Efficient protection of many-to-one. communications

Transcription

1 Efficient potection of many-to-one communications Miklós Molná, Alexande Guitton, Benad Cousin, and Raymond Maie Iisa, Campus de Beaulieu, Rennes Cedex, Fance Abstact. The dependability of a netwok is its ability to cope with failues, i.e., to maintain established connections even in case of failues. IP outing potocols (such as OSPF and RIP) do not fit the dependability objectives of today applications. Moeove, fowading techniques based on destination addess (like IP) induce many-to-one connections. If a dependable connection is needed, all pimay paths and potections having the same destination must be established in a coodinated way. In this pape, we popose a fault ecovey fo many-to-one connections based on a cold (peplanned) potection. The main advantage of ou appoach is that the ecovey in case of failues is achieved within a shot delay. Additionally, with espect to othe appoaches, the dependability of the outing scheme is inceased in the way that it statistically copes with many failues. The algoithm we popose computes an efficient backup fo an abitay pimay tee using an impoved multi-tee algoithm. Keywods: netwok, fault-toleant outing, many-to-one, cold potection, multi-tee algoithm

2 1 Intoduction High-speed netwoks ae becoming inceasingly impotant and allows the development of applications with eal-time constaints, such as multimedia sevices, coopeative systems, distibuted computing. These applications often ely on the suvivability of the netwok: communications should not be inteupted fo a long time by a failue of a link o of a oute. Indeed, the longe the communication is inteupted, the moe packets ae dopped. The poblem of fast ecovey has been well studied fo seveal types of communications, including boadcast (one-to-all), unicast (one-to-one) and multicast (one-to-many). Howeve, thee is no efficient poposition fo dependable incast (many-to-one) communications. Incast connections ae many-to-one, i.e., seveal souces send data to a single destination. An incast connection can be the suppot of homogeneous o heteogeneous communications. Examples of applications inducing homogeneous communications include log collection, data gatheing in senso netwoks, auction sales and massive submissions. In netwoks whee the fowading of packets is based on thei destination addess (such as IP netwoks), all the communications towad the same destination fom an incast connection. In this case, the connection is heteogeneous since it is composed of communications having diffeent equiements and potocols. Fo example, a FTP communication fom an host A to an host C

3 and a HTTP communication fom an host B to an host C fom an incast connection. Incast connections ae taditionally ealized using a tee 1. Implementing dependable communications is a majo thead fo cuent netwoks. Indeed, the netwok is supposed to be suvivable, i.e., it should withstand failues of links o outes. Two measues of the dependability of a netwok can be consideed: the ecovey delay and the numbe of failues managed [1]. It is theefoe citical to educe the ecovey delay as much as possible. Classical ecovey delays of IP potocols such as OSPF o RIP each tens of seconds (see [2] fo OSPF and the slow convegence poblem [3] fo RIP). The othe measue, the numbe of failues managed by a ecovey mechanism, impacts on the eliability of the netwok. In ou model, we conside two types of failues: independent failues and highly coelated failues. Thee is a tade-off fo the ecovey mechanisms in potecting efficiently against independent failues and highly coelated failues. Ou objective is to ecove quickly fom failues on an incast communication while coping with as much failues as possible. In this pape, we popose a cold peplanned potection that allows local ecovey using ac-disjoint tees. Ou constuction of ac-disjoint tees is a genealization of the algoithm descibed in [4]. Section 2 gives a state of the at on dependable communications. Section 3 descibes ou potection constuction based on ac-disjoint tees. 1 Multicast connections ae also ealized using a tee, but incast connections and multicast connections ae not symmetic: multicast connections and multicast outing potocols equie a paticula mechanism in outes, the duplication mechanism, while incast connections do not equie any.

