Congestion Control for Differentiated-Services using Non-linear Control Theory

Transcription

1 Congestion Contol fo Diffeentiated-Sevices using Non-linea Contol Theoy Andeas Pitsillides 1, Petos Ioannou 2, Loukas Rossides 1 Deatment of Comute Science, Univesity of Cyus, andeas.itsillides@ucy.ac.cy, 2 Deatment of Electical Engineeing, Univesity of Southen Califonia. Abstact The gowing demand of comute usage equies efficient ways of managing netwok taffic in ode to avoid o at least limit the level of congestion in cases whee inceases in bandwidth ae not desiable o ossible. Using non-linea contol theoy we develoed and analysed a geneic Integated Dynamic Congestion Contol (IDCC) scheme fo contolling taffic using infomation on the status of each queue in the netwok. The IDCC scheme is based on a nonlinea model of the netwok that is geneated using fluid flow consideations. The methodology used is geneal and indeendent of technology, as fo examle TCP/IP o ATM. We assume a diffeentiated-sevices netwok famewok and fomulate ou contol stategy in the same siit as IP Diff-Sev fo thee tyes of sevices: Pemium Sevice, Odinay Sevice, and Best Effot Sevice. The thee diffeentiated classes of taffic oeate at each outut ot of a oute/switch. An IDCC scheme is designed fo each outut ot, and a oweful, simle to imlement contolle is designed and analysed. The IDCC methodology has been alied to an ATM netwok. We use OPNET simulations to demonstate that the oosed contol methodology achieves the desied behaviou of the netwok, and ossesses imotant attibutes, such as: stable and obust behaviou, high utilisation with bounded delay and loss efomance, and good steady state and tansient behaviou. Index Tems Congestion contol, Non-linea Contol theoy, Diffeentiated-Sevices, DiffSev, Intenet, ATM. 1 Intoduction It is geneally acceted that the oblem of netwok congestion contol emains a citical issue and a high ioity, esecially given the gowing size, demand, and seed (bandwidth) of the inceasingly integated sevices netwok. One could ague that netwok congestion is a oblem unlikely to disaea in the nea futue. Futhemoe congestion may become unmanageable unless effective, obust, and efficient methods fo congestion contol ae develoed. The existing congestion contol solutions deloyed in the Intenet Tansot Contol Potocol (TCP) fo TCP tansoted taffic [1,2] ae inceasingly becoming ineffective, and it is geneally acceted that these solutions cannot easily scale u even with vaious oosed fixes [3,4,5], new aoaches [6.7] and achitectues [8, 9]. Newly develoed (lagely adhock) stategies [10,11,12,13] ae also not oven to be obust and effective. Also, ecently thee is inceasing inteest in non-tcp alications. As demand fo multimedia (steaming) alications inceases, it becomes inceasing imotant to ensue that these alications can co-exist with cuent TCP alications. It is becoming widely acceted that steaming media should be subjected to simila ate contols, as TCP taffic, and ecently a numbe of eseaches advocate that they should also exhibit TCP-fiendly behaviou [14,7,15,16]. This makes the contol of the congestion in the Intenet consideably moe difficult. Futhemoe, the congestion contol oblem in the Intenet is exacebated, as the Intenet is inceasingly tansfomed into a multi-sevices high-seed netwok, see fo examle Diffsev oosed achitectues [8,9]. Asynchonous Tansfe Mode (ATM) also witnessed a simila aoach, with the efomance of vaious congestion contol schemes oosed (e.g. [17,18]) fo solution of Available Bit Rate (ABR) oblem [19] not oven analytically. Most oosed schemes ae develoed using intuition and simle non-linea contol designs. These have been demonstated to be obust in a vaiety of scenaios that have been simulated (see e.g. [18]). Since these schemes ae designed with significant non-lineaities (e.g. twohase slow stat and congestion avoidance dynamic windows, binay feedback, additive-incease multilicative-decease flow contol etc), the analysis of the closed loo behaviou is difficult if at all ossible, even fo single contol loo netwoks. The inteaction of additional non-linea feedback loos can oduce unexected and eatic behaviou [20]. Emiical and analytical evidence demonstates the oo efomance and cyclic behaviou of the contolled TCP/IP Intenet [21,22]. This is exacebated as link seed inceases to satisfy demand (hence bandwidth-delay oduct, and thus feedback delay, inceases), and also as demand fo bette quality of sevice inceases. Clealy, moe effective congestion contol schemes ae needed to event seious economic losses and ossible "meltdown" of the Intenet.

