . . . . . . . . . . . . 19
4.4.1. Section Acknowledgement . . . . . . . . . . . . . . . 19
4.4.2. Stream Cancellation . . . . . . . . . . . . . . . . . 20
4.4.3. Insert Count Increment . . . . . . . . . . . . . . . 20
4.5. Field Line Representations . . . . . . . . . . . . . . . 20
4.5.1. Encoded Field Section Prefix . . . . . . . . . . . . 21
4.5.2. Indexed Field Line . . . . . . . . . . . . . . . . . 23
4.5.3. Indexed Field Line With Post-Base Index . . . . . . . 24
4.5.4. Literal Field Line With Name Reference . . . . . . . 24
4.5.5. Literal Field Line With Post-Base Name Reference . . 25
4.5.6. Literal Field Line Without Name Reference . . . . . . 25
5. Configuration . . . . . . . . . . . . . . . . . . . . . . . . 26
6. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 26
7. Security Considerations . . . . . . . . . . . . . . . . . . . 27
7.1. Probing Dynamic Table State . . . . . . . . . . . . . . . 27
7.2. Applicability to QPACK and HTTP . . . . . . . . . . . . . 28
7.3. Mitigation . . . . . . . . . . . . . . . . . . . . . . . 28
7.4. Never Indexed Literals . . . . . . . . . . . . . . . . . 29
7.5. Static Huffman Encoding . . . . . . . . . . . . . . . . . 30
7.6. Memory Consumption . . . . . . . . . . . . . . . . . . . 30
7.7. Implementation Limits . . . . . . . . . . . . . . . . . . 31
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
8.1. Settings Registration . . . . . . . . . . . . . . . . . . 31
8.2. Stream Type Registration . . . . . . . . . . . . . . . . 32
8.3. Error Code Registration . . . . . . . . . . . . . . . . . 32
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 33
9.1. Normative References . . . . . . . . . . . . . . . . . . 33
9.2. Informative References . . . . . . . . . . . . . . . . . 34
Appendix A. Static Table . . . . . . . . . . . . . . . . . . . . 34
Appendix B. Sample One Pass Encoding Algorithm . . . . . . . . . 39
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 40
C.1. Since draft-ietf-quic-qpack-15 . . . . . . . . . . . . . 40
C.2. Since draft-ietf-quic-qpack-14 . . . . . . . . . . . . . 41
C.3. Since draft-ietf-quic-qpack-13 . . . . . . . . . . . . . 41
C.4. Since draft-ietf-quic-qpack-12 . . . . . . . . . . . . . 41
C.5. Since draft-ietf-quic-qpack-11 . . . . . . . . . . . . . 41
C.6. Since draft-ietf-quic-qpack-10 . . . . . . . . . . . . . 41
C.7. Since draft-ietf-quic-qpack-09 . . . . . . . . . . . . . 41
C.8. Since draft-ietf-quic-qpack-08 . . . . . . . . . . . . . 41
C.9. Since draft-ietf-quic-qpack-06 . . . . . . . . . . . . . 41
C.10. Since draft-ietf-quic-qpack-05 . . . . . . . . . . . . . 41
C.11. Since draft-ietf-quic-qpack-04 . . . . . . . . . . . . . 42
C.12. Since draft-ietf-quic-qpack-03 . . . . . . . . . . . . . 42
C.13. Since draft-ietf-quic-qpack-02 . . . . . . . . . . . . . 42
C.14. Since draft-ietf-quic-qpack-01 . . . . . . . . . . . . . 42
C.15. Since draft-ietf-quic-qpack-00 . . . . . . . . . . . . . 42
C.16. Since draft-ietf-quic-qcram-00 . . . . . . . . . . . . . 43
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 43
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 44
1. Introduction
The QUIC transport protocol [QUIC-TRANSPORT] is designed to support
HTTP semantics, and its design subsumes many of the features of
HTTP/2 [RFC7540]. HTTP/2 uses HPACK [RFC7541] for compression of the
header and trailer sections. If HPACK were used for HTTP/3 [HTTP3],
it would induce head-of-line blocking for field sections due to
built-in assumptions of a total ordering across frames on all
streams.
QPACK reuses core concepts from HPACK, but is redesigned to allow
correctness in the presence of out-of-order delivery, with
flexibility for implementations to balance between resilience against
head-of-line blocking and optimal compression ratio. The design
goals are to closely approach the compression ratio of HPACK with
substantially less head-of-line blocking under the same loss
conditions.
1.1. 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:
HTTP fields: Metadata sent as part of an HTTP message. The term
encompasses both header and trailer fields. Colloquially, the
term "headers" has often been used to refer to HTTP header fields
and trailer fields; this document uses "fields" for generality.
HTTP field line: A name-value pair sent as part of an HTTP field
section. See Section 4 of [SEMANTICS].
HTTP field value: Data associated with a field name, composed from
all field line values with that field name in that section,
concatenated together and separated with commas.
Field section: An ordered collection of HTTP field lines associated
with an HTTP message. A field section can contain multiple field
lines with the same name. It can also contain duplicate field
lines. An HTTP message can include both header field and trailer
field sections.
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Representation: An instruction which represents a field line,
possibly by reference to the dynamic and static tables.
Encoder: An implementation which encodes field sections.
Decoder: An implementation which decodes encoded field sections.
Absolute Index: A unique index for each entry in the dynamic table.
Base: A reference point for relative and post-base indices.
Representations which reference dynamic table entries are relative
to a Base.
Insert Count: The total number of entries inserted in the dynamic
table.
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 4.1.1, 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 field lines
("headers") to indices. The static table (Section 3.1) is predefined
and contains common header field lines (some of them with an empty
value). The dynamic table (Section 3.2) is built up over the course
of the connection and can be used by the encoder to index both header
and trailer field lines in the encoded field sections.
QPACK defines unidirectional streams for sending instructions from
encoder to decoder and vice versa.
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2.1. Encoder
An encoder converts a header or trailer field section into a series
of representations by emitting either an indexed or a literal
representation for each field line in the list; see Section 4.5.
Indexed representations achieve high compression by replacing the
literal name and possibly the value with an index to either the
static or dynamic table. 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 field lines it is compressing.
QPACK preserves the ordering of field lines within each field
section. An encoder MUST emit field representations in the order
they appear in the input field section.
QPACK is designed to contain the more complex state tracking to the
encoder, while the decoder is relatively simple.
2.1.1. Limits on Dynamic Table Insertions
Inserting entries into the dynamic table might not be possible if the
table contains entries which cannot be evicted.
A dynamic table entry cannot be evicted immediately after insertion,
even if it has never been referenced. Once the insertion of a
dynamic table entry has been acknowledged and there are no
outstanding references to the entry in unacknowledged
representations, the entry becomes evictable. Note that references
on the encoder stream never preclude the eviction of an entry,
because those references are guaranteed to be processed before the
instruction evicting the entry.
If the dynamic table does not contain enough room for a new entry
without evicting other entries, and the entries which would be
evicted are not evictable, the encoder MUST NOT insert that entry
into the dynamic table (including duplicates of existing entries).
In order to avoid this, an encoder that uses the dynamic table has to
keep track of each dynamic table entry referenced by each field
section until those representations are acknowledged by the decoder;
see Section 4.4.1.
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2.1.1.1. Avoiding Prohibited Insertions
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 (Section 4.3.4), 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 non-blocking entries. To achieve
this, the encoder can maintain a draining index, which is the
smallest absolute index (Section 3.2.4) 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.
+--------+---------------------------------+----------+
| Unused | Referenceable | Draining |
| Space | Entries | Entries |
+--------+---------------------------------+----------+
^ ^ ^
| | |
Insertion Point Draining Index Dropping
Point
Figure 1: Draining Dynamic Table Entries
2.1.2. Blocked Streams
Because QUIC does not guarantee order between data on different
streams, a decoder might encounter a representation that references a
dynamic table entry that it has not yet received.
Each encoded field section contains a Required Insert Count
(Section 4.5.1), the lowest possible value for the Insert Count with
which the field section can be decoded. For a field section encoded
using references to the dynamic table, the Required Insert Count is
one larger than the largest absolute index of all referenced dynamic
table entries. For a field section encoded with no references to the
dynamic table, the Required Insert Count is zero.
