Header Compression for HTTP over QUIC
draft-krasic-quic-qcram-03
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| Author | Charles 'Buck' Krasic | ||
| Last updated | 2018-01-03 | ||
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draft-krasic-quic-qcram-03
QUIC C. Krasic
Internet-Draft Google
Intended status: Standards Track January 3, 2018
Expires: July 7, 2018
Header Compression for HTTP over QUIC
draft-krasic-quic-qcram-03
Abstract
The design of the core QUIC transport and the mapping of HTTP
semantics over it subsume many HTTP/2 features, prominent among them
stream multiplexing and HTTP header compression. A key advantage of
the QUIC transport is it provides stream multiplexing free of HoL
blocking between streams, while in HTTP/2 multiplexed streams can
suffer HoL blocking primarily due to HTTP/2's layering above TCP.
However if HPACK is used for header compression, HTTP over QUIC is
still vulnerable to HoL blocking, because of how HPACK exploits
header redundancies between multiplexed HTTP transactions. This
draft defines QCRAM, a variation of HPACK and mechanisms in the QUIC
HTTP mapping that allow QUIC implementations the flexibility to avoid
header-compression induced HoL blocking.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on July 7, 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. QCRAM overview . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Example of HoL blocking . . . . . . . . . . . . . . . . . 3
2.2. How QCRAM minimizes HoL blocking . . . . . . . . . . . . 3
3. HTTP over QUIC mapping extensions . . . . . . . . . . . . . . 4
3.1. HEADERS and PUSH_PROMISE . . . . . . . . . . . . . . . . 4
3.2. HEADERS_ACK . . . . . . . . . . . . . . . . . . . . . . . 4
4. HPACK extensions . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Header Block Prefix . . . . . . . . . . . . . . . . . . . 5
4.2. Hybrid absolute-relative indexing . . . . . . . . . . . . 6
4.3. Preventing Eviction Races . . . . . . . . . . . . . . . . 6
4.3.1. Blocked Evictions . . . . . . . . . . . . . . . . . . 6
4.4. Handling Stream Resets . . . . . . . . . . . . . . . . . 7
4.5. Refreshing Entries with Duplication . . . . . . . . . . . 7
4.5.1. Mandatory Entry De-duplication . . . . . . . . . . . 7
5. Performance considerations . . . . . . . . . . . . . . . . . 8
5.1. Speculative table updates . . . . . . . . . . . . . . . . 8
5.2. Fixed overhead. . . . . . . . . . . . . . . . . . . . . . 8
5.3. Co-ordinated Packetization . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
The QUIC transport protocol was designed from the outset to support
HTTP semantics, and its design subsumes most of the features of
HTTP/2. Two of those features, stream multiplexing and header
compression come into some conflict in QUIC. A key goal of the
design of QUIC is to improve stream multiplexing relative to HTTP/2,
by eliminating HoL (head of line) blocking that can occur in HTTP/2.
HoL blocking can happen because 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
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functionality in a fully stream-aware manner. However, the HTTP over
QUIC mapping is still subject to HoL blocking if HPACK is used
directly as in HTTP/2. 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 as will be described
more below (Section 2.1).
QUIC is described in [QUIC-TRANSPORT]. The HTTP over QUIC mapping is
described in [QUIC-HTTP]. For a full description of HTTP/2, see
[RFC7540]. The description of HPACK is [RFC7541].
2. QCRAM overview
Readers may wish to refer to [RFC7541] Section 1.3 to review HPACK
terminology, and [QUIC-HTTP], Sections 4 on "HTTP over QUIC stream
mapping" and 4.2.1 on "Header Compression". QCRAM extensions to
HPACK allow correctness in the presence of out-of-order delivery,
with flexibility to balance between resilience against HoL blocking
and compression ratio.
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 over QUIC.
