A Transparent Rate Adaptation Algorithm for Streaming Video over the Internet

Transcription

1 A Transparen Rae Adapaon Algorhm for Sreamng Vdeo over he Inerne L. S. Lam, Jack Y. B. Lee, S. C. Lew, and W. Wang Deparmen of Informaon Engneerng The Chnese Unversy of Hong Kong Shan, N.T., Hong Kong, Chna {lslam2, yblee, soung, ABSTRACT The lack of end-o-end qualy of servce suppor n he curren Inerne has caused sgnfcan dffcules o ensurng playback connuy n vdeo sreamng applcaons. Ths sudy addresses hs challenge by nvesgang a new adapaon algorhm o adjus he b-rae of vdeo daa n response o he nework bandwdh avalable o mprove playback connuy. Unlke prevous works, he proposed algorhm s ransparen o he vdeo clen, requres no parameer unng, and ye can ouperform exsng algorhms. Ths paper presens hs algorhm, evaluaes and compares s performance wh he bes algorhm currenly avalable usng exensve race-drven smulaons.. INTRODUCTION The lack of end-o-end qualy-of-servce (QoS) suppor n oday s Inerne has caused sgnfcan dffcules o he deploymen of vdeo sreamng servces such as vdeo broadcasng and vdeo-on-demand. In parcular, when he nework becomes congesed, sgnfcan packe losses wll arse, leadng o corruped or even dropped vdeo frames. Gven QoS suppor s unlkely o be wdely avalable n he near fuure, researchers have resored o anoher approach o ackle hs problem. Specfcally, a number of poneerng researchers have nvesgaed algorhms o adap he vdeo b-rae o he nework bandwdh avalable [-5]. For example, when he nework becomes congesed, he sender wll reduce he b-rae of he encoded vdeo o allevae he congeson. Clearly, reducng he b-rae wll also degrade he vsual qualy. Neverheless, reducng he vdeo b-rae n a conrolled manner a he sender wll resul n far beer vsual qualy han aempng o recover from daa loss a he recever. To perform vdeo adapaon we mus ackle wo fundamenal challenges. Frs, he sender mus be able o dynamcally conrol or conver he vdeo b-rae o he desred value. Ths can be accomplshed by means of scalable vdeo codng [6] and ranscodng [7-9]. Second, an adapaon algorhm s needed o esmae he nework bandwdh avalable, and subsequenly deermne he b-rae o be used for converng and ransmng he vdeo sream. Ths sudy focuses on he second challenge,.e., desgn of he rae adapaon algorhm. Ths problem has recenly been suded by a number of researchers, ncludng he sudes by Rejae, e al. [4] and Assuncao and Ghanbar [5] whch adoped UDP as he nework ranspor; and he sudes by Cueos and Ross [], Cueos, e al. [2], and Jacobs and Elefherads [3] whch adoped TCP as he nework ranspor. A common propery of hese adapaon algorhms s he exsence of a confgurable operang parameer [-2], whch s ypcally used n he feedback loop of he algorhms. No surprsngly, as wll be llusraed n Secon 5, he choce of hs operang parameer wll sgnfcanly affec he performance of he rae adapaon algorhm. Unforunaely, o opmze hs parameer for he bes performance wll requre a pror knowledge of he nework bandwdh avalable over he enre duraon of he vdeo sesson. Ths s clearly no possble n pracce and hus poses sgnfcan dffcules o deployng hese rae adapaon algorhms. In hs sudy, we address hs ssue by presenng a new rae adapaon algorhm ha does no have confgurable parameer a all. In oher words, no pror knowledge of he avalable nework bandwdh s needed nor requred o run he rae adapaon algorhm. Our resuls show ha compared o he exsng algorhms, he presened algorhm can acheve comparable or even beer performance and does so whou he need o weak any operang parameers. 2. SYSTEM MODEL In hs work we consder a vdeo sreamng sysem ha sreams pre-encoded vdeo daa usng TCP as he nework ranspor o he recever for playback. Despe he common noon ha TCP s unsuable for vdeo sreamng

