SLA-Based Inter-Domain QoS Routing

Transcription

1 SLA-Base Inter-Doman QoS Routng Ru Pror 1 an Susana Sargento 2 1 LIACC Unversty of Porto Portugal 2 Insttuto e Telecomuncações Unversty of Avero Portugal rpror@ncc.up.pt ssargento@et.ua.pt Abstract Ths techncal report escrbes a soluton for nter-oman QoS to be part of the framewor for en-to-en QoS control. The soluton s base on the use of vrtual-trun type aggregate efne by means of Servce Level Agreement for ata transport between peerng omans. The problem s formally state an formulate n Integer Lnear Programmng. As a practcal soluton we efne an extenson to the BGP routng protocol to transport three fferent QoS metrcs (lght loa elay assgne banwth an a congeston alarm) an a path selecton algorthm usng a combnaton of these metrcs. 1 Introucton Toay s commercal Internet has far surpasse the ntal concept of a ata transport networ mostly for acaemc an mltary purposes. Its mult-servce nature becomes event wth the ever ncreasng number of onlne auo an veo broacastng servce pocastng nstant messagng wth auo an veo extensons an nternet telephony servces le sype among others. Its success n replacng the tratonal networs to prove multmea servces wth real-tme requrements an n provng new servces however contone by the ablty to prove a hgh an guarantee qualty level. The ntroucton of Qualty of Servce (QoS) support n Internet routng therefore of maor mportance towars ths goal. Mnmzaton of the number of AS hops prove by the current non-qos-aware nter-oman routng protocol oes not guarantee avalablty of resources requre to transport the ata or better QoS: a path wth a smaller number of AS hops may traverse congeste lns or may comprse a larger span of fber an/or a hgher number of traverse routers than another one wth a larger number of AS hops. Other metrcs le elay loss probablty or avalable capacty nee to be taen nto account n choosng the paths. Internet routng s performe n two layers: ntra-oman routng wthn networs controlle by a sngle operator or amnstratve entty an nter-oman routng for the nterconnecton an transport servce between fferent amnstratve omans. Though much attenton has been pa to QoS n pacet swtchng networs n general an IP networs n partcular most of the effort has been centere on ntra-oman QoS. We have now realy avalable solutons base on the traffc engneerng capabltes of MultProtocol Label Swtchng (MPLS) [2] an on QoS extensons of ntra-oman routng protocols such as OSPF [3]. Much less has been one n the scope of nter-oman QoS though. Several fferent factors contrbute to the ffculty of solvng ths problem. The Internet s a complex entty comprse of Autonomous Systems (AS) manage by very verse operators. If t s to be wely eploye an nter-oman QoS routng mechansm must be capable of hanlng the heterogenety of the Internet an mpose mnmum requrements on ntraoman routng n orer to be appealng to the fferent operators. The ntroucton of QoS routng shoul not srupt the currently exstng nter-oman routng: both shoul nter-operate allowng for ncremental eployment among the fferent networ an the stablty of ntra-oman routng shoul not be sgnfcantly affecte by the QoS mechansms. Conclatng the requre stablty wth the ynamc nature of QoS nformaton s a maor challenge n nter-oman QoS routng. A relate ssue s that of scalablty: a soluton that oes not scale to the menson of the Internet cannot be eploye wely enough to be useful. A further requrement s the optmzaton of the use of networ resources. As ba as the current path selecton mechansm base mostly on the mnmzaton of the number of traverse AS t has the avantage of avong unnecessarly long path n general. A mnmzaton of the number of AS hops s not however guarantee to optmze the amount of resources requre to transport the ata from orgn to estnaton as a path wth a smaller number of AS hops may comprse a larger span of fber an/or a hgher number of traverse routers than another one wth a larger number of AS hops. In ths respect a better metrc for optmzaton woul be the expecte elay n lght loa contons that nowaay when the banwth n bacbones an transt networs s qute hgh correspons approxmately to the propagaton elay rectly proportonal to the traverse span of fber. Mnmzaton of ths metrc by the path selecton mechansm 1

2 woul lea to a more ratonal use of the networ resource as well as better QoS to ata pacet whch woul suffer smaller elay penalte as long as the path s not congeste 1. An approprate choce of the path selecton metrcs s ey to the behavor of QoS routng. Two funamentally stnct types of path metrcs may be use for the selecton: statc or ynamc. Statc metrc such as the bottlenec ln capacty o not vary over tme meanng that no nformaton exchange s requre after the ntal propagaton of routes to the entre networ an the chosen routes are stable. However they are unable to express the ynamc character of the networ an therefore QoS routng base solely on statc metrcs s unable to prevent an/or react to networ congeston. By usng ynamc metrc such as the avalable path banwth or the tme wnow average nstantaneous pacet elay t s possble to react to congeston. However a hgh overhea s ncurre n message exchange for upate metrcs an n path