2 Desite the successful alication of contol theoy to othe comlex systems the develoment of netwok congestion contol based on contol theoetic concets is quite unexloed. Most of the cuent congestion contol methods ae based on intuition and ad hoc contol techniques togethe with extensive simulations to demonstate thei efomance. The oblem with this aoach is that vey little is known why these methods wok and vey little exlanation can be given when they fail. Recently seveal attemts have been made to develo congestion contolles [20,23,24,25,26,27], mostly using linea contol theoy. Desite these effots the design of congestion contolles whose efomance can be analytically established and demonstated in actice is still a challenging unesolved oblem. Recent advances in non-linea adative contol theoy [28] offe otential fo develoing effective congestion netwok contolles whose oeties can be analytically established. This ae is oganised as follows. In section 2 we oveview ou oosed solution aoach, esent the contol oblem and objective, illustated by the fomal deivation of the integated dynamic congestion contolle (IDCC), deived using non-linea contol theoy based on a fluid flow model. In section 3 we evaluate the efomance of IDCC. Finally in section 4 we esent ou conclusions. 2 Netwok congestion and oosed solution Following the same siit adoted by the IETF Diff- Sev woking gou fo the Intenet [9] we define classes of aggegated behaviou. Cuently, the DiffSev woking gou is discussing two boad aggegate behaviou gous: the Exedited Fowading (EF) Pe-Ho Behaviou (PHB) [29] and the Assued Fowading (AF) PHB [30]. The EF- PHB can be used to build a low loss, low latency, low jitte, assued bandwidth end-to-end sevice, thus indiectly oviding some minimum quality of sevice. The AF-PHB gou ovides delivey of IP ackets in fou indeendently fowaded AF classes. Within each AF class, an IP acket can be assigned thee diffeent levels of do obabilities. Each class can be ovided with some minimum bandwidth and buffe guaantees. In this ae we define thee Sevices: Pemium Taffic Sevice, Odinay Taffic Sevice, and Best Effot Taffic Sevice. An analogy may ove useful hee to justify this choice. At many lage social events thee ae diffeent tyes of asses. Fo Vey Imotant Peole thee ae VIPs asses. VIP asses get efeential teatment. This categoy is likened to ou Pemie Taffic Sevice. Fo odinay eole thee ae common asses. To uchase these tickets eole may have to queue to get the best ossible seats, and thee is no efeential teatment, unless diffeent ices ae intoduced fo bette seats. This categoy may be likened to ou Odinay Taffic Sevice. Fo easons of economy, anothe ass may be offeed, at a discount ice fo the ootunists at the doo. The last (ootunistic, best effot) class will take any available seat, but also at the isk of not attending the event at all, due to a sold out event. This categoy is likened to ou Best Effot Taffic Sevice. The Pemium Taffic Sevice may belong to the EF-PHB in a Diff-Sev achitectue and is designed fo alications with stingent delay and loss equiements on e acket basis that can secify ue bounds on thei taffic needs and equied quality of sevice. Tyical alications include video on demand, audio, video confeencing, etc. The Odinay Taffic Sevice may belong to the fist class of the AF-PHB in a Diff-Sev achitectue. Note that diffeent ioities may be assigned, without comlicating geatly the design. The Odinay Taffic Sevice is intended fo alications that have elaxed delay equiements and allow thei ate into the netwok to be contolled. These Sevices use any left ove caacity fom the Pemium Taffic. Note that to ensue that bandwidth is leftove fom the Pemium Taffic Sevice a minimum bandwidth may be assigned, e.g. by using bandwidth allocation between sevices o connection admission. Tyical alications include web bowsing, image etieval, , ft, etc. Note that fo TCP tansot in a TCP/IP netwok a window-based congestion contolle is used [1]. To make this algoithm alicable in such a