Network management and QoS provisioning - QoS in Frame Relay. . packet switching with virtual circuit service (virtual circuits are bidirectional);

Transcription

1 QoS in Frame Relay Frame relay characerisics are:. packe swiching wih virual circui service (virual circuis are bidirecional);. labels are called DLCI (Daa Link Connecion Idenifier);. for connecion is mean virual circui, so QoS is provided per virual circui;. no error conrol (opional in edges), no flow conrol;. LAP-F proocol wih packe size up o 4096 Bye. Parameers negoiable a seup wih conrac basis are:. CIR (Commied Informaion Rae): his is he bi rae ha nework guaranees, measured in bi/s;. CBS (Commied Burs Size): his is he amoun of daa always acceped and rasmied a highes priory measured in seconds;. EBS (Excess Burs Size): his is he amoun of daa rasmied wih bes effor approach, acceped only if i is possible; i is measured in bi. Daa exceeding he size CBS+EBS is always discarded for defaul: his is an example of policing. he ime in which he CIR is measured is, defined as: = CBS CIR bi bi/s = s Graphically: CBS Pracically, is he period in which CBS is ransmied. here are wo fundamenals consrains have o be respeced:. he sum of all CIR of a source mus be lower han he access speed rae; 1

2 . he sum of all CIR of a link mus be lower han he link speed rae. he following picures describe he wo consrains: Consrain for source A C B Consrain for link A C B Congesion If he sum of CIR over a single link exceed he global capaciy his creae a long erm congesion. I can be done in order o increase nework uilizaion in wo ways:. ransmiing packes wih low prioriy: his raffic is sen only if here are avaiable resources so i does no creae unsaisfacion o users;. overbooking: his soluion violae he second consrain previously declared because sooner or laer high prioriy packes will be dropped creaing unsaisfacion o users. If i is raffic bursiness who creae congesion, here is shor erm congesion, so i is no criical beause his fac is well conrolled if bursiness is he small as possible. Congesion depends on CIR, link speed and. Look a he following examples: 2

3 No congesion Local congesion Algorihms Policing algorihms will be discussed are:. leaky bucke;. oken bucke. In order o manage congesion problem, here are wo possible algorims:. backward;. forward. Policing Confermance verificaion is realized differeniaing he raffic characerizaion over he period :. packes conforman o he CBS consrain are always sen a high prioriy (Discard Eligibiliy bi se o 0);. packes conforman o he EBS consrain are always sen a low prioriy (Discard Eligibiliy bi se o 1);. packes non conforman o he CBS+EBS consrain are always dropped. Policing works marking and dropping packes bu if insead perform drop operaion packes are delayed he same algorihm can be used as shaping in order o adap raffic o be conforman o he previous consrains. 3

4 Leaky Bucke Leaky bucke behaves like a raffic regulaor because ranslae in a CBR source a rae ρ if source is greedy (i means ha generaes packes couninously). If i is used like a policer, when packes arrive earlier respec ρ hey are dropped; hey are also dropped if user raffic exceed he buffer size B. ρ B Over a period is no possible o sen more raffic han ρ; wha concerning he bi rae:. if ρ can be approximae wih he average rae oo much raffic could be discared;. if ρ can be approximae wih he peak rae, resources are underuilized because here is a wase of link bi rae. oken Bucke If wih leaky bucke bursiness can no be generaed, oken bucke allows i: paying a silence for β packes (he size of he oken buffer) he buffer is full up, so he ransmission rae will be, over a local period, more han ρ (rae in which oken are generaed). Oherwise if he oken is released as soon as is generaed he behavior is, again, he one of a CBR source. he following picure show wha i is said before: silence β ρ Users can send daa only if here is a oken presen ino he buffer and he maximum raffic which hey can generae over a period is always lower or equal o ρ + β (β is relaed o he burs duraion). Measuremen problems ake measures of rae over an asynchronous packe flow is a complex problem: he simples soluion is akes measures over fixed lengh inervals: 4

5 I is imporan noice ha over each period is no possible send more han CBS and probably his consrain is respeced in average, bu maybe locally can no: he larger is period he mos criical is border effec beween adjacen inervals. Anoher soluion is a emporal sliding window: wih his approach rae is measured accouning for he amoun of bye received during he window as soon as a packe arrive. he mos imporan disadvanage is ha implemenaion is oo much complex because i is necessary o sore informaion abou packe ime arrivals. I is possible exploi fluid approximaion, approach in which i is no necessary sore informaion abou packe ime arrivals, bu only calculae and updae he rae hinking ha all byes arrived are spreaded over he sliding period. Congesion Conrol he scenario in which congesion conrol is discussed is he ouer buffer archiecure: i provides he bes performance because, in a packe ime, only one packe has o be forwarded o he proper ouer por. i λ j Congesion problem is complely avoided if all sources are sending a CIR, bu due o overbooking is possible ha here is congesion; in his case, firs of all, delay increases, hen buffers fill up and consequenly packes will be dropped causing a reduce of QoS. Algebraic condiion is: λi, j 1 if all pahs are normalized: 0 < λ < 1. his condiion may creae a long congesion problem or shor congesion problem: i depends on which ime i is measured and i is a criical parameer. When a node deecs congesion have o signal i seing one of he possible wo bis declared by he sandard: 5

