QUIC C. Krasic
Internet-Draft Netflix
Intended status: Standards Track M. Bishop
Expires: March 15, 2020 Akamai Technologies
A. Frindell, Ed.
Facebook
September 12, 2019
QPACK: Header Compression for HTTP/3
draft-ietf-quic-qpack-10
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 March 15, 2020.
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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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions and Definitions . . . . . . . . . . . . . . . 4
1.2. Notational Conventions . . . . . . . . . . . . . . . . . 4
2. Compression Process Overview . . . . . . . . . . . . . . . . 5
2.1. Encoder . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.1. Reference Tracking . . . . . . . . . . . . . . . . . 5
2.1.2. Blocked Dynamic Table Insertions . . . . . . . . . . 6
2.1.3. Blocked Streams . . . . . . . . . . . . . . . . . . . 7
2.1.4. Known Received Count . . . . . . . . . . . . . . . . 8
2.2. Decoder . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2.1. Blocked Decoding . . . . . . . . . . . . . . . . . . 8
2.2.2. State Synchronization . . . . . . . . . . . . . . . . 9
2.2.3. Invalid References . . . . . . . . . . . . . . . . . 10
3. Header Tables . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1. Static Table . . . . . . . . . . . . . . . . . . . . . . 10
3.2. Dynamic Table . . . . . . . . . . . . . . . . . . . . . . 11
3.2.1. Dynamic Table Size . . . . . . . . . . . . . . . . . 11
3.2.2. Dynamic Table Capacity and Eviction . . . . . . . . . 11
3.2.3. Maximum Dynamic Table Capacity . . . . . . . . . . . 12
3.2.4. Absolute Indexing . . . . . . . . . . . . . . . . . . 12
3.2.5. Relative Indexing . . . . . . . . . . . . . . . . . . 12
3.2.6. Post-Base Indexing . . . . . . . . . . . . . . . . . 14
4. Wire Format . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.1. Primitives . . . . . . . . . . . . . . . . . . . . . . . 14
4.1.1. Prefixed Integers . . . . . . . . . . . . . . . . . . 14
4.1.2. String Literals . . . . . . . . . . . . . . . . . . . 14
4.2. Encoder and Decoder Streams . . . . . . . . . . . . . . . 15
4.3. Encoder Instructions . . . . . . . . . . . . . . . . . . 15
4.3.1. Set Dynamic Table Capacity . . . . . . . . . . . . . 16
4.3.2. Insert With Name Reference . . . . . . . . . . . . . 16
4.3.3. Insert Without Name Reference . . . . . . . . . . . . 17
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4.3.4. Duplicate . . . . . . . . . . . . . . . . . . . . . . 17
4.4. Decoder Instructions . . . . . . . . . . . . . . . . . . 18
4.4.1. Header Acknowledgement . . . . . . . . . . . . . . . 18
4.4.2. Stream Cancellation . . . . . . . . . . . . . . . . . 18
4.4.3. Insert Count Increment . . . . . . . . . . . . . . . 19
4.5. Header Block Representations . . . . . . . . . . . . . . 19
4.5.1. Header Block Prefix . . . . . . . . . . . . . . . . . 19
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 . . . . . . . . . . . . . . . . . . . 25
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-09 . . . . . . . . . . . . . 35
C.2. Since draft-ietf-quic-qpack-08 . . . . . . . . . . . . . 35
C.3. Since draft-ietf-quic-qpack-06 . . . . . . . . . . . . . 35
C.4. Since draft-ietf-quic-qpack-05 . . . . . . . . . . . . . 35
C.5. Since draft-ietf-quic-qpack-04 . . . . . . . . . . . . . 35
C.6. Since draft-ietf-quic-qpack-03 . . . . . . . . . . . . . 35
C.7. Since draft-ietf-quic-qpack-02 . . . . . . . . . . . . . 36
C.8. Since draft-ietf-quic-qpack-01 . . . . . . . . . . . . . 36
C.9. Since draft-ietf-quic-qpack-00 . . . . . . . . . . . . . 36
C.10. Since draft-ietf-quic-qcram-00 . . . . . . . . . . . . . 36
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37
1. Introduction
The QUIC transport protocol [QUIC-TRANSPORT] is designed to support
HTTP semantics, and its design subsumes many of the features of
HTTP/2 [RFC7540]. HTTP/2 uses HPACK ([RFC7541]) for 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.
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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 and post-base indices.
