QUIC C. Krasic
Internet-Draft Netflix
Intended status: Standards Track M. Bishop
Expires: December 30, 2018 Akamai Technologies
A. Frindell, Ed.
Facebook
June 28, 2018
QPACK: Header Compression for HTTP over QUIC
draft-ietf-quic-qpack-01
Abstract
This specification defines QPACK, a compression format for
efficiently representing HTTP header fields, to be used in HTTP over
QUIC. This is a variation of HPACK header compression that seeks to
reduce head-of-line blocking.
Note 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 December 30, 2018.
Krasic, et al. Expires December 30, 2018 [Page 1]
Internet-Draft QPACK June 2018
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
2. Header Tables . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Static Table . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Dynamic Table . . . . . . . . . . . . . . . . . . . . . . 4
2.2.1. Absolute and Relative Indexing . . . . . . . . . . . 5
2.2.2. Post-Base Indexing . . . . . . . . . . . . . . . . . 6
2.3. Avoiding Head-of-Line Blocking in HTTP/QUIC . . . . . . . 7
2.3.1. State Synchronization . . . . . . . . . . . . . . . . 8
3. Conventions and Definitions . . . . . . . . . . . . . . . . . 8
3.1. Notational Conventions . . . . . . . . . . . . . . . . . 9
4. Configuration . . . . . . . . . . . . . . . . . . . . . . . . 9
5. Wire Format . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Primitives . . . . . . . . . . . . . . . . . . . . . . . 10
5.1.1. Prefixed Integers . . . . . . . . . . . . . . . . . . 10
5.1.2. String Literals . . . . . . . . . . . . . . . . . . . 10
5.2. QPACK Encoder Stream . . . . . . . . . . . . . . . . . . 11
5.2.1. Insert With Name Reference . . . . . . . . . . . . . 11
5.2.2. Insert Without Name Reference . . . . . . . . . . . . 11
5.2.3. Duplicate . . . . . . . . . . . . . . . . . . . . . . 12
5.2.4. Dynamic Table Size Update . . . . . . . . . . . . . . 12
5.3. QPACK Decoder Stream . . . . . . . . . . . . . . . . . . 13
5.3.1. Table State Synchronize . . . . . . . . . . . . . . . 13
5.3.2. Header Acknowledgement . . . . . . . . . . . . . . . 14
5.3.3. Stream Cancellation . . . . . . . . . . . . . . . . . 14
5.4. Request and Push Streams . . . . . . . . . . . . . . . . 15
5.4.1. Header Data Prefix . . . . . . . . . . . . . . . . . 15
5.4.2. Instructions . . . . . . . . . . . . . . . . . . . . 16
6. Encoding Strategies . . . . . . . . . . . . . . . . . . . . . 19
6.1. Single Pass Encoding . . . . . . . . . . . . . . . . . . 19
6.2. Preventing Eviction Races . . . . . . . . . . . . . . . . 19
6.3. Reference Tracking . . . . . . . . . . . . . . . . . . . 19
Krasic, et al. Expires December 30, 2018 [Page 2]
Internet-Draft QPACK June 2018
6.3.1. Blocked Eviction . . . . . . . . . . . . . . . . . . 20
6.3.2. Blocked Decoding . . . . . . . . . . . . . . . . . . 20
6.4. Speculative table updates . . . . . . . . . . . . . . . . 20
6.5. Sample One Pass Encoding Algorithm . . . . . . . . . . . 20
7. Security Considerations . . . . . . . . . . . . . . . . . . . 22
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
8.1. Settings Registration . . . . . . . . . . . . . . . . . . 22
8.2. Stream Type Registration . . . . . . . . . . . . . . . . 22
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
9.1. Normative References . . . . . . . . . . . . . . . . . . 22
9.2. Informative References . . . . . . . . . . . . . . . . . 23
9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 23
A.1. Since draft-ietf-quic-qpack-00 . . . . . . . . . . . . . 23
A.2. Since draft-ietf-quic-qcram-00 . . . . . . . . . . . . . 24
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
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 used HPACK ([RFC7541]) for header compression, but
QUIC's stream multiplexing comes into some conflict with HPACK. A
key goal of the design of QUIC is to improve stream multiplexing
relative to HTTP/2 by reducing head-of-line blocking. If HPACK were
used for HTTP/QUIC, it would induce head-of-line blocking due to
built-in assumptions of a total ordering across frames on all
streams.
