Movatterモバイル変換

Internet Engineering Task Force (IETF)                         C. RaiciuRequest for Comments: 6356                Univ. Politehnica of BucharestCategory: Experimental                                         M. HandlyISSN: 2070-1721                                             D. Wischik                                                    Univ. College London                                                            October 2011Coupled Congestion Control for Multipath Transport ProtocolsAbstract   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 2011Status 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; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 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 Contents1. Introduction ....................................................32. Requirements Language ...........................................53. Coupled Congestion Control Algorithm ............................54. Implementation Considerations ...................................74.1. Computing "alpha" in Practice ..............................7      4.2. Implementation Considerations when CWND is           Expressed in Packets .......................................85. Discussion ......................................................96. Security Considerations ........................................107. Acknowledgements ...............................................118. References .....................................................118.1. Normative References ......................................118.2. Informative References ....................................11Raiciu, et al.                Experimental                      [Page 2]

RFC 6356                MPTCP Congestion Control            October 20111.  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 20112.  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 inRFC 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 sectionSection 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 areRaiciu, 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 zeroRaiciu, 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 20117.  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.  References8.1.  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.ukRaiciu, et al.                Experimental                     [Page 12]