QPACK: Header Compression for HTTP over QUIC
draft-ietf-quic-qpack-07
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 9204.
|
|
|---|---|---|---|
| Authors | Charles 'Buck' Krasic , Mike Bishop , Alan Frindell | ||
| Last updated | 2019-03-11 (Latest revision 2019-01-23) | ||
| Replaces | draft-bishop-quic-http-and-qpack, draft-ietf-quic-qcram | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 9204 (Proposed Standard) | |
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-quic-qpack-07
QUIC C. Krasic
Internet-Draft Netflix
Intended status: Standards Track M. Bishop
Expires: September 12, 2019 Akamai Technologies
A. Frindell, Ed.
Facebook
March 11, 2019
QPACK: Header Compression for HTTP over QUIC
draft-ietf-quic-qpack-07
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 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 [1].
Working Group information can be found at https://github.com/quicwg
[2]; source code and issues list for this draft can be found at
https://github.com/quicwg/base-drafts/labels/-qpack [3].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 12, 2019.
Krasic, et al. Expires September 12, 2019 [Page 1]
Internet-Draft QPACK March 2019
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions and Definitions . . . . . . . . . . . . . . . 4
1.2. Notational Conventions . . . . . . . . . . . . . . . . . 5
2. Compression Process Overview . . . . . . . . . . . . . . . . 5
2.1. Encoder . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.1. Reference Tracking . . . . . . . . . . . . . . . . . 6
2.1.2. Blocked Dynamic Table Insertions . . . . . . . . . . 6
2.1.3. Avoiding Head-of-Line Blocking . . . . . . . . . . . 7
2.1.4. Known Received Count . . . . . . . . . . . . . . . . 8
2.2. Decoder . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2.1. State Synchronization . . . . . . . . . . . . . . . . 9
2.2.2. Blocked Decoding . . . . . . . . . . . . . . . . . . 9
3. Header Tables . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1. Static Table . . . . . . . . . . . . . . . . . . . . . . 9
3.2. Dynamic Table . . . . . . . . . . . . . . . . . . . . . . 10
3.2.1. Dynamic Table Size . . . . . . . . . . . . . . . . . 10
3.2.2. Dynamic Table Capacity and Eviction . . . . . . . . . 10
3.2.3. Maximum Dynamic Table Capacity . . . . . . . . . . . 11
3.2.4. Absolute Indexing . . . . . . . . . . . . . . . . . . 12
3.2.5. Relative Indexing . . . . . . . . . . . . . . . . . . 12
3.2.6. Post-Base Indexing . . . . . . . . . . . . . . . . . 13
3.2.7. Invalid References . . . . . . . . . . . . . . . . . 13
4. Wire Format . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.1. Primitives . . . . . . . . . . . . . . . . . . . . . . . 14
4.1.1. Prefixed Integers . . . . . . . . . . . . . . . . . . 14
4.1.2. String Literals . . . . . . . . . . . . . . . . . . . 14
4.2. Instructions . . . . . . . . . . . . . . . . . . . . . . 14
4.2.1. Encoder and Decoder Streams . . . . . . . . . . . . . 15
4.3. Encoder Instructions . . . . . . . . . . . . . . . . . . 15
4.3.1. Insert With Name Reference . . . . . . . . . . . . . 15
4.3.2. Insert Without Name Reference . . . . . . . . . . . . 16
Krasic, et al. Expires September 12, 2019 [Page 2]
Internet-Draft QPACK March 2019
4.3.3. Duplicate . . . . . . . . . . . . . . . . . . . . . . 16
4.3.4. Set Dynamic Table Capacity . . . . . . . . . . . . . 17
4.4. Decoder Instructions . . . . . . . . . . . . . . . . . . 17
4.4.1. Insert Count Increment . . . . . . . . . . . . . . . 17
4.4.2. Header Acknowledgement . . . . . . . . . . . . . . . 18
4.4.3. Stream Cancellation . . . . . . . . . . . . . . . . . 19
4.5. Header Block Instructions . . . . . . . . . . . . . . . . 19
4.5.1. Header Block Prefix . . . . . . . . . . . . . . . . . 20
4.5.2. Indexed Header Field . . . . . . . . . . . . . . . . 22
4.5.3. Indexed Header Field With Post-Base Index . . . . . . 23
4.5.4. Literal Header Field With Name Reference . . . . . . 23
4.5.5. Literal Header Field With Post-Base Name Reference . 24
4.5.6. Literal Header Field Without Name Reference . . . . . 24
5. Configuration . . . . . . . . . . . . . . . . . . . . . . . . 25
6. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 25
7. Security Considerations . . . . . . . . . . . . . . . . . . . 26
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
8.1. Settings Registration . . . . . . . . . . . . . . . . . . 26
8.2. Stream Type Registration . . . . . . . . . . . . . . . . 26
8.3. Error Code Registration . . . . . . . . . . . . . . . . . 26
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 27
