https://github.com/quicwg/base-drafts
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rfc9204.md
---
title: "QPACK: Field Compression for HTTP/3"
abbrev: QPACK
number: 9204
docname: draft-ietf-quic-qpack-latest
date: 2022-06
category: std
ipr: trust200902
area: Transport
workgroup: QUIC
keyword:
- compression
- "HTTP/3"
- HPACK
- header
- field
- trailer
stand_alone: yes
autolink-iref-cleanup: true
pi: [toc, sortrefs, symrefs, docmapping]
author:
-
ins: C. Krasic
name: Charles 'Buck' Krasic
email: krasic@acm.org
-
ins: M. Bishop
name: Mike Bishop
org: Akamai Technologies
email: mbishop@evequefou.be
-
ins: A. Frindell
name: Alan Frindell
org: Facebook
email: afrind@fb.com
role: editor
normative:
RFC9114:
display: HTTP/3
RFC9110:
display: HTTP
RFC2360:
informative:
RFC9113:
display: HTTP/2
CRIME:
target: http://en.wikipedia.org/w/index.php?title=CRIME&oldid=660948120
title: "CRIME"
author:
-
org: Wikipedia
date: May, 2015
PETAL:
target: http://www.pdl.cmu.edu/PDL-FTP/associated/CMU-PDL-13-106.pdf
title: "PETAL: Preset Encoding Table Information Leakage"
author:
-
ins: J. Tan
name: Jiaqi Tan
-
ins: J. Nahata
name: Jayvardhan Nahata
date: April, 2013
--- abstract
This specification defines QPACK: a compression format for efficiently
representing HTTP fields that is to be used in HTTP/3. This is a variation of
HPACK compression that seeks to reduce head-of-line blocking.
--- middle
# Introduction
The QUIC transport protocol ({{!QUIC-TRANSPORT=RFC9000}}) is designed to support
HTTP semantics, and its design subsumes many of the features of HTTP/2
({{?RFC9113}}). HTTP/2 uses HPACK ({{!RFC7541}}) for compression of the header
and trailer sections. If HPACK were used for HTTP/3 ({{RFC9114}}), 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.
## 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.
The following terms 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 {{Sections 6.3
and 6.5 of RFC9110}}.
HTTP field value:
: Data associated with a field name, composed from all field line values with
that field name in that section, concatenated together with
comma separators.
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
and trailer sections.
Representation:
: An instruction that represents a field line, possibly by reference to the
dynamic and static tables.
Encoder:
: An implementation that encodes field sections.
Decoder:
: An implementation that 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 that
reference dynamic table entries are relative to a Base.
Insert Count:
: The total number of entries inserted in the dynamic table.
Note that QPACK is a name, not an abbreviation.
## Notational Conventions
Diagrams in this document 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
{{prefixed-integers}}, beginning with an A-bit prefix.
x ...
: Indicates that x is variable length and extends to the end of the region.
# Compression Process Overview
Like HPACK, QPACK uses two tables for associating field lines ("headers") to
indices. The static table ({{header-table-static}}) is predefined and contains
common header field lines (some of them with an empty value). The dynamic table
({{header-table-dynamic}}) 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.
## Encoder
An encoder converts a header or trailer section into a series of representations
by emitting either an indexed or a literal representation for each field line in
the list; see {{field-line-representations}}. 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 acknowledgment 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 place the burden of optional state tracking on the encoder,
resulting in relatively simple decoders.
### Limits on Dynamic Table Insertions {#blocked-insertion}
Inserting entries into the dynamic table might not be possible if the table
contains entries that 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 that 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 {{header-acknowledgment}}.
#### 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
({{duplicate}}) 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 ({{indexing}}) 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.
~~~~~~~~~~ ascii-art
<-- Newer Entries Older Entries -->
(Larger Indicies) (Smaller Indicies)
+--------+---------------------------------+----------+
| Unused | Referenceable | Draining |
| Space | Entries | Entries |
+--------+---------------------------------+----------+
^ ^ ^
| | |
Insertion Point Draining Index Dropping
Point
~~~~~~~~~~
{:#fig-draining-index title="Draining Dynamic Table Entries"}
### 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 ({{header-prefix}}),
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.
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 {{blocked-decoding}}.
