Network Working Group                                           R. Shade
Internet-Draft                                                 M. Warres
Intended status: Informational                                    Google
Expires: January 9, 2017                                    July 8, 2016

           HTTP/2 Semantics Using The QUIC Transport Protocol


   The QUIC transport protocol has several features that are desirable
   in a transport for HTTP/2, such as stream multiplexing, per-stream
   flow control, and low-latency connection establishment.  This
   document describes a mapping of HTTP/2 semantics over QUIC.
   Specifically, this document identifies HTTP/2 features that are
   subsumed by QUIC, and describes how the other features can be
   implemented atop QUIC.

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 January 9, 2017.

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   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  QUIC advertisement  . . . . . . . . . . . . . . . . . . . . .   2
   3.  Connection establishment  . . . . . . . . . . . . . . . . . .   3
   4.  Sending a request on an HTTP/2-over-QUIC connection . . . . .   4
     4.1.  Terminating a stream  . . . . . . . . . . . . . . . . . .   5
   5.  Writing data to QUIC streams  . . . . . . . . . . . . . . . .   5
   6.  Stream Mapping  . . . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  Reserved Streams  . . . . . . . . . . . . . . . . . . . .   6
       6.1.1.  Stream 3: headers . . . . . . . . . . . . . . . . . .   6
       6.1.2.  Stream states . . . . . . . . . . . . . . . . . . . .   7
   7.  Stream Priorities . . . . . . . . . . . . . . . . . . . . . .   7
   8.  Flow Control  . . . . . . . . . . . . . . . . . . . . . . . .   8
   9.  Server Push . . . . . . . . . . . . . . . . . . . . . . . . .   8
   10. Error Codes . . . . . . . . . . . . . . . . . . . . . . . . .   9
   11. Other HTTP/2 frames . . . . . . . . . . . . . . . . . . . . .  10
     11.1.  GOAWAY frame . . . . . . . . . . . . . . . . . . . . . .  10
     11.2.  PING frame . . . . . . . . . . . . . . . . . . . . . . .  10
     11.3.  PADDING frame  . . . . . . . . . . . . . . . . . . . . .  11
   12. Normative References  . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   The QUIC transport protocol has several features that are desirable
   in a transport for HTTP/2, such as stream multiplexing, per-stream
   flow control, and low-latency connection establishment.  This
   document describes a mapping of HTTP/2 semantics over QUIC.
   Specifically, this document identifies HTTP/2 features that are
   subsumed by QUIC, and describes how the other features can be
   implemented atop QUIC.

   QUIC is described in [draft-hamilton-quic-transport-protocol].  For a
   full description of HTTP/2, see [RFC 7540].

2.  QUIC advertisement

   A server advertises that it can speak HTTP/2-over-QUIC via the Alt-
   Svc HTTP response header.  It does so by including the header in any
   response sent over a non-QUIC (e.g.  HTTP/2 over TLS) connection:

   Alt-Svc: quic=":443"

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   In addition, the list of QUIC versions supported by the server can be
   specified by the v= parameter.  For example, if a server supported
   both version 33 and 34 it would specify the following header:

   Alt-Svc: quic=":443"; v="34,33"

   On receipt of this header, a client may attempt to establish a QUIC
   connection on port 443 and, if successful, send HTTP/2 requests using
   the mapping described in this document.

   Connectivity problems (e.g. firewall blocking UDP) may result in QUIC
   connection establishment failure, in which case the client should
   gracefully fallback to HTTP/2-over-TLS/TCP.

3.  Connection establishment

   HTTP/2-over-QUIC connections are established as described in [draft-
   hamilton-quic-transport-protocol].  The QUIC crypto handshake MUST
   use TLS [draft-thomson-quic-tls].

   While connection-level options pertaining to the core QUIC protocol
   are set in the initial crypto handshake [Combined Crypto and
   Transport Handshake], HTTP/2-specific settings are conveyed in the
   HTTP/2 SETTINGS frame.  After the QUIC connection is established, an
   HTTP/2 SETTINGS frame may be sent as the initial frame of the QUIC
   headers stream (StreamID 3, See [Stream Mapping]).  As in HTTP/2,
   additional SETTINGS frames may be sent mid-connection by either

   TODO: decide whether to acknowledge receipt of SETTINGS through empty
   SETTINGS frames with ACK bit set, as in HTTP/2, or rely on transport-
   level acknowledgment.

   Some transport-level options that HTTP/2-over-TCP specifies via the
   SETTINGS frame are superseded by QUIC transport parameters in HTTP/2-
   over-QUIC.  Below is a listing of how each HTTP/2 SETTINGS parameter
   is mapped:


      *  Sent in HTTP/2 SETTINGS frame.


