    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.

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    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

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    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.

       Goals 1 and 2 together ensure fairness at the bottleneck.  Goal 3
       captures the concept of resource pooling [WISCHIK]: if each multipath
       flow sends more data through its least congested path, the traffic in
       the network will move away from congested areas.  This improves
       robustness and overall throughput, among other things.  The way to
       achieve resource pooling is to effectively "couple" the congestion
       control loops for the different subflows.

       We propose an algorithm that couples the additive increase function
       of the subflows, and uses unmodified TCP behavior in case of a drop.
       The algorithm relies on the traditional TCP mechanisms to detect
       drops, to retransmit data, etc.

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       Detecting shared bottlenecks reliably is quite difficult, but is just
       one part of a bigger question.  This bigger question is how much
       bandwidth a multipath user should use in total, even if there is no
       shared bottleneck.

       The congestion controller aims to set the multipath flow's aggregate
       bandwidth to be the same as that of a regular TCP flow would get on
       the best path available to the multipath flow.  To estimate the
       bandwidth of a regular TCP flow, the multipath flow estimates loss
       rates and round-trip times (RTTs) and computes the target rate.
       Then, it adjusts the overall aggressiveness (parameter alpha) to
       achieve the desired rate.

       While the mechanism above applies always, its effect depends on
       whether the multipath TCP flow influences or does not influence the
       link loss rates (low versus high statistical multiplexing).  If MPTCP
       does not influence link loss rates, MPTCP will get the same
       throughput as TCP on the best path.  In cases with low statistical
       multiplexing, where the multipath flow influences the loss rates on
       the path, the multipath throughput will be strictly higher than that
       a single TCP would get on any of the paths.  In particular, if using
       two idle paths, multipath throughput will be sum of the two paths'
       throughput.

       This algorithm ensures bottleneck fairness and fairness in the
       broader, network sense.  We acknowledge that current TCP fairness
       criteria are far from ideal, but a multipath TCP needs to be
       deployable in the current Internet.  If needed, new fairness criteria
       can be implemented by the same algorithm we propose by appropriately
       scaling the overall aggressiveness.

       It is intended that the algorithm presented here can be applied to
       other multipath transport protocols, such as alternative multipath
       extensions to TCP, or indeed any other congestion-aware transport
       protocols.  However, for the purposes of example, this document will,
       where appropriate, refer to the MPTCP.

       The design decisions and evaluation of the congestion control
       algorithm are published in [NSDI].

       The algorithm presented here only extends standard TCP congestion
       control for multipath operation.  It is foreseeable that other
       congestion controllers will be implemented for multipath transport to
       achieve the bandwidth-scaling properties of the newer congestion
       control algorithms for regular TCP (such as Compound TCP and Cubic).

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    2.  Requirements Language

       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] .

    3.  Coupled Congestion Control Algorithm

       The algorithm we present only applies to the increase phase of the
       congestion avoidance state specifying how the window inflates upon
       receiving an ACK.  The slow start, fast retransmit, and fast recovery
       algorithms, as well as the multiplicative decrease of the congestion
       avoidance state are the same as in standard TCP [RFC5681].

       Let cwnd_i be the congestion window on the subflow i.  Let cwnd_total
       be the sum of the congestion windows of all subflows in the
       connection.  Let p_i, rtt_i, and MSS_i be the loss rate, round-trip
       time (i.e., smoothed round-trip time estimate used by TCP), and
       maximum segment size on subflow i.

       We assume throughout this document that the congestion window is
       maintained in bytes, unless otherwise specified.  We briefly describe
       the algorithm for packet-based implementations of cwnd in section
       Section 4.2.

       Our proposed "Linked Increases" algorithm MUST:

       o  For each ACK received on subflow i, increase cwnd_i by

                    alpha * bytes_acked * MSS_i   bytes_acked * MSS_i
              min ( --------------------------- , ------------------- )  (1)
                             cwnd_total                   cwnd_i

       The increase formula (1) takes the minimum between the computed
       increase for the multipath subflow (first argument to min), and the
       increase TCP would get in the same scenario (the second argument).
       In this way, we ensure that any multipath subflow cannot be more
       aggressive than a TCP flow in the same circumstances, hence achieving
       Goal 2 (do no harm).

       "alpha" is a parameter of the algorithm that describes the
       aggressiveness of the multipath flow.  To meet Goal 1 (improve
       throughput), the value of alpha is chosen such that the aggregate
       throughput of the multipath flow is equal to the throughput a TCP
       flow would get if it ran on the best path.