The decoder specifies an upper bound on the number of streams that can be
blocked using the SETTINGS_QPACK_BLOCKED_STREAMS setting; see {{configuration}}.
An encoder MUST limit the number of streams that 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 that 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 that 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.
### 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 NOT write an instruction unless
sufficient stream and connection flow-control credit is available for the entire
instruction.
### 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 Acknowledgment instruction ({{header-acknowledgment}}) 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, the Known Received Count is
updated to that Required Insert Count value.
An Insert Count Increment instruction ({{insert-count-increment}}) increases the
Known Received Count by its Increment parameter. See {{new-table-entries}} for
guidance.
## 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.
### 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. This data is unusable until the stream becomes unblocked,
and releasing the flow control prematurely makes the decoder vulnerable to
memory exhaustion attacks. 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.
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 {{blocked-streams}}. If it
encounters a Required Insert Count smaller than expected, it MUST treat this as
a connection error of type QPACK_DECOMPRESSION_FAILED; see
{{invalid-references}}. If it encounters a Required Insert Count larger than
expected, it MAY treat this as a connection error of type
QPACK_DECOMPRESSION_FAILED.
### State Synchronization
The decoder signals the following events by emitting decoder instructions
({{decoder-instructions}}) on the decoder stream.
#### 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
Acknowledgment instruction ({{header-acknowledgment}}). A stream may carry
multiple field sections in the case of intermediate responses, trailers, and
pushed requests. The encoder interprets each Section Acknowledgment
instruction as acknowledging the earliest unacknowledged field section
containing dynamic table references sent on the given stream.
#### 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
{{stream-cancellation}}. 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 ({{maximum-dynamic-table-capacity}}) 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 Acknowledgment and Stream Cancellation instructions permit the
encoder to remove references to entries in the dynamic table. When an entry
with an absolute index lower than the Known Received Count has zero references,
then it is considered evictable; see {{blocked-insertion}}.
#### New Table Entries
After receiving new table entries on the encoder stream, the decoder chooses
when to emit Insert Count Increment instructions; see
{{insert-count-increment}}. 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
Acknowledgments; see {{header-acknowledgment}}. However, delaying too long
may lead to compression inefficiencies if the encoder waits for an entry to be
acknowledged before using it.
### Invalid References
If the decoder encounters a reference in a field line representation to a
dynamic table entry that has already been evicted or that has an absolute
index greater than or equal to the declared Required Insert Count
({{header-prefix}}), 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 that has already been evicted, it MUST treat this as a connection
error of type QPACK_ENCODER_STREAM_ERROR.
# 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.
## Static Table {#header-table-static}
The static table consists of a predefined list of field lines, each of which has
a fixed index over time. Its entries are defined in {{static-table}}.
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.
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.
## Dynamic Table {#header-table-dynamic}
The dynamic table consists of a list of field lines maintained in first-in,
first-out order. A QPACK encoder and decoder share a dynamic table that is
initially empty. The encoder adds entries to the dynamic table and sends them
to the decoder via instructions on the encoder stream; see
{{encoder-instructions}}.
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.
### 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 additional bytes. The size of an entry is calculated
using the length of its name and value without Huffman encoding applied.
### Dynamic Table Capacity and Eviction {#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
({{set-dynamic-capacity}}) 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
{{blocked-insertion}}. 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.
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
({{set-dynamic-capacity}}), 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.
### 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 {{configuration}}.
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
{{set-dynamic-capacity}}.
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 clients not using 0-RTT data (whether 0-RTT is not attempted or is rejected)
and for all HTTP/3 servers, 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.
### Absolute Indexing {#indexing}
Each entry possesses an absolute index that 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.
### 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 ({{encoder-instructions}}), 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.
~~~~~ ascii-art
+-----+---------------+-------+
| 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
~~~~~
{: title="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
{{header-prefix}}. 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.
~~~~~ ascii-art
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
~~~~~
{: title="Example Dynamic Table Indexing - Relative Index in Representation"}
### Post-Base Indexing {#post-base}
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.
~~~~~ ascii-art
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
~~~~~
{: title="Example Dynamic Table Indexing - Post-Base Index in Representation"}
# Wire Format
## Primitives
### 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.
### 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 bit flag, denoted as 'H' in this document (indicating whether the string
is Huffman encoded), followed by the string length encoded as a 7-bit prefix
integer, and finally the indicated number of bytes of data. When Huffman
encoding is enabled, the Huffman table from {{Section B of RFC7541}} is used
without modification and the indicated length is the size of the string after
encoding.