6 . FECN (Forward echnique);. BECN (Backward echnique). hese echniques are differen only for he direcion in which congesion is signaled. he wo consrains ha mus be respeced in order o avoid packe los are fairness and maximun nework uilizaion: hey are always in conflic because saisfying one of hem he oher will be violaed. A ypical example is he scenario of only one user in CSMA-CD: he nework uilizaion will be maximum because here are no collisions, bu his is complely unfair for ohers users. Max-min fairness his algorih ries o maximize he bandwidh allocaion o flows ha receive he minimum allocaion. Wih his algorihm, o increase he bandwidh already allocaed o a flow, i is necessary o decrease he bandwidh of anoher flow and his is a very criical disadvanage for QoS. he second disadvanage is ha, max-min algorihm can no be implemened looking locally in he nework oherwise i does no work: a global view is needed. Algorim sep:. a firs all flows are marked as unsaisfied;. bandwidh is increased over each flow by he same value unil he firs boleneck link is saisfied;. when a boleneck link is sauraed i is mark as saisfied and i can no increase more his bandwidh;. he procedure runs unil all flows are marked as saisfied. Forward congesion his approach is used when a node deecs congesion and, in order o signal i o he nework, ses FECN bi o 1. Congesion, in his way, is signaled o all nodes unil he receiver, and i has o redirec his informaion o he ransmier because only ha node can reduce is raffic in order o reac o he problem. he signalling from he ransmier o he receiver is sen hrough he same pah of inermediae nodes because virual circui are bidirecional. 6

7 Algorim propery are:. implemenaion very easy: he only ask have o be performed is he se of FECN bi;. slowly because if he node who deec congesion is closed o he source, a R is paid o propagae signalling o he receiver and hen o he ransmier;. no addiional raffic is inroduced because ACKs are used from desinaion o source, so he reverse signalling can be done in his way. Backward congesion his approach is used when a node deecs congesion seing BECN bi o 1. I is much more complex han forward algorim because an ad-hoc signalling is needed in order o reach he source. he oher issue is ha he signalling have o be propagaed o all congesed virual circuis, so i is required o mark all of hem. Main properies are:. fasly han forward algorim: in he wors case ime needed o signal congesion is half a R (if he node who deecs congesion is closed o he receiver);. much more complex (sore a lis of congesed DLCI and wai or creae raffic o reach he source). Source behavior he capaciy of nework o reac o congesion problem depends on he R and i is very differen chosing backward or forward echnique. In FECN echnique he reac is slow and he source does no know where and how congesion is deeced over he pah: i simply compue he percenage of packes received wih FECN bi se o 1 over a ime inerval. Noice ha if he source goes ou i can no receive packes wih FECN so congesion problem is no solved. BECN echnique reac isanly reducing he rae when a frame wih BECN bi is se o 1, oherwise rae in increased. Issues of congesion conrol he firs problem is fairness: here are los of differences among he wo echniques. For example, wih BECN approach, if a source receives a lo of congesion signals decrease a lo bi rae for all flows when is possible here 7

8 was only shor erm congesion. In general all flows who generae more are penalized, bu in some sense, can be posiive block anyway all virual circuis who creae oo much raffic. Oherwise, i is possible ha a source keep silen for a long period, wake up and generaes lo of packes for a shor period and hen come back silen: if in he period in which i wan o ransmi flows are block ha ransmier will be oo much penalized. Any kind of reacion o congesion is always slower han a R because he signalling akes a minimun half a R hen verificaion process (when source has already received he signalling and reduces is ransmission rae) akes anoher R. Anoher issue o ake care is he R duraion: for example, a source who generaes a lo of raffic and have a small R, in BECN echnique, receives a lo of signals, so reduce bi rae more han a source who generaes few packes wih bigger R. In his case, he firs one is penalized. Deec congesion is a very difficul problem o solve, infac if rae measures are used here are following consrain o keep in mind:. he inerval over which measures are aken depends on he R, bu nodes do no know his value; moreover o ake a relevan average measure he period considered have o be greaer han a R;. since here is only one bi o signal congesion, i is impossible o send informaion on he proper rae o use: he source knows only if congesion is presen or no; probably akes complex measures no really used is an useless operaion;. rae measures are no so relevan looking o he wide nework: a virual circui congesioned somewhere probably in a previous link should send a an higher speeed. Anoher approach o deec congesion is looking a he buffer size of nodes (indirec measure): considering a hreshold, if he occupancy is greaer han ha hreshold he node is congesioned. his is an indirec measure and is more performan since comparisons are done wih ineger numbers while rae measure wih real numbers. Moreover his approach does no disiguish abou flows bu looks a aggregae raffic. here are wo mehods who can implemen buffer occupancy:. isananeous buffer occupancy is faser bu unsable;. average buffer occupancy reacs slowly bu i is sable. 8

9 For he second mehod ime inerval have o be deerminaed and his is an issue while he isananeous buffer occupancy can no disinguish among siuaion very differen: assume ha a ime x here is congesion over wo links esimaed in 120%; he link A previously presened a siuaion of 80% a ime x 1 while link B was 200%: he firs implies ha load have increased oo much while in he second siuaion here is congesion bu he link is underloaded. Also average buffer occupancy can no disinguish among hese siuaions if he period considered is oo small. Anoher approach is derivaive occupancy which is more complex and precise han ohers approaches akes care o consider he period in which congesion problem is deeced. he hreshold value have o be chosen close o empy buffer in order o have more free space o collec packes bu in his way if in cerain ime arrive a lo of packes congesion is signaled alhough he siuaion is emporary. All parameers, hreshold, windows size, R change in ime so if algorihm is very fasly bu sensiive reacs no well han a slowly bu robus parameer. For buffer size he only consrain o be respeced is he cos since in Frame Relay here is no delay conrac; in order o avoid packes loss i is jus need o have large buffer size as possible. In his way if here is a large number of connecions, large R or large connecion raes packes will never be los. 9