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When the decoder receives an encoded field section with a Required
Insert Count greater than its own Insert Count, the stream cannot be
processed immediately, and is considered "blocked"; see
Section 2.2.1.
The decoder specifies an upper bound on the number of streams which
can be blocked using the SETTINGS_QPACK_BLOCKED_STREAMS setting; see
Section 5. An encoder MUST limit the number of streams which could
become blocked to the value of SETTINGS_QPACK_BLOCKED_STREAMS at all
times. If a decoder encounters more blocked streams than it promised
to support, it MUST treat this as a connection error of type
QPACK_DECOMPRESSION_FAILED.
Note that the decoder might not become blocked on every stream which
risks becoming 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 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
can avoid the risk of blocking by only referencing dynamic table
entries which have been acknowledged, but this could mean using
literals. Since literals make the encoded field section larger, this
can result in the encoder becoming blocked on congestion or flow
control limits.
2.1.3. Avoiding Flow Control Deadlocks
Writing instructions on streams that are limited by flow control can
produce deadlocks.
A decoder might stop issuing flow control credit on the stream that
carries an encoded field section until the necessary updates are
received on the encoder stream. If the granting of flow control
credit on the encoder stream (or the connection as a whole) depends
on the consumption and release of data on the stream carrying the
encoded field section, a deadlock might result.
More generally, a stream containing a large instruction can become
deadlocked if the decoder withholds flow control credit until the
instruction is completely received.
To avoid these deadlocks, an encoder SHOULD avoid writing an
instruction unless sufficient stream and connection flow control
credit is available for the entire instruction.
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2.1.4. Known Received Count
The Known Received Count is the total number of dynamic table
insertions and duplications acknowledged by the decoder. The encoder
tracks the Known Received Count in order to identify which dynamic
table entries can be referenced without potentially blocking a
stream. The decoder tracks the Known Received Count in order to be
able to send Insert Count Increment instructions.
A Section Acknowledgement instruction (Section 4.4.1) implies that
the decoder has received all dynamic table state necessary to decode
the field section. If the Required Insert Count of the acknowledged
field section is greater than the current Known Received Count, Known
Received Count is updated to the value of the Required Insert Count.
An Insert Count Increment instruction Section 4.4.3 increases the
Known Received Count by its Increment parameter. See Section 2.2.2.3
for guidance.
2.2. Decoder
As in HPACK, the decoder processes a series of representations and
emits the corresponding field sections. It also processes
instructions received on the encoder stream that modify the dynamic
table. Note that encoded field sections and encoder stream
instructions arrive on separate streams. This is unlike HPACK, where
encoded field sections (header blocks) can contain instructions that
modify the dynamic table, and there is no dedicated stream of HPACK
instructions.
The decoder MUST emit field lines in the order their representations
appear in the encoded field section.
2.2.1. Blocked Decoding
Upon receipt of an encoded field section, the decoder examines the
Required Insert Count. When the Required Insert Count is less than
or equal to the decoder's Insert Count, the field section can be
processed immediately. Otherwise, the stream on which the field
section was received becomes blocked.
While blocked, encoded field section data SHOULD remain in the
blocked stream's flow control window. A stream becomes unblocked
when the Insert Count becomes greater than or equal to the Required
Insert Count for all encoded field sections the decoder has started
reading from the stream.
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When processing encoded field sections, the decoder expects the
Required Insert Count to equal the lowest possible value for the
Insert Count with which the field section can be decoded, as
prescribed in Section 2.1.2. If it encounters a Required Insert
Count smaller than expected, it MUST treat this as a connection error
of type QPACK_DECOMPRESSION_FAILED; see Section 2.2.3. If it
encounters a Required Insert Count larger than expected, it MAY treat
this as a connection error of type QPACK_DECOMPRESSION_FAILED.
2.2.2. State Synchronization
The decoder signals the following events by emitting decoder
instructions (Section 4.4) on the decoder stream.
2.2.2.1. Completed Processing of a Field Section
After the decoder finishes decoding a field section encoded using
representations containing dynamic table references, it MUST emit a
Section Acknowledgement instruction (Section 4.4.1). A stream may
carry multiple field sections in the case of intermediate responses,
trailers, and pushed requests. The encoder interprets each
Section Acknowledgement instruction as acknowledging the earliest
unacknowledged field section containing dynamic table references sent
on the given stream.
2.2.2.2. Abandonment of a Stream
When an endpoint receives a stream reset before the end of a stream
or before all encoded field sections are processed on that stream, or
when it abandons reading of a stream, it generates a Stream
Cancellation instruction; see Section 4.4.2. 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
capacity (Section 3.2.3) 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 Section Acknowledgement and Stream Cancellation instructions
permit the encoder to remove references to entries in the dynamic
table. When an entry with absolute index lower than the Known
Received Count has zero references, then it is considered evictable;
see Section 2.1.1.
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2.2.2.3. New Table Entries
After receiving new table entries on the encoder stream, the decoder
chooses when to emit Insert Count Increment instructions; see
Section 4.4.3. Emitting this instruction after adding each new
dynamic table entry will provide the timeliest feedback to the
encoder, but could be redundant with other decoder feedback. By
delaying an Insert Count Increment instruction, the decoder might be
able to coalesce multiple Insert Count Increment instructions, or
replace them entirely with Section Acknowledgements; see
Section 4.4.1. However, delaying too long may lead to compression
inefficiencies if the encoder waits for an entry to be acknowledged
before using it.
2.2.3. Invalid References
If the decoder encounters a reference in a field line representation
to a dynamic table entry which has already been evicted or which has
an absolute index greater than or equal to the declared Required
Insert Count (Section 4.5.1), it MUST treat this as a connection
error of type QPACK_DECOMPRESSION_FAILED.
If the decoder encounters a reference in an encoder instruction to a
dynamic table entry which has already been evicted, it MUST treat
this as a connection error of type QPACK_ENCODER_STREAM_ERROR.
3. Reference Tables
Unlike in HPACK, entries in the QPACK static and dynamic tables are
addressed separately. The following sections describe how entries in
each table are addressed.
3.1. Static Table
The static table consists of a predefined static list of field lines,
each of which has a fixed index over time. Its entries are defined
in Appendix A.
All entries in the static table have a name and a value. However,
values can be empty (that is, have a length of 0). Each entry is
identified by a unique index.
Note that the QPACK static table is indexed from 0, whereas the HPACK
static table is indexed from 1.
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When the decoder encounters an invalid static table index in a field
line representation it MUST treat this as a connection error of type
QPACK_DECOMPRESSION_FAILED. If this index is received on the encoder
stream, this MUST be treated as a connection error of type
QPACK_ENCODER_STREAM_ERROR.
3.2. Dynamic Table
The dynamic table consists of a list of field lines maintained in
first-in, first-out order. Each HTTP/3 endpoint holds a dynamic
table that is initially empty. Entries are added by encoder
instructions received on the encoder stream; see Section 4.3.
The dynamic table can contain duplicate entries (i.e., entries with
the same name and same value). Therefore, duplicate entries MUST NOT
be treated as an error by the decoder.
Dynamic table entries can have empty values.
3.2.1. Dynamic Table Size
The size of the dynamic table is the sum of the size of its entries.
The size of an entry is the sum of its name's length in bytes, its
value's length in bytes, and 32. The size of an entry is calculated
using the length of its name and value without Huffman encoding
applied.
3.2.2. Dynamic Table Capacity and Eviction
The encoder sets the capacity of the dynamic table, which serves as
the upper limit on its size. The initial capacity of the dynamic
table is zero. The encoder sends a Set Dynamic Table Capacity
instruction (Section 4.3.1) with a non-zero capacity to begin using
the dynamic table.
Before a new entry is added to the dynamic table, entries are evicted
from the end of the dynamic table until the size of the dynamic table
is less than or equal to (table capacity - size of new entry). The
encoder MUST NOT cause a dynamic table entry to be evicted unless
that entry is evictable; see Section 2.1.1. The new entry is then
added to the table. It is an error if the encoder attempts to add an
entry that is larger than the dynamic table capacity; the decoder
MUST treat this as a connection error of type
QPACK_ENCODER_STREAM_ERROR.