This document expands the definition of string literals by permitting them to
begin other than on a byte boundary. An "N-bit prefix string literal" begins
mid-byte, with the first (8-N) bits allocated to a previous field. The string
uses one bit for the Huffman flag, followed by the length of the encoded string
as a (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.
## Encoder and Decoder Streams {#enc-dec-stream-def}
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.
The sender MUST NOT close either of these streams, and the receiver MUST NOT
request that the sender close either of these streams. 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 will not be used (for
example, if its encoder does not 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.
## Encoder Instructions {#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 that already exist in
the dynamic table, the full entry can also be used by reference, creating a
duplicate entry.
### Set Dynamic Table Capacity {#set-dynamic-capacity}
An encoder informs the decoder of a change to the dynamic table capacity using
an instruction that starts with the '001' 3-bit pattern. This is followed
by the new dynamic table capacity represented as an integer with a 5-bit prefix;
see {{prefixed-integers}}.
~~~~~~~~~~ ascii-art
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 1 | Capacity (5+) |
+---+---+---+-------------------+
~~~~~~~~~~
{:#fig-set-capacity title="Set Dynamic Table Capacity"}
The new capacity MUST be lower than or equal to the limit described in
{{maximum-dynamic-table-capacity}}. In HTTP/3, this limit is the value of the
SETTINGS_QPACK_MAX_TABLE_CAPACITY parameter ({{configuration}}) 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
{{eviction}}. This MUST NOT cause the eviction of entries that are not
evictable; see {{blocked-insertion}}. Changing the capacity of the dynamic
table is not acknowledged as this instruction does not insert an entry.
### 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' 1-bit pattern. The second ('T') bit
indicates whether the reference is to the static or dynamic table. The 6-bit
prefix integer ({{prefixed-integers}}) 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.
The field name reference is followed by the field value represented as a string
literal; see {{string-literals}}.
~~~~~~~~~~ ascii-art
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | T | Name Index (6+) |
+---+---+-----------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length bytes) |
+-------------------------------+
~~~~~~~~~~
{: title="Insert Field Line -- Indexed Name"}
### Insert with Literal Name
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' 2-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
{{string-literals}}.
~~~~~~~~~~ ascii-art
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) |
+-------------------------------+
~~~~~~~~~~
{: title="Insert Field Line -- New Name"}
### Duplicate {#duplicate}
An encoder duplicates an existing entry in the dynamic table using an
instruction that starts with the '000' 3-bit pattern. This is followed by
the relative index of the existing entry represented as an integer with a 5-bit
prefix; see {{prefixed-integers}}.
~~~~~~~~~~ ascii-art
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | Index (5+) |
+---+---+---+-------------------+
~~~~~~~~~~
{:#fig-index-with-duplication title="Duplicate"}
The existing entry is reinserted 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.
## Decoder Instructions {#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.
### Section Acknowledgment {#header-acknowledgment}
After processing an encoded field section whose declared Required Insert Count
is not zero, the decoder emits a Section Acknowledgment instruction. The
instruction starts with the '1' 1-bit pattern, followed by the field
section's associated stream ID encoded as a 7-bit prefix integer; see
{{prefixed-integers}}.
This instruction is used as described in Sections {{<known-received-count}} and
{{<state-synchronization}}.
~~~~~~~~~~ ascii-art
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | Stream ID (7+) |
+---+---------------------------+
~~~~~~~~~~
{:#fig-header-ack title="Section Acknowledgment"}
If an encoder receives a Section Acknowledgment instruction referring to a
stream on which every encoded field section with a non-zero Required Insert
Count has already been acknowledged, this MUST be treated as a connection error
of type QPACK_DECODER_STREAM_ERROR.
The Section Acknowledgment instruction might increase the Known Received Count;
see {{known-received-count}}.
### Stream Cancellation
When a stream is reset or reading is abandoned, the decoder emits a Stream
Cancellation instruction. The instruction starts with the '01' 2-bit
pattern, followed by the stream ID of the affected stream encoded as a
6-bit prefix integer.
This instruction is used as described in {{state-synchronization}}.
