https://github.com/quicwg/base-drafts
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draft-ietf-quic-qpack.md
---
title: "QPACK: Header Compression for HTTP/3"
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:
HTTP3:
title: "Hypertext Transfer Protocol Version 3 (HTTP/3)"
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/3. 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 {{QUIC-TRANSPORT}} is designed to support HTTP
semantics, and its design subsumes many of the features of HTTP/2 {{?RFC7540}}.
HTTP/2 uses HPACK ({{!RFC7541}}) for header compression. If HPACK were used for
HTTP/3 {{HTTP3}}, it would induce head-of-line blocking 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.
Definitions of terms that are used in this document:
Header field:
: A name-value pair sent as part of an HTTP message.
Header list:
: An ordered collection of header fields associated with an HTTP message. A
header list can contain multiple header fields with the same name. It can
also contain duplicate header fields.
Header block:
: The compressed representation of a header list.
Encoder:
: An implementation which transforms a header list into a header block.
Decoder:
: An implementation which transforms a header block into a header list.
Absolute Index:
: A unique index for each entry in the dynamic table.
Base:
: A reference point for relative indices. Dynamic references are made relative
to a Base in header blocks.
Insert Count:
: The total number of entries inserted in the dynamic table.
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.
# Compression Process Overview
Like HPACK, QPACK uses two tables for associating header fields to indices. 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}}) is built up over the course of the connection and can be used
by the encoder to index header fields in the encoded header lists.
QPACK defines unidirectional streams for sending instructions from encoder to
decoder and vice versa.
## Encoder
An encoder converts a header list into a header block by emitting either an
indexed or a literal representation for each header field in the list (see
{{header-block-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 acknowledgement indicating the entry
is available at the decoder.
An encoder MAY insert any entry in the dynamic table it chooses; it is not
limited to header fields it is compressing.
QPACK preserves the ordering of header fields within each header list. An
encoder MUST emit header field representations in the order they appear in the
input header list.
QPACK is designed to contain the more complex state tracking to the encoder,
while the decoder is relatively simple.
### Reference Tracking
An encoder MUST ensure that a header block which references a dynamic table
entry is not processed by the decoder after the referenced entry has been
evicted. Hence the encoder needs to track information about each compressed
header block that references the dynamic table until that header block is
acknowledged by the decoder (see {{header-acknowledgement}}).
### Blocked Dynamic Table Insertions {#blocked-insertion}
A dynamic table entry is considered blocking and cannot be evicted until its
insertion has been acknowledged and there are no outstanding unacknowledged
references to the entry. In particular, a dynamic table entry that has never
been referenced can still be blocking.
Note:
: A blocking entry is unrelated to a blocked stream, which is a stream that a
decoder cannot decode as a result of references to entries that are not yet
available. An encoder that uses the dynamic table has to keep track of
blocked entries.
An encoder MUST NOT insert an entry into the dynamic table (or duplicate an
existing entry) if doing so would evict a blocking entry.
#### Avoiding Blocked 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 (see
{{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 (see {{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.
~~~~~~~~~~ drawing
+----------+---------------------------------+--------+
| Draining | Referenceable | Unused |
| Entries | Entries | Space |
+----------+---------------------------------+--------+
^ ^ ^
| | |
Dropping Draining Index Insertion Point
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 header block that references a dynamic table entry
that it has not yet received.
Each header block contains a Required Insert Count (see {{header-prefix}}), the
lowest possible value for the Insert Count with which the header block can be
decoded. For a header block with 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 header block with no references to the dynamic
table, the Required Insert Count is zero.
When the decoder receives a header block 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 SETTINGS_QPACK_BLOCKED_STREAMS setting (see {{configuration}}) specifies an
upper bound on the number of streams which can be blocked. An encoder MUST limit
the number of streams which could become blocked to the value of
SETTINGS_QPACK_BLOCKED_STREAMS at all times. Note that the decoder might not
become blocked on every stream which risks becoming blocked.
