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Network Working Group                                             W. Zhu
INTERNET-DRAFT                                              Google, Inc.
Intended Status: Informational                               M. Jennings
Expires: November 18, 2010                                  May 17, 2010


                   Implications of Full-Duplex HTTP
                    draft-zhu-http-fullduplex-00.txt


Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

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Abstract

   Full-duplex HTTP follows the basic HTTP request-response semantics
   but also allows the server to send the response to the client at the
   same time when the client is transmitting the request to the server.
   Requirements for Full-duplex HTTP are under-specified in the existing
   HTTP specification, and this memo intends to clarify the behaviors of
   full-duplex HTTP under the standard HTTP protocol.


Table of Contents

   1  Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3
      1.1  Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
   2  Streaming in HTTP  . . . . . . . . . . . . . . . . . . . . . . . 3
      2.1  Request and Response Streaming  . . . . . . . . . . . . . . 3
      2.2  Full-Duplex Streaming . . . . . . . . . . . . . . . . . . . 4
   3  Protocol Considerations  . . . . . . . . . . . . . . . . . . . . 4
      3.1  Initialization  . . . . . . . . . . . . . . . . . . . . . . 4
      3.2  Termination . . . . . . . . . . . . . . . . . . . . . . . . 4
      3.3  Persistent Connections  . . . . . . . . . . . . . . . . . . 5
      3.4  Time-Out  . . . . . . . . . . . . . . . . . . . . . . . . . 5
      3.5  Proxies . . . . . . . . . . . . . . . . . . . . . . . . . . 5
      3.6  Errors  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   4  Application Considerations . . . . . . . . . . . . . . . . . . . 6
      4.1  Compatibility . . . . . . . . . . . . . . . . . . . . . . . 6
      4.2  Fall Back and Buffering . . . . . . . . . . . . . . . . . . 6
      4.2  Messaging . . . . . . . . . . . . . . . . . . . . . . . . . 6
   5  Security Considerations  . . . . . . . . . . . . . . . . . . . . 7
   6  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . . 7
   7  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . . 7
   8  Informative References . . . . . . . . . . . . . . . . . . . . . 7
   Author's Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 7


















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1  Introduction

   HTTP [RFC2616], being a RPC style protocol, requires communication
   between client and server follows the strict request-response
   pattern.

   HTTP may also be used to stream data from either client or server.
   When bi-directional streaming is required, two connections are often
   used to stream client and server data separately. Using two separate
   connections not only introduce overhead, but also make HTTP
   insufficient to be used as a standalone protocol, i.e. application-
   level protocols are required to handle the two connections.

   However, if the server is allowed to send response to the client at
   the same time when request is being transmitted from the client to
   the server, then effectively full-duplex streaming becomes possible
   under the standard HTTP protocol [RFC2616].

   Full-duplex streaming requires end-to-end support from both the
   client and server. More specifically, the client has to be explicitly
   designed to support such capability.

   Given the unique properties of full-duplex HTTP, special requirements
   exist for both the client and server. And those requirements need be
   clearly identified so that implementations are able to follow
   standard behaviors in adopting full-duplex HTTP.

1.1  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

2  Streaming in HTTP

2.1  Request and Response Streaming

   Request streaming requires the server to deliver to the application
   any data that has been received by the server.

   Response streaming requires the server to send to the client any data
   that has been generated by the application.

   Chunked transfer-encoding [RFC2616] is expected for both request and
   response streaming. However, client or server should not design the
   underlying streaming or messaging API based on the chunked transfer-
   encoding (which is generated hop-by-hop).




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   Most browsers don't support chunked transfer-encoding for requests.

2.2  Full-Duplex Streaming

   When full-duplex streaming is enabled, request and response are
   transmitted between client and server simultaneously over the same
   HTTP connection.

   Full-duplex streaming may be applied to any resource that is designed
   to concurrently stream request and response, for example a voice
   translator.

   Full-duplex streaming should still follow the standard request-
   response semantics and maintain the basic temporal dependency between
   request and response:

   1  Server SHOULD NOT generate any response until a request has been
      received. That is, unsolicited server-initiated response is not
      allowed.

