An Extended Fault-Tolerant Link-State Routing Protocol in the Internet

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1 An Extended Falt-Tolerant Link-State Roting Protocol in the Internet Jie W, Xiaola Lin, Jiannong Cao z, and Weijia Jia x Department of Compter Science and Engineering Florida Atlantic Uniersit Boca Raton, FL 3343 jie@cse.fa.ed Abstract Link-state roting protocols, sch as OSPF and IS-IS, are idel sed in the Internet noadas. In link-state roting protocols, global netork topolog is first collected at each node. A shortest path tree (SPT) is then constrcted b appling the Dijkstra s shortest path algorithm at each node. Link-state protocols normall reqire the flooding of ne information to the entire (sb)netork after changes in an link state (inclding link falts). Naraez et al. recentl proposed a falt-tolerant link-state roting protocol ithot flooding. The idea is to constrct a shortest restoration path for each ni-directional link falt. Falt link information is distribted onl to the nodes in the restoration path and onl one restoration path is constrcted. It is shon that this approach is loop-free. Hoeer, Naraez approach is inefficient hen a link failre is bi-directional, becase a restoration path is ni-directional and roting tables of nodes in the path are partiall pdated. In addition, to restoration paths ma be generated for each bi-directional link falt. In this paper, e extend the Naraez protocol to efficientl handle a bi-directional link falt b making the restoration path bi-directional. Seeral desirable properties of the proposed extended roting protocol are also explored. Ke ords: Falt tolerance, Internet, link-state, loopfree, roting Introdction Link-staterotingprotocols,schasOSPF[4,5,6,9] and IS-IS [, 8], are the dominant roting protocols in the This ork as spported in part b NSF grant CCR and NSF grant ANI Department of Electrical and Electronics Engineering, The Uniersit of Hong Kong, xlin@hkeee2.hk.hk. z Department of Compting, The Hong Kong Poltechnic Uniersit, csjcao@comp.pol.ed.hk. x Department of Compter Science, The Cit Uniersit of Hong Kong, jia@cs.cit.ed.hk. Internet [2]. There are to major phases in sch protocols: () each IP roter first collects the complete topological information of the nderling (sb)netork; (2) each roter then comptes the rotes according to the collected topological information. The first phase is performed distribtiel b all the roters in the netork throgh exchanging link state information ith its neighboring roters. In the second phase, each roter can constrct a roting table based on the shortest path tree (SPT) bilt sing the topological information. An SPT algorithm sch as the Dijkstra s shortest path algorithm [3] can be sed in bilding the SPT. Compared to other roting protocols sch as distanceector protocols, one of the major adantages of link-state protocols is that each roter comptes the rotes independentl sing the same original stats data; it does not depend on the comptation done in other roters in the netork. When link-states are changed in the netork, ne information onl needs to be sent once to each roter for pdating the roting table. Hitema [4] listed for good reasons that most netork specialists faor link state protocols oer the distance ector ariet: () fast, loopless conergenc; (2) spport of precise metrics and, if needed, mltiple metrics; (3) spport of mltiple paths to a destination; and (4) separate representation of external rotes. Hoeer, link-state protocols sall reqire flooding the netork hen an change occrs in the link states in the netork. Flooding ma be prohibitiel expensie, especiall hen the link states change too qickl or hen the nmber of links in the netork is too large. Limiting the freqenc of sch pdates can partiall sole the problem hen the effect of the change of cost metric is minor in terms of transmission dela. Hoeer, this approach is inefficient in coering a link falt becase certain paths ma be disconnected as a reslt of the link falt, dela in information pdate ill lead to n-delierable packets. In [7], Naraez et al. presented a roting algorithm based on the link state method to limit roting information that needs to be deliered in a link-state protocol hen a single link fails. Instead of sing the flooding method, the pro-

