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Dijkstra’s algorithm with a link failure The routers in the network on the top left use Dijkstra’s Algo- rithm to find the shortest path towards destination d .
Distance Vector Routing Based on Bellman-Ford Equation Define dx(y) := cost of least-cost path from x to y c(x,v) := cost of direct link from x to v Then, for all v that are neighbours of x dx(y) = min v {c(x,v) + dv(y) } y x v c(x,v) dx(y) dv(y)
Alyssa P. Hacker manages MIT's internal network that runs link-state routing. She wants to experiment with a few possible routing strategies. Of all possible paths available to a particular destination at a node, a routing strategy specifies the path that must be picked to create a routing table entry.
CSE 123 – Lecture 12: Link-state Routing 24 Getting consistent routing information to all nodes E.g., all nodes having the same link-state database
Link-state Routing! Two phases u Reliable flooding »Tell all routers what you know about your local topology u Path calculation (Dijkstra’s algorithm) »Each router computes best path over complete network! Motivation u Global information allows optimal route computation u Straightforward to implement and verify CSE 123–Lecture 14: Link ...
Router Tasks Forwarding u Move packet from input link to the appropriate output link u Purely local computation u Must go be very fast (executed for every packet) Routing u Make sure that the next hop actually leadsto the destination u Global decisions; distributed computation and communication u Can go slower (only important when topology changes) CSE 123 – Lecture 15: Link-State Routing 3
Link State Approach to Routing •Shortest paths in graph: classic theory problem •Classic centralized single-source shortest paths algorithm: Dijkstra’s Algorithm –requires map of entire network •Link State (LS) Routing: –push network map to every router –each router learns link state database –each router runs Dijkstra’s locally