9.1. Normative References . . . . . . . . . . . . . . . . . . 27
9.2. Informative References . . . . . . . . . . . . . . . . . 28
9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Appendix A. Static Table . . . . . . . . . . . . . . . . . . . . 28
Appendix B. Sample One Pass Encoding Algorithm . . . . . . . . . 33
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 35
C.1. Since draft-ietf-quic-qpack-06 . . . . . . . . . . . . . 35
C.2. Since draft-ietf-quic-qpack-05 . . . . . . . . . . . . . 35
C.3. Since draft-ietf-quic-qpack-04 . . . . . . . . . . . . . 35
C.4. Since draft-ietf-quic-qpack-03 . . . . . . . . . . . . . 35
C.5. Since draft-ietf-quic-qpack-02 . . . . . . . . . . . . . 35
C.6. Since draft-ietf-quic-qpack-01 . . . . . . . . . . . . . 36
C.7. Since draft-ietf-quic-qpack-00 . . . . . . . . . . . . . 36
C.8. Since draft-ietf-quic-qcram-00 . . . . . . . . . . . . . 36
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37
1. 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/3, it would induce head-of-line blocking due to built-
in assumptions of a total ordering across frames on all streams.
Krasic, et al. Expires September 12, 2019 [Page 3]
Internet-Draft QPACK March 2019
QUIC is described in [QUIC-TRANSPORT]. The HTTP/3 mapping is
described in [HTTP3]. 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.
1.1. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Definitions of terms that are used in this document:
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 indicies. 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.
Krasic, et al. Expires September 12, 2019 [Page 4]
Internet-Draft QPACK March 2019
1.2. Notational Conventions
Diagrams use the format described in Section 3.1 of [RFC2360], with
the following additional conventions:
x (A) Indicates that x is A bits long
x (A+) Indicates that x uses the prefixed integer encoding defined
in Section 5.1 of [RFC7541], beginning with an A-bit prefix.
x ... Indicates that x is variable-length and extends to the end of
the region.
2. Compression Process Overview
Like HPACK, QPACK uses two tables for associating header fields to
indices. The static table (see Section 3.1) is predefined and
contains common header fields (some of them with an empty value).
The dynamic table (see Section 3.2) is built up over the course of
the connection and can be used by the encoder to index header fields
in the encoded header lists.
QPACK instructions appear in three different types of streams:
o The encoder uses a unidirectional stream to modify the state of
the dynamic table without emitting header fields associated with
any particular request.
o HEADERS and PUSH_PROMISE frames on request and push streams
reference the table state without modifying it.
o The decoder sends feedback to the encoder on a unidirectional
stream. This feedback enables the encoder to manage dynamic table
state.
2.1. Encoder
An encoder compresses a header list by emitting either an indexed or
a literal representation for each header field in the list.
References to the static table and literal representations do not
require any dynamic state and never risk head-of-line blocking.
References to the dynamic table risk head-of-line blocking if the
encoder has not received an acknowledgement indicating the entry is
available at the decoder.
An encoder MAY insert any entry in the dynamic table it chooses; it
is not limited to header fields it is compressing.
Krasic, et al. Expires September 12, 2019 [Page 5]
Internet-Draft QPACK March 2019
QPACK preserves the ordering of header fields within each header
list. An encoder MUST emit header field representations in the order
they appear in the input header list.
QPACK is designed to contain the more complex state tracking to the
encoder, while the decoder is relatively simple.
2.1.1. Reference Tracking
An encoder MUST ensure that a header block which references a dynamic
table entry is not received by the decoder after the referenced entry
has been evicted. Hence the encoder needs to track information about
each compressed header block that references the dynamic table until
that header block is acknowledged by the decoder.
2.1.2. Blocked Dynamic Table Insertions
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. Any encoder that uses the
dynamic table has to keep track of blocked entries, whereas
blocked streams are optional.
An encoder MUST NOT insert an entry into the dynamic table (or
duplicate an existing entry) if doing so would evict a blocking
entry. In this case, the encoder can send literal representations of
header fields.