4 Section 4 descibes ou local ecovey mechanism. Section 5 analyzes the tade-off of potecting independent failues and highly coelated failues. Finally, we conclude ou wok in Section 6. 2 State of the at of dependable connections Seveal ways to cope with failues exist. In this section, we biefly suvey the existing appoaches to ealize dependable connections while concentating on ou main concen: the fastness of the ecovey. A detailed suvey on suvivability can be found in [5] (in the case of WDM netwoks). Hot and cold edundancy. Hot edundancy, denoted 1+1 edundancy, consists in sending each message on two disjoint paths simultaneously (cf. [6] fo an example). Hot edundancy allows fast ecovey since the destination eceives the packet fom one of the paths even if a failue on the othe path has occued and has not yet been detected. Howeve, hot edundancy wastes a lot of bandwidth. This majo dawback and the necessity of a selecting algoithm at the destination make hot edundancy not vey used. Cold edundancy, denoted 1:1 edundancy, consists in aising a ecovey mechanism once a failue is detected (cf. [6] again fo an example). Although 1:1 edundancies ae slowe than 1+1 edundancies because of the failue detection delay. They ae often pefeed since they save bandwidth. The estoation and potection ae the two main types of cold edundancy.

5 Restoation and potection. Restoation is a eactive appoach to cope with failues. At the time a failue is detected, the oute that detected the failue seaches fo a new path to eoute the taffic to the destination [7]. The advantage of estoation is that it adapts to the cuent state of the netwok. Howeve, intensive computations ae equied fo the oute to find a new path, which inceases the ecovey delay. Usual Intenet outing potocols use this appoach. Potection is a poactive appoach to cope with failues. The behavio of the outes in case of a failue is peplanned [8]. At the time the failue is detected, the oute eoutes the taffic to the peplanned potection path. This appoach has the advantage of being vey fast, since the ecovey is aised without any additional computation of the oute [9]. Howeve, less failues can be managed compaed to estoation since potection is poactive. Classic potections ae end-to-end o local. End-to-end and local. End-to-end ecovey consists in eouting the taffic at the souce on an ac-disjoint path, once a failue is detected. A typical example of end-to-end potection is the path-based potection. The delay induced by the end-to-end ecovey is high because the souce has fist to be infomed that a failue occued on the pimay path befoe aising the ecovey. Anothe dawback of path-based potection is that it cannot cope with two successive failues: if a failue occus on the pimay path and anothe occus on the altenate path, the connection is inteupted.

6 Local ecovey consists in eouting the taffic at the oute that detected a failue. A typical example of local ecovey is the link-based potection. The delay induced by the local ecovey is low because the ecovey is aised locally. Using link-based ecovey, seveal successive failues on the pimay path can be managed, as long as they concen diffeent links. A dawback of link-based potection (but not of local ecovey appoach) is that node failues ae not managed. A compaison of link-based potection and path-based potection can be found in [10]. In this pape, we popose a new local cold potection to ecove fom failues on incast connections within a shot delay, and without the dawback of the link-based potection. The specificity of incast connections is the lage numbe of souces; theefoe, taditional end-to-end ecovey mechanisms ae not suited to incast, whee all the souces have to be infomed of the failues that occued. 3 Poposed potection of incast tees Ou poposition aims the constuction of dependable incast connections. We show late that the dependability of the outing scheme is einfoced since the poposed potection statistically withstands many failues. Many-to-one connections equie the establishment of the pimay paths in a coodinated way. The backup paths should be synchonized togethe and also with the pimay paths. We call pimay tee the set of pimay paths.

7 Fo basic incast connections, the pimay tee is usually a shotest path tee. QoS awae incast connections may use diffeent patial spanning tees, depending on the netwok status. Fo this eason, we assume that the pimay incast tee is given to ou algoithm eithe by the application o by the netwok management. Often, this pimay tee spans only a sub-gaph of the given netwok. The objectives of the potection ae: (i) it should wok fo any topology and (ii) it should wok fo any given pimay tee (patial o not). In this section we pesent how ou potection can achieve these objectives. Since it efes to the multi-tee constuction algoithm poposed in [4], a bief desciption of the algoithm is done in the following. 3.1 Basic multi-tee constuction A multi-tee is a set of two diected tees that ae ac-disjoints. The algoithm pesented in [4] descibes a way to compute these two tees. The computation woks only in edge-edundant topologies and the two tees spans all the nodes of the netwok. The algoithm assumes that all links ae bidiectional. The multi-tee is built by adding successively extenal paths, as specified by Algoithm 1. An extenal path is a path stating in a spanned node u, ending in a spanned node v and such as all intemediate nodes ae not yet spanned by the multi-tee. Geneally, u v (except at the fist iteation o in the case of aticulation vetex whee u = v =, being eithe the oot o the aticulation node). Fom each extenal path, two

