QUIC C. Krasic
Internet-Draft Google, Inc
Intended status: Standards Track M. Bishop
Expires: November 24, 2018 Akamai Technologies
A. Frindell, Ed.
Facebook
May 23, 2018
QPACK: Header Compression for HTTP over QUIC
draft-ietf-quic-qpack-00
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 November 24, 2018.
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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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Head-of-Line Blocking in HPACK . . . . . . . . . . . . . 3
1.2. Avoiding Head-of-Line Blocking in HTTP/QUIC . . . . . . . 4
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 5
2.1. Notational Conventions . . . . . . . . . . . . . . . . . 5
3. Wire Format . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Primitives . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Indexing . . . . . . . . . . . . . . . . . . . . . . . . 6
3.3. QPACK Encoder Stream . . . . . . . . . . . . . . . . . . 8
3.3.1. Insert With Name Reference . . . . . . . . . . . . . 8
3.3.2. Insert Without Name Reference . . . . . . . . . . . . 9
3.3.3. Duplicate . . . . . . . . . . . . . . . . . . . . . . 9
3.3.4. Dynamic Table Size Update . . . . . . . . . . . . . . 10
3.4. QPACK Decoder Stream . . . . . . . . . . . . . . . . . . 10
3.4.1. Table State Synchronize . . . . . . . . . . . . . . . 11
3.4.2. Header Acknowledgement . . . . . . . . . . . . . . . 11
3.5. Request and Push Streams . . . . . . . . . . . . . . . . 11
3.5.1. Header Data Prefix . . . . . . . . . . . . . . . . . 12
3.5.2. Instructions . . . . . . . . . . . . . . . . . . . . 12
4. Encoding Strategies . . . . . . . . . . . . . . . . . . . . . 15
4.1. Single pass encoding . . . . . . . . . . . . . . . . . . 15
4.2. Preventing Eviction Races . . . . . . . . . . . . . . . . 15
4.3. Reference Tracking . . . . . . . . . . . . . . . . . . . 15
4.3.1. Blocked Eviction . . . . . . . . . . . . . . . . . . 16
4.3.2. Blocked Decoding . . . . . . . . . . . . . . . . . . 16
4.4. Speculative table updates . . . . . . . . . . . . . . . . 16
4.5. Sample One Pass Encoding Algorithm . . . . . . . . . . . 17
5. Security Considerations . . . . . . . . . . . . . . . . . . . 18
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
7.1. Normative References . . . . . . . . . . . . . . . . . . 18
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7.2. Informative References . . . . . . . . . . . . . . . . . 18
7.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 19
Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
B.1. Since draft-ietf-quic-qcram-00 . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
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. QUIC's stream multiplexing comes into some conflict with
header compression. A key goal of the design of QUIC is to improve
stream multiplexing relative to HTTP/2 by eliminating HoL (head of
line) blocking, which can occur in HTTP/2. HoL blocking can happen
because all HTTP/2 streams are multiplexed onto a single TCP
connection with its in-order semantics. QUIC can maintain
independence between streams because it implements core transport
functionality in a fully stream-aware manner. However, the HTTP/QUIC
mapping is still subject to HoL blocking if HPACK is used directly.
HPACK exploits multiplexing for greater compression, shrinking the
representation of headers that have appeared earlier on the same
connection. In the context of QUIC, this imposes a vulnerability to
HoL blocking (see Section 1.1).
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 modifies HPACK to allow correctness in the presence of out-of-
order delivery, with flexibility for implementations to balance
between resilience against HoL 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.
QPACK is intended to be a relatively non-intrusive extension to
HPACK; an implementation should be easily shared within stacks
supporting both HTTP/2 over (TLS+)TCP and HTTP/QUIC.
1.1. Head-of-Line Blocking in HPACK
HPACK enables several types of header representations, one of which
also adds the header to a dynamic table of header values. These
values are then available for reuse in subsequent header blocks
simply by referencing the entry number in the table.
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If the packet containing a header is lost, that stream cannot
complete header processing until the packet is retransmitted. This
is unavoidable. However, other streams which rely on the state
created by that packet _also_ cannot make progress. This is the
problem which QUIC solves in general, but which is reintroduced by
HPACK when the loss includes a HEADERS frame.