References to dynamic table entries in header blocks are relative
to a Base.
Insert Count: The total number of entries inserted in the dynamic
table.
QPACK is a name, not an acronym.
1.2. Notational Conventions
Diagrams use the format described in Section 3.1 of [RFC2360], with
the following additional conventions:
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x (A) Indicates that x is A bits long
x (A+) Indicates that x uses the prefixed integer encoding defined
in Section 4.1.1, beginning with an A-bit prefix.
x ... Indicates that x is variable-length and extends to the end of
the region.
2. Compression Process Overview
Like HPACK, QPACK uses two tables for associating 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 defines unidirectional streams for sending instructions from
encoder to decoder and vice versa.
2.1. 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 Section 4.5). Indexed representations achieve high
compression by replacing the literal name and possibly the value with
an index to either the static or dynamic table. References to the
static table and literal representations do not require any dynamic
state and never risk head-of-line blocking. References to the
dynamic table risk head-of-line blocking if the encoder has not
received an acknowledgement indicating the entry is available at the
decoder.
An encoder MAY insert any entry in the dynamic table it chooses; it
is not limited to 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.
2.1.1. 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
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about each compressed header block that references the dynamic table
until that header block is acknowledged by the decoder (see
Section 4.4.1).
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. 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.
2.1.2.1. 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 Section 4.3.4), and reference the duplicate instead.
Determining which entries are too close to eviction to reference is
an encoder preference. One heuristic is to target a fixed amount of
available space in the dynamic table: either unused space or space
that can be reclaimed by evicting non-blocking entries. To achieve
this, the encoder can maintain a draining index, which is the
smallest absolute index (see Section 3.2.4) in the dynamic table that
it will emit a reference for. As new entries are inserted, the
encoder increases the draining index to maintain the section of the
table that it will not reference. If the encoder does not create new
references to entries with an absolute index lower than the draining
index, the number of unacknowledged references to those entries will
eventually become zero, allowing them to be evicted.
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+----------+---------------------------------+--------+
| Draining | Referenceable | Unused |
| Entries | Entries | Space |
+----------+---------------------------------+--------+
^ ^ ^
| | |
Dropping Draining Index Insertion Point
Point
Figure 1: Draining Dynamic Table Entries
2.1.3. 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
Section 4.5.1), 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 decoder specifies an upper bound on the number of streams which
can be blocked using the SETTINGS_QPACK_BLOCKED_STREAMS setting (see
Section 5). An encoder MUST limit the number of streams which could
become blocked to the value of SETTINGS_QPACK_BLOCKED_STREAMS at all
times. If an decoder encounters more blocked streams than it
promised to support, it MUST treat this as a connection error of type
HTTP_QPACK_DECOMPRESSION_FAILED.
Note that the decoder might not become blocked on every stream which
risks becoming blocked.
An encoder can decide whether to risk having a stream become blocked.
If permitted by the value of SETTINGS_QPACK_BLOCKED_STREAMS,
compression efficiency can often be improved by referencing dynamic
table entries that are still in transit, but if there is loss or
reordering the stream can become blocked at the decoder. An encoder
avoids the risk of blocking by only referencing dynamic table entries
which have been acknowledged, but this could mean using literals.
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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
Acknowledgement instructions (Section 4.4.1) 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 Section 4.4.3).
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.
2.2.1. 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.
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When processing header blocks, the decoder expects the Required
Insert Count to exactly match the value defined in Section 2.1.3. If
it encounters a smaller value than expected, it MUST treat this as a
connection error of type HTTP_QPACK_DECOMPRESSION_FAILED (see
Section 2.2.3). If it encounters a larger value than expected, it
MAY treat this as a connection error of type
HTTP_QPACK_DECOMPRESSION_FAILED.
2.2.2. State Synchronization
The decoder signals the following events by emitting decoder
instructions (Section 4.4) on the decoder stream.
2.2.2.1. Completed Processing of a Header Block
After the decoder finishes decoding a header block containing dynamic
table references, it MUST emit a Header Acknowledgement instruction
(Section 4.4.1). 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.
2.2.2.2. 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 Section 4.4.2). This signals to the encoder that
all references to the dynamic table on that stream are no longer
outstanding. A decoder with a maximum dynamic table capacity 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.
The Header Acknowledgement and Stream Cancellation instructions
permit the encoder to remove references to entries in the dynamic
table. When an entry with absolute index lower than the Known
Received Count has zero references, then it is no longer considered
blocking (see Section 2.1.2).
