---
title: "QPACK: Header Compression for HTTP over QUIC"
abbrev: QPACK
docname: draft-ietf-quic-qpack-latest
date: {DATE}
category: std
ipr: trust200902
area: Transport
workgroup: QUIC

stand_alone: yes
pi: [toc, sortrefs, symrefs, docmapping]

author:
 -
    ins: C. Krasic
    name: Charles 'Buck' Krasic
    org: Netflix
    email: ckrasic@netflix.com
 -
    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:

  QUIC-HTTP:
    title: "Hypertext Transfer Protocol (HTTP) over QUIC"
    date: {DATE}
    seriesinfo:
      Internet-Draft: draft-ietf-quic-http-latest
    author:
      -
          ins: M. Bishop
          name: Mike Bishop
          org: Akamai Technologies
          role: editor

  QUIC-TRANSPORT:
    title: "QUIC: A UDP-Based Multiplexed and Secure Transport"
    date: {DATE}
    seriesinfo:
      Internet-Draft: draft-ietf-quic-transport-latest
    author:
      -
        ins: J. Iyengar
        name: Jana Iyengar
        org: Fastly
        role: editor
      -
        ins: M. Thomson
        name: Martin Thomson
        org: Mozilla
        role: editor


--- abstract

This specification defines QPACK, a compression format for efficiently
representing HTTP header fields, to be used in HTTP over QUIC. This is a
variation of HPACK header compression that seeks to reduce head-of-line
blocking.

--- note_Note_to_Readers

Discussion of this draft takes place on the QUIC working group mailing list
(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>.

--- middle

# Introduction

The QUIC transport protocol was designed from the outset to support HTTP
semantics, and its design subsumes many of the features of HTTP/2.  HTTP/2 uses
HPACK ({{!RFC7541}}) for header compression, but QUIC's stream multiplexing
comes into some conflict with HPACK.  A key goal of the design of QUIC is to
improve stream multiplexing relative to HTTP/2 by reducing head-of-line
blocking.  If HPACK were used for HTTP/QUIC, it would induce head-of-line
blocking due to built-in assumptions of a total ordering across frames on all
streams.

QUIC is described in {{QUIC-TRANSPORT}}.  The HTTP/QUIC mapping is described in
{{QUIC-HTTP}}. For a full description of HTTP/2, see {{?RFC7540}}. The
description of HPACK is {{!RFC7541}}.

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.

# Header Tables

Like HPACK, QPACK uses two tables for associating header fields to indexes.  The
static table (see {{table-static}}) is predefined and contains common header
fields (some of them with an empty value).  The dynamic table (see
{{table-dynamic}}) built up over the course of the connection and can be used by
the encoder to index header fields repeated in the encoded header lists.

Unlike in HPACK, entries in the QPACK static and dynamic tables are addressed
separately.  The following sections describe how entries in each table is
addressed.

## Static Table {#table-static}

The static table consists of a predefined static list of header fields, each of
which has a fixed index over time.  Its entries are defined in Appendix A of
{{!RFC7541}}. Note that because HPACK did not use zero-based references, there
is no value at index zero of the static table.

## Dynamic Table {#table-dynamic}

The dynamic table consists of a list of header fields maintained in first-in,
first-out order.  The dynamic table is initially empty.  Entries are added by
instructions on the encoder stream (see {{encoder-stream}}).

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 (maximum size - new entry size) or until the table is empty.

If the size of the new entry is less than or equal to the maximum size, that
entry is added to the table.  It is an error to attempt to add an entry that is
larger than the maximum size; this MUST be treated as a connection error of type
`HTTP_QPACK_DECOMPRESSION_FAILED`.

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 if the referenced entry is
evicted from the dynamic table prior to inserting the new entry.

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 a decoder.

The encoder decides how to update the dynamic table and as such can control how
much memory is used by the dynamic table.  To limit the memory requirements of
the decoder, the dynamic table size is strictly bounded.

The decoder determines the maximum size that the encoder is permitted to use for
the dynamic table.  In HTTP/QUIC, this value is determined by the
SETTINGS_HEADER_TABLE_SIZE setting (see {{configuration}}).

An encoder can choose to use less capacity than this maximum size (see
{{size-update}}), but the chosen size MUST stay lower than or equal to the
maximum set by the decoder.  Whenever the maximum size for the dynamic table is
reduced, entries are evicted from the end of the dynamic table until the size of
the dynamic table is less than or equal to the maximum size.