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       To get an idea of what the algorithm is trying to do, let's take the
       case where all the subflows have the same round-trip time and Maximum
       Segment Size (MSS).  In this case, the algorithm will grow the total
       window by approximately alpha*MSS per RTT.  This increase is
       distributed to the individual flows according to their instantaneous
       window size.  Subflow i will increase by alpha*cwnd_i/cwnd_total
       segments per RTT.

       Note that, as in standard TCP, when cwnd_total is large the increase
       may be 0.  In this case, the increase MUST be set to 1.  We discuss
       how to implement this formula in practice in the next section.

       We assume implementations use an approach similar to appropriate byte
       counting (ABC, [RFC3465]), where the bytes_acked variable records the
       number of bytes newly acknowledged.  If this is not the case,
       bytes_acked SHOULD be set to MSS_i.

       To compute cwnd_total, it is an easy mistake to sum up cwnd_i across
       all subflows: when a flow is in fast retransmit, its cwnd is
       typically inflated and no longer represents the real congestion
       window.  The correct behavior is to use the ssthresh (slow start
       threshold) value for flows in fast retransmit when computing
       cwnd_total.  To cater to connections that are app limited, the
       computation should consider the minimum between flight_size_i and
       cwnd_i, and flight_size_i and ssthresh_i, where appropriate.

       The total throughput of a multipath flow depends on the value of
       alpha and the loss rates, maximum segment sizes, and round-trip times
       of its paths.  Since we require that the total throughput is no worse
       than the throughput a single TCP would get on the best path, it is
       impossible to choose, a priori, a single value of alpha that achieves
       the desired throughput in every occasion.  Hence, alpha must be
       computed based on the observed properties of the paths.

       The formula to compute alpha is:

                            MAX (cwnd_i/rtt_i^2)
       alpha = cwnd_total * -------------------------           (2)
                            (SUM (cwnd_i/rtt_i))^2

       Note:

       MAX (x_i) means the maximum value for any possible value of i.

       SUM (x_i) means the summation for all possible values of i.

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       The formula (2) is derived by equalizing the rate of the multipath
       flow with the rate of a TCP running on the best path, and solving for
       alpha.

    4.  Implementation Considerations

       Equation (2) implies that alpha is a floating point value.  This
       would require performing costly floating point operations whenever an
       ACK is received.  Further, in many kernels, floating point operations
       are disabled.  There is an easy way to approximate the above
       calculations using integer arithmetic.

    4.1.  Computing "alpha" in Practice

       Let alpha_scale be an integer.  When computing alpha, use alpha_scale
       * cwnd_total instead of cwnd_total and do all the operations in
       integer arithmetic.

       Then, scale down the increase per ACK by alpha_scale.  The resulting
       algorithm is a simple change from Equation (1):

       o  For each ACK received on subflow i, increase cwnd_i by:

                    alpha * bytes_acked * MSS_i   bytes_acked * MSS_i
              min ( --------------------------- , ------------------- )  (3)
                     alpha_scale * cwnd_total              cwnd_i

       The alpha_scale parameter denotes the precision we want for computing
       alpha.  Observe that the errors in computing the numerator or the
       denominator in the formula for alpha are quite small, as the cwnd in
       bytes is typically much larger than the RTT (measured in ms).

       With these changes, all the operations can be done using integer
       arithmetic.  We propose alpha_scale be a small power of two, to allow
       using faster shift operations instead of multiplication and division.
       Our experiments show that using alpha_scale=512 works well in a wide
       range of scenarios.  Increasing alpha_scale increases precision, but
       also increases the risk of overflow when computing alpha.  Using 64-
       bit operations would solve this issue.  Another option is to
       dynamically adjust alpha_scale when computing alpha; in this way, we
       avoid overflow and obtain maximum precision.

       It is possible to implement the algorithm by calculating cwnd_total
       on each ack; however, this would be costly especially when the number
       of subflows is large.  To avoid this overhead, the implementation MAY
       choose to maintain a new per-connection state variable called
       "cwnd_total".  If it does so, the implementation will update the
       cwnd_total value whenever the individual subflow's windows are

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       updated.  Updating only requires one more addition or subtraction
       operation compared to the regular, per-subflow congestion control
       code, so its performance impact should be minimal.