An encoder can decide whether to risk having a stream become blocked. If
permitted by the value of SETTINGS_QPACK_BLOCKED_STREAMS, compression efficiency
can often be improved by referencing dynamic table entries that are still in
transit, but if there is loss or reordering the stream can become blocked at the
decoder. An encoder avoids the risk of blocking by only referencing dynamic
table entries which have been acknowledged, but this could mean using
literals. Since literals make the header block larger, this can result in the
encoder becoming blocked on congestion or flow control limits.
### Known Received Count
In order to identify which dynamic table entries can be safely used without a
stream becoming blocked, the encoder tracks the number of entries received by
the decoder. The Known Received Count tracks the total number of acknowledged
insertions.
When blocking references are permitted, the encoder uses Header Acknowledgement
instructions ({{header-acknowledgement}}) to maintain the Known Received
Count. 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 Required Insert Count of an acknowledged header block was
greater than the encoder's current Known Received Count, the block's Required
Insert Count becomes the new Known Received Count.
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 an Insert Count Increment instruction (see
{{insert-count-increment}}).
## Decoder
As in HPACK, the decoder processes header blocks and emits the corresponding
header lists. It also processes dynamic table modifications from encoder
instructions received on the encoder stream.
The decoder MUST emit header fields in the order their representations appear in
the input header block.
### Blocked Decoding
Upon receipt of a header block, the decoder examines the Required Insert Count.
When the Required Insert Count is less than or equal to the decoder's Insert
Count, the header block can be processed immediately. Otherwise, the stream on
which the header block was received becomes blocked.
While blocked, header block data SHOULD remain in the blocked stream's flow
control window. A stream becomes unblocked when the Insert Count becomes
greater than or equal to the Required Insert Count for all header blocks the
decoder has started reading from the stream.
<!-- doesn't the stream become unblocked when the encoder receives the acks? -->
When processing header blocks, the decoder expects the Required Insert Count to
exactly match the value defined in {{blocked-streams}. If it encounters a
smaller value than expected, it MUST treat this as a connection error of type
HTTP_QPACK_DECOMPRESSION_FAILED (see {{invalid-references}}). If it encounters a
larger value than expected, it MAY treat this as a connection error of type
HTTP_QPACK_DECOMPRESSION_FAILED.
If the decoder encounters more blocked streams than it promised to support, it
MUST treat this as a connection error of type HTTP_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 Header Block
When the decoder finishes decoding a header block containing dynamic table
references, it emits a Header Acknowledgement instruction
({{header-acknowledgement}}). A stream may carry multiple header blocks in the
case of intermediate responses, trailers, and pushed requests. The encoder
interprets each Header Acknowledgement instruction as acknowledging the earliest
unacknowledged header block 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 header blocks 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 equal to zero (see
{{maximum-dynamic-table-capacity}}) 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 Header Acknowledgement and Stream Cancellation instructions permit the
encoder to remove references to entries in the dynamic table. When entries
have zero references they are no longer considered blocking (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 Header
Acknowledgements (see {{header-acknowledgement}}). 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 header block representation to a
dynamic table entry which has already been evicted or which has an absolute
index greater than or equal to the declared Required Insert Count (see
{{header-prefix}}), it MUST treat this as a connection error of type
`HTTP_QPACK_DECOMPRESSION_FAILED`.
If the decoder encounters a reference in an encoder instruction to a dynamic
table entry which has already been evicted, it MUST treat this as a connection
error of type `HTTP_QPACK_ENCODER_STREAM_ERROR`.
# Header 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 {#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 {{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 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 header block
representation it MUST treat this as a connection error of type
`HTTP_QPACK_DECOMPRESSION_FAILED`. If this index is received on the encoder
stream, this MUST be treated as a connection error of type
`HTTP_QPACK_ENCODER_STREAM_ERROR`.
