Improving Congestion Control for Concurrent Multipath Transfer through Bandwidth Estimation based Resource Pooling

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1 Improvng Congeston Control for Concurrent Multpath Transfer through Bandwdth Estmaton based Resource Poolng Samar Shalendra, R. Bhattacharjee, Member IEEE, Sanjay K. Bose, Senor Member IEEE Department of Electroncs & Electrcal Engneerng, I.I.T. Guwahat, Guwahat, INDIA {samar, ratnajt, Abstract Stream Control Transmsson Protocol (SCTP) was ntroduced n 2001 as a multpath varant to tradtonal transport protocols,.e. Transmsson Control Protocol (TCP) and User Datagram Protocol (UDP). Concurrent Multpath Transfer (CMT) has been proposed as an extenson for SCTP to support concurrent usage of avalable multple paths. In ths paper, we propose a new congeston control algorthm for CMT-SCTP based on the prncple of resource poolng. We use the connecton bandwdth estmates to obtan the collecton of the network resources beng used by dfferent flows on multple paths. Based on these bandwdth estmates, we have used the bandwdth estmaton based resource poolng approach to adjust the congeston wndow of the respectve paths. We compare our proposed scheme wth CMT-SCTP through ns-2 based smulatons. Keywords-Stream Control Transmsson Protocol, Concurrent Multpath Transfer, Congeston Control, Bandwdth Estmates, Resource Poolng. I. INTRODUCTION Consderable research work s currently underway to support multpath transport [1-7] n networks so that resources can be effcently utlzed for data transfers. In 2001, IETF proposed the Stream Control Transmsson Protocol (SCTP) to support mult-homed end nodes [8-10] n a network. SCTP also has the potental to support mult-streamng though that s not part of the exstng standards. Ths would reduce the Head of Lne (HOL) blockng that s sometmes experenced by TCP [11]. Janardhan et al. [12] proposed a Concurrent Multpath Transfer (CMT) extenson to SCTP whch allows concurrent usage of multple dsjont end-to-end paths n the network. Implementaton of proper Congeston Control algorthms s necessary for the operaton of any transport protocol over a network and must be ncorporated n any Internet based mplementaton. The congeston control mechansm proposed for CMT s largely based on the mechansm followed by tradtonal TCP and s not effectve when multple paths are concurrently used. Whle proposng and mplementng new congeston control mechansms for SCTP, a prmary requrement would be that the new protocol mplementaton should not take undue advantage of the network and should not harm other fellow TCP and UDP flows wth whch t coexsts n the network. For provdng better QoS to an end user, load balancng has become a de-facto mplementaton strategy for any servce provder. The basc requrement for load balancng s to aggregate all the avalable resources. Thus load balancng s an example of resource poolng. The basc phlosophy of resource poolng s to look at all the avalable resources as a sngle pool of resources. The avalable bandwdth of the ndvdual paths s a typcal example of the resource under consderaton. Accordng to resource poolng prncple, the total bandwdth of the set of avalable paths wll be shared equally by the flows usng those paths. Currently IETF s workng on MultPath TCP, a TCP extenson to support multple path usage n parallel fashon. Racu et al. [13] have proposed a congeston control mechansm based upon resource poolng prncple [14]. Ths mechansm tres to acheve the followng three goals: ) Improved throughput.e. multpath flow should perform at least as good as the best of the path avalable n the pool; ) Do not harm.e. other fellow flows on the shared lnk should not unduly suffer; ) Balance Congeston.e. the congeston experenced by all the paths should be smlar. Besdes mprovng the congeston control algorthm, other efforts are also underway to mprove the performance of multpath flows. Drebholz et al. [15] have proposed schedulng of packets on multple paths n an optmzed fashon to mprove the throughput of the network and also provde for RTT compensaton on multple paths wth dssmlar RTTs. Tsa et al. [16] have proposed a multpath transmsson control scheme (MTCS) to mprove throughput for real tme data transmsson usng multple paths. They have proposed a packet schedulng polcy to handle out-of-order packets n multpath transmsson. Key et al. [17] have gven a mathematcal analyss of the benefts of dong coordnated congeston control wth actve state nformaton of the paths. In ths paper, we have proposed a modfed congeston control algorthm for CMT-SCTP. Our proposed algorthm apples the resource poolng prncple based upon the bandwdth estmates obtaned by observng the data flow on the paths. The rest of the paper s organzed as follows. In Secton II, we present the changes beng proposed n the congeston control algorthm. Secton III presents the network model of the scenaros where the proposed changes are tested and compares the results obtaned wth those obtaned by usng the CMT protocol. Secton IV concludes the paper followed by references used.

