QUIC C. Krasic
Internet-Draft Netflix
Intended status: Standards Track M. Bishop
Expires: June 7, 2019 Akamai Technologies
A. Frindell, Ed.
Facebook
December 04, 2018
QPACK: Header Compression for HTTP over QUIC
draft-ietf-quic-qpack-04
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 June 7, 2019.
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Copyright Notice
Copyright (c) 2018 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 . . . . . . . . . . . . . . . . . 4
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. Largest Known Received . . . . . . . . . . . . . . . 7
2.2. Decoder . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2.1. State Synchronization . . . . . . . . . . . . . . . . 8
2.2.2. Blocked Decoding . . . . . . . . . . . . . . . . . . 9
3. Header Tables . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1. Static Table . . . . . . . . . . . . . . . . . . . . . . 9
3.2. Dynamic Table . . . . . . . . . . . . . . . . . . . . . . 9
3.2.1. Maximum Table Size . . . . . . . . . . . . . . . . . 10
3.2.2. Calculating Table Size . . . . . . . . . . . . . . . 10
3.2.3. Absolute Indexing . . . . . . . . . . . . . . . . . . 11
3.2.4. Relative Indexing . . . . . . . . . . . . . . . . . . 11
3.2.5. Post-Base Indexing . . . . . . . . . . . . . . . . . 12
3.2.6. Invalid References . . . . . . . . . . . . . . . . . 12
4. Wire Format . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.1. Primitives . . . . . . . . . . . . . . . . . . . . . . . 13
4.1.1. Prefixed Integers . . . . . . . . . . . . . . . . . . 13
4.1.2. String Literals . . . . . . . . . . . . . . . . . . . 13
4.2. Stream Types . . . . . . . . . . . . . . . . . . . . . . 13
4.3. Encoder Stream . . . . . . . . . . . . . . . . . . . . . 14
4.3.1. Insert With Name Reference . . . . . . . . . . . . . 14
4.3.2. Insert Without Name Reference . . . . . . . . . . . . 15
4.3.3. Duplicate . . . . . . . . . . . . . . . . . . . . . . 15
4.3.4. Dynamic Table Size Update . . . . . . . . . . . . . . 16
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4.4. Decoder Stream . . . . . . . . . . . . . . . . . . . . . 16
4.4.1. Table State Synchronize . . . . . . . . . . . . . . . 16
4.4.2. Header Acknowledgement . . . . . . . . . . . . . . . 17
4.4.3. Stream Cancellation . . . . . . . . . . . . . . . . . 18
4.5. Request and Push Streams . . . . . . . . . . . . . . . . 18
4.5.1. Header Data Prefix . . . . . . . . . . . . . . . . . 18
4.5.2. Indexed Header Field . . . . . . . . . . . . . . . . 20
4.5.3. Indexed Header Field With Post-Base Index . . . . . . 21
4.5.4. Literal Header Field With Name Reference . . . . . . 21
4.5.5. Literal Header Field With Post-Base Name Reference . 22
4.5.6. Literal Header Field Without Name Reference . . . . . 22
5. Configuration . . . . . . . . . . . . . . . . . . . . . . . . 23
6. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 23
7. Security Considerations . . . . . . . . . . . . . . . . . . . 24
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
8.1. Settings Registration . . . . . . . . . . . . . . . . . . 24
8.2. Stream Type Registration . . . . . . . . . . . . . . . . 24
8.3. Error Code Registration . . . . . . . . . . . . . . . . . 24
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
9.1. Normative References . . . . . . . . . . . . . . . . . . 25
9.2. Informative References . . . . . . . . . . . . . . . . . 26
9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Appendix A. Static Table . . . . . . . . . . . . . . . . . . . . 26
Appendix B. Sample One Pass Encoding Algorithm . . . . . . . . . 31
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 33
C.1. Since draft-ietf-quic-qpack-03 . . . . . . . . . . . . . 33
C.2. Since draft-ietf-quic-qpack-02 . . . . . . . . . . . . . 33
C.3. Since draft-ietf-quic-qpack-01 . . . . . . . . . . . . . 33
C.4. Since draft-ietf-quic-qpack-00 . . . . . . . . . . . . . 33
C.5. Since draft-ietf-quic-qcram-00 . . . . . . . . . . . . . 34
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34
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.
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].
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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 Index: An absolute index in a header block from which relative
indices are made.
Largest Reference: The largest absolute index of an entry referenced
in a header block.
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:
x (A) Indicates that x is A bits long
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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.
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.