2 netork 2 Preliminar 2. Link-state protocols A tpical link-state protocol ses the folloing steps: restoration path initiated from. Topological information of the netork (link state) is first collected at each node b exchanging and accmlating adjacent link information among neighbors. Figre. A restoration path initiated from. posed scheme restores all the paths traersing the failed link b performing onl local pdates on the affected roters. Specificall, a shortest restoration path is constrcted that connects to for a falt link (see Figre ). Their method can restore loop-free roting after a link falt hile propagating information abot that failre to as fe roters as possible and onl to the ones along the shortest restoration path. This approach is also sefl to diert traffic for a congested link. Hoeer, Naraez approach is inefficient hen a link falt is bi-directional, becase a restoration path is ni-directional and roting tables of nodes in the path are partiall pdated. In addition, to restoration paths ma be generated for each bi-directional link falt. In this paper, e modif the Naraez protocol to efficientl handle the link falt b making the shortest restoration path bi-directional so that nodes on the restoration path is completel pdated and the constrction of the restoration path is initiated at the both end nodes of the falt link. Onl one shortest restoration path is constrcted if the shortest restoration path is niqe. When the shortest restoration path is not niqe, still one path is constrcted b either restricting the initiation process to onl one end node or extending the Dijkstra s algorithm. Seeral desirable properties of the proposed extended roting protocol are also explored. In the sbseqent discssion, e se nodes and roters interchangeabl. The rest of the paper is organized as follos: Section 2 reies the basic ideas sed in link-state roting protocols and briefl describes Naraez falt-tolerant link-state roting protocol ithot flooding. Section 3 proposes an extension of Naraez protocol for handling a bi-directional link falt. Section 4 discsses properties of the proposed extended roting protocol. Section 5 concldes this paper and discsses possible ftre ork. 2. A shortest path tree (SPT) is constrcted at each node b appling a SPT algorithm, sch as the Dijkstra s shortest path algorithm, on the graph representing the netork topolog. 3. For an roting ith a gien destination, a shortest path is selected from the SPT at the sorce node if the sorce roting approach is sed; otherise, a roting table is constrcted from the SPT if the distribted roting approach is applied. The roting table incldes next hop information for each destination. Note that in sorce roting, the sorce node decides the complete path hile in distribted roting onl the next hop is decided at each node along the roting path. In this paper, e se the distribted roting approach here a roting table is constrcted directl at each node based on the associated SPT. A (sb)netork that ses link-state roting protocols can be ieed as an ndirected graph G = (V; E), here V is a ertex (node) set and E is an edge (link) set. (; ) 2 E is a bi-directional link here and are to ertices in V. represents a directed link from to. Each (; ) is associated ith a cost (a positie nmber) representing the cost of traeling from to (and from to ). Clearl, (; ) can be ieed as to directed links and, and nodes and keep the link state of and, respectiel. Note that the aboe model is commonl sed in most roting protocol inclding OSPF [4]. Let (= 0 ),, 2, :::, (= n ) denote a path hich is a seqence of directed links from to here ertices i are distinct. It is assmed that a link falt is bidirectional. The end nodes and of a falt link (; ) can detect a bi-directional falt. The tnneling scheme is a possible soltion to handle link falts. Basicall, a ne path from to is constrcted hen link is broken (link is also broken). An path containing ill be replaced b a ne path from to. Once a packet arries at, it ill be encapslated in another packet ith destination and forardedalong the ne path ntil reaching. Then the packet is decapslated. The remaining roting process follos a reglar link-state roting protocol. Hoeer, encapslation/decapslation limits the efficienc of high-speed roters, since eer single data

3 2 x z (a) 4 x z (b) 5 Branch Update Algorithm [7] At node = 0 pon detecting a falt link, or at node i (here i 6=06= n) pon receiing a special packet indicating the failre of.. The set D i is defined as all the nodes that are descendents of in the shortest path tree SPT rooted at i (see Figre 3). 2. The link-state database is modified to incorporate the change of state of link (the link is don). 3. The Dijkstra s shortest path algorithm is applied to recomptes the next hop for reaching node onl. The ne next hop for is no some other node i+. 4. The next hop for all the nodes in D i is set to i+. Figre 2. To sample examples. 5. If i+ is not eqal to, send a special packet to i+ indicating the failre of. packet that goes throgh the ne path has to be encapslated at node. This method is not sitable to be sed in high-speed netorks. Naraez et al. [7] proposed a falt-tolerant link-state roting in the Internet ithot flooding. This approach can handle one ni-directional link falt at a time. The basic idea is to restore all the paths traersing the falt link b performing pdates onl in the neighborhood. First a shortest restoration path (a path ith the minimm cost) is constrcted that connects to (assming fails as in Figre ). Then onl nodes along the shortest restoration path need to pdate their roting tables. Specificall, e onl need to pdate next-hop information for those destinations