Modeling TCP Performance with Proxies

Transcription

1 Mdeling TCP Pefmance with Pxies Mingyan Liu, Navid Ehsan Electical Engineeing and Cmute Science Deatment Univesity f Michigan Ann Ab, MI , USA Abstact This ae investigates the TCP dynamics and efmance ve xies that shten the TCP feedback l by segmenting the end-t-end cnnectin. Such xies ae ften used t imve TCP efmance, e.g., a slitting/sfing xy in the satellite cmmunicatin, and me cmmnly, a web cache. By analysis, we attemt t devel a basic undestanding f the eties f TCP dynamics when such xies ae used, and futhe btain cetain design inciles f systems invlving such xies. We esent simle mdels catuing sme featues f the xy efmance in bth the lssless and lssy scenais. Due t the cmlexity invlved, detailed analysis is nly available in the lssless scenai, and u discussin in the lssy scenai is lagely limited t steady state behavi. weve we ae able t btain useful insight thugh such analysis. We identify cnditins unde which using a xy vides significant maginal efmance gain by investigating facts including initial windw size, cngestin level f the xy, and the level f asymmety between the links segegated by the xy. We als discuss hw these cnditins affect the delyment and visining f systems using xies. I. INTRODUCTION IS ae investigates cetain tye f xies that cause changes in the TCP dynamics and the esulting efmance imlicatins. In aticula, we fcus n xies that shten the TCP feedback l eithe by design as a by-duct. Such xies ae nmally used t educe the cnnectin esnse time and achieve highe link utilizatin. One tyical examle f such is a TCP cnnectin slitting and sfing xy that e-acknwledges the sende n behalf f the eceive (by sfing the eceive s addess), and fwads ackets t the eceive n behalf f the sende (by sfing the sende s addess). Such a scheme is usually called slit TCP, TCP sfing [1] indiect TCP (I-TCP) [2], [3]. It is cmmnly used in satellite cmmunicatin t imve TCP efmance ve the lage bandwidth-delay-duct f satellite link since it can seed u the windw gwth and achieve highe caacity utilizatin, esecially f sht cnnectins. It has als been sed f teestial wieless netwks (e.g., I-TCP) [3], [], [5] as a means f seaating the wied and the wieless at f the cnnectin, and seaating cngestin lsses and link failue lsses. The mtivatin behind this aach is TCP s efmance degadatin in a hetegeneus envinment. The idea is that if a cmmunicatin ath cnsists f hysical medium that have vey diffeent chaacteistics, the endt-end efmance is timized by islating ne tye f hysical link fm anthe and timizing each seaately. weve this aach geneally vilates the TCP end-t-end semantics, and will nt wk if the IP ackets aylad is encyted [6]. Anthe examle f such a xy, which may seem less bvius, is a cmmn web cache (e.g., with the web bwse set t xy mde). When thee is a hit at the cache, the file is diectly sent t the client fm the cache. When thee is a miss, the cache ens u a cnnectin t the emte seve and stats dwnlading the file t the cache (f cacheable bjects), while fwading ackets t the client at the same time. Thus the cache autmatically beaks the seve-client tansfe int tw seaate cnnectins [5]. In tems f TCP efmance f the file tansfe, this has exactly the same effect as slit TCP (althugh the cnnectin establishment is diffeent). weve, in this case the TCP semantics is eseved because the cache des nt sf the client s addess, and s it acknwledges the seve n behalf f itself athe than e-ack n behalf f the client. Caching nt nly educes latency by ushing the cntent clse t end uses but als esults in ediectin f taffic that is meant f web seves, and can achieve bette lad balancing. Thee has been imlementatin and exeimental study f the TCP efmance imvement using such xies, esecially slit TCP in satellite and teestial wieless cmmunicatins (e.g., [1], [2], [3]). In this ae we devel simle mathematical