recomputaton an the route stablty property s lost. QoS routng base on purely ynamc metrcs cannot therefore be apple to the Internet scale. Two man approches may be use for reucng the overhea assocate wth QoS routng wth ynamc metrc n orer to mae t scale [4]. The frst one s base on the stncton between sgnfcant changes n the metrcs that trgger QoS nformaton upates an path recomputatons from mnor changes that o not. The secon one s to convey longer term nformaton on the path characterstcs through statstcal characterzaton of the route metrcs. Both approaches prove coarser-grane nformaton: the frst one n terms of scale the secon one n terms of tme. In a commercal Internet ata transport s a servce subect to contracts between the ntereste partes. Networ operators establsh Servce Level Agreements (SLA) between them that specfy the contons for ata transport an these agreements are translate to path selecton polces n the routng protocol frequently confgure by han. The ablty of nter-oman routng to automatcally aust to these agreements shoul not be overlooe. The combne use of SLAs for transport servce wth an nter-oman QoS routng protocol s preconze n the DAIDALOS proect [5] n the scope of whch ths wor was evelope. In ths report we formally state the problem of SLA-aware nter-oman QoS routng an formulate t as an Integer Lnear Programmng (ILP) optmzaton problem provng that routes thus obtane are cycle-free. We propose a practcal soluton for nter-oman QoS routng base on both statc an coarse-grane ynamc metrcs: t uses the lght loa elay an assgne banwth (both statc) n orer to mprove the pacet QoS an mae better use of networ resource an a coarse-grane ynamc metrc for path congeston to avo overloae paths. We efne an extenson to the Borer Gateway Protocol (BGP) [6] routng protocol to transport these QoS metrc an a path selecton algorthm usng a combnaton of these metrcs. Ths report s organze as follows. The next secton escrbes relate wor. Secton 3 contans the formal escrpton of the problem an ts formulaton n ILP. Secton 4 presents the propose protocol an the assocate path selecton algorthm. Fnally secton 5 raws some conclusons an ponts out rectons for further evelopment of the wor heren presente. 2 Relate wor A framewor for QoS-base Internet routng aoptng the tratonal separaton between ntra- an nter-oman routng was efne by Crawley et al. [4]. They scusse the goals of an nter-oman QoS routng an the assocate ssues that must be aresse an prove general guelnes that shoul be followe by any vable soluton to QoS routng n the Internet. However they o not specfy the set of QoS metrcs to be transporte or the algorthms for usng such metrcs n the choce of nter-oman routes. A seres of statstcal metrcs for QoS nformaton avertsement an routng talore for nter-oman QoS routng though also applcable to ntra-oman routng were efne by Xao et al. [12] along wth algorthms to compute them along the path. These metrc the Avalable Banwth Inex (ABI) the Delay Inex (DI) the Avalable Banwth Hstogram (ABH) an the Delay Hstogram (DH) convey nformaton expresse n terms of one or more probablstc ntervals. Smulaton results show that by usng these metrc selecte routes are closer to optmalty than when usng statc metrcs; moreover the overhea s lower an the stablty hgher than when usng the corresponng nstantaneous (purely ynamc) metrcs. However these approaches conser only a sngle QoS parameter mang t ffcult to smultaneously satsfy fferent requrements. When optmzng by banwth paths wth large elay may be chosen whle 1 If the lns o not have these characterstcs an the lght loa elay verges sgnfcantly from the sum of propagaton elay the optmalty regarng resource utlzaton wll ecrease but the QoS optmzaton wll reman ntact. 2

3 others wth les yet suffcent avalable banwth an much lower elay may be avalable. Conversely when optmzng by elay a route wth low avalable banwth may be selecte; swtchng to ths route may cause congeston ncreasng the elay. When the elay nformaton s upate the prevous route mght be selecte agan an so on causng route flappng (though on longer tme scales than wth ynamc metrcs). Crstallo an Jacquenet propose an extenson to the BGP wth a new optonal an transtve attrbute QoS_NLRI for the transport of several types of QoS nformaton [13]. Ths wor s focuse on the specfcaton of the attrbute nclung the formats for transportng the fferent parameter such as reserve ata rate or mnmum one-way elay an oes not specfy how the nformaton s to be use n path selecton. Smulaton results emonstratng ts use wth (statc) nformaton on one-way pacet elay are prove though. 3 Inter-Doman QoS Routng wth Vrtual Truns In ths secton we formally escrbe the problem of nter-oman routng wth vrtual truns an formulate t as an Integer Lnear Programmng (ILP) problem. Ths formulaton can be use n a Mxe Interger Programmng (MIP) solver to obtan optmal route sets for a gven topology confguraton an traffc matrx. 