The Impact of Delayed Acknowledgement on E-TCP Performance In Wireless networks

Transcription

1 The mpact of Delayed Acknoledgement on E-TCP Performance n Wreless netorks Deddy Chandra and Rchard J. Harrs School of Electrcal and Computer System Engneerng Royal Melbourne nsttute of Technology Melbourne, Australa 3 {deddy.chandra@eee.org}{harrsr@rmt.edu.au} rmala Shenoy Department of nformaton Technology Rochester nsttute of Technology Rochester, Y 1463 {ns@t.rt.edu} Abstract The expanson of nternet nto reless netork has been a very actve area of research. One of the topcs many researches focus on s the performance of TCP over reless envronment. TCP s an nternet transport protocol, hch uses a cumulatve acknoledgement strategy to adjust ts transmsson rate. n ths paper e nvestgate the mpact of to acknoledgement strateges employed by recevers knon as basc acknoledgement and delayed acknoledgment on TCP performance. Thus, a smple analyss of mplementng delayed acknoledgement n standard TCP ll be presented. Further, e examne the mpact of both acknoledgement strateges on the performance of our proposed E-TCP (Enhanced-TCP). We sho that delayed acknoledgement mechansm has negatve mpact on E- TCP performance n reless netorks. Keyords TCP, E-TCP, delayed acknoledgements, reless netork. 1. ntroducton Transmsson Control Protocol (TCP) [6] s the desgnated relable transport protocol used on the nternet orld today. The TCP data sender transmts data to the TCP data recevers and the recever transmts acknoledgements (ACKs) back to the sender as segments arrve. The sender ould retransmt any segments that are not acknoledged by the recever and ths provdes relablty. Snce TCP uses a sldng ndo technque, any ncomng ACKs allo the transmsson of ne data. n addton, ACKs are also used by the TCP s congeston control mechansms [7][8][9] to ncrease the amount of data the sender s permtted to nject nto the netork. ntally, a TCP recever generated an ACK for every ncomng segment as t has been outlned n [5][6]. These ACKs are cumulatve and acknoledge all n-order segments that have arrved at the recever. f a recever has receved an out-of-order segment, an ACK s transmtted. Hoever, t ll not acknoledge the ncomng segment, but rather a duplcate ACK for the last n-order segment s generated. An optonal delayed acknoledgement (delack) strategy has been defned n RFC 11 [1]. Delayed ACKs allo a recever to refran from transmttng an ACK for every ncomng segment. Hoever, the recever must send an ACK for every second full-szed segment. n addton, an ACK should not be delayed for more than 5 msec hle atng for a second full-szed segment to arrve. Delayed acknoledgement strategy has been found to have a postve mpact on TCP performance for bulk transfers [11] and n reless mult-hop netorks [1]. Hoever, delacks can also reduce performance n certan stuatons (e.g. durng standard TCP s slo-start phase). TCP sender ncreases the amount of outstandng data njected nto the netork based on the number of receved ACKs. Therefore, the amount of data njected nto the netork s reduced as a result of reducng the number of ACKs. Lang and Floran have nvestgated the mpact of usng delack strategy on TCP performance over satellte lnks hch s usually charactersed th hgh propagaton delay and hgh bt-error rates [3]. They have shon that the utlzaton of delack causes orse TCP performance compared th the use of basc acknoledgements. Furthermore, t has also been shon that TCP th more sophstcated mechansms are mpacted more severely by the use of delack. Mark Allman [4] has nvestgated a standard verson of TCP and the to standard acknoledgement strateges employed by a recever: those that acknoledge each ncomng segment and those that mplement delayed acknoledgements. The results sho that the delack strategy hurts TCP performance, especally durng the slo start phase. Thus, three alternatve mechansms for generatng and usng acknoledgements desgned to mtgate the negatve mpact of mplementng delack ere proposed.