This mechanism can be used to completely clear entries from the dynamic table by
setting a maximum size of 0, which can subsequently be restored.

### Absolute and Relative Indexing {#indexing}

Each entry possesses both an absolute index which is fixed for the lifetime of
that entry and a relative index which changes over time based on the context of
the reference. The first entry inserted has an absolute index of "1"; indices
increase sequentially with each insertion.

The relative index begins at zero and increases in the opposite direction from
the absolute index.  Determining which entry has a relative index of "0" depends
on the context of the reference.

On the encoder stream, a relative index of "0" always 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.

~~~~~ drawing
    +---+---------------+-----------+
    | n |      ...      |   d + 1   |  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 - Control Stream"}

Because frames from request streams can be delivered out of order with
instructions on the encoder stream, relative indices are relative to the Base
Index at the beginning of the header block (see {{absolute-index}}). The Base
Index is an absolute index. When interpreting the rest of the frame, the entry
identified by Base Index has a relative index of zero.  The relative indices of
entries do not change while interpreting headers on a request or push stream.

~~~~~ drawing
             Base Index
                 |
                 V
    +---+-----+-----+-----+-------+
    | n | n-1 | n-2 | ... |  d+1  |  Absolute Index
    +---+-----+  -  +-----+   -   +
              |  0  | ... | n-d-3 |  Relative Index
              +-----+-----+-------+

n = count of entries inserted
d = count of entries dropped
~~~~~
{: title="Example Dynamic Table Indexing - Request Stream"}

### Post-Base Indexing

A header block on the request stream can reference entries added after the entry
identified by the Base Index. This allows an encoder to process a header block
in a single pass and include references to entries added while processing this
(or other) header blocks. Newly added entries are referenced using Post-Base
instructions. Indices for Post-Base instructions increase in the same direction
as absolute indices, but the zero value is one higher than the Base Index.

~~~~~ drawing
             Base Index
                 |
                 V
    +---+-----+-----+-----+-----+
    | n | n-1 | n-2 | ... | d+1 |  Absolute Index
    +---+-----+-----+-----+-----+
    | 1 |  0  |                    Post-Base Index
    +---+-----+

