IP Header. Link State and Distance Vector Routing. Internet Routing. What is Routing? Intra-domain Routing Protocols. Example

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1 IP Header Link State and istance Vector Routing S 9 omputer etworks Z. Morley Mao Tuesday Sept, cknowledgement: Some slides adapted from Kurose&Ross and Katz&Stoica Vers: IP versions HL: Header length (in - bits) Type: Type of service Length: size of datagram (header + data; in bytes) Identification: fragment I ragment offset: offset of current fragment (x bytes) TTL: number of network hops Protocol: protocol type (e.g., TP, UP) Source IP addresses estination IP address What is Routing? Internet Routing Routing is the core function of a network It ensures that information accepted for transfer at a source node is delivered to the correct set of destination nodes, at reasonable levels of performance. Internet organized as a two level hierarchy irst level autonomous systems (S s) - S region of network under a single administrative domain S s run an intra-domain routing protocols - istance Vector, e.g., Routing Information Protocol (RIP) - Link State, e.g., Open Shortest Path irst (OSP) etween S s runs inter-domain routing protocols, e.g., order Gateway Routing (GP) - e facto standard today, GP- xample Intra-domain Routing Protocols S- S- S- Interior router GP router ased on unreliable datagram delivery istance vector - Routing Information Protocol (RIP), based on ellman-ord - ach neighbor periodically exchange reachability information to its neighbors - Minimal communication overhead, but it takes long to converge, i.e., in proportion to the maximum path length Link state - Open Shortest Path irst (OSP), based on ijkstra - ach network periodically floods immediate reachability information to other routers - ast convergence, but high communication and computation overhead

2 Routing Interplay between routing and forwarding Goal: determine a good path through the network from source to destination - Good means usually the shortest path etwork modeled as a graph - Routers nodes - Link edges dge cost: delay, congestion level, value in arriving packet s header routing algorithm local forwarding table header value output link u Graph: G = (,) Graph abstraction v w z x y = set of routers = { u, v, w, x, y, z } = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) } Remark: Graph abstraction is useful in other network contexts xample: PP, where is set of peers and is set of TP connections u v x Graph abstraction: costs w y z c(x,x ) = cost of link (x,x ) - e.g., c(w,z) = cost could always be, or inversely related to bandwidth, or inversely related to congestion ost of path (x, x, x,, x p ) = c(x,x ) + c(x,x ) + + c(x p-,x p ) Question: What s the least-cost path between u and z? Routing algorithm: algorithm that finds least-cost path 9 Routing lgorithm classification Outline Global or decentralized information? Global: all routers have complete topology, link cost info link state algorithms ecentralized: router knows physicallyconnected neighbors, link costs to neighbors iterative process of computation, exchange of info with neighbors distance vector algorithms Static or dynamic? Static: routes change slowly over ynamic: routes change more quickly - periodic update - in response to link cost changes Link State istance Vector

3 Link State Routing lgorithm Link State looding xample ijkstra s algorithm et topology, link costs known to all nodes - ccomplished via link state flooding - ll nodes have same info ompute least cost paths from one node ( source ) to all other nodes Iterative: after k iterations, know least cost paths to k closest destinations otations c(i,j): link cost from node i to j; cost infinite if not direct neighbors (v): current value of cost of path from source to destination v p(v): predecessor node along path from source to v, that is next to v S: set of nodes whose least cost path definitively known Link State looding xample Link State looding xample Link State looding xample ijsktra s lgorithm Initialization: S = {}; for all nodes v if v adjacent to then (v) = c(,v); else (v) = ; 9 find w not in S such that (w) is a minimum; add w to S; update (v) for all v adjacent (v) = min( (v), (w) + c(w,v) ); // new cost to v is either old cost to v or known // shortest path cost to w plus cost from w to v until all nodes in S; otations c(i,j): link cost from node i to j; cost infinite if not direct neighbors (v): current value of cost of path from source to destination v p(v): predecessor node along path from source to v, that is next to v S: set of nodes whose least cost path definitively known

4 xample: ijkstra s lgorithm xample: ijkstra s lgorithm (),p(), (),p() (),p(),, (),p() (),p() Initialization: S = {}; for all nodes v if v adjacent to then (v) = c(,v); else (v) = ; (),p(), (),p() (),p(),,, (),p() (),p(), 9 find w not in S s.t. (w) is a minimum; add w to S; update (v) for all v adjacent (v) = min( (v), (w) + c(w,v) ); until all nodes in S; 9 xample: ijkstra s lgorithm xample: ijkstra s lgorithm (),p(), (),p() (),p(),,,, (),p() (),p(),, 9 find w not in S s.t. (w) is a minimum; add w to S; update (v) for all v adjacent (v) = min( (v), (w) + c(w,v) ); until all nodes in S; (),p(), (),p() (),p(),,,, (),p() (),p(),, 9 find w not in S s.t. (w) is a minimum; add w to S; update (v) for all v adjacent (v) = min( (v), (w) + c(w,v) ); until all nodes in S; xample: ijkstra s lgorithm xample: ijkstra s lgorithm (),p(), (),p() (),p(),,,, (),p() (),p(),, 9 find w not in S s.t. (w) is a minimum; add w to S; update (v) for all v adjacent (v) = min( (v), (w) + c(w,v) ); until all nodes in S; (),p(), (),p() (),p(),,,, (),p() (),p(),, 9 find w not in S s.t. (w) is a minimum; add w to S; update (v) for all v adjacent (v) = min( (v), (w) + c(w,v) ); until all nodes in S;