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A new entry can reference an entry in the dynamic table that will be
evicted when adding this new entry into the dynamic table.
Implementations are cautioned to avoid deleting the referenced name
or value if the referenced entry is evicted from the dynamic table
prior to inserting the new entry.
Whenever the dynamic table capacity is reduced by the encoder
(Section 4.3.1), entries are evicted from the end of the dynamic
table until the size of the dynamic table is less than or equal to
the new table capacity. This mechanism can be used to completely
clear entries from the dynamic table by setting a capacity of 0,
which can subsequently be restored.
3.2.3. Maximum Dynamic Table Capacity
To bound the memory requirements of the decoder, the decoder limits
the maximum value the encoder is permitted to set for the dynamic
table capacity. In HTTP/3, this limit is determined by the value of
SETTINGS_QPACK_MAX_TABLE_CAPACITY sent by the decoder; see Section 5.
The encoder MUST not set a dynamic table capacity that exceeds this
maximum, but it can choose to use a lower dynamic table capacity; see
Section 4.3.1.
For clients using 0-RTT data in HTTP/3, the server's maximum table
capacity is the remembered value of the setting, or zero if the value
was not previously sent. When the client's 0-RTT value of the
SETTING is zero, the server MAY set it to a non-zero value in its
SETTINGS frame. If the remembered value is non-zero, the server MUST
send the same non-zero value in its SETTINGS frame. If it specifies
any other value, or omits SETTINGS_QPACK_MAX_TABLE_CAPACITY from
SETTINGS, the encoder must treat this as a connection error of type
QPACK_DECODER_STREAM_ERROR.
For HTTP/3 servers and HTTP/3 clients when 0-RTT is not attempted or
is rejected, the maximum table capacity is 0 until the encoder
processes a SETTINGS frame with a non-zero value of
SETTINGS_QPACK_MAX_TABLE_CAPACITY.
When the maximum table capacity is zero, the encoder MUST NOT insert
entries into the dynamic table, and MUST NOT send any encoder
instructions on the encoder stream.
3.2.4. Absolute Indexing
Each entry possesses an absolute index which is fixed for the
lifetime of that entry. The first entry inserted has an absolute
index of "0"; indices increase by one with each insertion.
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3.2.5. Relative Indexing
Relative indices begin at zero and increase in the opposite direction
from the absolute index. Determining which entry has a relative
index of "0" depends on the context of the reference.
In encoder instructions (Section 4.3), a relative index of "0" refers
to the most recently inserted value in the dynamic table. Note that
this means the entry referenced by a given relative index will change
while interpreting instructions on the encoder stream.
+-----+---------------+-------+
| n-1 | ... | d | Absolute Index
+ - - +---------------+ - - - +
| 0 | ... | n-d-1 | Relative Index
+-----+---------------+-------+
^ |
| V
Insertion Point Dropping Point
n = count of entries inserted
d = count of entries dropped
Figure 2: Example Dynamic Table Indexing - Encoder Stream
Unlike in encoder instructions, relative indices in field line
representations are relative to the Base at the beginning of the
encoded field section; see Section 4.5.1. This ensures that
references are stable even if encoded field sections and dynamic
table updates are processed out of order.
In a field line representation, a relative index of "0" refers to the
entry with absolute index equal to Base - 1.
Base
|
V
+-----+-----+-----+-----+-------+
| n-1 | n-2 | n-3 | ... | d | Absolute Index
+-----+-----+ - +-----+ - +
| 0 | ... | n-d-3 | Relative Index
+-----+-----+-------+
n = count of entries inserted
d = count of entries dropped
In this example, Base = n - 2
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Figure 3: Example Dynamic Table Indexing - Relative Index in
Representation
3.2.6. Post-Base Indexing
Post-Base indices are used in field line representations for entries
with absolute indices greater than or equal to Base, starting at 0
for the entry with absolute index equal to Base, and increasing in
the same direction as the absolute index.
Post-Base indices allow an encoder to process a field section in a
single pass and include references to entries added while processing
this (or other) field sections.
Base
|
V
+-----+-----+-----+-----+-----+
| n-1 | n-2 | n-3 | ... | d | Absolute Index
+-----+-----+-----+-----+-----+
| 1 | 0 | Post-Base Index
+-----+-----+
n = count of entries inserted
d = count of entries dropped
In this example, Base = n - 2
Figure 4: Example Dynamic Table Indexing - Post-Base Index in
Representation
4. Wire Format
4.1. Primitives
4.1.1. Prefixed Integers
The prefixed integer from Section 5.1 of [RFC7541] is used heavily
throughout this document. The format from [RFC7541] is used
unmodified. Note, however, that QPACK uses some prefix sizes not
actually used in HPACK.
QPACK implementations MUST be able to decode integers up to and
including 62 bits long.
4.1.2. String Literals
The string literal defined by Section 5.2 of [RFC7541] is also used
throughout. This string format includes optional Huffman encoding.
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HPACK defines string literals to begin on a byte boundary. They
begin with a single bit flag, denoted as 'H' in this document
(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.
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 prefix size, N, can have
a value between 2 and 8 inclusive. 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.
4.2. Encoder and Decoder Streams
QPACK defines two unidirectional stream types:
* An encoder stream is a unidirectional stream of type 0x02. It
carries an unframed sequence of encoder instructions from encoder
to decoder.
* A decoder stream is a unidirectional stream of type 0x03. It
carries an unframed sequence of decoder instructions from decoder
to encoder.
HTTP/3 endpoints contain a QPACK encoder and decoder. Each endpoint
MUST initiate at most one encoder stream and at most one decoder
stream. Receipt of a second instance of either stream type MUST be
treated as a connection error of type H3_STREAM_CREATION_ERROR.
These streams MUST NOT be closed. Closure of either unidirectional
stream type MUST be treated as a connection error of type
H3_CLOSED_CRITICAL_STREAM.
An endpoint MAY avoid creating an encoder stream if it's not going to
be used (for example if its encoder doesn't wish to use the dynamic
table, or if the maximum size of the dynamic table permitted by the
peer is zero).
An endpoint MAY avoid creating a decoder stream if its decoder sets
the maximum capacity of the dynamic table to zero.
An endpoint MUST allow its peer to create an encoder stream and a
decoder stream even if the connection's settings prevent their use.
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4.3. Encoder Instructions
An encoder sends encoder instructions on the encoder stream to set
the capacity of the dynamic table and add dynamic table entries.
Instructions adding table entries can use existing entries to avoid
transmitting redundant information. The name can be transmitted as a
reference to an existing entry in the static or the dynamic table or
as a string literal. For entries which already exist in the dynamic
table, the full entry can also be used by reference, creating a
duplicate entry.
This section specifies the following encoder instructions.
4.3.1. Set Dynamic Table Capacity
An encoder informs the decoder of a change to the dynamic table
capacity using an instruction which begins with the '001' three-bit
pattern. This is followed by the new dynamic table capacity
represented as an integer with a 5-bit prefix; see Section 4.1.1.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 1 | Capacity (5+) |
+---+---+---+-------------------+
Figure 5: Set Dynamic Table Capacity
The new capacity MUST be lower than or equal to the limit described
in Section 3.2.3. In HTTP/3, this limit is the value of the
SETTINGS_QPACK_MAX_TABLE_CAPACITY parameter (Section 5) received from
the decoder. The decoder MUST treat a new dynamic table capacity
value that exceeds this limit as a connection error of type
QPACK_ENCODER_STREAM_ERROR.
Reducing the dynamic table capacity can cause entries to be evicted;
see Section 3.2.2. This MUST NOT cause the eviction of entries which
are not evictable; see Section 2.1.1. Changing the capacity of the
dynamic table is not acknowledged as this instruction does not insert
an entry.
4.3.2. Insert With Name Reference
An encoder adds an entry to the dynamic table where the field name
matches the field name of an entry stored in the static or the
dynamic table using an instruction that starts with the '1' one-bit
pattern. The second ('T') bit indicates whether the reference is to
the static or dynamic table. The 6-bit prefix integer
(Section 4.1.1) that follows is used to locate the table entry for
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the field name. When T=1, the number represents the static table
index; when T=0, the number is the relative index of the entry in the
dynamic table.