~~~~~~~~~~ ascii-art
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 1 | Stream ID (6+) |
+---+---+-----------------------+
~~~~~~~~~~
{:#fig-stream-cancel title="Stream Cancellation"}
### Insert Count Increment
The Insert Count Increment instruction starts with the '00' 2-bit pattern,
followed by the Increment encoded as a 6-bit prefix integer. This instruction
increases the Known Received Count ({{known-received-count}}) 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.
~~~~~~~~~~ ascii-art
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | Increment (6+) |
+---+---+-----------------------+
~~~~~~~~~~
{:#fig-size-sync title="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.
## 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 they do not modify that state.
Encoded field sections are carried in frames on streams defined by the enclosing
protocol.
### Encoded Field Section Prefix {#header-prefix}
Each encoded field section is prefixed with two integers. The Required Insert
Count is encoded as an integer with an 8-bit prefix using the encoding described
in {{ric}}. The Base is encoded as a Sign bit ('S') and a Delta Base value
with a 7-bit prefix; see {{base}}.
~~~~~~~~~~ ascii-art
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Required Insert Count (8+) |
+---+---------------------------+
| S | Delta Base (7+) |
+---+---------------------------+
| Encoded Field Lines ...
+-------------------------------+
~~~~~~~~~~
{:#fig-base-index title="Encoded Field Section"}
#### Required Insert Count {#ric}
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:
~~~ pseudocode
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:
~~~ pseudocode
MaxEntries = floor( MaxTableCapacity / 32 )
~~~
`MaxTableCapacity` is the maximum capacity of the dynamic table as specified by
the decoder; see {{maximum-dynamic-table-capacity}}.
This encoding limits the length of the prefix on long-lived connections.
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.
~~~ pseudocode
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.
#### Base {#base}
The Base is used to resolve references in the dynamic table as described in
{{relative-indexing}}.
To save space, the Base is encoded relative to the Required Insert Count using a
one-bit Sign ('S' in {{fig-base-index}}) 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 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:
~~~ pseudocode
if Sign == 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 and is referencing them, 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 that 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 encoded difference will be
positive and the Sign bit is set to 0.
The value of Base MUST NOT be negative. Though the protocol might operate
correctly with a negative Base using post-Base indexing, it is unnecessary and
inefficient. An endpoint MUST treat a field block with a Sign bit of 1 as
invalid if the value of Required Insert Count is less than or equal to the value
of Delta Base.
An encoder that produces table updates before encoding a field section might set
Base to the value of Required Insert Count. In such a 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 a Sign 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 fifth entry that was added to the table; a post-Base index of 1 refers to
the eighth entry.
### 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.
~~~~~~~~~~ ascii-art
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | T | Index (6+) |
+---+---+-----------------------+
~~~~~~~~~~
{: title="Indexed Field Line"}
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 ({{prefixed-integers}}) 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.
### 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.
~~~~~~~~~~ ascii-art
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 1 | Index (4+) |
+---+---+---+---+---------------+
~~~~~~~~~~
{: title="Indexed Field Line with Post-Base Index"}
This representation starts with the '0001' 4-bit pattern. This is followed
by the post-Base index ({{post-base}}) of the matching field line, represented
as an integer with a 4-bit prefix; see {{prefixed-integers}}.
### Literal Field Line with Name Reference {#literal-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.
~~~~~~~~~~ ascii-art
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 1 | N | T |Name Index (4+)|
+---+---+---+---+---------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length bytes) |
+-------------------------------+
~~~~~~~~~~
{: title="Literal Field Line with Name Reference"}
This representation starts with the '01' 2-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 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 {{probing-dynamic-table-state}} for more
details.
The fourth ('T') bit indicates whether the reference is to the static or dynamic
table. The 4-bit prefix integer ({{prefixed-integers}}) 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 {{string-literals}}.
### 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.
~~~~~~~~~~ ascii-art
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 0 | N |NameIdx(3+)|
+---+---+---+---+---+-----------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length bytes) |
+-------------------------------+
~~~~~~~~~~
{: title="Literal Field Line with Post-Base Name Reference"}
This representation starts with the '0000' 4-bit pattern. The fifth bit is
the 'N' bit as described in {{literal-name-reference}}. This is followed by a
post-Base index of the dynamic table entry ({{post-base}}) encoded as an
integer with a 3-bit prefix; see {{prefixed-integers}}.