QUIC is described in [QUIC-TRANSPORT]. The HTTP/QUIC mapping is
described in [QUIC-HTTP]. For a full description of HTTP/2, see
[RFC7540]. The description of HPACK is [RFC7541], with important
terminology in Section 1.3.
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.
2. Header Tables
Like HPACK, QPACK uses two tables for associating header fields to
indexes. The static table (see Section 2.1) is predefined and
contains common header fields (some of them with an empty value).
Krasic, et al. Expires December 30, 2018 [Page 3]
Internet-Draft QPACK June 2018
The dynamic table (see Section 2.2) built up over the course of the
connection and can be used by the encoder to index header fields
repeated in the encoded header lists.
Unlike in HPACK, entries in the QPACK static and dynamic tables are
addressed separately. The following sections describe how entries in
each table is addressed.
2.1. 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 of [RFC7541]. Note that because HPACK did not
use zero-based references, there is no value at index zero of the
static table.
2.2. 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 5.2).
Before a new entry is added to the dynamic table, entries are evicted
from the end of the dynamic table until the size of the dynamic table
is less than or equal to (maximum size - new entry size) or until the
table is empty.
If the size of the new entry is less than or equal to the maximum
size, that entry is added to the table. It is an error to attempt to
add an entry that is larger than the maximum size; this MUST be
treated as a connection error of type
"HTTP_QPACK_DECOMPRESSION_FAILED".
A new entry can reference an entry in the dynamic table that will be
evicted when adding this new entry into the dynamic table.
Implementations are cautioned to avoid deleting the referenced name
if the referenced entry is evicted from the dynamic table prior to
inserting the new entry.
The dynamic table can contain duplicate entries (i.e., entries with
the same name and same value). Therefore, duplicate entries MUST NOT
be treated as an error by a decoder.
The encoder decides how to update the dynamic table and as such can
control how much memory is used by the dynamic table. To limit the
memory requirements of the decoder, the dynamic table size is
strictly bounded.
Krasic, et al. Expires December 30, 2018 [Page 4]
Internet-Draft QPACK June 2018
The decoder determines the maximum size that the encoder is permitted
to use for the dynamic table. In HTTP/QUIC, this value is determined
by the SETTINGS_HEADER_TABLE_SIZE setting (see Section 4.2.5.2 of
[QUIC-HTTP]).
An encoder can choose to use less capacity than this maximum size
(see Section 5.2.4), 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.
2.2.1. Absolute and Relative Indexing
Each entry possesses both an absolute index which is fixed for the
lifetime of that entry and a relative index which changes over time
based on the context of the reference. The first entry inserted has
an absolute index of "1"; indices increase sequentially with each
insertion.
The relative index begins at zero and increases in the opposite
direction from the absolute index. Determining which entry has a
relative index of "0" depends on the context of the reference.
On the control 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
Krasic, et al. Expires December 30, 2018 [Page 5]
Internet-Draft QPACK June 2018
Because frames from request streams can be delivered out of order
with instructions on the control stream, relative indices are
relative to the Base Index at the beginning of the header block (see
Section 5.4.1). 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 - Request Stream
2.2.2. 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
Dynamic Table Indexing - Post-Base References
If the decoder encounters a reference to an entry which has already
been dropped from the table or which is greater than the declared
Krasic, et al. Expires December 30, 2018 [Page 6]
Internet-Draft QPACK June 2018
Largest Reference (see Section 5.4.1), this MUST be treated as a
stream error of type "HTTP_QPACK_DECOMPRESSION_FAILED" error code.
If this reference occurs on the control stream, this MUST be treated
as a session error.