3.1 Vrtual trun moel of the Autonomous Systems Though the use of some nner nformaton of the ASs s mportant for nter-oman QoS routng the exact topology an confguraton of the ASs shoul not be use for two reasons: (1) the level of etal woul be excessve complcatng the route computaton tas an most mportant (2) networ operators usually want to sclose the mnmum possble amount of nternal nformaton about ther networs. In ths wor we use a blac box moel where only externally observable AS nformaton s sclose. The ntra-oman connectons between ege routers are replace by vrtual truns wth specfc characterstcs nterconnectng the peerng ASs. Each vrtual trun has a specfc amount of reserve banwth an an expecte elay. These values epen on the nternal topology of the AS on the ntraoman routng an on resource management performe by the operator an usually reflect Servce Level Agreements establshe between the operator of the AS an the operators of the peerng ASs. The vrtual trun moel of ASs s llustrate n fgure 1. Each vrtual trun correspons to a partcular (ngress pont egress pont) par. Fgure 1 llustrates the concept: an SLS between oman S1 an oman T1 specfes that X traffc may flow between S1 an oman T3; an SLS between oman T1 an oman T3 specfes that Y traffc may flow between T1 an oman D1. Aggregates are manage nternally wthn each (transt) oman ensurng that enough resources are assgne an no mposton s mae regarng mechansm use to ths en. Fgure 1 - Vrtual trun type SLSs The confguraton of the vrtual truns must be consstent wth the nter-oman lns. In partcular the summe banwth of all vrtual truns traversng AS an gong to AS must be less than the banwth of the nter-oman ln connectng ASs an ; smlarly the summe banwth of all vrtual truns comng from AS an traversng AS must be less than the banwth of the nter-oman ln connectng ASs an. 3

4 3.2 Problem statement Let G = ( V E) be an unrecte graph wth ege capactes c an ege elays w. Each noe represents an AS an the eges correspon to the nter-oman lns. Atonally we efne a set F of aggregate flows between pars of noes an a corresponng matrx of traffc emans a for all ( ) V 2 where s an enote the source an estnaton noe respectvely. Gven any three noes an where s rectly connecte to an s rectly connecte to there may be a traffc contract (SLS) statng that proves a vrtual trun between an wth r reserve capacty. The amount of ata transporte from to va therefore boune by. If no such contract exst we say that r 0. Snce each vrtual trun s mappe to an actual path nse the AS t has an assocate elay truns ( ). y = The vrtual truns must satsfy the contons corresponng to the elay of that path. Call L the set of all vrtual o r c for traffc orgnate at noe an estne to or traversng noe an s r + where o s the capacty reserve + where t s t r c the capacty reserve for traffc estne to noe an orgnate at or traversng noe. The expecte total elay suffere by pacets of a gven flow s the sum of the w an y parameters along the path followe by the flow. Our goal s to fn the set of hop-by-hop routes that mnmze the elay whle guaranteeng that nteroman ln an vrtual trun capactes are not exceee. 3.3 Problem statement transform In orer to formulate the state problem as an ILP problem we frst transform the orgnal graph nto a transforme graph where the vrtual truns are explctly accounte for Transform graph It s possble to transform the graph nto a recte multgraph where each ege correspons to a vrtual trun; however t s ffcult to account for the elays of all lns n the orgnal graph (nter-oman lns) wthout countng some of them twce. Moreover graphs are better stue an easer to eal wth than multgraphs. Therefore we a vrtual noes to the recte multgraph n orer to obtan a resultng recte graph. Vrtual truns are establshe between an entry ln an an ext ln. Therefore we a two vrtual vertces per ln of the orgnal graph one for each recton an vrtual truns are represente by eges connectng these vrtual noes. Moreover n orer to forb a noe of the orgnal graph from beng traverse rectly (nstea of va a vrtual trun) we splt each orgnal noe nto two: one source vrtual noe wth outgong eges only an one estnaton vrtual noe wth ncomng eges only. Flows on the transform graph exst between source vrtual noes an estnaton vrtual noes. A very smple example of an orgnal graph an ts transform wth all possble vrtual truns s shown n fgure 2. Ln (AB) on the orgnal graph s represente by vrtual noes AB an BA vrtual trun (ABC) s represente by an ege connectng AB to BC noe A s represente by the vrtual source an estnaton noes A S an A D an flow (AD) s represente by flow (A S D D ) for example. a) Orgnal graph b) Transform graph Fgure 2: Smple networ wth 4 noes 4

5 The sol ege connectng the vrtual noes an correspon to the vrtual trun for senng traffc from noe to noe va noe an has capacty (that of the vrtual trun) an elay y + w where y s the nternal elay of the vrtual trun an w the elay of the nter-oman ext ln. Each ashe ege ( S ) correspons to the nter-oman ext ln from noe to noe an has elay w an capacty r o. Each otte ege ) correspons to the nter-oman entry ln n noe from noe an has zero elay an capacty value they may be compute usng ( D t. Notce that even f there s no explct = c r an = c o t r. o an t In the example of fgure 2 there s only one possble path from A to C corresponng to the vrtual trun through noe B. Traffc sent from A to C s subect to a elay equal to the sum of w (from the y a b c wb c ashe ege A S AB) an + (from the sol ege