ARTICLE IN PRESS. Computer Networks xxx (2008) xxx xxx. Contents lists available at ScienceDirect. Computer Networks

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1 Computer Networks xxx (28) xxx xxx Contents lsts avalable at ScenceDrect Computer Networks journal homepage: On applyng far queung dscplne to schedule requests at access gateway for downlnk dfferental QoS Shh-Chang Tsao a, *, Yuan-Cheng La b, Le-Ch Tsao a, Yng-Dar Ln a a Department of Computer and Informaton Scence, Natonal Chao Tung Unversty, Hsnchu 3, Tawan b Department of Informaton Management, Natonal Tawan Unversty of Scence and Technology, Tape 16, Tawan artcle nfo abstract Artcle hstory: Receved 17 December 27 Receved n revsed form 13 August 28 Accepted 4 September 28 Avalable onlne xxxx Responsble Edtor: Nelson Fonseca Keywords: Request schedulng Access gateway Far queung Schedulng packets s a usual soluton to allocate the bandwdth on a bottleneck lnk. However, ths soluton cannot be used to manage the downlnk bandwdth at the user-sde access gateway, snce the traffc s queued at the ISP-sde gateway but not the user-sde gateway. An dea s schedulng the requests at the user-sde gateway to control the amount of the responses queued n the ISP-sde gateway. Ths work frst nvestgates the possblty of applyng the class-based far queung dscplne, whch was wdely and maturely used n schedulng packets, to schedule requests. However, we found that smply applyng ths dscplne to schedule requests would encounter the tmng and orderng problems at releasng requests and may not satsfy hgh-class users. Thus, we propose a mnmum-servce frst request schedulng (MSF-RS) scheme. MSF-RS always selects the next request from the class recevng the mnmum servce to provde user-based weghted farness, whch ensures more bandwdth for hgh-class users. Next, MSF-RS uses a wndow-based rate control on releasng requests to mantan full lnk utlzaton and reduce the user-perceved latency. The results of analyss, smulaton and feld tral demonstrate that MSF-RS provdes farness whle reducng 23 3% of user-perceved latency on average. Besdes, a MSF-RS gateway can save 25% of CPU loadng. Ó 28 Elsever B.V. All rghts reserved. 1. Introducton Numerous enterprses connect to the Internet wth the access lnk of Internet servce provder (ISP), a typcal topology of whch s depcted as Fg. 1. In general, ISPs are wllng to nvest money n expandng the backbone bandwdth to provde ther customers better servce. However, to mnmze costs, ther customers often delay upgradng the bandwdth of the access lnk, whch consequently becomes a potental bottleneck to access the Internet. To guarantee key traffc gettng enough bandwdth when passng through the bottlenecked lnk, ther customers may employ a class-based far queung (FQ) dscplne * Correspondng author. Tel.: E-mal addresses: weafon@cs.nctu.edu.tw (S.-C. Tsao), layc@cs.ntust. edu.tw (Y.-C. La), lctsao@cs.nctu.edu.tw (L.-C. Tsao), ydln@cs.nctu. edu.tw (Y.-D. Ln). or other packet-based bandwdth management [1] at the user-sde access gateway to schedulng packets. Unfortunately, these packet schedulng solutons fal to provde such guarantee for key traffc when the downlnk s the bottleneck. In ths case, packets are queued at the ISP-sde gateway, not at the user-sde gateway, for traversng the bottleneck. Schedulng packets at the user-sde access gateway s useless because the packets have passed the bottleneck. On the other hand, although schedulng packets at the ISP-sde gateway s useful, classfyng packets at ths gateway may be troublesome because of the network address translaton (NAT), whch s wdely deployed at the user-sde gateway to allow multple users n an ntranet sharng a publc IP address. The packets whch ntend to enter the ntranet cannot be classfed by the ISPsde gateway because the classfcaton needs to refer to the destnaton IP address of these packets but unfortunately they own the same one before they enter the NATembedded user-sde gateway /$ - see front matter Ó 28 Elsever B.V. All rghts reserved. do:1.116/j.comnet

2 2 S.-C. Tsao et al. / Computer Networks xxx (28) xxx xxx User-sde access gateway ISP-sde edge gateway C1 1 C n G Uplnk requests -> EG access lnk <- Downlnk responses Queung packets Internet S 1 EG S 2 EG S 3 Fg. 1. A typcal network topology that an enterprse accesses the Internet through ISP. Schedulng requests, nstead of packets, at the user-sde access gateway may solve the above mentoned falure of packet schedulng. Such an dea s based on that applcatons runnng over the Internet mostly adopt the clent server model,.e. the request/response model, such as HTTP, FTP, and E-mal. Requests sent from clents go through the access gateway and the uplnk of the access lnk to remote servers, and the correspondng responses answered by the remote servers return to clents through the downlnk of the access lnk and the access gateway. The downlnk bandwdth could be managed by controllng the releasng of uplnk requests. Request schedulng was used n several studes to provde dfferental Web QoS for the requests of dfferent types or the users of dfferent classes. Such a usage was frst ntroduced n [2] to schedule requests at a Web server. Then, the earlest deadlne frst schedulng was taken n [3] to ensure that requests of dfferent types can be served wthn ther specfc deadlnes. Besdes, requests of a new sesson may be blocked from gettng servce n [4] to prevent the server from overloadng and thus ensure the servce qualty of exstng sessons. Compared wth [2 4], whch deployed request schedulng at a Web server, [5 7] deployed t at a Web-sde gateway,.e. a gateway n front of a group of Web servers. Requests were scheduled based on ther resource requrements for load balance n [6] whle ths schedulng was for performance guarantee of users n dfferent classes n [7]. Although many request schedulng algorthms were proposed for a Web server or a Web-sde gateway, no publshed studes dscussed how to schedule requests at the user-sde access gateway to provde dfferental servces for users. The key dfference of schedulng requests at a server or a Web-ste gateway from at a user-sde gateway s that the destnaton Web servers n the former are specfc and ther statuses are easy to be measured or controlled for assstng n the schedulng operaton. However, the servers n the latter are nfnte n number, dstrbuted over the Internet, and cannot be controlled. In order to provde bandwdth sharng and weghted farness among users of dfferent classes on ther downlnk responses, ths work studes how to schedule uplnk requests at the user-sde access gateway. We frst nvestgate the possblty of applyng the class-based FQ dscplne, whch was wdely and maturely used n schedulng packets, to schedule requests. However, we found that smply applyng ths dscplne to schedule requests would encounter three problems. The frst two are determnng the tmng of releasng requests and selecton of the next released request. The last one s that