2 II. PROPOSED CHANGES IN CONGESTION CONTROL Whle technologes lke ATM can ensure guaranteed QoS to the end users, Internet typcally supports a best effort delvery servce. TCP and UDP are the two transport protocols that are manly used n the current Internet. UDP s a datagram based protocol whch does not provde any relablty. TCP does end-to-end error correcton and provdes relablty to the underlyng Internet Protocol (IP) layer whch s connectonless and unrelable n nature. Recently, CMT-SCTP and Multpath TCP (MPTCP) have been proposed to use multple parallel paths for data transfer. Ths should provde better resource utlzaton and ncrease the error reslence of the underlyng network. The congeston control of SCTP s based on the TCP congeston control. TCP based congeston control works well wth standard SCTP because n spte of beng a multhomng protocol, t uses only one path at a tme and swtches to an alternate path only f the prmary path fals. However, CMT- SCTP uses all the paths concurrently. Hence t takes undue advantage of shared lnks on multple paths whch wll create ssue of farness and TCP frendlness. In lterature, t has been demonstrated that CMT flows unduly harm other network flows on the shared lnks and captures more bandwdth n the rato of the number of concurrent paths beng establshed across any shared bottleneck lnk en-route. These problems motvated us to reexamne the congeston control algorthm of CMT-SCTP and suggest an approach whch wll allow multple paths to be used wthout ncurrng these dffcultes. In ths paper, we have proposed a modfed congeston control algorthm usng Bandwdth Estmaton based Resource Poolng (BERP) to overcome the above sad shortcomngs n CMT congeston control. Wschk et al. ntroduced the concept of resource poolng n [14]. Ths prncple consders all the ndependent resources avalable on the network as a sngle pool of resources. Recently, Racu et al. [13] have nvestgated fully coupled and lnk coupled congeston control algorthms for Multpath TCP (MPTCP). In ths work they demonstrate that lnked congeston control based algorthm exhbts better performance n terms of throughput and farness on paths wth dssmlar round trp tme (RTT). However, they stll use a TCP lke approach to decrease the congeston wndow. Ths dfference n polcy for congeston wndow evoluton causes unfarness. Hence to balance the ncrease n congeston wndow wth the decrease of the same at equlbrum, a factor of aggressveness (α) was ntroduced. Drebholz et al. [18] have compared an MPTCPlke algorthm wth CMT-SCTP. In ths paper, we also consder the resources beng used by CMT as a sngle pool of resources. However, we argue that ths approach s not optmum on heterogeneous networks wth wreless lnks. Unlke wred lnks, losses n wreless lnks may happen because of reasons other than congeston (e.g. nose or lnk qualty). Hence we consder the resources to be as a sngle pool of resources durng the congeston detecton phase as well. Currently the CMT congeston control algorthm s based upon the standard TCP based congeston control algorthm wth some mnor modfcatons. Note that TCP s congeston control algorthm conssts of three phases, ) Slow Start phase, ) Congeston Avodance phase and ) Congeston detecton phase. CMT starts the connecton wth the slow-start phase. It exhbts exponental growth n the congeston wndow tll the slow start threshold (ssthresh) s reached. Once the cwnd value crosses ssthresh, t gets nto the congeston avodance phase. Durng ths congeston avodance phase, CMT ncreases ts congeston wndow by one Maxmum Transmsson Unt (MTU) for every RTT. We propose to modfy