that are descendents of the falt link in the SPT. The a these roting tables shold be pdated remains a challenge. A simple pdate that recomptes roting tables of nodes along the restoration path does not ork, becase packets might leae the restoration path too soon. For the example illstrated in Figre 2 (a), sppose that onl the roting tables associated ith nodes and are pdated and,, forms a shortest restoration path for falt link. A packet from to ill exit at to z. Becase the roting table associated ith z is not pdated, path z,,,, is still considered the shortest. Therefore, is selected as the next hop, and conseqentl, a roting loop beteen and z is formed. Forcing all the packets that old hae had to traerse the falt link to trael throgh the entire restoration path old not ork either. For the example of Figre 2 (b), assming that a packet need to be forarded from to z, once the packet reaches at ia,,, the next hop ill be since,, z is the shortest path to z. Again a roting loop occrs beteen and. In this sitation, a packet exits the restoration path too late. 2.2 Branch pdate algorithm The branch pdate algorithm proposed b Naraez et al. as designed in sch a a that a packet exits a restoration path at a right time. This protocol constrcts a shortest restoration path (= 0 ),, 2,:::,(= n ) initiated from for falt link. Note that an path connecting to, not necessaril the shortest one, can be sed as a restoration path. Naraez protocol has to desirable properties: It garantees loop-free roting after the link falt. If a minimal restoration path (in terms of hop cont) is sed, it garantees the minimm nmber of nodes that need to be informed of the link falt. Hoeer, the branch pdate algorithm can onl handle a ni-directional link falt. Using the algorithm, to restoration paths are needed for each link falt that is bidirectional. For the example of Figre, an otdated SPT rooted at can reach a destination ia either,, or,, (other destinations hose shortest paths do not inclde or are of no interest here). In the branch pdate algorithm here is the initiator, it onl pdates the path of tpe,, to a destination and the sccessor of in the restoration path is selected as the next hop to reach the destination. When the path is of tpe,,, itis taken care of b another restoration path initiated from.

4 i other branches netork D i Figre 3. A SPT rooted at i : triangle D i contains descendents of hich belongs to a branch of i and triangle other branches" contains other branches of i. x When these to paths share the same node set, each node in the set is isited tice, one for each packet initiated from each end node of the falt link. When to restoration paths do not share the same node set, the sitation is more complex. Consider the example of Figre 4 here link (; ) fails, the restoration path initiated from is, x,, and the one initiated from is,,,. (The case for can be treated in a similar a.) Sppose x does not appear in,,,, then the roting table of x is partiall pdated, i.e., x knos the failre of bt not that of. In fact, an shortest path of tpe x,, to a destination is not pdated at x, ths roting proceeds based on the otdated roting table ithot ne information abot the falt link (; ) ntil the packet reaches a node on the restoration path,,, (initiated from ). Then node is reached b folloing the restoration path,,,. 3 Extended Branch Update Algorithm In the extended roting protocol proposed in this paper, onl one restoration path needs to be constrcted for a bidirectional link falt as long as the shortest path is niqe. This is done b making the path bi-directional. Initiall, still to restoration paths are initiated, one from each end node of a falt link. When to restoration paths for the same link falt meet at an intermediate node, both processes stop. In the examples of Figres and 4, hen to restoration paths meet at node, both processes stop. A bi-directional restoration path P(; ; ) is constrcted for Figre and P(; x; ; ; ) for Figre 4. To make the restoration path bi-directional, e distingish the orientation of the path to a destination that goes throgh the falt link (; ). Sppose is a node in the restoration path initiated from,ifapath deried from the SPT that is initiated from to a destination contains link (i.e., the path is of tpe,, ), the corresponding destination is kept in set D. If the path is of tpe,,, the corresponding destination is kept in D 0. Figre 4. To restoration paths for falt link (; ). The next hop of a destination in D (D 0 ) is the sccessor (predecessor) of in the restoration path. To relate to restoration paths that are intended for the same link falt, a special marker is sed for each falt link. A node in the restoration path is marked once isited. Note that the shortest path tree (SPT) propert implies that at least one of D and D 0 is empt. The extended branch pdate algorithm is also applied to the other end node of the falt link (; ) b exchanging theroleof and. The extended roting protocol garantees a