the class-based weghted farness, acheved by a class-based FQ dscplne, does not sut for the user-level dfferentaton,.e. may not guarantee hgh-class users to get more bandwdth than low-class ones when more users appear n the hgh class. Based on the above nvestgaton, we further propose a mnmum-servce frst request schedulng (MSF-RS) scheme to provde bandwdth sharng and user-based weghted farness,.e. a polcy that the rato of the bandwdth allocated for each hgh-class user to that for each low-class user matches the rato of ther weghts. MSF-RS conssts of a mnmum-servce order arbter (MOA) and a wndow-based rate controller (WRC). MOA always selects the next request from the class whch receves the lowest amount of responses whle WRC determnes the tmng n releasng a request by montorng the downlnk utlzaton to control the number of outstandng responses. A response s regarded as outstandng f ts correspondng request s released, but the response has not been receved completely. MSF-RS s orgnally desgned based on the assumpton that the uplnk traffc conssts of requests only and the downlnk traffc conssts of ther correspondng responses. However, MSF-RS also works well under the envronment where the exceptve traffc coexsts wth the assumed traffc. We would further dscuss the traffc-mxed case and show the robustness of MSF-RS by smulaton n Secton 6. The remander of the paper s organzed as follows. Secton 2 dentfes the three problems occurrng n schedulng requests wth the class-based far queung dscplne. Also, the user-based weghted farness s ntroduced heren. Secton 3 proposes the MSF-RS scheme. Secton 4 proves that MSF-RS does shorten the user-perceved latency and also analyzes the worst-case farness of MSF-RS, whch are further demonstrated through the smulaton results n Secton 5. Besdes, the affecton of excepton traffc on MSF-RS s dscussed n Secton 6. Secton 7 demonstrates the effect of MSF-RS through feld tral, where MSF-RS s mplemented n Squd [8], an open-source Web proxy package. Secton 8 gves the conclusons and future work. 2. Problems on usng class-based far queung Three problems would occur when the class-based FQ s used to schedule requests. The former two are related to the FQ dscplne whle the last s about the class-based weghted farness polcy The tmng for releasng requests A FQ-based packet scheduler selects and sends the next packet va a lnk rght after the last packet has fnshed ts

3 S.-C. Tsao et al. / Computer Networks xxx (28) xxx xxx 3 transmsson. The bandwdth of the lnk would be totally consumed by the scheduled packets themselves. That s, each packet transmsson fully uses the lnk bandwdth. However, a FQ-based request scheduler cannot send the next request mmedately followng the last request, because a request does not drectly consume the bandwdth of the bottleneck downlnk, whch actually wll be consumed when recevng ts responses. Releasng requests one-by-one may brng a large number of concurrent response transmssons at the bottleneck downlnk, because the transmsson tme of a response, due to ts sze, s often longer than that of a request. Each transmsson, under such a condton, only shares small bandwdth, resultng n the serous congeston or the long user-perceved latency. On the other hand, the request schedulng cannot just wat to send the next request tll the precedng request completely gets ts response, because such a watng may waste the downlnk bandwdth. After a request s sent out, untl the frst packet of ts response returns, the downlnk wll be dle. Besdes, even when the response s transmttng, the transmsson may not run out the whole downlnk bandwdth, because the Internet bandwdth avalable for the transmsson may be smaller than the downlnk bandwdth. Snce requests cannot be sent out as packets, a mechansm s necessary to control the release of uplnk requests based on the utlzaton of downlnk bandwdth, n order to avod the downlnk from congeston and to keep t on hgh utlzaton The determnaton of the next request A FQ-based packet scheduler selects the next packet whch s the earlest one to be completely served,.e., fully transmtted, n the flud-based general processor sharng (GPS) model [9]. The order of servce completon s easly determned when a packet arrves because the determnaton only nvolves two known parameters, packet arrval tme and packet sze. For two packets arrvng at the same tme, the packet sze decdes the order of servce completon tme. A smaller packet fnshes servce earler. However, n a FQ-based request scheduler, although the arrval tme of each request s known, the sze of a request does not affect the servce tme of the request, whch however s counted from releasng a request to recevng ts whole response and manly contrbuted by the transmsson tme of the response. Because the transmsson tme s determned by the response sze and the avalable bandwdth n Internet, t s uncertan at the request-schedulng moment. Therefore, the completon tme s uncertan too and the request schedulng cannot select the next request smply by ts completon order. Hence, ths selecton s a problem when the FQ dscplne s appled to request schedulng The class-based weghted farness polcy The class-based weghted farness polcy s orgnally proposed to provde dfferental QoS for dfferent servce types of connectons. For example, the real-tme connectons and the best-effort connectons would be classfed nto two dstnct classes. Then, accordng to the weghts of these classes, each class s allocated wth a fxed proporton of bandwdth. When the polcy s appled, to guarantee that each connecton n a class gets enough bandwdth, controllng the number of the actve connectons n the class s necessary. Establshng a new connecton wll be rejected when the number of actve connectons exceeds the expected value. However, when the class-based weghted farness polcy s appled on provdng dfferental QoS for dfferent levels of users, t may expose undesrable characterstcs for hghclass users. For example, a hgh-class user may be rejected from gettng servce when the number of users now n the hgh class exceeds the expected value. Besdes, f the number of users n the hgh class s much more than that n the low class, each hgh-class user may get lower bandwdth than the low-class user. Therefore, another polcy may be necessary to always provde the hgh-class users enough bandwdth partcularly when more users are actve n the hgh class than the low class. We call such a polcy the user-based weghted farness. The polcy guarantees that the rato of bandwdth allocated for each hgh-class user to that for each low-class user matches the rato of ther weghts. 