The field name reference is followed by the field value represented
as a string literal; see Section 4.1.2.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | T | Name Index (6+) |
+---+---+-----------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length bytes) |
+-------------------------------+
Figure 6: Insert Field Line -- Indexed Name
4.3.3. Insert Without Name Reference
An encoder adds an entry to the dynamic table where both the field
name and the field value are represented as string literals using an
instruction that starts with the '01' two-bit pattern.
This is followed by the name represented as a 6-bit prefix string
literal, and the value represented as an 8-bit prefix string literal;
see Section 4.1.2.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 1 | H | Name Length (5+) |
+---+---+---+-------------------+
| Name String (Length bytes) |
+---+---------------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length bytes) |
+-------------------------------+
Figure 7: Insert Field Line -- New Name
4.3.4. Duplicate
An encoder duplicates an existing entry in the dynamic table using an
instruction that begins with the '000' three-bit pattern. This is
followed by the relative index of the existing entry represented as
an integer with a 5-bit prefix; see Section 4.1.1.
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0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | Index (5+) |
+---+---+---+-------------------+
Figure 8: Duplicate
The existing entry is re-inserted into the dynamic table without
resending either the name or the value. This is useful to avoid
adding a reference to an older entry, which might block inserting new
entries.
4.4. Decoder Instructions
A decoder sends decoder instructions on the decoder stream to inform
the encoder about the processing of field sections and table updates
to ensure consistency of the dynamic table.
This section specifies the following decoder instructions.
4.4.1. Section Acknowledgement
After processing an encoded field section whose declared Required
Insert Count is not zero, the decoder emits a Section Acknowledgement
instruction. The instruction begins with the '1' one-bit pattern
which is followed by the field section's associated stream ID encoded
as a 7-bit prefix integer; see Section 4.1.1.
This instruction is used as described in Section 2.1.4 and in
Section 2.2.2.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | Stream ID (7+) |
+---+---------------------------+
Figure 9: Section Acknowledgement
If an encoder receives a Section Acknowledgement instruction
referring to a stream on which every encoded field section with a
non-zero Required Insert Count has already been acknowledged, that
MUST be treated as a connection error of type
QPACK_DECODER_STREAM_ERROR.
The Section Acknowledgement instruction might increase the Known
Received Count; see Section 2.1.4.
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4.4.2. Stream Cancellation
When a stream is reset or reading is abandoned, the decoder emits a
Stream Cancellation instruction. The instruction begins with the
'01' two-bit pattern, which is followed by the stream ID of the
affected stream encoded as a 6-bit prefix integer.
This instruction is used as described in Section 2.2.2.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 1 | Stream ID (6+) |
+---+---+-----------------------+
Figure 10: Stream Cancellation
4.4.3. Insert Count Increment
The Insert Count Increment instruction begins with the '00' two-bit
pattern, followed by the Increment encoded as a 6-bit prefix integer.
This instruction increases the Known Received Count (Section 2.1.4)
by the value of the Increment parameter. The decoder should send an
Increment value that increases the Known Received Count to the total
number of dynamic table insertions and duplications processed so far.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | Increment (6+) |
+---+---+-----------------------+
Figure 11: Insert Count Increment
An encoder that receives an Increment field equal to zero, or one
that increases the Known Received Count beyond what the encoder has
sent MUST treat this as a connection error of type
QPACK_DECODER_STREAM_ERROR.
4.5. Field Line Representations
An encoded field section consists of a prefix and a possibly empty
sequence of representations defined in this section. Each
representation corresponds to a single field line. These
representations reference the static table or the dynamic table in a
particular state, but do not modify that state.
Encoded field sections are carried in frames on streams defined by
the enclosing protocol.
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4.5.1. Encoded Field Section Prefix
Each encoded field section is prefixed with two integers. The
Required Insert Count is encoded as an integer with an 8-bit prefix
after the encoding described in Section 4.5.1.1). The Base is
encoded as a sign bit ('S') and a Delta Base value with a 7-bit
prefix; see Section 4.5.1.2.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Required Insert Count (8+) |
+---+---------------------------+
| S | Delta Base (7+) |
+---+---------------------------+
| Encoded Field Lines ...
+-------------------------------+
Figure 12: Encoded Field Section
4.5.1.1. Required Insert Count
Required Insert Count identifies the state of the dynamic table
needed to process the encoded field section. Blocking decoders use
the Required Insert Count to determine when it is safe to process the
rest of the field section.
The encoder transforms the Required Insert Count as follows before
encoding:
if ReqInsertCount == 0:
EncInsertCount = 0
else:
EncInsertCount = (ReqInsertCount mod (2 * MaxEntries)) + 1
Here "MaxEntries" is the maximum number of entries that the dynamic
table can have. The smallest entry has empty name and value strings
and has the size of 32. Hence "MaxEntries" is calculated as
MaxEntries = floor( MaxTableCapacity / 32 )
"MaxTableCapacity" is the maximum capacity of the dynamic table as
specified by the decoder; see Section 3.2.3.
This encoding limits the length of the prefix on long-lived
connections.
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The decoder can reconstruct the Required Insert Count using an
algorithm such as the following. If the decoder encounters a value
of EncodedInsertCount that could not have been produced by a
conformant encoder, it MUST treat this as a connection error of type
QPACK_DECOMPRESSION_FAILED.
TotalNumberOfInserts is the total number of inserts into the
decoder's dynamic table.
FullRange = 2 * MaxEntries
if EncodedInsertCount == 0:
ReqInsertCount = 0
else:
if EncodedInsertCount > FullRange:
Error
MaxValue = TotalNumberOfInserts + MaxEntries
# MaxWrapped is the largest possible value of
# ReqInsertCount that is 0 mod 2*MaxEntries
MaxWrapped = floor(MaxValue / FullRange) * FullRange
ReqInsertCount = MaxWrapped + EncodedInsertCount - 1
# If ReqInsertCount exceeds MaxValue, the Encoder's value
# must have wrapped one fewer time
if ReqInsertCount > MaxValue:
if ReqInsertCount <= FullRange:
Error
ReqInsertCount -= FullRange
# Value of 0 must be encoded as 0.
if ReqInsertCount == 0:
Error
For example, if the dynamic table is 100 bytes, then the Required
Insert Count will be encoded modulo 6. If a decoder has received 10
inserts, then an encoded value of 4 indicates that the Required
Insert Count is 9 for the field section.
4.5.1.2. Base
The Base is used to resolve references in the dynamic table as
described in Section 3.2.5.
To save space, the Base is encoded relative to the Required Insert
Count using a one-bit sign ('S') and the Delta Base value. A sign
bit of 0 indicates that the Base is greater than or equal to the
value of the Required Insert Count; the decoder adds the value of
Delta Base to the Required Insert Count to determine the value of the
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Base. A sign bit of 1 indicates that the Base is less than the
Required Insert Count; the decoder subtracts the value of Delta Base
from the Required Insert Count and also subtracts one to determine
the value of the Base. That is:
if S == 0:
Base = ReqInsertCount + DeltaBase
else:
Base = ReqInsertCount - DeltaBase - 1
A single-pass encoder determines the Base before encoding a field
section. If the encoder inserted entries in the dynamic table while
encoding the field section, Required Insert Count will be greater
than the Base, so the encoded difference is negative and the sign bit
is set to 1. If the field section was not encoded using
representations which reference the most recent entry in the table
and did not insert any new entries, the Base will be greater than the
Required Insert Count, so the delta will be positive and the sign bit
is set to 0.
An encoder that produces table updates before encoding a field
section might set Base to the value of Required Insert Count. In
such case, both the sign bit and the Delta Base will be set to zero.
A field section that was encoded without references to the dynamic
table can use any value for the Base; setting Delta Base to zero is
one of the most efficient encodings.
For example, with a Required Insert Count of 9, a decoder receives an
S bit of 1 and a Delta Base of 2. This sets the Base to 6 and
enables post-base indexing for three entries. In this example, a
relative index of 1 refers to the 5th entry that was added to the
table; a post-base index of 1 refers to the 8th entry.
