Network Working Group                                         O. Mazahir
Internet-Draft                                                 J. Padhye
Expires: May 18, 2013                                           R. Trace
                                                               S. Loreto
                                                           G. Montenegro
                                                       November 14, 2012

                  HTTP 2.0 Principles for Flow Control


   This document states the principles for flow control in HTTP 2.0.

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   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on May 18, 2013.

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   Copyright (c) 2012 IETF Trust and the persons identified as the
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Principles for Flow Control in HTTP 2.0 Multiplexing  . . . . . 4
   3.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 5
   4.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     4.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     4.2.  Informative References  . . . . . . . . . . . . . . . . . . 6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 7

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1.  Introduction

   HTTP/2.0 introduces multiplexed streams over a given TCP connection.
   In HTTP 1.X, there is no interleaving of Request/Response pairs.
   Thus, any flow control issues are mostly left to the underlying TCP
   implementation.  In HTTP 2.0, each Request/Response pair uses a
   separate stream, sharing the same TCP connection with other such
   pairs over different streams.  All such streams will be vying for a
   common underlying resource of a single TCP connection.  Given that
   this interaction among all the streams is not visible to the TCP
   implementation, handling the interaction among them has to be solved
   at the HTTP 2.0 multiplexing layer.  There are issues of
   prioritization, head-of-line blocking and flow control.  Perhaps the
   most complex aspect is that of flow control.  It seems likely that
   flow control will follow a path similar to what TCP's complex
   dynamics have followed throughout the years.  In particular, TCP
   congestion control has seen a constant progress of improved
   specifications based on measurements and research of the networking
   community.  What the TCP community recognized early on was that this
   was a hard problem.  Thus, the best course of action was to agree on
   a minimal set of rules or principles (e.g., TCP "friendliness").
   Many TCP algorithms are then possible as a (mostly) local
   implementation issue giving rise to TCP Reno, Tahoe, Vegas, CTCP, and
   many more.

   Flow control for HTTP 2.0 multiplexing over TCP is also a complex
   issue.  This document proposes (1) a set of principles aimed at
   preventing egregious behavior, while allowing for future and ongoing
   improvement of flow control algorithms, and (2) a simple flow control
   algorithm that could be implemented in the absence of better schemes
   (TBD).  Other flow control algorithms with subsequent improvements
   should be specified in separate documents without encumbering nor
   delaying the base HTTP 2.0 specification.  This is similar to how the
   myriad TCP congestion algorithms published so far have been specified
   separately from the base TCP documents.

   The goal of this document is to propose additional text to the
   HTTP/2.0 specification.  The starting point for HTTP/2.0, the SPDY
   [I-D.mbelshe-httpbis-spdy] protocol, does not have much language with
   respect to flow control.  Hence, the text below is offered as a new
   section or sections within the HTTP/2.0 document.

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2.  Principles for Flow Control in HTTP 2.0 Multiplexing

   Flow control for Multiplexing in HTTP 2.0 must follow these

   1.  Flow control is hop by hop (where "hop" means an HTTP 2.0 hop),
       and not end-to-end.

   2.  Flow control is based on window update messages.  It is
       essentially a credit-based scheme.

   3.  Flow control is directional.  The client and server independently
       advertise their flow control preference.  It MAY be declared by
       the receiver and MUST be heeded by the sender.

   4.  Flow control can be OFF or ON.  It is OFF if no flow control is
       advertised by a receiver, or if it declares "infinite" credit to
       the sender.

   5.  HTTP 2.0 should only standardize the format of the window update
       message and its semantics.  In particular, the algorithms used by
       the receiver to decide when to send window update messages, and
       how much to update the window by, are not mandated in the spec.
       The draft should, however, provide some illustrative examples.

   NOTE: Whether flow control operates on a per-stream basis, on a per-
   session (per-TCP connection) basis or on both a per-stream and a per-
   session basis is TBD.

   The spec will not define the algorithms the sender will use to manage
   priorities among streams and to minimize head of the line blocking.
   This is included for completeness, but is essentially independent of

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3.  Acknowledgements

   This document was produced using the xml2rfc tool [RFC2629].

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4.  References

4.1.  Normative 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.

              Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Message Syntax and Routing",
              draft-ietf-httpbis-p1-messaging-21 (work in progress),
              October 2012.

              Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Semantics and Content",
              draft-ietf-httpbis-p2-semantics-21 (work in progress),
              October 2012.

4.2.  Informative References

   [RFC2629]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
              June 1999.

   [RFC6455]  Fette, I. and A. Melnikov, "The WebSocket Protocol",
              RFC 6455, December 2011.

              Belshe, M. and R. Peon, "SPDY Protocol",
              draft-mbelshe-httpbis-spdy-00 (work in progress),
              February 2012.

              Trace, R., Foresti, A., Singhal, S., Mazahir, O., Nielsen,
              H., Raymor, B., Rao, R., and G. Montenegro, "HTTP Speed+
              Mobility", draft-montenegro-httpbis-speed-mobility-02
              (work in progress), June 2012.

              Tarreau, W., Jeffries, A., and A. Croy, "Proposal for a
              Network-Friendly HTTP Upgrade",
              draft-tarreau-httpbis-network-friendly-00 (work in
              progress), March 2012.

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Authors' Addresses

   Osama Mazahir


   Jitu Padhye


   Rob Trace


   Salvatore Loreto


   Gabriel Montenegro


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