To ensure that the encoder is not prevented from adding new entries,
the encoder can avoid referencing entries that are close to eviction.
Rather than reference such an entry, the encoder can emit a Duplicate
instruction (see Section 4.3.3), and reference the duplicate instead.
Determining which entries are too close to eviction to reference is
an encoder preference. One heuristic is to target a fixed amount of
available space in the dynamic table: either unused space or space
that can be reclaimed by evicting non-blocking entries. To achieve
this, the encoder can maintain a draining index, which is the
smallest absolute index in the dynamic table that it will emit a
reference for. As new entries are inserted, the encoder increases
the draining index to maintain the section of the table that it will
not reference. If the encoder does not create new references to
Krasic, et al. Expires September 12, 2019 [Page 6]
Internet-Draft QPACK March 2019
entries with an absolute index lower than the draining index, the
number of unacknowledged references to those entries will eventually
become zero, allowing them to be evicted.
+----------+---------------------------------+--------+
| Draining | Referenceable | Unused |
| Entries | Entries | Space |
+----------+---------------------------------+--------+
^ ^ ^
| | |
Dropping Draining Index Insertion Point
Point
Figure 1: Draining Dynamic Table Entries
2.1.3. Avoiding Head-of-Line Blocking
Because QUIC does not guarantee order between data on different
streams, a header block might reference an entry in the dynamic table
that has not yet been received.
Each header block contains a Required Insert Count, 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.
If the decoder encounters a header block with a Required Insert Count
value larger than defined above, it MAY treat this as a stream error
of type HTTP_QPACK_DECOMPRESSION_FAILED. If the decoder encounters a
header block with a Required Insert Count value smaller than defined
above, it MUST treat this as a stream error of type
HTTP_QPACK_DECOMPRESSION_FAILED as prescribed in Section 3.2.7.
When the Required Insert Count is zero, the frame contains no
references to the dynamic table and can always be processed
immediately.
If the Required Insert Count is greater than the number of dynamic
table entries received, the stream is considered "blocked." While
blocked, header field 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.
Krasic, et al. Expires September 12, 2019 [Page 7]
Internet-Draft QPACK March 2019
The SETTINGS_QPACK_BLOCKED_STREAMS setting (see Section 5) specifies
an upper bound on the number of streams which can be blocked. An
encoder MUST limit the number of streams which could become blocked
to the value of SETTINGS_QPACK_BLOCKED_STREAMS at all times. Note
that the decoder might not actually become blocked on every stream
which risks becoming blocked. If the decoder encounters more blocked
streams than it promised to support, it MUST treat this as a stream
error of type HTTP_QPACK_DECOMPRESSION_FAILED.
An encoder can decide whether to risk having a stream become blocked.
If permitted by the value of SETTINGS_QPACK_BLOCKED_STREAMS,
compression efficiency can often be improved by referencing dynamic
table entries that are still in transit, but if there is loss or
reordering the stream can become blocked at the decoder. An encoder
avoids the risk of blocking by only referencing dynamic table entries
which have been acknowledged, but this could mean using literals.
Since literals make the header block larger, this can result in the
encoder becoming blocked on congestion or flow control limits.
2.1.4. 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 block
acknowledgement to maintain the Known Received Count, as described in
Section 4.4.2.
To acknowledge dynamic table entries which are not referenced by
header blocks, for example because the encoder or the decoder have
chosen not to risk blocked streams, the decoder sends an Insert Count
Increment instruction (see Section 4.4.1).
2.2. Decoder
As in HPACK, the decoder processes header blocks and emits the
corresponding header lists. It also processes dynamic table
modifications from encoder instructions received on the encoder
stream.
The decoder MUST emit header fields in the order their
representations appear in the input header block.
Krasic, et al. Expires September 12, 2019 [Page 8]
Internet-Draft QPACK March 2019
2.2.1. State Synchronization
The decoder instructions (Section 4.4) signal key events at the
decoder that permit the encoder to track the decoder's state. These
events are:
o Complete processing of a header block
o Abandonment of a stream which might have remaining header blocks
o Receipt of new dynamic table entries
Knowledge that a header block with references to the dynamic table
has been processed permits the encoder to evict entries to which no
unacknowledged references remain (see Section 2.1.2). When a stream
is reset or abandoned, the indication that these header blocks will
never be processed serves a similar function (see Section 4.4.3).