1.2. 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 (see Section 3.5.1)
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.
A decoder can permit the possibility of blocked streams by setting
SETTINGS_QPACK_BLOCKED_STREAMS to a non-zero value. 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.
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2. 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.
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.
2.1. 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.
3. Wire Format
QPACK instructions occur in three locations, each of which uses a
separate instruction space:
o Table updates are carried by a unidirectional stream from encoder
to decoder. Instructions on this stream modify the dynamic table
state without generating output to any particular request.
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o Acknowledgements of table modifications and header processing are
carried by a unidirectional stream from decoder to encoder.
o Finally, the contents of HEADERS and PUSH_PROMISE frames on
request streams reference the QPACK table state.
This section describes the instructions which are possible on each
stream type.
All table updates occur on the control stream. Request streams only
carry header blocks that do not modify the state of the table.
3.1. Primitives
The prefixed integer from Section 5.1 of [RFC7541] is used heavily
throughout this document. The string literal, defined by Section 5.2
of [RFC7541], is used with the following modification.
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.
QPACK permits strings 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.
3.2. Indexing
Entries in the QPACK static and dynamic tables are addressed
separately.
Entries in the static table have the same indices at all times. The
static table is 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.
Entries are inserted into the dynamic table over time. 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.
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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 can change while
interpreting a HEADERS frame as new entries are inserted.
+---+---------------+-------+
| 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 control stream, relative indices are
relative to the Base Index at the beginning of the header block (see
Section 3.5.1). The Base Index is the absolute index of the entry
which has the relative index of zero when interpreting the frame.
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
Entries with an absolute index greater than a frame's Base Index can
be referenced using specific Post-Base instructions. The relative
indices of Post-Base references count up from Base Index.
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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
Largest Reference, 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.
3.3. 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.
Each set of encoder instructions is prefaced by its length, encoded
as a variable length integer with an 8-bit prefix. Instructions MUST
NOT span more than one block.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Block Length (8+) |
+-------------------------------+
| Instruction Block (*) ...
+-------------------------------+
Encoder instruction block
3.3.1. 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
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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
3.3.2. 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 3.1) 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 octets) |
+---+---------------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
Insert Header Field -- New Name
3.3.3. 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.
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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.
3.3.4. 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 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
limit MUST be treated as a decoding error. In HTTP/QUIC, this limit
is the value of the SETTINGS_HEADER_TABLE_SIZE parameter (see
[QUIC-HTTP]) 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 4.3).
3.4. 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.
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3.4.1. Table State Synchronize
After processing a set of instructions on the encoder stream, the
decoder will emit a Table State Synchronize instruction on the
decoder stream. The instruction begins with the '1' one-bit pattern.
The instruction specifies the total number of dynamic table inserts
and duplications since the last Table State Synchronize, encoded as a
7-bit prefix integer. The encoder uses this value to determine which
table entries are vulnerable to head-of-line blocking. A decoder MAY
coalesce multiple synchronization updates into a single update.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | Insert Count (7+) |
+---+---------------------------+
Figure 3: Table Size Synchronize
3.4.2. 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 '0' 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
+---+---+---+---+---+---+---+---+
| 0 | Stream ID (7+) |
+---+---------------------------+
Figure 4: Header Acknowledgement
3.5. 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.
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3.5.1. 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 5: 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 3.2. To save space, Base Index is encoded
relative to Largest Reference using a one-bit sign flag.
baseIndex = largestReference + deltaBaseIndex
If the encoder inserted entries to the table while the encoding the
block, Largest Reference will be greater than Base Index, so
deltaBaseIndex will be negative and encoded with S=1. If the block
did not reference the most recent entry in the table and did not
insert any new entries, Largest Reference will be less than Base
Index, so deltaBaseIndex will be positive and encoded with S=0. When
Largest Reference and Base Index are equal, deltaBaseIndex is 0 and
encoded with S=0.
3.5.2. Instructions
3.5.2.1. 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].
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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]).