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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
An encoder MUST NOT insert an entry into the dynamic table (or
duplicate an existing entry) if doing so would evict an entry with
unacknowledged references. For header blocks that might rely on the
newly added entry, the encoder can use a literal representation and
maybe insert the entry later.
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 unreferenced 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
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 Base Index /
Point Insertion Point
Figure 1: Draining Dynamic Table Entries
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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 Largest Reference (Section 4.5.1) which
identifies the table state necessary for decoding. If the greatest
absolute index in the dynamic table is less than the value of the
Largest Reference, the stream is considered "blocked." While
blocked, header field data SHOULD remain in the blocked stream's flow
control window. When the Largest Reference is zero, the frame
contains no references to the dynamic table and can always be
processed immediately. A stream becomes unblocked when the greatest
absolute index in the dynamic table becomes greater than or equal to
the Largest Reference for all header blocks the decoder has started
reading from the stream. If the decoder encounters a header block
where the actual largest reference is not equal to the Largest
Reference declared in the prefix, it MAY treat this as a stream error
of type HTTP_QPACK_DECOMPRESSION_FAILED.
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. Largest Known Received
In order to identify which dynamic table entries can be safely used
without a stream becoming blocked, the encoder tracks the absolute
index of the decoder's Largest Known Received entry.
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When blocking references are permitted, the encoder uses header block
acknowledgement to identify the Largest Known Received index, 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 a Table State
Synchronize 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 instructions on the encoder stream.
The decoder MUST emit header fields in the order their
representations appear in the input header block.
2.2.1. State Synchronization
The decoder stream (Section 4.4) signals 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, regardless of whether those
references were potentially blocking (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 Table State Synchronize 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 a Table State Synchronize instruction, the decoder might be
able to coalesce multiple Table State Synchronize 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.
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2.2.2. Blocked Decoding
To track blocked streams, the necessary Largest Reference 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.
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 on a
request stream or push stream 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. The dynamic table is initially empty.
Entries are added by instructions on the encoder stream (see
Section 4.3).
The maximum size of the dynamic table can be modified by the encoder,
subject to a decoder-controlled limit (see Section 5 and
Section 4.3.4). The initial maximum size is determined by the
corresponding setting when HTTP requests or responses are first
permitted to be sent. For clients using 0-RTT data in HTTP/3, the
table size is the remembered value of the setting, even if the server
later specifies a larger maximum in its SETTINGS frame. For HTTP/3
servers and HTTP/3 clients when 0-RTT is not attempted or is
rejected, the initial maximum table size is the value of the setting
in the peer's SETTINGS frame.
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
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is less than or equal to (maximum size - new entry size) or until the
table is empty. The encoder MUST NOT evict a dynamic table entry
unless it has first been acknowledged by the decoder.
If the size of the new entry is less than or equal to the maximum
size, that entry is added to the table. It is an error to attempt to
add an entry that is larger than the maximum size; this MUST be
treated as a connection error of type
"HTTP_QPACK_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
if the referenced entry is evicted from the dynamic table prior to
inserting the new entry.
The dynamic table can contain duplicate entries (i.e., entries with
the same name and same value). Therefore, duplicate entries MUST NOT
be treated as an error by the decoder.
3.2.1. Maximum Table Size
The encoder decides how to update the dynamic table and as such can
control how much memory is used by the dynamic table. To limit the
memory requirements of the decoder, the dynamic table size is
strictly bounded.
The decoder determines the maximum size that the encoder is permitted
to use for the dynamic table. In HTTP/3, this value is determined by
the SETTINGS_HEADER_TABLE_SIZE setting (see Section 5).
An encoder can choose to use less capacity than this maximum size
(see Section 4.3.4), but the chosen size MUST stay lower than or
equal to the maximum set by the decoder. Whenever the maximum size
for the dynamic table is reduced, entries are evicted from the end of
the dynamic table until the size of the dynamic table is less than or
equal to the maximum size.
This mechanism can be used to completely clear entries from the
dynamic table by setting a maximum size of 0, which can subsequently
be restored.
3.2.2. Calculating 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.
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The size of an entry is calculated using the length of its name and
value without any Huffman encoding applied.
"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. The MaxEntries is calculated as
MaxEntries = floor( MaxTableSize / 32 )
MaxTableSize is the maximum size of the dynamic table as specified by
the decoder (see Section 3.2.1).
3.2.3. 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 "1"; indices increase sequentially with each
insertion.
3.2.4. 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.
On the encoder stream, a relative index of "0" always refers to the
most recently inserted value in the dynamic table. Note that this
means the entry referenced by a given relative index will change
while interpreting instructions on the encoder stream.