The decoder chooses when to emit Insert Count Increment instructions
(see Section 4.4.1). Emitting an instruction after adding each new
dynamic table entry will provide the most timely feedback to the
encoder, but could be redundant with other decoder feedback. By
delaying 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
Section 4.4.2). However, delaying too long may lead to compression
inefficiencies if the encoder waits for an entry to be acknowledged
before using it.
2.2.2. Blocked Decoding
To track blocked streams, the Required Insert Count value for each
stream can be used. Whenever the decoder processes a table update,
it can begin decoding any blocked streams that now have their
dependencies satisfied.
3. Header Tables
Unlike in HPACK, entries in the QPACK static and dynamic tables are
addressed separately. The following sections describe how entries in
each table are addressed.
3.1. Static Table
The static table consists of a predefined static list of header
fields, each of which has a fixed index over time. Its entries are
defined in Appendix A.
Krasic, et al. Expires September 12, 2019 [Page 9]
Internet-Draft QPACK March 2019
All entries in the static table have a name and a value. However,
values can be empty (that is, have a length of 0).
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 instruction it MUST treat this as a stream error of type
"HTTP_QPACK_DECOMPRESSION_FAILED". If this index is received on the
encoder stream, this MUST be treated as a connection error of type
"HTTP_QPACK_ENCODER_STREAM_ERROR".
3.2. Dynamic Table
The dynamic table consists of a list of header fields maintained in
first-in, first-out order. Each HTTP/3 endpoint holds a dynamic
table that is initially empty. Entries are added by encoder
instructions received on the encoder stream (see Section 4.3).
The dynamic table can contain duplicate entries (i.e., entries with
the same name and same value). Therefore, duplicate entries MUST NOT
be treated as an error by the decoder.
3.2.1. Dynamic Table Size
The size of the dynamic table is the sum of the size of its entries.
The size of an entry is the sum of its name's length in bytes (as
defined in Section 4.1.2), its value's length in bytes, and 32.
The size of an entry is calculated using the length of its name and
value without Huffman encoding applied.
3.2.2. Dynamic Table Capacity and Eviction
The encoder sets the capacity of the dynamic table, which serves as
the upper limit on its size. The initial capcity of the dynamic
table is zero.
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) or
until the table is empty. The encoder MUST NOT evict a blocking
dynamic table entry (see Section 2.1.2).
If the size of the new entry is less than or equal to the dynamic
table capacity, then that entry is added to the table. It is an
error if the encoder attempts to add an entry that is larger than the
Krasic, et al. Expires September 12, 2019 [Page 10]
Internet-Draft QPACK March 2019
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,
entries are evicted from the end of the dynamic table until the size
of the dynamic table is less than or equal to the new table capacity.
This mechanism can be used to completely clear entries from the
dynamic table by setting a capacity of 0, which can subsequently be
restored.
3.2.3. Maximum Dynamic Table Capacity
To bound the memory requirements of the decoder, the decoder limits
the maximum value the encoder is permitted to set for the dynamic
table capacity. In HTTP/3, this limit is determined by the value of
SETTINGS_QPACK_MAX_TABLE_CAPACITY sent by the decoder (see
Section 5). The encoder MUST not set a dynamic table capacity that
exceeds this maximum, but it can choose to use a lower dynamic table
capacity (see Section 4.3.4).
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.
Krasic, et al. Expires September 12, 2019 [Page 11]
Internet-Draft QPACK March 2019
3.2.4. Absolute 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.
3.2.5. Relative Indexing
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.
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.
+-----+---------------+-------+
| 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
Example Dynamic Table Indexing - Control Stream
Unlike encoder instructions, relative indices in header block
instructions are relative to the Base at the beginning of the header
block (see Section 4.5.1). This ensures that references are stable
even if the dynamic table is updated while decoding a header block.
The Base is encoded as a value relative to the Required Insert Count.
The Base identifies which dynamic table entries can be referenced
using relative indexing, starting with 0 at the last entry added.
Post-Base references are used for entries inserted after base,
starting at 0 for the first entry added after the Base, see
Section 3.2.6.
Krasic, et al. Expires September 12, 2019 [Page 12]
Internet-Draft QPACK March 2019
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
Example Dynamic Table Indexing - Relative Index in Header Block
3.2.6. Post-Base Indexing
A header block can reference entries added after the entry identified
by the Base. 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, with the zero
value being the first entry inserted after the Base.
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
Example Dynamic Table Indexing - Post-Base Index in Header Block
3.2.7. Invalid References
If the decoder encounters a reference in a header block instruction
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 Section 4.5.1), it MUST treat this as a stream
error of type "HTTP_QPACK_DECOMPRESSION_FAILED".