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 1 | 0 | 0 | Index (4+) |
+---+---+-----------------------+
Indexed Header Field
If the entry is in the dynamic table with an absolute index greater
than Base Index, the representation starts with the '0100' 4-bit
pattern, followed by the post-base index (see Section 3.2) of the
matching header field, represented as an integer with a 4-bit prefix
(see Section 5.1 of [RFC7541]).
3.5.2.2. 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 '00' two-bit pattern. If the entry is in the dynamic
table with an absolute index greater than Base Index, the
representation starts with the '0101' 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
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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 | 0 | N | S |Name Index (4+)|
+---+---+-----------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
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.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 1 | 0 | 1 | N |NameIdx(3+)|
+---+---+-----------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
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) encoded as an integer with a
3-bit prefix.
3.5.2.3. 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 3.1) starts with the '011' 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
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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.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 1 | 1 | N | H |NameLen(3+)|
+---+---+---+-------------------+
| Name String (Length octets) |
+---+---------------------------+
| H | Value Length (7+) |
+---+---------------------------+
| Value String (Length octets) |
+-------------------------------+
Literal Header Field Without Name Reference
4. Encoding Strategies
4.1. 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.
4.2. 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.
4.3. 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.
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In order to enable this, the encoder will need to track outstanding
(unacknowledged) header blocks and table updates using feedback
received from the decoder.
4.3.1. 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 3.3.3).
4.3.2. 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 3.4. 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.
4.4. 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 3.3.3), ensuring they have small indices and hence minimal
size on the wire.
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4.5. Sample One Pass Encoding Algorithm
Pseudo-code for single pass encoding, excluding handling of
duplicates, non-blocking mode, and reference tracking.
baseIndex = dynamicTable.baseIndex
largestReference = 0
for header in headers:
staticIdx = staticTable.getIndex(header)
if staticIdx:
encodeIndexReference(streamBuffer, staticIdx)
continue
dynamicIdx = dynamicTable.getIndex(header)
if !dynamicIdx:
# No matching entry. Either insert+index or encode literal
nameIdx = getNameIndex(header)
if shouldIndex(header) and dynamicTable.canIndex(header):
encodeLiteralWithIncrementalIndex(controlBuffer, nameIdx,
header)
dynamicTable.add(header)
dynamicIdx = dynamicTable.baseIndex
if !dynamicIdx:
# Couldn't index it, literal
if nameIdx <= staticTable.size:
encodeLiteral(streamBuffer, nameIndex, header)
else:
# encode literal, possibly with nameIdx above baseIndex
encodeDynamicLiteral(streamBuffer, nameIndex, baseIndex,
header)
largestReference = max(largestReference,
dynamicTable.toAbsolute(nameIdx))
else:
# Dynamic index reference
assert(dynamicIdx)
largestReference = max(largestReference, dynamicIdx)
# Encode dynamicIdx, possibly with dynamicIdx above baseIndex
encodeDynamicIndexReference(streamBuffer, dynamicIdx,
baseIndex)
# encode the prefix
encodeInteger(prefixBuffer, 0x00, largestReference, 8)
delta = largestReference - baseIndex
sign = delta > 0 ? 0x80 : 0
encodeInteger(prefixBuffer, sign, delta, 7)
return controlBuffer, prefixBuffer + streamBuffer
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5. Security Considerations
TBD.
6. IANA Considerations
None.
7. References
7.1. Normative References
[QUIC-HTTP]
Bishop, M., "Hypertext Transfer Protocol (HTTP) over
QUIC", draft-ietf-quic-http-12 (work in progress), April
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>.
7.2. Informative References
[QUIC-TRANSPORT]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-11 (work
in progress), April 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>.
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7.3. 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
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
Change Log
*RFC Editor's Note:* Please remove this section prior to
publication of a final version of this document.
B.1. 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)
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Authors' Addresses
Charles 'Buck' Krasic
Google, Inc
Email: ckrasic@google.com
Mike Bishop
Akamai Technologies
Email: mbishop@evequefou.be
Alan Frindell (editor)
Facebook
Email: afrind@fb.com
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