Distance Vector and Link State Routing Pragyaditya Das.

Distance Vector Routing Algorithm. This is one of the two major class of intra domain routing algorithms. The other one is Link State Routing. Distance-vector routing protocols use the Bellman–Ford algorithm, Ford–Fulkerson algorithm, or DUAL FSM (in the case of Cisco Systems' protocols) to calculate paths. Definition : A distance vector routing algorithm operates by having each router maintain a table (i.e., a vector) giving the best known distance to each destination and which link to use to get there. These tables are updated by exchanging information with the neighbours. Eventually, every router knows the best link to reach each destination.

It was the original ARPANET routing algorithm and was also used in the Internet under the name RIP(Routing Information Protocol). The router is assumed to know the ‘‘distance’’ to each of its neighbours. If the metric is hops, the distance is just one hop. If the metric is propagation delay, the router can measure it directly with special ECHO packets that the receiver just timestamps and sends back as fast as it can.

Some theory about the Bellman–Ford algorithm. Bellman-Ford algorithm is used to find the shortest path from a single source vertex to all other vertices of the structure in a graph. The graph used is a Weighted Digraph (Basically a Weighted Directed Graph). It is slower than Dijkstra’s Algorithm, but more versatile (Can handle weights with negative weights too). Also called Bellman–Ford–Moore algorithm because Edward F. Moore also published the same algorithm in 1957.

An example :

The cost of immediate nodes:

The “?” are filled with the cheapest path values.

In practice, each node's forwarding table consists of a set of triples of the form: ( Destination, Cost,

The Count-to-Infinity Problem. Networks using distance-vector routing are susceptible to loops and issues with count to infinity. Problems can happen with your routing protocol when a link or a router fails. Basically, unreachable nodes in divided network.

Initially. Without the breakage between R2 and R3. After first iteration. It happens because R2, R3 is no longer connected, so R2 "thinks" it can redirect packages to R3 through R1, which has a path of 2 - so it will get a path of weight 3.

This will continue forever. This is the count-to-infinity problem. Tanenbaum says : Good news travels quickly, bad news travels slowly.

Link State Routing. Distance vector routing was used in the ARPANET until 1979, when it was replaced by link state routing. The primary problem that caused its demise was that the algorithm often took too long to converge after the network topology changed (due to the count-to-infinity problem). Consequently, it was replaced by an entirely new algorithm, now called link state routing. Variants of link state routing called IS-IS and OSPF are the routing algorithms that are most widely used inside large networks and the Internet today. open shortest path first (OSPF) and intermediate system to intermediate system (IS-IS).

This contrasts with distance-vector, which work by having each node share its routing table with its neighbours. In a link-state protocol the only information passed between nodes is connectivity related. Each router must do the following things to make it work: 1. Discover its neighbours and learn their network addresses. 2. Set the distance or cost metric to each of its neighbours. 3. Construct a packet telling all it has just learned. 4. Send this packet to and receive packets from all other routers. 5. Compute the shortest path to every other router. In effect, the complete topology is distributed to every router. Then Dijkstra’s algorithm can be run at each router to find the shortest path to every other router.

Proceeds in two phases: Nodes flood topology in the form of link state packets. Each node computes its own forwarding table by any shortest path finding algorithm.

Phase 1: Topology Flooding. Courtesy : University Of Coursera.

Phase 2 : Route Computation. Courtesy : University Of Coursera

One important terminology: Forwarding Table: This says about the next node in your journey towards the destination node.

Updating after Changes Link Failure and Node Failure: In this case, we remove the old/failed node and flood the Link State Packet and re-compute routes. Nodes adjacent to the failed node will notice the change. There are certain problems we encounter: 1. Detection Delay : Delay in detection of failure, information sent into a Black Hole. 2. Some routers will know about the disruptions before other, causing inconsistency. Tackle this problems: 1. Faster Flooding. 2. Smaller HELLO timers. 3. Faster processors on routers.

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