Internet DRAFT - draft-krasic-quic-qcram
draft-krasic-quic-qcram
QUIC C. Krasic
Internet-Draft Google
Intended status: Standards Track January 23, 2018
Expires: July 27, 2018
Header Compression for HTTP over QUIC
draft-krasic-quic-qcram-04
Abstract
The design of the core QUIC transport subsumes many HTTP/2 features,
prominent among them stream multiplexing. A key advantage of the
QUIC transport is stream multiplexing free of head-of-line (HoL)
blocking between streams. In HTTP/2, multiplexed streams can suffer
HoL blocking due to TCP.
If HTTP/2's HPACK is used for header compression, HTTP/QUIC is still
vulnerable to HoL blocking, because of HPACK's assumption of in-order
delivery. This draft defines QCRAM, a variation of HPACK and
mechanisms in the HTTP/QUIC mapping that allow the flexibility to
avoid header-compression-induced HoL blocking.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on July 27, 2018.
Copyright Notice
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Head-of-Line Blocking in HPACK . . . . . . . . . . . . . 3
1.2. Avoiding Head-of-Line Blocking in HTTP/QUIC . . . . . . . 3
2. HTTP over QUIC mapping extensions . . . . . . . . . . . . . . 4
2.1. HEADERS and PUSH_PROMISE . . . . . . . . . . . . . . . . 4
2.2. HEADER_ACK . . . . . . . . . . . . . . . . . . . . . . . 4
3. HPACK extensions . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Allowed Instructions . . . . . . . . . . . . . . . . . . 5
3.2. Header Block Prefix . . . . . . . . . . . . . . . . . . . 5
3.3. Hybrid absolute-relative indexing . . . . . . . . . . . . 6
3.4. Preventing Eviction Races . . . . . . . . . . . . . . . . 6
3.4.1. Blocked Evictions . . . . . . . . . . . . . . . . . . 7
3.5. Refreshing Entries with Duplication . . . . . . . . . . . 7
4. Performance considerations . . . . . . . . . . . . . . . . . 7
4.1. Speculative table updates . . . . . . . . . . . . . . . . 7
4.2. Fixed overhead. . . . . . . . . . . . . . . . . . . . . . 8
4.3. Co-ordinated Packetization . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
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
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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.
QCRAM 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.
QCRAM 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.
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
In the example above, the second stream contained a reference to data
which might not yet have been processed by the recipient. Such
references are called "vulnerable," because the loss of a different
packet can keep the reference from being usable.
The encoder can choose on a per-header-block basis whether to favor
higher compression ratio (by permitting vulnerable references) or HoL
resilience (by avoiding them). This is signaled by the BLOCKING flag
in HEADERS and PUSH_PROMISE frames (see Section 2).
If a header block contains no vulnerable header fields, BLOCKING MUST
be 0. This implies that the header fields are represented either as
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references to dynamic table entries which are known to have been
received, or as Literal header fields (see [RFC7541] Section 6.2).
If a header block contains any header field which references dynamic
table state which the peer might not have received yet, the BLOCKING
flag MUST be set. If the peer does not yet have the appropriate
state, such blocks might not be processed on arrival.
The header block contains a prefix (Section 3.2). This prefix
contains table offset information that establishes total ordering
among all headers, regardless of reordering in the transport (see
Section 3.3). In blocking mode, the prefix additionally identifies
the minimum state required to process any vulnerable references in
the header block (see "Depends" in Section Section 3.3). When the
necessary state has arrived, the header block can be processed.
Notice that while blocked, HB's header field data remains in stream
B's flow control window.
2. HTTP over QUIC mapping extensions
2.1. HEADERS and PUSH_PROMISE
HEADERS and PUSH_PROMISE frames define a new flag.
BLOCKING (0x01): Indicates the stream might need to wait for
dependent headers before processing. If 0, the frame can be
processed immediately upon receipt.
HEADERS frames can be sent on the Connection Control Stream as well
as on request / push streams.
2.2. HEADER_ACK
The HEADER_ACK frame (type=0x8) is sent from the decoder to the
encoder on the Control Stream when the decoder has fully processed a
header block. It is used by the encoder to determine whether
subsequent indexed representations that might reference that block
are vulnerable to HoL blocking.
The HEADER_ACK frame indicates the stream on which the header block
was processed by encoding the Stream ID as a variable-length integer.
The same Stream ID can be identified multiple times, as multiple
header-containing blocks can be sent on a single stream in the case
of intermediate responses, trailers, pushed requests, etc. as well as
on the Control Streams.
Since header frames on each stream are received and processed in
order, this gives the encoder precise feedback on which header blocks
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within a stream have been fully processed. This information can then
be used to correctly track outstanding stream references to
checkpoints.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Stream ID [i] |
+---+---------------------------+
HEADER_ACK frame
The HEADER_ACK frame does not define any flags.
