Interior Gateway Protocols (RIP and OSPF)

Transcription

1 Interior Gateway Protocols (RIP, OSPF) Interior Gateway Protocols (RIP and OSPF) What is Routing? To ensure information is delivered to the correct destination at a reasonable level of performance Forwarding Given a forwarding table, move information from input ports to output ports of a router Local mechanical operations Routing cquires information in the forwarding tables Requires knowledge of the network Requires distributed coordination of routers

2 Viewing Routing as a Policy Given multiple alternative paths, how to route information to destinations should be viewed as a policy decision What are some possible policies? Shortest path (RIP, OSPF) Most load-balanced QoS routing (satisfies app requirements) etc E F Internet Routing Internet topology roughly organized as a two level hierarchy First lower level autonomous systems (S s) S: region of network under a single administrative domain Each S runs an intra-domain routing protocol istance Vector, e.g., Routing Information Protocol (RIP) Link State, e.g., Open Shortest Path First (OSPF) Possibly others Second level inter-connected S s etween S s runs inter-domain routing protocols, e.g., order Gateway Routing (GP) e facto standard today, GP-

3 Example Interior router GP router S- S- S- Why eed the oncept of S or omain? Routing algorithms are not efficient enough to deal with the size of the entire Internet ifferent organizations may want different internal routing policies llow organizations to hide their internal network configurations from outside llow organizations to choose how to route across multiple organizations (GP) asically, easier to compute routes, more flexibility, more autonomy/independence

4 Outline Two intra-domain routing protocols oth try to achieve the shortest path routing policy Quite commonly used OSPF: ased on Link-State routing algorithm RIP: ased on istance-vector routing algorithm Intra-domain Routing Protocols ased on unreliable datagram delivery istance vector Routing Information Protocol (RIP), based on ellman-ford algorithm Each 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 First (OSPF), based on ijkstra s algorithm Each router periodically floods immediate reachability information to other routers Fast convergence, but high communication and computation overhead

5 Routing on a Graph Goal: determine a good path through the network from source to destination Good often means the shortest path etwork modeled as a graph Routers nodes Link edges Edge cost: delay, congestion level, E F Link State Routing (OSPF): Flooding Each node knows its connectivity and cost to a direct neighbor Every node tells every other node this local connectivity/cost information Via flooding In the end, every node learns the complete topology of the network E.g. floods message connected to cost connected to cost connected to cost E F

6 Flooding etails Each node periodically generates Link State Packet (LSP) contains I of node created LSP List of direct neighbors and costs Sequence number (6 bit, assume to never wrap around) Time to live Flood is reliable Use acknowledgement and retransmission Sequence number used to identify *newer* LSP n older LSP is discarded What if a router crash and sequence number reset to 0? Receiving node flood LSP to all its neighbors except the neighbor where the LSP came from LSP is also generated when a link s state changes (failed or restored) Peterson & avie P.8 [the crashed node] will eventually receive a copy of its own LSP with a higher sequence number,

7 Link State Flooding Example Link State Flooding Example 8 6 0

8 Link State Flooding Example Link State Flooding Example 8 6 0

9 Link State Routing lgorithm 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 Repeat for all sources 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 node 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 ijsktra s lgorithm ( Greedy lgorithm) Initialization: S = {}; for all nodes v if v adjacent to then (v) = c(,v); 6 else (v) = ; 8 Loop 9 find w not in S such that (w) is a minimum; 0 add w to S; update (v) for all v adjacent to w and not in S: (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;

10 Example: ijkstra s lgorithm Step 0 start S (),p(), (),p(), (),p(), (E),p(E) (F),p(F) E F Initialization: S = {}; for all nodes v if v adjacent to then (v) = c(,v); 6 else (v) = ; Example: ijkstra s lgorithm Step 0 start S (),p(), E F (),p(),, (),p(), (E),p(E), (F),p(F) 8 Loop 9 find w not in S s.t. (w) is a minimum; 0 add w to S; update (v) for all v adjacent to w and not in S: (v) = min( (v), (w) + c(w,v) ); until all nodes in S;

11 Example: ijkstra s lgorithm Step 0 start S E (),p(), (),p(),,,e (),p(), (E),p(E), (F),p(F),E E F 8 Loop 9 find w not in S s.t. (w) is a minimum; 0 add w to S; update (v) for all v adjacent to w and not in S: (v) = min( (v), (w) + c(w,v) ); until all nodes in S; Example: ijkstra s lgorithm Step 0 start S E E (),p(), (),p(),,,e (),p(), (E),p(E), (F),p(F),E E F 8 Loop 9 find w not in S s.t. (w) is a minimum; 0 add w to S; update (v) for all v adjacent to w and not in S: (v) = min( (v), (w) + c(w,v) ); until all nodes in S;