mdels t deive the TCP efmance (mainly latency) when such a xy is used, and analyze the level f efmance imvement unde diffeent scenais. Ou mtivatin is thee-fld: t have an analytical and quantitative study t gain insights int the dynamics f a shtened TCP l in additin t simulatin and exeimental studies; t investigate the use f xy as a geneal slutin t blems invlving hetegeneus links and lage amunts f taffic; and me imtantly, t aly such undestanding t system level design issues. In subsequent sectins we will igne whethe the xy sfs addesses nt since it des nt affect u analysis, and instead fcus n a geneal mdel f sevexy-client cmmunicatin. Due t the cmlexity invlved, detailed analysis is nly available in the lssless scenai, and u discussin in the lssy scenai is lagely limited t steady state behavi. weve we ae able t btain useful insight thugh such analysis. In

2 Seve Pxy Fig. 1. Netwk Mdel Client summay, we fund that veall using the xy esults in highe utilizatin f the link caacity and lwe latency. weve, when the xy becmes cngested this efmance gain is limited. In additin, when a cnnectin is bken in tw, the slwe ne always dminates the veall efmance, and as this dminance inceases, the gain fm using the xy is again educed. These esults imly that while timizatin f seaate ats f a cnnectin (segegated by the xy) is imtant, it is equally imtant t minimize the asymmety between these ats, esecially in a hetegeneus envinment. The ganizatin f the ae is as fllws. In Sectin 2 we esent the netwk mdel and descibe hw the xy functins. In Sectins 3 thugh 5 we analyze the latency in file tansfe with withut using the xy. Tw cases ae investigated by assuming the links ae lssless and lssy, esectively. The accuacy f u mdel is discussed. We then analyze the effect f initial windw size, the cngestin level f the xy and the asymmety between the tw segments segegated by the xy. Sectin 6 summaizes u esults and cncludes the ae. II. SPLIT CONNECTION AND TE NETWORK MODEL A. Netwk Mdel Ou analysis is based n a tw link mdel with ne end hst n each side and a xy in the middle, as shwn in Figue 1. In eality each f the tw links may cntain multile intemediate utes and hysical links, but ae abstacted int a single link with a single und-ti time (RTT) aamete and a single lss ate aamete. In a eal netwk, a sfing xy is usually laced between the wied at and the wieless ( satellite) link, and the client is usually lcated at the end f the wieless link. The lcatin f a cache xy is me abitay. File tansfe is u main alicatin f inteest, and withut lss f geneality ae cnsideed t be fm the seve t the client. When an end-t-end cnnectin is established, the xy functins as a nmal ute that fwads ackets fm the seve t the client and vice vesa. When cnnectin slitting is used, the xy acknwledges t the seve, the client acknwledges t the xy, and the xy elays ackets fm the seve t the client. Same cedue is used f the the diectin f the cnnectin. The tw cnnectins ae inevitably culed, but they kee seaate sequence numbes and queues, and the xy des nt elay ut-f-de ackets fm ne t the the thus acting as a vitual suce f the file. In geneal, with a sfing xy the initial cnnectin establishment (thee-way handshake) and the final clsing ae dne in an end-t-end fashin. The cnnectin is nly slit in tw duing the data tansfe eid, as shwn in Figue 2. With a cache xy thee ae tw seaate cnnectin fm the vey beginning, i.e., thee-way handshake is fist cnducted between the client and the xy, and if thee is a miss, anthe thee-way handshake is cnducted between the xy and the seve, as shwn in Figue 3. Bth situatins esult in aximately the same delay in cnnectin establishment f a single cnnectin. We theefe d n include this initial delay in u analysis and instead fcus n the delay slely f data tansfe, which is the duatin between when the seve sends the fist data acket f a file and the