2 for s aggressve congeson conrol and full relably, does possess a number of appealng feaures. Frs, TCP s nrnscally TCP-frendly and hus farness wh oher TCP raffcs s auomacally guaraneed. Second, usng TCP he sender can sream vdeo usng say he sandard HTTP proocol o he clen. As mos, f no all, vdeo players n he marke suppors HTTP-based vdeo sreamng and playback, compably s grealy enhanced. Thrd, for secury reasons, many company and ISP blocks UDP raffc a her gaeways, hus makng UDP-based vdeo sreamng mpossble. By conras, TCP/HTTP sreamng can pass hrough frewalls n he same way as web raffc. Fnally, o perform bandwdh esmaon he sender wll need some form of feedbacks from he clen. Thus wh UDP ranspor he clen wll need o be modfed o send explc feedbacks o he sender o enable bandwdh esmaon and subsequenly rae adapaon o be performed. By conras, TCP wh s bul-n flow conrol already can provde mplc feedbacks o he sender and hus no modfcaon o he clen s necessary. Agan hs wll grealy enhance he compably of he rae-adapaon algorhm o he exsng vdeo player sofware. Neverheless, he rae-adapaon algorhm presened n hs sudy can also be appled o UDP-based vdeo sreamng wh approprae suppor from he clen s player sofware (e.g. sendng explc feedbacks). Fgure shows he key componens n he vdeo sreamng sysem. Assumng he vdeo daa are encoded a a consan b rae of r max bps. The rae conroller can conver he encoded vdeo o any b-rae beween r max and r mn (e.g., usng scalable vdeo codng [6] or ranscodng [7-9]). Noe ha here s a lower lm r mn on he achevable vdeo b-rae o model, for example, he b-rae of he base layer n FGS encoded vdeo [6] or he lowes achevable b rae n ranscodng [7-9]. In pracce, even wh a ranscoder he vdeo b-rae may no be changed a arbrary me due o he srucure of he codng algorhm (e.g. group of pcures, ec.). Thus n he sysem model we assume vdeo ranscodng s performed n dscree vdeo segmens of fxed playback duraon, denoed by M seconds. The rae conroller wll hen deermne he arge b-rae for he nex vdeo segmen based on esmaon of he clen s buffer occupancy. We denoe he average b rae for he k h vdeo segmen by r k. The ranscoded vdeo segmens are hen ransmed o he clen usng TCP. Noe ha he server does no lm he ransmsson rae here and smply sends he ranscoded vdeo daa as fas as TCP allows. Ths ensures ha avalable nework bandwdh s fully ulzed. A he recever, many exsng vdeo players wll prefech a ceran amoun of vdeo daa before sarng playback o absorb he nevable bandwdh flucuaons. Brae r max Encoded Vdeo Brae r max r mn Rae Conroller TCP Nework Fgure : Block dagram of he sysem model. Clen Buffer We denoe he playback duraon of he prefeched vdeo daa by B p seconds. Dependng on he specfc player sofware, B p can be a fxed value known o he server, or can be confgurable by he users. If s he laer case and he exsng player sofware does no repor hs value o he server, he server wll smply assume he wors case of no prefech,.e. B p = 0 sec, n performng rae adapaon. Our resuls show ha he performance dfference s nsgnfcan (c.f. Secon 5-A). 3. CLIENT BUFFER OCCUPANCY AND NETWORK BANDWIDTH ESTIMATION The objecve of he rae adapaon algorhm s o preven playback sarvaon caused by clen buffer underflow. To preven buffer underflow, he server wll need o esmae he avalable nework bandwdh as well as he clen buffer occupancy, n erms of second s worh of vdeo daa. Specfcally, we make wo assumpons on he recever and he server. Frs, we assume ha he clen wll no decode and playback a vdeo frame unl s compleely receved. Thus f a frame arrves lae mssng he playback schedule, hen he player wll pause playback unl he whole frame s receved. We call he perod of me when he playback s salled due o lae frame arrval underflow me. Second, we assume ha he oal sze of he buffer n beween he server applcaon and he nework (e.g., ncludng he buffer nsde he socke lbrary and TCP) s a known consan, denoed by Z. Esmaon of he clen buffer occupancy s hen performed every me he server complees submng a vdeo frame o he nework ranspor for delvery. For example, f he common socke lbrary s used hen hs s equvalen o compleng all send() funcon calls for he vdeo frame. Le be he compleon me of submng vdeo frame for ransmsson, and le f be he ndex of he oldes frame (.e. wh he smalles ndex number) ha has no ye been compleely receved by he clen a me. Now as he server wll subm daa for ransmsson as fas as he ranspor allows, we can assume ha he nermedae buffer a he server s always full,.e., here are Z byes of daa accumulaed awang for ransmsson. Thus we can esmae f from Decoder