3. A request schedulng scheme n user-sde gateway The secton proposes a mnmum-servce frst request schedulng (MSF-RS) scheme, whch s deployed at the user-sde access gateway and can provde user-based weghed farness, bandwdth sharng, full bandwdth utlzaton, and short user-perceved latency. As shown n Fg. 2, the MSF-RS scheme conssts of a mnmum-servce order arbter (MOA) and a wndow-based rate controller (WRC). The former decdes whch request s the next one whle the latter determnes the tmng to release requests Mnmum-servce order arbter (MOA) As shown n Fg. 2, MOA ncludes a request selector, a request recever, and three groups of varables. There are N classes and each class s allocated a queue Q, a user counter (UC), a servce counter (SC), and a weght w. MOA selects the next request from one of the class queues,.e. Q 1 to Q N, based on the value n SCs, whch are updated by consderng UCs and w s. (1) UC and SC: The UC of a class keeps the number of the actve users now n the class, where the actve user means an ntranet user who has requests or outstandng responses n MSF-RS. The SC of a class keeps the amount of servces whch the class has receved. Heren the servce represents the receved responses n bytes after normalzed wth w and UC. That s, every tme when one class, say the class, receves ts partal response of length L p, ts SC, SC, s updated as SC new ¼ SC old L p þ : w UC ð1þ By normalzng L p wth w and UC durng the SC update, for any two classes wth the same SC, the rato of ther receved responses wll match the rato of ther weghts

4 4 S.-C. Tsao et al. / Computer Networks xxx (28) xxx xxx Requests classfer request recever Mnmum-servce order arbter (MOA) Q1 request Q 2 selector Q N Wndow-based rate controller (WRC) request releaser w 1 w N SC 1 W W + UC 1 UC N SC N U End of RI End of Rsp. classfer Response Mnmum-Servce Frst Request Schedulng (MSF-RS) SC: Servce Counter UC: User Counter w: Weght A A B B Data flow A changes B A s referenced by B Fg. 2. The nternal archtecture of MSF-RS. and ther actve users, respectvely. That s, even when the number of actve users n the hgh class s much more than that n the low class, the rato of the responses got by one hgh-class user to that by one low-class user stll matches the rato of ther pre-assgned weghts. (2) Next request selecton: As shown n Fg. 2, when a request arrves, the classfer forwards the request nto the correspondng Q. On the other hand, the request selector selects the head-of-lne (HOL) request from the queue wth the mnmum SC. A class wth the smaller SC represents that t receved less servces than other classes. Selectng a request from such a queue mproves the farness between classes, because t mnmzes the dfference of ther SCs. Besdes, f multple classes have the same SC, the request selector selects the class wth the hghest product of the weght and UC. An dle class represents that the class has no outstandng responses and ts request queue s empty. When a class dles for a long perod, ts SC may be far smaller than the SCs of actve classes. Once the dle class has ncomng requests, ts far-small SC may cause the starvaton of other classes. That s, untl ts SC s larger than any one SC of other classes, no request can be selected from other classes. To avod ths unfavorable condton, once the dle class becomes busy, ts SC s updated to the mnmal SC among all actve classes. Such an update lets MOA follow the sharng concept used n the far queung dscplne: the bandwdth freed from the dle classes would be shared by actve classes, and these actve classes wll not be punshed for the sharng, e.g. get less bandwdth when the dle classes become actve. Notably, f all classes are dle, all SCs wll be reset to. (3) Basc procedures: Fg. 3 lsts the pseudo codes of the two components n MOA. The request_selector pcks the class queue wth the mnmum SC and releases the HOL request of ths queue. The request_recever classfes and en-queues all ncomng requests. If an arrval request s classfed nto an dle class,.e. the class s UC s zero, the request recever resets the SC of ths class. Next, the request s put nto the belongng queue. If the request comes from a user j who has no request watng for servce or beng Fg. 3. Two procedures n MOA: request selector and request recever. served,.e. ReqFromUser[j] =, then the UC of ts belongng class wll be added one, mplyng one more user arrves n ths class. If the system s dle,.e. no responses are outstandng, the request_recever actvely asks the request_ selector to release the just comng request mmedately Wndow-based rate controller (WRC) As shown n Fg. 2, WRC controls the maxmum of outstandng responses, W +, accordng to the bandwdth utlzaton of the lnk, denoted by U. The varable W s used

5 S.-C. Tsao et al. / Computer Networks xxx (28) xxx xxx 5 to record the current number of outstandng responses. U s updated by the expresson: U m ¼ S m=t C ; ð2þ where U m s the utlzaton between the mth and (m + 1)th updates, C s the lnk capacty, T s the tme nterval between two updates, and S m s the responses n bytes receved durng T. Next, once U m s changed, WRC updates W + by the equaton W þ mþ1 ¼ mn Uþ U m ; K W þ m ; where W þ m and Wþ mþ1 are the maxmum of outstandng responses allowed after the mth and (m + 1)th update, respectvely. Also, W þ ¼ 1 and Uþ s the target utlzaton. K s a constant and assgned to 2 to avod WRC from overestmatng the new W + partcularly when the old W + s small. When U m s lower than U +, W þ mþ1 wll be set to a larger value so that more outstandng responses can use the bandwdth and then rase the utlzaton U m+1. For example, f the current W + s sx and U s 6%, then the next W + wll be set to 1 when U + = 99%. On the contrary, when U m s hgher than U +, W þ mþ1 wll be decreased so that fewer responses compete for the downlnk bandwdth. U + s constant and should be smaller than 1% (U + < 1%). Otherwse, when U + = 1% and U m = U +, t cannot be dstngushed whether the bandwdth requred by the responses s larger than or just equal to the lnk capacty. Notably, W + should be recomputed only when W = W +. When W < W +, t s wrong to expect the rase of U by ncreasng W +, because a low U s caused by nsuffcent arrval requests, but not too small W. Fg. 4 lsts the pseudo code of WRC. When WRC receves any part of a response, t looks for the class and the user ð3þ Fg. 4. Procedure of wndow-based servce-rate controller (WRC). whch ths response belongs to, and updates ts class s SC. Once the receved data ncludes the last packet of a response, W s decreased by one to mply that a request has been fully answered. Also, ReqFromUser of ths user s decreased by one. If ths s the last request, UC of ths class s decreased by one also, because one user leaves the class. Next, the request_selector s nvoked to release requests as more as possble, tll W = W + or all queues are empty. 