8 ac-disjoint banches ae extacted such as one of them ends at u while the othe ends at v. Algoithm 1 Multi-tee constuction. initialize the multi-tee with the destination node while an extenal path of the cuent multi-tee exists do select an extenal path p extact two ac-disjoint diected paths fom p add the two diected paths to the multi-tee end while Figue 1 shows the successive steps of the algoithm. In this example, an extenal path is chosen abitaily at each step. The coesponding two ac-disjoint banches ae shown on the figue. One of the tee of the multi-tee is epesented in solid lines (and is efeed to as the blue tee in [4]) while the othe is epesented in dashed lines (and is efeed to as the ed tee in [4]). Advantages. With the help of the multi-tee, tee-based communications can be potected against node and link failues in edge-edundant gaphs, and the multi-tee is elatively easy to compute. It can be applied to ealize hot-edundancy fo boadcast o incast communications o as a peplanned potection fo cold-edundancy. Dawbacks. One of the dawbacks of the descibed multi-tee potection is that the algoithm does not deal with abitay topologies, only with edge-edundant topologies.

9 PSfag eplacements u 1 v 1 u 2 v AA ,, --,, --,, -- v 3 FF GG FF GG FF GG :: ;; :: ;; :: ;; "" ## "" ## "" ## //.. //.. // u 3 not spanned node spanned node HH II HH II HH II JJ KK JJ KK JJ KK << == << == << == >>?? >>?? >>?? BB CC v BB 4 CC BB CC DD EE u DD 4 EE DD EE link edge of p $$ %% $$ %% $$ %% && '' && '' && ''!!!!!! u 5 v 5 (( )) (( )) (( )) ** ++ ** ++ ** ++ ac of one tee ac of the othe tee Fig. 1. The multi-tee constuction. A moe impotant dawback follows fom the fact that the selection of successive extenal paths is not detemined. Thus, the diamete of the tees (and as a consequence, the length of the pimay paths) can be abitay lage (fo example, it can be seen on Figue 1 that the distance in the solid tee fom node v 5 to the destination is vey long). The QoS equiement of the applications o the netwok management often impose the use of paticula pimay tees (e.g., of shotest path tees o QoS awae tees). Finally, in most incast communications, only a sub-set of nodes belongs to the set of the souces and so the incast tee is a patial spanning tee. The algoithm should be adapted to patial spanning tees.

10 In the following, we will specify how the diffeent dawbacks of the multi-tee based potection can be eliminated. 3.2 Extension to abitay topologies Often, in eal netwok, aticulation nodes o edges ae possible. Even in initially edundant netwoks, aticulations can be poduced due to pesistent failues. The potection computation should wok also in these cases. To build a multi-tee in an abitay connected netwok, we popose to add to the pevious algoithm a paticula case when aticulation edges ae found. In this case, we popose the ceation of two diected acs on the aticulation edge towad the destination. This edge is not potected against failue but no algoithm could have potected it. Howeve, if failues occu on othe pats of the netwok, they can be ecoveed. It can be noticed that the two tees of the multi-tee ae not ac-disjoints anymoe. 3.3 Potection of a given incast spanning tee In netwok, failues ae ae. Theefoe, the chosen tee used if no failue occus has to ensue an efficient delivey of data packets and cetain QoS citeion. Geneally, none of the diected tees built by the multi-tee algoithm does coespond to a good pimay tee and can not be used fo the communications. Howeve, computing a multi-tee can help in