setting a convesion fom ate to window will be equied [31].Thisissueisnottackledinthisae;itisleftfo futue investigation. Note howeve, that fo eal time steaming taffic the cuently advocated aoach is to use ate based contol [15, 16]. Finally, the Best Effot Taffic Sevice may belong to the last class of the EF-PHB in a Diff-Sev achitectue. It has no delay o loss exectations. It ootunistically uses any instantaneous leftove caacity fom both Pemium and Odinay Taffic Sevices. It should be noted that the methodology used is geneal and indeendent of technology. The oosed IDCC algoithm can be classified as Netwok-assisted Congestion Contol [32] and uses queue length infomation fo feedback. IDCC oeates locally fo the Pemium Taffic Sevice (note concet similaity with RED [6]) and fo the Odinay Taffic Sevice it sends feedback to the souces to egulate thei ate. Note some ecent woks fo DiffSev aly feedback contol [33,34]. 2.1 Poosed integated dynamic congestion contol aoach using non-linea contol theoy We assume a geneic K inut, K outut ot, outut buffeed switch as a efeence model. Each outut ot has numbe of hysical o logical queues: one fo each taffic class. A congestion contolle is installed at each outut ot. At each outut ot of the switch we assume that dedicated buffe sace is allocated fo each one of the thee sevices and that the seve can be shaed between the thee in a contolled fashion (see Figue 1). Each sevice tansmits ackets to destination teminals. The ackets

3 fom seveal Oigin-Destination (OD) ais tavese a numbe of switches on oute to the destination. Each OD flow may be classified as Pemium Sevice, Odinay Sevice, o Best Effot Sevice. Pemium Sevice equies stict guaantees of delivey, within given delay and loss bounds. It does not allow egulation of its ate (o at least egulation that will affect the given delay bounds). Any egulation of this tye of taffic has to be achieved at the connection hase. Once admitted into the netwok the netwok has to offe sevice in accodance with the given guaantees. This is the task of the Pemium Taffic Contolle. Odinay Taffic on the othe hand allows the netwok to egulate its flow (ace it) into the netwok. It cannot toleate loss of ackets. It can howeve toleate queueing delays. This is the task of the Odinay Taffic Contolle. Best Effot Sevice on the othe hand offes no guaantees on eithe loss o delay. It makes use of any instantaneous leftove caacity. λ Allowed common ate sent to the Odinay Taffic souces Incoming taffic Pemium Taffic λ Odinay in Taffic λ Best effot taffic λ b() t Integated Dynamic Congestion Contolle (IDCC) x () t () t ( ) C ( ) t Instantaneous left-ove caacity Fixed sevice ate C seve (e.g. 155 Mb/s) Schedule with seve buffe ef x ef efeences Figue 1. Imlementation of contol stategy at each switch outut ot 2.2 Dynamic Netwok Models Most of the cuent congestion contol techniques use intuition and ad hoc contol techniques togethe with extensive simulations to demonstate thei efomance. The oblem with this aoach is that vey little is known why these methods wok and vey little exlanation can be given when they fail. The use of dynamic models could ovide a bette undestanding of how the netwok oeates and can be used to develo contol techniques whose oeties can be established analytically even when such techniques ae based on intuition and ad hoc guesses. Fo contol design uoses the model does not need to be accuate. It is because of the inability of modelling the eal wold accuately that feedback was invented and contol theoy is widely used. A good feedback contol design should be able to deal with consideable uncetainties and inaccuacies that ae not accounted fo in the model. A dynamic model is sought, in a fom suitable fo a distibuted contol solution. The objective is to find a model which catues the essential dynamic behaviou, but has low ode comlexity elative to detailed obabilistic models such as the Chaman-Kolmogoov equations fo detemining the time-deendent state obability distibution fo a Makovian