4. Analyss for latency and farness In the secton, we frst demonstrate that a MSF-RS gateway provdes users shorter latency than an ordnary gateway on the average. An ordnary gateway means that t drectly forwards the requests or responses once recevng them. Then, we analyze the farness provded by MSF-RS n the worst case User-perceved latency In general, the user-perceved latency of a request represents the tme nterval begnnng when the clent host sends out the request and endng when t receves the response of the request. As shown n Fg. 1, when a user-sde gateway G s deployed at the gate of the Intranet to the Internet, we dvde such a latency nto four parts: (1) the transmsson tme of the request and ts response between the clent host and G, (2) the queung tme T q of the request n G, (3) the transmsson tme of the request from G to the Internet destnaton ste, and (4) the servce tme T s of the request,.e. the tme of recevng ts response from the ste. The tme of the frst and thrd parts are gnored n the followng comparson because they are small and unchanged no matter MSF-RS s deployed or not n G. Besdes, the sum of T q and T s s called the actve tme of the request n G, denoted as T a. Frst, we conceptually explan why the T s of a response n MSF-RS s shorter than that n an ordnary gateway when both gateways acheve the same hgh downlnk utlzaton. In general, the lnk utlzaton s the most domnated factor to affect the packet s T s, the transmsson tme over the lnk. However, snce a Web response conssts of a crowd of packets and ts T s spans from transmttng the frst packet of the response to the end of ts last packet s transmsson, thus T s depends on not only the utlzaton but also the concurrent number of the response transmssons. That s, a large number of concurrent transmssons would also ncrease the T s of a response. Recall that the number of concurrent responses n MSF-RS s controlled to run out but not overload the downlnk, whle that n an ordnary gateway s uncontrolled and ncreased wth the arrval rate of requests. When both MSF-RS and the ordnary gateway operate at the hgh lnk utlzaton, MSF-RS would have a smaller number of concurrent transmssons and thus provde a shorter T s. For example, assume that 1 concurrent transmssons of responses can rght exhaust the downlnk bandwdth. Then, when 2 responses are concurrently transmtted through an ordnary gateway, although the downlnk s exhausted too, the transmsson tme of the 2 responses would prolong doubly at least,

6 6 S.-C. Tsao et al. / Computer Networks xxx (28) xxx xxx because each response transmsson n the former only gets half bandwdth of that n the latter on average. The followng formally proves that MSF-RS provdes shorter T a on average than an ordnary gateway under a case that a batch of m requests arrves nto the gateways. Assume MSF-RS has a fxed W + and the m requests would ask the responses of sze L. Besdes, the maxmum bandwdth that W + responses can use would approxmate to the downlnk bandwdth C, because f a MSF-RS gateway allows the current transmsson of W + responses, then these responses are expected to completely occupy but not overload the downlnk. Therefore, the maxmum bandwdth of each response can be expressed as C/W +. Although the assumpton of C/W + may be unrealstc, t s for the convenence n analyss and does not mpose any constrant on the conclusons, whch are valdated by our smulatons and experments where no such an assumpton s gven. Let us frst consder the stuaton under an ordnary gateway. Snce ths work concerns the condton when the downlnk s the bottleneck for an enterprse to connect the Internet, we assume the uplnk bandwdth s far hgher than the bandwdth necessary for the transmsson of requests. Also, because the ordnary gateway smply forwards any receved requests wthout consderng the utlzaton of downlnk and a request s usually far smaller, n length, than ts correspondng response, the T q s of these requests are closed to zero, compared to ther T s s. Then, snce the responses of the m requests wll concurrently share the downlnk bandwdth, the bandwdth got by each response can be wrtten as C. Therefore, the average T a of requests under an ordnary m gateway s avgðt ordnary a Þ¼þ L ¼ ml C C : ð4þ m Next, consder the case under MSF-RS. The T q s of the frst W + requests are zero because they are forwarded mmedately. Then, others requests wll be queued untl any of the W + requests have been served. In the worst case, the W + requests end concurrently. Thus, the T q s of the next W + requests would equal to the T s s of the frst W + requests,.e. Wþ L. Next, the T C q s of the followng W + requests would equal to 2 Wþ L. The T C q s of the resdual requests could be derved from the same way. Thus, by summng up the T q s of W + requests n each round and consderng the possblty that the number of requests n the last round may be less than W +, the average T q of the m requests could be calculated as q Þ¼ 1 m Wþ þ 1 þ 2 þþmax þðmmodw þ m Þ W þ avgðt MSF-RS m W þ 1; W þ L C : ð5þ Smlarly, the mean T s of the m requests could be expressed as avgðt MSF-RS s Þ¼ 1 m ðm ðmmodw þ ÞÞ Wþ L C þðmmodw þ Þ ðmmodwþ ÞL : ð6þ C Therefore, we get the average T a of requests under a MSF- RS by summng up Eqs. (5) and (6). To compare the average T a of requests over the two gateways, Fg. 5 plots the rato of the MSF-RS gateway to an ordnary gateway on T a over dfferent m and W +. Fg. 5 shows that the rato s smaller than 1 always,.e. the average tme to queue and serve requests under MSF-RS s no more than that under an ordnary gateway. For example, as plotted by the dotted lne, MSF-RS can reduce 25% of T a when the number of arrval requests s two tmes of W +. These results demonstrates that MSF-RS does not cause addtonal delay on T a through MSF-RS does queue requests,.e prolong T q, to prevent the downlnk from beng the bottleneck of the response transmsson. The followng gves an example to further clarfy the fact. Assume there are 12 requests watng for beng forwarded. Also, we suppose that the downlnk bandwdth can support four response transmssons n 1 s,.e. T s = 1, when four responses are receved concurrently. When an ordnary gateway s deployed, all the 12 requests would be mmedately forwarded. Snce there are 12, nstead of four, response transmssons concurrently competng the downlnk, each transmsson wll get only 1/12, nstead of 1/4, of downlnk bandwdth. Therefore, T s of the 12 requests becomes three and ther user-perceved latency s T q + T s =+3=3s on average. In contrast, when a MSF-RS gateway s deployed, snce t montors the downlnk utlzaton to control the number of the concurrent response transmssons, only four responses would be allowed to be transmtted concurrently. Thus, the frst four requests are forwarded wthout any delay and ther responses are receved n one second; meanwhle, the other eght requests are queued n the gateway for 1 s. Then, the next four requests are forwarded and ther responses are receved n the 2nd second, whle the last four requests are stll queued. Fnally, the last four requests are forwarded. Therefore, the average T q for the 12 requests can be easly calculated as ( )/12 = 1 s. Besdes, T s of the 12 request s 1 s. Then, ther average user-perceved latency s equal to = 2 s, whch s obvously shorter than that acheved by an ordnary gateway. Fg. 5. The rato on T a of a MSF-RS gateway to an ordnary gateway.