Only the field name is taken from the dynamic table entry; the field value is
encoded as an 8-bit prefix string literal; see {{string-literals}}.
### Literal Field Line with Literal Name
The literal field line with literal name representation encodes a
field name and a field value as string literals.
~~~~~~~~~~ ascii-art
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) |
+-------------------------------+
~~~~~~~~~~
{: title="Literal Field Line with Literal Name"}
This representation starts with the '001' 3-bit pattern. The fourth bit is
the 'N' bit as described in {{literal-name-reference}}. The name follows,
represented as a 4-bit prefix string literal, then the value, represented as an
8-bit prefix string literal; see {{string-literals}}.
# Configuration
QPACK defines two settings for the HTTP/3 SETTINGS frame:
SETTINGS_QPACK_MAX_TABLE_CAPACITY (0x01):
: The default value is zero. See {{header-table-dynamic}} for usage. This is
the equivalent of the SETTINGS_HEADER_TABLE_SIZE from HTTP/2.
SETTINGS_QPACK_BLOCKED_STREAMS (0x07):
: The default value is zero. See {{blocked-streams}}.
# Error Handling {#error-handling}
The following error codes are defined for HTTP/3 to indicate failures of
QPACK that prevent the stream or connection from continuing:
QPACK_DECOMPRESSION_FAILED (0x0200):
: The decoder failed to interpret an encoded field section and is not able to
continue decoding that field section.
QPACK_ENCODER_STREAM_ERROR (0x0201):
: The decoder failed to interpret an encoder instruction received on the
encoder stream.
QPACK_DECODER_STREAM_ERROR (0x0202):
: The encoder failed to interpret a decoder instruction received on the
decoder stream.
# 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.
## Probing Dynamic Table State
QPACK reduces the encoded size of field sections 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 and trailer fields can be probed
by an attacker who can both define 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=RFC8446}}) and the QUIC Transport Protocol ({{QUIC-TRANSPORT}}), because
while TLS and QUIC provide confidentiality protection for content, they only
provide a limited amount of protection for the length of that content.
{:aside}
> 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.
### 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 field line rather than
individual characters. An attacker can only learn whether a guess is correct or
not, so the attacker is reduced to a brute-force guess for the field values
associated with a given field name.
Therefore, the viability of recovering specific field values 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 refer to those entries while encoding
chosen field lines, then the attacker (the second entity) can learn the state
of the table by observing the length of the encoded output.
For example, requests or responses from mutually distrustful entities can occur
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.
Other scenarios involving mutually distrustful entities are also possible.
### Mitigation
Users of HTTP that require confidentiality for header or trailer 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 the message. 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.
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 field values might
instead introduce a penalty for many field lines with the same field name and
different values. This penalty could cause a large number of attempts to guess
a field value to result in the field not being compared to the dynamic table
entries in future messages, effectively preventing further guesses.
This response might be made inversely proportional to the length of the
field value. Disabling access to the dynamic table for a given field name might
occur for shorter values more quickly or with higher probability than for longer
values.
This mitigation is most effective between two endpoints. If messages are
re-encoded by an intermediary without knowledge of which entity constructed a
given message, the intermediary could inadvertently merge compression contexts
that the original encoder had specifically kept separate.
<aside><t>
Note: Simply removing entries corresponding to the 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 websites can easily force an image to be loaded, thereby
refreshing the entry in the dynamic table.
</t></aside>
### Never-Indexed Literals
Implementations can also choose to protect sensitive fields by not compressing
them and instead encoding their value as literals.
Refusing to insert a field line into the dynamic table is only effective if
doing so is avoided on all hops. The never-indexed literal bit (see
{{literal-name-reference}}) 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 field value should never be indexed depends on several
factors. Since QPACK does not protect against guessing an entire 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.
An encoder might also choose not to index values for 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 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.
## 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}}).
## 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.
QPACK uses the definition of the maximum size of the dynamic table and the
maximum number of blocking streams to limit the amount of memory the encoder can
cause the decoder to consume. 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
{{maximum-dynamic-table-capacity}} and {{blocked-streams}}). 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 choosing a smaller dynamic table size than the decoder
allows and signaling this to the decoder (see {{set-dynamic-capacity}}).