11 finding the backup suppot of the desied pimay tee as pesented in the following 2. To adapt the potection to a given (total) pimay tee (fo example to the shotest-path tee), we popose the constuction of a multi-tee spanning all of the acs of the given pimay tee. Ou dependable incast connection computation contains two steps: (i) the computation of a multi-tee spanning a given pimay tee and (ii) the constuction of a backup foest on the basis of the multi-tee. Multi-tee constuction fo a given pimay tee To ensue that the pimay tee T p is coveed by the multi-tee, we have to ensue that T p is coveed by the union of the extenal paths selected duing the multitee constuction. In othe wods, we have to ensue that evey ac of T p is coveed by an ac of the multi-tee. Let us denote by dist(t p,, n) the hop distance fom to a node n in T p. At each iteation, the algoithm selects an extenal path containing exactly one edge (n 1, n 2 ) that is not in the pimay tee T p and with at least one node of {n 1, n 2 } not spanned by the multi-tee. One can pove that this kind of extenal path exists if the topology is edundant and the pimay tee spans it. If thee ae seveal candidates, then the extenal path minimizing dist(t p,, n 1 )+dist(t p, n 2, ) is selected. Figue 2 shows such an edge on an example whee the pimay tee T p coesponds to the 2 To simplify let us suppose hee that, similaly to the multi-tee computation, all nodes of the netwok will paticipate to the incast communication. The case of patial spanning tees is discussed in the next sub-section.

12 shotest-path tee. Having chosen the not yet spanned edge (n 1, n 2 ), the extenal path p fom u to v can be found as follows: u and v ae spanned by the multi-tee, p contains the path in the pimay tee fom n 1 to u, the edge (n 1, n 2 ) and the path fom n 2 to v. %% && %% && %% && PSfag eplacements ++,, ++,, ++,,!! ""!! ""!! "" '' (( '' (( '' (( --.. ## $$ )) ** --.. ## $$ )) ** --.. ## $$ )) ** u v u v // 00 // 00 // 00 [3+3] n 1 n 2 n 1 n 2 Fig. 2. An extenal path on a shotest-path tee. Minimizing the hop distance dist(t p,, n 1 ) + dist(t p, n 2, ) on T p allows the potection to be dense. A dense potection is moe obust in the case of multiple failues than a spase potection. Popety 1 shows that ou selection of extenal paths ensue that the pimay tee T p is coveed by the multi-tee. Popety 1. The acs of the diected pimay tee T p ae in the multi-tee built by ou algoithm. Poof. Thee is only one outgoing ac fo each node of the tee T p diected to the oot. The computed multi-tee spans all the nodes by successive extenal paths. To pove the popety, it is sufficient to show that adding a node to the multi-tee implicates also adding its outgoing ac in the pimay tee T p. Let p be an extenal path fom the node u to v selected

13 at any iteation of the algoithm. In a fist time, we show that the outgoing ac of any node n p is in p and in a second time we show that thee is an ac diected to the same diection in the multi-tee. Let us assume that the outgoing ac of n in T p is not in p. In this case, the adjacent edge of n p ae not in T p but this is impossible because thee is only one edge in p which is not in T p. The multi-tee algoithm associates two outgoing acs (one in each diected tee of the multi-tee) to each node, except u and v. The outgoing ac on T p of any node diffeent fom u and v coesponds to an ac in the ceated multi-tee. Constuction of the backup foest Ou impoved multi-tee algoithm builds a multi-tee M = (T 1, T 2 ) coveing the pimay tee T p. To obtain the backup suppot of the pimay tee, Algoithm 2 is poposed: Algoithm 2 Backup foest constuction. Requie: T p a pimay tee, M = (T 1, T 2 ) a multi-tee coveing T p F acs(t 1 ) acs(t 2 ) F F \acs(t p ) The fist step of the algoithm meges the acs of T 1 and T 2 into the diected set F. T 1 and T 2 ae disjoint, except fom the acs on the aticulation edges. Then, in F, only one ac exists on the aticulation edges. The second step of the algoithm emoves in F the pimay acs of T p. The emaining acs ae the backup potection of T p. It can be noticed that F is a foest.

