1. Review: routing algorithms. –Choose the appropriate paths. –Routing algorithms Flooding Shortest path routing (example). –Dijkstra algorithm. –Bellman-Ford algorithm. Shortest path routing algorithm is perfect if we know the precise network topology. –How to maintain the global network state information? This is also an important part of practical routing algorithms.

2. Distance Vector Routing: –used at ARPANET till 1979, still used in WAN(RIP) Each router maintains a routing table, where each node in the network has an entry in the table. Each entry contains two parts, the outgoing line for the destination (node) and the distance (time, cost, whatever) to the destination. Each router knows the distance to its neighbors In a certain period of time, each router sends the table to each of its neighbors. When a router receives a table from each of its neighbors, it updates its routing table, determines the new minimum distance and the outgoing link for the destination.

3. A B C D E A B C D E A (0, -) (8, C) (5, A) (4, E) (3, A) B (4, B) (0, - ) (3, B) (3, C) (3, D) C (2, C) (5, C) (0, - ) (2, C) (2, D) D (5, E) (3, D) (3, B) (0, - ) (1, D) E (2, E) (9, A) (8, A) (3, E) (0, - ) Routing table for node A Routing table for node D. Let dist(B, A) = 4, dist(B, D) = 1, dist (B, C) = 10, what is B’s routing table in the next cycle?

4. The count-to-infinity problem. A B C D E # # # # 1 2 3 4 1 # # # 3 2 3 4 1 2 # # 3 4 3 4 1 2 3 # 5 4 5 4 1 2 3 4 5 6 5 6 7 6 7 6 7 8 7 8 …... Good news propagates fast, bad news propagates slow. Some heuristics (split horizon). But the heuristics do not solve the problem.

5. link state routing ( currently used in the internet, OSPF ): »The distance vector algorithm converges too slow. 1) discover its neighbors 2) measure the distance to each neighbor 3) construct a packet telling all that is learned 4) send the packet to all other nodes -- flooding 5) each node computes the shortest path to all other nodes -- Dijsktra algorithm.

6. A B C D E 4 2 4 1 5 6 2 Link state packets Node A: (A, seq, age, ((B, 4), (C, 4),(E, 2)) Node B: (B, seq, age, ((A, 4), (D, 2), (C, 1)) …. Node E: (E, seq, age, ((A, 2), (D, 5))

7. If everything goes ok, every node will have the whole information of the network in the next step, algorithm converges faster than the distance vector method. The count to infinite problem? Some of current research on the link state algorithm. –Use broadcast tree instead of flooding –Use local update to the fix the routing table instead of running the full shortest path algorithm –When the send the link state packet »periodical or sudden change?