shortest restoration path (see Theorem 3). In addition, it garantees that onl a minimal nmber of nodes needs to be informed hen the proposed roting protocol tries to search for a minimal restoration path (see Theorem 4). Note that the restoration path initiated from ma or ma not be the reerse of the one initiated from, becase seeral shortest paths ma exist in a gien netork. To bi-directional restoration paths ill be constrcted for a bidirectional link falt (; ). The sitation here a marked node is encontered deseres more discssion. Sppose i is a marked node ( i cold be ) of the restoration path initiated from enconters (see Figre 5). That means the restoration path initiated from has selected i in its restoration path; that is, the restoration path initiated from has passed throgh node i and a signal has been sent to either i, or a node other than i,, sa. In either case, the process initiated from simpl stops at i as shon in Figre 5. Eentall, the path initiated from ill reach a marked node j ( j cold be ). Again, the process simpl stops at j. The restoration paths constrcted in the aboe sitation are called oerlapping paths. In the special case here j = and i =, the resltant paths are called node-disjoint paths. To restoration paths exist for each direction, one

5 Extended Branch Update Algorithm At node = 0 pon detecting a falt link (; ), orat node i (here i 6=06= n) pon receiing a special packet indicating the failre of (; (= n )): j i i+. The set D i (and D 0 i ) is defined as all the nodes that are descendents of (and ) in the shortest path tree (SPT) rooted at i. 2. If i has been marked for (; ), exit; otherise, i is marked. 3. The link-state database is modified to incorporate the change of state of link (; ) (the link is don). 4. The next hop for all the nodes in D 0 i is set to i,. 5. If i is, exit; otherise, the Dijkstra s shortest path algorithm is applied to recomptes the next-hop for node onl. The ne next hop for is no some other node i+. 6. The next hop for all the nodes in D i is set to i+. 7. Send a special packet to i+ indicating the failre of (; ). complete (that connects and ) and one incomplete. In Figre 5,, j, j+, i, and 0,, i, are for D hile, i,, 0, j, and i,, j+, j, are for D 0. Althogh to restoration paths are constrcted for each bi-directional link falt, each node in a restoration path is completel pdated rather than partiall pdated as in the original branch pdate algorithm. That is, ith the same nmber of informed nodes, the extended roting protocol proides roting information more accratel than that of the branch pdate algorithm. The net effect is that the proposed roting protocol proides shorter rotes for some destinations. Consider again the example of Figre 2, sppose that node z intends to forard a packet to. The packet is forarded to, becase z does not hae ne information abot the link falt (; ) and z,,,, is still considered the shortest path from z to. If belongs to a restoration path,, initiated from, the packet is forarded to based on the extended roting protocol (the predecessorof in the restoration path). Hoeer, sing the original branch pdate algorithm here to separate restoration paths are constrcted:, x, (initiated from )and,, (initiated from ), since path,,, is not pdated at, the packet ill be sent to and then forarded to along the restoration path, x,. Finall, j+ i- Figre 5. Oerlapping restoration paths. the packet is sent to ia. The resltant roting path is z,,, x,,. Note that the path,, is jst the optimal replacement for falt link (; ), ths the resltant roting path z,,, generated from the extended roting protocol is not shortest. In fact, the shortest path from z to is path z, ith a cost of. Hoeer, if the shortest path does not contain the falt link, it ill remain the shortest. If the reqirement is to generate a single restoration path for each link falt, the extended roting protocol can be easil modified to ensre that one and onl one end node of each link falt initiates the constrction of a restoration path. This can be accomplished b comparing the IDs of to end nodes and letting the one ith the larger ID initiate the process. Still another approach exists hich initiates the constrction from both end nodes bt garantees single restoration path. It is based on a simple modification of the Dijkstra s algorithm hich allos s to detect all the eqal length paths [4]. Here e frther modif the extended Dijkstra s algorithm b keeping the ID of the last hop of each path dring the formation of SPT. Dring the formation of SPT, hen to eqal length paths are detected the one ith a larger ID of the last hop sries. In addition, both and tr to find a restoration path from and, and the restoration path from is jst the reerse of the one from. 4 Properties In this section, e std seeral desirable properties of the extended branch pdate algorithm. Let G and G 0 be graphs (representing netork topolog) before and after link falt (; ). d G (; ) and d G 0(; ) are distance beteen and in G and G 0, respectiel. Since and are directl connected in G, d G (; ) is the cost of link (; ) in G. Unless otherise specified, the restoration path here refers to either a single restoration path or oerlapping restoration paths (inclding node-disjoint restoration paths). The folloing reslt shos that shortest paths in G remains the shortest in G 0 if the are not affected b the falt link; otherise, the lengths of these paths increase b a predictable ale. Theorem : The extended branch pdate algorithm ensres

