[Docs] [txt|pdf|xml|html] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02

QUIC                                                           C. Krasic
Internet-Draft                                                    Google
Intended status: Standards Track                          August 3, 2017
Expires: February 4, 2018


                 Header Compression for HTTP over QUIC
                       draft-krasic-quic-qcram-02

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.

   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 http://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 February 4, 2018.

Copyright Notice

   Copyright (c) 2017 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



Krasic                  Expires February 4, 2018                [Page 1]


Internet-Draft                    QCRAM                      August 2017


   (http://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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  QCRAM overview  . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Example of HoL blocking . . . . . . . . . . . . . . . . .   3
     2.2.  How QCRAM minimizes HoL blocking  . . . . . . . . . . . .   3
   3.  HPACK extensions  . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Header Block Prefix . . . . . . . . . . . . . . . . . . .   4
     3.2.  Hybrid absolute-relative indexing . . . . . . . . . . . .   4
     3.3.  Preventing Eviction Races . . . . . . . . . . . . . . . .   5
       3.3.1.  Blocked Evictions . . . . . . . . . . . . . . . . . .   5
     3.4.  Handling Stream Resets  . . . . . . . . . . . . . . . . .   5
     3.5.  Refreshing Entries with Duplication . . . . . . . . . . .   6
       3.5.1.  Mandatory Entry De-duplication  . . . . . . . . . . .   6
   4.  Performance considerations  . . . . . . . . . . . . . . . . .   6
     4.1.  Speculative table updates . . . . . . . . . . . . . . . .   6
     4.2.  Fixed overhead. . . . . . . . . . . . . . . . . . . . . .   7
     4.3.  Co-ordinated Packetization  . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     8.2.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

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
   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



Krasic                  Expires February 4, 2018                [Page 2]


Internet-Draft                    QCRAM                      August 2017


   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[j]" was delivered
       in a prior round trip.  To identify this case, QCRAM assumes that
       QUIC transport surfaces acknowledgment notifications to the HTTP
       layer, and that the QCRAM encoder can rely that acknowledged
       headers have been received by the decoder.




Krasic                  Expires February 4, 2018                [Page 3]


Internet-Draft                    QCRAM                      August 2017


   2.  "HB[i]" may be represented with one of the Literal variants (see
       [RFC7541] Section 6.2), trading lower compression ratio for HoL
       resilience.

   3.  "HB[i]" may be represented with an Indexed Representation.  This
       favors compression ratio, but the decoder MUST ensure that HB is
       not decoded until after HA (see blocking in Section 3.2)).

3.  HPACK extensions

3.1.  Header Block Prefix

   In HEADERS and PUSH_PROMISE frames, HPACK Header data should be
   prefixed by a 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.  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

   Section 3.2 describes the role of "Fill" and Section 3.3 covers the
   role of "Evictions".

3.2.  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 3.1 is used by the decoder to interpret all
   subsequent HPACK instructions at absolute positions for indexed
   lookups and insertions.  It is also used for evictions (Section 3.3).





Krasic                  Expires February 4, 2018                [Page 4]


Internet-Draft                    QCRAM                      August 2017


   As was defined in Section 2.2 case 3, the encoder has the option to
   select indexed representations that are vulnerable to HoL blocking.
   Decoder processing of indexed header fields MUST block the
   encompassing header block if the referenced entry has not been added
   to the table yet.

   To protect against buggy or malicious peers, a timer should be used
   to set an upper bound on such blocking and in treat expiration of the
   timer as a decoding error.  However, if the implementation chooses
   not to abort the connection, the remainder of the header block MUST
   be decoded and output discarded.

3.3.  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 3.1).  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.

3.3.1.  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,
   the encoder SHOULD use an Indexed-Duplicate representation instead
   (see Section 3.5).

3.4.  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



Krasic                  Expires February 4, 2018                [Page 5]


Internet-Draft                    QCRAM                      August 2017


   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 3.2).

3.5.  Refreshing Entries with Duplication

       0 1 2 3 4 5 6 7
      +-+-+-+-+-+-+-+-+
      |0|0|1|Index(5+)|
      +-+-+-+---------+

              Figure 2: 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._

3.5.1.  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.

4.  Performance considerations

4.1.  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




Krasic                  Expires February 4, 2018                [Page 6]


Internet-Draft                    QCRAM                      August 2017


   (Section 3.1), ensuring they have small indices and hence minimal
   size on the wire.

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 rank, and requires mechanisms to de-duplicate strings.  A
   larger value than 32 might be more accurate for QCRAM.

4.3.  Co-ordinated Packetization

   In Section 2.2 case 3, 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.

5.  Security Considerations

   TBD.

6.  IANA Considerations

   This document currently makes no request of IANA, and might not need
   to.

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





Krasic                  Expires February 4, 2018                [Page 7]


Internet-Draft                    QCRAM                      August 2017


8.  References

8.1.  Normative References

   [QUIC-HTTP]
              Bishop, M., Ed., "Hypertext Transfer Protocol (HTTP) over
              QUIC", August 2017.

   [QUIC-TRANSPORT]
              Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", August 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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc7541>.

8.2.  URIs

   [1] https://tools.ietf.org/html/rfc7541#section-2.3.3

Author's Address

   Charles 'Buck' Krasic
   Google

   Email: ckrasic@google.com




















Krasic                  Expires February 4, 2018                [Page 8]


Html markup produced by rfcmarkup 1.124, available from https://tools.ietf.org/tools/rfcmarkup/