congeston avodance rule as follows. Congeston Avodance Phase: Increase the value of cwnd of th path by: where mn(α * P a * MTU/w T, P a * MTU/w ) P a = P a - w P a = Partal Bytes Acked w T = Total congeston wndow w = Congeston wndow of th path srtt = smoothed RTT of th path β * w 2* wt *max 2 srtt α 2 w srtt We have adopted the same methodology to calculate α, the factor of aggressveness as the one suggested n [13]. The value of β s gven by (2). Whenever the sender experences any loss n packet, tradtonal SCTP algorthm nfers that the path s n congeston and to reduce the congeston on that path t cuts the congeston wndow by half. However, n wreless lnks ths may not always be the case. A wreless lnk may experence hgher packet loss even when the lnk s not n congeston. It has also been demonstrated n lterature that CMT may be unfar to fellow TCP flows on a shared lnk and may capture unduly more bandwdth. We plan to avod ths n our approach by usng resource poolng durng congeston detecton for adjustng the congeston wndow. To apply resource poolng, we need to measure the avalable resources,.e. bandwdth used by the flow. We use the bandwdth estmaton for measurng the lnk bandwdth avalable to the user. Several technques for bandwdth estmaton have been proposed n lterature. We use the same approach for the bandwdth estmaton as the one beng used n TCP Westwood. The modfed rule for decreasng the congeston wndow wll be as follows. Congeston Detecton Phase: f (four duplcates are receved) s = max(w β *w, 4* MTU) w = s f (Tmeout occurred) s = max(w β *w, 4* MTU) (1)

3 where w = 1 * MTU s = slow start threshold of th path BWE β (2) BWE where, BWE = Bandwdth estmate of th path Snce CMT uses all the paths concurrently, we can use the SACKs on each path for measurng the bandwdth estmate on respectve path. If the k th SACK s receved at tme nstant t k and acknowledges d k new bytes then the current bandwdth sample B k s gven by B k dk t (3) k tk 1 For our tests, we have assumed that the source s transferrng a 60MB fle to the destnaton usng ftp. We have measured the tme taken to transfer the fle from the source to the destnaton whle the packet loss probablty of lnks s vared from 1% to 10% (Fg.2) for both the CMT congeston control (CMT-CC) algorthm and our proposed algorthm (CMT-BERP). The results ndcate that when the losses are purely due to error and there s no congeston n the lnks, CMT-BERP algorthm performs better n comparson to the CMT-CC algorthm. Ths s because CMT-BERP algorthm does not abruptly decrease the congeston wndow on every loss by assumng that ths s beng caused by congeston. Instead, t estmates the bandwdth of the connecton and decreases the bandwdth accordng to the resource poolng prncple. In Fg. 3, we show how the congeston wndow at the source evolves for the case when the packet loss probablty s 10%. In order to average the sampled bandwdth, a low pass flter s appled to these samples as n [19]. The fltered bandwdth estmate s gven as B B ^ ^ k k 1 Bk pb. k 1 (1 p). 2 (4) where p s a constant whose value s typcally set to 0.9. Note that MPTCP congeston control s also a specal case of our algorthm wth β=0.5. III. PERFORMANCE ANALYSIS To study the performance of the proposed congeston control, we have consdered three scenaros. A. Dsjont Paths wth no congeston We consder the network graph as shown n Fg.1. Ths network conssts of a dual homed source (S) and a dual-homed destnaton (D). Both the paths have several dsjonts lnks as shown n the fgure. The lnks between the source nterfaces and the Node-1 (N1) and Node-2 (N2) are wred lnks wth bandwdth 