2 The results presented n ths paper am to evaluate the effects of mplementng delacks on our proposed Enhanced-TCP scheme [1][]. Snce the use of a delayed acknoledgement s recommended and frequently mplemented, ths study shall nvestgate the effectveness of utlzng delacks n the case hen our E-TCP scheme, hch has been proposed n our earler ork s mplemented. Thus, t s possble to provde a suggeston on hch acknoledgement strategy should be mplemented th E-TCP scheme. The rest of ths paper s organsed as follos. Secton presents a smple analyss of TCP performance hen the basc or delayed acknoledgment s mplemented. The smulaton study and results of utlzng delack and basc acknoledgement strategy n our proposed E-TCP scheme ll be presented n secton, thus, the smulaton results allo authors to provde suggeston on hch acknoledgement strategy should be mplemented for E-TCP scheme. Fnally, the concluson of ths paper ll be presented n secton V.. TCP Congeston Control TCP uses a sldng ndo mechansm as a form of flo control. That s, the sender assgns a sequence number to each packet sent and expects acknoledgement from the recever for each of these packets. The sender ll ensure that no more than W data packets are outstandng thout beng acknoledged, here W s knon as the ndo sze. The recever uses cumulatve acknoledgements. For example, an acknoledgement for packet j mplctly acknoledges all packets from to j. Hence, f j s the last acknoledged packet, the sender may send packet 1 j and so on untl t reaches j W. The sender uses any receved acknoledgement as a self-clockng mechansm. Assume that the recever returns an acknoledgement packet for every packet receved from sender. When a sender receves an acknoledgement, t ndcates that one packet has left the netork and mples that one ne packet can be sent thout causng congeston. There are many mplementatons of TCP and the current standard s knon as TCP Reno. Reno TCP has more refned loss recovery algorthms hen compared to older versons of TCP such as TCP Tahoe [4][7][9]. The Reno TCP mplementaton as ncorporated nto Tahoe, but modfed the Fast Retransmt operaton to nclude fast recovery [6]. The follong s a bref descrpton of slo start and congeston avodance phase of TCP Reno. A. Slo Start Phase n the slo start phase, the tme that a TCP sender spends n ncreasng congeston ndo (cnd) sze from 1 segment to the advertsed ndo sze hen the recever uses a basc acknoledgement technque (acknoledge every ncomng data segment) s gven by: Slo start tme R. log W A... (1) Where, R s the round trp tme (RTT) beteen the sender and recever, W A s the recever s advertsed ndo (n segments). For comparson, the next equaton gves the approxmate tme spent by the TCP sender usng a slo start algorthm hen the recever mplements delayed acknoledgements: Slo start tme R log W A () ote that both equatons [4] assume that the sender s cnd s able to reach the advertsed ndo before any types of packet losses are detected. As llustrated by the above equatons, the requred tme to ncrease cnd from 1 to the sze of the advertsed ndo s roughly doubled by usng delack. Ths ncrease n tme s caused by the reducton of the number ACKs beng sent by the recever and snce TCP ncreases cnd by 1 segment for each ACK receved durng the slo start phase, therefore, reducng the number of ACKs by roughly half ncreases the tme requred to open cnd by a factor of. The ncrease n tme requred durng the slo start phase has a negatve affect on performance especally for short transfers that complete thn the slo start perod as ell as transfer over long-delay channel (e.g. satellte lnks). The follong s an nvestgaton of TCP slo start phase th and thout the mplementaton of delayed acknoledgement strategy. 1) Delayed acknoledgement s consdered Durng the slo start phase, TCP ll ncrease ts congeston ndo sze () by one TCP segment for each receved acknoledgement from the recever n each cycle, that s: ack (3) 1 Snce delayed acknoledgement has been consdered n ths case, the recever ll send back one acknoledgement packet (ack ) roughly for every y receved packets. Therefore: ack (4) y Then (3) ll become: 1 y y φ (5) Let φ represents geometrc seres factor, thus the total number of packets transmtted durng the slo start phase s approxmately gven by: s n n 1 1 φ φ... φ φ (6)

3 Let t start phase to reach represents tme, hch s used by the slo (slo start ndo sze, here ). Let represents the total number of segments sent durng slo start phase and represents the total number of cycles to complete the slo start phase. t T φ (7) s( ) φ After solvng (7), e obtan: log 1 φ t (log φ 1 T (8) ) ( φ ) ) Delayed acknoledgement s not consdered f the delayed acknoledgement s not consdered, then sender TCP ll ncrease cnd by one segment for each receved acknoledgement packet from the recever (equaton 3). And snce a sngle receved ack means that the congeston ndo sze s ncreased by one segment, therefore: ack (9) 1 The total number of packets that can be transmtted on each slo start cycle s: s n n (1) Wth smlar representaton from the prevous secton, e obtan: t T s ( ) (11) By solvng equaton 11, e obtan the follong equatons: log 1 t ( log ) T (1) 1 log 1 B. Congeston Avodance Phase The Congeston Avodance algorthm [7][8][9] s used to probe the netork for addtonal capacty after the congeston s detected. For each ncomng ACK, cnd s ncreased by 1/cnd up to the advertsed ndo. Therefore, the congeston avodance algorthm ncreases cnd more conservatvely than the slo start algorthm. The congeston avodance phase ll start from the moment cnd sze has reached the slo start threshold ) and ll end hen the ndo has reached ( the mum alloed ndo sze ( ). 