7 S.-C. Tsao et al. / Computer Networks xxx (28) xxx xxx Farness The farness parameter that we use s based on the defnton presented by Golestan [1] for the analyss of selfclocked far queung. The parameter s defned as the maxmum dfference of the servce got by any two backlogged classes over arbtrary tme ntervals, whch can be wrtten n a mathematcal formula as MAX D ðt 1 ; t 2 Þ D jðt 1 ; t 2 Þ ðt 2 > t 1 Þ; w w j where t 1 and t 2 are two arbtrary nstants when the two classes are backlogged. w s the weght of Class, and D (t 1,t 2 ) represents the receved servce of Class durng [t 1,t 2 ]. A schedulng algorthm has a zero value of farness f t always provdes equal servce for any two classes even n a short tme nterval. We consder two classes, Class and Class j, through the followng analyss because the defnton of farness only concerns two classes. The exstence of more backlogged classes does not affect the dfference of servces got between two classes. As shown n Fg. 6, assume the MSF-RS s dle before the tme t,.e. W =. Then, at the tme t more than W + requests of Class arrve. Let v denotes the tmestamp of the frst request of Class, where the tmestamp represents the value n SC when the request arrves nto Class. Upon these requests arrves, MSF-RS wll forward the frst W + ones and W s set to W +. Let request k be the (W + + 1)th one of Class,.e. t would be the frst request n the queue of Class after t. Snce W + requests of Class would be served before k, k wll have a tmestamp v k expressed by v k ¼ v þ 1 X W þ w L h h¼1 6 v þ Wþ L þ w ; where L h s the sze of the hth response of Class and L þ s the maxmum sze of response of Class. Assume that the requests of Class j arrve rght after the tme t, and the frst request should have the tmestamp v snce the tmestamp of the request latest released by the server rght after the tme t s equal to v. However, although the frst request of Class j has tmestamp v smaller than that of Class,.e. v k, no requests can be forwarded from Class j because all W + sub-lnks are busy for Class. v t t 1 t 2 Tme t ~t 1 t 1 ~t 2 t 2 ~ Max. dfference of servce between Class and j Tmestamp v k + + W L w L W L w + w Class j Class Fg. 6. The dfference of the servce between Class and Class j. + L w Tme Let the sub-lnks become dle at t 1. Then, n the worst case all requests of Class j wth a tmestamp no larger than v k wll be forwarded before k. Assume the Class-j request wth v k asks a response wth sze equal to L þ j. Then, between t 1 and t 2 the maxmum of the total responses receved by Class j wll be 1 w j W þ L þ þ L þ j w : ð7þ However, as shown n Fg. 6, Class does not get any servce durng the perod. Thus, between t 1 and t 2, the dfference of the servce between the two classes s Wþ L þ þ L þ j, got by w w j dvdng (7) by w j. For the tme later than t 2, when the servce of a class already equals to another one, the addtonal servce whch the class can get must smaller than Lþ snce Lþ P L þ j s supposed. Snce the dfference of servce after t 2 and before t 1 w w w j s smaller than that between t 1 and t 2, the dfference of the servce between t 1 and t 2 s the worst-case farness of MSF- RS, that s W þ L þ w þ Lþ j w j : 5. Smulaton results Ths secton frst verfes the effects of MSF-RS by ns-2 [11] n terms of the farness and bandwdth sharng, user-perceved latency and the relatonshp between U and W +. Then, the effect of U + on latency s nvestgated Topology The HTTP/Cache n ns-2 acts as a Web proxy cache and sts between clents and Web servers. It ntercepts the requests sent from clents and forwards them to the remote servers f the requested data s not cached yet. Ths work dsables the cache functon and mplements MSF-RS n HTTP/Cache. Fg. 7 shows the topology used n the smulaton. The MSF-RS gateway provdes three classes, Class 1, Class 2, and Class 3, wth the weghts, 4, 2, and 1, respectvely. Each class nvolves four clents and each clent repeatedly requests pages from the 12 remote Web servers through the MSF-RS gateway. For each clent, the tme nterval between two requests s an exponental dstrbuton wth mean equal to 5 s. The lnk between the MSF-RS gateway and every clent s 1 Mbps wth 2 ms propagaton delay. The ISP gateway connects to twelve servers wth twelve ndependent lnks. These servers are classfed nto two equal numbers of groups, representng overseas servers and domestc servers. Lnks between the gateway and these servers have a unform dstrbuton, as shown n Fg. 7. By the statstcs from the real Internet [12], the Web response sze has a lognormal dstrbuton wth M = and S = 1.318, where the probablty densty functon P(x) of the lognormal dstrbuton can be wrtten as PðxÞ ¼ p 1 S ffffffffffff e ðln x MÞ2 =ð2s 2Þ : x 2p

8 8 S.-C. Tsao et al. / Computer Networks xxx (28) xxx xxx Class 2 Class 1 Class 3 1Mbps 2ms User-sde Gateway 2Mbps 1ms ISP-sde Gateway 6 Overseas Servers BW: unform [1-5] Mbps RTT: unform [4-14] ms 6 Domestc Servers BW: unform [1-2] Mbps RTT: unform [-4] ms Fg. 7. Smulaton topology for three classes wth servce rato 4:2:1. The average response sze s e MþS2 =2 bytes,.e. 27,656 bytes. The U + n WRC s set to 98% and the tme nterval between two updates s set to 5 s. Secton 5.6 would further reveal the effects of dfferent U + s on lnk utlzaton, but that of dfferent update ntervals are gnored to show because of ther nsgnfcant effects. Besdes, we use TCP SACK and assume no delayed acknowledgments. Over the smulaton the packet sze s 1 bytes and the maxmum congeston wndow of TCP s 2. The queues at the two gateways are managed by Drop-Tal and ther szes are 1.5 bandwdthdelay products Weghted farness and bandwdth sharng Frst, we demonstrate that when all classes have the same users, MSF-RS provdes weghted farness between classes and the dle bandwdth s shared by actve classes. Four phases are ncluded n the smulaton and the duraton of each phase s 2 s. In the frst phase, all of the three classes have backlogged requests. In the next two phases, Class 1 and 2 stop requestng ndvdually, and then both of them have backlogged requests agan n the last phase. Fg. 8a shows the average throughput under MSF-RS n each phase. Durng the frst phase, the three classes get proportonal bandwdth n rato 3.96:1.98:1, whch s close to the expected rato 4:2:1. In the second phase, the dle bandwdth freed by Class 1 s shared by Class 2 and Class 3 proportonally. Both of the bandwdth obtaned by Class 2 and Class 3 ncrease n ths phase, and the usage rato between them s stll 2:1. After Class 2 stops requestng n the thrd phase, Class 3 occupes all bandwdth untl the end of ths phase. Durng the second and the thrd phases, Class 1 and Class 2 stll obtan a bt of bandwdth separately due to ther unfnshed responses at the 2th and 4th s, respectvely. Once all dle classes have requests agan n the last phase, the three classes obtan the bandwdth n the expected proporton, 4:2:1, agan. a BW(kbps) 2 18 Class 1 Class 2 16 Class 3 14 All Phase b BW (kbps) 2 Class 1 18 Class 2 16 Class 3 14 All Phase Fg. 8. The average throughput of three classes over the four phases under (a) MSF-RS and (b) DRR.

