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

Versions: (draft-krasic-quic-qcram) 00 draft-ietf-quic-qpack

QUIC                                                           C. Krasic
Internet-Draft                                               Google, Inc
Intended status: Standards Track                               M. Bishop
Expires: August 24, 2018                             Akamai Technologies
                                                        A. Frindell, Ed.
                                                                Facebook
                                                       February 20, 2018


                 Header Compression for HTTP over QUIC
                        draft-ietf-quic-qcram-00

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 August 24, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




Krasic, et al.           Expires August 24, 2018                [Page 1]


Internet-Draft                    QCRAM                    February 2018


   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
   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  . . . . . . . . . . . . . . . . . . . . . . .   5
   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 . . . . . . . . . . . . . . . .   7
       3.4.1.  Blocked Evictions . . . . . . . . . . . . . . . . . .   7
     3.5.  Refreshing Entries with Duplication . . . . . . . . . . .   8
   4.  Performance considerations  . . . . . . . . . . . . . . . . .   8
     4.1.  Speculative table updates . . . . . . . . . . . . . . . .   8
     4.2.  Additional state beyond HPACK.  . . . . . . . . . . . . .   8
       4.2.1.  Vulnerable Entries  . . . . . . . . . . . . . . . . .   8
       4.2.2.  Safe evictions  . . . . . . . . . . . . . . . . . . .   9
       4.2.3.  Decoder Blocking  . . . . . . . . . . . . . . . . . .   9
       4.2.4.  Fixed overhead. . . . . . . . . . . . . . . . . . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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



Krasic, et al.           Expires August 24, 2018                [Page 2]


Internet-Draft                    QCRAM                    February 2018


   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.

   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.





Krasic, et al.           Expires August 24, 2018                [Page 3]


Internet-Draft                    QCRAM                    February 2018


   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
   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 Index" in Section 3.3).  The decoder keeps track
   of which entries have been added to its dynamic table.  The stream
   for a header with BLOCKING flag set is considered blocked by the
   decoder and can not be processed until all entries in the range "[1,
   Depends Index]" have been added.  While blocked, header field data
   MUST remain in the blocked stream'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.  The value of BLOCKING MUST be 0 for
   HEADERS frames on the Connection Control Stream, since they can only
   depend on previous HEADERS on the same stream.








Krasic, et al.           Expires August 24, 2018                [Page 4]


Internet-Draft                    QCRAM                    February 2018


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, and to prevent eviction races (see
   Section 3.4).

   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 within a stream have been fully
   processed.

     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 Stream SHOULD contain only Literal with
   Incremental Indexing and Indexed with Duplication (see Section 3.5)
   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 and Indexed with
   Duplication 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

   For request and push promise streams, 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 dynamic



Krasic, et al.           Expires August 24, 2018                [Page 5]


Internet-Draft                    QCRAM                    February 2018


   table prior to encoding the current block, including any entries
   already evicted.  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':

       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 (see Section 1.2).
   The encoder computes a value "Depends Index" which is the largest
   (absolute) index referenced by the following header block.  To help
   keep the prefix smaller, "Depends Index" is converted to a relative
   value: "Depends = Base Index - Depends Index".

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.

   When the encoder adds a new entry to its header table, it can compute
   an absolute index:

   "entry.absoluteIndex = baseIndex++; "




Krasic, et al.           Expires August 24, 2018                [Page 6]


Internet-Draft                    QCRAM                    February 2018


   Since literals with indexing are only sent on the control stream, the
   decoder can be guaranteed to compute the same absolute index values
   when it adds corresponding entries to its table, just as in HPACK and
   HTTP/2.

   When encoding indexed representations, the following holds for
   (relative) HPACK indices:

   "relative index = baseIndex - entry.absoluteIndex + staticTable.size"

   Header blocks on request and push streams do not modify the dynamic
   table state, so they never change the "baseIndex".  However, since
   ordering between streams is not guaranteed, the value of "baseIndex"
   can not be synchronized implicitly.  Instead then, QCRAM sends
   encoder's "Base Index" explicitly as part of the prefix (see
   Section 3.2), so that the decoder can compute the same absolute
   indices that the encoder used:

   "absoluteIndex = prefix.baseIndex + staticTable.size -
   relativeIndex;"

   In this way, even if request or push stream headers are decoded in a
   different order than encoded, the absolute indices will still
   identify the correct table entries.