## Dynamic Table {#table-dynamic}
The dynamic table consists of a list of header fields maintained in first-in,
first-out order. Each HTTP/3 endpoint holds a dynamic table that is initially
empty. Entries are added by encoder instructions received on the encoder stream
(see {{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 (as defined in
{{string-literals}}), its value's length in bytes, and 32.
The size of an entry is calculated using the length of its name and value
without Huffman encoding applied.
### 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 evict a
blocking dynamic table entry (see {{blocked-insertion}}). The 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 `HTTP_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 (see
{{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 0, 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
`HTTP_QPACK_DECODER_STREAM_ERROR`.
For HTTP/3 servers and HTTP/3 clients when 0-RTT is not attempted or is
rejected, the maximum table capacity is 0 until the encoder processes a SETTINGS
frame with a non-zero value of SETTINGS_QPACK_MAX_TABLE_CAPACITY.
When the maximum table capacity is 0, 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 both an absolute index which is fixed for the lifetime of
that entry and a relative index which changes based on the context of the
reference. 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, 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-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 encoder instructions, relative indices in header block representations
are relative to the Base at the beginning of the header block (see
{{header-prefix}}). This ensures that references are stable even if header
blocks and dynamic table updates are processed out of order.
In a header block a relative index of "0" refers to the entry with absolute
index equal to Base - 1.
~~~~~ drawing
Required
Insert
Count Base
| |
V V
+-----+-----+-----+-----+-------+
| n-1 | n-2 | n-3 | ... | d | Absolute Index
+-----+-----+ - +-----+ - +
| 0 | ... | n-d-3 | Relative Index
+-----+-----+-------+
n = count of entries inserted
d = count of entries dropped
~~~~~
{: title="Example Dynamic Table Indexing - Relative Index in Header Block"}
### Post-Base Indexing {#post-base}
Post-Base indices are used for entries with absolute indexes 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 header block in a single pass
and include references to entries added while processing this (or other) header
blocks.
~~~~~ drawing
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
~~~~~
{: title="Example Dynamic Table Indexing - Post-Base Index in Header Block"}
# 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. QPACK
implementations MUST be able to decode integers up to 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 flag (indicating whether the string is Huffman-coded), followed by the
Length encoded as a 7-bit prefix integer, and finally Length bytes of data.
When Huffman encoding is enabled, the Huffman table from Appendix B of [RFC7541]
is used without modification.
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.
## 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 HTTP_WRONG_STREAM_COUNT. These streams MUST NOT be closed. Closure of
either unidirectional stream type MUST be treated as a connection error of type
HTTP_CLOSED_CRITICAL_STREAM.
An endpoint MAY avoid creating its own encoder stream if it's not going to be
used (for example if the endpoint doesn't wish to use the dynamic table, or if
the maximum size of the dynamic table permitted by the peer is zero).
An endpoint MAY avoid creating its own decoder stream if the maximum size of
its own dynamic table is zero.
An endpoint MUST allow its peer to create both encoder and decoder streams
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 which already exist in
the dynamic table, the full entry can also be used by reference, creating a
duplicate entry.
This section specifies the following encoder instructions.
### Set Dynamic Table Capacity {#set-dynamic-capacity}
An encoder informs the decoder of a change to the dynamic table capacity using
an instruction which begins with the '001' three-bit pattern. The new dynamic
table capacity 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 | 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 (see {{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
`HTTP_QPACK_ENCODER_STREAM_ERROR`.
Reducing the dynamic table capacity can cause entries to be evicted (see
{{eviction}}). This MUST NOT cause the eviction of blocking entries (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 header field name
matches the header field name of an entry stored in the static or the dynamic
table using an instruction that starts with the '1' one-bit pattern. The second
(`S`) bit indicates whether the reference is to the static or dynamic 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 bytes) |
+-------------------------------+
~~~~~~~~~~
{: title="Insert Header Field -- Indexed Name"}
### Insert Without Name Reference
An encoder adds an entry to the dynamic table where both the header field name
and the header field value are represented as string literals (see
{{primitives}}) using an instruction that 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 bytes) |
+---+---------------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length bytes) |
+-------------------------------+
~~~~~~~~~~
{: title="Insert Header Field -- New Name"}
### Duplicate {#duplicate}
An encoder duplicates an existing entry in the dynamic table using an
instruction that 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.