time when the client eceives the last data acket f a file. F a cache xy if thee is a hit n the file equest, the cntent is etieved diectly fm the cache. In this case the cnnectin mdel is simly end-t-end fm the client t the xy, with a factin f the entie seve-client und-ti time. Ou analysis theefe nly alies t situatins whee thee is a miss. Fig. 2. File tansfe using a slitting xy B. Assumtins and Paametes We assume that a file cntains exactly segments f the maximum segment size (MSS). This is an aximatin t an abitay file size whse last segment may be a factin f MSS. weve, this des nt affect u methd f analysis, and als des nt affect the cmaisn between with withut using the xy. We assume that bth the end-t-end cnnectin (seve-client) and the slit cnnectins (seve-xy and xy-client) have the same value f slw stat theshld (ssthesh),, and the maximum windw size. These tw values ae als

3 Q " i Fig. 3. File tansfe using a cache un miss assumed t be in numbe f segments athe than numbe f bytes t simlify the analysis. The seve, the xy and the client each has a tansmissin ate f,, and, esectively. Assuming acket length f, (including bth TCP and IP heades) the time it takes f the seve t tansmit a acket is, and,! f the xy and the client, esectively. When thee ae tw seaate cnnectins, we assume a e-acket cessing delay f " at the xy. All the cessing delays ae igned. We assume that each link has the same agatin delay and tansmissin ate in bth diectins. The ne-way agatin delay n the seve-xy link and the xy-client link ae dented by # and #, esectively. Thughut u analysis, we assume that the tansmissin time f an ACK is negligible. We futhe assume that the TCP sende is nly cnstained by the cngestin windw and nt the advetised eceive windw size. Mst wk in TCP analysis assumes an infinite suce, e.g., [7], [8], [9]. weve, when we have tw cnnectins, the windw f the secnd cnnectin (xy-client) evlves nt nly accding t the windw dynamics f TCP, but als accding t the availability f ackets (fm the seve-xy cnnectin), i.e., the fist cnnectin may nt catch u with the secnd cnnectin due t facts like initial windw size, tansmissin ate, etc.. Theefe the windw f the secnd cnnectin will be fced t gw at a slwe ate. We will discuss bth in subsequent sectins. III. LOSSLESS LINKS Assuming that the windw gws in the slw stat and cngestin avidance stages until the maximum windw size is achieved, the numbe f windws that is needed t cve a file f segments can be calculated by extending the methd esented in [10]. We als assume that delayed ACK is imlemented. As shwn in [11], since ne ACK is geneated f evey $ ackets eceived befe the time exies, the ate f exnential gwth f the cngestin windw is %'&( ), which equals 2 when n delayed ACK is used. Let *(+ dente the initial windw size. Let be such that *(+,%.-0/ *+7%8-, if :9<;?> - * + % /, i.e., the slw stat theshld ssthesh is eached duing the AB&DC 3FE windw if the file is big enugh. Similaly, let be such that 3GI2J /K-/ ) 1 L5 3MN2J ) /-, i.e., the maximum windw size is achieved duing the O&PC 3FE windw if the file is big enugh. All subsequent windws have the same windw size f. The numbe f windws needed t tansfe a file is then given by the fllwing: VXW?YKZP[]\ ;_ `> *a+7% / 2bdc if [ 5e SSSSSR T USSSSS VXW?YKZP[]\ ; `> - * + % / ;?> -gf 3 /K-/ ) C bdc if 1 [ 5 VXW?YKZP[]\ ; `> - *a+7% / ;?> 2J -gf 3M /K-/ ) C h;?