2.3. Avoiding Head-of-Line Blocking in HTTP/QUIC
Because QUIC does not guarantee order between data on different
streams, a header block might reference an entry in the dynamic table
that has not yet been received.
Each header block contains a Largest Reference which identifies the
table state necessary for decoding. If the greatest absolute index
in the dynamic table is less than the value of the Largest Reference,
the stream is considered "blocked." While blocked, header field data
should remain in the blocked stream's flow control window. When the
Largest Reference is zero, the frame contains no references to the
dynamic table and can always be processed immediately. A stream
becomes unblocked when the greatest absolute index in the dynamic
table becomes greater than or equal to the Largest Reference for all
header blocks the decoder has started reading from the stream. If a
decoder encounters a header block where the actual largest reference
is not equal to the largest reference declared in the prefix, it MAY
treat this as a stream error of type HTTP_QPACK_DECOMPRESSION_FAILED.
A decoder can permit the possibility of blocked streams by setting
SETTINGS_QPACK_BLOCKED_STREAMS to a non-zero value (see Section 4).
This setting specifies an upper bound on the number of streams which
can be blocked.
An encoder can decide whether to risk having a stream become blocked.
If permitted by the value of SETTINGS_QPACK_BLOCKED_STREAMS,
compression efficiency can be improved by referencing dynamic table
entries that are still in transit, but if there is loss or reordering
the stream can become blocked at the decoder. An encoder avoids the
risk of blocking by only referencing dynamic table entries which have
been acknowledged, but this means using literals. Since literals
make the header block larger, this can result in the encoder becoming
blocked on congestion or flow control limits.
An encoder MUST limit the number of streams which could become
blocked to the value of SETTINGS_QPACK_BLOCKED_STREAMS at all times.
Note that the decoder might not actually become blocked on every
stream which risks becoming blocked. If the decoder encounters more
blocked streams than it promised to support, it SHOULD treat this as
a stream error of type HTTP_QPACK_DECOMPRESSION_FAILED.
Krasic, et al. Expires December 30, 2018 [Page 7]
Internet-Draft QPACK June 2018
2.3.1. State Synchronization
The decoder stream signals key events at the decoder that permit the
encoder to track the decoder's state. These events are:
o Successful processing of a header block
o Abandonment of a stream which might have remaining header blocks
o Receipt of new dynamic table entries
Regardless of whether a header block contained blocking references,
the knowledge that it was processed successfully permits the encoder
to avoid evicting entries while references remain outstanding; see
Section 6.3.1. When a stream is reset or abandoned, the indication
that these header blocks will never be processed serves a similar
function; see Section 5.3.3.
For the encoder to identify which dynamic table entries can be safely
used without a stream becoming blocked, the encoder tracks the
absolute index of the decoder's Largest Known Received entry.
When blocking references are permitted, the encoder uses
acknowledgement of header blocks to identify the Largest Known
Received index, as described in Section 5.3.2.
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 5.3.1).
3. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Definitions of terms that are used in this document:
Header: A name-value pair sent as part of an HTTP message.
Header set: The full collection of headers associated with an HTTP
message.
Header block: The compressed representation of a header set.
Krasic, et al. Expires December 30, 2018 [Page 8]
Internet-Draft QPACK June 2018
Encoder: An implementation which transforms a header set into a
header block.
Decoder: An implementation which transforms a header block into a
header set.
QPACK is a name, not an acronym.
3.1. Notational Conventions
Diagrams use the format described in Section 3.1 of [RFC2360], with
the following additional conventions:
x (A) Indicates that x is A bits long
x (A+) Indicates that x uses the prefixed integer encoding defined
in Section 5.1 of [RFC7541], beginning with an A-bit prefix.
x ... Indicates that x is variable-length and extends to the end of
the region.
4. Configuration
QPACK defines two settings which are included in the HTTP/QUIC
SETTINGS frame.
SETTINGS_HEADER_TABLE_SIZE (0x1): An integer with a maximum value of
2^30 - 1. The default value is 4,096 bytes. See (TODO: reference
PR#1357) for usage.