       Computing alpha per ACK is also costly.  We propose alpha be a per-
       connection variable, computed whenever there is a drop and once per
       RTT otherwise.  More specifically, let cwnd_new be the new value of
       the congestion window after it is inflated or after a drop.  Update
       alpha only if the quotient of cwnd_i/MSS_i differs from the quotient
       of cwnd_new_i/MSS_i.

       In certain cases with small RTTs, computing alpha can still be
       expensive.  We observe that if RTTs were constant, it is sufficient
       to compute alpha once per drop, as alpha does not change between
       drops (the insight here is that cwnd_i/cwnd_j = constant as long as
       both windows increase).  Experimental results show that even if
       round-trip times are not constant, using average round-trip time per
       sawtooth instead of instantaneous round-trip time (i.e., TCP's
       smoothed RTT estimator) gives good precision for computing alpha.
       Hence, it is possible to compute alpha only once per drop using a
       modified version of equation (2) where rtt_i is replaced with
       rtt_avg_i.

       If using average round-trip time, rtt_avg_i will be computed by
       sampling the rtt_i whenever the window can accommodate one more
       packet, i.e., when cwnd / MSS < (cwnd+increase)/MSS.  The samples are
       averaged once per sawtooth into rtt_avg_i.  This sampling ensures
       that there is no sampling bias for larger windows.

       Given cwnd_total and alpha, the congestion control algorithm is run
       for each subflow independently, with similar complexity to the
       standard TCP increase code [RFC5681].

    4.2.  Implementation Considerations when CWND is Expressed in Packets

       When the congestion control algorithm maintains cwnd in packets
       rather than bytes, the algorithms above must change to take into
       account path MSS.

       To compute the increase when an ACK is received, the implementation
       for multipath congestion control is a simple extension of the
       standard TCP code.  In standard, TCP cwnd_cnt is an additional state
       variable that tracks the number of segments acked since the last cwnd
       increment; cwnd is incremented only when cwnd_cnt > cwnd; then,
       cwnd_cnt is set to 0.

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       In the multipath case, cwnd_cnt_i is maintained for each subflow as
       above, and cwnd_i is increased by 1 when cwnd_cnt_i > max(alpha_scale
       * cwnd_total / alpha, cwnd_i).

       When computing alpha for packet-based stacks, the errors in computing
       the terms in the denominator are larger (this is because cwnd is much
       smaller and rtt may be comparatively large).  Let max be the index of
       the subflow used in the numerator.  To reduce errors, it is easiest
       to move rtt_max (once calculated) from the numerator to the
       denominator, changing equation (2) to obtain the equivalent formula
       below.

                                                                      (4)

                                                   cwnd_max
     alpha = alpha_scale * cwnd_total * ------------------------------------
                                        (SUM ((rtt_max * cwnd_i) / rtt_i))^2

       Note that the calculation of alpha does not take into account path
       MSS and is the same for stacks that keep cwnd in bytes or packets.
       With this formula, the algorithm for computing alpha will match the
       rate of TCP on the best path in B/s for byte-oriented stacks, and in
       packets/s in packet-based stacks.  In practice, MSS rarely changes
       between paths so this shouldn't be a problem.

       However, it is simple to derive formulae allowing packet-based stacks
       to achieve byte rate fairness (and vice versa) if needed.  In
       particular, for packet-based stacks wanting byte-rate fairness,
       equation (4) above changes as follows: cwnd_max is replaced by
       cwnd_max * MSS_max * MSS_max, while cwnd_i is replaced with cwnd_i *
       MSS_i.

    5.  Discussion

       The algorithm we've presented fully achieves Goals 1 and 2, but does
       not achieve full resource pooling (Goal 3).  Resource pooling
       requires that no traffic should be transferred on links with higher
       loss rates.  To achieve perfect resource pooling, one must couple
       both increase and decrease of congestion windows across subflows, as
       in [KELLY].

       There are a few problems with such a fully coupled controller.
       First, it will insufficiently probe paths with high loss rates and
       will fail to detect free capacity when it becomes available.  Second,
       such controllers tend to exhibit "flappiness": when the paths have
       similar levels of congestion, the congestion controller will tend to
       allocate all the window to one random subflow and allocate zero

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       window to the other subflows.  The controller will perform random
       flips between these stable points.  This doesn't seem desirable in
       general, and is particularly bad when the achieved rates depend on
       the RTT (as in the current Internet): in such a case, the resulting
       rate with fluctuate unpredictably depending on which state the
       controller is in, hence violating Goal 1.