n = count of entries inserted
d = count of entries dropped
~~~~~
{: title="Dynamic Table Indexing - Post-Base References"}

If the decoder encounters a reference to an entry which has already been dropped
from the table or which is greater than the declared Largest Reference (see
{{absolute-index}}), this MUST be treated as a stream error of type
`HTTP_QPACK_DECOMPRESSION_FAILED` error code.  If this reference occurs on the
encoder stream, this MUST be treated as a session error.

## Avoiding Head-of-Line Blocking in HTTP/QUIC {#overview-hol-avoidance}

Because QUIC does not guarantee order between data on different streams, a
header block might reference an entry in the dynamic table that has not yet been
received.

Each header block contains a Largest Reference which identifies the table state
necessary for decoding. If the greatest absolute index in the dynamic table is
less than the value of the Largest Reference, the stream is considered
"blocked."  While blocked, header field data should remain in the blocked
stream's flow control window.  When the Largest Reference is zero, the frame
contains no references to the dynamic table and can always be processed
immediately. A stream becomes unblocked when the greatest absolute index in the
dynamic table becomes greater than or equal to the Largest Reference for all
header blocks the decoder has started reading from the stream.  If a decoder
encounters a header block where the actual largest reference is not equal to the
largest reference declared in the prefix, it MAY treat this as a stream error of
type HTTP_QPACK_DECOMPRESSION_FAILED.

A decoder can permit the possibility of blocked streams by setting
SETTINGS_QPACK_BLOCKED_STREAMS to a non-zero value (see {{configuration}}).
This setting specifies an upper bound on the number of streams which can be
blocked.

An encoder can decide whether to risk having a stream become blocked. If
permitted by the value of SETTINGS_QPACK_BLOCKED_STREAMS, compression efficiency
can be improved by referencing dynamic table entries that are still in transit,
but if there is loss or reordering the stream can become blocked at the decoder.
An encoder avoids the risk of blocking by only referencing dynamic table entries
which have been acknowledged, but this means using literals. Since literals make
the header block larger, this can result in the encoder becoming blocked on
congestion or flow control limits.

An encoder MUST limit the number of streams which could become blocked to the
value of SETTINGS_QPACK_BLOCKED_STREAMS at all times. Note that the decoder
might not actually become blocked on every stream which risks becoming blocked.
If the decoder encounters more blocked streams than it promised to support, it
SHOULD treat this as a stream error of type HTTP_QPACK_DECOMPRESSION_FAILED.

### State Synchronization

The decoder stream signals key events at the decoder that permit the encoder to
track the decoder's state.  These events are:

- Complete processing of a header block
- Abandonment of a stream which might have remaining header blocks
- Receipt of new dynamic table entries

Regardless of whether a header block contained blocking references, the
knowledge that it has been processed permits the encoder to evict
entries to which no unacknowledged references remain; see {{blocked-eviction}}.
When a stream is reset or abandoned, the indication that these header blocks
will never be processed serves a similar function; see {{stream-cancellation}}.

For the encoder to identify which dynamic table entries can be safely used
without a stream becoming blocked, the encoder tracks the absolute index of the
decoder's Largest Known Received entry.

When blocking references are permitted, the encoder uses acknowledgement of
header blocks to identify the Largest Known Received index, as described in
{{header-acknowledgement}}.

To acknowledge dynamic table entries which are not referenced by header blocks,
for example because the encoder or the decoder have chosen not to risk blocked
streams, the decoder sends a Table State Synchronize instruction (see
{{table-state-synchronize}}).

# Conventions and Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14 {{!RFC2119}} {{!RFC8174}}
when, and only when, they appear in all capitals, as shown here.

Definitions of terms that are used in this document:

Header:

: A name-value pair sent as part of an HTTP message.

Header set:

: The full collection of headers associated with an HTTP message.

Header block:

: The compressed representation of a header set.

Encoder:

: An implementation which transforms a header set into a header block.

Decoder:

: An implementation which transforms a header block into a header set.

QPACK is a name, not an acronym.

## Notational Conventions

Diagrams use the format described in Section 3.1 of {{?RFC2360}}, with the
following additional conventions:

x (A)
: Indicates that x is A bits long

x (A+)
: Indicates that x uses the prefixed integer encoding defined in Section 5.1 of
  [RFC7541], beginning with an A-bit prefix.

x ...
: Indicates that x is variable-length and extends to the end of the region.

#  Configuration

QPACK defines two settings which are included in the HTTP/QUIC SETTINGS frame.

  SETTINGS_HEADER_TABLE_SIZE (0x1):
  : An integer with a maximum value of 2^30 - 1.  The default value is 4,096
    bytes.  See {{table-dynamic}} for usage.

  SETTINGS_QPACK_BLOCKED_STREAMS (0x7):
  : An integer with a maximum value of 2^16 - 1.  The default value is 100.  See
    {{overview-hol-avoidance}}.

# Wire Format

QPACK instructions occur in three locations, each of which uses a separate
instruction space:

 - The encoder stream is a unidirectional stream of type `0x48` (ASCII 'H')
   which carries table updates from encoder to decoder. Instructions on this
   stream modify the dynamic table state without generating output to any
   particular request.

 - The decoder stream is a unidirectional stream of type `0x68` (ASCII 'h')
   which carries acknowledgements of table modifications and header processing
   from decoder to encoder.

 - Finally, the contents of HEADERS and PUSH_PROMISE frames on request streams
   and push streams reference the QPACK table state.

There MUST be exactly one of each unidirectional stream type in each direction.
Receipt of a second instance of either stream type MUST be treated as a
connection error of HTTP_WRONG_STREAM_COUNT.  Closure of either unidirectional
stream MUST be treated as a connection error of type
HTTP_CLOSED_CRITICAL_STREAM.

This section describes the instructions which are possible on each stream type.

All table updates occur on the encoder stream.  Request streams and push streams
only carry header blocks that do not modify the state of the table.

## 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.

### String Literals

The string literal defined by Section 5.2 of [RFC7541] is also used throughout.
This string format includes optional Huffman encoding.

HPACK defines string literals to begin on a byte boundary.  They begin with a
single flag (indicating whether the string is Huffman-coded), followed by the
Length encoded as a 7-bit prefix integer, and finally Length octets of data.
When Huffman encoding is enabled, the Huffman table from Appendix B of [RFC7541]
is used without modification.

This document expands the definition of string literals and permits them to
begin other than on a byte boundary.  An "N-bit prefix string literal" begins
with the same Huffman flag, followed by the length encoded as an (N-1)-bit
prefix integer.  The remainder of the string literal is unmodified.

A string literal without a prefix length noted is an 8-bit prefix string literal
and follows the definitions in [RFC7541] without modification.

## QPACK Encoder Stream {#encoder-stream}

Table updates can add a table entry, possibly using 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.

The contents of the encoder stream are an unframed sequence of the following
instructions.

### Insert With Name Reference

An addition to the header table where the header field name matches the header
field name of an entry stored in the static table or the dynamic table starts
with the '1' one-bit pattern.  The `S` bit indicates whether the reference is to
the static (S=1) or dynamic (S=0) table. The 6-bit prefix integer (see Section
5.1 of [RFC7541]) that follows is used to locate the table entry for the header
name.  When S=1, the number represents the static table index; when S=0, the
number is the relative index of the entry in the dynamic table.

The header name reference is followed by the header field value represented as a
string literal (see Section 5.2 of [RFC7541]).