3 f=max n s.. sk Z () k= n where s s he sze of frame. Smlarly, afer frame + s submed for ransmsson, we can compue f + usng (). Now f f + > f, hen we know ha frame f o frame f + mus have arrved a he clen durng he me from o +. Assumng n hs shor nerval he frames arrve a he clen a a consan rae. Then we can esmae he arrval me of frame k, denoed by T k, from k + f Tk = + ( + ) k [ f, f+ ] (2) f+ f Noe ha we gnored n (2) nework and processng delay n recevng ACKs from he clen. Our smulaons show ha hs does no have sgnfcan mpac on he algorhm s performance. Knowng he arrval me of each vdeo frame, we can hen proceed o esmae he clen buffer occupancy. Le B (n seconds of vdeo daa) be he clen buffer occupancy when frame arrves a he clen and G be he frame rae of he vdeo. Then we can esmae he clen buffer occupancy B accordng o he followng rules: Case - Bp G In hs case he frame belongs o he nal prefech par of he vdeo,.e., he player has no ye sared decodng he receved vdeo daa. Thus he buffer occupancy s equal o he duraon of vdeo daa receved: B = / G (3) Case 2 - > Bp G In hs case, he way o esmae B depends on wheher or no he frame has arrved before all he daa n he clen buffer s consumed as llusraed n Fgure 2. If (T +B ) T 0, ha means frame has arrved before he clen buffer becomes empy, hen B s esmaed as: B = ( B + T ) T + / G (4) Oherwse, f (T +B ) T 0, ha means he clen buffer has been empy for a perod of me before frame arrved, hen B s smply equal o he me value of a frame,.e.: = / G (5) B From he above dervaon, we can esmae B when frame has jus arrved a he clen. However, snce he vdeo b rae of a segmen has o be deermned when all he daa of he prevous segmen has been submed no he server buffer, some frames of he prevous segmen are sll n he server buffer. Therefore, o predc he clen buffer occupancy afer all he daa of he prevous segmen has arrved a he clen, we need o predc he arrval mes of he frames n he server buffer. T - T B = (B - + T - )- T + /G B - T B = /G Fgure 2: Two ways o esmae B when > B p x G. B Perod when buffer s empy Frame Le n be he ndex of he las frame of segmen, we have o predc B a me n whle frame n f o n frame n are sll n he server buffer and hen use he predced B o perform adapaon of segmen +. n Assumng he remanng daa n he server buffer a me wll arrve a he clen a a consan rae of D n +, whch s also he esmaed TCP hroughpu for sendng he segmen +, he arrval mes of he remanng frames are esmaed as follows: k Tk = n + ( ), Fj n k f n n D + ' (6) j= fn where F () s he remanng amoun of daa of frame a me. Wh Tk, k [ f, n n ], we can esmae B. n To esmae D +, we smply ake he rae a whch segmen was submed no he server buffer as he esmaed value,.e., n ' D+ = sk ( n ) m (7) k= m where m s he ndex of he frs frame of segmen. Ths s because he rae a whch daa are submed no he server buffer s equal o he rae a whch daa leave he server buffer. 4. RATE ADAPTATION Armed wh a mean o esmae he clen buffer occupancy and nework bandwdh, he nex challenge s o devse an adapaon algorhm o conrol he vdeo b-rae o preven clen buffer underflow. A. Segmen-based Rae Conrol As vdeo daa are ranscoded and ransmed n fxed-duraon segmens, he server mus deermne he arge b-rae before converng a vdeo segmen for ransmsson. The server deermnes he arge b-rae based on wo facors, namely he esmaed clen buffer occupancy and he esmaed nework bandwdh avalable whch could be esmaed usng echnques descrbed n Secon 3.