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
SETTINGS_QPACK_BLOCKED_STREAMS parameter. Streams that risk becoming blocked
consume no additional state memory on the encoder.
An encoder allocates memory to track all dynamic table references in
unacknowledged field sections. An implementation can directly limit the amount
of state memory by only using as many references to the dynamic table as it
wishes to track; no signaling to the decoder is required. However, limiting
references to the dynamic table will reduce compression effectiveness.
The amount of temporary memory consumed by an encoder or decoder can be limited
by processing field lines sequentially. A decoder implementation does not need
to retain a complete list of field lines while decoding a field section. An
encoder implementation does not need to retain a complete list of field lines
while encoding a field section if it is using a single-pass algorithm. Note
that it might be necessary for an application to retain a complete list of field
lines 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 that 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.
## 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 {{prefixed-integers}}. In the same way, it
has to set a limit to the length it accepts for string literals; see
{{string-literals}}. These limits SHOULD be large enough to process the
largest individual field the HTTP implementation can be configured to accept.
If an implementation encounters a value larger than it is able to decode, this
MUST be treated as a stream error of type QPACK_DECOMPRESSION_FAILED if on a
request stream or a connection error of the appropriate type if on the encoder
or decoder stream.
# IANA Considerations
This document makes multiple registrations in the registries defined by
{{RFC9114}}. The allocations created by this document are all assigned permanent
status and list a change controller of the IETF and a contact of the HTTP
working group (ietf-http-wg@w3.org).
## Settings Registration
This document specifies two settings. The entries in the following table are
registered in the "HTTP/3 Settings" registry established in {{RFC9114}}.
|------------------------------|--------|---------------------------| ------- |
| Setting Name | Code | Specification | Default |
| ---------------------------- | :----: | ------------------------- | ------- |
| QPACK_MAX_TABLE_CAPACITY | 0x01 | {{configuration}} | 0 |
| QPACK_BLOCKED_STREAMS | 0x07 | {{configuration}} | 0 |
| ---------------------------- | ------ | ------------------------- | ------- |
{: title="Additions to the HTTP/3 Settings Registry"}
For formatting reasons, the setting names here are abbreviated by removing the
'SETTINGS_' prefix.
## Stream Type Registration
This document specifies two stream types. The entries in the following table are
registered in the "HTTP/3 Stream Types" registry established in {{RFC9114}}.
| ---------------------------- | ------ | ------------------------- | ------ |
| Stream Type | Code | Specification | Sender |
| ---------------------------- | :----: | ------------------------- | ------ |
| QPACK Encoder Stream | 0x02 | {{enc-dec-stream-def}} | Both |
| QPACK Decoder Stream | 0x03 | {{enc-dec-stream-def}} | Both |
| ---------------------------- | ------ | ------------------------- | ------ |
{: title="Additions to the HTTP/3 Stream Types Registry"}
## Error Code Registration
This document specifies three error codes. The entries in the following table
are registered in the "HTTP/3 Error Codes" registry established in {{RFC9114}}.
| --------------------------------- | ------ | ---------------------------------------- | ---------------------- |
| Name | Code | Description | Specification |
| --------------------------------- | ------ | ---------------------------------------- | ---------------------- |
| QPACK_DECOMPRESSION_FAILED | 0x0200 | Decoding of a field section failed | {{error-handling}} |
| QPACK_ENCODER_STREAM_ERROR | 0x0201 | Error on the encoder stream | {{error-handling}} |
| QPACK_DECODER_STREAM_ERROR | 0x0202 | Error on the decoder stream | {{error-handling}} |
| --------------------------------- | ------ | ---------------------------------------- | ---------------------- |
{: title="Additions to the HTTP/3 Error Codes Registry"}
--- back
# Static Table
This table was generated by analyzing actual Internet traffic in 2018 and
including the most common header fields, 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 header fields with
the smallest number of bytes.
| 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 |
| 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 |
| 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 |
| 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 | |
| 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 |
{: title="Static Table"}
Any line breaks that appear within field names or values are due to formatting.
# Encoding and Decoding Examples
The following examples represent a series of exchanges between an encoder and a
decoder. The exchanges are designed to exercise most QPACK instructions and
highlight potentially common patterns and their impact on dynamic table state.