SETTINGS_QPACK_BLOCKED_STREAMS (0x7): An integer with a maximum
value of 2^16 - 1. The default value is 100. See Section 2.3.
5. Wire Format
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.
Instructions on this stream modify the dynamic table state without
generating output to any particular request.
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.
Krasic, et al. Expires December 30, 2018 [Page 9]
Internet-Draft QPACK June 2018
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.
All table updates occur on the encoder stream. Request streams and
push streams only carry header blocks that do not modify the state of
the table.
5.1. Primitives
5.1.1. Prefixed Integers
The prefixed integer from Section 5.1 of [RFC7541] is used heavily
throughout this document. The format from [RFC7541] is used
unmodified.
5.1.2. 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 octets of data. When Huffman encoding is enabled, the
Huffman table from Appendix B of [RFC7541] is used without
modification.
This document expands the definition of string literals and permits
them to begin other than on a byte boundary. An "N-bit prefix string
literal" begins with the same Huffman flag, followed by the length
encoded as an (N-1)-bit prefix integer. The remainder of the string
literal is unmodified.
A string literal without a prefix length noted is an 8-bit prefix
string literal and follows the definitions in [RFC7541] without
modification.
Krasic, et al. Expires December 30, 2018 [Page 10]
Internet-Draft QPACK June 2018
5.2. QPACK 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.
5.2.1. 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 header field name is represented using the relative
index of that entry, which is represented as an integer with a 6-bit
prefix (see Section 5.1 of [RFC7541]).
The header name reference is followed by the header field value
represented as a string literal (see Section 5.2 of [RFC7541]).
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | S | Name Index (6+) |
+---+---+-----------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
Insert Header Field -- Indexed Name
5.2.2. 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 5.1) 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.
Krasic, et al. Expires December 30, 2018 [Page 11]
Internet-Draft QPACK June 2018
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 1 | H | Name Length (5+) |
+---+---+---+-------------------+
| Name String (Length octets) |
+---+---------------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
Insert Header Field -- New Name
5.2.3. 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 1: Duplicate
The existing entry is re-inserted into the dynamic table without
resending either the name or the value. This is useful to mitigate
the eviction of older entries which are frequently referenced, both
to avoid the need to resend the header and to avoid the entry in the
table blocking the ability to insert new headers.
5.2.4. 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 2: 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
Krasic, et al. Expires December 30, 2018 [Page 12]
Internet-Draft QPACK June 2018
limit MUST be treated as a decoding error. In HTTP/QUIC, this limit
is the value of the SETTINGS_HEADER_TABLE_SIZE parameter (see
Section 4) 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 6.3).
Changing the size of the dynamic table is not acknowledged as this
instruction does not insert an entry.
5.3. QPACK Decoder Stream
The decoder stream carries information used to ensure consistency of
the dynamic table. Information is sent from the QPACK decoder to the
QPACK encoder; that is, the server informs the client about the
processing of the client's header blocks and table updates, and the
client informs the server about the processing of the server's header
blocks and table updates.
The contents of the decoder stream are an unframed sequence of the
following instructions.
5.3.1. 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
Header Acknowledgement that increased the Largest Known Received
dynamic table entry. This is encoded as a 6-bit prefix integer. The
encoder uses this value to determine which table entries might cause
a stream to become blocked, as described in Section 2.3.1.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | Insert Count (6+) |
+---+---+-----------------------+
Figure 3: Table State Synchronize
A decoder chooses when to emit Table State Synchronize instructions.
Emitting a Table State Synchronize after adding each new dynamic
table entry will provide the most timely feedback to the encoder, but
could be redundant with other decoder feedback. By delaying a
Table State Synchronize, a decoder might be able to coalesce multiple
Table State Synchronize instructions, or replace them entirely with
Header Acknowledgements. However, delaying too long may lead to
compression inefficiencies if the encoder waits for an entry to be
acknowledged before using it.
Krasic, et al. Expires December 30, 2018 [Page 13]
Internet-Draft QPACK June 2018
5.3.2. Header Acknowledgement
After processing a header block on a request or push stream, the
decoder emits a Header Acknowledgement instruction on the decoder
stream. The instruction begins with the '1' one-bit pattern and
includes the request stream's stream ID, encoded as a 7-bit prefix
integer. It is used by the peer's QPACK encoder to know when it is
safe to evict an entry.