Krasic, et al. Expires September 12, 2019 [Page 13]
Internet-Draft QPACK March 2019
If the decoder encounters a reference in an encoder instruction to a
dynamic table entry which has already been dropped, it MUST treat
this as a connection error of type "HTTP_QPACK_ENCODER_STREAM_ERROR".
4. Wire Format
4.1. Primitives
4.1.1. Prefixed Integers
The prefixed integer from Section 5.1 of [RFC7541] is used heavily
throughout this document. The format from [RFC7541] is used
unmodified. QPACK implementations MUST be able to decode integers up
to 62 bits long.
4.1.2. String Literals
The string literal defined by Section 5.2 of [RFC7541] is also used
throughout. This string format includes optional Huffman encoding.
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.
4.2. Instructions
There are three separate QPACK instruction spaces. Encoder
instructions (Section 4.3) carry table updates, decoder instructions
(Section 4.4) carry acknowledgments of table modifications and header
processing, and header block instructions (Section 4.5) convey an
encoded representation of a header list by referring to the QPACK
table state.
Encoder and decoder instructions appear on the unidirectional stream
types described in this section. Header block instructions are
Krasic, et al. Expires September 12, 2019 [Page 14]
Internet-Draft QPACK March 2019
contained in HEADERS and PUSH_PROMISE frames, which are conveyed on
request or push streams as described in [HTTP3].
4.2.1. Encoder and Decoder Streams
QPACK defines two unidirectional stream types:
o An encoder stream is a unidirectional stream of type "0x02". It
carries an unframed sequence of encoder instructions from encoder
to decoder.
o 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 a single encoder stream and decoder stream. Receipt of
a second instance of either stream type be MUST 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.
4.3. Encoder Instructions
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.
This section specifies the following encoder instructions.
4.3.1. 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]).
Krasic, et al. Expires September 12, 2019 [Page 15]
Internet-Draft QPACK March 2019
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | S | Name Index (6+) |
+---+---+-----------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length bytes) |
+-------------------------------+
Insert Header Field -- Indexed Name
4.3.2. 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
Section 4.1) 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.
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) |
+-------------------------------+
Insert Header Field -- New Name
4.3.3. 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.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | Index (5+) |
+---+---+---+-------------------+
Figure 2: Duplicate
The existing entry is re-inserted into the dynamic table without
resending either the name or the value. This is useful to mitigate
Krasic, et al. Expires September 12, 2019 [Page 16]
Internet-Draft QPACK March 2019
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.
4.3.4. Set Dynamic Table Capacity
An encoder informs the decoder of a change to the dynamic table
capacity using an instruction which begins with the '001' three-bit
pattern. The new dynamic table capacity is represented as an integer
with a 5-bit prefix (see Section 5.1 of [RFC7541]).
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 1 | Capacity (5+) |
+---+---+---+-------------------+
Figure 3: Set Dynamic Table Capacity
The new capacity MUST be lower than or equal to the limit described
in Section 3.2.3. In HTTP/3, this limit is the value of the
SETTINGS_QPACK_MAX_TABLE_CAPACITY parameter (see Section 5) received
from the decoder. The decoder MUST treat a new dynamic table
capacity value that exceeds this limit as a connection error of type
"HTTP_QPACK_ENCODER_STREAM_ERROR".
Reducing the dynamic table capacity can cause entries to be evicted
(see Section 3.2.2). This MUST NOT cause the eviction of blocking
entries (see Section 2.1.2). Changing the capacity of the dynamic
table is not acknowledged as this instruction does not insert an
entry.
4.4. 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.
4.4.1. 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
Krasic, et al. Expires September 12, 2019 [Page 17]
Internet-Draft QPACK March 2019
the dynamic table (see Section 2.1.4). 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 Section 2.2.1.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | Increment (6+) |
+---+---+-----------------------+
Figure 4: 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".
4.4.2. Header Acknowledgement
After processing a header block whose declared Required Insert Count
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 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, and possibly update the Known
Received Count.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | Stream ID (7+) |
+---+---------------------------+
Figure 5: Header Acknowledgement
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 HEADERS
and PUSH_PROMISE 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.
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".
When blocking references are permitted, the encoder uses
acknowledgement of header blocks to update the Known Received Count.
Krasic, et al. Expires September 12, 2019 [Page 18]
Internet-Draft QPACK March 2019
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.