9 S.-C. Tsao et al. / Computer Networks xxx (28) xxx xxx 9 As mentoned n Secton 1, packet schedulng algorthms fal to allocate the downlnk bandwdth at the user-sde gateway, because the packets of responses have passed the bottleneck and can be mmedately forwarded to the clents. Under the stuaton, a packet schedulng algorthm would degrade nto a frst-n-frst-out schedulng. To demonstrate such degradaton, we employ a defct round robn (DRR) [13] nstead of MSF-RQ at the user-sde gateway. DRR s a wdely used packet schedulng algorthm because t s easy to be mplemented. Fg. 8b shows the bandwdth allocaton managed by DRR. Obvously, over the four phases, Class 1 and Class 2 do not get hgher bandwdth than Class 3, even though both classes have larger weghts than Class 3. Further observaton shows that the request queues of three classes n DRR are empty durng the smulaton, whch verfes that requests are forwarded upon ther arrval,.e. a FIFO order User-perceved latency The smulaton scenaro here s the same as that used n the frst phase n Secton 5.2. Fg. 9 llustrates the user-perceved latency for the three classes, the average latency of all classes, and the latency f no MSF-RS s deployed, denoted as non-msf-rs. The latency s decomposed nto the queung tme and the servce tme of the requests, as ntroduced n Secton 4.1. Frst, by comparng the left three bars, the three classes n MSF-RS experence the dfferent user-perceved latences, manly caused from dfferent queung tme snce they have dfferent weghts. Second, by comparng the rght two bars, the average latency (6.76 s) n MSF-RS s shorter than that n non-msf-rs (8.83 s) by 23.44%. It s because the average servce tme n MSF-RS (1.44 s) s far shorter than that n non-msf-rs (8.83 s). The servce tme n MSF-RS s reduced because t has the well-controlled number of concurrent outstandng responses and thus each response can be receved n a short tme User-based weghted farness Next, we show the MSF-RS gateway provdes the hghclass users more bandwdth than the low-class users regardless of the number of users n the hgh class. The same testng scenaro as that n Secton 5.2 s used, but the number of users n Class 1 s ncreased from 4 to 24. Also, all of the three classes have backlogged requests durng the whole testng tme, 8 s. Fg. 1a plots the dfference of the average bandwdth allocated for the users n each class. When there are 4 users n Class 1, each user n ths class owns 27 Kbps, whch s the two and four tmes of that allocated for the user n Class 2 and Class 3, respectvely. The fxed rato of the allocated bandwdth among the three classes s kept even when the number of users n the frst class s ncreased. Fg. 1b plots the result provded by the MSF-RS gateway wthout consderng the number of users when updatng SC,.e. the L p n Eq. (1) s not dvded by UC. Obvously, under such a gateway, the users n Class 1 cannot be ensured to get more bandwdth than that n other classes when more users are actve n Class Adjustment of outstandng responses The subsecton observes the adjustment of W + when the arrval of requests s not backlogged always,.e. MSF-RS may be dle sometmes. In a 15-s test, clents n Class 1 and 2 send requests every 1 s whle that n Class 3 sends one every 1 s. Besdes, durng the mddle 5 s, clents n Class 1 and 2 stop sendng requests, whch results n nsuffcent requests so that the MSF-RS gateway has no request to send out. Fg. 11 reveals the relaton between U and W +. The utlzaton of access lnk stays around.98 as the expected U + n the frst and last 5-s perods because of suffcent arrval requests. In the 5th 1th s, the utlzaton falls apparently and the value of W + keeps constant as descrbed n Secton 3.2. Increasng W + for rasng the utlzaton durng the perod s n van because the low utlzaton results from the fact that the ncomng requests are too few to occupy all sub-lnks. Besdes, the value of W vares wth a wde range, determned by the dynamcally arrval requests. Durng the perod, any requests are forwarded mmedately once they arrve, snce there are always free sub-lnks. Notably, at the 1th s, once the two stopped classes restart sendng requests, all requests can be released soon and the utlzaton jumps to the expected value Effect of U + on latency Fg. 12 depcts the user-perceved latency and the queung tme spent n the MSF-RS access gateway and n the Tme (sec) Servce tme Queung tme Class 1 Class 2 Class 3 Average 8.83 Non-MSF-RS Fg. 9. The comparson on the user-perceved latency among classes and between MSF-RS and non-msf-rs.

10 1 S.-C. Tsao et al. / Computer Networks xxx (28) xxx xxx a 3 b Bandwdth (Kbps) Class 1 25 Class 2 Class 3 2 Bandwdth (Kbps) 15 1 Class 1 Class 2 Class Hosts n Class Hosts n Class 1 Fg. 1. The dfference on the bandwdth allocated for the hgh-class host between the (a) host-based and (b) class-based weghted farness. Utl(%) Tme (sec) Latency Queung Tme 1 * Avg. pkt queung tme n ISP U W + W Tme (sec) Wn 3 Fg. 11. The sze of W + s fxed n the perod wth nsuffcent traffc (the 5th 1th s) U + Fg. 12. The user-perceved latency, queung tme, and the packet queung tme n ISP-sde gateway under the dfferent U +. ISP-sde gateway when U + s assgned from.65 to.99. Rasng U + follows shorter user-perceved latency because more responses can be concurrently transmtted and the bandwdth can be more utlzed. However, the rase also causes packets to be queued n the ISP-sde gateway because of less free bandwdth to elmnate the queued packets as U + s hgh. By the observaton n Fg. 12, the value of U + s suggested to be set to 98% Affecton of exceptve traffc MSF-RS s desgned under the assumpton that the uplnk traffc comprses requests only and the downlnk traffc comprses ther correspondng responses only. However, the exceptve traffc does coexst wth the assumed traffc n the real envronment. We classfy the exceptve traffc nto three types and explan why they do not affect the farness or lnk utlzaton provded by MSF-RS. (1) Uplnk exceptve traffc: The type of traffc may nclude the uplnk responses and the packets actvely sent from the nternal users. If ths traffc s heavy enough to turn the uplnk to a bottleneck, a packet scheduler wth the FQ dscplne s suggested to be deployed at the access gateway frst. MSF-RS can coexst wth the uplnk FQ dscplne well, as shown n Fg. 13a. Also, f the weghted farness on uplnk s not a concern, the combnaton of MSF-RS and prorty queues s a smple soluton, as shown n Fg. 13b. The soluton gves the request traffc hgher prorty snce they are smaller than responses usually. (2) Downlnk exceptve traffc belongng to some classes: Such traffc s stll the downlnk responses, but ther requests are not recognzed by the mplementaton of MSF-RS. For example, POP3 mal traffc for someone s host belongs to Class. It s possble snce only the Web request s recognzable for the present mplementaton of MSF- RS.The MOA n MSF-RS regards these exceptve packets as the receved servce of classes. That s, when the packets not trggered by an (recognzed) request arrve from the Internet, ther szes are accumulated nto the SC of the class whch the packets belong to, as other response packets trggered by requests. That s, f a user of Class receves a crowd of such packets from the Internet, the szes of packets would be accumulated nto SC. Although the addtonal value n SC caused by these exceptve packets brngs the fewer requests sent out from Class by MSF-RS, t does not affect the weghted farness and the lnk utlzaton.