4 Suppose segmen has jus been submed o he server buffer, wh he esmaed D + and B, we can predc n he clen buffer occupancy afer ransmng he segmen + o he clen,.e., from: B n + Mr = + (8) + Bn B n M + D ' + where he las erm s he predced me aken o send he whole + h segmen o he clen. By rearrangng (8), we oban: Bn B n + r+ = D+ ' (9) M From (9), we can relae he vdeo b-rae r + wh he esmaed clen buffer occupancy (represened by B ). n + Our goal s o adjus he vdeo b-rae o manan he clen buffer occupancy o above a gven hreshold denoed by B T such ha shor-erm bandwdh varaons can be absorbed. In pracce, B T =B p when B p s known, oherwse s se o 5 seconds. Specfcally, f Bn < B + T hen mples he clen buffer occupancy s below he hreshold. Hence he server wll reduce he vdeo b-rae o rase he buffer occupancy o B T by subsung Bn = B n (9) o oban: + T BT Bn r+ = D+ ' (0) M Oherwse f Bn B T, hen mples he clen buffer occupancy s above he hreshold. In hs case he server wll smply manan he curren clen buffer occupancy by seng B = n + B n (9) o oban r n +. Ths s a conservave sraegy o reduce he possbly of buffer underflow. Thus, we have r ' + = D+ () Fnally, he server checks and lms he compued vdeo b rae o he feasble range [r mn, r max ] by r+ = mn { rmax, max { rmn, r+ } } (2) Noe ha n conras o prevous works [-3], hs adapaon algorhm has no conrol parameer ha requres eher offlne or onlne opmzaon. Ths has praccal sgnfcance as opmzng he conrol parameers n he exsng algorhms [-3] requres a pror knowledge of he avalable nework bandwdh over he enre duraon of he vdeo sesson, clearly mpossble n pracce. B. Preempve Rae Conrol In our expermens, we found ha he avalable nework bandwdh can occasonally drops drascally o a very low value. These sudden bandwdh drops do no appear o be predcable and hus can resul n clen vdeo playback sarvaon. The fundamenal problem s ha he adapaon algorhm s execued only when a new vdeo segmen s o be ransmed. Thus f bandwdh drops sgnfcanly, hen he ransmsson of he curren vdeo segmen wll sall. The adapaon algorhm canno reac n hs case as he curren vdeo segmen has no ye been compleely ransmed. Meanwhle he clen wll connue consumng vdeo daa for playback and hus may evenually runs no buffer underflow. To ackle hs problem, we propose a preempve schedulng echnque o shoren he me a whch he adapaon algorhm can reac o changng nework condons. Insead of wang for a vdeo segmen o be compleely submed no he server buffer, he scheduler wll meou afer Mr + /D + seconds, whch s he expeced me requred o subm he + h vdeo segmen no he server buffer, even f no all vdeo daa have been submed. In hs case, any daa no ye submed for ransmsson wll be dscarded and he remanng vdeo segmen ranscoded agan accordng o he new esmaes on clen buffer occupancy and avalable nework bandwdh. Noe ha preempve rae conrol requres he vdeo ranscoder o be able o adjus he vdeo b rae n beween a vdeo segmen. The mplemenaon wll be hghly dependen on he vdeo compresson employed and furher sudy s requred o denfy he consrans and radeoffs of hs requremen. 5. PERFORMANCE EVALUATION In hs secon, we use race-drven smulaon wren n ns-2 [0] o evaluae he performance of he proposed adapaon algorhm (denoed by AVS) and compare wh he curren sae-of-he-ar algorhm proposed by Cueos and Ross [-2] (denoed by CR). Fgure 3 depcs he smulaed nework opology. We use he common NewReno TCP [-2] as he ranspor proocol o delver he vdeo daa o he clen. Cross raffc s generaed from a packe race fle obaned from Bell Labs [3-4]. The race fle capured 07 hours of nework raffc passng hrough a frewall. We dvde he 07-hour race fle no 07 -hour race fles and run a smulaon for each -hour race fle o evaluae he algorhms performance under dfferen cross raffcs. Boh he sreamng raffc and he cross raffc share a lnk of R Mbps as shown n Fgure 3. For each smulaon, we adjus R such ha he nework has jus suffcen Nework races used n he smulaons belongs o NLANR projec sponsored by he Naonal Scence Foundaon and s ANIR dvson under Cooperave Agreemen No. ANI , and he Naonal Laboraory for Appled Nework Research.