10Mpbs and fxed packet loss probablty of 1%. The lnks between N1 & Node-3 (N3) and also N2 & Node-4 (N4) are lnks wth 1Mbps bandwdth each and the packet loss probablty of these lnks s vared n the range 1-10%. The lnks between destnaton node nterface and N3 and N4 are also wred lnks wth 10Mbps bandwdth and 1% packet loss probablty. Fgure 2. Average Transmsson Tme for dsjont paths Fgure 3. Congeston Wndow evoluton for CMT congeston control and CMT-BERP algorthm for packet loss probablty of 10%. Fgure 1. Network Graph for dsjont path wth smlar characterstcs

4 B. Paths wth Shared Bottleneck lnk Fg. 4 shows an example of a scenaro where there s a shared bottleneck lnk. The lnk characterstcs are same as n (A) except that the shared bottleneck lnk between N1 and N2 has 1Mpbs bandwdth wth a packet loss probablty that s vared from 1% to 10%. Fgure 4. Network Graph for shared bottleneck The test results for transferrng a 60MB fle are shown n Fg.5 whch shows the average fle transmsson tme for varyng packet loss probabltes n the bottleneck lnk. The results ndcate that CMT-BERP algorthm performs better than the CMT-CC algorthm for ths scenaro. However, because of the shared bottleneck lnk, the average transmsson tme s hgher than the earler case where dsjont paths were used. The way the Congeston Wndow evolves n ths scenaro has been shown n Fg. 6 for both the congeston control schemes when the bottleneck lnk has 10% packet loss probablty. C. Dsjont paths wth only congeston losses For ths scenaro, as n Fg. 7, we consder lnks between N1, N2 and N3, N4 to be error free. We assume that U1 and U2 are UDP sources generatng CBR traffc wth U3 and U4 as ther respectve snk nodes. These UDP sources are ntroduced to create congeston n the network between the source S and destnaton D. Fgure 7. Network Graph for dsjont lnks wth congeston losses Fgure 5. Average Transmsson Tme for Shared bottleneck lnk. The packet sze assumed for these UDP sources s 512 bytes. In ths case, we have obtaned average throughput acheved for both CMT-CC algorthm and CMT-BERP algorthm. The results are shown n Fg. 8. Fgure 6. Congeston Wndow evoluton for CMT congeson control and CMT-BERP algorthm for packet loss probablty of 10%. Fgure 8. Average Throughput for dfferent Average load of UDP traffc. We observe that n ths scenaro, both congeston control strateges show almost dentcal performances. Ths s

5 expected, as n ths system, vrtually all the losses are caused by congeston. The evoluton of the congeston wndow for ths scenaro has also been shown n Fg. 9 for average UDP traffc load of 0.9. IV. CONCLUSION We have proposed and studed the performance of a modfed congeston control algorthm. Ths algorthm s based on the multpath TCP congeston control algorthm. Though we have tested t only for CMT-SCTP, we expect that ths can be sutably adapted for use wth any multpath transport protocol. Our proposed algorthm (CMT-BERP) uses resource poolng durng congeston detecton phase for better overall performance where the bandwdth resource avalable s obtaned usng a sutable bandwdth estmaton approach. The results ndcate that CMT-BERP algorthm adapts better to packet drops than CMT congeston control algorthm and therefore provdes better resource utlzaton. Snce ths algorthm s based upon the MPTCP algorthm, t s lkely to nhert the farness and frendlness property of MPTCP as well. Fgure 9. Congeston Wndow evoluton for average UDP traffc load of 0.9. REFERENCES [1] Huazhong Han, Srnvas Shakkotta, C.V. Hollot, R. Srkant, Don Towsley, Mult-Path TCP: A jont Congeston Control and Routng Scheme to Explot