AB BC); the otte ege BC C D has zero elay. Regarng banwth t s constrane by o a b r a b c an t b c all of them share wth other traffc. Fgure 3 prove a somewhat more complex example wth a cyclc graph an ts transform contanng all possble vrtual truns. Though the transform graph loos overly complex when compare to the orgnal one the number of varables an constrants n the ILP formulaton s not ncrease snce a formulaton base on the orgnal graph woul requre varable unfolng n orer to be lnear. Also eep n mn that an unrecte graph has half the number of eges of the equvalent recte graph. a b a) Orgnal graph b) Transform graph Fgure 3: Cyclc networ wth 5 noes Generaton of the transform graph In ths secton we present an algorthm for the generaton of the tranform graph G = ( V E ) from the orgnal graph an the set of vrtual trun nformally escrbe above. The algorthm s as follows: 1. For each noe V 1.1. A noe S to the set S of sources an to the set 1.2. A noe D to the set D of estnatons an to 2. For each (unrecte) ege { } E 2.1. A noe to V 2.2. A noe to V 2.3. A ege ( D ) to the set E of eges Set capacty c = t D Set elay w = 0 D V V of noes 5

6 2.4. A ege ( D ) to E Set capacty c = t D Set elay w = A ege ( S to E Set capacty c S = o Set elay w S = w 2.6. A ege ( S ) to E D Set capacty c S = o Set elay w S = w 3. For each (recte) vrtual trun ( ) L 3.1. A ege ( ) to E an to the set L of vrtual trun eges Set capacty c = r Set elay w = y + w 4. For each flow ( F 4.1. A flow ) to the set F of flows ( S D 4.2. Set traffc eman a = a S D When the algorthm fnshe we have the transform graph G = ( V E ) the assocate ege capacty an ege elay matrces C an W a set L E of vrtual trun ege a set S V of source noes an a set D V of estnaton noe a set F of flow an the respectve traffc eman matrx A Complexty of the transform graph The number of noes an eges of the transform graph G s relate to the orgnal (unrecte) graph G an the set of vrtual truns n the followng way. The number of noes s two per noe of the orgnal graph (one source an one estnaton e.g. A S an A D ) plus two per ege of the orgnal graph (one for each recton e.g. AB an BA). The number of eges s four per ege of the orgnal graph (combnatons of source/estnaton an transmsson/recepton e.g. (A S AB) (AB B D ) (B S BA) an (BAA D )) plus one per vrtual trun (e.g. (ABBC)). In the example of fgure 3 the orgnal graph has 5 noe 5 eges an 12 possble vrtual truns. The transform therefore has 20 noes (2*5+2*5) an 32 eges (4*5+12) Converson of routes from the transform to the orgnal graph A route p on the transform graph may be converte bac to a route p on the orgnal graph by analysng the traverse eges. Each traverse ege on the transform graph correspons to a traverse noe on the orgnal graph accorng to the followng table: Ege on the transform graph Noe on the orgnal graph ( S ( ) ) ( D For example the route (B S BCCEE D ) on the transform graph of fgure 3.b) correspons to the route (BCE) on the orgnal graph. 3.4 Formulaton as ILP problem We now formulate our banwth-constrane global route an elay optmzaton hop-by-hop routng problem as an ILP problem wth boolean varables usng the transform graph. In the transform graph 6

7 formulaton s somewhat smpler than n a generc graph snce some restrctons are alreay enforce by the topology (snce there are no ncomng eges n source noe t s not necessary to use a restrcton sallowng ncomng traffc for flows orgnate at those noe an smlarly for estnaton noes). Our obectve s to mnmze the global elay whle respectng the banwth lmt assumng that the networ has enough capacty to satsfy all emans. In aton to the transform ata obtane by the above escrbe algorthm let us efne a set of postve flow weghts b for all ( ) F. Two fferent ns of optmzaton may be obtane by usng fferent weght values. The frst alternatve uses = 1 ( ) statng that all flows have equal mportance; n ths case optmzaton s performe on b s F a per-route bass. The secon alternatve uses bs a s ( ) F statng that a flow s mportance s proportonal to ts traffc eman; n ths case optmzaton s performe on a traffc volume bass. Let us efne the boolean ecson varables x whch tae the value 1 f the flow ( ) F s route through the ege ( E an the value 0 otherwse. The problem can thu be formulate as follows: w ( ( ) F Mnmze b x subect to { 01 } ( ) F( x s E s (1) a s ( ) F ( ) ( ( ) x c ( E ( V { s } ) x x = 0 ( ) F ( x = 1 ( ) F x = 1 ( ) F V :( t S t t x S x ( ) F S : ( ) E x = ( :( ) s S V :( ) s S s s x D s (2) (3) (4) (5) (6) (7) Constrant set (1) mposes boolean ecson varable meanng that flows cannot be splt over multple paths. Constrant set (2) means that the sum of all flows traversng an ege wll not excee ts capacty. Constrant sets (3) (4) an (5) are the mass balance equatons: (3) means that each flow enterng a noe that s nether source nor estnaton for that flow must leave t an vce-versa; (4) means that each flow leaves the source noe once an conversely (5) means that each flow enters the estnaton noe once. Constrant set (6) mposes hop-by-hop routng on the orgnal graph: t means that f a flow from a gven noe to a certan estnaton leaves that noe by a gven ln no flow to the same estnaton traversng that noe may leave that noe by a fferent ln. On the transform graph t means that f a flow from a source to a estnaton traverses a gven vrtual noe rectly connecte to that source no other flows to the same estnaton may traverse a fferent vrtual noe connecte to the same source. Fnally constrant set (7) prevents routng loops at the estnaton noes of flows n the orgnal graph by forcng flows arrvng at a noe rectly connecte to ther estnaton vrtual noe to use that rect path. Falng th a flow woul be counte twce (or more) on the left han se an only once on the rght han se nvalatng the equalty. Theorem 1: Paths obtane through ths optmzaton proceure are guarantee to be cycle-free on the transform graph. 7