+---+---------------+-----------+
| n | ... | d + 1 | Absolute Index
+ - +---------------+ - - - - - +
| 0 | ... | n - d - 1 | Relative Index
+---+---------------+-----------+
^ |
| V
Insertion Point Dropping Point
n = count of entries inserted
d = count of entries dropped
Example Dynamic Table Indexing - Control Stream
Because frames from request streams can be delivered out of order
with instructions on the encoder stream, relative indices are
relative to the Base Index at the beginning of the header block (see
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Section 4.5.1). The Base Index is an absolute index. When
interpreting the rest of the frame, the entry identified by Base
Index has a relative index of zero. The relative indices of entries
do not change while interpreting headers on a request or push stream.
Base Index
|
V
+---+-----+-----+-----+-------+
| n | n-1 | n-2 | ... | d+1 | Absolute Index
+---+-----+ - +-----+ - +
| 0 | ... | n-d-3 | Relative Index
+-----+-----+-------+
n = count of entries inserted
d = count of entries dropped
Example Dynamic Table Indexing - Relative Index on Request Stream
3.2.5. Post-Base Indexing
A header block on the request stream can reference entries added
after the entry identified by the Base Index. This allows an encoder
to process a header block in a single pass and include references to
entries added while processing this (or other) header blocks. Newly
added entries are referenced using Post-Base instructions. Indices
for Post-Base instructions increase in the same direction as absolute
indices, but the zero value is one higher than the Base Index.
Base Index
|
V
+---+-----+-----+-----+-----+
| n | n-1 | n-2 | ... | d+1 | Absolute Index
+---+-----+-----+-----+-----+
| 1 | 0 | Post-Base Index
+---+-----+
n = count of entries inserted
d = count of entries dropped
Example Dynamic Table Indexing - Post-Base Index on Request Stream
3.2.6. Invalid References
If the decoder encounters a reference on a request or push stream to
a dynamic table entry which has already been evicted or which has an
absolute index greater than the declared Largest Reference (see
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Section 4.5.1), it MUST treat this as a stream error of type
"HTTP_QPACK_DECOMPRESSION_FAILED".
If the decoder encounters a reference on the encoder stream 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. Stream Types
QPACK instructions occur in three locations, each of which uses a
separate instruction space:
o The encoder stream is a unidirectional stream of type "0x48"
(ASCII 'H') which carries table updates from encoder to decoder.
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o The decoder stream is a unidirectional stream of type "0x68"
(ASCII 'h') which carries acknowledgements of table modifications
and header processing from decoder to encoder.
o Finally, the contents of HEADERS and PUSH_PROMISE frames on
request streams and push streams reference the QPACK table state.
There MUST be exactly one of each unidirectional stream type in each
direction. Receipt of a second instance of either stream type MUST
be treated as a connection error of HTTP_WRONG_STREAM_COUNT. Closure
of either unidirectional stream MUST be treated as a connection error
of type HTTP_CLOSED_CRITICAL_STREAM.
This section describes the instructions which are possible on each
stream type.
4.3. Encoder Stream
Table updates can add a table entry, possibly using existing entries
to avoid transmitting redundant information. The name can be
transmitted as a reference to an existing entry in the static or the
dynamic table or as a string literal. For entries which already
exist in the dynamic table, the full entry can also be used by
reference, creating a duplicate entry.
The contents of the encoder stream are an unframed sequence of the
following instructions.
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]).
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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
4.3.2. 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
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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. Dynamic Table Size Update
An encoder informs the decoder of a change to the size of the dynamic
table using an instruction which begins with the '001' three-bit
pattern. The new maximum table size is represented as an integer
with a 5-bit prefix (see Section 5.1 of [RFC7541]).
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 1 | Max size (5+) |
+---+---+---+-------------------+
Figure 3: Maximum Dynamic Table Size Change
The new maximum size MUST be lower than or equal to the limit
determined by the protocol using QPACK. A value that exceeds this
limit MUST be treated as a connection error of type
"HTTP_QPACK_ENCODER_STREAM_ERROR". In HTTP/3, this limit is the
value of the SETTINGS_HEADER_TABLE_SIZE parameter (see Section 5)
received from the decoder.
Reducing the maximum size of the dynamic table can cause entries to
be evicted (see Section 4.3 of [RFC7541]). This MUST NOT cause the
eviction of entries with outstanding references (see Section 2.1.1).
Changing the size of the dynamic table is not acknowledged as this
instruction does not insert an entry.