11 S.-C. Tsao et al. / Computer Networks xxx (28) xxx xxx 11 uplnk traffc uplnk traffc Req. (a) The archtecture provdng weghted farness between request traffc and exceptve traffc Y Classfer REQ MSF-RS N MSF-RS w 1 w 2 w 3 w n PKT-FQ L P H downlnk traffc (b) The archtecture gvng the hgher prorty for request traffc (3) Downlnk exceptve traffc not belongng to any class: Exceptve traffc not belongng to any class may nclude requests from outsde network for the responses provded by the nternal servers, or the malcous attacks. The former case s possble for the enterprses havng Web servers for ther customers. The exceptve traffc contrbuted from such requests s usually small, compared wth other response traffc runnng on the downlnk. The latter case may rudely and fully occupy the downlnk, resultng n the falure for all transmssons. However, the latter case s a securty problem and out of the scope of ths work. Snce ths exceptve traffc s not belongng to any class and not counted n any SC, t wll not affect the weghted farness between classes. Besdes, although t would be counted by WRC n the utlzaton of the downlnk, t does not degrade the hgh lnk utlzaton whch WRC can ensure. Such traffc only causes WRC to have a smaller W + than that n the case wthout the exceptve traffc. That s, WRC may thnk such a small W + s enough to fully utlze the downlnk bandwdth. Moreover, when the exceptve traffc passed, because WRC would quckly adjust W + accordng to the new U, the lnk utlzaton s not affected n ths case. We use the followng smulaton to demonstrate that the utlzaton s not affected by the downlnk exceptve traffc. The on off CBR traffc not belongng to any classes wth dfferent rates durng dfferent perods are generated to demonstrate the responsveness of WRC s fast enough to keep the hgh lnk utlzaton. Fve on/off perods are tested: 4, 8, 16, 32, and 64 s. Durng the on-perod, three rates of CBR traffc are tested ndvdually: 2%, 4%, and 6% of the downlnk capacty. Each test s run for 3 seconds and U + s set to 98%. Fg. 14 shows the case where on/off perod s 64 s and CBR rate s 4% of lnk capacty,.e..8 Mbps. At 128 and 256, once the CBR traffc stops, WRC mmedately resets W + from 7 to 13 n order to release more requests. The bandwdth freed by CBR traffc s fully and fast occuped by the response traffc. In fact, the results n all tests, as shown n Table 1, reveal S downlnk traffc Fg. 13. Two potental ntegrated archtectures for handlng the network where the uplnk s a bottleneck. Utl (%) Tme (sec) that WRC keeps the utlzaton on 97.84% averagely, closng to the desgned goal, 98%. Although the exceptve traffc does not degrade the farness and lnk utlzaton acheved by MSF-RS, t reduces the bandwdth avalable for response transmssons and thus ncreases the user-perceved latency of these responses. As shown n Secton 4.1, MSF-RS has the advantage of shorter user-perceved latency than an ordnary gateway under no exceptve traffc. Actually, the advantage stll holds when exceptve traffc appears, as explaned n the followng. When the exceptve traffc exsts, the downlnk bandwdth C used n Eqs. (4) (6) s decreased to a smaller value, and thus a smaller W + s adopted n MSF-RS, as shown n Fg. 14. From Fg. 5, we know that the rato on T a of a MSF-RS gateway to an ordnary gateway under a small W + s smaller than that under a large W +. That s, a MSF-RS gateway can provde a much shorter user-perceved latency than an ordnary gateway when exceptve traffc exsts, although the latences n both gateways are ndeed ncreased n ths crcumstance. 7. Feld tral Utlzaton W + W + 25 Fg. 14. Fast-responsve W + and full utlzaton of access lnk under oscllate CBR traffc. We mplemented MSF-RS n Squd [8], whch s an opensource package of Web proxy cache, and performed a feld tral n an open network envronment. Fg. 15 llustrates the test bed for evaluatng MSF-RS n Squd. An applcaton-layer traffc generator named Avalanche [14] s used to emulate the behavors of multple clents and send requests to the Web servers n the Internet. Avalanche s mported wth a URL lst, a hstorcal record logged by an enterprse n a couple of days. The access gateway nstalled wth MSF-RS s confgured as a transparent proxy wth ptables [15]. All HTTP requests destned to port 8 are drected to port 3128, the servce port of Squd. A layer 3 swtch s acted as the ISPsde gateway. The bandwdth of the access lnk between the access gateway and the layer 3 swtch s lmted to 2 Mbps. As the confguraton n smulaton, three classes are provded wth servce rato 4:2:1. Notably, the cache functon s dsabled to avod gettng responses drectly from caches. The effects of MSF-RS Squd are observed n terms of weghted farness, user-perceved latency, and CPU loadng as follows

12 12 S.-C. Tsao et al. / Computer Networks xxx (28) xxx xxx Table 1 The utlzaton of lnk under oscllatng CBR traffc (U + = 98%) Table 2 User-perceved latency comparsons On/off perod (s) The rate of CBR durng on-perod.4 Mbps (%).8 Mbps (%) 1.2 Mbps (%) Item User-perceved latency Case Orgnal Squd MSF-RS Squd ms ms (nclude queung tme ms) (1) Weghted farness: The amounts of bandwdth allocated to three classes for a 2-s test are 1.3,.52, and.26 Mbps, respectvely, when backlogged requests are appled. The result qute obeys the confgured servce rato 4:2:1. (2) User-perceved latency: Table 2 shows the latency provded by the orgnal Squd and the MSF-RS Squd. The orgnal Squd case represents all requests are mmedately released by the proxy. The MSF-RS Squd reduces ( )/1686, or 3%, of the average user-perceved latency n the orgnal Squd case, although the user-perceved latency n MSF-RS ncludes the addtonal queung tme, ms. (3) CPU loadng: Table 3 shows the benchmark results on CPU tme occuped by the MSF-RS Squd process and the orgnal Squd process when both processes provde the same lnk-speed throughput durng 2 s. As expected, the CPU tme ncreases as the number of classes or the access lnk bandwdth ncreases. Notably, the tme under MSF-RS s always lower than that under the orgnal Squd. Under the orgnal Squd, all requests are mmedately released by the proxy, brngng great number of concurrent responses. However, a proper number of concurrent responses are allowed by MSF-RS. It s beleved that the number of concurrent responses domnates the cost of CPU computng. 8. Conclusons and future work Schedulng the uplnk requests s a potental method to manage the bottlenecked downlnk at the user-sde access gateway. Because the class-based far queung (FQ) dscplne s wdely and maturely used n schedulng packets, Table 3 Comparson between MSF-RS and the orgnal Squd on CPU tme Case Lnk capacty 2 Mbps 1 Mbps MSF-RS Squd wth 1 Classes 44.8 s s MSF-RS Squd wth 1 Classes 46.2 s 58.4 s Orgnal Squd s 84.9 s we frst nvestgate the possblty of applyng ths dscplne to schedule requests. However, we found that three problems occur at applyng the class-based FQ dscplne to schedule requests: the tmng and orderng to release requests and the sutablty of class-based weghted farness for user-level dfferentaton. Based on the nvestgaton on the three problems, we propose the mnmum-servce frst request schedulng (MSF-RS) scheme to manage the access lnk bandwdth at user-sde access gateway. To acheve hgh bandwdth utlzaton whle avodng congestng the lnk, the wndow rate control module n MSF-RS determnes the releasng rate of requests and the number of outstandng responses. To perform user-based weghted farness and bandwdth sharng, the mnmum-servce arbter module n MSF-RS always selects the request from the class recevng the least normalzed responses. The analyss frst proves that MSF-RS shortens 25% of the user-perceved latency on average, compared wth an ordnary gateway, because the number of concurrent transmssons s controlled, even though ths control may queue requests n the MSF-RS gateway. Besdes, the analyss on worst-case farness represents that the MSF-RS gateway does provde the dfferental servces among classes whle avodng the low-class users from long latency. The results n the smulaton and n the feld tral show that the bandwdth usage between classes conforms to the targeted rato and the dle bandwdth s proportonally shared Avalanche (Clents) eth user space :3128 MSF-RS Squd kernel space Lnux Port Redrect (ptables): ptables -t nat -A PREROUTING - eth1 -s /24 -p tcp --dport 8 -j REDIRECT --to-port 3128 eth swtch Realstc Servers n Internet Rate Lmtng by swtch: Input 2Mbps / Output 2Mbps Fg. 15. The test bed for feld tral n the Internet.