2.1. Example of HoL blocking
The following is an example of how HPACK can induce HoL blocking in
QUIC. Assume two HTTP message exchange streams "A" and "B", and
corresponding header blocks "HA" and "HB". Stream "B" experiences
HoL blocking due to "A" as follows:
1. HPACK encodes header field "HB[i]" using an index that refers to
a table entry that resulted from header field "HA[j]".
2. "HA" and "HB" are delivered via distinct packets that are
inflight in the same round trip.
3. "HB"'s packet is delivered but "HA"'s is dropped. HPACK can not
decode "HB" until "HA"'s packet is successfully retransmitted.
2.2. How QCRAM minimizes HoL blocking
Continuing the example, QCRAM's approach is as follows.
1. "HB[i]" will not introduce HoL blocking if "HA" has been
acknowledged, otherwise it is vulnerable. A new HQ frame type
HEADERS_ACK is defined (see Section 3). When the decoder has
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processed a header block, HEADERS_ACK is sent from the decoder
back to the encoder.
The encoder can choose on a per header block basis whether to favor
higher compression ratio or HoL resilience, signaled by the BLOCKING
flag in HEADERS and PUSH_PROMISE frames (see Section 3).
If HB contains no vulnerable header fields, BLOCKING MUST be 0.
If BLOCKING is not set, then for each "HB[i]" that is vulnerable:
1. "HB[i]" is represented with one of the Literal variants (see
[RFC7541] Section 6.2), trading lower compression ratio for HoL
resilience.
If BLOCKING is set then HB is encoded in blocking mode:
1. "HB[i]" is represented with an Indexed Representation. This
favors compression ratio.
In blocking mode, after reading HB's prefix stream B might block.
Stream B proceeds with reading and processing the rest of HB only
once all HB's dependencies are satisfied. The header prefix contains
table offset information that establishes total ordering among all
headers, regardless of reordering in the transport (see Section 4.1).
In blocking mode, the prefix additionally identifies the largest
(absolute) index I that HB depends on (see "Depends" in
Section Section 4.2). HB's dependencies are satisfied when all
entries less than or equal to I have been inserted into the table.
Notice that while blocked, HB's header field data remains in stream
B's flow control window.
3. HTTP over QUIC mapping extensions
3.1. HEADERS and PUSH_PROMISE
HEADER 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 header can always be processed immediately
upon receipt.
3.2. HEADERS_ACK
The HEADERS_ACK frame (type=0x8) is sent by the decoder side to the
encoder when a 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
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blocking. The HEADERS_ACK frame does not define any flags, and has
no payload.
4. HPACK extensions
4.1. Header Block Prefix
In HEADERS and PUSH_PROMISE frames, HPACK Header data are prefixed by
a pair of integers pair of integers: "Fill" and the "Evictions".
"Fill" is the number of entries in the table, and "Evictions" is the
cumulative number entries that have been evicted from the table.
Their sum is the cumulative number of entries inserted before the
following header block was encoded. Each is encoded as a single
HPACK integer (8-bit prefix):
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|Fill (8+)|
+---------------+
|Evictions (8+)|
+---------------+
Figure 1: Absolute indexing (BLOCKING=0x0)
Section 4.2 describes the role of "Fill" and Section 4.3 covers the
role of "Evictions".
When BLOCKING flag is 0x1, a the prefix additionally contains a third
HPACK integer (8-bit prefix) 'Depends':
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|Fill (8+)|
+---------------+
|Evictions (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 2.2. The
largest entry index is "Evictions + Fill - Depends".
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4.2. Hybrid absolute-relative indexing
HPACK indexed entries refer to an entry by its current position in
the dynamic table. As Figure 1 of RFC7541 [1] illustrates, newest
entries have smallest indices, and oldest 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 it is is problematic in the out-of-order context
of QUIC.
QCRAM uses a hybrid absolute-relative indexing approach. The prefix
defined in Section 4.1 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 stream 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.