      *  Sent in HTTP/2 SETTINGS frame [TBD, currently set using QUIC
         "SPSH" connection option]


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      *  QUIC requires the maximum number of incoming streams per
         connection to be specified in the initial crypto handshake,
         using the "MSPC" tag.  Specifying
         an error.


      *  QUIC requires both stream and connection flow control window
         sizes to be specified in the initial crypto handshake, using
         the "SFCW" and "CFCW" tags, respectively.  Specifying


      *  This setting has no equivalent in QUIC.  Specifying it in the
         HTTP/2 SETTINGS frame is an error.


      *  Sent in HTTP/2 SETTINGS frame.

   As with HTTP/2-over-TCP, unknown SETTINGS parameters are tolerated
   but ignored.  SETTINGS parameters are acknowledged by the receiving
   peer, by sending an empty SETTINGS frame in response with the ACK bit

4.  Sending a request on an HTTP/2-over-QUIC connection

   A high level overview of sending an HTTP/2 request on an established
   QUIC connection is as follows, with further details in later sections
   of this document.  A client should first encode any HTTP headers
   using HPACK [RFC7541] and frame them as HTTP/2 HEADERS frames.  These
   are sent on StreamID 3 (see [Stream Mapping]).  The exact layout of
   the HEADERS frame is described in Section 6.2 of [RFC7540].  No
   HTTP/2 padding is required: QUIC provides a PADDING frame for this

   While HEADERS are sent on stream 3, the mandatory stream identifier
   in each HEADERS frame indicates the QUIC StreamID on which a
   corresponding request body may be sent.  If there is no non-header
   data, the specified QUIC data stream will never be used.

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4.1.  Terminating a stream

   A stream can be terminated in one of three ways:

   o  the request/response is headers only, in which case a HEADERS
      frame with the END_STREAM bit set ends the stream specified in the
      HEADERS frame

   o  the request/response has headers and body but no trailing headers,
      in which case the final QUIC STREAM frame will have the FIN bit

   o  the request/response has headers, body, and trailing headers, in
      which case the final QUIC STREAM frame will not have the FIN bit
      set, and the trailing HEADERS frame will have the END_STREAM bit

   (TODO: Describe mapping of HTTP/2 stream state machine to QUIC stream
   state machine.)

5.  Writing data to QUIC streams

   A QUIC stream provides reliable in-order delivery of bytes, within
   that stream.  On the wire, data is framed into QUIC STREAM frames,
   but this framing is invisible to the HTTP/2 layer.  A QUIC receiver
   buffers and orders received STREAM frames, exposing the data
   contained within as a reliable byte stream to the application.

   Bytes written to Stream 3 must be HTTP/2 HEADERS frames (or other
   HTTP/2 non-data frames), whereas bytes written to data streams should
   simply be request or response bodies.  No further framing is required
   by HTTP/2 (i.e. no HTTP/2 DATA frames are used).

   If data arrives on a data stream before the corresponding HEADERS
   have arrived on stream 3, then the data is buffered until the HEADERS

6.  Stream Mapping

   When HTTP/2 headers and data are sent over QUIC, the QUIC layer
   handles most of the stream management.  HTTP/2 StreamIDs are replaced
   by QUIC StreamIDs.  HTTP/2 does not need to do any explicit stream
   framing when using QUIC---data sent over a QUIC stream simply
   consists of HTTP/2 headers or body.  Requests and responses are
   considered complete when the QUIC stream is closed in the
   corresponding direction.

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   Like HTTP/2, QUIC uses odd-numbered StreamIDs for client initiated
   streams, and even-numbered IDs for server initiated (i.e. server
   push) streams.  Unlike HTTP/2 there are a couple of reserved (or
   dedicated) StreamIDs in QUIC.

6.1.  Reserved Streams

   StreamID 1 is reserved for crypto operations (the handshake, crypto
   config updates), and MUST NOT be used for HTTP/2 headers or body, see
   [core protocol doc].  StreamID 3 is reserved for sending and
   receiving HTTP/2 HEADERS frames.  Therefore the first client
   initiated data stream has StreamID 5.

   There are no reserved server initiated StreamIDs, so the first server
   initiated (i.e. server push) stream has an ID of 2, followed by 4,

6.1.1.  Stream 3: headers

   HTTP/2-over-QUIC uses HPACK header compression as described in
   [RFC7541].  HPACK was designed for HTTP/2 with the assumption of in-
   order delivery such as that provided by TCP.  A sequence of encoded
   header blocks must arrive (and be decoded) at an endpoint in the same
   order in which they were encoded.  This ensures that the dynamic
   state at the two endpoints remains in sync.