14 3.4 Patial spanning tees Geneally, only a sub-set of the nodes paticipates to the incast communication. To ceate a patial multi-tee which coves at least the patial pimay tee, we popose two algoithms. A simple solution can be obtained by computing the multi-tee spanning all the nodes and by puning the pats that ae not used o edundant. This solution equies an impotant computation even if thee ae few nodes in the set of souces 3. A moe efficient computation can be obtained if only the equied pat of the multi-tee is built. The patial multi-tee can be built with the help of successive shotest loops fom the destination to the souces following the pimay paths in the evese diection. The extenal path selection and the stop condition of the multi-tee algoithm should be modified accodingly. Nodes which ae in the patial multi-tee but not in the pimay tee have two diected paths to the destination. The outgoing acs which is not in the pimay tee can be deleted because the potection of these nodes is not needed. Othe outgoing acs ae used as backup segments fo the pimay tee 4. Figue 3 illustates the patial multi-tee constuction and the obtained potection scheme. The left pat of the figue shows the patial multi-tee constuction. The pimay tee T p, in thick lines, is contained in M = (T 1, T 2 ). The ight pat of the figue, the 3 In this case, poof of Popety 1 emains the same, as long as the puning does not alte neithe the pimay tee no the backup foest. 4 In this case, the poof of Popety 1 is not valid anymoe. Howeve, a simila poof can be done by noticing that the last pat of the extenal path coesponds to the path in the pimay tee T p.

15 obtained potection scheme, shows the pimay tee T p in solid lines and its backup F in dashed lines. PSfag eplacements node node paticipating to outing 2 ** ++ ** ++ ** ++!!!!!! $$ %% $$ %% $$ %% "" ##,, -- "" ##,, -- a "" ##,, -- && '' b && (( '' )) (( )) && '' (( )) souce patial tee solid tee dashed tee pimay tee backup ac Fig. 3. Multi-tee and backup constuction fo a patial incast tee. 4 Poposed ecovey mechanism The pevious section descibed how to compute a potection fo a given tee. Once the potection has been configued, a local ecovey mechanism has to be implemented in outes to potect the connection against failues. Let us ecall that the potection ensues that, fo evey node n of the pimay tee, thee exists two ac-disjoint paths fom n to the destination of the incast connection. We assume that all the outes can stoe two enties fo evey destination. The pimay enty fo the node n coesponds to the fist edge on the pimay path towad the destination and the altenate enty coesponds to the fist edge of the backup path. If a oute detects a failue

16 on the pimay enty fo a destination, it switches its pimay enty to its altenate enty. A oute detects a failue popagation need when it eceives a packet fo a destination on an inteface which coesponds to the next hop fo on the pimay path. In this case, it switches its pimay enty to its altenate enty to avoid loops. This failue popagation occus only when the backup path uses the edges of the pimay path in the evese diection. Fo example, if node a of Figue 3 detects that the link (a, ) failed, it switches to its altenate enty (failue detection). Then, the packets that each a ae fowaded to b. When eceiving a packet fom a fo, b detects that a is the next hop to on its pimay path. Then, b switches to its altenate enty (failue popagation). The packets follow the backup path up to. Now that we descibed ou potection and ou ecovey, the next section analyzes its behavio in the case of independent o highly coelated failues. 5 Analysis of the dependability in the case of multiple failues In this section, we analyze the dependability of seveal potections in case of multiple failues. The numbe of failues that can be managed by a potection geatly depends on the netwok topology and on the incast connection. To study the dependability of a potection in the case of