8 Proof: Satsfacton of contons (4) an (5) mples that each flow leaves the source vrtual noe exactly once (4) an arrves at the estnaton vrtual noe exactly once (5) therefore the source an estnaton vrtual noes belong to the path. There are no ncent eges to source vrtual noe therefore these noes cannot be n a cycle. Conversely there are no ncent eges from estnaton vrtual noe therefore these noes cannot be part of a cycle ether. Now let p be a path wth a cycle from a gven source s S to a gven estnaton D an p * the same path wth the cycle remove. The cycle may only nclue ntermeate noe snce source an estnaton noes cannot be part of cycles. If the above constrants (notably the capacty constrants) are satsfe wth path p from s to then they are also satsfe wth path p * from s to. Snce > 0 ( ) an > 0 ( E : S D the cost of usng p * woul be lower than b s F w the cost of usng p therefore p coul not be n the optmal route set as a route set nclung t woul not mnmze the cost functon. Theorem 2: Paths obtane through ths optmzaton proceure are also guarantee to be cycle-free on the orgnal graph. Proof: The proof s base on three lemmas regarng cycles wthout the estnaton noe cycles wth the estnaton noe an the preceng ege an cycles wth the estnaton noe wthout the preceng ege. Lemma 1: A path satsfyng the above contons cannot contan cycles that o not nclue the estnaton noe on the orgnal graph. Proof: Let p 1 be a path on the orgnal graph from source s to estnaton contanng a cycle that oes not nclue noe an p 1 the equvalent path n the transform graph. Snce t oes not contan the estnaton the cycle on p 1 contans a noe left by the flow twce by fferent ege ( an ( ) towars the cycle an towars the estnaton noe. Therefore n the transform graph p 1 contans vrtual noes an. Constrant (6) mples that x 1 an x 1. However ths contracts constrant (4) = = therefore p 1 can only contan cycles that nclue the estnaton noe. Lemma 2: A path satsfyng the above contons cannot contan cycles that nclue the estnaton noe an the preceng ege on the orgnal graph. Proof: Let p 2 be a path on the orgnal graph from source s to estnaton contanng a cycle that contans noe an the ege ncent to ( ) an p 2 the equvalent path n the transform graph. In ths case the flow enters twce by the ege ( ) from the source an from the cycle. Therefore n the transform graph p 2 contans a cycle contanng the vrtual noes. Ths contracts theorem 1 whch states that p 2 s guarantee to be cycle-free on the tranform graph meanng that a cycle contanng noe an the ege ncent to cannot exst n the returne route set. Lemma 3: A path satsfyng the above contons cannot contan cycles that nclue the estnaton noe but not the preceng ege on the orgnal graph. Proof: Let p 3 be a path on the orgnal graph from source s to estnaton contanng a cycle that p the equvalent path n the transform graph. In ths nclues noe but not the ege ncent to an 3 case the flow enters noe twce by fferent ege ( ) an ( ) from the source an from the cycle. Therefore n the transform graph p 3 contans vrtual noes an contrbutng two unts to the left han se of constrant (7). Accorng to constrant (5) the estnaton vrtual noe can only be entere once therefore ths flow s contrbuton to the rght han se of constrant (7) can only be one unt. Snce the same apples to all flow no flow on the orgnal graph can have a cycle contanng the estnaton noe but not the ege ncent to. 8

9 The preceng lemmas cover all possble cycles on the orgnal graph therefore all paths satsfyng the above contons are guarantee to be cycle-free on the orgnal graph Reucng the number of varables The transform graphs have partcular characterstcs that allow us to sgnfcantly reuce the number of ecson varables. Notce that the source vrtual noes have only out-eges (ashe eges n fgure 3.b)) an the estnaton vrtual noes have only n-eges (otte eges n fgure 3.b)). As such the followng contons hol: x ( S {} s ) ( D { } ) s = 0 ( ) F( E : (8) x s = 0 ( ) F( E : (9) Therefore we may restrct the oman of ( n varables s x to L {( E : = s = }. 