GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc6356

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.
 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.

Raiciu, et al. Experimental [Page 3] RFC 6356 MPTCP Congestion Control October 2011

 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).

Raiciu, et al. Experimental [Page 4] RFC 6356 MPTCP Congestion Control October 2011

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.

Raiciu, et al. Experimental [Page 5] RFC 6356 MPTCP Congestion Control October 2011

 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.

Raiciu, et al. Experimental [Page 6] RFC 6356 MPTCP Congestion Control October 2011

 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

Raiciu, et al. Experimental [Page 7] RFC 6356 MPTCP Congestion Control October 2011

 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.

Raiciu, et al. Experimental [Page 8] RFC 6356 MPTCP Congestion Control October 2011

 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

Raiciu, et al. Experimental [Page 9] RFC 6356 MPTCP Congestion Control October 2011

 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].

Raiciu, et al. Experimental [Page 10] RFC 6356 MPTCP Congestion Control October 2011

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.

Raiciu, et al. Experimental [Page 11] RFC 6356 MPTCP Congestion Control October 2011

 [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

Raiciu, et al. Experimental [Page 12]

/data/webs/external/dokuwiki/data/pages/rfc/rfc6356.txt · Last modified: 2011/10/15 01:16 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki