© 2017 ACM. Multi-Path TCP (MPTCP) is a new standardized transport protocol that enables devices to utilize multiple network interfaces. The default MPTCP path scheduler prioritizes paths with the smallest round trip time (RTT). In this work, we examine whether the default MPTCP path scheduler can provide applications the ideal aggregate bandwidth, i.e., the sum of available bandwidths of all paths. Our experimental results show that heterogeneous paths cause under-utilization of the fast path, resulting in undesirable application behaviors such as lower video streaming quality than can be obtained using the available aggregate bandwidth. To solve this problem, we propose and implement a new MPTCP path scheduler, ECF (Earliest Completion First), that utilizes all relevant information about a path, not just RTT. Our results show that ECF consistently utilizes all available paths more efficiently than other approaches under path heterogeneity, particularly for streaming video

Gibbens, RJ

Lim, YS

Nahum, EM

Towsley, D

English

© 2017 Association for Computing Machinery. Multi-Path TCP (MPTCP) is a new standardized transport protocol that enables devices to utilize multiple network interfaces. The default MPTCP path scheduler prioritizes paths with the smallest round trip time (RTT).In this work, we examine whether the default MPTCP path scheduler can provide applications the ideal aggregate bandwidth, i.e., the sum of available bandwidths of every paths. Our experimental results show that heterogeneous paths cause underutilization of the fast path, resulting in undesirable application behaviors such as lower streaming quality in a video than can be obtained using the available aggregate bandwidth. To solve this problem, we propose and implement a new MPTCP path scheduler, ECF (Earliest Completion First), that utilizes all relevant information about a path, not just RTT. We compare ECF with both the default and other MPTCP path schedulers, using both an experimental testbed and in-the-wild measurements. Our results show that ECF consistently utilizes all available paths more efficiently than other approaches under path heterogeneity, particularly for streaming video. In Web browsing workloads, ECF also does better in some scenarios and never does worse

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ECF: An MPTCP path scheduler to manage heterogeneous paths

