Evaluation of TCP Variants and Bandwidth on Demand over Next Generation Satellite Network

Transcription

1 Evaluaton of TCP Varants and Bandwdth on Demand over Next Generaton Satellte Network Songrth Kttperachol, Zhl Sun and Hatham Cruckshank Centre for Communcaton Systems Research Unversty of Surrey Guldford, Surrey, GU2 7XH, UK Abstract the Internet has become an mportant part of day to day actvtes. There s hardly a day wthout usng Internet, such as readng Emals and artcles as well as enjoyng musc and vdeo. Thus, t s very mportant for the Internet to be provded to anyone anywhere. Terrestral network has been the underlyng nfrastructure for the Internet. However, terrestral network by tself cannot always satsfy all of the growng demands for the Internet, partcularly n the remote areas. Thus, the deployment of the Next Generaton Satellte Network (NGSN) s needed to fll n the gap and break the dgtal dvde. Ths paper evaluates how the performances of TCP over NGSN wth dynamc bandwdth allocaton mechansm. The TCP used n ths work s a real-world TCP based on both Lnux and Wndow Vsta mplementatons whch have been ntegrated nto a network smulator, INET. The study reveals that the TCP performances n terms of utlzaton and robustness, frendlness and farness, and user s perceved Qualty of Servce are clearly affected by the dynamc bandwdth allocaton mechansm. Keywords Lnux, TCP, Next Generaton Satellte Network, Bandwdth on Demand I. INTRODUCTION Over the Internet, transmsson Control Protocol or TCP [1] s the most domnant protocol snce t s the protocol on whch legacy Internet applcatons are based. Although NewReno, the latest IETF standard TCP [2], performs exceptonally well over the tradtonal network, ts performances are qute lmted n the next generaton network havng large Bandwdth Delay Product (BDP). Thus, varous developments have been carred out to mprove TCP performances. Lnux TCP s a part of the Lnux TCP/IP network stack whch provdes ten other TCP varants,.e. bc, cubc, hghspeed, hamlton, hybla, scalable, vegas, veno and westwood, n addton to the standard TCP, reno. Detals of these varants can be found n lteratures. In addton, the Wndow Vsta based TCP, known as compound, s ntegrated nto Lnux kernel by [3]. Thus, to obtan realstc TCP dynamc, TCP functonaltes of Lnux TCP/IP network stack (ver ) are ntegrated nto a network smulator, INET [4, 5]. As a consequence, more accurate TCP dynamc can be observed n smulated network envronments of INET. The advance n satellte technologes,.e. powerful codng, hghspeed lnk, multple spotbeam and on-board processor, presents NGSN. Moreover, the adopton of the latest standards n Dgtal Vdeo Broadcastng (DVB),.e. DVB-S/S2 [6, 7] standard clent enhanced clent Fg. 1 Smulaton setup NGSN standard server enhanced server and DVB-RCS [8] enables fully nteractve Internet servces over NGSN at a relatvely low cost. Wth dynamc bandwdth allocaton,.e. Bandwdth on Demand (BoD), NGSN s able to utlze bandwdth effcently. Nonetheless, BoD when coupled wth network characterstcs, such as large BDP and hgh data loss rate, can consderably affect TCP performances and thus Internet servces over NGSN. Ths work s to evaluate the performances of non standard TCP, based on Lnux and Mcrosoft Vsta mplementatons, aganst reno n terms of utlzaton and robustness, frendlness and farness, and user s perceved Qualty of Servce over the NGSN wth BoD. Smulaton setup s outlned n Secton II whereas smulaton results and dscussons are gven n secton III. Fnally, secton IV presents conclusons and future works. II. SIMULATION SETUP The setup s based on NGSN. The satellte tself s the next generaton geostatonary satellte havng multple spotbeams, hghspeed lnks and on-board processor. The downlnk/uplnk s based on TDM/TDMA and both have the same bandwdth. The uplnk bandwdth s segmented nto frames. Each frame contans 48 tme slots; one of whch s a preamble slot and the rest are data slots. A data slot s equvalent to eght MPEG-II (188-bytes) frames or 154 bytes; thus supportng Ethernet frames or 15 bytes. In general, gven bt rate (R) and slot length (SL), slot duraton (SD) can be calculated as follow. SD = SL (1) R /8/$ IEEE IWSSC 28 Authorzed lcensed use lmted to: Unversty of Surrey. Downloaded on Aprl 12,21 at 9::4 UTC from IEEE Xplore. Restrctons apply.