   QUIC streams provide in-order delivery of data sent on those streams,
   but there are no guarantees about order of delivery between streams.
   To achieve in-order delivery of HEADERS frames in QUIC, they are all
   sent on the reserved Stream 3.  Data (request/response bodies) which
   arrive on other data streams are buffered until the corresponding
   HEADERS arrive and are read out of Stream 3.

   This does introduce head-of-line blocking: if the packet containing
   HEADERS for stream N is lost or reordered then stream N+2 cannot be
   processed until they it has been retransmitted successfully, even
   though the HEADERS for stream N+2 may have arrived.

   Trailing headers (trailers) can also be sent on stream 3.  These are
   sent as HTTP/2 HEADERS frames, but MUST have the END_STREAM bit set,
   and MUST include a ":final-offset" pseudo-header.  Since QUIC
   supports out of order delivery, receipt of a HEADERS frame with the
   END_STREAM bit set does not guarantee that the entire request/
   response body has been fully received.  Therefore, the extra ":final-
   offset" pseudo-header is included in trailing HEADERS frames to
   indicate the total number of body bytes sent on the corresponding
   data stream.  This is used by the QUIC layer to determine when the
   full request has been received and therefore when it is safe to tear

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   down local stream state.  The ":final-offset" pseudo header is
   stripped from the HEADERS before passing to the HTTP/2 layer.

6.1.2.  Stream states

   The mapping of HTTP/2-over-QUIC with potential out of order delivery
   of HEADERS frames results in some changes to the HTTP/2 stream state
   transition diagram [].
   Specifically the transition from "open" to "half closed (remote)",
   and the transition from "half closed (local)" to "closed" takes place
   only when:

   o  the peer has explicitly ended the stream via either

      *  an HTTP/2 HEADERS frame with END_STREAM bit set and, in the
         case of trailing headers, the :final-offset pseudo-header

      *  or a QUIC stream frame with the FIN bit set.

   o  and the full request or response body has been received.

7.  Stream Priorities

   HTTP/2-over-QUIC uses the HTTP/2 priority scheme described in
   [RFC7540 Section 5.3].  In the HTTP/2 priority scheme, a given stream
   can be designated as dependent upon another stream, which expresses
   the preference that the latter stream (the "parent" stream) be
   allocated resources before the former stream (the "dependent"
   stream).  Taken together, the dependencies across all streams in a
   connection form a dependency tree.  The structure of the dependency
   tree changes as HTTP/2 HEADERS and PRIORITY frames add, remove, or
   change the dependency links between streams.

   Implicit in this scheme is the notion of in-order delivery of
   priority changes (i.e., dependency tree mutations): since operations
   on the dependency tree such as reparenting a subtree are not
   commutative, both sender and receiver must apply them in the same
   order to ensure that both sides have a consistent view of the stream
   dependency tree.  HTTP/2 specifies priority assignments in PRIORITY
   frames and (optionally) in HEADERS frames.  To achieve in-order
   delivery of HTTP/2 priority changes in HTTP/2-over-QUIC, HTTP/2
   PRIORITY frames, in addition to HEADERS frames, are also sent on
   reserved stream 3.  The semantics of the Stream Dependency, Weight, E
   flag, and (for HEADERS frames) PRIORITY flag are the same as in

   Since HEADERS and PRIORITY frames are sent on a different stream than
   the STREAM frames for the streams they reference, they may be

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   delivered out-of-order with respect to the STREAM frames.  There is
   no special handling for this--the receiver should simply assign
   resources according to the most recent stream priority information
   that it has received.

   ALTERNATIVE DESIGN: if the core QUIC protocol implements priorities,
   then this document should map the HTTP/2 priorities scheme to that
   provided by the core protocol.  This would likely involve prohibiting
   the sending of HTTP/2 PRIORITY frames and setting of the PRIORITY
   flag in HTTP/2 HEADERS frames, to avoid conflicting directives.

8.  Flow Control

   QUIC provides stream and connection level flow control, similar in
   principle to HTTP/2's flow control but with some implementation
   differences.  As flow control is handled by QUIC, the HTTP/2 mapping
   need not concern itself with maintaining flow control state, or how/
   when to send flow control frames to the peer.  The HTTP/2 mapping
   must not send HTTP/2 WINDOW_UPDATE frames.

   The initial flow control window sizes (stream and connection) are
   communicated during the crypto handshake (see [Connection
   establishment]).  Setting these values to the maximum size (2^31 - 1)
   effectively disables flow control.