The encoder sends three encoded field sections containing one field line each,
as well as two speculative inserts that are not referenced.
The state of the encoder's dynamic table is shown, along with its
current size. Each entry is shown with the Absolute Index of the entry (Abs),
the current number of outstanding encoded field sections with references to that
entry (Ref), along with the name and value. Entries above the 'acknowledged'
line have been acknowledged by the decoder.
## Literal Field Line with Name Reference
The encoder sends an encoded field section containing a literal representation
of a field with a static name reference.
~~~
Data | Interpretation
| Encoder's Dynamic Table
Stream: 0
0000 | Required Insert Count = 0, Base = 0
510b 2f69 6e64 6578 | Literal Field Line with Name Reference
2e68 746d 6c | Static Table, Index=1
| (:path=/index.html)
Abs Ref Name Value
^-- acknowledged --^
Size=0
~~~
## Dynamic Table
The encoder sets the dynamic table capacity, inserts a header with a dynamic
name reference, then sends a potentially blocking, encoded field section
referencing this new entry. The decoder acknowledges processing the encoded
field section, which implicitly acknowledges all dynamic table insertions up to
the Required Insert Count.
~~~
Stream: Encoder
3fbd01 | Set Dynamic Table Capacity=220
c00f 7777 772e 6578 | Insert With Name Reference
616d 706c 652e 636f | Static Table, Index=0
6d | (:authority=www.example.com)
c10c 2f73 616d 706c | Insert With Name Reference
652f 7061 7468 | Static Table, Index=1
| (:path=/sample/path)
Abs Ref Name Value
^-- acknowledged --^
0 0 :authority www.example.com
1 0 :path /sample/path
Size=106
Stream: 4
0381 | Required Insert Count = 2, Base = 0
10 | Indexed Field Line With Post-Base Index
| Absolute Index = Base(0) + Index(0) = 0
| (:authority=www.example.com)
11 | Indexed Field Line With Post-Base Index
| Absolute Index = Base(0) + Index(1) = 1
| (:path=/sample/path)
Abs Ref Name Value
^-- acknowledged --^
0 1 :authority www.example.com
1 1 :path /sample/path
Size=106
Stream: Decoder
84 | Section Acknowledgment (stream=4)
Abs Ref Name Value
0 0 :authority www.example.com
1 0 :path /sample/path
^-- acknowledged --^
Size=106
~~~
## Speculative Insert
The encoder inserts a header into the dynamic table with a literal name.
The decoder acknowledges receipt of the entry. The encoder does not send
any encoded field sections.
~~~
Stream: Encoder
4a63 7573 746f 6d2d | Insert With Literal Name
6b65 790c 6375 7374 | (custom-key=custom-value)
6f6d 2d76 616c 7565 |
Abs Ref Name Value
0 0 :authority www.example.com
1 0 :path /sample/path
^-- acknowledged --^
2 0 custom-key custom-value
Size=160
Stream: Decoder
01 | Insert Count Increment (1)
Abs Ref Name Value
0 0 :authority www.example.com
1 0 :path /sample/path
2 0 custom-key custom-value
^-- acknowledged --^
Size=160
~~~
## Duplicate Instruction, Stream Cancellation
The encoder duplicates an existing entry in the dynamic table, then sends an
encoded field section referencing the dynamic table entries including the
duplicated entry. The packet containing the encoder stream data is delayed.
Before the packet arrives, the decoder cancels the stream and notifies the
encoder that the encoded field section was not processed.
~~~
Stream: Encoder
02 | Duplicate (Relative Index = 2)
| Absolute Index =
| Insert Count(3) - Index(2) - 1 = 0
Abs Ref Name Value
0 0 :authority www.example.com
1 0 :path /sample/path
2 0 custom-key custom-value
^-- acknowledged --^
3 0 :authority www.example.com
Size=217
Stream: 8
0500 | Required Insert Count = 4, Base = 4
80 | Indexed Field Line, Dynamic Table
| Absolute Index = Base(4) - Index(0) - 1 = 3
| (:authority=www.example.com)
c1 | Indexed Field Line, Static Table Index = 1
| (:path=/)
81 | Indexed Field Line, Dynamic Table
| Absolute Index = Base(4) - Index(1) - 1 = 2
| (custom-key=custom-value)
Abs Ref Name Value
0 0 :authority www.example.com
1 0 :path /sample/path
2 1 custom-key custom-value
^-- acknowledged --^
3 1 :authority www.example.com
Size=217
Stream: Decoder
48 | Stream Cancellation (Stream=8)
Abs Ref Name Value
0 0 :authority www.example.com
1 0 :path /sample/path
2 0 custom-key custom-value
^-- acknowledged --^
3 0 :authority www.example.com
Size=217
~~~
## Dynamic Table Insert, Eviction
The encoder inserts another header into the dynamic table, which evicts the
oldest entry. The encoder does not send any encoded field sections.