5 bandwdh o sream he vdeo R = rmax + c, where c s he average daa rae of he cross raffc. We summarze he sysem sengs n Table. We use wo performance mercs, namely underflow rao and bandwdh ulzaon, o evaluae he algorhms performance. Underflow rao s defned as he rao of underflow duraon (.e. he duraon of me ha playback sarvaon occurs) o he vdeo lengh. Bandwdh ulzaon s defned as: N max{ P+ L, S} 0 Ulzaon s v() d = (3) = 0 where N s he oal number of frames, P s he nal prefech delay, L s he move lengh, S s he oal me aken o sream he vdeo and v() s he TCP hroughpu a me. The value of bandwdh ulzaon s n he range of [0,]. Ths merc measures how well an algorhm ulzes he avalable nework bandwdh. A. Sensvy o prefech duraon The proposed rae adapaon algorhm makes use of knowledge of he clen s nal prefech duraon n esmang he clen buffer occupancy. However f hs s no known hen smply assumes no prefech s performed. To nvesgae he performance mpac of such knowledge we run wo ses of smulaons for all 07 raffc races, one se wh he prefech duraon known o he server and he oher se smply assumng no prefech. In boh cases he clen has a prefech duraon of 5 seconds. Table 2 shows he underflow rao and bandwdh averaged over all 07 races for he wo cases. In boh cases he dfferences are nsgnfcan and hus mplyng ha he proposed rae adapaon algorhm s nsensve o he knowledge of he prefech duraon. Therefore n pracce we can smply assume no prefech f he prefech duraon s no known. B. Effecveness of preempve rae conrol To nvesgae how much performance gans can be obaned from preempve rae conrol, we run wo ses of smulaons for all 07 raffc races, one wh segmen-based rae conrol and he oher wh preempve rae conrol. In all 07 races, preempve rae conrol acheves lower underflow raos compared o segmen-based rae conrol. On average, he underflow rao s reduced by 20% when preempve rae conrol s used. Neverheless preempve rae conrol does requre more complex ranscoders and hus furher nvesgaon s needed o quanfy he gans and he radeoffs. Sreamng Server 00Mbps, 0ms 00Mbps, 0ms Cross Traffc Generaor R Mbps, 00ms 00Mbps, 0ms 00Mbps, 0ms Sreamng Clen Cross Traffc Recever Fgure 3: The nework opology n he smulaon. Table : Sysem sengs for smulaons. Parameer Symbol Value Prefech duraon B p 5 seconds Vdeo segmen lengh M seconds Orgnal vdeo b-rae r max. Mbps Lowes vdeo b-rae r mn 200 kbps Vdeo Lengh 3000 seconds TCP MSS 500 byes Table 2: Effec of knowledge of he prefech duraon. Prefech Duraon Known Unknown Dfference Bandwdh Ulzaon ~0.0% Underflow Rao ~.48% C. Comparson wh he CR algorhm In hs secon, we compare he proposed rae adapaon algorhm (he AVS algorhm) wh he curren sae-of-he-ar algorhm proposed by Cueos and Ross [-2] (he CR algorhm). In he CR algorhm, here s a conrol parameer α (0 α ) ha can subsanally affec he performance. To fnd he opmal value for α s necessary o know he nework bandwdh avalably over he enre duraon of he vdeo sesson. Ths s clearly no possble n pracce and he auhors dd no explan how o adjus he parameer n pracce. Thus o oban performance resuls for he CR algorhm we run 2,000 smulaons wh he conrol parameer α vared from 0 o wh a sep sze of We found ha he opmal value for α depends heavly on he parcular raffc race chosen, and can range from 0 o 0.7 over he 07 races. As he opmal α s no known a pror, we compare CR wh AVS by compung he proporon of he 2,000 smulaon runs ha resul n hgher underflow rao han he AVS algorhm, whch does no need any parameer unng. The resuls are summarzed n Fgure 4, whch also plos he bandwdh ulzaon rao, defned as (bandwdh ulzaon of AVS)/(average bandwdh ulzaon of CR over all α values).

