RFC 6356: Coupled Congestion Control for Multipath Transport Protocols | RFC Editor

Skip to content Primary navigationThe RFC SeriesFor AuthorsAbout UsHomeRFC 6356 View as: Plain Text, Compact HTML Info RFC 6356: Coupled Congestion Control for Multipath Transport Protocolscongestion controlMPTCPOctober 2011. C. Raiciu, M. Handley, D. WischikExperimentalSubscribe4 subscribed Add to Set Read aloud 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 of separate "subflows" across potentially disjoint paths. How should congestion control be performed for multipath TCP? First, each subflow must have its own congestion control state (i.e., cwnd) so that capacity on that path is matched by offered load. The simplest way to achieve this goal is to simply run standard TCP congestion control on each subflow. However, this solution is unsatisfactory as it gives the multipath flow an unfair share when the paths taken by its different subflows share a common bottleneck. Bottleneck fairness is just one requirement multipath congestion control should meet. The following three goals capture the desirable properties of a practical multipath congestion control algorithm: o Goal 1 (Improve Throughput) A multipath flow should perform at least as well as a single path flow would on the best of the paths available to it. o Goal 2 (Do no harm) A multipath flow should not take up more capacity from any of the resources shared by its different paths than if it were a single flow using only one of these paths. This guarantees it will not unduly harm other flows. o Goal 3 (Balance congestion) A multipath flow should move as much traffic as possible off its most congested paths, subject to meeting the first two goals. G