~~~
Stream: Encoder
810d 6375 7374 6f6d | Insert With Name Reference
2d76 616c 7565 32 | Dynamic Table, Relative Index = 1
| Absolute Index =
| Insert Count(4) - Index(1) - 1 = 2
| (custom-key=custom-value2)
Abs Ref Name Value
1 0 :path /sample/path
2 0 custom-key custom-value
^-- acknowledged --^
3 0 :authority www.example.com
4 0 custom-key custom-value2
Size=215
~~~
# Sample Single-Pass Encoding Algorithm
Pseudocode for single-pass encoding, excluding handling of duplicates,
non-blocking mode, available encoder stream flow control and reference tracking.
~~~ pseudocode
# Helper functions:
# ====
# Encode an integer with the specified prefix and length
encodeInteger(buffer, prefix, value, prefixLength)
# Encode a dynamic table insert instruction with optional static
# or dynamic name index (but not both)
encodeInsert(buffer, staticNameIndex, dynamicNameIndex, fieldLine)
# Encode a static index reference
encodeStaticIndexReference(buffer, staticIndex)
# Encode a dynamic index reference relative to Base
encodeDynamicIndexReference(buffer, dynamicIndex, base)
# Encode a literal with an optional static name index
encodeLiteral(buffer, staticNameIndex, fieldLine)
# Encode a literal with a dynamic name index relative to Base
encodeDynamicLiteral(buffer, dynamicNameIndex, base, fieldLine)
# Encoding Algorithm
# ====
base = dynamicTable.getInsertCount()
requiredInsertCount = 0
for line in fieldLines:
staticIndex = staticTable.findIndex(line)
if staticIndex is not None:
encodeStaticIndexReference(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)
dynamicIndex = dynamicTable.add(line)
if dynamicIndex is None:
# Could not index it, literal
if dynamicNameIndex is not None:
# Encode literal with dynamic name, possibly above Base
encodeDynamicLiteral(streamBuffer, dynamicNameIndex,
base, line)
requiredInsertCount = max(requiredInsertCount,
dynamicNameIndex)
else:
# Encodes a literal with a static name or literal name
encodeLiteral(streamBuffer, staticNameIndex, line)
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:
encodeInteger(prefixBuffer, 0x00, 0, 8)
encodeInteger(prefixBuffer, 0x00, 0, 7)
else:
wireRIC = (
requiredInsertCount
% (2 * getMaxEntries(maxTableCapacity))
) + 1;
encodeInteger(prefixBuffer, 0x00, wireRIC, 8)
if base >= requiredInsertCount:
encodeInteger(prefixBuffer, 0x00,
base - requiredInsertCount, 7)
else:
encodeInteger(prefixBuffer, 0x80,
requiredInsertCount - base - 1, 7)
return encoderBuffer, prefixBuffer + streamBuffer
~~~
# Acknowledgments
{:numbered="false"}
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 version of this document. 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:
{:compact}
- <t>{{{Bence Béky}}}</t>
- <t>{{{Alessandro Ghedini}}}</t>
- <t>{{{Ryan Hamilton}}}</t>
- <t>{{{Robin Marx}}}</t>
- <t>{{{Patrick McManus}}}</t>
- <t>{{{Kazuho Oku}{奥 一穂}}}</t>
- <t>{{{Lucas Pardue}}}</t>
- <t>{{{Biren Roy}}}</t>
- <t>{{{Ian Swett}}}</t>
This document draws heavily on the text of {{!RFC7541}}. The indirect input of
those authors is also gratefully acknowledged.
{{{Buck Krasic}}}'s contribution was supported by Google during his employment
there.
A portion of {{{Mike Bishop}}}'s contribution was supported by Microsoft during
his employment there.