2.2.2.3. New Table Entries
After receiving new table entries on the encoder stream, the decoder
chooses when to emit Insert Count Increment instructions (see
Section 4.4.3). Emitting this instruction after adding each new
dynamic table entry will provide the timeliest feedback to the
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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 4.4.1). However, delaying too long may lead to compression
inefficiencies if the encoder waits for an entry to be acknowledged
before using it.
2.2.3. Invalid References
If the decoder encounters a reference in a 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 Section 4.5.1), 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".
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.
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".
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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.
Dynamic table entries can have empty values.
3.2.1. Dynamic Table Size
The size of the dynamic table is the sum of the size of its entries.
The size of an entry is the sum of its name's length in bytes (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 capacity of the dynamic
table is zero. The encoder sends a Set Dynamic Table Capacity
instruction (Section 4.3.1) with a non-zero capacity to begin using
the dynamic table.
Before a new entry is added to the dynamic table, entries are evicted
from the end of the dynamic table until the size of the dynamic table
is less than or equal to (table capacity - size of new entry). The
encoder MUST NOT evict a blocking dynamic table entry (see
Section 2.1.2). 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
Section 4.3.1), entries are evicted from the end of the dynamic table
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until the size of the dynamic table is less than or equal to the new
table capacity. This mechanism can be used to completely clear
entries from the dynamic table by setting a capacity of 0, which can
subsequently be restored.
3.2.3. Maximum Dynamic Table Capacity
To bound the memory requirements of the decoder, the decoder limits
the maximum value the encoder is permitted to set for the dynamic
table capacity. In HTTP/3, this limit is determined by the value of
SETTINGS_QPACK_MAX_TABLE_CAPACITY sent by the decoder (see
Section 5). The encoder MUST not set a dynamic table capacity that
exceeds this maximum, but it can choose to use a lower dynamic table
capacity (see Section 4.3.1).
For clients using 0-RTT data in HTTP/3, the server's maximum table
capacity is the remembered value of the setting, or zero if the value
was not previously sent. When the client's 0-RTT value of the
SETTING is 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.
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
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.
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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 - Encoder Stream
Unlike encoder instructions, relative indices in header block
representations 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 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.
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
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3.2.6. Post-Base Indexing
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.
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
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.
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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. 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 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_STREAM_CREATION_ERROR.
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.
4.3. Encoder Instructions
An encoder sends encoder instructions on the encoder stream to set
the capacity of the dynamic table and add dynamic table entries.
Instructions adding table entries can use existing entries to avoid
transmitting redundant information. The name can be transmitted as a
reference to an existing entry in the static or the dynamic table or
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as a string literal. For entries which already exist in the dynamic
table, the full entry can also be used by reference, creating a
duplicate entry.
This section specifies the following encoder instructions.
4.3.1. Set Dynamic Table Capacity
An encoder informs the decoder of a change to the dynamic table
capacity using an instruction which begins with the '001' three-bit
pattern. This is followed by the new dynamic table capacity
represented as an integer with a 5-bit prefix (see Section 4.1.1).
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 1 | Capacity (5+) |
+---+---+---+-------------------+
Figure 2: 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.
4.3.2. 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 4.1.1) 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 4.1.2).
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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
4.3.3. 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 using an instruction that starts with the '01' two-bit
pattern.
This is followed by the name represented as a 6-bit prefix string
literal, and the value represented as an 8-bit prefix string literal
(see Section 4.1.2).
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 1 | H | Name Length (5+) |
+---+---+---+-------------------+
| Name String (Length bytes) |
+---+---------------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length bytes) |
+-------------------------------+
Insert Header Field -- New Name
4.3.4. Duplicate
An encoder duplicates an existing entry in the dynamic table using an
instruction that begins with the '000' three-bit pattern. This is
followed by the relative index of the existing entry represented as
an integer with a 5-bit prefix (see Section 4.1.1.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 0 | Index (5+) |
+---+---+---+-------------------+
Figure 3: Duplicate
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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.
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. 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 which is followed
by the header block's associated stream ID encoded as a 7-bit prefix
integer (see Section 4.1.1).
This instruction is used as described in Section 2.1.4 and in
Section 2.2.2.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | Stream ID (7+) |
+---+---------------------------+
Figure 4: 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".