2 utlzaton utlzaton utlzaton Utlzaton (%) (a) 2-Mbps lnk wth 1-packet buffer (b) 4-Mbps lnk wth 2-packet buffer (c) 8-Mbps lnk wth 4-packet buffer Fg. 2 Utlzaton of TCP over the satellte lossy lnk wth BoD The total frame duraton s 48 SD. The bandwdth s allocated dynamcally accordng to a request/allocaton scheme of BoD [9]. The request s based on Volume Based Dynamc Capacty (VDBC) and the allocaton s based on proportonal allocator. Bandwdth s requested and allocated n numbers of slots and s performed once every superframe,.e. multple frames. The allocator s located on board the satellte where all the requests are collected and data slots are computed and then allocated n a round robn manner. Let R and A be slot request and slot allocaton whereas Q ( t) and D ( t) be queue occupancy and slot defct snce the last request for termnal, Q ( Δt) + L ( ) R = D Δt and R f R DS A = (3) R R f otherwse and s.t. A DS where x s the smallest nteger greater than x, DS s the data slots per frame and s.t. s the abbrevaton for subject to. The slot allocaton,.e. Termnal Burst Tme Plan (TBTP), s broadcasted at the begnnng of the next superframe. Once recevng the TBTP, satellte termnals can begn transmttng data but only n the desgnated data slots of the correspondng superframe. The allocaton tme T,.e. tme needed for a slot request to become effectve, follows ths expresson, T = n F DS SL T P (4) ( ) s.t. where n s the smallest nteger that satsfes the condton, F s frames per superframe and P s the propagaton delay, roughly 5 ms for geostatonary satellte. If the request s not entrely satsfed, slot defct s added to the next request. In ths study, the request/allocaton s performed every three frames and the network s setup as dsplayed n Fg. 1. Clent network conssts of standard and enhanced peers and connects to a satellte gateway. Two correspondng servers are (2) on the other sde of the network and connect to two gateways. A TCP applcaton (2 MB fle download) s requested by peers at a random tme whch s exponentally dstrbuted and one second on average. The uplnks are setup for three cases (a) 2-Mbps wth 1-packet, (b) 4-Mbps wth 2-packet and (c) 8-Mbps lnk wth 4-packet buffer. The choces of buffer szes are smply to accommodate the BDP of the satellte lnk. The network smulator used s the modfed INET [5]. III. RESULTS AND DISCUSSIONS A. Utlzaton and Robustness In general, utlzaton and robustness ndcate the effcency of TCP when utlzng a bottleneck lnk and when dealng wth packet losses. In ths study, the satellte lnks are assumed to be lossy wth bt error rate (BER) rangng from 1-1 to 1-7. Utlzatons and robustness of TCP over the satellte lnk wth BoD for the three lnk setups are dsplayed n Fg. 2. Fg. 2 (a) llustrates that wth 2-Mbps lnk and 1-packet buffer, all varants acheve utlzaton at 4 percents or lower. For low BER,.e. 1-1 to 1-9, reno, bc, cubc, hghspeed and scalable acheve utlzaton at roughly 3-4 percents whereas compound, hamlton, hybla, veno and westwood are at 2-3 percents. For hgh BER,.e. 1-8 to 1-7, cubc, hamlton and hybla are the only varants that do not suffer sgnfcant drop n utlzaton. However, vegas dsplays the lowest utlzaton at 1 percents or less regardless of BER. Fg. 2 (b) llustrates that wth 4-Mbps lnk and 2-packet buffer, all varants dsplay mprovement n utlzaton but stll lmted to 6 percents. It s clear that varants havng the hghest and the lowest utlzaton are veno and vegas respectvely. For low BER, bc and cubc acheve utlzaton at 5 percents whereas the rest are around 4-5 percents. For hgh BER, cubc, hamlton and hybla are the only varants havng utlzaton mantaned at 2 percents or more. Fg. 2 (c) llustrates that wth 8-Mbps lnk and 4- packet buffer, all varants appear to dsplay utlzaton smlar Authorzed lcensed use lmted to: Unversty of Surrey. Downloaded on Aprl 12,21 at 9::4 UTC from IEEE Xplore. Restrctons apply.