queue [35]. Using the aoximate fluid flow modelling aoach oosed by Agnew [36], vaious dynamic models have been used by a numbe of eseaches [35,37,38,39] to model a wide ange of queueing and contention systems, e.g. see [35,38,37,39]. Note that seveal vaiants of the fluid flow model have been extensively used fo netwok efomance evaluation and contol, see fo examle an ealy efeence that stimulated a lot of inteest theeafte [40], and a ecent efeence of the esent inteest [41]. Fo an M/M1 queue it can be shown [42] that xt () xt &( ) = Ct ( ) +λ( t), x(0) = xo (1) 1 + xt ( ) whee x(t) is the state of the queue, given by the ensemble aveage of the numbe of cells N(t) in the system (i.e. queue + seve) at time t; λ(t) is the ate ackets aive at the queue, and C(t) is the caacity of queue seve. Note that this equation is valid fo 0 xt ( ) xbuffe size and 0 Ct ( ) Cseve whee xbuffe size is the maximum ossible queue size and Cseve the maximum ossible seve ate. The validity of this model has been veified by a numbe of eseaches, including [37,38,42]. Note that simila fluid flow models in both the discete and continuous time fom have been used by a numbe of eseaches fo designing o analysing the behaviou of netwok systems unde contol [20,23,22,43,44]. 2.3 Poosed integated congestion contol stategy: Design and analysis At each outut ot of a switch, we imlement IDCC, the integated congestion contol stategy (see Figue 1). IDCC is an integated stategy develoed fo Pemium Taffic, Odinay Taffic and Best Effot Taffic. It is basedonthefluidflowmodel(1)usedtomodeltheinutoutut chaacteistics of the switch (see Figue 1). Pemium Taffic contol stategy: Fo Pemium Taffic Sevice, ou aoach is to tightly contol the length of the Pemium Taffic queue to be always close to a efeence value, chosen by the netwok oeato, so as to indiectly guaantee accetable bounds fo the maximum delay and loss. The caacity fo the Pemium Taffic is dynamically allocated, u to the hysical seve limit, o a given maximum. In this way, the Pemium Taffic is always given esouces, u to the allocated maximum (C max : maximum available o assigned caacity, and x max : maximum buffe size) to ensue the ovision of Pemium Taffic Sevice with known bounds. Wheneve this sevice does not equie the use of maximum caacity to maintain its QoS at the escibed levels it offes the excess caacity to the Odinay Taffic Sevice. The contol objective is to choose the caacity

4 C (t) to be allocated to the Pemium Taffic unde the constaint that the incoming taffic ate λ( t) is unknown but bounded by k ) so that the aveaged buffe size x( t) ef is as close to the desied value x (chosen by the oeato o designe) as ossible. In mathematical tems we need to ef choose C (t) sothat x() t 0 ( x = x() t x )unde ) the constaints that C Cseve and λ() t k < Cseve. Based on the fluid flow equation (1), feedback lineaization, and obust adative contol ideas: 1 + x( t) C = max 0, min Cseve, ρ α x + k() t x() t (2) whee: 0 if x ( t) 0.01 ρ ( t) = 1.01 x( t) 0.01 if 0.01 < x( t) 1 1 if x ( t) > 1 and k & = P δ x() t, 0 k (0) k ) (3) whee P[ ] is a ojection oeato defined as: ) if (0 k ( t) k ) ) δ x ( t) o ( k ( t) = k and x ( t) 0) P δ x( t) = (4) o ( k ( t) = 0 and x ( t) 0) 0 else whee k ) is a constant indicating the maximum ate that could be allocated to incoming Pemium Taffic (e.g. though a connection admission olicy), and α > 0,and δ > 0, ae design constants that affect the convegence ate and efomance. Due to sace limitations, the deivation, and analysis ofthe stabilityofthe above contol stategy will not be esented hee. Details aea in [45]. Odinay Taffic contol stategy: The Odinay Taffic Sevice Contolle egulates the flow of Odinay Taffic into the netwok, by monitoing the length of the Odinay Taffic queue and the available caacity. The length of the Odinay Taffic queue is comaed with the efeence value and using a non-linea contol stategy it infoms the souces of the maximum allowed ate they can tansmit ove the next contol inteval. This