4.4.2. Stream Cancellation
When a stream is reset or reading is abandoned, the decoder emits a
Stream Cancellation instruction. The instruction begins with the
'01' two-bit pattern, which is followed by the stream ID of the
affected stream encoded as a 6-bit prefix integer.
This instruction is used as described in Section 2.2.2.
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0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 1 | Stream ID (6+) |
+---+---+-----------------------+
Figure 5: Stream Cancellation
4.4.3. Insert Count Increment
The Insert Count Increment instruction begins with the '00' two-bit
pattern, followed by the Increment encoded as a 6-bit prefix integer.
The value of the Increment is the total number of dynamic table
insertions and duplications processed by the decoder since the last
time it sent a Header Acknowledgement instruction that increased the
Known Received Count (see Section 2.1.4) or an Insert Count Increment
instruction. The encoder uses this value to update the Known
Received Count, as described in Section 2.2.2.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | Increment (6+) |
+---+---+-----------------------+
Figure 6: 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".
4.5. Header Block Representations
A header block consists of a prefix and a possibly empty sequence of
representations defined in this section. Each representation
corresponds to a single header field. These representations
reference the static table or the dynamic table in a particular
state, but do not modify that state.
Header blocks are carried in frames on streams defined by the
enclosing protocol.
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).
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These two values are followed by representations for compressed
headers.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Required Insert Count (8+) |
+---+---------------------------+
| S | Delta Base (7+) |
+---+---------------------------+
| Compressed Headers ...
+-------------------------------+
Figure 7: Header Block
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.
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".
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TotalNumberOfInserts is the total number of inserts into the
decoder's dynamic table.
FullRange = 2 * MaxEntries
if EncodedInsertCount == 0:
ReqInsertCount = 0
else:
if EncodedInsertCount > FullRange:
Error
MaxValue = TotalNumberOfInserts + MaxEntries
# MaxWrapped is the largest possible value of
# ReqInsertCount that is 0 mod 2*MaxEntries
MaxWrapped = floor(MaxValue / FullRange) * FullRange
ReqInsertCount = MaxWrapped + EncodedInsertCount - 1
# If ReqInsertCount exceeds MaxValue, the Encoder's value
# must have wrapped one fewer time
if ReqInsertCount > MaxValue:
if ReqInsertCount <= FullRange:
Error
ReqInsertCount -= FullRange
# Value of 0 must be encoded as 0.
if ReqInsertCount == 0:
Error
For example, if the dynamic table is 100 bytes, then the Required
Insert Count will be encoded modulo 6. If a decoder has received 10
inserts, then an encoded value of 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:
if S == 0:
Base = ReqInsertCount + DeltaBase
else:
Base = ReqInsertCount - DeltaBase - 1
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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
relative index of 1 refers to the 5th entry that was added to the
table; a post-base index of 1 refers to the 8th entry.
4.5.2. Indexed 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 or dynamic table. The 6-bit prefix
integer (see Section 4.1.1) that follows is used to locate the table
entry for the header field. 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.
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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 4.1.1).
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.
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 for more details).
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
The fourth ("S") bit indicates whether the reference is to the static
or dynamic table. The 4-bit prefix integer (see Section 4.1.1) that
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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.
Only the header field name is taken from the dynamic table entry; the
header field value is encoded as an 8-bit prefix string literal (see
Section 4.1.2).
4.5.5. Literal Header Field With Post-Base Name Reference
A literal header field with post-base name reference represents a
header field where the name matches the header field name of a
dynamic table entry with an absolute index greater than or equal to
the Base.
This representation starts with the '0000' four-bit pattern. The
fifth bit is the 'N' bit as described in Section 4.5.4. This is
followed by a post-base index of the dynamic table entry (see
Section 3.2.6) encoded as an integer with a 3-bit prefix (see
Section 4.1.1).
Only the header field name is taken from the dynamic table entry; the
header field value is encoded as an 8-bit prefix string literal (see
Section 4.1.2).
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
The literal header field without name reference representation
encodes a header field name and header field value as string
literals.
This representation begins with the '001' three-bit pattern. The
fourth bit is the 'N' bit as described in Section 4.5.4. The name
follows, represented as a 4-bit prefix string literal, then the
value, represented as an 8-bit prefix string literal (see
Section 4.1.2).
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0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 1 | N | H |NameLen(3+)|
+---+---+---+---+---+-----------+
| Name String (Length bytes) |
+---+---------------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length bytes) |
+-------------------------------+
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): The default value is zero.