12 Example: ijkstra s lgorithm Step 0 start S E E E (),p(), (),p(),,,e (),p(), (E),p(E), (F),p(F),E E F 8 Loop 9 find w not in S s.t. (w) is a minimum; 0 add w to S; update (v) for all v adjacent to w and not in S: (v) = min( (v), (w) + c(w,v) ); until all nodes in S; Example: ijkstra s lgorithm Step 0 start S E E E EF (),p(), E F (),p(),,,e (),p(), (E),p(E), (F),p(F),E 8 Loop 9 find w not in S s.t. (w) is a minimum; 0 add w to S; update (v) for all v adjacent to w and not in S: (v) = min( (v), (w) + c(w,v) ); until all nodes in S;

13 istance Vector Routing (RIP) What is a distance vector? urrent best known cost to get to a destination Idea: Exchange distance vectors among neighbors to learn about lowest cost paths ode est. E F G ost ote no vector entry for itself t the beginning, distance vector only has information about directly attached neighbors, all other dests have cost Eventually the vector is filled istance Vector Routing lgorithm Iterative: continues until no nodes exchange info synchronous: nodes need not exchange info/iterate in lock steps istributed: each node communicates only with directlyattached neighbors Each router maintains Row for each possible destination olumn for each directly-attached neighbor to node Entry in row Y and column Z of node X best known distance from X to Y, via Z as next hop ote: for simplicity in this lecture examples we show only the shortest distances to each destination

14 istance Vector Routing Each local iteration caused by: Local link cost change Message from neighbor: its least cost path change from neighbor to destination Each node notifies neighbors only when its least cost path to any destination changes eighbors then notify their neighbors if necessary Each 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: 8 wait (until sees a link cost change to neighbor V 9 or until receives update from neighbor V) 0 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) 6 send (, Y) to all neighbors forever

15 Example: istance Vector lgorithm ode ode est. ost exthop - est. ost exthop Initialization: for all neighbors V do if V adjacent to (, V) = c(,v); else 6 (, V) = ; ode est. ost exthop ode est. ost exthop - Example: st Iteration ( ) loop: else if (update (V, Y) received from V) (,Y) = (,V) + (V, Y); if (there is a new min. for destination Y) 6 send (, Y) to all neighbors forever ode est. ode est. ost 8 ost exthop exthop ode est. ((,), (,), (,)) ode est. ost ost exthop (, ) = (, ) + (,) = + = 8 exthop -

16 Example: st Iteration (, ) ode est. ost exthop ode est. ost exthop (,) = (,) + (,) = + = (,) = (,) + (,) = + = loop: else if (update (V, Y) received from V) (,Y) = min((,v), (,V) + (V, Y)) if (there is a new min. for destination Y) 6 send (, Y) to all neighbors forever ode est. ost exthop ode est. ost exthop - Example: End of st Iteration loop: else if (update (V, Y) received from V) (,Y) = (,V) + (V, Y); if (there is a new min. for destination Y) 6 send (, Y) to all neighbors forever ode est. ode est. ost ost exthop exthop ode est. ost ode est. ost exthop exthop

17 Example: End of nd Iteration loop: else if (update (V, Y) received from V) (,Y) = (,V) + (V, Y); if (there is a new min. for destination Y) 6 send (, Y) to all neighbors forever ode est. ost ode est. ost exthop exthop ode est. ost ode est. ost exthop exthop Example: End of rd Iteration ode est. loop: else if (update (V, Y) received from V) (,Y) = (,V) + (V, Y); if (there is a new min. for destination Y) 6 send (, Y) to all neighbors forever ode ost exthop ode est. ost ode exthop est. ost exthop est. ost exthop othing changes algorithm terminates

18 istance Vector: Link ost hanges loop: 8 wait (until sees a link cost change to neighbor V 9 or until receives update from neighbor V) 0 if ((,V) changes by d) for all destinations 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) 6 send (, Y) to all neighbors forever 0 ode ode good news travels fast Link cost changes here time lgorithm terminates istance Vector: ount to Infinity Problem loop: 8 wait (until sees a link cost change to neighbor V 9 or until receives update from neighbor V) 0 if ((,V) changes by d) for all destinations 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) 6 send (, Y) to all neighbors forever 60 0 ode ode Link cost changes here; recall that also maintains shortest distance to through, which is 6. Thus (, ) becomes 6! bad news travels slowly time

19 istance Vector: Poisoned Reverse If routes through to get to : - tells its ( s) distance to is infinite (so won t route to via ) - Will this completely solve count to infinity problem? 60 0 ode ode time Link cost changes here; updates (, ) = 60 as has advertised (, ) = lgorithm terminates Link State vs. istance Vector Per node message complexity LS: O(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 time varies may be routing loops count-to-infinity problem Robustness: what happens if router malfunctions? LS: node can advertise incorrect link cost V: each node computes only its own table node can advertise incorrect path cost each node s table used by others; error propagate through network

20 Oscillations ssume link cost = amount of carried traffic +e e e initially +e 0 0 +e 0 recompute routing 0 +e 0 0 +e recompute +e 0 0 +e e recompute How can you avoid oscillations? Reference T. S. Eugene g Slides on Interior Gateway Protocols, Rice University