13 S.-C. Tsao et al. / Computer Networks xxx (28) xxx xxx 13 by all actve classes. Besdes, MSF-RS reduces 23.44% and 3% of user-perceved latency n the smulaton and the feld tral, respectvely. Currently, http traffc s growng due to the exploson of vdeo streamng servces. For streamng servces, MSF-RS s stll better than an ordnary gateway. The reason s explaned as follows. MSF-RS does not provde a hgh-class user to use a sgnfcantly hgh throughput to download the vdeo. In fact, t smply releases requests from hghclass queue more frequently than low-class queue, but does not control the downloadng rate of a sngle transmsson. If the vdeo server can properly determne transmttng rate, the transmsson wll not overuse the bandwdth. Actually, many exstng servers can adjust ths rate accordng to the sze of clent s buffer. In contrast, f the vdeo server sends data as soon as possble, only a few hgh-class requests wll be released snce MSF-RS provdes weghted farness among hgh and low classes. Thus the low-class users do not encounter the starvaton. Therefore, for streamng servces, MSF-RS s stll better than an ordnary gateway where all requests are released and no dfferentaton exsts. Under an ordnary gateway, ether hgh-class or low-class users wll suffer nsuffcent bandwdth. The vewng of vdeos wll be suspended to wat more data arrval, although users may be quckly ntalzed. In the future, we wll further enhance MSF-RS to support other request response protocols, such as FTP and POP3. Fnally, to further reduce the overhead, mplementng MSF-RS n the kernel space may be consdered. [12] P. Barford, M. Crovella, Generatng representatve Web workloads for network and server performance evaluaton, ACM SIGMETRICS Performance Evaluaton Revew 26 (1) (1998) [13] M. Shreedhar, G. Varghese, Effcent far queung usng defct round- Robn, IEEE/ACM Transactons on Networkng 4 (3) (1996) [14] Avalanche. < 2&az-c=dc>. [15] The Netflter/Iptables project. < Shh-Chang Tsao was born n Tawan n He receved the B.S. and M.S. n Computer and Informaton Scence from Natonal Chao Tung Unversty n 1997 and 1999, respectvely. He worked as an assocate researcher n Chung-Hwa Telecom from 1999 to 23, manly to capture and analyze swtch performance. He receved the Ph.D. degree n Computer Scence from Natonal Chao Tung Unversty n 27, and s n Hgh Performance Computng Research Department of Lawrence Berkeley Laboratory for hs postdoctoral research. Hs research nterests nclude TCP-frendly congeston control algorthms, far queung algorthms, and Web QoS. Yuan-Cheng La receved the Ph.D. degree n Computer Scence from Natonal Chao Tung Unversty n He joned the faculty of the Department of Informaton Management at Natonal Tawan Unversty of Scence and Technology n 21 and has been a professor snce 28. Hs research nterests nclude wreless networks, network performance evaluaton, network securty, and content networkng. References [1] H.Y. We, S.C. Tsao, Y.D. Ln, Assessng and mprovng TCP rate shapng over edge gateways, IEEE Transactons on Computers 53 (3) (24) [2] R. Pandey, J. Frtz Barnes, R. Frtz Barnes, Supportng qualty of servce n HTTP servers, n: Proceedngs of the 17th Annual ACM Symposum on Prncples of Dstrbuted Computng, 1998, pp [3] N. Bhatt, A. Bouch, A. Kuchnsky, Integratng user-perceved qualty nto Web server desgn, n: Proceedngs of the 9th Internatonal World Wde Web Conference, 2. [4] L. Cherkasova, P. Phaa, Sesson based admsson control: a mechansm for Web QoS, n: Proceedngs of the Internatonal Workshop on Qualty of Servce, [5] V. Cardelln, E. Casalccho, M. Colajann, M. Mambell, Enhancng a Web-server cluster wth qualty of servce mechansms, n: Proceedngs of IEEE Internatonal Performance Computng and Communcatons Conference, 22. [6] E. Casalccho, M. Colajann, A clent-aware dspatchng algorthm for Web clusters provdng multple servces, n: Proceedngs of the 1th Internatonal World Wde Web Conference, 21. [7] C. L, G. Peng, K. Gopalan, T. Chuch, Performance Guarantee for Cluster-Based Internet Servces, State Unversty of Stony Brook (21). [8] Squd Web Proxy Cache. < [9] A.K. Parekh, R.G. Gallager, A generalzed processor sharng approach to flow control n ntegrated servces networks: the sngle-node case, IEEE/ACM Transactons on Networkng (1993) [1] J. Golestan, A self-clocked far queueng scheme for broadband applcatons, n: Proceedngs of the IEEE INFOCOM, Toronto, June [11] The Network Smulator ns-2. < Le-Ch Tsao was born n Tape, Tawan n She receved the B.S. and M.S. degree n Computer Scence from Natonal Chao Tung Unversty n 1999 and 25, respectvely. Her research nterests nclude network QoS and request schedulng. She can be reached at lctsao@cs.nctu.edu.tw. Yng-Dar Ln s a professor of Computer Scence at Natonal Chao Tung Unversty, where he s also the drector of Computer and Network Center and Network Benchmarkng Lab (NBL). Hs research nterests nclude qualty of servces, deep packet nspecton, and hardware software co-desgn. He s spendng hs sabbatcal year, from July 27, at Csco, San Jose. Dr. Ln graduated from Natonal Tawan Unversty (B.S. 1988) and UCLA (Ph.D. 1993).