The same Stream ID can be identified multiple times, as multiple
header blocks can be sent on a single stream in the case of
intermediate responses, trailers, and pushed requests. Since header
frames on each stream are received and processed in order, this gives
the encoder precise feedback on which header blocks within a stream
have been fully processed.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | Stream ID (7+) |
+---+---------------------------+
Figure 4: Header Acknowledgement
When blocking references are permitted, the encoder uses
acknowledgement of header blocks to update the Largest Known Received
index. If a header block was potentially blocking, the
acknowledgement implies that the decoder has received all dynamic
table state necessary to process the header block. If the Largest
Reference of an acknowledged header block was greater than the
encoder's current Largest Known Received index, the block's Largest
Reference becomes the new Largest Known Received.
5.3.3. Stream Cancellation
A stream that is reset might have multiple outstanding header blocks.
A decoder that receives a stream reset before the end of a stream
generates a Stream Cancellation instruction on the decoder stream.
Similarly, a decoder that abandons reading of a stream needs to
signal this using the Stream Cancellation instruction. This signals
to the encoder that all references to the dynamic table on that
stream are no longer outstanding.
An encoder cannot infer from this instruction that any updates to the
dynamic table have been received.
The instruction begins with the '01' two-bit pattern. The
instruction includes the stream ID of the affected stream - a request
or push stream - encoded as a 6-bit prefix integer.
Krasic, et al. Expires December 30, 2018 [Page 14]
Internet-Draft QPACK June 2018
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 1 | Stream ID (6+) |
+---+---+-----------------------+
Figure 5: Stream Cancellation
5.4. 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.
5.4.1. Header Data Prefix
Header data is prefixed with two integers, "Largest Reference" and
"Base Index".
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Largest Reference (8+) |
+---+---------------------------+
| S | Delta Base Index (7+) |
+---+---------------------------+
| Compressed Headers ...
+-------------------------------+
Figure 6: Frame Payload
"Largest Reference" identifies the largest absolute dynamic index
referenced in the block. Blocking decoders use the Largest Reference
to determine when it is safe to process the rest of the block.
"Base Index" is used to resolve references in the dynamic table as
described in Section 2.2.1.
To save space, Base Index is encoded relative to Largest Reference
using a one-bit sign and the "Delta Base Index" value. A sign bit of
0 indicates that the Base Index has an absolute index that is greater
than or equal to the Largest Reference; the value of Delta Base Index
is added to the Largest Reference to determine the absolute value of
the Base Index. A sign bit of 1 indicates that the Base Index is
less than the Largest Reference. That is:
if sign == 0:
baseIndex = largestReference + deltaBaseIndex
else:
baseIndex = largestReference - deltaBaseIndex
Krasic, et al. Expires December 30, 2018 [Page 15]
Internet-Draft QPACK June 2018
A single-pass encoder is expected to determine the absolute value of
Base Index before encoding a header block. If the encoder inserted
entries in the dynamic table while encoding the header block, Largest
Reference will be greater than Base Index, so the encoded difference
is negative and the sign bit is set to 1. If the header block did
not reference the most recent entry in the table and did not insert
any new entries, Base Index will be greater than the Largest
Reference, so the delta will be positive and the sign bit is set to
0.
An encoder that produces table updates before encoding a header block
might set Largest Reference and Base Index to the same value. When
Largest Reference and Base Index are equal, the Delta Base Index is
encoded with a zero sign bit. A sign bit set to 1 when the Delta
Base Index is 0 MUST be treated as a decoder error.
A header block that does not reference the dynamic table can use any
value for Base Index; setting both Largest Reference and Base Index
to zero is the most efficient encoding.
5.4.2. Instructions
5.4.2.1. 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)
table. Finally, the relative index of the matching header field is
represented as an integer with a 6-bit prefix (see Section 5.1 of
[RFC7541]).