ECF: An MPTCP Path Scheduler to Manage Heterogeneous PathsYeon-sup LimUniversity of Massachusetts Amherstylim@cs.umass.eduErich M. NahumIBM Researchnahum@us.ibm.comDon TowsleyUniversity of Massachusetts Amhersttowsley@cs.umass.eduRichard J. GibbensUniversity of Cambridgerichard.gibbens@cl.cam.ac.ukABSTRACTMulti-Path TCP (MPTCP) is a new standardized transport protocolthat enables devices to utilize multiple network interfaces. Thedefault MPTCP path scheduler prioritizes paths with the small-est round trip time (RTT). In this work, we examine whether thedefault MPTCP path scheduler can provide applications the idealaggregate bandwidth, i.e., the sum of available bandwidths of allpaths. Our experimental results show that heterogeneous pathscause under-utilization of the fast path, resulting in undesirable ap-plication behaviors such as lower video streaming quality than canbe obtained using the available aggregate bandwidth. To solve thisproblem, we propose and implement a new MPTCP path scheduler,ECF (Earliest Completion First), that utilizes all relevant informationabout a path, not just RTT. Our results show that ECF consistentlyutilizes all available paths more efficiently than other approachesunder path heterogeneity, particularly for streaming video.1 INTRODUCTIONOne significant factor that affects MPTCP performance is the de-sign of the path scheduler, which distributes traffic across avail-able paths according to a particular scheduling policy. The defaultpath scheduler of MPTCP is based on round trip time (RTT) esti-mates, that is, given two paths with available congestion windowspace, it prefers to send traffic over the path with the smallest RTT.While simple and intuitive, this scheduling policy does not care-fully consider path heterogeneity, where available bandwidths andround trip times of the two paths differ considerably. This pathheterogeneity is common in mobile devices with multiple interfaces[2, 4, 5, 9, 12] and can cause significant reorderings at the receiver-side [1–3, 7, 13]. To prevent this, MPTCP includes opportunisticretransmission and penalization mechanisms along with the defaultscheduler [10]. In long-lived flows, e.g., large file transfer, MPTCPis able to enhance performance using these mechanisms. However,a large number of Internet applications such as Web browsing andvideo streaming usually generate traffic which consists of multipleuploads/downloads for relatively short durations. We find that inthe presence of path heterogeneity, the default MPTCP scheduler isPermission to make digital or hard copies of all or part of this work for personal orclassroom use is granted without fee provided that copies are not made or distributedfor profit or commercial advantage and that copies bear this notice and the full citationon the first page. Copyrights for components of this work owned by others than ACMmust be honored. Abstracting with credit is permitted. To copy otherwise, or republish,to post on servers or to redistribute to lists, requires prior specific permission and/or afee. Request permissions from permissions@acm.org.SIGMETRICS’17, Urbana-Champaign, Illinois, USA© 2017 ACM. 123-4567-24-567/08/06. . . $15.00DOI: 1234567898unable to efficiently utilize some paths with such a traffic pattern.In particular it does not fully utilize the highest bandwidth paths,which should be prioritized to achieve the highest performance andlowest response time.In this work, we propose a novel MPTCP path scheduler to maxi-mize fast path utilization, called ECF (Earliest Completion First). Tothis end, ECF monitors not only RTT estimates, but also the currentsubflow bandwidths (i.e., congestion windows) and the amountof data available to send (i.e., the send buffer). By determiningwhether using a slow path for the injected traffic will cause fasterpaths to become idle, ECF more efficiently utilizes the faster paths,maximizing throughput, minimizing download time, and reducingout-of-order packet delivery. Our experimental results demonstratethat ECF successfully avoids undesirable idle periods, achievinggreater throughput with higher path utilization than the defaultscheduler. At the same time, it performs as well as other schedulersunder symmetric path conditions.2 THE EFFECT OF PATH HETEROGENEITYWe examine the effect of path heterogeneity on application perfor-mance using adaptive video streaming, since it is currently one ofthe dominant applications in use over the Internet [11]. Wemeasurethe average video bit rate obtained by an Android DASH (DynamicAdaptive Streaming over HTTP) streaming client while limiting thebandwidth of the WiFi and LTE subflows on the server-side usingthe Linux traffic control utility tc [8]. The streaming client usesa state-of-art adaptive bit rate selection (ABR) algorithm [6]. Thechoice of ABR does not significantly affect the results in this exper-iment as we use fixed bandwidths for each interface. The oppor-tunistic retransmission and penalizationmechanisms are enabled bydefault. Each experiment consists of five runs, where a run consistsof the playout of the 20 minute video of which available resolutionsare 144p to 1080p. Table 1 presents the bit rates correspondingto each resolution. We choose bandwidth amounts slightly largerthan those listed in Table 1, i.e., {0.3, 0.7, 1.1, 1.7, 4.2, 8.6} Mbps, toensure there is sufficient bandwidth for that video encoding.Figure 1(a) presents the ratio of the average bit rate achievedversus the ideal average bit rate available, based on the bandwidthcombinations, when using the default MPTCP path scheduler. Thefigure is a grey-scale heat map where the darker the area is, thecloser to the ideal bit rate the streaming client experiences. Thecloser the ratio is to one, the better the scheduler does in achievingthe potential available bandwidth. The values are averaged overfive runs. In a streaming workload, we define the ideal averagebit rate as the minimum of the aggregate total bandwidth and theSIGMETRICS’17, June 