4.5.2. Indexed Field Line
An indexed field line representation identifies an entry in the
static table, or an entry in the dynamic table with an absolute index
less than the value of the Base.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | T | Index (6+) |
+---+---+-----------------------+
Figure 13: Indexed Field Line
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This representation starts with the '1' 1-bit pattern, followed by
the 'T' bit indicating whether the reference is into the static or
dynamic table. The 6-bit prefix integer (Section 4.1.1) that follows
is used to locate the table entry for the field line. When T=1, the
number represents the static table index; when T=0, the number is the
relative index of the entry in the dynamic table.
4.5.3. Indexed Field Line With Post-Base Index
An indexed field line with post-base index representation identifies
an entry in the dynamic table with an absolute index greater than or
equal to the value of the Base.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 1 | Index (4+) |
+---+---+---+---+---------------+
Figure 14: Indexed Field Line with Post-Base Index
This representation starts with the '0001' 4-bit pattern. This is
followed by the post-base index (Section 3.2.6) of the matching field
line, represented as an integer with a 4-bit prefix; see
Section 4.1.1.
4.5.4. Literal Field Line With Name Reference
A literal field line with name reference representation encodes a
field line where the field name matches the field name of an entry in
the static table, or the field name of an entry in the dynamic table
with an absolute index less than the value of the Base.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 1 | N | T |Name Index (4+)|
+---+---+---+---+---------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length bytes) |
+-------------------------------+
Figure 15: Literal Field Line With Name Reference
This representation starts with the '01' two-bit pattern. The
following bit, 'N', indicates whether an intermediary is permitted to
add this field line to the dynamic table on subsequent hops. When
the 'N' bit is set, the encoded field line MUST always be encoded
with a literal representation. In particular, when a peer sends a
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field line that it received represented as a literal field line with
the 'N' bit set, it MUST use a literal representation to forward this
field line. This bit is intended for protecting field values that
are not to be put at risk by compressing them; see Section 7 for more
details.
The fourth ('T') bit indicates whether the reference is to the static
or dynamic table. The 4-bit prefix integer (Section 4.1.1) that
follows is used to locate the table entry for the field name. When
T=1, the number represents the static table index; when T=0, the
number is the relative index of the entry in the dynamic table.
Only the field name is taken from the dynamic table entry; the field
value is encoded as an 8-bit prefix string literal; see
Section 4.1.2.
4.5.5. Literal Field Line With Post-Base Name Reference
A literal field line with post-base name reference representation
encodes a field line where the field name matches the field name of a
dynamic table entry with an absolute index greater than or equal to
the value of the Base.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 0 | N |NameIdx(3+)|
+---+---+---+---+---+-----------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length bytes) |
+-------------------------------+
Figure 16: Literal Field Line With Post-Base Name Reference
This representation starts with the '0000' four-bit pattern. The
fifth bit is the 'N' bit as described in Section 4.5.4. This is
followed by a post-base index of the dynamic table entry
(Section 3.2.6) encoded as an integer with a 3-bit prefix; see
Section 4.1.1.
Only the field name is taken from the dynamic table entry; the field
value is encoded as an 8-bit prefix string literal; see
Section 4.1.2.
4.5.6. Literal Field Line Without Name Reference
The literal field line without name reference representation encodes
a field name and a field value as string literals.
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0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 1 | N | H |NameLen(3+)|
+---+---+---+---+---+-----------+
| Name String (Length bytes) |
+---+---------------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length bytes) |
+-------------------------------+
Figure 17: Literal Field Line Without Name Reference
This representation begins with the '001' three-bit pattern. The
fourth bit is the 'N' bit as described in Section 4.5.4. The name
follows, represented as a 4-bit prefix string literal, then the
value, represented as an 8-bit prefix string literal; see
Section 4.1.2.
5. Configuration
QPACK defines two settings which are included in the HTTP/3 SETTINGS
frame.
SETTINGS_QPACK_MAX_TABLE_CAPACITY (0x1): The default value is zero.
See Section 3.2 for usage. This is the equivalent of the
SETTINGS_HEADER_TABLE_SIZE from HTTP/2.
SETTINGS_QPACK_BLOCKED_STREAMS (0x7): The default value is zero.
See Section 2.1.2.
6. Error Handling
The following error codes are defined for HTTP/3 to indicate failures
of QPACK which prevent the connection from continuing:
QPACK_DECOMPRESSION_FAILED (0x200): The decoder failed to interpret
an encoded field section and is not able to continue decoding that
field section.
QPACK_ENCODER_STREAM_ERROR (0x201): The decoder failed to interpret
an encoder instruction received on the encoder stream.
QPACK_DECODER_STREAM_ERROR (0x202): The encoder failed to interpret
a decoder instruction received on the decoder stream.
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7. Security Considerations
This section describes potential areas of security concern with
QPACK:
* Use of compression as a length-based oracle for verifying guesses
about secrets that are compressed into a shared compression
context.
* Denial of service resulting from exhausting processing or memory
capacity at a decoder.
7.1. Probing Dynamic Table State
QPACK reduces the length of header field encodings by exploiting the
redundancy inherent in protocols like HTTP. The ultimate goal of
this is to reduce the amount of data that is required to send HTTP
requests or responses.
The compression context used to encode header fields can be probed by
an attacker who can both define header fields to be encoded and
transmitted and observe the length of those fields once they are
encoded. When an attacker can do both, they can adaptively modify
requests in order to confirm guesses about the dynamic table state.
If a guess is compressed into a shorter length, the attacker can
observe the encoded length and infer that the guess was correct.
This is possible even over the Transport Layer Security Protocol
(TLS, see [RFC5246]), because while TLS provides confidentiality
protection for content, it only provides a limited amount of
protection for the length of that content.
Note: Padding schemes only provide limited protection against an
attacker with these capabilities, potentially only forcing an
increased number of guesses to learn the length associated with a
given guess. Padding schemes also work directly against
compression by increasing the number of bits that are transmitted.
Attacks like CRIME [CRIME] demonstrated the existence of these
general attacker capabilities. The specific attack exploited the
fact that DEFLATE [RFC1951] removes redundancy based on prefix
matching. This permitted the attacker to confirm guesses a character
at a time, reducing an exponential-time attack into a linear-time
attack.
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7.2. Applicability to QPACK and HTTP
QPACK mitigates but does not completely prevent attacks modeled on
CRIME [CRIME] by forcing a guess to match an entire header field
value, rather than individual characters. An attacker can only learn
whether a guess is correct or not, so is reduced to a brute force
guess for the header field values.
The viability of recovering specific header field values therefore
depends on the entropy of values. As a result, values with high
entropy are unlikely to be recovered successfully. However, values
with low entropy remain vulnerable.
Attacks of this nature are possible any time that two mutually
distrustful entities control requests or responses that are placed
onto a single HTTP/3 connection. If the shared QPACK compressor
permits one entity to add entries to the dynamic table, and the other
to access those entries, then the state of the table can be learned.
Having requests or responses from mutually distrustful entities
occurs when an intermediary either:
* sends requests from multiple clients on a single connection toward
an origin server, or
* takes responses from multiple origin servers and places them on a
shared connection toward a client.
Web browsers also need to assume that requests made on the same
connection by different web origins [RFC6454] are made by mutually
distrustful entities.
7.3. Mitigation
Users of HTTP that require confidentiality for header fields can use
values with entropy sufficient to make guessing infeasible. However,
this is impractical as a general solution because it forces all users
of HTTP to take steps to mitigate attacks. It would impose new
constraints on how HTTP is used.
Rather than impose constraints on users of HTTP, an implementation of
QPACK can instead constrain how compression is applied in order to
limit the potential for dynamic table probing.
An ideal solution segregates access to the dynamic table based on the
entity that is constructing header fields. Header field values that
are added to the table are attributed to an entity, and only the
entity that created a particular value can extract that value.
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To improve compression performance of this option, certain entries
might be tagged as being public. For example, a web browser might
make the values of the Accept-Encoding header field available in all
requests.
An encoder without good knowledge of the provenance of header fields
might instead introduce a penalty for a header field with many
different values, such that a large number of attempts to guess a
header field value results in the header field not being compared to
the dynamic table entries in future messages, effectively preventing
further guesses.
Note: Simply removing entries corresponding to the header field from
the dynamic table can be ineffectual if the attacker has a
reliable way of causing values to be reinstalled. For example, a
request to load an image in a web browser typically includes the
Cookie header field (a potentially highly valued target for this
sort of attack), and web sites can easily force an image to be
loaded, thereby refreshing the entry in the dynamic table.
This response might be made inversely proportional to the length of
the header field value. Disabling access to the dynamic table for a
header field might occur for shorter values more quickly or with
higher probability than for longer values.
7.4. Never Indexed Literals
Implementations can also choose to protect sensitive header fields by
not compressing them and instead encoding their value as literals.
Refusing to insert a header field into the dynamic table is only
effective if doing so is avoided on all hops. The never indexed
literal bit (see Section 4.5.4) can be used to signal to
intermediaries that a particular value was intentionally sent as a
literal.
An intermediary MUST NOT re-encode a value that uses a literal
representation with the 'N' bit set with another representation that
would index it. If QPACK is used for re-encoding, a literal
representation with the 'N' bit set MUST be used. If HPACK is used
for re-encoding, the never indexed literal representation (see
Section 6.2.3 of [RFC7541]) MUST be used.
The choice to mark that a header field should never be indexed
depends on several factors. Since QPACK doesn't protect against
guessing an entire header field value, short or low-entropy values
are more readily recovered by an adversary. Therefore, an encoder
might choose not to index values with low entropy.
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An encoder might also choose not to index values for header fields
that are considered to be highly valuable or sensitive to recovery,
such as the Cookie or Authorization header fields.
On the contrary, an encoder might prefer indexing values for header
fields that have little or no value if they were exposed. For
instance, a User-Agent header field does not commonly vary between
requests and is sent to any server. In that case, confirmation that
a particular User-Agent value has been used provides little value.
Note that these criteria for deciding to use a never indexed literal
representation will evolve over time as new attacks are discovered.
7.5. Static Huffman Encoding
There is no currently known attack against a static Huffman encoding.
A study has shown that using a static Huffman encoding table created
an information leakage, however this same study concluded that an
attacker could not take advantage of this information leakage to
recover any meaningful amount of information (see [PETAL]).
7.6. Memory Consumption
An attacker can try to cause an endpoint to exhaust its memory.
QPACK is designed to limit both the peak and stable amounts of memory
allocated by an endpoint.
The amount of memory used by the encoder is limited by the protocol
using QPACK through the definition of the maximum size of the dynamic
table, and the maximum number of blocking streams. In HTTP/3, these
values are controlled by the decoder through the settings parameters
SETTINGS_QPACK_MAX_TABLE_CAPACITY and SETTINGS_QPACK_BLOCKED_STREAMS,
respectively (see Section 3.2.3 and Section 2.1.2). The limit on the
size of the dynamic table takes into account the size of the data
stored in the dynamic table, plus a small allowance for overhead.
The limit on the number of blocked streams is only a proxy for the
maximum amount of memory required by the decoder. The actual maximum
amount of memory will depend on how much memory the decoder uses to
track each blocked stream.
A decoder can limit the amount of state memory used for the dynamic
table by setting an appropriate value for the maximum size of the
dynamic table. In HTTP/3, this is realized by setting an appropriate
value for the SETTINGS_QPACK_MAX_TABLE_CAPACITY parameter. An
encoder can limit the amount of state memory it uses by signaling a
lower dynamic table size than the decoder allows (see Section 3.2.2).
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A decoder can limit the amount of state memory used for blocked
streams by setting an appropriate value for the maximum number of
blocked streams. In HTTP/3, this is realized by setting an
appropriate value for the QPACK_BLOCKED_STREAMS parameter. An
encoder can limit the amount of state memory by only using as many
blocked streams as it wishes to support; no signaling to the decoder
is required.
The amount of temporary memory consumed by an encoder or decoder can
be limited by processing header fields sequentially. A decoder
implementation does not need to retain a complete list of header
fields while decoding a header block. An encoder implementation does
not need to retain a complete list of header fields while encoding a
header block if it is using a single-pass algorithm. Note that it
might be necessary for an application to retain a complete header
list for other reasons; even if QPACK does not force this to occur,
application constraints might make this necessary.
While the negotiated limit on the dynamic table size accounts for
much of the memory that can be consumed by a QPACK implementation,
data which cannot be immediately sent due to flow control is not
affected by this limit. Implementations should limit the size of
unsent data, especially on the decoder stream where flexibility to
choose what to send is limited. Possible responses to an excess of
unsent data might include limiting the ability of the peer to open
new streams, reading only from the encoder stream, or closing the
connection.
7.7. Implementation Limits
An implementation of QPACK needs to ensure that large values for
integers, long encoding for integers, or long string literals do not
create security weaknesses.
An implementation has to set a limit for the values it accepts for
integers, as well as for the encoded length (see Section 4.1.1). In
the same way, it has to set a limit to the length it accepts for
string literals (see Section 4.1.2).
8. IANA Considerations
8.1. Settings Registration
This document specifies two settings. The entries in the following
table are registered in the "HTTP/3 Settings" registry established in
[HTTP3].
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+--------------------------+------+---------------+---------+
| Setting Name | Code | Specification | Default |
+==========================+======+===============+=========+
| QPACK_MAX_TABLE_CAPACITY | 0x1 | Section 5 | 0 |
+--------------------------+------+---------------+---------+
| QPACK_BLOCKED_STREAMS | 0x7 | Section 5 | 0 |
+--------------------------+------+---------------+---------+
Table 1
8.2. Stream Type Registration
This document specifies two stream types. The entries in the
following table are registered in the "HTTP/3 Stream Type" registry
established in [HTTP3].
+----------------------+------+---------------+--------+
| Stream Type | Code | Specification | Sender |
+======================+======+===============+========+
| QPACK Encoder Stream | 0x02 | Section 4.2 | Both |
+----------------------+------+---------------+--------+
| QPACK Decoder Stream | 0x03 | Section 4.2 | Both |
+----------------------+------+---------------+--------+
Table 2
8.3. Error Code Registration
This document specifies three error codes. The entries in the
following table are registered in the "HTTP/3 Error Code" registry
established in [HTTP3].
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+----------------------------+-------+-------------+---------------+
| Name | Code | Description | Specification |
+============================+=======+=============+===============+
| QPACK_DECOMPRESSION_FAILED | 0x200 | Decoding of | Section 6 |
| | | a field | |
| | | section | |
| | | failed | |
+----------------------------+-------+-------------+---------------+
| QPACK_ENCODER_STREAM_ERROR | 0x201 | Error on | Section 6 |
| | | the encoder | |
| | | stream | |
+----------------------------+-------+-------------+---------------+
| QPACK_DECODER_STREAM_ERROR | 0x202 | Error on | Section 6 |
| | | the decoder | |
| | | stream | |
+----------------------------+-------+-------------+---------------+
Table 3
9. References
9.1. Normative References
[HTTP3] Bishop, M., Ed., "Hypertext Transfer Protocol Version 3
(HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
quic-http-29, 9 June 2020,
<https://tools.ietf.org/html/draft-ietf-quic-http-29>.
[QUIC-TRANSPORT]
Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", Work in Progress,
Internet-Draft, draft-ietf-quic-transport-29, 9 June 2020,
<https://tools.ietf.org/html/draft-ietf-quic-transport-
29>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC7541] Peon, R. and H. Ruellan, "HPACK: Header Compression for
HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
<https://www.rfc-editor.org/info/rfc7541>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
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[SEMANTICS]
Fielding, R., Nottingham, M., and J. Reschke, "HTTP
Semantics", Work in Progress, Internet-Draft, draft-ietf-
httpbis-semantics-08, 26 May 2020, <http://www.ietf.org/
internet-drafts/draft-ietf-httpbis-semantics-08.txt>.
9.2. Informative References
[CRIME] Wikipedia, "CRIME", May 2015, <http://en.wikipedia.org/w/
index.php?title=CRIME&oldid=660948120>.
