Coupled Congestion Control for Multipath Transport Protocols
RFC 6356

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Internet Engineering Task Force (IETF)                         C. Raiciu
Request for Comments: 6356                Univ. Politehnica of Bucharest
Category: Experimental                                         M. Handly
ISSN: 2070-1721                                             D. Wischik
                                                    Univ. College London
                                                            October 2011

      Coupled Congestion Control for Multipath Transport Protocols

Abstract

   Often endpoints are connected by multiple paths, but communications
   are usually restricted to a single path per connection.  Resource
   usage within the network would be more efficient were it possible for
   these multiple paths to be used concurrently.  Multipath TCP is a
   proposal to achieve multipath transport in TCP.

   New congestion control algorithms are needed for multipath transport
   protocols such as Multipath TCP, as single path algorithms have a
   series of issues in the multipath context.  One of the prominent
   problems is that running existing algorithms such as standard TCP
   independently on each path would give the multipath flow more than
   its fair share at a bottleneck link traversed by more than one of its
   subflows.  Further, it is desirable that a source with multiple paths
   available will transfer more traffic using the least congested of the
   paths, achieving a property called "resource pooling" where a bundle
   of links effectively behaves like one shared link with bigger
   capacity.  This would increase the overall efficiency of the network
   and also its robustness to failure.

   This document presents a congestion control algorithm that couples
   the congestion control algorithms running on different subflows by
   linking their increase functions, and dynamically controls the
   overall aggressiveness of the multipath flow.  The result is a
   practical algorithm that is fair to TCP at bottlenecks while moving
   traffic away from congested links.

Raiciu, et al.                Experimental                      [Page 1]
RFC 6356                MPTCP Congestion Control            October 2011

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6356.

Copyright Notice

   Copyright (c) 2011 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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
   2. Requirements Language ...........................................5
   3. Coupled Congestion Control Algorithm ............................5
   4. Implementation Considerations ...................................7
      4.1. Computing "alpha" in Practice ..............................7
      4.2. Implementation Considerations when CWND is
           Expressed in Packets .......................................8
   5. Discussion ......................................................9
   6. Security Considerations ........................................10
   7. Acknowledgements ...............................................11
   8. References .....................................................11
      8.1. Normative References ......................................11
      8.2. Informative References ....................................11

Raiciu, et al.                Experimental                      [Page 2]
RFC 6356                MPTCP Congestion Control            October 2011

1.  Introduction

   Multipath TCP (MPTCP, [MPTCP-MULTIADDRESSED]) is a set of extensions
   to regular TCP [RFC0793] that allows one TCP connection to be spread
   across multiple paths.  MPTCP distributes load through the creation