algoithm takes into account the leftove caacity C = max 0, Cseve C, uses the eo between the queue length x of the Odinay Taffic ef queue and the efeence queue length x as the ef feedback infomation ( x = x x ), and calculates the common ate λ to be allocated to the Odinay Taffic uses once evey contol inteval T s ms, based on the contol algoithm esented below. Once the common ate is calculated it is sent to all usteam souces. The imlementation details of the feedback scheme ae left fo futhe study. In this ae we evaluate using simulation a scheme wheeby exlicit feedback is ovided by udating secial fields in ackets (RM cells in an ATM setting). The souce does not allow its tansmission ate ove the next contol inteval T s ms to exceed the allowed calculated common ate eceived. Note that any excess souce demand (above calculated common ate) is queued at the souce queues, athe than be allowed to ente the netwok, and thus congest the netwok. Based on the fluid flow equation (1) and feedback lineaization the contolled taffic inut ate is x() t λ = max 0, min C(), t C αx() t (5) 1 + x ( t) whee α > 0 is a design constant. Due to sace limitations, the deivation, and analysis of the above contol stategy will not be esented hee. Best Effot taffic contol stategy: The Best Effot taffic contolle oeates on an instantaneous (acket o cell) time scale. It utilizes any instantaneous left ove caacity to tansmit a acket fom the Best Effot buffe. This inceases the netwok utilisation duing eiods of insufficient suly of ackets fom both the Pemium and Odinay Taffic Sevices. Details aea in [45]. 3 Pefomance evaluation In this section we use simulations to evaluate the efomance of IDCC. Analytical oofs aea in [45]. We fistly esent the imlementation details of the contol algoithm, and then the simulation scenaios and simulation esults. 3.1 Imlementation of integated contol stategy At each switch outut ot (Figue 1) we imlement the integated contol stategy deived in the evious section. ef ef The efeences x, x, and the design constants α, α, δ, and k % ae fist selected. At each instant n (n=0,1,2, ) which coesonds to time t=nt s,wheet s is the samling eiod, we calculate C ( n ), C ( n ), and λ ( n) (comuted in discete fom). Note that C ( n) is used at the switch outut ot by the schedule to dynamically allocate caacity to the Pemium Taffic queue (see Figue 1). The allocated caacity is held constant ove the eiod of the contol inteval T s ms. The calculated common ate λ ( n) is sent to each of the

5 Odinay Taffic souces using this outut ot of the switch to each the destination. 3.2 Simulations Fo the evaluation of the efomance using simulation, we use a netwok comising a numbe of ATM switches. As discussed ealie, if the ATM switches ae elaced by outes simila efomance is exected, if the calculated common ate is sent to the souces. Ou ATM netwok model is shown in Figue 2. We assume all of the queueing occus at the outut buffes of the switches and thee is no intenal blocking in the switch. In each ATM switch thee ae thee seaate logical buffes (e outut ot) collecting Pemium Taffic (128 cells), Odinay Taffic (1024 cells) and Best Effot Taffic (infinite buffe sace). The queues ae seviced in accodance with the stategy, outlined in section 2. Figue 2. Simulation Netwok Model We use same netwok model fo LAN and WAN, but the distances between switches ae changed to eflect the diffeent geogahic sans of the two netwok tyes. In WAN, delay between each switch, due to link distance is set at 20 msec fo each link and 20 msec between the last switch and the destination station (thus a ound ti delay of 120 msec is esent). All of the links ae assumed to tansmit at 155 Mbits/sec. Fo the Odinay Taffic we conside 40 connections at the edge of the netwok (20 ae connected diectly to ATM switch 0, and 10 in each of ATM switches 1 and 2), which can have thei tansmission ate contolled by the netwok. Thee of the Odinay Taffic flow aths ae 1-ho aths, and one is a 3-ho ath. Also fou VBR and two