4.4.3. Stream Cancellation
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.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 1 | Stream ID (6+) |
+---+---+-----------------------+
Figure 6: Stream Cancellation
A stream that is reset might have multiple outstanding header blocks
with dynamic table references. When an endpoint receives a stream
reset before the end of a stream, it generates a Stream Cancellation
instruction on the decoder stream. Similarly, when an endpoint
abandons reading of a stream it needs to signal this using the Stream
Cancellation instruction. This signals to the encoder that all
references to the dynamic table on that stream are no longer
outstanding. A decoder with a maximum dynamic table capacity equal
to zero (see Section 3.2.3) MAY omit sending Stream Cancellations,
because the encoder cannot have any dynamic table references.
An encoder cannot infer from this instruction that any updates to the
dynamic table have been received.
4.5. Header Block Instructions
HTTP/3 endpoints convert header lists to headers blocks and exchange
them inside HEADERS and PUSH_PROMISE frames. A decoder interprets
header block instructions in order to construct a header list. These
instructions reference the static table, or dynamic table in a
particular state without modifying it.
This section specifies the following header block instructions.
Krasic, et al. Expires September 12, 2019 [Page 19]
Internet-Draft QPACK March 2019
4.5.1. Header Block 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 Section 4.5.1.1). The Base is encoded as sign-
and-modulus integer, using a single sign bit and a value with a 7-bit
prefix (see Section 4.5.1.2).
These two values are followed by instructions for compressed headers.
The entire block is expected to be framed by the using protocol.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Required Insert Count (8+) |
+---+---------------------------+
| S | Delta Base (7+) |
+---+---------------------------+
| Compressed Headers ...
+-------------------------------+
Figure 7: Frame Payload
4.5.1.1. Required Insert Count
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 Section 3.2.3).
This encoding limits the length of the prefix on long-lived
connections.
Krasic, et al. Expires September 12, 2019 [Page 20]
Internet-Draft QPACK March 2019
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 stream 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.
4.5.1.2. Base
The "Base" is used to resolve references in the dynamic table as
described in Section 3.2.5.
To save space, the Base is encoded relative to the 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:
Krasic, et al. Expires September 12, 2019 [Page 21]
Internet-Draft QPACK March 2019
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 an 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.
4.5.2. 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].
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | S | Index (6+) |
+---+---+-----------------------+
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 (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]).
Krasic, et al. Expires September 12, 2019 [Page 22]
Internet-Draft QPACK March 2019
4.5.3. 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 Section 3.2.6) of
the matching header field, represented as an integer with a 4-bit
prefix (see Section 5.1 of [RFC7541]).
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 1 | Index (4+) |
+---+---+---+---+---------------+
Indexed Header Field with Post-Base Index
4.5.4. 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 the Base, this representation starts with
the '01' two-bit pattern. If the 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.
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).
Krasic, et al. Expires September 12, 2019 [Page 23]
Internet-Draft QPACK March 2019
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 1 | N | S |Name Index (4+)|
+---+---+---+---+---------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length bytes) |
+-------------------------------+
Literal Header Field With Name Reference
For entries in the static table or in the dynamic table with an
absolute index less than the Base, 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.
4.5.5. Literal Header Field With Post-Base Name Reference
For entries in the dynamic table with an absolute index greater than
or equal to the Base, the header field name is represented using the
post-base index of that entry (see Section 3.2.6) encoded as an
integer with a 3-bit prefix.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | 0 | N |NameIdx(3+)|
+---+---+---+---+---+-----------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length bytes) |
+-------------------------------+
Literal Header Field With Post-Base Name Reference
4.5.6. 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
Section 4.1) 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
Krasic, et al. Expires September 12, 2019 [Page 24]
Internet-Draft QPACK March 2019
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.
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) |
+-------------------------------+
Literal Header Field Without Name Reference
5. Configuration
QPACK defines two settings which are included in the HTTP/3 SETTINGS
frame.
SETTINGS_QPACK_MAX_TABLE_CAPACITY (0x1): An integer with a maximum
value of 2^30 - 1. The default value is zero bytes. See
Section 3.2 for usage. This is the equivalent of the
SETTINGS_HEADER_TABLE_SIZE from HTTP/2.
SETTINGS_QPACK_BLOCKED_STREAMS (0x7): An integer with a maximum
value of 2^16 - 1. The default value is zero. See Section 2.1.3.
6. Error Handling
The following error codes are defined for HTTP/3 to indicate failures
of QPACK which prevent the stream or connection from continuing:
HTTP_QPACK_DECOMPRESSION_FAILED (TBD): The decoder failed to
interpret a header block instruction and is not able to continue
decoding that header block.