Path Dversty n the Internet, IEEE/ACM Transactons On Networkng, vol. 14, no. 6, pp , December [2] S. H. Low, Optmzaton flow control wth on-lne measurement or multple paths, n Proceedngs of the 16th Internatonal Teletraffc Congress, Ednburgh,U.K., [3] We-Hua Wang, M. Palanswam, Steven H. Low, Optmal flow control and routng n mult-path networks, Performance Evaluaton, Volume 52, Issues 2-3, Internet Performance and Control of Network Systems, pp , Aprl [4] Henrk Abrahamsson, Bengt Ahlgren, Juan Alonso, Anders Andersson, Per Kreuger, A Mult-path Routng Algorthm for IP Networks Based on Flow Optmsaton, Proceedngs of the 3rd nternatonal conference on qualty of future nternet servces and nternet chargng and QoS technologes 2nd nternatonal conference on From QoS provsonng to QoS chargng, pp , [5] V. Sharma, S. Kalyanaraman, K. Kar, K.K. Ramakrshnan, V. Subramanan, MPLOT: A Transport Protocol Explotng Multpath Dversty usng Erasure Codes, Proceedngs of 27th Conference on Computer Communcatons (INFOCOM 2008), pp , Aprl [6] V. Sharmat, K. Kart, S. Kalyanaraman, K.K. Ramakrshnan, Explotng Multple Paths and Dversty n Wreless Networks for Hgh Goodput and Low Latency, Frst Internatonal conference on Communcaton Systems and Networks and Workshops (COMSNETS 2009), pp. 1-10, January, [7] Yashar Ganjal, Abtn Keshavarzan, Load Balancng n Ad Hoc Networks:Sngle-path Routng vs. Mult-path Routng, Twenty-thrd Annual Jont Conference of the IEEE Computer and Communcatons Socetes (INFOCOM 2004), vol. 2, pp , March [8] RFC4960, Stream Control Transmsson Protocol,2007. [9] Shaojan Fu, Mohammed Atquzzaman, SCTP: State of the Art n Research, Products, and Techncal Challenges, IEEE Communcaton Magazne, vol. 42, ssue: 4, pp , [10] Preeth Natarajan, Fred Baker, Paul D. Amer, Jonathan T. Leghton, SCTP: what, why, and how, IEEE Internet Computng Magazne, vol. 13, no. 5, pp , September [11] Scharf, M. and Kesel, S., Head-of-lne Blockng n TCP and SCTP: Analyss and Measurements, IEEE Global Telecommuncatons Conference, pp. 1-5, November [12] Janardhan R. Iyengar, Paul D. Amer, Randall Stewart, Concurrent Multpath Transfer Usng SCTP Multhomng Over Independent Endto-End Paths, IEEE/ACM Transactons On Networkng, vol. 14, no. 5, pp , October [13] Costn Racu, Damon Wschk, Mark Handley, Practcal Congeston Control for Multpath Transport Protocols, Tech Report, Unversty College London, UK. [14] Damon Wschnk, The Resource Poolng Prncple, Proceddngs of SIGCOMM Computer Communcaton, pp , September [15] Thomas Drebholz, R. Segglemann, M. Tuxen, Erwn P. Rathgeb, Transmsson Schedulng Optmzaton for Concurrent Multpath Transfer, Proceddngs of the 8 th Internatonal Workshop on Protocols for Furure, Large-Scale and Dveerse Network Transports (PFLDNet), vol. 8, November [16] M.F.Tsa, N.Chlamkurt, J.H.Park, C.K. Sheh, Mult-path transmsson control scheme combnng bandwdth aggregaton and packet schedulng for real-tme streamng n mult-path envronment, IET Communcatons, vol. 4, no. 8, pp , May, [17] Peter key, Laurent Massoule, Don Towsley, Path Selecton and multpath congeston control, 26th IEEE Internatonal Conference on Computer Communcatons (INFOCOM 2007), pp , May [18] Thomas Drebholz, Martn Becke, Hakm Adhar, Erwn P. Rathgeb, On the Impact of Congeston Control for Concurrent Multpath Transfer on the Transport Layer, Proceedngs of 11th Internatonal Conference on Telecommuncatons, Austra, June [19] Caludo Casett, Enahancng TCP Congeston Control Va Connecton bandwdth Estmaton, A Performance Study, CteSeerX - Scentfc Lterature Dgtal Lbrary and Search Engne (Unted States), tcpw.pdf, 2007.