## Decoder Instructions {#decoder-instructions}
Decoder instructions provide information used to ensure consistency of the
dynamic table. They are sent from the decoder to the encoder on a decoder
stream; 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.
This section specifies the following decoder instructions.
### Header Acknowledgement
After processing a header block whose declared Required Insert Count is not
zero, the decoder emits a Header Acknowledgement instruction. The instruction
begins with the '1' one-bit pattern and includes the header block's associated
stream ID, encoded as a 7-bit prefix integer. It is used by the peer's encoder
to know when it is safe to evict an entry ({{blocked-insertion}}), and possibly
update the Known Received Count ({{known-received-count}}).
~~~~~~~~~~ drawing
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | Stream ID (7+) |
+---+---------------------------+
~~~~~~~~~~
{:#fig-header-ack title="Header Acknowledgement"}
If an encoder receives a Header Acknowledgement instruction referring to a
stream on which every header block with a non-zero Required Insert Count has
already been acknowledged, that MUST be treated as a connection error of type
`HTTP_QPACK_DECODER_STREAM_ERROR`.
### Stream Cancellation
When a stream is reset or reading is abandoned, the decoder emits a Stream
Cancellation instruction. The instruction begins with the '01' two-bit
pattern. The instruction includes the stream ID of the affected stream encoded
as a 6-bit prefix integer. See {{state-synchronization}}.
~~~~~~~~~~ drawing
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 begins with the '00' two-bit pattern.
The instruction specifies the total number of dynamic table inserts and
duplications since the last Insert Count Increment or Header Acknowledgement
that increased the Known Received Count for the dynamic table (see
{{known-received-count}}). The Increment field 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 | 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 `HTTP_QPACK_DECODER_STREAM_ERROR`.
## Header Block Representations
Header blocks contain compressed representations of header lists and are carried
in frames on streams defined by the enclosing protocol. These representations
reference the static table, or dynamic table in a particular state without
modifying it.
### Header Block Prefix {#header-prefix}
Each header block is prefixed with two integers. The Required Insert Count is
encoded as an integer with an 8-bit prefix after the encoding described in
{{ric}}). The Base is encoded as sign-and-modulus integer, using a single sign
bit and a value with a 7-bit prefix (see {{base}}).
These two values are followed by representations for compressed headers.
~~~~~~~~~~ drawing
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Required Insert Count (8+) |
+---+---------------------------+
| S | Delta Base (7+) |
+---+---------------------------+
| Compressed Headers ...
+-------------------------------+
~~~~~~~~~~
{:#fig-base-index title="Header Block"}
#### Required Insert Count {#ric}
Required Insert Count identifies the state of the dynamic table needed to
process the header block. Blocking decoders use the Required Insert Count to
determine when it is safe to process the rest of the block.
The encoder transforms the Required Insert Count as follows before encoding:
~~~
if ReqInsertCount == 0:
EncInsertCount = 0
else:
EncInsertCount = (ReqInsertCount mod (2 * MaxEntries)) + 1
~~~
Here `MaxEntries` is the maximum number of entries that the dynamic table can
have. The smallest entry has empty name and value strings and has the size of
32. Hence `MaxEntries` is calculated as
~~~
MaxEntries = floor( MaxTableCapacity / 32 )
~~~
`MaxTableCapacity` is the maximum capacity of the dynamic table as specified by
the decoder (see {{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 `HTTP_QPACK_DECOMPRESSION_FAILED`.
TotalNumberOfInserts is the total number of inserts into the decoder's dynamic
table.