4.4. Decoder Stream
The decoder stream carries information used to ensure consistency of
the dynamic table. Information is sent from the decoder to the
encoder; that is, the server informs the client about the processing
of the client's header blocks and table updates, and the client
informs the server about the processing of the server's header blocks
and table updates.
The contents of the decoder stream are an unframed sequence of the
following instructions.
4.4.1. Table State Synchronize
The Table State Synchronize instruction begins with the '00' two-bit
pattern. The instruction specifies the total number of dynamic table
inserts and duplications since the last Table State Synchronize or
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Header Acknowledgement that increased the Largest Known Received
dynamic table entry (see Section 2.1.4). This is encoded as a 6-bit
prefix integer. The encoder uses this value to determine which table
entries might cause a stream to become blocked, as described in
Section 2.2.1.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | Insert Count (6+) |
+---+---+-----------------------+
Figure 4: Table State Synchronize
An encoder that receives an Insert Count equal to zero or one that
increases Largest Known Received 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 Largest Reference is
not zero, the decoder emits a Header Acknowledgement instruction on
the decoder stream. The instruction begins with the '1' one-bit
pattern and includes the request stream's stream ID, encoded as a
7-bit prefix integer. It is used by the peer's encoder to know when
it is safe to evict an entry, and possibly update Largest Known
Received.
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 header
frames on each stream are received and processed in order, this gives
the encoder precise feedback on which header blocks within a stream
have been fully processed.
If an encoder receives a Header Acknowledgement instruction referring
to a stream on which every header block with a non-zero Largest
Reference has already been acknowledged, that MUST be treated as a
connection error of type "HTTP_QPACK_DECODER_STREAM_ERROR".
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When blocking references are permitted, the encoder uses
acknowledgement of header blocks to update the Largest Known Received
index. If a header block was potentially blocking, the
acknowledgement implies that the decoder has received all dynamic
table state necessary to process the header block. If the Largest
Reference of an acknowledged header block was greater than the
encoder's current Largest Known Received index, the block's Largest
Reference becomes the new Largest Known Received.
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 size equal to
zero MAY omit sending Stream Cancellations, because the encoder
cannot have any dynamic table references.
An encoder cannot infer from this instruction that any updates to the
dynamic table have been received.
4.5. Request and Push Streams
HEADERS and PUSH_PROMISE frames on request and push streams reference
the dynamic table in a particular state without modifying it. Frames
on these streams emit the headers for an HTTP request or response.
4.5.1. Header Data Prefix
Header data is prefixed with two integers, "Largest Reference" and
"Base Index".
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0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Largest Reference (8+) |
+---+---------------------------+
| S | Delta Base Index (7+) |
+---+---------------------------+
| Compressed Headers ...
+-------------------------------+
Figure 7: Frame Payload
4.5.1.1. Largest Reference
"Largest Reference" identifies the largest absolute dynamic index
referenced in the block. Blocking decoders use the Largest Reference
to determine when it is safe to process the rest of the block. If
Largest Reference is greater than zero, the encoder transforms it as
follows before encoding:
LargestReference = (LargestReference mod (2 * MaxEntries)) + 1
The decoder reconstructs the Largest Reference using the following
algorithm:
if LargestReference > 0:
LargestReference -= 1
CurrentWrapped = TotalNumberOfInserts mod (2 * MaxEntries)
if CurrentWrapped >= LargestReference + MaxEntries:
# Largest Reference wrapped around 1 extra time
LargestReference += 2 * MaxEntries
else if CurrentWrapped + MaxEntries < LargestReference
# Decoder wrapped around 1 extra time
CurrentWrapped += 2 * MaxEntries
LargestReference += TotalNumberOfInserts - CurrentWrapped
TotalNumberOfInserts is the total number of inserts into the
decoder's dynamic table. This encoding limits the length of the
prefix on long-lived connections.
4.5.1.2. Base Index
"Base Index" is used to resolve references in the dynamic table as
described in Section 3.2.4.
To save space, Base Index is encoded relative to Largest Reference
using a one-bit sign and the "Delta Base Index" value. A sign bit of
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0 indicates that the Base Index has an absolute index that is greater
than or equal to the Largest Reference; the value of Delta Base Index
is added to the Largest Reference to determine the absolute value of
the Base Index. A sign bit of 1 indicates that the Base Index is
less than the Largest Reference. That is:
if sign == 0:
baseIndex = largestReference + deltaBaseIndex
else:
baseIndex = largestReference - deltaBaseIndex
A single-pass encoder determines the absolute value of Base Index
before encoding a header block. If the encoder inserted entries in
the dynamic table while encoding the header block, Largest Reference
will be greater than Base Index, so the encoded difference is
negative and the sign bit is set to 1. If the header block did not
reference the most recent entry in the table and did not insert any
new entries, Base Index will be greater than the Largest Reference,
so the delta will be positive and the sign bit is set to 0.