3 Throughput (Mbps) vs Tme (s) reno-bc reno-compound reno-cubc reno-hghspeed reno-hamlton reno-hybla (a) 2-Mbps lnk wth 1-packet buffer competng flows reno-bc reno-compound reno-cubc reno-hghspeed reno-hamlton reno-hybla (b) 4-Mbps lnk wth 2-packet buffer competng flows reno-bc reno-compound reno-cubc reno-hghspeed reno-hamlton reno-hybla to the results prevously found n Fg. 2 (b). But, compound and hghspeed are the only varants able to acheve utlzaton at 8 and 7 percents respectvely whereas vegas stll dsplays the lowest utlzaton. These low utlzatons are rather uncommon when a large fle download s consdered but t s all due to the nteracton between TCP and BoD. Pror to data transmsson, bandwdth has to be frst allocated. Accordng to (2) and (4), the numbers of data slots that can be requested gven the buffer sze of 1, 2 and 4 are 12, 141 and 141 slots per superframe and the allocaton tme gven the lnks of 2, 4 and 8 Mbps are.866,.866 and.649 s; therefore, ncreasng the overall latency and resultng n 1, 2 and 3 dle superframes pror to transmsson (c) 8-Mbps lnk wth 4-packet buffer Fg. 3 Aggregate throughput of two reno and two competng flows over the satellte lnk wth BoD competng flows Thus, to have all 141 data slots allocated per superframe, the mnmum buffer sze of 141 s needed. Lkewse, to request all data slots n the subsequent superframe, a buffer sze of 282 or more s requred n order to avod any packet drops f another 141 packets arrve before any of the prevous 141 packets can be transmtted. Evdently, when BoD s used, queued packets do not necessarly mply full utlzaton and larger buffers are essental to mantan full utlzaton. Accordng to the results, TCP usng an ncrease n delay as a sgn for network congeston, lke vegas, s lkely to dsplay sgnfcantly degraded performances due to large varatons n delay caused by BoD mechansm. In addton, the buffer sze of 1 or 2 packets are nsuffcent to request all 141 data Authorzed lcensed use lmted to: Unversty of Surrey. Downloaded on Aprl 12,21 at 9::4 UTC from IEEE Xplore. Restrctons apply.

4 throughput 1..5 Throughput (Mbps) (a) 2-Mbps lnk wth 1-packet buffer throughput throughput (b) 4-Mbps lnk wth 2-packet buffer (c) 8-Mbps lnk wth 4-packet buffer Fg. 4 Average throughput of reno and other competng flows over the satellte lnk wth BoD. slots for two consecutve superframes, therefore resultng n poor utlzatons. Although two consecutve superframes can be requested by the 4-packet buffer, three superframes are dle and cannot be utlzed before TBTP arrves. Gven that TCP requres feedbacks,.e. acknowledgement, to ncrease ts transmsson rate, havng more dle superframes can lead to even lower utlzaton. Clearly, BoD adversely affects the performances of TCP by ncreasng both round trp tme and delay varaton as well as leavng a number of superframes unutlzed. In addton to BoD, lower BER s also necessary to obtan hgher utlzaton partcularly for larger bandwdth. Accordng to the results, compound, hghspeed and veno acheve hgher utlzaton for very low BER whereas cubc, hamlton, hybla and scalable appear to be more robust when compared to reno. B. Frendlness and Farness In general, TCP s sad to be frendly f ts throughput s no hgher than that of reno under the same condtons and to be far f ts throughput s relatvely the same as that of reno when competng on the same bottleneck lnk. In ths study, four concurrent fle downloads are used; two of whch are reno whle the other two are other competng varants. The satellte lnks are assumed to be error free and the smoothed aggregate throughputs of reno flows and competng varants for the three lnk setups are dsplayed n Fg. 3. Fg. 3 (a) llustrates that wth 2-Mbps lnk and 1-packet buffer, all varants are frendly and far except for vegas that s frendly but unfar. In addton, even wth four concurrent flows,.e. two reno and two competng flows, 1-percent utlzaton s yet to be acheved. Ths s due to the effects of BoD on TCP mentoned earler. As