Coupled congestion control for RTP media
draft-welzl-rmcat-coupled-cc-00

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Last updated 2013-01-19
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RTP Media Congestion Avoidance                                  M. Welzl
Techniques (rmcat)                                    University of Oslo
Internet-Draft                                          January 19, 2013
Intended status: Experimental
Expires: July 23, 2013

                Coupled congestion control for RTP media
                    draft-welzl-rmcat-coupled-cc-00

Abstract

   When multiple congestion controlled RTP sessions traverse the same
   network bottleneck, it can be beneficial to combine their controls
   such that the total on-the-wire behavior is improved.  This document
   describes such a method for flows that have the same sender, in a way
   that is as flexible and simple as possible while minimizing the
   amount of changes needed to existing RTP applications.

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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   This Internet-Draft will expire on July 23, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   (http://trustee.ietf.org/license-info) in effect on the date of
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   include Simplified BSD License text as described in Section 4.e of

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

1.  Introduction

   When there is enough data to send, a congestion controller must
   increase its sending rate until the path's available capacity has
   been reached; depending on the controller, sometimes the rate is
   increased further, until packets are ECN-marked or dropped.  In the
   public Internet, this is currently the only way to get any feedback
   from the network that can be used as an indication of congestion.
   This process inevitably creates undesirable queuing delay -- an
   effect that is amplified when multiple congestion controlled
   connections traverse the same network bottleneck.  When such
   connections originate from the same host, it would therefore be ideal
   to use only one single sender-side congestion controller which
   determines the overall allowed sending rate, and then use a local
   scheduler to assign a proportion of this rate to each RTP session.
   This way, priorities could also be implemented quite easily, as a
   function of the scheduler; honoring user-specified priorities is, for
   example, required by rtcweb [rtcweb-usecases].

   The Congestion Manager (CM) [RFC3124] provides a single congestion
   controller with a scheduling function just as described above.  It
   is, however, hard to implement because it requires an additional
   congestion controller and removes all per-connection congestion
   control functionality, which is quite a significant change to
   existing RTP based applications.  This document presents a method
   that is easier to implement than the CM and also requires less
   significant changes to existing RTP based applications.  It attempts
   to roughly approximate the CM behavior by sharing information between
   existing congestion controllers, akin to "Ensemble Sharing" in
   [RFC2140].

2.  Definitions

   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].

   Available Bandwidth:
         The available bandwidth is the nominal link capacity minus the
         amount of traffic that traversed the link during a certain time
         interval, divided by that time interval.

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   Bottleneck:
         The first link with the smallest available bandwidth along the
         path between a sender and receiver.

   Flow:
         A flow is the entity that congestion control is operating on.
         It could, for example, be a transport layer connection, an RTP
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