[PETAL] Tan, J. and J. Nahata, "PETAL: Preset Encoding
Table Information Leakage", April 2013,
<http://www.pdl.cmu.edu/PDL-FTP/associated/CMU-PDL-
13-106.pdf>.
[RFC1951] Deutsch, P., "DEFLATE Compressed Data Format Specification
version 1.3", RFC 1951, DOI 10.17487/RFC1951, May 1996,
<https://www.rfc-editor.org/info/rfc1951>.
[RFC2360] Scott, G., "Guide for Internet Standards Writers", BCP 22,
RFC 2360, DOI 10.17487/RFC2360, June 1998,
<https://www.rfc-editor.org/info/rfc2360>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC6454, December 2011,
<https://www.rfc-editor.org/info/rfc6454>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.
Appendix A. Static Table
This table was generated by analyzing actual internet traffic in 2018
and including the most common headers, after filtering out some
unsupported and non-standard values. Due to this methodology, some
of the entries may be inconsistent or appear multiple times with
similar but not identical values. The order of the entries is
optimized to encode the most common headers with the smallest number
of bytes.
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+-------+----------------------------------+-----------------------+
| Index | Name | Value |
+=======+==================================+=======================+
| 0 | :authority | |
+-------+----------------------------------+-----------------------+
| 1 | :path | / |
+-------+----------------------------------+-----------------------+
| 2 | age | 0 |
+-------+----------------------------------+-----------------------+
| 3 | content-disposition | |
+-------+----------------------------------+-----------------------+
| 4 | content-length | 0 |
+-------+----------------------------------+-----------------------+
| 5 | cookie | |
+-------+----------------------------------+-----------------------+
| 6 | date | |
+-------+----------------------------------+-----------------------+
| 7 | etag | |
+-------+----------------------------------+-----------------------+
| 8 | if-modified-since | |
+-------+----------------------------------+-----------------------+
| 9 | if-none-match | |
+-------+----------------------------------+-----------------------+
| 10 | last-modified | |
+-------+----------------------------------+-----------------------+
| 11 | link | |
+-------+----------------------------------+-----------------------+
| 12 | location | |
+-------+----------------------------------+-----------------------+
| 13 | referer | |
+-------+----------------------------------+-----------------------+
| 14 | set-cookie | |
+-------+----------------------------------+-----------------------+
| 15 | :method | CONNECT |
+-------+----------------------------------+-----------------------+
| 16 | :method | DELETE |
+-------+----------------------------------+-----------------------+
| 17 | :method | GET |
+-------+----------------------------------+-----------------------+
| 18 | :method | HEAD |
+-------+----------------------------------+-----------------------+
| 19 | :method | OPTIONS |
+-------+----------------------------------+-----------------------+
| 20 | :method | POST |
+-------+----------------------------------+-----------------------+
| 21 | :method | PUT |
+-------+----------------------------------+-----------------------+
| 22 | :scheme | http |
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+-------+----------------------------------+-----------------------+
| 23 | :scheme | https |
+-------+----------------------------------+-----------------------+
| 24 | :status | 103 |
+-------+----------------------------------+-----------------------+
| 25 | :status | 200 |
+-------+----------------------------------+-----------------------+
| 26 | :status | 304 |
+-------+----------------------------------+-----------------------+
| 27 | :status | 404 |
+-------+----------------------------------+-----------------------+
| 28 | :status | 503 |
+-------+----------------------------------+-----------------------+
| 29 | accept | */* |
+-------+----------------------------------+-----------------------+
| 30 | accept | application/dns- |
| | | message |
+-------+----------------------------------+-----------------------+
| 31 | accept-encoding | gzip, deflate, br |
+-------+----------------------------------+-----------------------+
| 32 | accept-ranges | bytes |
+-------+----------------------------------+-----------------------+
| 33 | access-control-allow-headers | cache-control |
+-------+----------------------------------+-----------------------+
| 34 | access-control-allow-headers | content-type |
+-------+----------------------------------+-----------------------+
| 35 | access-control-allow-origin | * |
+-------+----------------------------------+-----------------------+
| 36 | cache-control | max-age=0 |
+-------+----------------------------------+-----------------------+
| 37 | cache-control | max-age=2592000 |
+-------+----------------------------------+-----------------------+
| 38 | cache-control | max-age=604800 |
+-------+----------------------------------+-----------------------+
| 39 | cache-control | no-cache |
+-------+----------------------------------+-----------------------+
| 40 | cache-control | no-store |
+-------+----------------------------------+-----------------------+
| 41 | cache-control | public, max- |
| | | age=31536000 |
+-------+----------------------------------+-----------------------+
| 42 | content-encoding | br |
+-------+----------------------------------+-----------------------+
| 43 | content-encoding | gzip |
+-------+----------------------------------+-----------------------+
| 44 | content-type | application/dns- |
| | | message |
+-------+----------------------------------+-----------------------+
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| 45 | content-type | application/ |
| | | javascript |
+-------+----------------------------------+-----------------------+
| 46 | content-type | application/json |
+-------+----------------------------------+-----------------------+
| 47 | content-type | application/x-www- |
| | | form-urlencoded |
+-------+----------------------------------+-----------------------+
| 48 | content-type | image/gif |
+-------+----------------------------------+-----------------------+
| 49 | content-type | image/jpeg |
+-------+----------------------------------+-----------------------+
| 50 | content-type | image/png |
+-------+----------------------------------+-----------------------+
| 51 | content-type | text/css |
+-------+----------------------------------+-----------------------+
| 52 | content-type | text/html; |
| | | charset=utf-8 |
+-------+----------------------------------+-----------------------+
| 53 | content-type | text/plain |
+-------+----------------------------------+-----------------------+
| 54 | content-type | text/ |
| | | plain;charset=utf-8 |
+-------+----------------------------------+-----------------------+
| 55 | range | bytes=0- |
+-------+----------------------------------+-----------------------+
| 56 | strict-transport-security | max-age=31536000 |
+-------+----------------------------------+-----------------------+
| 57 | strict-transport-security | max-age=31536000; |
| | | includesubdomains |
+-------+----------------------------------+-----------------------+
| 58 | strict-transport-security | max-age=31536000; |
| | | includesubdomains; |
| | | preload |
+-------+----------------------------------+-----------------------+
| 59 | vary | accept-encoding |
+-------+----------------------------------+-----------------------+
| 60 | vary | origin |
+-------+----------------------------------+-----------------------+
| 61 | x-content-type-options | nosniff |
+-------+----------------------------------+-----------------------+
| 62 | x-xss-protection | 1; mode=block |
+-------+----------------------------------+-----------------------+
| 63 | :status | 100 |
+-------+----------------------------------+-----------------------+
| 64 | :status | 204 |
+-------+----------------------------------+-----------------------+
| 65 | :status | 206 |
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+-------+----------------------------------+-----------------------+
| 66 | :status | 302 |
+-------+----------------------------------+-----------------------+
| 67 | :status | 400 |
+-------+----------------------------------+-----------------------+
| 68 | :status | 403 |
+-------+----------------------------------+-----------------------+
| 69 | :status | 421 |
+-------+----------------------------------+-----------------------+
| 70 | :status | 425 |
+-------+----------------------------------+-----------------------+
| 71 | :status | 500 |
+-------+----------------------------------+-----------------------+
| 72 | accept-language | |
+-------+----------------------------------+-----------------------+
| 73 | access-control-allow-credentials | FALSE |
+-------+----------------------------------+-----------------------+
| 74 | access-control-allow-credentials | TRUE |
+-------+----------------------------------+-----------------------+
| 75 | access-control-allow-headers | * |
+-------+----------------------------------+-----------------------+
| 76 | access-control-allow-methods | get |
+-------+----------------------------------+-----------------------+
| 77 | access-control-allow-methods | get, post, options |
+-------+----------------------------------+-----------------------+
| 78 | access-control-allow-methods | options |
+-------+----------------------------------+-----------------------+
| 79 | access-control-expose-headers | content-length |
+-------+----------------------------------+-----------------------+
| 80 | access-control-request-headers | content-type |
+-------+----------------------------------+-----------------------+
| 81 | access-control-request-method | get |
+-------+----------------------------------+-----------------------+
| 82 | access-control-request-method | post |
+-------+----------------------------------+-----------------------+
| 83 | alt-svc | clear |
+-------+----------------------------------+-----------------------+
| 84 | authorization | |
+-------+----------------------------------+-----------------------+
| 85 | content-security-policy | script-src 'none'; |
| | | object-src 'none'; |
| | | base-uri 'none' |
+-------+----------------------------------+-----------------------+
| 86 | early-data | 1 |
+-------+----------------------------------+-----------------------+
| 87 | expect-ct | |
+-------+----------------------------------+-----------------------+
| 88 | forwarded | |
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+-------+----------------------------------+-----------------------+
| 89 | if-range | |
+-------+----------------------------------+-----------------------+
| 90 | origin | |
+-------+----------------------------------+-----------------------+
| 91 | purpose | prefetch |
+-------+----------------------------------+-----------------------+
| 92 | server | |
+-------+----------------------------------+-----------------------+
| 93 | timing-allow-origin | * |
+-------+----------------------------------+-----------------------+
| 94 | upgrade-insecure-requests | 1 |
+-------+----------------------------------+-----------------------+
| 95 | user-agent | |
+-------+----------------------------------+-----------------------+
| 96 | x-forwarded-for | |
+-------+----------------------------------+-----------------------+
| 97 | x-frame-options | deny |
+-------+----------------------------------+-----------------------+
| 98 | x-frame-options | sameorigin |
+-------+----------------------------------+-----------------------+
Table 4
Appendix B. Sample One Pass Encoding Algorithm
Pseudo-code for single pass encoding, excluding handling of
duplicates, non-blocking mode, available encoder stream flow control
and reference tracking.