> 2J f bdc if 1 [ A. Delay Mdels We fist cnside an end-t-end cnnectin between the seve and the client. Assuming that the links ae lssless and that cnnectins ae nly cnstained by cngestin windw size, afte the seve sends a windw s ackets it waits f the fist ACK t cme back, if it takes lnge f the ACK t aive than it takes t tansmit the windw s wth f data. The time it takes t tansmit the [ 3FE windw is a functin f the acket tansmissin time at the sende given by R T j C U *a+,% / if [ 5e 3M /K-0/ ) Ck if 1 [ 5 if 1 [ Theefe if b i (1), which indicates that the xy tansmits at least as fast as the seve and thus ackets will nt exeience queueing delay at the xy, the und-ti time f the end-t-end cnnectin is la#pmn#7dc. We define 2 t be the time it takes f the fist ACK t aive afte the fist acket was sent, thus q < _la# # C NA$2&PCk s$t u BlA# B# C. Nte that this exessin assumes that thee ae at least $ ackets in a windw s that the eceive can immediately etun an ACK un eceit f the $ 3FE acket. If f examle * + v& and $Oẅl, then the eceive may have t wait f the delayed ACK time t exied t etun an ACK. In the est f u analysis we will igne this diffeence,

4 which can be easily taken int accunt. The ttal time it takes t tansfe the file is then C B whee fb / > " j7c f # #7 (2) f sitive and thewise. This latency eflects the ttal tansmissin time, the time that the seve sends waiting f ACKs, and the time f the last windw t eached the client. When 1, ackets culd build u at the xy waiting t be tansmitted int the slwe link and exeience additinal queueing delay at the xy. In this case the ACKs f the same windw aive at the seve aximately aat instead f, thus the seve may need t wait f evey ACK f the same windw instead f stalling afte sending ut the entie windw. We deive the latency by examining fm the client s side. Since 9, the client eceives ackets f the same windw cntinuusly at ate & P. The time that the client is idle is theefe ] OlA# 0 #, C $&DC " j C f, whee " j C is the time it takes the client t eceive the [ 3FE windw, and " C is given by (1). The latency is then C B a # #7 / > l0f# # C $G " j C f (3) which eflects the time the client sends eceiving the file, waiting f the next windw, and the time f the fist windw t each the client. Redefining VnW?Y F CM $ V j mchlf#d#7dc, (2) and (3) can be cmbined int C V j C # # VnW?Y F 7CM / 2 > " V F mc C f i When the xy is used, we have tw seial cnnectins. Nte that these tw cnnectins ae nt indeendent but culed by data. This is because the secnd cnnectin (xy-client) cannt send any data ackets it has nt eceived fm the fist cnnectin (seve-xy) and theefe be cnstained. This can be caused by a much lage initial windw size and/ a much shte undti time n the secnd cnnectin. In this scenai the secnd cnnectin has a limited suce based n the sending f the fist cnnectin. In [12] we develed a detailed mdel f this scenai. Due t sace limit we d nt esent it hee. weve, simila qualitative insight can be btained withut having t g thugh the detailed analysis. F the est f u discussin we will assume that the secnd cnnectin is neve cnstained by the fist cnnectin, which culd imly * + b * # <5 #7, whee * + 25 and/ + is the xy s initial windw size. Latency in Secnds ETE sim ETE md Pxy sim Pxy md File Size in Bytes, W1 Latency in Secnds ETE sim ETE md Pxy sim Pxy md File Size in Bytes, W Fig.. Latency vs. file sizes, with initial windw size f 1 and, esectively. The xy eceives the fist acket fm the seve at time #. Assuming thee is " delay f cessing at the xy, the xy stats sending this acket t the client at time 6<# 6e". Fm this int n, we nly need t fcus n the secnd cnnectin since the latency is nly detemined by this cnnectin. By fllwing the same analysis, we have the ttal latency f the xy case whee " C # " B / > " j C f # () C is given in (1), Q is the ttal numbe f win- dws needed f the tansfe, and ü l8# _A$ &DC $t l8# is the time it takes f the ACK t cme back t the xy. This latency eflects the initial delay f the fist acket t aive at the xy, the ttal tansmissin time at the xy, stall time and the time f the last acket t each the client. B. Validatin Figue cmaes the numeical esults fm u mdel with NS2 simulatin, f bth the end-t-end and xy schemes. In this case, N N q& Mbs. The initial windw size is set t 1 and, esectively. Unless inted ut exlicitly, u numeical esults and simulatin thughut this ae ae based n the fllwing aametes: MSS512 bytes, ssthesh128 segments. In bth cases, # &8 ms, # l8 ms. Each gah cntains fu cuves, tw fm ns simulatin (sim) and tw fm u mdel (md). We see that each ai (sim and md) velas almst cmletely. IV. LINKS WIT RANDOM LOSSES When lsses (eithe due t cngestin link failue) ae esent the analysis becmes me cmlicated. Meve the analysis is lagely limited t the steady state