CBR souces ae diectly connected to ATM switch 2 (1 ho-ath). Each Odinay Taffic teminal geneates taffic based on a 3-state model; see Table 1 fo the selected aametes. We have consideed the Odinay Taffic souce teminal buffes as infinite. Each VBR souce is simulated by using the autoegessive model oosed by Maglais et al [46]. The CBR souce geneates 25 Mbits/sec and aces the cells into the netwok unifomly. In case of cell losses, which occu duing the eiods of congestion, we use a simle etansmission otocol. A acket is esumed to be lost even if a single cell is lost. Packets that ae eceived by the eceive teminal with missing cells ae etansmitted by the souce until successful delivey. The "useful netwok thoughut" eesents the actual thoughut of ackets (in Mbits/sec) that ae eventually deliveed to the destination without cell loss (afte etansmission if necessay). PARAMETER DISTRIBUTION MEAN VALUE Idle eiod Geometic Chosen to adjust netwok load Geneated ackets Geometic 20 in an active eiod Packet size Geometic 8 Kbytes Pause eiod Exonential sec Table 1. Odinay Taffic souce model aametes We have used Onet simulation tools fo ou exeiments. Using simle undestanding of the ATM, we set the contolle design constants as follows: Fo the ef Pemium Taffic contolle x =50 o 100 fo a buffe size of 128, α =2000, k ) =210000, k (0) =2000, and min ef k =1000. Fo the Odinay Taffic contolle x =900 o 300 o 600 fo a buffe of 1024, and α =2000. The contol udate eiod T s msec was set at seveal values (32 celltimes msec, 75 celltimes, 175 celltimes, and 353 celltimes 1 msec) in ode to investigate the sensitivity of contol to the value of the contol udate eiod. 3.3 Simulation esults Steady state and tansient behavio Using the simulation model we evaluate the efomance of ATM LAN and ATM WAN. As noted eviously Pemium Taffic (CBR/VBR souces) has a ioity. We can guaantee them maximum delays not exceeding in an aveage sense the efeence value on the buffe length set by the netwok administato. In ode to test the esonsiveness of ou contolle (tansient behaviou) we set a vaiable efeence oint fo this sevice. At the beginning we set the efeence oint to 100 cells. Afte t=0.4 sec it is set to 50 cells and afte t=0.8 sec it is again aised to 100 cells (whee t stands fo time in seconds). In this way not only we show that ou contolle can match the efeence values but that it can also coe with dynamic changes that occu in the netwok (e.g. anothe connection is set-u, moe bandwidth is equied fo eal-time sevices etc). To simulate a moe ealistic scenaio, we also change the efeence on the Odinay Taffic. Since we can accet highe delays the efeence values ae set at 900 cells fo t<0.5 sec. Afte that time it is set to 300 cells until t=1 sec and afte t=1 sec it is aised to 600 cells. It can be noticed that the efeence oint changes between the Pemium and Odinay Taffic ae not synchonised between them. Note that the Pemium and Odinay Taffic souces geneate taffic accoding to a

6 ealistic scenaio (they ae not satuated souces). The contolled system efomance fo the case of an ATM unde heavy load (140%) is demonstated hee, fo both a LAN and a WAN. Figue 3 shows the behaviou of the Pemium Taffic fo both a LAN and WAN. As exected the contolled system behaviou is the same, as the feedback is local fo both WAN and LAN netwoks. In Figue 4 the behaviou of the Odinay Taffic queue length is shown fo the WAN configuation with vaying contol eiods (as exected, the LAN exhibited a slightly bette behaviou and is not shown hee). The most heavily congested switch (Switch 2) is selected, whee Odinay Taffic cometes with Pemium Taffic fo the scace netwok esouces. The figue shows that the contolle adats vey quickly to efeence oint changes (could be likened to abut changing netwok conditions), as well as showing a easonable insensitivity to contol eiods anging fom msec to 1 msec (a moe than 10 tenfold incease). It is vey imotant to notice that thee ae no obsevable oveshoots and undeshoots (excet fo the undeshoot