   2  Server SHOULD NOT complete the response before the client
      completes the request. That is, the server can only complete the
      response after the server has received the complete request from
      the client

3  Protocol Considerations

3.1  Initialization

   Full-duplex streaming may be started as soon as the server receives
   the first byte of the request body. This behavior is significantly
   different from the technique commonly-known as Hanging GET, in which
   a separate GET request is issued to initiate streaming to the client.

3.2  Termination

   The client terminates request streaming by completing the request,
   i.e. sending out the last-chunk [RFC2616]. And the server may choose
   to continue streaming the response after it receives the complete
   request from the client, as decided by the application. Eventually
   the server terminates response streaming by sending out the last-
   chunk of response.

   Before the server receives the complete request from the client, the
   only way for the server to terminate the response streaming is to
   close the connection. It is considered an illegal state for an HTTP
   connection to have a pending request when the response has already
   been completed. The client should close the connection immediately if



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   the client receives a complete response when the client is still
   streaming the request.

3.3  Persistent Connections

   It's important that HTTP keep-alive and pipelining still work with
   full-duplex HTTP. To achieve that, the client and server should
   respect the original semantics of HTTP persistent connections
   [RFC2616].

3.4  Time-Out

   HTTP server or client may time-out connections while waiting for
   request or response. Full-duplex HTTP should not override this
   behavior. A connection may be closed due to time-out if either
   request or response streaming is inactive.

   Full-duplex HTTP introduces no special requirement on timeout of the
   underlying TPC connection. When time-out does happen both request and
   response streaming will be terminated.

3.5  Proxies

   HTTP proxies may not support concurrent responses, and one of the
   purposes of this document is to increase awareness of full-duplex
   HTTP communication in proxies.

   Proxies may also buffer streamed requests or responses, or have
   problems to handle chunked transfer-encoding, especially for
   requests.


3.6  Errors

   According to RFC2616 [RFC2616], a client or proxy should close the
   connection if the client receives an error response as it is
   transmitting the request. Full-duplex HTTP must respect this
   requirement.

   In cases when a server is incapable of streaming response or decides
   to timeout, the server should close the connection. This is also true
   for a client when it is streaming request.

   A client or server should stop streaming any new data after it
   notices that the underlying TCP connection has been closed by the
   other party. In-flight data will be discarded under this kind of
   half-close behavior.




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4  Application Considerations

4.1  Compatibility

   Full-duplex streaming is completely controlled by the server
   application, and should only be enabled to clients that have been
   explicitly identified by the server.

   It is not sufficient to enable full-duplex HTTP solely based on User-
   Agent.

   For non-controlled client applications, the client needs advise its
   capability of full-duplex streaming via URL parameters or headers
   (for example, "X-Content-Streaming: full-duplex, timeout=30").
   Otherwise, full-duplex streaming should be disabled, or the server
   should return 404 (Not Found) status if full-duplex streaming is
   mandatory for the requested resource.

4.2  Fall Back and Buffering

   Proxies may disallow early responses, or buffer requests or
   responses. In such a case, the application may have to switch to a
   different protocol that uses two connections and rely on polling
   techniques.

   One efficient way to trigger a fall back will be for the client to
   wait for initial response for a short time-out period.

   When a proxy or client is unable to consume early response data, the
   server may suffer from buffer overflow. And the server should close
   the connection as soon as it detects such a condition. Generally
   there is no reliable way for a server to distinguish between ill-
   behaved clients and non-compatible proxies.

   Full-duplex HTTP expects minimized buffering from both client and
   server. However, applications may choose to buffer certain amount of
   data for optimization or application specific purposes.

4.2  Messaging

   Since chunked transfer-encoding isn't a reliable way to provide
   message framing, messaging support has to be provided by the
   application stack, along with any required message delivery
   guarantee.







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5  Security Considerations

   Full-duplex HTTP introduces no new security concerns beyond those
   known with regular HTTP communication.

6  IANA Considerations

   This document does not require any actions by the IANA.

7  Acknowledgments

   Thanks to Henrik Nordstrom, Jamie Lokier, and Mark Nottingham for
   their feedbacks when the subject of this document was originally
   brought up on httpbis.

8  Informative References

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2616]   Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
               Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
               Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

Author's Addresses


   Wenbo Zhu
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA 94043
   US

   Phone: +1 650 214 5894
   Email: wenboz@google.com


   Mike Jennings

   Email: mike.c.jennings@gmail.com











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