3. HPACK extensions
3.1. Allowed Instructions
HEADERS frames on the Control Streams SHOULD contain only Literal
with Incremental Indexing representations. Frames on this stream
modify the dynamic table state without generating output to any
particular request.
HEADERS and PUSH_PROMISE frames on request and push streams MUST NOT
contain Literal with Incremental Indexing representations. Frames on
these streams reference the dynamic table in a particular state
without modifying it, but emit the headers for an HTTP request or
response.
3.2. Header Block Prefix
In HEADERS and PUSH_PROMISE frames, HPACK Header data is prefixed by
an integer: "Base Index". "Base index" is the cumulative number of
entries added to the table prior to encoding the current block, it is
encoded as a single 8-bit prefix integer:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|Base Index (8+)|
+---------------+
Figure 1: Absolute indexing (BLOCKING=0x0)
Section 3.3 describes the role of "Base Index".
When the BLOCKING flag is 0x1, a the prefix additionally contains a
second HPACK integer (8-bit prefix) 'Depends':
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0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|Base Index (8+)|
+---------------+
|Depends (8+)|
+---------------+
Figure 2: Absolute indexing (BLOCKING=0x1)
Depends is used to identify header dependencies, namely the largest
table entry referred to by indexed representations within the
following header block. Its usage is described in Section 1.2. The
largest index referenced is "Base Index - Depends".
3.3. Hybrid absolute-relative indexing
HPACK indexed entries refer to an entry by its current position in
the dynamic table. As Figure 1 of [RFC7541] illustrates, newer
entries have smaller indices, and older entries are evicted first if
the table is full. Under this scheme, each insertion to the table
causes the index of all existing entries to change (implicitly).
Implicit index updates are acceptable for HTTP/2 because TCP is
totally ordered, but are problematic in the out-of-order context of
QUIC.
QCRAM uses a hybrid absolute-relative indexing approach. The prefix
defined in Section 3.2 is used by the decoder to interpret all
subsequent HPACK instructions at absolute positions for indexed
lookups and insertions.
Since QCRAM handles blocking at the header block level, it is an
error if the HPACK decoder encounters an indexed representation that
refers to an entry missing from the table, and the connection MUST be
closed with the "HTTP_HPACK_DECOMPRESSION_FAILED" error code.
3.4. Preventing Eviction Races
Due to out-of-order arrival, QCRAM'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. QCRAM employs a two-phase eviction algorithm, in which the
encoder will not evict entries that have outstanding (unacknowledged)
references.
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3.4.1. Blocked Evictions
The decoder 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 Indexed-Duplicate
representation instead (see Section 3.5).
3.5. Refreshing Entries with Duplication
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|0|0|1|Index(5+)|
+-+-+-+---------+
Figure 3: Indexed Header Field with Duplication
_Indexed-Duplicates_ insert a new entry into the dynamic table which
duplicates an existing entry. [RFC7541] allows duplicate HPACK table
entries, that is entries that have the same name and value.
This replaces the HPACK instruction for Dynamic Table Size Update
(see Section 6.3 of [RFC7541], which is not supported by HTTP over
QUIC.
4. Performance considerations
4.1. 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 Indexed-Duplicate representations for popular header fields
(Section 3.2), ensuring they have small indices and hence minimal
size on the wire.
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4.2. Fixed overhead.
HPACK defines overhead as 32 bytes ([RFC7541] Section 4.1). QCRAM
adds some per-entry state, to track acknowledgment status and
eviction reference count. A larger value than 32 might be more
accurate for QCRAM.
4.3. Co-ordinated Packetization
When a dynamic table entry is both defined and referenced by header
blocks within the same packet, there is no risk of HoL blocking and
using an indexed representation is strictly better than using a
literal. An implementation could attempt to exploit this exception
by employing co-ordination between QCRAM compression and QUIC
transport packetization. However, if the packet is lost, the
transport might choose a different packetization when retransmitting
the missing data.
5. Security Considerations
TBD.
6. IANA Considerations
This document registers a new frame type, HEADER_ACK, for HTTP/QUIC.
This will need to be added to the IANA Considerations of [QUIC-HTTP].
7. 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 Mike Bishop
o Patrick McManus
o Biren Roy
o Alan Frindell
o Ian Swett
o Ryan Hamilton
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8. References
8.1. Normative References
[QUIC-HTTP]
Bishop, M., "Hypertext Transfer Protocol (HTTP) over
QUIC", draft-ietf-quic-http-08 (work in progress),
December 2017.
[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>.
8.2. Informative References
[QUIC-TRANSPORT]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-08 (work
in progress), December 2017.
[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>.
Author's Address
Charles 'Buck' Krasic
Google
Email: ckrasic@google.com
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