5 ) C study f TCP cnnectins which is alicable in the case f an unlimited file tansfe, less accuate in the case f a finite TCP cnnectin, and much less in the case f a xy as shwn in this sectin. A. The Seve-Pxy Link is lssless If we assume that the seve-xy is lssless, then the methds intduced in [9], [8], [7] can be alied t detemine the thughut and delay f the xy-client cnnectin. In aticula, the TCP bulk data tansfe thughut is shwn t be well aximated bym Cn, whee is the bability f lss f a single acket at lw lss ate [9], and is me accuately aximated in [8] by cnsideing timeuts. These esults wee develed f bulk TCP tansfes and wee based nly n analysis f the TCP cngestin avidance hase. In [7] it was shwn that they can be equally effective when alied t sht TCP cnnectins if cmbined with slw stat analysis. The efmance imlicatin f using a xy when lsses ae esent immediately fllws: these esults shw that the thughut f a TCP tansfe is invesely tinal t the cnnectin und-ti time and the squae t f the lss ate. If lsses ae cncentated n the xy-client link, then using the xy effectively islates the at f the cnnectin that invlves lss, and educes the und-ti time equied t ecve the lsses, thus achieve highe thughut and lwe latency. The same key cncet can be seen in schemes such as Sn TCP [13], [1], WTCP [15], [16], and [17], [18] that use lcal etansmissin (sme at the link laye, sme at the tanst laye). Secifically, denting the lss ate n the xy-client link by, and the thughut by,c, the tansfe latency f a file f size is given by whee MCa > MC, using end-t-end cnnectin MCM C > MC MCM + AlA# #7PC C &(C f 1 ; MCa & C (5) f dentes the bability that ackets ae successfully sent befe the fist lss ccus. + / /! /!! & is the exected numbe f ackets sent befe the fist lss ccus. C is the latency functin f an end-t-end cnnectin shwn in Sectin 3. When using the xy, assuming that the xy-client cnnectin is nt cnstained by the seve-xy cnnectin (e.g., # 1 #, 1 ), > MC MCM + M l8#7 C i (6) The diffeence f the tw (using esults fm Sectin 3) is > MC, > MC MC C MC7 [ &DC +,C lm$ + 7C l$ l8#,c C7 l8#,c whee [ is the numbe f windw needed t cve a file f segments. The aximatin in the last equatin is based n the assumtin that the link caacity is nt filled duing the tansfe f ackets. F a given file size and lss ate n the xy-client link, the fist tem f the abve equatin is a cnstant, and the amunt f gain in using the xy deends n the und-ti time f the seve-xy cnnectin. Figue 5 cmaes the latency btained using this analysis with the esult fm simulatin. Simulatin is the aveage ve unning 50 indeendent simulatins. Thee is an bvius disceancy between the tw cuves. This is mainly due t the fact that the delay mdel basically assumes that the cnnectin ges int steady state ight afte the fist lss. The tw cuves eventually aach each the as the file size inceases (numbe f ackets sent in this figue). This is because f a lage file tansfe the effect f the abve assumtin is diluted (since the effect f steady state will dminate). numbe f ackets sent time (sec) (a) simulatin analysis Fig.!#"%$&'('*),+-.*/0!21#$3('('*)+-.*/65718$':9 5. Latency when slitting is used, whee'('(;:/<> the fist?$ab' link lssless, B. Randm Lsses n Bth Links When lsses ae esent n bth links, using the evius analysis f bth cnnectins vides nly gss aximatin. Suse the lss ates n the sevexy link and the xy-client link ae M and, esectively. Assuming lsses ae indeendent, the veall lss ate exeienced by an end-t-end cnnectin is CKmD E0. F an infinite file tansfe, in the lng