a result, the aggregate throughputs n any cases are unable to acheve the far share of bandwdth at 1 Mbps,.e. half of the avalable bandwdth. Fg. 3 (b) llustrates that wth 4-Mbps lnk and 2-packet buffer, all varants are frendly and far except for vegas that s frendly but unfar. In addton, n the cases of reno, bc, compound, cubc, hghspeed, scalable, veno and westwood beng competng flows, the aggregate throughputs are able to acheve the far share of 2-Mbps bandwdth but not n the cases of hamlton and hybla. Fg. 3 (c) llustrates that wth 8-Mbps lnk and 4-packet buffer, all varants stll reman frendly and far except for vegas that remans frendly but unfar. In addton, the aggregate throughputs n all cases are able to acheve the far share of 4-Mbps bandwdth. A closer nvestgaton reveals that the throughput dsplays trajectory wth three dstnct characterstcs when convergng to the far share. The frst one s the sudden convergence, formng a rectangle-lke shape,.e. reno, compound, hghspeed and westwood. The second one s the slow convergence, formng a trangle-lke shape,.e. bc, cubc and scalable. The thrd one s smlar to the second one except wth hgher varaton durng convergence,.e. hamlton and hybla. Despte the fact that some varants, such as bc, cubc, hamlton, hybla and scalable, are known to be unfrendly and unfar to reno n the terrestral network, the study reveals that BoD mechansm s able to entrely change ther characterstcs when competng wth reno such that they become frendly and far. Accordng to the results, all TCP varants, except vegas, are found to be both frendly and far whereas vegas s found to be frendly but fals to be far. C. User s Preceved Qualty of Servce User s perceved QoS (UQoS) s used to ndcate on how well the gven servces perform from the user pont of vew. In general, hgher throughput often mples better UQoS, partcularly for downloadng servces. Based on the results of Fg. 3, average throughput of ndvdual competng flows s dsplayed n Fg. 4. In each group, the frst two bars are the average throughput of reno whereas the other two bars are that of other competng varants. The tcp? ndcates the competng varant that s labeled on the x-axs. In ths study, the average throughput s loosely used as UQoS. Fg. 4 (a) llustrates that wth 2-Mbps lnk and 1-packet Authorzed lcensed use lmted to: Unversty of Surrey. Downloaded on Aprl 12,21 at 9::4 UTC from IEEE Xplore. Restrctons apply.

5 bc TABLE I. TCP PERFORMANCES compound cubc hghspeed 1 flow utlzaton o ++ o ++ - o o robustness + o ++ o o -- o o when compared to reno + means better o means comparable - means lower buffer, bc, compound, hghspeed, hamlton, hybla and veno, when competng aganst reno, acheve relatvely the same throughput as reno does whereas cubc and scalable acheve slghtly hgher throughput than reno. However, vegas and westwood dsplay lowest and slghtly lower throughput respectvely. Fg. 4 (b) llustrates that wth 4-Mbps lnk and 2-packet buffer, bc, compound, hghspeed, hamlton, hybla and veno acheve throughput smlar to reno whereas cubc and scalable stll acheve slghtly hgher throughput than reno. But, vegas and westwood dsplay throughput that s lower than reno. Fg. 4 (c) llustrates that wth 8-Mbps lnk and 4-packet buffer, besdes vegas and westwood, all varants acheve throughput comparable to reno. Stll, vegas and westwood dsplays lower throughput. Accordng to the results, UQoS based on bc, compound, hghspeed, hamlton, hybla and veno are at the same level as reno whereas UQoS based on cubc and scalable s slghtly better. However, UQoS provded by westwood s margnally lower and UQoS provded by vegas s the lowest. IV. CONCLUSIONS AND FUTURE WORKS Ths paper evaluates the performances of real-world TCP based on Lnux and Mcrosoft Vsta mplementatons over a satellte lnk of NGSN wth BoD. The metrcs of nterest are utlzaton and robustness, frendlness and farness, and UQoS. The fndngs based on the study are summarzed n TABLE I. The Noveltes of ths work