3.5 Varant formulaton The problem formulaton above assume that a certan amount of capacty s reserve at the nteroman lns for traffc generate at the transmttng AS (ntal noe) an estne to or traversng the recevng AS (termnal noe) therefore not corresponng to any vrtual trun an smlarly that at the ngress polcng a gven amount of capacty was assgne to traffc estne to the AS tself. In terms of constrant they are represente respectvely by the capactes of the eges ncent from the vrtual source noes (ashe) an by the capactes of the eges ncent to vrtual estnaton noes (otte). A perhaps more reasonable assumpton s that traffc outse the vrtual truns may use all the capacty left unuse by traffc nse the vrtual truns (nclung reserve but unuse vrtual trun capacty). Aaptng the problem formulaton to ths new assumpton nvolves the followng changes: 1. Restrctng the oman of ( n constrant (2) to L nstea of E that to the set of vrtual trun ege removng the fxe capacty constrants outse the vrtual truns. 2. Introucng an atonal constrant at each vrtual noe corresponng to an nter-oman ln a x c ( V S D) (10) V :( ( ) F where c s the capacty of the corresponng nter-oman ln n the orgnal graph. Ths constrant set states that the sum of all flows traversng (leavng) the nter-oman ln must be less than the ln s capacty. Usually c =. c V :( 4 Propose protocol an assocate algorthms Whle the ILP formulaton of the nter-oman QoS routng problem presente n the prevous secton s useful as a baselne for comparson wth real protocols n controlle envronments where all the nput ata s nown t cannot be use n the mplementaton of a real protocol tself for several reasons: frst the problem of 0-1 nteger programmng s nown to be NP-complete [14]; secon because t requres nowlege of the traffc matrx whch s not easy to obtan real utlzaton scenaros; an thr because t requres nowlege of the vrtual trun SLA whch are usually sclose only to the nvolve peers. In ths secton we propose a vrtual-trun-aware nter-oman QoS routng protocol base on an extenson of BGP for use practcal mplementaton n real nternetwors. 4.1 QoS routng Currently verson 4 of BGP has become the e facto stanar for nter-oman routng n the Internet. BGP s a path vector protocol for the exchange of reachablty nformaton between connecte ASs (ebgp - External BGP). It s also use to share nformaton on learne routes among the fferent ege routers of a gven oman (BGP - Internal BGP). The routes selecte by BGP are propagate to the ntra-oman routng protocol use n the AS (usually referre to as the Interor Gateway Protocol IGP) by the ege routers. The 9

10 reachablty nformaton s conveye n UPDATE message each contanng an avertsement of a new or change route to a gven networ estnaton an/or a set of wthrawals of routes to estnatons that may no longer be reache va the AS orgnatng the UPDATE. Beses the estnaton prefx nformaton on the traverse ASs (AS_PATH) an on the next hop (NEXT_HOP) s prove for avertse routes. The recepton of an UPDATE message trggers a three step ecson process: (1) a egree of preference s assgne to the new route (f any) base on a set of polces; (2) one of the avalable routes to the estnaton s selecte an propagate to the IGP; (3) f the route s fferent from the prevously nstalle one t s propagate to the peerng ASs. The most common polcy for path selecton s the mnmum number of hops n the AS_PATH. Though the AS_PATH length metrc bears only a very loose relaton to QoS parameter BGP can easly be extene to convey vrtually any n of relevant QoS nformaton. The ecson processes may also be change to use the QoS nformaton (f present) for path selecton wthout breang bacwar compatblty. We extene BGP to transport an use three QoS metrcs: assgne banwth (statc) path elay uner lght loa (statc) an a ynamc metrc for path congeston escrbe below Metrcs Vrtual trun nformaton s explctly nclue usng BGP to carry nformaton on the amount of banwth contracte between two omans regarng ata transport to a thr one. The assgne banwth reflectng traffc contract s essentally statc. It s upate along the path to be the mnmum that the bottlenec banwth (concave metrc). Notce that our moel oes not requre explct an quantfe agreement only that transport operators assgn a certan capacty for ata transport between ther connecte peers; explct SLAs are ust a means to guarantee that reasonable assgnments are performe. Informaton on the expecte elay n lght loa contons s also carre. Ths nformaton not only allows for the mnmzaton of pacet elay but also for a more ratonal use of the networ resource snce n hgh capacty lns wth sgnfcant length such as those foun n toay s nter-oman connecton t conssts mostly on the sum of propagaton elays [15] rectly proportonal to the traverse span of fber as long as there s no congeston. It also proves a lower boun for the expecte pacet elay. We argue that the mnmzaton of ths metrc by the path selecton mechansm wll lea to a more ratonal use of the networ resource as well as better QoS to ata pacet whch woul suffer smaller elay penaltes. The lght loa elay metrc s statc an s summe along the path (atve metrc). The thr QoS metrc conveye by our propose extenson s path congeston. The concept of congeston s elberately vague an may therefore be translate nto a coarse obectve metrc mnmzng the nherent overhea n message exchange an path re-computaton typcal of ynamc metrcs. In our case the congeston alarm s expresse by an nteger wth three possble value whose meanng s the followng: 0 not congeste; 1 very lghtly congeste; 2 congeste. Ths metrc s upate along the path to the maxmum value (convex metrc). In the most basc verson congeston may be nferre from the utlzaton of