~~~
FullRange = 2 * MaxEntries
if EncodedInsertCount == 0:
ReqInsertCount = 0
else:
if EncodedInsertCount > FullRange:
Error
MaxValue = TotalNumberOfInserts + MaxEntries
# MaxWrapped is the largest possible value of
# ReqInsertCount that is 0 mod 2*MaxEntries
MaxWrapped = floor(MaxValue / FullRange) * FullRange
ReqInsertCount = MaxWrapped + EncodedInsertCount - 1
# If ReqInsertCount exceeds MaxValue, the Encoder's value
# must have wrapped one fewer time
if ReqInsertCount > MaxValue:
if ReqInsertCount < FullRange:
Error
ReqInsertCount -= FullRange
~~~
For example, if the dynamic table is 100 bytes, then the Required Insert Count
will be encoded modulo 6. If a decoder has received 10 inserts, then an encoded
value of 3 indicates that the Required Insert Count is 9 for the header block.
#### Base {#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 Insert Count using a one-bit
sign and the `Delta Base` value. A sign bit of 0 indicates that the Base is
greater than or equal to the value of the Insert Count; the value of Delta Base
is added to the Insert Count to determine the value of the Base. A sign bit of
1 indicates that the Base is less than the Insert Count. That is:
~~~
if S == 0:
Base = ReqInsertCount + DeltaBase
else:
Base = ReqInsertCount - DeltaBase - 1
~~~
A single-pass encoder determines the Base before encoding a header block. If
the encoder inserted entries in the dynamic table while encoding the header
block, 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 header block did
not reference the most recent entry in the table and did not insert any new
entries, the Base will be greater than the Required Insert Count, so the delta
will be positive and the sign bit is set to 0.
An encoder that produces table updates before encoding a header block might set
Required Insert Count and the Base to the same value. In such case, both the
sign bit and the Delta Base will be set to zero.
A header block that does not reference the dynamic table can use any value for
the Base; setting Delta Base to zero is the most efficient encoding.
For example, with a Required Insert Count of 9, a decoder receives a S bit of 1
and a Delta Base of 2. This sets the Base to 6 and enables post-base indexing
for three entries. In this example, a regular index of 1 refers to the 5th
entry that was added to the table; a post-base index of 1 refers to the 8th
entry.
### 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].
<!-- is the 7541 reference still helpful here -->
~~~~~~~~~~ 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 the Base, this representation starts with the '1' 1-bit pattern,
followed by the `S` bit indicating whether the reference is into the static or
dynamic 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.
### Indexed Header Field With Post-Base Index
If the entry is in the dynamic table with an absolute index greater than or
equal to the Base, the representation starts with the '0001' 4-bit pattern,
followed by the post-base index (see {{post-base}}) 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 {#literal-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 the Base, this representation starts with the '01' two-bit
pattern.
Only the header field name stored in the static or dynamic table is used. Any
header field value MUST be ignored.
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 bytes) |
+-------------------------------+
~~~~~~~~~~
{: title="Literal Header Field With Name Reference"}
The fourth (`S`) bit indicates whether the reference is to the static or dynamic
table. The 4-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.
### Literal Header Field With Post-Base Name Reference
If the name entry is in the dynamic table with an absolute index greater than or
equal to the Base, the representation starts with the '0000' four-bit
pattern. The fifth bit is the 'N' bit as described in
{{literal-name-reference}}. Finally, the header field name is represented using
the post-base index of that entry (see {{post-base}}) 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 bytes) |
+-------------------------------+
~~~~~~~~~~
{: 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 bytes) |
+---+---------------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length bytes) |
+-------------------------------+
~~~~~~~~~~
{: title="Literal Header Field Without Name Reference"}
# Configuration
QPACK defines two settings which are included in the HTTP/3 SETTINGS frame.
SETTINGS_QPACK_MAX_TABLE_CAPACITY (0x1):
: The default value is zero. See {{table-dynamic}} for usage. This is
the equivalent of the SETTINGS_HEADER_TABLE_SIZE from HTTP/2.
SETTINGS_QPACK_BLOCKED_STREAMS (0x7):
: The default value is zero. See {{blocked-streams}}.
