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Versions: 00 01 02 03 04 draft-ietf-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

   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 July 27, 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



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   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  . . . . . . . . . . . . . . . . . . . . . . . .   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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