the aggregates; n a more avance verson other parameter such as pacet los average length of traverse router queues or measure elay may be use as nputs to the functon for computng the alarm level of vrtual trun aggregates. The man requrement for the congeston alarm the sole ynamc metrc n our proposal s that changes shoul be nfrequent for scalablty an stablty reasons; hysteress an relate technques may be apple n the assgnment of alarm levels to ths en. An effectve value of the congeston alarm s use for path selecton nstea of the receve value amng at reucng the fluctuatons n the usage of the aggregates. Ths effectve value s the same as the receve value unless the receve value s 1 an the route s alreay n use n whch case the effectve alarm s 0. Ths means that when level 1 (lght congeston) s reache no oman shoul use that route to replace a prevously establshe one but omans that were alreay usng the route shoul not swtch to a fferent one unless a hgher congeston level s reache. Ths behavor s meant to avo the synchronze route flappng problem. The next secton escrbes the propagaton an upatng of these metrcs an ther use n path selecton Path selecton algorthm The three above mentone QoS metrcs are conveye n the UPDATE messages by a newly efne Path Attrbute QoS_INFO whch s optonal an transtve (meanng that ASs whch o not yet support the extenson smply forwar the receve value) an are upate by the BGP-speang routers at each transt oman tang nto account the vrtual truns between the oman to whch the route s avertse an the next hop oman for the route. Notce that these vrtual truns are share among fferent source to 10

11 estnaton routes: n fgure 1 for example all traffc transporte from T1 to D1 va T3 shares the T1:D1 vrtual trun nepenently of beng orgnate at S1 or S2. Fgure 4 - Propagaton of metrcs n the QoS_INFO attrbute Fgure 4 llustrates the propagaton of the elay banwth an congeston alarm metrcs n the QoS_INFO attrbute of UPDATE messages. When the estnaton AS (AD2) frst announces the route to an nternal networ t may omt the QoS_INFO attrbute f ths networ s rectly connecte to the announce NEXT_HOP. On recevng the UPDATE the ege router at transt oman TD2 creates (or upate f alreay present) the QoS_INFO attrbute wth metrcs of the outgong ln for route selecton purposes (ths step s omtte n the fgure). If the route s selecte t s propagate to all peerng omans; the QoS_INFO attrbute sent to the fferent upstream omans s fferent snce the metrcs are upate wth respect to the vrtual trun aggregates. The same process s repeate at transt oman TD1. Notce that n the UPDATE sent from TD1 to AD1 the elay metrc (17ms) s the sum of the elays of the concatenate vrtual truns (2ms an 15ms) the reserve banwth s the mnmum along the path (300Mbps) an the congeston alarm the maxmum (1). The vrtual trun values that contrbute to the fnal values receve by AD1 for ths route are unerlne n the fgure. set Traffc to est = Local traffc to est + Transt traffc to est for both routes f Alarm rcv = 1 an route n use then set Alarm eff = 0 else set Alarm eff = Alarm rcv f both routes have Assgne BW < traffc to est choose the one wth larger Assgne BW else f one route has Assgne BW < traffc to est choose the other one else f Alarm eff s fferent choose the route wth lower Alarm eff else f Delay s fferent choose the route wth least Delay else f Assgne BW s fferent choose the route wth larger Assgne BW else use normal BGP rules (AS_PATH length etc.) Fgure 5 - Route comparson/selecton functon Fgure 5 shows the algorthm for route comparson use n the ecson processes n pseuo-coe. Delay nformaton s use to select the fastest/shortest route. The benefts of ong th as prevously mentone are twofol: pacets wll suffer lower elays an networ resource usage wll be more ratonal. The nformaton on the reserve banwth s use to elmnate from the set of possble choce routes wth nsuffcent banwth to support the current outgong traffc aggregate from the local AS to the estnaton (nclung flows generate at the local AS an flows traversng t); t s also use as te breaer when two routes for the same estnaton have the same announce elay. The alarm levels are use to elmnate congeste routes from the set of possble choces. The elmnaton of routes wth nsuffcent capacty from the set of possble choces prevents congeston of those routes to a certan egree contrbutng to lower message an processng overhea an to route stablty Route aggregaton A very mportant aspect n nter-oman routng s the possblty of aggregatng routes. Wthout the eployment of route aggregaton an Classless Inter-Doman Routng (CIDR) [16] n the 1990 the routers woul not have been able to support the ncreasng number of avertse routes. Paraoxcally lttle attenton s gven to aggregaton n nter-oman QoS routng proposal n general. The use of a metrc so coarse-grane as the congeston alarm n ths proposal s aggregaton-frenly. Whle the ntroucton of new metrcs reuces the possbltes of aggregaton compare to the stanar non- QoS-aware BGP congeston alarm values wll almost always be ether 0 or 1 meanng that much aggregaton s stll possble. Ths s partcularly true f congeston s ntrouce n transt oman snce t s 11