~~~~~~~~~~ drawing
     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 1 | S |    Name Index (6+)    |
   +---+---+-----------------------+
   | H |     Value Length (7+)     |
   +---+---------------------------+
   | Value String (Length octets)  |
   +-------------------------------+
~~~~~~~~~~
{: title="Insert Header Field -- Indexed Name"}


### Insert Without Name Reference

An addition to the header table where both the header field name and the header
field value are represented as string literals (see {{primitives}}) starts with
the '01' two-bit pattern.

The name is represented as a 6-bit prefix string literal, while the value is
represented as an 8-bit prefix string literal.

~~~~~~~~~~ drawing
     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 0 | 1 | H | Name Length (5+)  |
   +---+---+---+-------------------+
   |  Name String (Length octets)  |
   +---+---------------------------+
   | H |     Value Length (7+)     |
   +---+---------------------------+
   | Value String (Length octets)  |
   +-------------------------------+
~~~~~~~~~~
{: title="Insert Header Field -- New Name"}


### Duplicate {#duplicate}

Duplication of an existing entry in the dynamic table starts with the '000'
three-bit pattern.  The relative index of the existing entry is represented as
an integer with a 5-bit prefix.

~~~~~~~~~~ drawing
     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 0 | 0 | 0 |    Index (5+)     |
   +---+---+---+-------------------+
~~~~~~~~~~
{:#fig-index-with-duplication title="Duplicate"}

The existing entry is re-inserted into the dynamic table without resending
either the name or the value. This is useful to mitigate the eviction of older
entries which are frequently referenced, both to avoid the need to resend the
header and to avoid the entry in the table blocking the ability to insert new
headers.

### Dynamic Table Size Update {#size-update}

An encoder informs the decoder of a change to the size of the dynamic table
using an instruction which begins with the '001' three-bit pattern.  The new
maximum table size is represented as an integer with a 5-bit prefix (see Section
5.1 of [RFC7541]).

~~~~~~~~~~ drawing
  0   1   2   3   4   5   6   7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 1 |   Max size (5+)   |
+---+---+---+-------------------+
~~~~~~~~~~
{:#fig-size-change title="Maximum Dynamic Table Size Change"}

The new maximum size MUST be lower than or equal to the limit determined by the
protocol using QPACK.  A value that exceeds this limit MUST be treated as a
decoding error.  In HTTP/QUIC, this limit is the value of the
SETTINGS_HEADER_TABLE_SIZE parameter (see {{configuration}}) received from the
decoder.

Reducing the maximum size of the dynamic table can cause entries to be evicted
(see Section 4.3 of [RFC7541]).  This MUST NOT cause the eviction of entries
with outstanding references (see {{reference-tracking}}).  Changing the size of
the dynamic table is not acknowledged as this instruction does not insert an
entry.


## QPACK Decoder Stream {#feedback}

The decoder stream carries information used to ensure consistency of the dynamic
table. Information is sent from the QPACK decoder to the QPACK encoder; that is,
the server informs the client about the processing of the client's header blocks
and table updates, and the client informs the server about the processing of the
server's header blocks and table updates.

The contents of the decoder stream are an unframed sequence of the following
instructions.

### Table State Synchronize

The Table State Synchronize instruction begins with the '00' two-bit pattern.
The instruction specifies the total number of dynamic table inserts and
duplications since the last Table State Synchronize or Header Acknowledgement
that increased the Largest Known Received dynamic table entry.  This is encoded
as a 6-bit prefix integer. The encoder uses this value to determine which table
entries might cause a stream to become blocked, as described in
{{state-synchronization}}.