Krasic, et al. Expires December 30, 2018 [Page 16]
Internet-Draft QPACK June 2018
5.4.2.2. 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 2.2.1) 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
5.4.2.3. 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).
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 1 | N | S |Name Index (4+)|
+---+---+---+---+---------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
Literal Header Field With Name Reference
Krasic, et al. Expires December 30, 2018 [Page 17]
Internet-Draft QPACK June 2018
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.
5.4.2.4. 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 2.2.1) 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 octets) |
+-------------------------------+
Literal Header Field With Post-Base Name Reference
5.4.2.5. 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 5.1) 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
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.
Krasic, et al. Expires December 30, 2018 [Page 18]
Internet-Draft QPACK June 2018
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 0 | 1 | N | H |NameLen(3+)|
+---+---+---+---+---+-----------+
| Name String (Length octets) |
+---+---------------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
Literal Header Field Without Name Reference
6. Encoding Strategies
6.1. Single Pass Encoding
An encoder making a single pass over a list of headers must choose
Base Index before knowing Largest Reference. When trying to
reference a header inserted to the table after encoding has begun,
the entry is encoded with different instructions that tell the
decoder to use an absolute index greater than the Base Index.
6.2. Preventing Eviction Races
Due to out-of-order arrival, QPACK's eviction algorithm requires
changes (relative to HPACK) to avoid the possibility that an indexed
representation is decoded after the referenced entry has already been
evicted. QPACK employs a two-phase eviction algorithm, in which the
encoder will not evict entries that have outstanding (unacknowledged)
references.
6.3. Reference Tracking
An encoder MUST ensure that a header block which references a dynamic
table entry is not received by the decoder after the referenced entry
has already been evicted. An encoder also respects the limit set by
the decoder on the number of streams that are allowed to become
blocked. Even if the decoder is willing to tolerate blocked streams,
the encoder might choose to avoid them in certain cases.
In order to enable this, the encoder will need to track outstanding
(unacknowledged) header blocks and table updates using feedback
received from the decoder.
Krasic, et al. Expires December 30, 2018 [Page 19]
Internet-Draft QPACK June 2018
6.3.1. Blocked Eviction
The encoder MUST NOT permit an entry to be evicted while a reference
to that entry remains unacknowledged. If a new header to be inserted
into the dynamic table would cause the eviction of such an entry, the
encoder MUST NOT emit the insert instruction until the reference has
been processed by the decoder and acknowledged.
The encoder can emit a literal representation for the new header in
order to avoid encoding delays, and MAY insert the header into the
table later if desired.
To ensure that the blocked eviction case is rare, references to the
oldest entries in the dynamic table SHOULD be avoided. When one of
the oldest entries in the table is still actively used for
references, the encoder SHOULD emit an Duplicate representation
instead (see Section 5.2.3).
6.3.2. Blocked Decoding
For header blocks encoded in non-blocking mode, the encoder needs to
forego indexed representations that refer to table updates which have
not yet been acknowledged with Section 5.3. Since all table updates
are processed in sequence on the control stream, an index into the
dynamic table is sufficient to track which entries have been
acknowledged.
To track blocked streams, the necessary Base Index value for each
stream can be used. Whenever the decoder processes a table update,
it can begin decoding any blocked streams that now have their
dependencies satisfied.
6.4. Speculative table updates
Implementations can _speculatively_ send header frames on the HTTP
Control Streams which are not needed for any current HTTP request or
response. Such headers could be used strategically to improve
performance. For instance, the encoder might decide to _refresh_ by
sending Duplicate representations for popular header fields
(Section 5.2.3), ensuring they have small indices and hence minimal
size on the wire.
6.5. Sample One Pass Encoding Algorithm
Pseudo-code for single pass encoding, excluding handling of
duplicates, non-blocking mode, and reference tracking.