at 0.5 seconds fo the longest contolle eiod of 1 msec), no oscillations o cyclic behaviou, and that the contolled system esonds vey quickly to the changes intoduced in both queues. In othe wods, the system exhibits a good tansient behaviou. So we can say that we can dynamically contol the buffe state and the souces sending ate, which in tun imlies that the netwok is well contolled and congestion is avoided, o quickly contolled. Note that the case of the WAN exhibits comaable efomance with the LAN, even though the oagation delay (and theefoe the fowad and feedback delay have substantially inceased in the ode of 120 msec RTT, ound ti time). Also, the obseved deteioation due to the ten-fold incease in contol eiod is vey accetable. Also note that a sizeable offset fom the efeence value is esent, howeve this can be ectified by intoducing integating action into the contolle design. Figue 4. Switch 2 (last switch) time evolution of the Odinay Taffic queue length fo WAN fo 140% load demand. (The contol eiod vaies between 32 celltimes msec to 353 celltimes celltimes 0.94 msec) We have also monitoed the queue length behaviou fo Switch 0 and Switch 1. We obseve that both queues ae well contolled with no oveshoots o undeshoots exceeding 2%. The thoughut fo the bottlenecked switch was also monitoed, exhibiting a constant and vey close to 100% utilisation; ~98 % fo tyical simulation uns. This is vey imotant since the contolle not only avoids congestion but also fully utilises the available esouces, even fo demands consideably exceeding the available link caacity (140% in this case). The time evolution of the calculated common allowed cell ate fo the congested switch is shown in Figue 5 fo WAN, fo 140% load demand. Note that this common allowed cell ate is sent to all souces using this switch. This ate is used by the souces as the maximum ate that they ae allowed to tansmit ove the next contol udate eiod. The time evolution of the tansmission ate of a numbe of contolled souces is shown in Figue 6. In the figue 3-ho and 1 ho c souces ae shown fo WAN netwoks. Note that the souces ae not satuated. The souce ates quickly adat in a fai way to thei steady-state values, even though the 3-ho souces, in the case of the WAN, ae located about km away (equivalently 60 msec delay fo cells to aive at the switch). Figue 3. Switch 2 (last switch) time evolution of Pemium Taffic queue length fo a LAN and WAN fo 140% load demand. Note that as the feedback infomation is local, thee is no deteioation in efomance due to the inceased WAN oagation delay. Figue 5. Tyical behaviou of the time evolution of the common calculated allowed cell ate at Switch 2 fo WAN.

7 Figue 6. Tyical behavio of the time evolution of the tansmission ate of contolled souces using Switch 2 fo WAN configuations. Fainess is anothe imotant attibute of any congestion contol system. The fainess shown by ISCC can be infeed fom Figue 6 fo a numbe of tyical on-off souces fo the Odinay Taffic. Additional scenaios ae esented in [45]. Also the oosed contolle has demonstated insensitivity of contol to the value of the contol udate eiod (T s msec was set at seveal values fom 32 celltimes msec to 353 celltimes 1 msec). As shown in Figue 4, the contolled netwok efomance does not degade consideably, consideing the 10-fold incease in the value of the contol udate eiod (and thus 10-fold eduction in contol infomation ovehead). Futhemoe obustness of contol design constant to changing netwok conditions was obseved fo divese taffic demands anging fom 50%-140% and souce location (feedback delays) u to about 250 msec RTT, as well as contol eiods anging fom msec to 1 msec. Fo all simulations the behaviou of the netwok emains vey well contolled, without any unaccetable degadation. This demonstates the obustness of the oosed congestion contolle. Given also that thee was no change in the selected design constants the oosed scheme has demonstated its univesality and suitability to oeate effectively and efficiently unde divese netwok conditions in both LAN and WAN configuations. 