HTTP_QPACK_ENCODER_STREAM_ERROR (TBD): The decoder failed to
interpret an encoder instruction received on the encoder stream.
HTTP_QPACK_DECODER_STREAM_ERROR (TBD): The encoder failed to
interpret a decoder instruction received on the decoder stream.
Krasic, et al. Expires September 12, 2019 [Page 25]
Internet-Draft QPACK March 2019
Upon encountering an error, an implementation MAY elect to treat it
as a connection error even if this document prescribes that it MUST
be treated as a stream error.
7. Security Considerations
TBD.
8. IANA Considerations
8.1. Settings Registration
This document specifies two settings. The entries in the following
table are registered in the "HTTP/3 Settings" registry established in
[HTTP3].
+--------------------------+------+---------------+
| Setting Name | Code | Specification |
+--------------------------+------+---------------+
| QPACK_MAX_TABLE_CAPACITY | 0x1 | Section 5 |
| | | |
| QPACK_BLOCKED_STREAMS | 0x7 | Section 5 |
+--------------------------+------+---------------+
8.2. Stream Type Registration
This document specifies two stream types. The entries in the
following table are registered in the "HTTP/3 Stream Type" registry
established in [HTTP3].
+----------------------+------+---------------+--------+
| Stream Type | Code | Specification | Sender |
+----------------------+------+---------------+--------+
| QPACK Encoder Stream | 0x02 | Section 4.2.1 | Both |
| | | | |
| QPACK Decoder Stream | 0x03 | Section 4.2.1 | Both |
+----------------------+------+---------------+--------+
8.3. Error Code Registration
This document specifies three error codes. The entries in the
following table are registered in the "HTTP/3 Error Code" registry
established in [HTTP3].
Krasic, et al. Expires September 12, 2019 [Page 26]
Internet-Draft QPACK March 2019
+------------------------------+------+--------------+--------------+
| Name | Code | Description | Specificatio |
| | | | n |
+------------------------------+------+--------------+--------------+
| HTTP_QPACK_DECOMPRESSION_FAI | TBD | Decompressio | Section 6 |
| LED | | n of a | |
| | | header block | |
| | | failed | |
| | | | |
| HTTP_QPACK_ENCODER_STREAM_ER | TBD | Error on the | Section 6 |
| ROR | | encoder | |
| | | stream | |
| | | | |
| HTTP_QPACK_DECODER_STREAM_ER | TBD | Error on the | Section 6 |
| ROR | | decoder | |
| | | stream | |
+------------------------------+------+--------------+--------------+
9. References
9.1. Normative References
[HTTP3] Bishop, M., Ed., "Hypertext Transfer Protocol Version 3
(HTTP/3)", draft-ietf-quic-http-19 (work in progress),
March 2019.
[QUIC-TRANSPORT]
Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", draft-ietf-quic-
transport-18 (work in progress), March 2019.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC7541] Peon, R. and H. Ruellan, "HPACK: Header Compression for
HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
<https://www.rfc-editor.org/info/rfc7541>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
Krasic, et al. Expires September 12, 2019 [Page 27]
Internet-Draft QPACK March 2019
9.2. Informative References
[RFC2360] Scott, G., "Guide for Internet Standards Writers", BCP 22,
RFC 2360, DOI 10.17487/RFC2360, June 1998,
<https://www.rfc-editor.org/info/rfc2360>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.