6 roting optimalit as long as the path constrcted before the link falt does not contain the falt link; otherise, the increase in the length of the path is pper bonded b d G 0(; ) - d G (; ). Proof: It is clear that a link falt (; ) ill not decrease the distance beteen to nodes. Therefore, a shortest path in G remains the shortest in G 0 if the path does not contain the falt link. On the other hand, if a shortest path in G contains (; ), the falt link ill be replaced b a shortest restoration path beteen and in G 0. Consider a destination hich is a descendant of in the SPT. Sppose the packet to be roted reaches node ithot reaching annodealongtherestorationpath, thenlink is replaced b the restoration path and the length of the path increases b d G 0(; ) - d G (; ). Note that the packet ma exit the restoration path becase a shorter path is fonded to the destination. In this case, the increase in the length of the path ill be less than d G 0(; ) - d G (; ). Sppose the packet reaches node hich is along the restoration path before it reaches node. In this case, the restoration path can be simpl expressed as,,. The packet directl follos the restoration path at node to reach node. Folloing the similar argment in the first case, the increase in the length of the path ill be pper bonded b d G 0(; ) - d G (; ) - d G 0(; ) <d G 0(; ) - d G (; ). Becase information abot the falt link is distribted to nodes along the restoration path, link state information associated ith different nodes are different, i.e., link state information is either pdated inclding the location of the falt link or otdated ithot inclding the location of the falt link. The folloing reslt shos that the roting process is still loop-free een ith inconsistent ies of link states among nodes in a netork. Theorem 2: The extended roting protocol ensres that loop-free roting ill contine after the link falt. Proof: We first reie a concept sed in [7]. A packet at node s is affected b a falt link if its intended destination d is a descendent of the falt link in the SPT rooted at s; otherise, it is n-affected. If there is onl one restoration path for falt link (; ), ithot loss of generalit, e assme that a packet is affected becase of (not ) as shon in Figre 3. The folloing three cases are considered (see Figre 6):. If a packet at node s is n-affected, it ill remain naffected and reach the destination d based on the otdated SPT that is loop-free. 2. If a packet at node s is affected and s is on the restoration path, then as long as the packet is affected, the packet ill sta on the restoration path ntil it becomes A similar reslt is gien in [7]; hoeer the proof in [7] is flaed. s i j d netork Figre 6. Loop-free roting. n-affected at j ( j cold be node ). Path s, j is loop-free. Since the packet is n-affected at j,it remains naffected and eentall reaches its intended destination d. Again, j, d is loop-free. In addition, paths s, j and j, d do not share an intermediate node. Therefore, path s, j, d is loop-free. 3. If a packet at node s is affected bt s is not on the restoration path, then the packet is roted based on the otdated SPT ntil reaching node i (inclding node ) hich is on the restoration path for (; ). Path s, i is clearl loop-free. As long as the packet is affected, the packet ill sta along the restoration path ntil it becomes n-affected at j (i<jand j cold be node ). Path i, j is loop-free. Since the packet is n-affected at j, it remains naffected and eentall reaches its intended destination d. Again, path j, d is loop-free. It is obios that i, j does not share an intermediate node ith either s, i or j, d, since intermediate nodes in i, j belong to the restoration path. We se proof b contradiction to sho that s, i and j, d do not share an intermediate node. Assme that these to paths share node hich has an otdated SPT (see Figre 6). Node in s, i is affected b the falt link hile node in j, d is n-affected. This is a contradiction. Therefore, s, i, j, d is loop-free. To oerlapping or node-disjoint restoration paths ma be constrctedfor a link falt (; ). This occrs in the extended branch pdate algorithm hen bi-directional restoration paths initiated from and do not select the same set of intermediate nodes. After oerlapping or node-disjoint restoration paths are constrcted, there are to restoration paths for each direction. Since each packet ill se at most one restoration path, the same argment sed for the single restoration path case still applies.