~~~~~~~~~~ drawing
  0   1   2   3   4   5   6   7
+---+---+---+---+---+---+---+---+
| 0 | 0 |   Insert Count (6+)   |
+---+---+-----------------------+
~~~~~~~~~~
{:#fig-size-sync title="Table State Synchronize"}

A decoder chooses when to emit Table State Synchronize instructions. Emitting a
Table State Synchronize after adding each new dynamic table entry will provide
the most timely feedback to the encoder, but could be redundant with other
decoder feedback. By delaying a Table State Synchronize, a decoder might be able
to coalesce multiple Table State Synchronize instructions, or replace them
entirely with Header Acknowledgements. However, delaying too long may lead to
compression inefficiencies if the encoder waits for an entry to be acknowledged
before using it.

### Header Acknowledgement

After processing a header block whose declared Largest Reference is not zero,
the decoder emits a Header Acknowledgement instruction on the decoder stream.
The instruction begins with the '1' one-bit pattern and includes the request
stream's stream ID, encoded as a 7-bit prefix integer.  It is used by the
peer's QPACK encoder to know when it is safe to evict an entry.

The same Stream ID can be identified multiple times, as multiple header blocks
can be sent on a single stream in the case of intermediate responses, trailers,
and pushed requests.  Since header frames on each stream are received and
processed in order, this gives the encoder precise feedback on which header
blocks within a stream have been fully processed.

~~~~~~~~~~ drawing
  0   1   2   3   4   5   6   7
+---+---+---+---+---+---+---+---+
| 1 |      Stream ID (7+)       |
+---+---------------------------+
~~~~~~~~~~
{:#fig-header-ack title="Header Acknowledgement"}

When blocking references are permitted, the encoder uses acknowledgement of
header blocks to update the Largest Known Received index.  If a header block was
potentially blocking, the acknowledgement implies that the decoder has received
all dynamic table state necessary to process the header block.  If the Largest
Reference of an acknowledged header block was greater than the encoder's current
Largest Known Received index, the block's Largest Reference becomes the new
Largest Known Received.


### Stream Cancellation

A stream that is reset might have multiple outstanding header blocks.  A decoder
that receives a stream reset before the end of a stream generates a Stream
Cancellation instruction on the decoder stream.  Similarly, a decoder that
abandons reading of a stream needs to signal this using the Stream Cancellation
instruction.  This signals to the encoder that all references to the dynamic
table on that stream are no longer outstanding.

An encoder cannot infer from this instruction that any updates to the dynamic
table have been received.

The instruction begins with the '01' two-bit pattern. The instruction includes
the stream ID of the affected stream - a request or push stream - encoded as a
6-bit prefix integer.

~~~~~~~~~~ drawing
  0   1   2   3   4   5   6   7
+---+---+---+---+---+---+---+---+
| 0 | 1 |     Stream ID (6+)    |
+---+---+-----------------------+
~~~~~~~~~~
{:#fig-stream-cancel title="Stream Cancellation"}


## Request and Push Streams

HEADERS and PUSH_PROMISE frames on request and push streams reference the
dynamic table in a particular state without modifying it.  Frames on these
streams emit the headers for an HTTP request or response.

### Header Data Prefix {#absolute-index}

Header data is prefixed with two integers, `Largest Reference` and `Base Index`.

~~~~~~~~~~  drawing
  0   1   2   3   4   5   6   7
+---+---+---+---+---+---+---+---+
|     Largest Reference (8+)    |
+---+---------------------------+
| S |   Delta Base Index (7+)   |
+---+---------------------------+
|      Compressed Headers     ...
+-------------------------------+
~~~~~~~~~~
{:#fig-base-index title="Frame Payload"}

`Largest Reference` identifies the largest absolute dynamic index referenced in
the block.  Blocking decoders use the Largest Reference to determine when it is
safe to process the rest of the block.

`Base Index` is used to resolve references in the dynamic table as described in
{{indexing}}.

To save space, Base Index is encoded relative to Largest Reference using a
one-bit sign and the `Delta Base Index` value.  A sign bit of 0 indicates that
the Base Index has an absolute index that is greater than or equal to the
Largest Reference; the value of Delta Base Index is added to the Largest
Reference to determine the absolute value of the Base Index.  A sign bit of 1
indicates that the Base Index is less than the Largest Reference.  That is:

~~~
   if sign == 0:
      baseIndex = largestReference + deltaBaseIndex
   else:
      baseIndex = largestReference - deltaBaseIndex
~~~

A single-pass encoder is expected to determine the absolute value of Base Index
before encoding a header block.  If the encoder inserted entries in the dynamic
table while encoding the header block, Largest Reference will be greater than
Base Index, so the encoded difference is negative and the sign bit is set to 1.
If the header block did not reference the most recent entry in the table and did
not insert any new entries, Base Index will be greater than the Largest
Reference, so the delta will be positive and the sign bit is set to 0.

An encoder that produces table updates before encoding a header block might set
Largest Reference and Base Index to the same value.  When Largest Reference and
Base Index are equal, the Delta Base Index is encoded with a zero sign bit.  A
sign bit set to 1 when the Delta Base Index is 0 MUST be treated as a decoder
error.

A header block that does not reference the dynamic table can use any value for
Base Index; setting both Largest Reference and Base Index to zero is the most
efficient encoding.


### Instructions

#### Indexed Header Field

An indexed header field representation identifies an entry in either the static
table or the dynamic table and causes that header field to be added to the
decoded header list, as described in Section 3.2 of [RFC7541].

~~~~~~~~~~ drawing
  0   1   2   3   4   5   6   7
+---+---+---+---+---+---+---+---+
| 1 | S |      Index (6+)       |
+---+---+-----------------------+
~~~~~~~~~~
{: title="Indexed Header Field"}

If the entry is in the static table, or in the dynamic table with an absolute
index less than or equal to Base Index, this representation starts with the '1'
1-bit pattern, followed by the `S` bit indicating whether the reference is into
the static (S=1) or dynamic (S=0) table. Finally, the relative index of the
matching header field is represented as an integer with a 6-bit prefix (see
Section 5.1 of [RFC7541]).


#### Indexed Header Field With Post-Base Index

If the entry is in the dynamic table with an absolute index greater than Base
Index, the representation starts with the '0001' 4-bit pattern, followed by the
post-base index (see {{post-base-indexing}}) of the matching header field,
represented as an integer with a 4-bit prefix (see Section 5.1 of [RFC7541]).

~~~~~~~~~~ drawing
  0   1   2   3   4   5   6   7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 1 |  Index (4+)   |
+---+---+---+---+---------------+
~~~~~~~~~~
{: title="Indexed Header Field with Post-Base Index"}


#### Literal Header Field With Name Reference

A literal header field with a name reference represents a header where the
header field name matches the header field name of an entry stored in the static
table or the dynamic table.

If the entry is in the static table, or in the dynamic table with an absolute
index less than or equal to Base Index, this representation starts with the '01'
two-bit pattern.  If the entry is in the dynamic table with an absolute index
greater than Base Index, the representation starts with the '0000' four-bit
pattern.

The following bit, 'N', indicates whether an intermediary is permitted to add
this header to the dynamic header table on subsequent hops. When the 'N' bit is
set, the encoded header MUST always be encoded with a literal representation. In
particular, when a peer sends a header field that it received represented as a
literal header field with the 'N' bit set, it MUST use a literal representation
to forward this header field.  This bit is intended for protecting header field
values that are not to be put at risk by compressing them (see Section 7.1 of
[RFC7541] for more details).

~~~~~~~~~~ drawing
     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 0 | 1 | N | S |Name Index (4+)|
   +---+---+---+---+---------------+
   | H |     Value Length (7+)     |
   +---+---------------------------+
   | Value String (Length octets)  |
   +-------------------------------+
~~~~~~~~~~
{: title="Literal Header Field With Name Reference"}

For entries in the static table or in the dynamic table with an absolute index
less than or equal to Base Index, the header field name is represented using the
relative index of that entry, which is represented as an integer with a 4-bit
prefix (see Section 5.1 of [RFC7541]). The `S` bit indicates whether the
reference is to the static (S=1) or dynamic (S=0) table.

#### Literal Header Field With Post-Base Name Reference

For entries in the dynamic table with an absolute index greater than Base Index,
the header field name is represented using the post-base index of that entry
(see {{post-base-indexing}}) encoded as an integer with a 3-bit prefix.