base = dynamicTable.getInsertCount()
requiredInsertCount = 0
for line in field_lines:
staticIndex = staticTable.findIndex(line)
if staticIndex is not None:
encodeIndexReference(streamBuffer, staticIndex)
continue
dynamicIndex = dynamicTable.findIndex(line)
if dynamicIndex is None:
# No matching entry. Either insert+index or encode literal
staticNameIndex = staticTable.findName(line.name)
if staticNameIndex is None:
dynamicNameIndex = dynamicTable.findName(line.name)
if shouldIndex(line) and dynamicTable.canIndex(line):
encodeInsert(encoderBuffer, staticNameIndex,
dynamicNameIndex, line)
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dynamicIndex = dynamicTable.add(line)
if dynamicIndex is None:
# Couldn't index it, literal
if nameIndex is None or isStaticName:
# Encodes a literal with a static name or literal name
encodeLiteral(streamBuffer, nameIndex, line)
else:
# encode literal with dynamic name, possibly above base
encodeDynamicLiteral(streamBuffer, nameIndex, base, line)
requiredInsertCount = max(requiredInsertCount, nameIndex)
else:
# Dynamic index reference
assert(dynamicIndex is not None)
requiredInsertCount = max(requiredInsertCount, dynamicIndex)
# Encode dynamicIndex, possibly above base
encodeDynamicIndexReference(streamBuffer, dynamicIndex, base)
# encode the prefix
if requiredInsertCount == 0:
encodeIndexReference(prefixBuffer, 0, 0, 8)
encodeIndexReference(prefixBuffer, 0, 0, 7)
else:
wireRIC = (
requiredInsertCount
% (2 * getMaxEntries(maxTableCapacity))
) + 1;
encodeInteger(prefixBuffer, 0x00, wireRIC, 8)
if base >= requiredInsertCount:
encodeInteger(prefixBuffer, 0, base - requiredInsertCount, 7)
else:
encodeInteger(prefixBuffer, 0x80,
requiredInsertCount - base - 1, 7)
return encoderBuffer, prefixBuffer + streamBuffer
Appendix C. Change Log
*RFC Editor's Note:* Please remove this section prior to
publication of a final version of this document.
C.1. Since draft-ietf-quic-qpack-15
No changes
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C.2. Since draft-ietf-quic-qpack-14
Added security considerations
C.3. Since draft-ietf-quic-qpack-13
No changes
C.4. Since draft-ietf-quic-qpack-12
Editorial changes only
C.5. Since draft-ietf-quic-qpack-11
Editorial changes only
C.6. Since draft-ietf-quic-qpack-10
Editorial changes only
C.7. Since draft-ietf-quic-qpack-09
* Decoders MUST emit Header Acknowledgements (#2939)
* Updated error code for multiple encoder or decoder streams (#2970)
* Added explicit defaults for new SETTINGS (#2974)
C.8. Since draft-ietf-quic-qpack-08
* Endpoints are permitted to create encoder and decoder streams even
if they can't use them (#2100, #2529)
* Maximum values for settings removed (#2766, #2767)
C.9. Since draft-ietf-quic-qpack-06
* Clarify initial dynamic table capacity maximums (#2276, #2330,
#2330)
C.10. Since draft-ietf-quic-qpack-05
* Introduced the terms dynamic table capacity and maximum dynamic
table capacity.
* Renamed SETTINGS_HEADER_TABLE_SIZE to
SETTINGS_QPACK_MAX_TABLE_CAPACITY.
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C.11. Since draft-ietf-quic-qpack-04
* Changed calculation of Delta Base Index to avoid an illegal value
(#2002, #2005)
C.12. Since draft-ietf-quic-qpack-03
* Change HTTP settings defaults (#2038)
* Substantial editorial reorganization
C.13. Since draft-ietf-quic-qpack-02
* Largest Reference encoded modulo MaxEntries (#1763)
* New Static Table (#1355)
* Table Size Update with Insert Count=0 is a connection error
(#1762)
* Stream Cancellations are optional when
SETTINGS_HEADER_TABLE_SIZE=0 (#1761)
* Implementations must handle 62 bit integers (#1760)
* Different error types for each QPACK stream, other changes to
error handling (#1726)
* Preserve header field order (#1725)
* Initial table size is the maximum permitted when table is first
usable (#1642)
C.14. Since draft-ietf-quic-qpack-01
* Only header blocks that reference the dynamic table are
acknowledged (#1603, #1605)
C.15. Since draft-ietf-quic-qpack-00
* Renumbered instructions for consistency (#1471, #1472)
* Decoder is allowed to validate largest reference (#1404, #1469)
* Header block acknowledgments also acknowledge the associated
largest reference (#1370, #1400)
* Added an acknowledgment for unread streams (#1371, #1400)
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* Removed framing from encoder stream (#1361,#1467)
* Control streams use typed unidirectional streams rather than fixed
stream IDs (#910,#1359)
C.16. Since draft-ietf-quic-qcram-00
* Separate instruction sets for table updates and header blocks
(#1235, #1142, #1141)
* Reworked indexing scheme (#1176, #1145, #1136, #1130, #1125,
#1314)
* Added mechanisms that support one-pass encoding (#1138, #1320)
* Added a setting to control the number of blocked decoders (#238,
#1140, #1143)
* Moved table updates and acknowledgments to dedicated streams
(#1121, #1122, #1238)
Acknowledgments
The IETF QUIC Working Group received an enormous amount of support
from many people.
The compression design team did substantial work exploring the
problem space and influencing the initial draft. The contributions
of design team members Roberto Peon, Martin Thomson, and Dmitri
Tikhonov are gratefully acknowledged.
The following people also provided substantial contributions to this
document:
* Bence Beky
* Alessandro Ghedini
* Ryan Hamilton
* Robin Marx
* Patrick McManus
* 奥 一穂 (Kazuho Oku)
* Lucas Pardue
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* Biren Roy
* Ian Swett
This draft draws heavily on the text of [RFC7541]. The indirect
input of those authors is also gratefully acknowledged.
Buck's contribution was supported by Google during his employment
there.
A portion of Mike's contribution was supported by Microsoft during
his employment there.
Authors' Addresses
Charles 'Buck' Krasic
Netflix
Email: ckrasic@netflix.com
Mike Bishop
Akamai Technologies
Email: mbishop@evequefou.be
Alan Frindell (editor)
Facebook
Email: afrind@fb.com
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