See Section 3.2 for usage. This is the equivalent of the
SETTINGS_HEADER_TABLE_SIZE from HTTP/2.
SETTINGS_QPACK_BLOCKED_STREAMS (0x7): The default value is zero.
See Section 2.1.3.
6. 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.
7. Security Considerations
TBD.
While the negotiated limit on the dynamic table size accounts for
much of the memory that can be consumed by a QPACK implementation,
data which cannot be immediately sent due to flow control is not
affected by this limit. Implementations should limit the size of
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unsent data, especially on the decoder stream where flexibility to
choose what to send is limited. Possible responses to an excess of
unsent data might include limiting the ability of the peer to open
new streams, reading only from the encoder stream, or closing the
connection.
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 | Default |
+--------------------------+------+---------------+---------+
| QPACK_MAX_TABLE_CAPACITY | 0x1 | Section 5 | 0 |
| | | | |
| QPACK_BLOCKED_STREAMS | 0x7 | Section 5 | 0 |
+--------------------------+------+---------------+---------+
8.2. Stream Type Registration
This document specifies two stream types. The entries in the
following table are registered in the "HTTP/3 Stream Type" registry
established in [HTTP3].
+----------------------+------+---------------+--------+
| Stream Type | Code | Specification | Sender |
+----------------------+------+---------------+--------+
| QPACK Encoder Stream | 0x02 | Section 4.2 | Both |
| | | | |
| QPACK Decoder Stream | 0x03 | Section 4.2 | Both |
+----------------------+------+---------------+--------+
8.3. Error Code Registration
This document specifies three error codes. The entries in the
following table are registered in the "HTTP/3 Error Code" registry
established in [HTTP3].
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+-----------------------------+-------+--------------+--------------+
| Name | Code | Description | Specificatio |
| | | | n |
+-----------------------------+-------+--------------+--------------+
| HTTP_QPACK_DECOMPRESSION_FA | 0x200 | Decompressio | Section 6 |
| ILED | | n of a | |
| | | header block | |
| | | failed | |
| | | | |
| HTTP_QPACK_ENCODER_STREAM_E | 0x201 | Error on the | Section 6 |
| RROR | | encoder | |
| | | stream | |
| | | | |
| HTTP_QPACK_DECODER_STREAM_E | 0x202 | Error on the | Section 6 |
| RROR | | decoder | |
| | | stream | |
+-----------------------------+-------+--------------+--------------+
9. References
9.1. Normative References
[HTTP3] Bishop, M., Ed., "Hypertext Transfer Protocol Version 3
(HTTP/3)", draft-ietf-quic-http-23 (work in progress),
September 2019.
[QUIC-TRANSPORT]
Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", draft-ietf-quic-
transport-23 (work in progress), September 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>.
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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 | |
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| | | |
| 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 | |
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| | | |
| 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 |
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| | | |
| 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 |
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| | 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 |
+------+-----------------------------+------------------------------+
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Appendix B. Sample One Pass Encoding Algorithm
Pseudo-code for single pass encoding, excluding handling of
duplicates, non-blocking mode, and reference tracking.
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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
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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-09
o Decoders MUST emit Header Acknowledgements (#2939)
o Updated error code for multiple encoder or decoder streams (#2970)
o Added explicit defaults for new SETTINGS (#2974)
C.2. Since draft-ietf-quic-qpack-08
o Endpoints are permitted to create encoder and decoder streams even
if they can't use them (#2100, #2529)
o Maximum values for settings removed (#2766, #2767)
C.3. Since draft-ietf-quic-qpack-06
o Clarify initial dynamic table capacity maximums (#2276, #2330,
#2330)
C.4. 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.5. Since draft-ietf-quic-qpack-04
o Changed calculation of Delta Base Index to avoid an illegal value
(#2002, #2005)
C.6. Since draft-ietf-quic-qpack-03
o Change HTTP settings defaults (#2038)
o Substantial editorial reorganization
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C.7. 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)
o Initial table size is the maximum permitted when table is first
usable (#1642)
C.8. Since draft-ietf-quic-qpack-01
o Only header blocks that reference the dynamic table are
acknowledged (#1603, #1605)
C.9. 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.10. Since draft-ietf-quic-qcram-00
o Separate instruction sets for table updates and header blocks
(#1235, #1142, #1141)
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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
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
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Alan Frindell (editor)
Facebook
Email: afrind@fb.com
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