       By only coupling increases our proposal probes high loss paths,
       detecting free capacity quicker.  Our proposal does not suffer from
       flappiness but also achieves less resource pooling.  The algorithm
       will allocate window to the subflows such that p_i * cwnd_i =
       constant, for all i.  Thus, when the loss rates of the subflows are
       equal, each subflow will get an equal window, removing flappiness.
       When the loss rates differ, progressively more windows will be
       allocated to the flow with the lower loss rate.  In contrast, perfect
       resource pooling requires that all the window should be allocated on
       the path with the lowest loss rate.  Further details can be found in
       [NSDI].

    6.  Security Considerations

       One security concern relates to what we call the traffic-shifting
       attack: on-path attackers can drop packets belonging to a multipath
       subflow, which, in turn, makes the path seem congested and will force
       the sender's congestion controller to avoid that path and push more
       data over alternate subflows.

       The attacker's goal is to create congestion on the corresponding
       alternative paths.  This behavior is entirely feasible but will only
       have minor effects: by design, the coupled congestion controller is
       less (or similarly) aggressive on any of its paths than a single TCP
       flow.  Thus, the biggest effect this attack can have is to make a
       multipath subflow be as aggressive as a single TCP flow.

       Another effect of the traffic-shifting attack is that the new path
       can monitor all the traffic, whereas before it could only see a
       subset of traffic.  We believe that if privacy is needed, splitting
       traffic across multiple paths with MPTCP is not the right solution in
       the first place; end-to-end encryption should be used instead.

       Besides the traffic-shifting attack mentioned above, the coupled
       congestion control algorithm defined in this document adds no other
       security considerations to those found in [MPTCP-MULTIADDRESSED] and
       [RFC6181].  Detailed security analysis for the Multipath TCP protocol
       itself is included in [MPTCP-MULTIADDRESSED] and [RFC6181].

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    7.  Acknowledgements

       We thank Christoph Paasch for his suggestions for computing alpha in
       packet-based stacks.  The authors are supported by Trilogy
       (http://www.trilogy-project.org), a research project (ICT-216372)
       partially funded by the European Community under its Seventh
       Framework Program.  The views expressed here are those of the
       author(s) only.  The European Commission is not liable for any use
       that may be made of the information in this document.

    8.  References

    8.1.  Normative References

       [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
                  RFC 793, September 1981.

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

       [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
                  Control", RFC 5681, September 2009.

    8.2.  Informative References

       [KELLY]    Kelly, F. and T. Voice, "Stability of end-to-end
                  algorithms for joint routing and rate control", ACM
                  SIGCOMM CCR vol. 35 num. 2, pp. 5-12, 2005,
                  <http://portal.acm.org/citation.cfm?id=1064415>.

       [MPTCP-MULTIADDRESSED]
                  Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
                  "TCP Extensions for Multipath Operation with Multiple
                  Addresses", Work in Progress, July 2011.

       [NSDI]     Wischik, D., Raiciu, C., Greenhalgh, A., and M. Handley,
                  "Design, Implementation and Evaluation of Congestion
                  Control for Multipath TCP", Usenix NSDI , March 2011, <htt
                  p://www.cs.ucl.ac.uk/staff/c.raiciu/files/mptcp-nsdi.pdf>.

       [RFC3465]  Allman, M., "TCP Congestion Control with Appropriate Byte
                  Counting (ABC)", RFC 3465, February 2003.

       [RFC6181]  Bagnulo, M., "Threat Analysis for TCP Extensions for
                  Multipath Operation with Multiple Addresses", RFC 6181,
                  March 2011.

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       [WISCHIK]  Wischik, D., Handley, M., and M. Bagnulo Braun, "The
                  Resource Pooling Principle", ACM SIGCOMM CCR vol. 38 num.
                  5, pp. 47-52, October 2008,
                  <http://ccr.sigcomm.org/online/files/p47-handleyA4.pdf>.

    Authors' Addresses

       Costin Raiciu
       University Politehnica of Bucharest
       Splaiul Independentei 313
       Bucharest
       Romania

       EMail: costin.raiciu@cs.pub.ro

       Mark Handley
       University College London
       Gower Street
       London  WC1E 6BT
       UK

       EMail: m.handley@cs.ucl.ac.uk

       Damon Wischik
       University College London
       Gower Street
       London  WC1E 6BT
       UK

       EMail: d.wischik@cs.ucl.ac.uk

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