Krasic, et al. Expires December 30, 2018 [Page 20]
Internet-Draft QPACK June 2018
baseIndex = dynamicTable.baseIndex
largestReference = 0
for header in headers:
staticIdx = staticTable.getIndex(header)
if staticIdx:
encodeIndexReference(streamBuffer, staticIdx)
continue
dynamicIdx = dynamicTable.getIndex(header)
if !dynamicIdx:
# No matching entry. Either insert+index or encode literal
nameIdx = getNameIndex(header)
if shouldIndex(header) and dynamicTable.canIndex(header):
encodeLiteralWithIncrementalIndex(controlBuffer, nameIdx,
header)
dynamicTable.add(header)
dynamicIdx = dynamicTable.baseIndex
if !dynamicIdx:
# Couldn't index it, literal
if nameIdx <= staticTable.size:
encodeLiteral(streamBuffer, nameIndex, header)
else:
# encode literal, possibly with nameIdx above baseIndex
encodeDynamicLiteral(streamBuffer, nameIndex, baseIndex,
header)
largestReference = max(largestReference,
dynamicTable.toAbsolute(nameIdx))
else:
# Dynamic index reference
assert(dynamicIdx)
largestReference = max(largestReference, dynamicIdx)
# Encode dynamicIdx, possibly with dynamicIdx above baseIndex
encodeDynamicIndexReference(streamBuffer, dynamicIdx,
baseIndex)
# encode the prefix
encodeInteger(prefixBuffer, 0x00, largestReference, 8)
if baseIndex >= largestReference:
encodeInteger(prefixBuffer, 0, baseIndex - largestReference, 7)
else:
encodeInteger(prefixBuffer, 0x80,
largestReference - baseIndex, 7)
return controlBuffer, prefixBuffer + streamBuffer
Krasic, et al. Expires December 30, 2018 [Page 21]
Internet-Draft QPACK June 2018
7. Security Considerations
TBD.
8. IANA Considerations
8.1. Settings Registration
This document creates two new settings in the "HTTP/QUIC Settings"
registry established in [QUIC-HTTP].
The entries in the following table are registered by this document.
+-----------------------+------+---------------+
| Setting Name | Code | Specification |
+-----------------------+------+---------------+
| HEADER_TABLE_SIZE | 0x1 | Section 4 |
| | | |
| QPACK_BLOCKED_STREAMS | 0x7 | Section 4 |
+-----------------------+------+---------------+
8.2. Stream Type Registration
This document creates two new settings in the "HTTP/QUIC Stream Type"
registry established in [QUIC-HTTP].
The entries in the following table are registered by this document.
+----------------------+------+---------------+--------+
| Stream Type | Code | Specification | Sender |
+----------------------+------+---------------+--------+
| QPACK Encoder Stream | 0x48 | Section 5 | Both |
| | | | |
| QPACK Decoder Stream | 0x68 | Section 5 | Both |
+----------------------+------+---------------+--------+
9. References
9.1. Normative References
[QUIC-HTTP]
Bishop, M., Ed., "Hypertext Transfer Protocol (HTTP) over
QUIC", draft-ietf-quic-http-13 (work in progress), June
2018.
Krasic, et al. Expires December 30, 2018 [Page 22]
Internet-Draft QPACK June 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>.
9.2. Informative References
[QUIC-TRANSPORT]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-12 (work
in progress), May 2018.
[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. Change Log
*RFC Editor's Note:* Please remove this section prior to
publication of a final version of this document.
A.1. Since draft-ietf-quic-qpack-00
o Renumbered instructions for consistency (#1471, #1472)
o Decoder is allowed to validate largest reference (#1404, #1469)
Krasic, et al. Expires December 30, 2018 [Page 23]
Internet-Draft QPACK June 2018
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)
A.2. 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.
Krasic, et al. Expires December 30, 2018 [Page 24]
Internet-Draft QPACK June 2018
A substantial portion of Mike's contribution was supported by
Microsoft during his employment there.
Authors' Addresses
Charles 'Buck' Krasic
Netflix
Email: ckrasic@netflix.com
Mike Bishop
Akamai Technologies
Email: mbishop@evequefou.be
Alan Frindell (editor)
Facebook
Email: afrind@fb.com
Krasic, et al. Expires December 30, 2018 [Page 25]