   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.

3.4.1.  Blocked Evictions

   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.





Krasic, et al.           Expires August 24, 2018                [Page 7]


Internet-Draft                    QCRAM                    February 2018


   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.

4.2.  Additional state beyond HPACK.

4.2.1.  Vulnerable Entries

   For header blocks encoded in non-blocking mode, the encoder needs to
   forego indexed representations that refer to vulnerable entries (see
   Section 1.2).  An implementation could extend the header table entry
   with a boolean to track vulnerability.  However, the number of
   entries in the table that are vulnerable is likely to be small in



Krasic, et al.           Expires August 24, 2018                [Page 8]


Internet-Draft                    QCRAM                    February 2018


   practice, much less than the total number of entries, so a data
   tracking only vulnerable (un-acknowledged) entries, separate from the
   main header table, might be more space efficient.

4.2.2.  Safe evictions

   Section Section 3.4 describes how QCRAM avoids invalid references
   that might result from out-of-order delivery.  When the encoder
   processes a HEADER_ACK, it dereferences table entries that were
   indexed in the acknowledged header.  To track which entries must be
   dereferenced, it can maintain a map from unacknowledged headers to
   lists of (absolute) indices.  The simplest place to store the actual
   reference count might be the table entries.  In practice the number
   of entries in the table with a non-zero reference count is likely to
   stay quite small.  A data structure tracking only entries with non-
   zero reference counts, separate from the main header table, could be
   more space efficient.

4.2.3.  Decoder Blocking

   To support blocking, the decoder needs to keep track of entries it
   has added to the dynamic table (see Section 1.2), and it needs to
   track blocked streams.

   Tracking added entries might be done in a brute force fashion without
   additional space.  However, this would have O(N) cost where N is the
   number of entries in the dynamic table.  Alternatively, a dedicated
   data structure might improve on brute force in exchange a small
   amount of additional space.  For example, a set of pairs (of
   indices), representing non-overlapping sub-ranges can be used.  Each
   operation (add, or query) can be done within O(log M) complexity.
   Here set size M is the number of sub-ranges.  In practice M would be
   very small, as most table entries would be concentrated in the first
   sub-range [1, M].

   To track blocked streams, an ordered map (e.g. multi-map) from
   "Depends Index" values to streams can be used.  Whenever the decoder
   processes a header block, it can drain any members of the blocked
   streams map that have "Depends Index <= M" where "[1,M]" is the first
   member of the added- entries sub-ranges set.  Again, the complexity
   of operations would be at most O(log N), N being the number of
   concurrently blocked streams.

4.2.4.  Fixed overhead.

   HPACK defines overhead as 32 bytes ([RFC7541] Section 4.1).  As
   described above, QCRAM adds some per-connection state, and possibly
   some per-entry state to track acknowledgment status and eviction



Krasic, et al.           Expires August 24, 2018                [Page 9]


Internet-Draft                    QCRAM                    February 2018


   reference count.  A larger value than 32 might be more accurate for
   QCRAM.

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  Alan Frindell

   o  Ryan Hamilton

   o  Patrick McManus

   o  Kazuho Oku

   o  Biren Roy

   o  Ian Swett

   o  Dmitri Tikhonov

8.  References

8.1.  Normative References

   [QUIC-HTTP]
              Bishop, M., "Hypertext Transfer Protocol (HTTP) over
              QUIC", draft-ietf-quic-http-09 (work in progress), January
              2018.

   [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>.





Krasic, et al.           Expires August 24, 2018               [Page 10]


Internet-Draft                    QCRAM                    February 2018


8.2.  Informative References

   [QUIC-TRANSPORT]
              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-09 (work
              in progress), 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>.

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





















Krasic, et al.           Expires August 24, 2018               [Page 11]


Html markup produced by rfcmarkup 1.127, available from https://tools.ietf.org/tools/rfcmarkup/