Performance Analysis ofCHOKewithMultiple UDPFlows

Several active queuemanagement schemes havebeenproposed toprovide thefairness amongflows. In particular, CHOKeisstateless andsimple toimplement, butitcaneffectively penalize UDPflows whichusually obtain morebandwidth thanTCPflows. However, its performance hasnotbeenanalyzed interms oftheUDPthroughput and theJain's fairness index intheextended caseofmultiple UDPflows. Inthis paper, wederive theUDPthroughput and Jain's fairness index under CHOKe.Thesimulation results illustrate theaccuracy ofouranalysis andverify ourderived result. SinceTCPwithcongestion control mechanism pro- vides a reliable transmission service, themajority of thetraffic overtheInternet hasbeenusually carried by TCP.However, real-time multimedia services like Inter- netphoneandvideo conference have grown rapidly dur- ingthepastseveral years.Suchapplications aremostly based on UDP whichdoesnotemploy themechanism to adapt itstransmission rateaccording tothenetwork sta- tus.Thus,since TCPcontrols thetransmission rateac- cording toavailable bandwidth, theUDP flows compet- ingwithTCPflows can consume amuchlarger portion of thebandwidth unfairly (1). Inorder toovercome this un- fairness problem, several schemes, whicharedeployed ata router, havebeenproposed. Theyaimatavoiding congestion, penalizing aggressive flows, andguarantee- ingthefairness among flows (2). Thescheme, CHOKe (3), isparticulary interesting among them.Itdoesnotrequire any per-flow statein- formation, butitcan penalize theaggressive flows such asUDP flows effectively andimprove thefairness. The basic ideaofitisasfollows (3): Whenapacket arrives atacongested router, CHOKe draws apacket atrandom fromtheFIFObuffer andcompares itwiththearriving packet. Iftheybothbelong tothesame flow, thenthey arebothdropped; else therandomly chosen packet isleft intact andthearriving packet isadmitted into thebuffer witha probability thatdepends on thelevel ofconges- tion(this probability iscomputed exactly as inRED). Inthis simple case,thethroughput characteristic ofone UDP flowunderCHOKe isanalyzed in(4). However, in general, CHOKe chooses multiple packets, called mul- tiple dropcandidates, fromthebuffer, andcomparesall ofthemwithan incoming packet anddrops thepackets thathavethesame flowIDastheincoming packet. It isknownthat CHOKewithmultiple dropcandidates has better control overthemultiple UDP flows(5). However, itisnotstill knownhowthenumberofdropcandidates Thisworkwas supported byHWRS Engineering Research Center (sponsored byKOSEF) influences ontheUDPthroughput andthefairness. Al- though (5)adjusts thesuitable number ofdropcandidates according tothecurrent traffic status, itisheuristic and doesnotguarantee maximizing thefairness amongflows. Thepurpose ofthis paperistoanalyze theUDP throughput characteristic underCHOKe withmultiple UDP flows andtoinvestigate theeffect ofthenumber ofdropcandidates onthefairness. Inorder toestimate thefairness, weusetheJain's fairness index whichis widely used. TheJain's fairness index Xisdefined asfol- tN+K Sa) 2