4.3. 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 is already
evicted. QCRAM employs a two-phase eviction algorithm, in which the
encoder will not evict entries that have outstanding (unacknowledged)
references. The QCRAM encoder maintains a counter as entries are
evicted, which is the cumulative number of evictions so far,
"Evictions" (Section 4.1). On arrival at the decoder, if "Evictions"
is higher than previously seen, the decoder MUST evict all entries at
or below. Unlike HPACK where the decoder follows the same logic as
the encoder to perform evictions, in QCRAM the decoder evicts
exclusively based on the encoder's explicit guidance.
4.3.1. Blocked Evictions
In some cases, the encoder must forgo eviction by selecting a literal
representation (blocked eviction), namely in the event that the entry
subject to eviction _is_ referenced by one or more unacknowledged
header frames. To assure that the blocked eviction case is rare, a
form of thresholding MAY be applied that constrains selection of
Indexed representations, such that the oldest entries in the dynamic
table will largely be evictable. The constraint is applied when
encoding header fields: comparing the cumulative position (in bytes)
of the matching entry to a threshold, categorizing oldest entries
(past threshold) as at-risk. Avoiding references to at-risk entries,
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the encoder SHOULD use an Indexed-Duplicate representation instead
(see Section 4.5).
4.4. Handling Stream Resets
The QCRAM encoder has the option to select representations that might
require blocking (Section 2.2 case 3), but the decoder must be
prevented from becoming hung if the stream associated with the
referenced entry is reset. On stream reset, the QCRAM encoder MUST
check if the stream has unacknowledged headers, and if so resend them
on the Control Stream ([QUIC-HTTP] Section 4.1). If header blocks
are resent on the control stream, duplicate arrivals are possible due
to reset-acknowledgment races. The decoder MUST ignore duplicate
header block arrivals, which is straightforward because of
unambiguous indexing (see Section 4.2).
4.5. 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_ are treated as an Indexed Header Field
Representation (see [RFC7541] Section 6.1), additionally inserting a
new duplicate entry. [RFC7541] allows duplicate HPACK table entries,
that is entries that have the same name and value.
_Figure 2 annexes the representation for HPACK Dynamic Table Size
Update (see Section 6.3 of RFC7541), which is not supported by HTTP
over QUIC._
4.5.1. Mandatory Entry De-duplication
To help mitigate memory consumption due to duplicate entries, HPACK
for QCRAM is required to de-duplicate strings in the dynamic table.
The table insertion logic should check if the new entry matches any
existing entries (name and value), and if so, table accounting MUST
charge only the overhead portion ([RFC7541] Section 4.1) to the new
entry.
Specific de-duplication mechanisms are left to implementations, but
using a map in conjunction with reference counted pointers to strings
would be typical.
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5. Performance considerations
5.1. Speculative table updates
Implementations can _speculatively_ send header frames on the HTTP
Connection Control Stream. Such headers would not be associated with
any HTTP transaction, but 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 4.1), ensuring they have small indices and hence minimal
size on the wire.
5.2. 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, and requires mechanisms to de-duplicate
strings. A larger value than 32 might be more accurate for QCRAM.
5.3. Co-ordinated Packetization
In Section 2.2, an exception exists when the representation of
"HA[i]" and "HB[j]" are delivered within the same transport packet.
If so, there is no risk of HoL blocking and using an indexed
representation is strictly better than using a literal. An
implementation could exploit this exception by employing co-
ordination between QCRAM compression and QUIC transport
packetization.
6. Security Considerations
TBD.
7. IANA Considerations
This document currently makes no request of IANA, and might not need
to.
8. 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
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o Biren Roy
o Alan Frindell
o Ian Swett
o Ryan Hamilton
9. References
9.1. Normative References
[QUIC-HTTP]
Bishop, M., Ed., "Hypertext Transfer Protocol (HTTP) over
QUIC", January 2018.
[QUIC-TRANSPORT]
Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", January 2018.
[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>.
[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>.
9.2. URIs
[1] https://tools.ietf.org/html/rfc7541#section-2.3.3
Author's Address
Charles 'Buck' Krasic
Google
Email: ckrasic@google.com
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