are frst the use of the exstng protocol mplementaton n smulated envronment n whch more realstc protocol behavors can be produced and captured, and second the dscovery that real-world TCP may not perform optmally over a typcal NGSN partcularly wth BoD. Evdently, the performances of TCP can easly be degraded by a smple BoD mechansm. The result suggests that gven the satellte envronments, compound and hghspeed are best at utlzaton whle cubc, hamlton and hybla are best at robustness. Addtonally, all varants, other than vegas, are both frendly and far when competng aganst reno. Lastly, cubc and scalable delvers slghtly better UQoS when sharng the same lnk wth reno. Although not havng the best utlzaton, cubc and scalable hamlton hybla scalable 4 concurrent flows (2 reno and 2 competng varants) frendlness o o o o o o o ++ o o farness o o o o o o o -- o o vegas veno UQoS o o + o o o + -- o - westwood appear to be the best TCP from the user s pont of vews. Wth BoD, the presence of queue occupancy no longer mples full utlzaton and addtonal slot watng tme has to be added to the overall round trp tme. Therefore, larger buffer sze s requred not only to accommodate larger BDP but also to allow a number of superframes to be requested. It s clear that BoD causes several superframes to be dle and unutlzed n whch t contrbutes to even lower utlzaton. Thus, to optmze TCP performances over NGSN wth BoD, buffer sze, bandwdth and frame structure have to carefully be fne tuned, snce they are related to one another. The fnal am s to optmze the slot watng tme and the number of unutlzed superframes caused by BoD. In addton to BoD, Adaptve Codng and Modulaton (ACM), a mechansm that adapts transmsson rate to best sut wreless envronment, also affect TCP performances by causng the total avalable bandwdth to vary. Moreover, f nterworkng between the next generaton terrestral network and NGSN s mportant, IP QoS archtecture, such as Dfferentated Servce, has to be consdered n order to provde end-to-end QoS across the ntegrated next generaton network. The future works nclude further study on the mpacts of BoD,.e. dfferent buffer szes, dfferent request/allocaton setups and dfferent BoD schemes, on TCP performances. Next, the mpact of ACM and later the combned effects of both BoD and ACM on TCP performances wll be studed. Fnally, the performances of TCP over the ntegrated next generaton satellte-terrestral network, utlzng BoD, ACM and IP QoS, wll be nvestgated. ACKNOWLEDGMENT Songrth Kttperachol thanks the Royal Tha Navy and EPSRC for fnancal support. REFERENCE [1] M. Allman, V. Paxson, and W. Stevens, "TCP congeston control," RFC 2581, Aprl, [2] S. Floyd, T. Henderson, and A. Gurtov, "The NewReno modfcaton to TCP's fast recovery algorthm," RFC 3782, Aprl, 24. [3] D. X. We and P. Cao, "NS-2 TCP-Lnux: an NS-2 TCP mplementaton wth congeston control algorthms from Lnux," n Proceedng of 26 workshop of ns-2: the IP network smulator, Psa, Italy, 1 October 26. [4] A. Varga, "OMNeT++ dscrete event smulaton system," [5] S. Kttperachol, Z. Sun, and H. Cruckshank, "Integraton of Lnux TCP mplementaton: verfcaton and valdaton," n Proceedng of SPECTS '8, Ednburgh, UK, June 28, to be publshed. [6] ETSI, "Dgtal Vdeo Broadcastng (DVB) - framng structure, channel codng and modulaton for 11/12 GHz satellte servces," EN 3 421, August, [7] ETSI, "Dgtal Vdeo Broadcastng (DVB): user gudelnes for the second generaton system for broadcastng, nteractve servces, news gatherng and other broadband satellte applcatons (DVB-S2)," TR , February, 25. [8] ETSI, "Dgtal Vdeo Broadcastng (DVB): nteracton channel for satellte dstrbuton systems," EN 31 79, 23. [9] A. Gotta, F. Potort, and R. Secch, "Smulatng dynamc bandwdth allocaton on satellte lnks," n Proceedng of 26 workshop on ns- 2: the IP network smulator, Psa, Italy, 1 October 26. Authorzed lcensed use lmted to: Unversty of Surrey. Downloaded on Aprl 12,21 at 9::4 UTC from IEEE Xplore. Restrctons apply.