An encoder that produces table updates before encoding a header block
might set Largest Reference and Base Index to the same value. When
Largest Reference and Base Index are equal, the Delta Base Index is
encoded with a zero sign bit. A sign bit set to 1 when the Delta
Base Index is 0 MUST be treated as a decoder error.
A header block that does not reference the dynamic table can use any
value for Base Index; setting both Largest Reference and Base Index
to zero is the most efficient encoding.
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 or equal to Base Index, this representation
starts with the '1' 1-bit pattern, followed by the "S" bit indicating
whether the reference is into the static (S=1) or dynamic (S=0)
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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]).
4.5.3. Indexed Header Field With Post-Base Index
If the entry is in the dynamic table with an absolute index greater
than Base Index, the representation starts with the '0001' 4-bit
pattern, followed by the post-base index (see Section 3.2.5) 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 or equal to Base Index, this representation
starts with the '01' two-bit pattern. If the entry is in the dynamic
table with an absolute index greater than Base Index, the
representation starts with the '0000' four-bit pattern.
The following bit, 'N', indicates whether an intermediary is
permitted to add this header to the dynamic header table on
subsequent hops. When the 'N' bit is set, the encoded header MUST
always be encoded with a literal representation. In particular, when
a peer sends a header field that it received represented as a literal
header field with the 'N' bit set, it MUST use a literal
representation to forward this header field. This bit is intended
for protecting header field values that are not to be put at risk by
compressing them (see Section 7.1 of [RFC7541] for more details).
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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 or equal to Base Index, the header field
name is represented using the relative index of that entry, which is
represented as an integer with a 4-bit prefix (see Section 5.1 of
[RFC7541]). The "S" bit indicates whether the reference is to the
static (S=1) or dynamic (S=0) table.
4.5.5. Literal Header Field With Post-Base Name Reference
For entries in the dynamic table with an absolute index greater than
Base Index, the header field name is represented using the post-base
index of that entry (see Section 3.2.5) 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
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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_HEADER_TABLE_SIZE (0x1): An integer with a maximum value of
2^30 - 1. The default value is zero bytes. See Section 3.2 for
usage.
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 an instruction on a request or push stream and is not
able to continue decoding that header block.
HTTP_QPACK_ENCODER_STREAM_ERROR (TBD): The decoder failed to
interpret an instruction on the encoder stream.
HTTP_QPACK_DECODER_STREAM_ERROR (TBD): The encoder failed to
interpret an instruction on the decoder stream.
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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 creates two new settings in the "HTTP/3 Settings"
registry established in [HTTP3].
The entries in the following table are registered by this document.
+-----------------------+------+---------------+
| Setting Name | Code | Specification |
+-----------------------+------+---------------+
| HEADER_TABLE_SIZE | 0x1 | Section 5 |
| | | |
| QPACK_BLOCKED_STREAMS | 0x7 | Section 5 |
+-----------------------+------+---------------+
8.2. Stream Type Registration
This document creates two new settings in the "HTTP/3 Stream Type"
registry established in [HTTP3].
The entries in the following table are registered by this document.
+----------------------+------+---------------+--------+
| Stream Type | Code | Specification | Sender |
+----------------------+------+---------------+--------+
| QPACK Encoder Stream | 0x48 | Section 4 | Both |
| | | | |
| QPACK Decoder Stream | 0x68 | Section 4 | Both |
+----------------------+------+---------------+--------+
8.3. Error Code Registration
This document establishes the following new error codes in the
"HTTP/3 Error Code" registry established in [HTTP3].
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+------------------------------+------+--------------+--------------+
| 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-16 (work in progress),
December 2018.
[QUIC-TRANSPORT]
Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", draft-ietf-quic-
transport-16 (work in progress), December 2018.
[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-03
Substantial editorial reorganization; no technical changes.
C.2. 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.3. Since draft-ietf-quic-qpack-01
o Only header blocks that reference the dynamic table are
acknowledged (#1603, #1605)
C.4. 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)
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o Control streams use typed unidirectional streams rather than fixed
stream IDs (#910,#1359)
C.5. 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
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
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Charles 'Buck' Krasic
Netflix
Email: ckrasic@netflix.com
Mike Bishop
Akamai Technologies
Email: mbishop@evequefou.be
Alan Frindell (editor)
Facebook
Email: afrind@fb.com
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