6 ) ) ) f Q + un the seve-xy cnnectin has an aveage thughutn M and the xy-client cnnectin has an aveage thughutn! K! if uncnstainted by the seve-xy cnnectin. The slwe ne f the tw is ging t dminate the cmbined thughut and delay. weve, bth values ae geate than the thughut f the end-t-end cnnectin! since it has a lage lss ate and a lage und-ti time. Theefe by segegating the seve-client cnnectin int ats that each has a smalle lss ate and und-ti time, using the xy achieves highe thughut and thus lwe latency. F a finite file tansfe, the latency f the end-t-end cnnectin is given by (5) with lss ate. When using the xy, we cnside O, the exected numbe f ackets sent successfully befe the fist acket lss ccus t the tw cnnectins, esectively. The tw cnnectins have bulk tansfe thughutm lf#d2 #7PC,C Alm#7 DC, esectively. Fllwing this we can chaacteize the sending cess and cngestin windw evlutin f the seve f the fist ackets, and as an aximatin t the emaining f the tansfe. The chaacteizatin f the xy can be btained by listing all ssible cases cmaing and,,c and I7C, and0. Figue 6 cmaes the esults btained fm this analysis t the simulatin esults. It can be seen that the tw d nt match f a wide ange f file sizes. Indeed the disceancy seems t divege. Ou cnclusin is the cuent mdel f TCP thughut and latency is insufficient t accuately edict the xy efmance with lsses n bth segments. The develment f a me suitable mdel is at f u n-ging eseach. ackets tansmitted simulatin analysis latency Fig. 3('('*)+-.*/0! 6. Latency when 1 $AB'('*),+-.*/65 slitting is used " $ with ':9 ':&(/ lsses 5 1 $':9 n'('(; bth links,! "2$ V. ANALYSIS AND DISCUSSIONS In this sectin we fcus n the mdel develed f the lssless scenai. This is nt a ealistic scenai, nev- TABLE I INITIAL WINDOW SIZE OF TE END-TO-END CONNECTION File Size (KBytes) * etheless insights ae btained via cetain simlificatin and aximatin esecially f sht file tansfes. A. Initial Windw Size C, Fm the definitin f and, ü$8,c. Cnside a file that finishes tansfeing within the slw stat hase, Q the ttal numbe f windws needed t cve the file,, wuld be such Q that 5v. Since 5 ; `> *a+g% / *a+ / /, this means 5 / / Q. is the smallest intege that satisfies 5q* + / / Q, theefe, j%x &DCGX&PC j%0 &DCG &DCGX&. Assume (5, i.e., the link ( ie caacity) is nt filled duing slw stat, and 1 that the xy is slwe than the seve, we have B u &DC 2 *a+ / > *a+7% / a # #7 F% &PC &PC (7) whee <# #. This last equatin is the same as esented in [7], but deived in a diffeent way. Similaly, f an initial windw size * + used by the xy, assuming 5 Q, we have j% &PC &PC F% &DCM<&DC<& and * j% &DC<&DC (8) whee # # ". Nte that and bth ae clse t ne half f. Since 9, *a+ has t be geat than * + in de t achieve the same delay. Me secifically, F% _&DC &DC j%nb&dc K! &DC leads t * + / / f! /. Table I shws sme values f and *(+ based n this aximatin f + N& i and &Dl! segments. %n l * We see that in de t achieve simila latency even f elatively small files we need significantly lage initial windw size f the end-t-end cnnectin. B. Slw Cngested Pxy When the same initial windw size *(+ is used, the diffeence in delay between the tw is C *a+ j% &PC &PC l8#,c *a+ F% &DC &DC (9)

7 + C Q Q Q Latency in Secnds ETE Pxy Pxy Rate (bs) Latency in Secnds ETE Pxy Pxy Rate (bs) Fig. 7. Latency vs., the tansmissin ate f the xy. File size is 11 Kbytes and " $ 51 Kbytes, 1 $ & f the gah n the left and ight, esectively. Mbs in bth cases. These gahs ae deived fm u mdel. C. File Size M /, simila analysis aly and we have B [ d&pc2 ; > / " j mci# K #7MI and B 8 [ &DC ; > / " j mc e# #7(6e", whee [ and [ ae the ttal numbe f windw sent befe the ie In the case whee the file tansfe entes the cngestin avidance stage, i.e., 9 / becmes full f the end-t-end and the slit