17 multiple failues, we popose to discuss on the numbe of failues that do not inteupt communications. 5.1 Independent failues In ou model of independent failues, we assume that failues occu successively on the pimay path. Indeed, failues that do not occu on a pimay path do not impact communications. Path-based potection When the fist failue occus on the pimay path and the souce is infomed, the taffic is swapped to the backup path at the souce. Then, if an independent failue touches the backup path, the communication is inteupted. Link-based potection Link-based potections do not cope with node failues. In the case of a link failue, the bypass is used to each the next node while the est of the pimay path is used. If an ulteio link failue occus on the pimay path, anothe bypass can be used if and even if the second bypass is link-disjoint with the fist bypass. The link-based potection allows independent link failues if the bypasses ae independent. Howeve, it does not cope with failues which occus on the bypasses. Ou potection The poposed incast tee potection copes with a link o a node failue. If a second failue touches the backup used to ecove fom a fist failue, thee ae two possibilities. If this backup coesponds to a pimay path of anothe communication and its backup is diffeent fom

18 % & % & % & % & % & $ $ $ $ $ G H G H G H @ > > > > > the fist pimay path, then the local ecovey is possible. If the failed pat of the backup does not coespond to a pimay path of anothe communication o if the backup of this failed point uses the fist failed pimay path, then the mechanism cannot cope with the failue. The mechanism is illustated on Figue 4 (A) and (B) in the case of PSfag eplacements link and node failues. node node paticipating to outing souce patial tee solid tee dashed tee pimay tee backup ac QQ RR QQ RR QQ RR SS TT SS TT SS TT MM NN MM NN MM NN II JJ II JJ II JJ V KK LL KK LL KK LL (A) WW XX WW XX WW XX OO PP OO PP OO PP ecovey path UU VV UU VV UU VV '' (( aa bb aa bb aa bb cc dd cc dd cc dd ]] ^^ ]] ^^ ]] ^^ YY ZZ YY ZZ YY!!! ZZ!!!!!!!!!!!! """ ### """ ### """ ### """ ### """ ### [[ \\ `` ))) *** [[ \\ `` +++,,, [[ \\ `` (B) gg hh gg hh gg hh ee ff ee ff ee ff AAA BBB qq qq qq ss tt ss tt ss tt CCC DDD mm nn... mm nn... mm //// nn //// //// //// //// //// ii jj ////... ii jj ::: ii ;;; jj ::: ;;; ::: ;;; ::: ;;; ::: ;;; kk ll kk ll kk ll (C) ww xx ww xx ww xx oo pp oo pp oo pp <<< === <<< === <<< === <<< === <<< === EEE FFF uu vv uu vv uu vv s Fig. 4. Recovey capability of ou mechanism. 5.2 Highly coelated failues In ou model of highly coelated failues, the failues occu simultaneously on adjacent links o nodes of the netwok: a connected subgaph of the netwok fails. The highe the potection esists to highly coelated failues, the highe can be the diamete of the failed subgaph without inteupting communications. In the same way than peviously, we assume that at least a link o a node of pimay tee fails. Path-based potection Simultaneous failues can occu on the links and nodes of a pimay path. The path based potection copes with lage

19 diamete of failues when all intenal nodes and edges of the pimay path failed. Howeve, the altenate path should be intact. Link-based potection Since link-based potection does not cope with node failues, the maximal diamete of a ecoveed failue is one link. Ou potection In edundant topologies, the poposed tee-based potection gives a link-disjoint altenate path fom all the nodes of the pimay path to the destination. Highly coelated failues on a pimay path with lage diamete can be ecoveed. Moeove in the case of dense incast communications, the mechanism copes with failues which occu on the altenate paths. Figue 4 (C) shows the maximal sub-set of the netwok which can fail with ecovey fom a souce s. 6 Conclusion In this pape, we pesented a cold potection fo many-to-one communications. The potection computation is based on the constuction of a backup foest, given an abitay pimay tee. The ecovey uses a simple failue popagation mechanism and can be ealized by a local switch opeation in the concened outes. Thus, it poduces low failue ecovey delay. We studied the impact on the potection of two scenaios of failues: independent failues and highly coelated failues. We wee able to show that ou method can popose a tade-off between managing independent failues and highly coelated failues. In the futue, we intend to evaluate quantitatively ou potection though simulations.

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