2017, Urbana-Champaign, Illinois, USA Y. Lim et. al.Resolution 144p 240p 360p 480p 760p 1080pBit Rate (Mbps) 0.26 0.64 1.00 1.60 4.14 8.47Table 1: Video Bit Rates vs. Resolution0.30.71.11.74.28.60.3 0.7 1.1 1.7 4.2 8.6LTE (Mbps)WiFi (Mbps) 0 0.2 0.4 0.6 0.8 1(a) Default0.30.71.11.74.28.60.3 0.7 1.1 1.7 4.2 8.6LTE (Mbps)WiFi (Mbps) 0 0.2 0.4 0.6 0.8 1(b) ECFFigure 1: Ratio of Measured Average Bit Rate vs. Ideal Aver-age Bit Rate (darker is better)bandwidth required for the highest resolution at that bandwidth.For example, in the 8.6 MbpsWiFi and 8.6 Mbps LTE pair (the upperright corner in Figure 1(a)), the ideal average bit rate is 8.47 Mbps,since the ideal aggregate bandwidth (8.6+8.6 = 17.2Mbps) is largerthan the required bandwidth for the highest resolution of 1080p(8.47 Mbps). Since the full bit rate is achieved, the value is one andthe square is black.Figure 1(a) shows that, when significant path heterogeneity ex-ists, the streaming client fails to obtain the ideal bit rate. For exam-ple, whenWiFi and LTE provide 0.3Mbps and 8.6Mbps, respectively(the upper left box in Figure 1), the streaming client retrieves 480pvideo chunks, which requires only 2 Mbps, even though the idealaggregate bandwidth is larger than 8.47 Mbps. Thus, the value isonly 25% of the ideal bandwidth and the square is light grey. Thisproblem becomes even more severe when the primary path (WiFi)becomes slower (compare the 0.3 Mbps & [0.3 – 8.6] Mbps and 8.6Mbps & [0.3 – 8.6Mbps] pairs), as shown by the grey areas in theupper left and lower right corners.3 ECF SCHEDULERTo solve the performance degradation problem with path hetero-geneity, we propose a new MPTCP path scheduler, called ECF (Ear-liest Completion First). An MPTCP sender stores packets both in itsconnection-level send buffer and in the subflow level send buffer(if the packet is assigned to that subflow). Assume that there are kpackets in the connection level send buffer, which have not beenassigned (scheduled) to any subflow. If the fastest subflow in termsof RTT has available CWND, the packet can simply be scheduledto that subflow. If the fastest subflow does not have available space,the packet needs to be scheduled to the second fastest subflow.We denote the fastest and the second fastest subflows as xfand xs , respectively. Let RTTf , RTTs andCWNDf ,CWNDs be theRTTs and CWNDs of xf and xs , respectively. If the sender waitsuntil xf becomes available and then transfers k packets through xf ,it will take approximately RTTf + kCW NDf ×RTTf , i.e., the waitingand transmission time of k packets. Otherwise, if the sender sendssome packets over xs , the transmission will finish after RTTs withCWND full……… subflow 𝑥𝑓subflow 𝑥𝑠CWND availablesend𝑅𝑇𝑇𝑠𝑘Connection level sndbuf𝑘𝐶𝑊𝑁𝐷𝑓times transfer𝑅𝑇𝑇𝑓< 𝑅𝑇𝑇𝑓SchedulingFigure 2: The case for waiting for the fast subflowor without completing k packet transfers. Thus, as shown in Figure2, in the case of RTTf + kCW NDf × RTTf < RTTs , using xf afterit becomes available can complete the transmission earlier thanusing xs immediately. If RTTf + kCW NDf ×RTTf ≥ RTTs , there aresufficient number of packets to send, so that usingxs at thatmomentcan decrease the transmission time by utilizing more bandwidththan just by using xf .ECF checks the above inequality to decide whether it will wait forxf or immediately use xs . Figure 1(b) shows that ECF successfullyenables the streaming client to obtain average bit rates closest tothe ideal average bit rate, and does substantially better than thedefault when paths are not symmetric.REFERENCES[1] Y.-C. Chen, Y.-S. Lim, R. J. Gibbens, E. Nahum, R. Khalili, and D. Towsley. Ameasurement-based study of Multipath TCP performance in wireless networks.In Proc. of ACM IMC, pages 455–468, Nov 2013.[2] S. Deng, R. Netravali, A. Sivaraman, and H. Balakrishnan. WiFi, LTE, or both?Measuring multi-homed wireless Internet performance. In Proc. of ACM IMC,2014.[3] S. Ferlin, O. Alay, O. Mehani, and R. Boreli. BLEST: Blocking estimation-basedMPTCP scheduler for heterogeneous networks. In Proc. of IFIP Networking, pages1222–1227, 2016.[4] B. Han, F. Qian, L. Ji, and V. Gopalakrishnan. MP-DASH: Adaptive video stream-ing over preference-aware multipath. In Proc. of ACM CoNEXT, 2016.[5] J. Huang, Q. Feng, A. Gerber, Z. M. Mao, S. Sen, and O. Spatscheck. A closeexamination of performance and power characteristics of 4G LTE networks. InProc. of ACM MobiSys, pages 225–238, 2012.[6] T.-Y. Huang, R. Johari, N. McKeown, M. Trunnell, and M. Watson. A buffer-basedapproach to rate adaptation: Evidence from a large video streaming service. InProc. of ACM SIGCOMM, pages 187–198, 2014.[7] N. Kuhn, E. Lochin, A. Mifdaoui, G. Sarwar, O. Mehani, and R. Boreli. DAPS:Intelligent delay-aware packet scheduling for multipath transport. In Proc. ofIEEE ICC, pages 1222–1227, 2014.[8] Linux Foundation. Linux advanced routing and traffic control. http://lartc.org/howto/.[9] A. Nikravesh, Y. Goo, F. Qian, Z. M. Mao, and S. Sen. An in-depth understandingof multipath TCP on mobile devices: Measurement and system design. In Proc.of ACM MobiCom, 2016.[10] C. Raiciu, C. Paasch, S. Barre, A. Ford, M. Honda, F. Duchene, O. Bonaventure,and M. Handley. How hard can it be? Designing and implementing a deployablemultipath TCP. In Proc. of USENIX NSDI, pages 399–412, 2012.[11] Sandvine. Global Internet phenomena report, Latin Amer-ica and North America 2016. https://www.sandvine.com/downloads/general/global-internet-phenomena/2016/global-internet-phenomena-report-latin-america-and-north-america.pdf.[12] J. Sommers and P. Barford. Cell vs. wifi: on the performance of metro area mobileconnections. In Proceedings of the 2012 ACM conference on Internet measurementconference, pages 301–314. ACM, 2012.[13] F. Yang, P. Amer, and N. Ekiz. A scheduler for multipath TCP. In Proc. of ICCCN,pages 1–7, 2013.

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ECF: An MPTCP path scheduler to manage heterogeneous paths

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