~~~~~~~~~~ drawing
     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 0 | 0 | 0 | 0 | N |NameIdx(3+)|
   +---+---+---+---+---+-----------+
   | H |     Value Length (7+)     |
   +---+---------------------------+
   | Value String (Length octets)  |
   +-------------------------------+
~~~~~~~~~~
{: title="Literal Header Field With Post-Base Name Reference"}


#### Literal Header Field Without Name Reference

An addition to the header table where both the header field name and the header
field value are represented as string literals (see {{primitives}}) starts with
the '001' three-bit pattern.

The fourth bit, 'N', indicates whether an intermediary is permitted to add this
header to the dynamic header table on subsequent hops. When the 'N' bit is set,
the encoded header MUST always be encoded with a literal representation. In
particular, when a peer sends a header field that it received represented as a
literal header field with the 'N' bit set, it MUST use a literal representation
to forward this header field.  This bit is intended for protecting header field
values that are not to be put at risk by compressing them (see Section 7.1 of
[RFC7541] for more details).

The name is represented as a 4-bit prefix string literal, while the value is
represented as an 8-bit prefix string literal.

~~~~~~~~~~ drawing
     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 0 | 0 | 1 | N | H |NameLen(3+)|
   +---+---+---+---+---+-----------+
   |  Name String (Length octets)  |
   +---+---------------------------+
   | H |     Value Length (7+)     |
   +---+---------------------------+
   | Value String (Length octets)  |
   +-------------------------------+
~~~~~~~~~~
{: title="Literal Header Field Without Name Reference"}


# Error Handling

The following error code is defined for HTTP/QUIC to indicate all failures of
QPACK which prevent the stream or connection from continuing:

HTTP_QPACK_DECOMPRESSION_FAILED (0x06):
: QPACK failed to decompress a frame and cannot continue.


# Encoding Strategies

## Single Pass Encoding

An encoder making a single pass over a list of headers must choose Base Index
before knowing Largest Reference.  When trying to reference a header inserted to
the table after encoding has begun, the entry is encoded with different
instructions that tell the decoder to use an absolute index greater than the
Base Index.

## Preventing Eviction Races {#evictions}

Due to out-of-order arrival, QPACK's eviction algorithm requires changes
(relative to HPACK) to avoid the possibility that an indexed representation is
decoded after the referenced entry has already been evicted.  QPACK employs a
two-phase eviction algorithm, in which the encoder will not evict entries that
have outstanding (unacknowledged) references.

## Reference Tracking

An encoder MUST ensure that a header block which references a dynamic table
entry is not received by the decoder after the referenced entry has already been
evicted.  An encoder also respects the limit set by the decoder on the number of
streams that are allowed to become blocked. Even if the decoder is willing to
tolerate blocked streams, the encoder might choose to avoid them in certain
cases.

In order to enable this, the encoder will need to track outstanding
(unacknowledged) header blocks and table updates using feedback received from
the decoder.

### Blocked Eviction

The encoder MUST NOT permit an entry to be evicted while a reference to that
entry remains unacknowledged.  If a new header to be inserted into the dynamic
table would cause the eviction of such an entry, the encoder MUST NOT emit the
insert instruction until the reference has been processed by the decoder and
acknowledged.

The encoder can emit a literal representation for the new header in order to
avoid encoding delays, and MAY insert the header into the table later if
desired.

To ensure that the blocked eviction case is rare, references to the oldest
entries in the dynamic table SHOULD be avoided.  When one of the oldest entries
in the table is still actively used for references, the encoder SHOULD emit an
Duplicate representation instead (see {{duplicate}}).


### Blocked Decoding

For header blocks encoded in non-blocking mode, the encoder needs to forego
indexed representations that refer to table updates which have not yet been
acknowledged with {{feedback}}.  Since all table updates are processed in
sequence on the encoder stream, an index into the dynamic table is sufficient to
track which entries have been acknowledged.

To track blocked streams, the necessary Base Index value for each stream can be
used.  Whenever the decoder processes a table update, it can begin decoding any
blocked streams that now have their dependencies satisfied.


## Speculative table updates {#speculative-updates}

Implementations can *speculatively* send header frames on the HTTP Control
Streams which are not needed for any current HTTP request or response.  Such
headers could be used strategically to improve performance.  For instance, the
encoder might decide to *refresh* by sending Duplicate representations for
popular header fields ({{duplicate}}), ensuring they have small indices and
hence minimal size on the wire.

## Sample One Pass Encoding Algorithm

Pseudo-code for single pass encoding, excluding handling of duplicates,
non-blocking mode, and reference tracking.