9.3. URIs
[1] https://mailarchive.ietf.org/arch/search/?email_list=quic
[2] https://github.com/quicwg
[3] https://github.com/quicwg/base-drafts/labels/-qpack
Appendix A. Static Table
+------+-----------------------------+------------------------------+
| Inde | Name | Value |
| x | | |
+------+-----------------------------+------------------------------+
| 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 | |
Krasic, et al. Expires September 12, 2019 [Page 28]
Internet-Draft QPACK March 2019
| | | |
| 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- | cache-control |
| | headers | |
| | | |
| 34 | access-control-allow- | content-type |
| | headers | |
Krasic, et al. Expires September 12, 2019 [Page 29]
Internet-Draft QPACK March 2019
| | | |
| 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 |
Krasic, et al. Expires September 12, 2019 [Page 30]
Internet-Draft QPACK March 2019
| | | |
| 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- | FALSE |
| | credentials | |
| | | |
| 74 | access-control-allow- | TRUE |
| | credentials | |
| | | |
| 75 | access-control-allow- | * |
| | headers | |
| | | |
| 76 | access-control-allow- | get |
| | methods | |
| | | |
| 77 | access-control-allow- | get, post, options |
| | methods | |
| | | |
| 78 | access-control-allow- | options |
Krasic, et al. Expires September 12, 2019 [Page 31]
Internet-Draft QPACK March 2019
| | methods | |
| | | |
| 79 | access-control-expose- | content-length |
| | headers | |
| | | |
| 80 | access-control-request- | content-type |
| | headers | |
| | | |
| 81 | access-control-request- | get |
| | method | |
| | | |
| 82 | access-control-request- | post |
| | method | |
| | | |
| 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 |
+------+-----------------------------+------------------------------+
Krasic, et al. Expires September 12, 2019 [Page 32]
Internet-Draft QPACK March 2019
Appendix B. Sample One Pass Encoding Algorithm
Pseudo-code for single pass encoding, excluding handling of
duplicates, non-blocking mode, and reference tracking.
Krasic, et al. Expires September 12, 2019 [Page 33]
Internet-Draft QPACK March 2019
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
Krasic, et al. Expires September 12, 2019 [Page 34]
Internet-Draft QPACK March 2019
Appendix C. Change Log
*RFC Editor's Note:* Please remove this section prior to
publication of a final version of this document.
C.1. Since draft-ietf-quic-qpack-06
o Clarify initial dynamic table capacity maximums (#2276, #2330,
#2330)
C.2. Since draft-ietf-quic-qpack-05
o Introduced the terms dynamic table capacity and maximum dynamic
table capacity.
o Renamed SETTINGS_HEADER_TABLE_SIZE to
SETTINGS_QPACK_MAX_TABLE_CAPACITY.
C.3. Since draft-ietf-quic-qpack-04
o Changed calculation of Delta Base Index to avoid an illegal value
(#2002, #2005)
C.4. Since draft-ietf-quic-qpack-03
o Change HTTP settings defaults (#2038)
o Substantial editorial reorganization
C.5. Since draft-ietf-quic-qpack-02
o Largest Reference encoded modulo MaxEntries (#1763)
o New Static Table (#1355)
o Table Size Update with Insert Count=0 is a connection error
(#1762)
o Stream Cancellations are optional when
SETTINGS_HEADER_TABLE_SIZE=0 (#1761)
o Implementations must handle 62 bit integers (#1760)
o Different error types for each QPACK stream, other changes to
error handling (#1726)
o Preserve header field order (#1725)
Krasic, et al. Expires September 12, 2019 [Page 35]
Internet-Draft QPACK March 2019
o Initial table size is the maximum permitted when table is first
usable (#1642)
C.6. Since draft-ietf-quic-qpack-01
o Only header blocks that reference the dynamic table are
acknowledged (#1603, #1605)
C.7. Since draft-ietf-quic-qpack-00
o Renumbered instructions for consistency (#1471, #1472)
o Decoder is allowed to validate largest reference (#1404, #1469)
o Header block acknowledgments also acknowledge the associated
largest reference (#1370, #1400)
o Added an acknowledgment for unread streams (#1371, #1400)
o Removed framing from encoder stream (#1361,#1467)
o Control streams use typed unidirectional streams rather than fixed
stream IDs (#910,#1359)
C.8. Since draft-ietf-quic-qcram-00
o Separate instruction sets for table updates and header blocks
(#1235, #1142, #1141)
o Reworked indexing scheme (#1176, #1145, #1136, #1130, #1125,
#1314)
o Added mechanisms that support one-pass encoding (#1138, #1320)
o Added a setting to control the number of blocked decoders (#238,
#1140, #1143)
o Moved table updates and acknowledgments to dedicated streams
(#1121, #1122, #1238)
Acknowledgments
This draft draws heavily on the text of [RFC7541]. The indirect
input of those authors is gratefully acknowledged, as well as ideas
from:
o Ryan Hamilton
Krasic, et al. Expires September 12, 2019 [Page 36]
Internet-Draft QPACK March 2019
o Patrick McManus
o Kazuho Oku
o Biren Roy
o Ian Swett
o 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.
Authors' Addresses
Charles 'Buck' Krasic
Netflix
Email: ckrasic@netflix.com
Mike Bishop
Akamai Technologies
Email: mbishop@evequefou.be
Alan Frindell (editor)
Facebook
Email: afrind@fb.com
Krasic, et al. Expires September 12, 2019 [Page 37]