6 .20 ACKNOWLEDGEMENTS Porporon/Rao Ths research s funded n par by an Earmarked Gran (CUHK4229/00E) from he HKSAR Research Gran Councl and n par by he Area of Excellence Scheme, esablshed under he Unversy Grans Councl of he Hong Kong Specal Admnsrave Regon, Chna (Projec No. AoE/E-0/99). REFERENCES Trace No. Proporon of alpha values CR gves larger underflow rao Bandwdh ulzaon rao Fgure 4: Comparson of underflow raos and bandwdh ulzaon of CR and AVS for dfferen races. The resuls n Fgure 4 show ha he proposed AVS algorhm ouperforms CR n more han half of he smulaon runs wh dfferen α values. Averagng over all 07 races, he proposed AVS algorhm can acheve lower underflow rao han he CR algorhm for 77% of he α values. Ths shows ha n pracce, he proposed AVS algorhm s lkely o perform beer and ye does no requre any a pror knowledge of he nework bandwdh avalable nor unng of any conrol parameer. Despe he reducon n he underflow rao, he proposed AVS algorhm can sll make effcen use of he nework bandwdh, and achevng bandwdh ulzaon smlar o ha of he CR algorhm. 6. CONCLUSIONS In hs sudy we presened a new rae adapaon algorhm for vdeo sreamng over he Inerne. The algorhm has wo unque feaures o maxmze s compably wh exsng vdeo player sofware. Frs, we show ha he rae adapaon algorhm can be appled o sreamng vdeo over TCP/HTTP, whch s compable wh mos of he exsng vdeo player sofware. Second, he rae adapaon algorhm performs nework bandwdh and clen buffer occupancy esmaons usng only local nformaon. Thus explc feedbacks from he clen s no needed and hence exsng vdeo player sofware can be suppored. More mporanly, unlke prevous approaches he proposed algorhm does no need any parameer unng o operae nor requres a pror knowledge of he nework bandwdh avalable o perform well, hus smplfyng he deploymen of he adapaon algorhm n pracce. Our resuls show ha he proposed algorhm can ouperform exsng algorhm and ye sll acheve effcen bandwdh ulzaon. [] P. de Cueos and K.W. Ross, Adapve Rae Conrol for Sreamng Sored Fne-Graned Scalable Vdeo, Proc. NOSSDAV, May 2002, pp.3-2. [2] P. de Cueos, P. Gulloel, K.W. Ross and D. Thoreau, Implemenaon of Adapve Sreamng Of Sored MPEG-4 FGS Vdeo Over TCP, Proc. IEEE Mulmeda and Expo, 2002, pp [3] S. Jacobs and A. Elefherads, Sreamng Vdeo usng Dynamc Rae Shapng and TCP Congeson Conrol, Journal of Vsual Communcaon and Image Represenaon, Vol. 9, No. 3, 998, pp [4] R. Rejae, M. Handley and D. Esrn, Archecural consderaons for playback of qualy adapve vdeo over he Inerne, Techncal Repor , USC-CS, Nov [5] P.A.A. Assuncao and M. Ghanbar, Congeson conrol of vdeo raffc wh ranscoders, Proc. IEEE In. Conf. Communcaons, Vol., June 997, pp [6] W. L, Overvew of Fne Granulary Scalably n MPEG-4 Vdeo Sandard, IEEE Trans. Crcus and Sysems for Vdeo Tech. Vol., No. 3, March 200, pp [7] P. A. A. Assunção and G. Mohammed, A Frequency-Doman Vdeo Transcoder for Dynamc B-Rae Reducon of MPEG-2 B Sreams, IEEE Trans. Crcus and Sysems for Vdeo Tech., Vol. 8, No. 8, Dec. 998, pp [8] B. K. Naarajan and B. Vasudev, A Fas Approxmae Algorhm for Scalng down Dgal Images n he DCT Doman, Proc. IEEE In. Conf. Image Processng, Vol. 2, Oc. 995, pp [9] H. Sun, W. Kwok and J.W. Zdepsk, Archecure for MPEG compressed bsream scalng, IEEE Trans. Crcus and Sysems for Vdeo Technology, Vol. 6, No. 2, Aprl 996, pp [0] The nework smulaor ns-2. [Onlne]. Avalable: hp:// [] S. Floyd and V. Paxson, Dffcules n Smulang he Inerne, IEEE/ACM Trans. Neworkng, Vol. 9, No. 4, Augus 200, pp [2] S. Floyd and T. Henderson, The NewReno Modfcaon o TCP s Fas Recovery Algorhm, RFC 2582, Aprl 999. [3] NLANR Measuremen and Nework Analyss Group. [Onlne]. Avalable: hp://pma.nlanr.ne/traces/long/bell.hml [4] Bell Labs Inerne Traffc Research. [Onlne]. Avalable: hp://cm.bell-labs.com/cm/ms/deparmens/sa/inernetra ffc/