   Relatively small initial windows can be used, as QUIC will attempt to
   auto-tune the flow control windows based on usage.  See [draft-
   hamilton-quic-transport-protocol] for more details.

9.  Server Push

   HTTP/2-over-QUIC supports HTTP/2 server push.  During connection
   establishment, the client indicates whether or it is willing to
   receive server pushes via the SETTINGS_ENABLE_PUSH setting in the
   HTTP/2 SETTINGS frame (see [Connection Establishment]), which
   defaults to 1 (true).

   As with server push for HTTP/2-over-TCP, the server initiates a
   server push by sending an HTTP/2 PUSH_PROMISE frame containing the
   StreamID of the stream to be pushed, as well as request header fields
   attributed to the request.  The PUSH_PROMISE frame is sent on stream
   3, to ensure proper ordering with respect to other HEADERS and non-
   data frames.  Within the PUSH_PROMISE frame, the StreamID in the
   common HTTP/2 frame header indicates the associated (client-
   initiated) stream for the new push stream, while the Promised Stream
   ID field specifies the StreamID of the new push stream.

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   The server push response is conveyed in the same way as a non-server-
   push response, with response headers and (if present) trailers
   carried by HTTP/2 HEADERS frames sent on reserved stream 3, and
   response body (if any) sent via QUIC stream frames on the stream
   specified in the corresponding PUSH_PROMISE frame.

10.  Error Codes

   The HTTP/2 error codes defined in [RFC7540 Section 7] map to QUIC
   error codes as follows:

   o  NO_ERROR (0x0)

      *  Maps to QUIC_NO_ERROR

   o  PROTOCOL_ERROR (0x1)

      *  No single mapping?

   o  INTERNAL_ERROR (0x2)

      *  QUIC_INTERNAL_ERROR? (not currently defined in core protocol


         defined in core protocol spec)


      *  ? (depends on whether we support SETTINGS acks)

   o  STREAM_CLOSED (0x5)


   o  FRAME_SIZE_ERROR (0x6)


   o  REFUSED_STREAM (0x7)

      *  ?

   o  CANCEL (0x8)

      *  ?

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      *  QUIC_DECOMPRESSION_FAILURE (not currently defined in core spec)

   o  CONNECT_ERROR (0xa)

      *  ? (depends whether we decide to support CONNECT)


      *  ?



   o  HTTP_1_1_REQUIRED (0xd)

   TODO: fill in missing error code mappings.

11.  Other HTTP/2 frames

   QUIC includes some features (e.g. flow control) which are also
   present in HTTP/2.  In these cases the HTTP/2 mapping need not re-
   implement them.  As a result some HTTP/2 frame types are not required
   when using QUIC, as they either are directly implemented in the QUIC
   layer, or their functionality is provided via other means.  This
   section of the document describes these cases.

11.1.  GOAWAY frame

   QUIC has its own GOAWAY frame, and QUIC implementations may to expose
   the sending of a GOAWAY to the application.  The semantics of sending
   a GOAWAY in QUIC are identical to HTTP/2: an endpoint sending a
   GOAWAY will continue processing open streams, but will not accept
   newly created streams.

   QUIC's GOAWAY frame is described in detail in the [draft-hamilton-

11.2.  PING frame

   QUIC has its own PING frame, which is currently exposed to the
   application.  QUIC clients send periodic PINGs to servers if there
   are no currently active data streams on the connection.

   QUIC's PING frame is described in detail in the [draft-hamilton-quic-

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11.3.  PADDING frame

   There is no HTTP/2 padding in this mapping; padding is instead
   provided at the QUIC layer by including QUIC PADDING frames in a
   packet payload.  An HTTP/2 over QUIC mapping should treat any HTTP/2
   level padding as an error, to avoid any possibility of inconsistent
   flow control states between endpoints (e.g. client sends HTTP/2
   padding, counts it against flow control, server ignores).

12.  Normative References

   [RFC2119]  Bradner, S., "Key Words for use in RFCs to Indicate
              Requirement Levels", March 1997.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, "Hypertext Transfer
              Protocol Version 2 (HTTP/2)", May 2015.

   [RFC7541]  Peon, R. and H. Ruellan, "HPACK: Header Compression for
              HTTP/2", May 2015.

              Hamilton, R., Iyengar, J., Swett, I., and A. Wilk, "QUIC:
              A UDP-Based Multiplexed and Secure Transport", July 2016.

              Thomson, M. and R. Hamilton, "Porting QUIC to TLS", March

              Iyengar, J. and I. Swett, "QUIC Loss Recovery and
              Congestion Control", July 2016.

Authors' Addresses

   Robbie Shade


   Mike Warres


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