What is routing? Determining the best path to reach a destination. Routing protocols form the control plane of the network layer, and responsible for coming up with a set of routes for any destination, and for picking the best route.
Any routing protocol roughly consists of two phases: route advertisements using which information about destinations is exchanged via messages, and best route computation where a best route is chosen to be installed into the forwarding table.
What are the criteria to pick a path? Usually, intra-domain routing protocols pick the shortest cost path. Every link has a metric, and the path that minimizes the sum of all link metrics is chosen as best path. How are metrics chosen? Can be simple hop count, based on bandwidth, latency, physical distance etc. That is, based on desirability of using the link. Inter-domain routing protocols may use other criteria like policy. Network operators may also wish to balance load across links (traffic engineering). But usually, the simplest way to pick path is shortest path or least cost path.
Load-sensitive vs load-insensitive routing: most routing protocols do not pick best path based on congestion, as it can lead to oscillations and instability. That is, link metrics do not depend on current load level. For example, if some link is loaded and its metric reduces, all flows may move away from it, leading to load on another link, and all flows will move back again.
Static vs dynamic routing: for small networks and/or networks that rarely change, manually setting the route once is sufficient. This is called static routing. For more complicated networks, dynamic routing protocols are used, that adapt routes in response to changes in network topology. We will study two classes of dynamic routing protocols: link state (LS) routing protocols, and distance vector (DV) routing protocols.
Link state routing: “tell about your neighbors to everyone”. Each node collects information about all its neighbors and the link metrics. This LSA (link state announcement) of every node is flooded through the entire network. So, at the end of it, each node has complete view of the network graph. Each node then independently runs Dijkstra’s shortest path algorithm to get the shortest path to every destination, based on which it figures out the next hop for every destination.
Distance vector routing: “tell about everyone to your neighbors”. Every node exchanges a distance vector (a vector containing its estimate of distance to each destination) with its neighbors. Upon receiving a neighbor’s distance vector, a node updates its distance vector by adding its link cost to neighbor’s distance vector, and if a better path is found through neighbor, it updates its best route. This is called the Bellman-Ford update algorithm.
Distance vector has “count to infinity” problem. Simple solution is “split horizon” or “poisoned reverse”.
OSPF (Open Shortest Paths First) is a popular intra-domain routing protocols that uses simple LS routing logic. RIP (Routing Information Protocol) is a simple DV protocol.
Work out examples of LS and DV routing protocols in action.