cnnectin, esectively. In case when the file finishes tansfe befe the ie is full, [ [, and the diffeence between the tw is mainly PC7 &DC, which inceases as inceases ( Q inceases with ). weve, if is lage enugh and the ie is filled u befe the tansfe cmletes, then the diffeence between the tw stays cnstant, and bth incease with ate as the file size inceases. This can be bseved in Figue 8. This diffeence incease as and # incease, but seems invaiant t changes in K. As inceases, which cesnds t a slwe xy, the diffeence in delay emains. cnstant s lng as * + % / 5 f any [ 5 weve, as kees inceasing t the int whee the CM<& and ie is filled befe the file tansfe cmletes, the diffeence quickly educes. In aticula, if the ie ca- windw f the end- acity is achieved duing the [ 3FE t-end cnnectin, i.e., * + % / _b, then [ K B a 2 2 * + C.*+ %2 & 2# #7 i (10) We can get a simila exessin f, and thus K 2 * + C * j*a+6* + C l8# 6 % & i (11) This exessin deceases as inceases. This esult can be clealy bseved in Figue 7. A slwe xy (inceased and deceased tansmissin ate) can be viewed as an aximatin t a busie me cngested xy, because unde such situatin each TCP cnnectin nly gets a factin f the ttal xy caacity (assuming the xy has sufficient buffe), and queuing is inceased. This esult shws that as the xy becmes busy, the gain fm using seaate cnnectins educes because the bttleneck dminates the veall efmance n matte which scheme we use. In a system whee a xy is laced at the aggegatin int f incming taffic, adequate visining f such a xy becmes vey imtant since thewise vey little is gained fm using a xy. D. Cnnectin With Asymmetic Segments, i.e., the xy-client cnnectin is cnstained by the seve-xy cnnectin thughut the entie file tansfe. Futhe assume that the file tansfe is nly limited t slw stat hase. Fllwing u evius analysis, we can shw Suse *a+ 5<* + and 9 C j% &PC * + j% &PC *a+ &PC #D #, &PC with an e within half f. Suse we nw let 1, but kee unchanged, and let *(+q9 * +, then using u ealie analysis we get C / d&dc while the latency f end-t-end cnnectin emain the same C C / &PC. We see that when V using the xy, the lnge cnnectin f the tw ( Z c ) detemines the ttal latency. As the diffeence between the tw und-ti times inceases, the gain fm using the xy educes. Unde this scenai the efmance f the xy is maximized when the tw * + * cnnectins ae simila +. Same agument can als be deived f file tansfes that ente cngestin avidance hase. This is an inteesting bsevatin cnsideing the fact that many such xies ae used sed t be used in a hetegeneus envinment whee links have vey diffeent eties. This indicates that while it is vey imtant t timize each link seaately, it is equally imtant t minimize the asymmety between links since if seaate timizatin nly inceases the diffeence, e.g., making the fast link even faste, the esulting efmance might nt be imved.

8 Latency in Secnds ETE Pxy File Size in Bytes 6 8 Latency in Secnds ETE Pxy File Size in Bytes x 10 6 Fig. 8. Latency vs. file size. F small files the latency f an end-t-end cnnectin inceases faste than that f slit cnnectins. weve as file size gw big enugh t fill u the caacity, the tw have same gwth ate and the diffeence stays cnstant. These gahs ae deived fm u mdel. VI. DISCUSSIONS AND CONCLUSIONS In this study we examined using xy as a way f imving TCP efmance in vaius situatins. Such xies tyically beak an end-t-end cnnectin int tw segments, such as a sfing xy. We develed mdels t investigate the TCP dynamics when xies ae used and cmaed its efmance with end-t-end TCP cnnectin. We summaize u bsevatins and cnclusins as fllws. In geneal using xy ( seaate TCP cnnectins) esults in lwe latency fm u analysis. F an endt-end cnnectin this can be cmensated by inceasing the initial windw size. weve, as we shw in Table I it equies significantly lage initial windw size even f easnably small