4 Conclusions This ae ooses a geneic scheme fo congestion contol. It uses an integated dynamic congestion contol aoach (IDCC). A secific oblem fomulation fo handling multile diffeentiated classes of taffic, oeating at each outut ot of a switch is illustated. IDCC is deived fom non-linea contol theoy using a fluid flow model. The fluid flow model deicts the dynamical system behaviou, using acket flow consevation consideations and by matching the queue behaviou at equilibium. In this way analytical efomance bounds can be deived, fo ovable contolled netwok behaviou. We divide taffic into thee basic tyes of sevice (in the same siit as those adoted fo the Intenet by the IETF Diff-Sev woking gou, i.e. Pemium, Odinay, and Best Effot). The oosed contol algoithm ossesses a numbe of imotant attibutes such as ovable stable and obust behaviou, ovable high utilisation with bounded delay and loss efomance (can be set by efeence values), good steady state and tansient behaviou. It uses minimal infomation to contol the system and avoids additional measuements and noisy estimates. That is, it uses only one imay measue, namely the queue length, it does not equie e connection state infomation, queuing, o sevicing at the switch, o any state infomation about the set of connections bottlenecked elsewhee in the netwok (not even a count of these connections). The contolle fo Odinay Taffic comutes the common allowable tansmission ate only once evey T s msec (the contol udate eiod) theeby educing ocessing ovehead. The contolle is insensitive to the value of T s, and achieves max/min fainess. It guaantees a minimum ageeable sevice ate, and it is obust in the sense that it woks ove a wide ange of netwok conditions, such as ound ti delays, taffic attens, and contol udate intevals, without any change in the contol aametes. Futhemoe, the contolle woks in an integated way with diffeent sevices and has simle imlementation and low comutational ovehead, as well as featuing a vey small set of design constants that can be easily set (tuned) fom simle undestanding of the system behaviou. These attibutes make the oosed contol algoithm aealing fo imlementation in eal, lage-scale heteogeneous netwoks. In this ae full exlicit feedback was used in the simulations, signalled using RM cells in an ATM setting. A challenging task is to investigate othe exlicit (e.g. single bit feedback as in ECN oosal fo IP [10]) and imlicit (end-to-end) feedback and signalling schemes. A comaative analytic and simulative evaluation between the diffeent feedback and signalling schemes is a toic fo futue eseach. 5 Refeences: [1] V. Jacobson, Congestion Avoidance and Contol, ACM SIGCOMM88, [2] W. Stevens, "TCP Slow Stat, Congestion Avoidance, Fast Retansmit, and Fast Recovey Algoithms", RFC 2001, Jan [3] P. Kan, C. Patidge, ``Imoving Round-Ti Time Estimates in Reliable Tansot Potocol'', ACM SIGCOMM87, Octobe, 1987.

8 [4] W. Stevens, TCP/IP Illustated, Volume 1 The Potocols', Addison-Wesley, [5] V. Jacobson, R. Baden, D. Boman, TCP Extensions fo High Pefomance, RFC 1323, May [6] S. Floyd, V. Jacobson, Random Ealy Detection gateways fo congestion avoidance, IEEE/ACM Tansactions on Netwoking, August [7] S. Floyd, K. Fall, Pomoting the use of end-to-end congestion contol in the Intenet, IEEE/ACM Tansactions on Netwoking, vol. 7, no. 4, August 1999, [8] R. Baden, D. Clak, S.Shenke, Integated sevices in the intenet achitectue: an oveview, RFC 1633, July [9]D.Black,S.Blake,M.Calson,E.Davies,Z.Wang,andW. Weiss, "An Achitectue fo Diffeentiated Sevices", RFC 2475, Decembe (also see IETF Diffeentiated Sevices Woking Gou htt:// [10] K.K. Ramakishnan, and S. Floyd, A oosal to add exlicit congestion notification (ECN) to IP, daft-kksjf-ecn-03.txt, Octobe (RFC2481, Januay 1999). [11] Baden et al, Recommendations on Queue Management and Congestion Avoidance in the Intenet, RFC2309, Ail [12] K.K. 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