7 The next to theorems sho properties related to restoration path(s) constrcted from the extended roting protocol. Theorem 3: The extended roting protocol ensres shortest restoration path(s) from to (from to ). Proof: The reslt applies to complete restoration paths (hich connect and ). Based on the extended branch pdate algorithm, each node i has pdated link state information hen it selects node i+ on the restoration path. When to restoration paths are constrcted, the are done independentl from to end nodes of the falt link. Therefore, each path is a shortest one. If one restoration path from to is constrcted from combining to (sb)paths: one from each end node of the falt link (; ), ithot loss of generalit, e assme that these to paths are merged at node. Clearl, and, are the shortest paths beteen and and beteen and, respectiel.,, is the bi-directional path formed b combining, and, and it is a shortest restoration path beteen and ith being an intermediate node. In addition, there ill be no shorter restoration path beteen and ithot sing as an intermediate node; otherise ill not be selected dring the constrction of,. The case for the restoration path from to can be proed in a similar a. Theorem 4: If a minimal restoration path is sed and a single restoration path is constrcted, the extended roting protocol garantees sch a minimal path and the nmber of nodes along the path corresponds to the minimm nmber of nodes that need to be informed of the link falt. Proof: In [7], it has been proed that the nmber of nodes in a minimal restoration path corresponds to the minimm nmber of nodes that need to be informed of the link falt ithot casing loop-free, i.e., if the nmber of nodes to be informed is less than the minimm nmber, there is alas a set of metrics for the links of the netork that ill case an scheme to create roting loops after the link failre. Let P(; ) and P(; ) be the sections of bi-directional paths beteen and and beteen and, respectiel. We onl need to proe that hen the resltant restoration path is constrcted b combining P(; ) and P(; ) at node, path P(; ; ) has the minimm nmber of nodes. Clearl, both P(; ) and P(; ) contain minimm nmbers of nodes beteen and and beteen and, respectiel. P(; ; ) is the path formed b combining P(; ) and P(; ) and it is a minimm restoration path beteen and ith being an intermediate node. In addition, there ill be no restoration path beteen and ithot sing as an intermediate node that has a feer nmber of nodes; otherise ill not be selected dring the constrction of P(; ). 5 Conclsions In this paper, e hae extended a falt-tolerant link-state roting protocol in the Internet. This approach trades optimalit for lo oerhead. A shortest path is maintained as long as it does not contains a falt link; otherise, the falt link is replaced b a shortest restoration path and the cost increase of the resltant path is pper bonded b the cost difference beteen the shortest restoration path and the falt link. Or approach is based the premises that link falts are bi-directional. Therefore, instead of constrcting to ni-directional restoration paths (one from each end node of a falt link), one bi-directional restoration path is constrcted. Or ftre ork incldes inestigating other possible trade-offs beteen optimalit and lo oerhead. Different trade-offs ill be backed p throgh simlation std. Like Naraez approach the proposed approach can garantee loop-free roting onl hen one failre occrs at a time in the netork. The efficient a of handling mltiple falts still remains an open problem. References [] R. Callon. Use of OSI IS-IS for roting in TCP/IP and dal enironments. RFC-95, Dec. 9, 990. [2] D.E.Comer. Internetorking ith TCP/IP. Prentice Hall, Inc. ol., 2nd Edition, 99. [3] E. W. Dijkstra. A note on to problems in conexion ith graphs. Nmerische Mathematik. ol., 959, pp [4] C.Hitema. Roting in the Internet. Prentice Hall PTR. 2nd Edition, [5] J. McQillan, I. Richer, and E. Rosen. The ne roting algorithm for the ARPANET. IEEE Transactions on Commnications. ol. COM-28, no. 5, Ma 980, pp [6] J. Mo. OSPF ersion 2. Internet Draft, RFC-278, Jl 997. [7] P. Naraez, K.-Y. Si, and H.-Y. Tzeng. Falt-tolerant roting in the internet ithot flooding. in Dependable Netork Compting, Aersk (ed.), Kler Academic Pblishers, 2000, pp [8] R. Perlman. A comparison beteen to roting protocols: OSPF and IS-IS. IEEE Netorks. ol.5, Sept. 99, pp [9] M. Streenstrp. Roting in Commnications Netorks. Prentice Hall. 995.