~~~
baseIndex = dynamicTable.baseIndex
largestReference = 0
for header in headers:
  staticIdx = staticTable.getIndex(header)
  if staticIdx:
    encodeIndexReference(streamBuffer, staticIdx)
    continue

  dynamicIdx = dynamicTable.getIndex(header)
  if !dynamicIdx:
    # No matching entry.  Either insert+index or encode literal
    nameIdx = getNameIndex(header)
    if shouldIndex(header) and dynamicTable.canIndex(header):
      encodeLiteralWithIncrementalIndex(controlBuffer, nameIdx,
                                        header)
      dynamicTable.add(header)
      dynamicIdx = dynamicTable.baseIndex

  if !dynamicIdx:
    # Couldn't index it, literal
    if nameIdx <= staticTable.size:
      encodeLiteral(streamBuffer, nameIndex, header)
    else:
      # encode literal, possibly with nameIdx above baseIndex
      encodeDynamicLiteral(streamBuffer, nameIndex, baseIndex,
                           header)
      largestReference = max(largestReference,
                             dynamicTable.toAbsolute(nameIdx))
  else:
    # Dynamic index reference
    assert(dynamicIdx)
    largestReference = max(largestReference, dynamicIdx)
    # Encode dynamicIdx, possibly with dynamicIdx above baseIndex
    encodeDynamicIndexReference(streamBuffer, dynamicIdx,
                                baseIndex)

# encode the prefix
encodeInteger(prefixBuffer, 0x00, largestReference, 8)
if baseIndex >= largestReference:
  encodeInteger(prefixBuffer, 0, baseIndex - largestReference, 7)
else:
  encodeInteger(prefixBuffer, 0x80,
                largestReference  - baseIndex, 7)

return controlBuffer, prefixBuffer + streamBuffer
~~~

# Security Considerations

TBD.

# IANA Considerations

## Settings Registration

This document creates two new settings in the "HTTP/QUIC Settings" registry
established in {{QUIC-HTTP}}.

The entries in the following table are registered by this document.

|------------------------------|--------|---------------------------|
| Setting Name                 | Code   | Specification             |
| ---------------------------- | :----: | ------------------------- |
| HEADER_TABLE_SIZE            | 0x1    | {{configuration}}         |
| QPACK_BLOCKED_STREAMS        | 0x7    | {{configuration}}         |
| ---------------------------- | ------ | ------------------------- |

## Stream Type Registration

This document creates two new settings in the "HTTP/QUIC Stream Type" registry
established in {{QUIC-HTTP}}.

The entries in the following table are registered by this document.

| ---------------------------- | ------ | ------------------------- | ------ |
| Stream Type                  | Code   | Specification             | Sender |
| ---------------------------- | :----: | ------------------------- | ------ |
| QPACK Encoder Stream         | 0x48   | {{wire-format}}           | Both   |
| QPACK Decoder Stream         | 0x68   | {{wire-format}}           | Both   |
| ---------------------------- | ------ | ------------------------- | ------ |

## Error Code Registration

This document establishes one new error code in the "HTTP/QUIC Error Code"
registry established in {{QUIC-HTTP}}.

Name:
: HTTP_QPACK_DECOMPRESSION_FAILED

Code:
: 0x06

Description:
: QPACK failed to interpret an instruction and cannot continue.


--- back

# Change Log

> **RFC Editor's Note:** Please remove this section prior to publication of a
> final version of this document.

## Since draft-ietf-quic-qpack-01

- Only header blocks that reference the dynamic table are acknowledged (#1603,
  #1605)

## 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)
- Removed framing from encoder stream (#1361,#1467)
- Control streams use typed unidirectional streams rather than fixed stream IDs
  (#910,#1359)

## 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
{:numbered="false"}

This draft draws heavily on the text of {{!RFC7541}}.  The indirect input of
those authors is gratefully acknowledged, as well as ideas from:

* Ryan Hamilton

* Patrick McManus

* Kazuho Oku

* Biren Roy

* Ian Swett

* Dmitri Tikhonov

Buck's contribution was supported by Google during his employment there.

A substantial portion of Mike's contribution was supported by Microsoft during
his employment there.