1) Delayed Acknoledgement s not consdered When delayed acknoledgement s not consdered, every acknoledgement packet from the recever ll cause the sender TCP to ncrease cnd by 1 packet per ndo. Thus, the ndo sze ll ncrease lnearly from to. The number of packets that have been sent durng congeston avodance phase s gven by: s (13) 4 Approxmately from equatons (1) and (13), the total number of packets sent durng the slo start and the congeston avodance phase are gven by: total ss ca log ( ) 1 8 log (14) 4 ) Delayed acknoledgement s consdered When delayed acknoledgements are employed by the recever, roughly only half of ACKs arrve at the sender and the congeston avodance algorthm ll ncrease cnd by roughly half a segment every RTT. Approxmately, the number of packets that have been sent durng congeston avodance s gven by: 1 s (15) 8 4

4 From equatons (8) and (15), the total number of packets sent durng the slo start and the congeston avodance phase are gven approxmately by: total ss ca φ 5 8 (16) 3. Smulaton As mentoned before, ths secton ll nvestgate the effects of deployng basc acknoledgement and delayed acknoledgement strateges on E-TCP performance through smulaton study. Thus, the results allo us to provde suggeston on hch acknoledgement strategy should be mplemented together th our E-TCP scheme. The smulatons ere performed usng OPET TM netork smulator. Snce e are not nterested n the effects caused by routng, our smulaton setup does not nclude any routng scheme n the red netork model. Furthermore, the follong smulaton parameter settngs are adequately chosen snce e only concentrate on the performance mprovement based on the motvaton mentoned above. The topology of the netork studed s gven n Fgure 1. Let S represents the data source n ths fgure, the data recever D (destnaton), and the base staton, BTS. The bottleneck router, R, n the smulated topology utlzes a drop-tal queueng strategy th a queue that s lmted to serve a mum of 15 packets. The packet sze used n ths smulaton s relatvely small and set at 17 bts. The red lnk capacty s set to 1 Mbps, and the reless lnk to Mbps. A propagaton delay of 3 msec n the red lnks and delay free reless lnk. The am of ths paper s to analyse hether t s effectve to mplement delacks n reless netorks or t s not. Snce, the mpact of delayed acknoledgements on the varous types of TCP has been studed n [3][4]. Therefore, e only analyse the mpact of usng delack on E-TCP performance. To analyse the mpact of each acknoledgement strategy, e have run the smulaton for approxmately n 9 seconds. The comparson of average throughput hen the smulaton as tested th reless bt-error rates (BER) of 1-5 and 1-4 s shon n Fgure. t shos that E-TCP has suffered from poorer performance hen the delack strategy s mplemented. t shos that the basc ACK strategy has a sgnfcant postve effect to the performance of E-TCP. S 1 Mbps, 3 ms R BTS Mbps, ms Fgure 1: Smulated etork Topology D Throughput (pkts/sec) Wth delack Wthout delack Wth delack Wthout delack BER 1e-5 BER 1e-4 Fgure : Comparson of Average Throughput Snce, our proposed E-TCP can dfferentate the varous types of packet loss (.e. congeston, corrupton or handoff loss). The sooner the nformaton can be delvered to E-TCP, the better the performance a sender ll have. By acknoledgng every packet, E-TCP ll have more accurate status nformaton of every packet, hch has been transmtted by a sender. The reason for a loer throughput s the hgh number of tmeouts, hch mght be caused by usng delack. n some cases, the number of tmeouts could be tce the basc acknoledgement case. Loer throughput s also caused by the fact that a connecton usng a delayed ACK on average has a smaller cnd sze than a connecton hose recever acknoledges every segment. The smaller number of segments n flght durng any gven RTT ll be the result of a small cnd sze. f the tmeout occurs more frequently, E-TCP ll frequently enter the slo start phase and sgnfcantly reduce ts transmsson rate. We can see the number of packets receved as a result of comparng the utlzaton of the delayed acknoledgement and the basc acknoledgement over a reless lnk th BER of 1-5 n Fgure 3. Under such reless lnk error parameter, t shos that the basc ACK strategy causes more packets can be successfully receved by the recever. n contrast, the delack has forced E-TCP to lmt ts transmsson rate. The expry of TCP tmer forces TCP to reduce ts cnd sze don to 1 packet. Therefore f multple tmeout occurs, TCP can only transmt 1 packet untl t s free from the tmeout. Fgure 4 shos the number of packet receved as the comparson of mplementng delacks and basc ACK over a reless lnk th BER of 1-4. t also shos that the total number of receved packets hen E-TCP mplements delack s less than the total number of receved packets f E-TCP mplements the basc acknoledgement strategy. Both Fgure 5 and 6 sho the sequence number trace of packets that have been successfully receved. t shos that n both condtons, the mplementaton of a basc acknoledgement strategy alloed the E-TCP sender to transmt more packets n comparson to the delayed acknoledgement strategy. The mplementaton of delayed acknoledgements refran the data transmsson and ths has been shon n both fgures.