5 omplexity ssume a network consisting of n nodes - ach iteration: need to check all nodes, w, not in S - n*(n+)/ comparisons: O(n ) - More efficient implementations possible: O(n*log(n)) Oscillations ssume link cost = amount of carried traffic +e e e initially +e +e recompute routing +e +e recompute +e +e e recompute How can you avoid oscillations? Outline istance Vector Routing lgorithm Link State istance Vector Iterative: continues until no nodes exchange info synchronous: nodes need not exchange info/iterate in lock steps istributed: each node communicates only with directly - attached neighbors ach router maintains - Row for each possible destination - olumn for each directly-attached neighbor to node - ntry in row Y and column Z of node X best known distance from X to Y, via Z as next hop (remember this!) ote: for simplicity in this lecture examples we show only the shortest distances to each destination istance Vector Routing ach local iteration caused by: - Local link cost change - Message from neighbor: its least cost path change from neighbor to destination ach node notifies neighbors only when its least cost path to any destination changes - eighbors then notify their neighbors if necessary ach node: wait for (change in local link cost or msg from neighbor) recompute distance table if least cost path to any dest has changed, notify neighbors istance Vector lgorithm (cont d) Initialization: for all neighbors V do if V adjacent to (, V) = c(,v); else (, V) = ; loop: wait (until sees a link cost change to neighbor V 9 or until receives update from neighbor V) if ((,V) changes by d) for all destinations Y through V do (,Y) = (,Y) + d else if (update (V, Y) received from V) /* shortest path from V to some Y has changed */ (,Y) = (,V) + (V, Y); if (there is a new minimum for destination Y) send (, Y) to all neighbors forever 9

6 xample: istance Vector lgorithm xample: st Iteration ( ) ode est. ost - ode est. ost ode est. ost ode est. ost (, ) = (, ) + (,) = + = Initialization: for all neighbors V do if V adjacent to (, V) = c(,v); else (, V) = ; ode est. ost ode est. ost - loop: else if (update ( V, Y) received from V) (,Y) = (,V) + (V, Y); if (there is a new min. for destination Y) send (, Y) to all neighbors forever ((,), (,), (,)) ode ode est. ost est. ost - xample: st Iteration (, ) xample: nd of st Iteration ode est. ost ode est. ost ode est. ost ode est. ost (,) = (,) + (,) = + = (,) = (,) + (,) = + = loop: else if (update ( V, Y) received from V) (,Y) = min((,v), (,V) + ( V, Y)) if (there is a new min. for destination Y) send (, Y) to all neighbors forever ode est. ost ode est. ost - loop: else if (update ( V, Y) received from V) (,Y) = (,V) + (V, Y); if (there is a new min. for destination Y) send (, Y) to all neighbors forever ode est. ost ode est. ost xample: nd of nd Iteration xample: nd of rd Iteration ode est. ost ode est. ost ode est. ost ode est. ost loop: else if (update ( V, Y) received from V) (,Y) = (,V) + (V, Y); if (there is a new min. for destination Y) send (, Y) to all neighbors forever ode est. ost ode est. ost loop: else if (update ( V, Y) received from V) (,Y) = (,V) + (V, Y); if (there is a new min. for destination Y) send (, Y) to all neighbors forever ode est. ost ode est. ost othing changes algorithm terminates

7 istance Vector: Link ost hanges istance Vector: ount to Infinity Problem loop: wait (until sees a link cost change to neighbor V 9 or until receives update from neighbor V) if ((, V) changes by d) for alldestinations Y through V do (,Y) = (,Y) + d else if (update ( V, Y) received from V) (,Y) = (,V) + (V, Y); if (there is a new minimum for destination Y) send (, Y) to all neighbors forever loop: wait (until sees a link cost change to neighbor V 9 or until receives update from neighbor V) if ((, V) changes by d) for alldestinations Y through V do (,Y) = (,Y) + d ; else if (update ( V, Y) received from V) (,Y) = (,V) + (V, Y); if (there is a new minimum for destination Y) send (, Y) to all neighbors forever ode ode good news travels fast ode ode bad news travels slowly Link cost changes here Link cost changes here; recall that also maintains lgorithm terminates shortest distance to through, which is. Thus (, ) becomes! istance Vector: Poisoned Reverse Look at this example If routes through to get to : ode ode - tells its ( s) distance to is infinite (so won t route to via ) - Will this completely solve count to infinity problem? Link cost changes here; updates (, ) = as has advertised (, ) = lgorithm terminates 9 Initially, both and have a distance to, has a distance ssume goes down Using split horizon, and tell that they cannot get to concludes is unreachable, reports to and hears that has a path of length to concludes it can get to via in hops and set their distance to to ount to infinity???? Link State vs. istance Vector Per node message complexity LS: O(n*e) messages; n number of nodes; e number of edges V: O(d) messages; where d is node s degree omplexity LS: O(n ) with O(n*e) messages V: convergence varies - may be routing loops - count-to-infinity problem Robustness: what happens if router malfunctions? LS: - node can advertise incorrect link cost - each node computes only its own table V: - node can advertise incorrect path cost - each node s table used by others; error propagate through network