# Error Handling {#error-handling}
The following error codes are defined for HTTP/3 to indicate failures of
QPACK which prevent the connection from continuing:
HTTP_QPACK_DECOMPRESSION_FAILED (0x200):
: The decoder failed to interpret a header block and is not able to continue
decoding that header block.
HTTP_QPACK_ENCODER_STREAM_ERROR (0x201):
: The decoder failed to interpret an encoder instruction received on the
encoder stream.
HTTP_QPACK_DECODER_STREAM_ERROR (0x202):
: The encoder failed to interpret a decoder instruction received on the
decoder stream.
# Security Considerations
TBD.
# IANA Considerations
## Settings Registration
This document specifies two settings. The entries in the following table are
registered in the "HTTP/3 Settings" registry established in {{HTTP3}}.
|------------------------------|--------|---------------------------|
| Setting Name | Code | Specification |
| ---------------------------- | :----: | ------------------------- |
| QPACK_MAX_TABLE_CAPACITY | 0x1 | {{configuration}} |
| QPACK_BLOCKED_STREAMS | 0x7 | {{configuration}} |
| ---------------------------- | ------ | ------------------------- |
## Stream Type Registration
This document specifies two stream types. The entries in the following table are
registered in the "HTTP/3 Stream Type" registry established in {{HTTP3}}.
| ---------------------------- | ------ | ------------------------- | ------ |
| Stream Type | Code | Specification | Sender |
| ---------------------------- | :----: | ------------------------- | ------ |
| QPACK Encoder Stream | 0x02 | {{enc-dec-stream-def}} | Both |
| QPACK Decoder Stream | 0x03 | {{enc-dec-stream-def}} | Both |
| ---------------------------- | ------ | ------------------------- | ------ |
## Error Code Registration
This document specifies three error codes. The entries in the following table
are registered in the "HTTP/3 Error Code" registry established in {{HTTP3}}.
| --------------------------------- | ----- | ---------------------------------------- | ---------------------- |
| Name | Code | Description | Specification |
| --------------------------------- | ----- | ---------------------------------------- | ---------------------- |
| HTTP_QPACK_DECOMPRESSION_FAILED | 0x200 | Decompression of a header block failed | {{error-handling}} |
| HTTP_QPACK_ENCODER_STREAM_ERROR | 0x201 | Error on the encoder stream | {{error-handling}} |
| HTTP_QPACK_DECODER_STREAM_ERROR | 0x202 | Error on the decoder stream | {{error-handling}} |
| --------------------------------- | ----- | ---------------------------------------- | ---------------------- |
--- back
# Static Table
| 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 |
# 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
~~~
# 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-08
- Endpoints are permitted to create encoder and decoder streams even if they
can't use them (#2100, #2529)
- Maximum values for settings removed (#2766, #2767)
## Since draft-ietf-quic-qpack-06
- Clarify initial dynamic table capacity maximums (#2276, #2330, #2330)
## Since draft-ietf-quic-qpack-05
- Introduced the terms dynamic table capacity and maximum dynamic table
capacity.
- Renamed SETTINGS_HEADER_TABLE_SIZE to SETTINGS_QPACK_MAX_TABLE_CAPACITY.
## Since draft-ietf-quic-qpack-04
- Changed calculation of Delta Base Index to avoid an illegal value (#2002,
#2005)
## Since draft-ietf-quic-qpack-03
- Change HTTP settings defaults (#2038)
- Substantial editorial reorganization
## Since draft-ietf-quic-qpack-02
- Largest Reference encoded modulo MaxEntries (#1763)
- New Static Table (#1355)
- Table Size Update with Insert Count=0 is a connection error (#1762)
- Stream Cancellations are optional when SETTINGS_HEADER_TABLE_SIZE=0 (#1761)
- Implementations must handle 62 bit integers (#1760)
- Different error types for each QPACK stream, other changes to error
handling (#1726)
- Preserve header field order (#1725)
- Initial table size is the maximum permitted when table is first usable (#1642)
## 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.