file sizes, which makes it less actical in eal alicatins. When the xy becmes the bttleneck, the gain fm using the xy quickly diminishes. In systems whee such a xy is sitined at a lace that all cnnectins have t g thugh, e.g., in a satellite system the xy is c-lcated with the satellite gateway s that all cnnectins g thugh the xy, the efmance gain fm the xy can be limited esecially duing busy hus. This may als cause buffe veflw at the xy, cause the xy t advetise smalle eceive windw size, which we did nt cnside exlicitly in this ae. When this is the case, queueing becmes sevee and ackets quickly build u at the xy, esecially if in additin # is much smalle than #. It is theefe imtant t ely visin such systems and imlement sme fm f dynamic flw cntl at the xy. This may be evented by using a much lage initial windw size ve the xy-client link. A xy achieves the effect f lcalizing e/lss ecvey and in geneal imves the thughut and educes latency f a cnnectin when lsses ae esent. One f the cmmn situatins whee xies ae used is a hetegeneus envinment whee ats segegated by the xy have vey diffeent link chaacteistics, e.g., agatin delay, lss ate, etc.. Inteestingly, the efmance gain in using a xy is maximized when bth ats have simila eties. Since the slwe at always dminates the veall efmance, as the level f asymmety inceases, the efmance ga between using a xy and using an end-t-end cnnectin becmes smalle. This imlies that while it is imtant t seaately timize these hetegeneus ats f the cnnectin, it is als imtant that such timizatin educes the asymmety between them. REFERENCES [1] V. G. Bhaadwaj, Imving TCP Pefmance ve igh-bandwidth Gestatinay Satellite Links, Tech. Re. MS 99-12, Institute f Systems Reseach, Univesity f Mayland, Cllege Pak, 1999, htt://htt:// [2] A. Bake and B.R. Badinath, I-TCP: Indiect TCP f Mbile sts, Pc. IEEE ICDCS, , [3] A. V. Bake and B. R. Badinath, Imlementatin and Pefmance Evaluatin f Indiect TCP, IEEE Tansactins n Cmutes, vl. 6, n. 3, , [] K. Bwn and S. Singh, A Netwk Achitectue f Mbile Cmuting, IEEE INFOCOM, , [5] S. Sibal P. Rdiguez and O. Satscheck, TPOT: Tanslucent Pxying f TCP, Tech. Re., AT & T labs-reseach and EURE- COM Technical Ret, [6] M. Kai, IPSEC and the Intenet, Tech. Re. MS 99-1, Institute f Systems Reseach, Univesity f Mayland, Cllege Pak, 1999, htt://htt:// [7] N. Cadwell, S. Savage, and T. Andesn, Mdeling TCP Latency, IEEE INFOCOM, [8] J. Padhye, V. Fiiu, D. F. Twsley, and J. F. Kuse, Mdeling TCP Ren Pefmance: A Simle Mdel and Its Emiical Validatin, IEEE Tans. Netwking, vl. 8, n. 2, , [9] T. V. Lakshman and U. Madhw, The Pefmance f TCP/IP f Netwks with igh Bandwidth-Delay Pducts and Randm Lss, IEEE Tans. Netwking, vl. 5, n. 3, , [10] J. Kuse and K. Rse, Cmute Netwking, A T-Dwn Aach Featuing the Intenet. [11] M. Allman and V. Paxsn, On Estimating End-t-end Netwk Path Peties, SIGCOMM, [12] M. Liu and N. Ehsan, Mdeling TCP efmance with xies, Technical Ret, EECS Deatment, Univesity f Michigan, Ann Ab, [13]. Balakishnan, S. Seshan, E. Ami, and R.. Katz, Imving TCP/IP Pefmance Ove Wieless Netwks, ACM/IEEE Intenatinal Cnfeence n Mbile Cmuting and Netwking (MbiCm 95), vl. 2, n. 11, [1]. Balakishnan, V. N. Padmanabhan, S. Seshan, and R.. Katz, A Cmaisn f Mechanisms f Imving TCP Pefmance ve Wieless Links, IEEE/ACM Tans. n Netwking, vl. 5, n. 6, , [15] K. Ratnam and Ibahim Matta, WTCP: An Efficient Mechanism f Imving TCP Pefmance Ove Wieless Links, Pc. IEEE ISCC,. 7 78, [16] K. Ratnam and Ibahim Matta, Effect f Lcal Retansmissin at Wieless Access Pints n The Rund Ti Time Estimatin f TCP, Pc. 31st Annual Simulatin Sym., , [17] C. Pasa and J. J. Gacia-Luna-Aceves, Imving TCP Pefmance Ove Wieless Netwk at The Link Laye, ACM Mbile Netwks & Alicatins Junal, [18] C. Pasa and J. J. Gacia-Luna-Aceves, TULIP: A Link-Level Ptcl f Imving TCP ve Wieless Links, Pc. IEEE WCNC 99, , 1999.