12 common to all routes sharng the congeste vrtual trun. The lght loa expecte elay metrc may easly be mae compatble wth aggregaton f assume to be an ncator rather than an exact value: n ths case two routes may be aggregate f the smaller elay s more than a certan fracton (say 75%) of the larger one announcng the larger value for the aggregate route. The herarchcal structure of the Internet allows for a large egree of aggregaton wth ths approach. The banwth metrc however s more ffcult to eal wth even wth a herarchcal structure. We are currently worng on how to conclate the banwth metrc wth hgh levels of aggregaton. We are also evaluatng the use of the congeston metrc alone an of a combnaton of congeston an lght loa elay metrcs wthout the banwth metrc. An evaluaton of the aggregaton possbltes s out of the scope of ths report though. 5 Conclusons Ths report aresse the problem of nter-oman QoS routng. Our proposal s base on the use of vrtual trun type aggregates for the nrect transport of traffc between two fferent amnstratve omans across a thr one. These vrtual truns are usually (though not necessarly) efne by means of SLSs between the operators of peerng oman an each nter-oman path conssts on a concatenaton of vrtual truns. Each vrtual trun efne by a trplet of AS s share by all actve routes contanng that sequence of ASs n the AS path. We formally state the problem of SLA-aware nter-oman QoS routng an formulate t as an Integer Lnear Programmng (ILP) optmzaton problem provng a proof that routes thus obtane o not contan cycles. As a practcal soluton we propose the QoS_INFO extenson to the BGP protocol usng of a combnaton of three fferent metrcs (assgne banwth expecte lght loa elay an congeston alarm) n orer to smultaneously acheve fferent an conflctng goals: fnng non-congeste paths that satsfy the QoS requrements of the ata flow mnmze the networ resources use to transport the flow nteroman route stablty an mnmzaton of the message exchange an path computaton overheas. Though one of the metrc congeston alarm s ynamc ts coarse granularty an the rules for ts use n the path selecton algorthm are such that the mpact overhea n message exchange an n path re-computaton s mnmze an route stablty ncrease. There are several topcs for further mprovement of ths wor. Devsng a more avance algorthm for the assgnment of alarm levels to aggregates s one of the topcs. Beses aggregate usage ths algorthm woul use parameters such as measure elay pacet losse queue lengths an hstorcal recors as nput mprovng the assessment of congeston whle reucng even further the number of requre upates. Another topc s the ntroucton of route aggregaton an ts conclaton wth accurate QoS nformaton n orer to mprove scalablty. Fnally we want to smulate the propose QoS routng mechansm n orer to evaluate t an compare t to stanar BGP wthout QoS support to BGP wth the QoS_NLRI extenson an to the optmal results prove by the ILP formulaton. Smulatons wll be performe frst wth a sngle representatve topology an then wth a large number of ranomly generate topologe n orer to better ascertan ts behavor n the real wor. 6 Acnowlegement Ths wor s base on results of the IST FP6 Integrate Proect DAIDALOS [5] fune by the European Communtys Sxth Framewor Programme. It reflects the authors vew an the EC s not lable for any use that may be mae of ths nformaton. 7 References [1] S. Blae (e.) et al. An Archtecture for Dfferentate Servces. IETF RFC 2475 December [2] E. Rosen A. Wswanathan an R. Callon Multprotocol Label Swtchng Archtecture. IETF RFC 3031 January [3] R. Guern A. Ora an D. Wllam QoS Routng Mechansms an OSPF Extensons. In Proceengs of IEEE Globecom 97 November [4] E. Crawley R. Nar B. Raagopalan an H. Sanc A Framewor for QoS-base Routng n the Internet. IETF RFC 2386 August [5] DAIDALOS proect (IST ) homepage [6] Y. Rehter an T. L (es.) A Borer Gateway Protocol 4 (BGP-4). IETF RFC 1771 March [7] M. Wnter (e.) et al. Fnal System Specfcaton. AQUILA Consortum Delverable D1203 (v. b1) February [8] P. Pan E. Hahne H. Schulzrnne: "BGRP: Sn-Tree-Base Aggregaton for Inter-Doman Reservatons." In Journal of Communcatons an Networ Vol. 2 No. 2 June 2000 pp

13 [9] T. Damlats (e.) et al. D3.4: Fnal System Evaluaton. Part B: D1.4: Fnal Archtecture Protocol an Algorthm Specfcaton. TEQUILA Consortum Delverable October [10] P. Flegas (e.) et al. Specfcaton of Busness Moels an a Functonal Archtecture for Inter-oman QoS Delvery. MESCAL Consortum Delverable D1.1 June [11] P. Chmento et al. QBone Sgnallng Desgn Team Fnal Report. Internet2 QoS Worng Group July [12] L. Xao J. Wang K. Lu an K. Nahrstet Avertsng Inter-Doman QoS Routng Informaton. In IEEE Journal on Selecte Areas n Communcaton Vol. 22 No. 10 December [13] G. Crstallo an C. Jacquenet The BGP QOS_NLRI Attrbute. IETF Internet Draft February [14] R. M. Karp Reucblty among combnatoral problems. In Complexty of Computer Computaton pages Plenum Pres [15] K. Papaganna et al. Measurement an analyss of sngle-hop elay on an p bacbone networ. IEEE JSAC Specal Issue on Internet an WWW Measurement Mappng an Moelng Vol. 21 No. 6 August [16] V. Fuller et al. Classless Inter-Doman Routng (CIDR): an Aress Assgnment an Aggregaton Strategy. IETF RFC 1519 September