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httpbis Working Group                                             G. Xie
Internet-Draft                                               F. Frindell
Intended status: Standards Track                           Facebook Inc.
Expires: September 25, 2019                               March 24, 2019

     An HTTP/2 extension for bidirectional messaging communication


   This draft proposes a http2 protocol extension, which enables
   bidirectional messaging communication between client and server.

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
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   time.  It is inappropriate to use Internet-Drafts as reference
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   This Internet-Draft will expire on September 25, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   HTTP/2 is the de facto application protocol in Internet.  The
   optimizations developed in HTTP/2, like stream multiplexing, header
   compression, and binary message framing are very generic.  They can
   be useful in non web browsing applications, for example, Publish/
   Subscribe, RPC.  However, the request/response from client to server
   communication pattern limits HTTP/2 from wider use in these
   applications.  This draft proposes a HTTP/2 protocol extension, which
   enables bidirectional messaging between client and server.

   The only mechanism HTTP/2 provides for server to client communication
   is PUSH

   PROMISE and the bi-directionality of HEADERS.  Further, clients are
   also able group streams together for routing purposes, such that each
   individual stream does not need to carry additional routing


   The keywords









   , and

   , when they appear in this document, are to be interpreted as
   described in [RFC2119].

   All the terms defined in the Conventions and Terminology section in
   [RFC7540] apply to this document.

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   A routing stream (RStream) is a long lived HTTP/2 stream in nature.
   RStreams are initiated by clients, and can be routed independently by
   any intermediaries.  Though an RStream is effectively a regular
   HTTP/2 stream, RStreams are recommended for exchanging metadata, but
   not user data.

   A new HTTP/2 stream called ExStream is introduced for exchanging user
   data.  ExStreams are recommended for short lived transactions, so
   intermediaries and servers can gracefully shutdown ExStreams within a
   short time.  The typical use case can be a subscription or publish
   request/response in Publish/Subscribe use case, or an RPC call
   between two endpoints.

   An ExStream is opened by an EX_HEADERS frame, and continued by
   CONTINUATION and DATA frames.  An ExStream

   be associated with an open RStream, and

   be associated with any other ExStream.  ExStreams are routed
   according to their RStreams by intermediaries and servers.
   Effectively, all ExStreams with the same RStream form a logical
   stream group, and are routed to the same endpoint.


   With RStreams and ExStreams, HTTP/2 can be used for bidirectional
   messaging communication.  As shown in the follow diagrams, after an
   RStream is open from client to server, either endpoint can initiate
   an ExStreams to its peer.

   +--------+   RStream (5)   +---------+    RStream (1)   +--------+
   | client |>--------------->|  proxy  |>---------------->| server |
   +--------+                 +---------+                  +--------+
       v                        ^     v                        ^
       |    ExStream(7, RS=5)   |     |    ExStream(3, RS=1)   |
       +------------------------+     +------------------------+

                                 Figure 1

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   +--------+   RStream (5)   +---------+    RStream (1)   +--------+
   | client |>--------------->|  proxy  |>---------------->| server |
   +--------+                 +---------+                  +--------+
        ^                        v     ^                        v
        |    ExStream(4, RS=5)   |     |    ExStream(2, RS=1)   |
        +------------------------+     +------------------------+

                                 Figure 2

   Beyond that, clients can multiplex RStreams, ExStreams and regular
   HTTP/2 streams into one HTTP/2 connection.  This enables clients to
   access different services without initiating new TCP connections.
   This avoids the latency cost of setting up new connections.  This is
   more desirable for mobile devices because they usually have longer
   RTT and battery constraints.  Multiplexing these services also allows
   them to share a single TCP connection congestion control context.

   As shown in the following diagram, the client can exchange data with
   PubSub, RPC and CDN three different services with one TCP connection.

   +--------+   RStream (5)   +---------+    RStream (1)   +----------+
   | client |>--------------->|  proxy  |>---------------->|  PUBSUB  |
   +--------+                 +---------+                  +----------+
     v   v                     ^ ^  v  v
     |   |     RStream (7)    /  |  |   \    RStream (5)   +----------+
     |   +-------------------+   |  |    +---------------->|    RPC   |
     |                           |  |                      +----------+
     |                           |  |
     |         Stream (9)        |  |      Stream (7)      +----------+
     +---------------------------+  +--------------------->|    CDN   |

                                 Figure 3


   RStreams are regular HTTP/2 streams that follow the stream lifecycle
   in [RFC7540], section 5.1.  ExStreams use the same lifecycle as
   regular HTTP/2 streams, but also depend on their RStreams.  If a
   RStream is reset, endpoints

   reset the ExStreams associated with that RStream.  If the RStream is
   closed, endpoints SHOULD allow existing ExStreams complete normally.
   The RStream SHOULD remain open while communication is ongoing.
   Endpoints SHOULD refresh any timeouts on the RStream while associated
   ExStreams are open.

   A sender

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   initiate new ExStreams if on an RStream that is in the open or half
   closed (remote) state.

   Endpoints process new ExStreams only when the RStream is open or half
   closed (local) state.  If an endpoint receives an EX



   The extension SHOULD be disabled by default.  Endpoints can negotiate
   the use of the extension through the SETTINGS frame.  If an
   implementation supports the extension, it is RECOMMENDED to include
   the ENABLE

   HEADERS setting in the initial SETTINGS frame.  HTTP/2 compliant
   implementations will ignore the setting if it is unknown.  An
   endpoint can send EX


   Endpoints MUST NOT send out EX


   HEADERS will be ignored, making the header compression contexts
   inconsistent between sender and receiver.

   If an endpoint supports this extension, but receives EX




   Intermediaries SHOULD send the ENABLE

   HEADERS setting to clients, only if intermediaries and their upstream
   servers can support this extension.  If an intermediary receives an
   ExStream but discovers the destination endpoint does not support the
   extension, it MUST reset the stream with EX



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   The extension implementation should apply stream and connection level
   flow control, maximum concurrent streams limit, GOAWAY logic to both
   RStreams and ExStreams.


   The EX

   HEADERS has one extra field, RStream ID.  It is used to open an
   ExStream, and additionally carries a header block fragment.
   EX_HEADERS frames can be sent on a stream in the "idle", "open", or
   "half-closed (remote)" state.

   Like HEADERS, the CONTINUATION frame (type=0x9) is used to continue a
   sequence of header block fragments, if the headers do not fit into
   one EX_HEADERS frame.

    |Pad Length? (8)|
    |E|                 Stream Dependency? (31)                     |
    |  Weight? (8)  |
    |R|                 Routing Stream ID (31)                      |
    |                   Header Block Fragment (*)                 ...
    |                           Padding (*)                       ...

                                 Figure 4

   The RStream specified in EX


   ERROR, if the specified RStream is missing; or is an ExStream rather
   than a stream; or is closed or half-closed (remote).  Otherwise, the
   states maintained for header compression or flow control) may be out
   of sync.

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   This document establishes a registry for a new frame type, setting,
   and error code.


   The entry in the following table are registered by this document.


   The entry in the following table are registered by this document.


   The entry in the following table are registered by this document.

6.  References

Authors' Addresses

   Guowu Xie
   Facebook Inc.
   1 Hacker Way
   Menlo Park  CA 94025

   Email: woo@fb.com

   Alan Frindell
   Facebook Inc.

   Email: afrind@fb.com

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