5 o. of packets teraton th delack thout delack Fgure 3: umber of Receved Packets (BER 1e -5 ) Sequence umber tme (sec) th delack thout delack Fgure 5: Sequence umber Trace (BER 1e -5 ) o. of packets teraton th delack thout delack Fgure 4: umber of Receved Packets (BER 1e -4 ) 4. Conclusons n ths paper e have analysed the mpact of mplementng the delack strategy on TCP performance through the use of a mathematcal model. Furthermore, e have also nvestgated the effect of mplementng delayed acknoledgement on the performance of our E- TCP scheme over reless lnk. Our smulaton results sho that the mplementaton of a delayed acknoledgement n the E-TCP scheme has poorer performance than the E-TCP performance hen the basc acknoledgement strategy s used. The degradaton of E-TCP performance s bascally caused by the mplementaton of delack hch holds the ncrease of cnd by roughly half per RTT. The hgh amount of tmeouts also becomes another cause of performance degradaton. Although, the mplementaton of a delayed acknoledgement has been found to have a postve mpact on certan condtons, t s suggested that the basc acknoledgement strategy n data transmsson over reless netorks should be mplemented hen the E- TCP scheme s used. References [1] D. Chandra, R.J. Harrs and. Shenoy, TCP Performance for future P-based Wreless etorks, The 3 rd ASTED nternatonal Conference on Communcatons Systems and Applcatons, July 3. [] D. Chandra, R.J. Harrs and. Shenoy, Congeston and Corrupton Loss Detecton th Enhanced-TCP, The Australan Telecommuncatons etorks and Sequence umber tme (sec) th delack thout delack Fgure 6: Sequence umber Trace (BER 1e -4 ) Applcaton Conference (ATAC 3), December, 3. [3] T Lang and D. Floran, The mpact of delayed Acknoledgements on TCP performance over Satellte Lnks, Proc. ACM Workshop on Wreless Moble nternet, July 1. [4] M. Allman, On the generaton and use of TCP acknoledgements, Computer Communcaton Reve, vol.8, no.5, pp.4-1, Oct [5] J.C. Hoe, mprovng the start-up behavour of a congeston control scheme for TCP, Proceedngs of SGCOMM 96, (ACM), pp. 7-8, August [6] J. Postel, Transmsson Control Protocol, September 1981, RFC 793. [7] V. Jacobson, Modfed TCP congeston Avodance Algorthms, Techncal report, Apr 199. [8] V. Jacobson, Congeston Avodance and Control, Computer Communcaton Reve, vol18, no.4, pp , August [9] W. Steven, M. Allman and V. Paxson, TCP Congeston Control, nternet-draft draft-etf-tcpmpl-cong-control-.txt (ork n progress), August [1] R. Braden, Requrements for nternet Hosts Communcaton Layers, October 1989, RFC 11. [11] S. Johnson, ncreasng TCP Throughput by usng an extended acknoledgement nterval, Master s thess, Oho Unversty, June [1] S. Xu and T. Saada, Evaluaton for TCP th Delayed ACK Opton n Wreless mult-hop etorks, n EEE VTS 54 th Vehcular Technology Conference, vol. 1, pp , 1.