Intra-domain Routing

This posts talks about Intra-domain Routing protocols, used to communicate two computers in Internet. Mainly, there are three routing strategies:

• Source Routing: source keeps a cache with the whole path towards each destination.
• Shortest path Routing: the source finds the next neighbor and so on until the destination is reached.
• Delivery semantic: defines the set of nodes that will receive the packet.

On the actual Internet, the number of nodes in the net is the order of millions, so source and short path routing are not practical. The strategy used is hierarchical routing, which is based in organizations that design networks to provide services. Inter-domain routing are those routing mechanisms that connect different organizations, and Intra-domain routing are the ones that are internally run by run organization.

To determinate the path which may forward a datagram, it must be used information introduced by networks administrators (static information), or from automatic information received by the routers (dynamic), i.e. hops, delay, bandwidth, load… This information exchanged by the routers is particular to any protocol, but in general is defined format and content of routing packets, periodicity of the packet exchange and associated algorithms that allow calculating the best path.

All this exchanged information is susceptible to topological changes, where routers should re-calculate routes and updating routing tables, so the shorter the time, called convergence time, which all routers reach a common knowledge of the net, fewer packets will be lost.

The Router Architecture is based on three planes: the Management plane offers an API to configure any feature offered by the router. The Control plane, where any protocol or drawing network maps are located. Finally, the Forwarding plane is the part of the router that decides what to do with the incoming data packets.

There are many ways to classify routing protocols, the first one could be into static; those are the ones that network administrators manually set the routing entries, or dynamic, which set automatically the routing table. Dynamic protocols are used in medium-large networks, and are classified in three groups:

• Vector-distance protocols: they determine direction and distance towards any subnet in the network.
• Link-state protocols: they use the network topology.
• Hybrids: a combination of both.

Another way to classify routing protocols is based on the advertisement of masks. Class-full routing does not advertise the mask, so subnetting is not allowed. In the other hand, Class-less routing advertise the mask, and subnetting is allowed using VLSM.