Routing

The Internet is constantly changing and growing. New networks are continually added, and links between networks are added and removed, fail and come back. It is the job of routing to determine the best path to the destination, and to create a routing table listing the best path for all the different destinations.

Static routing is the term used when the routing table is created by manual configuration. This is sometimes convenient for small networks but can easily become very difficult and error prone for large networks. Worse, if the best path to a network becomes unusable because of equipment failure or other reasons, static routing will not make use of the next best path.

Dynamic routing is a method in which network elements, in particular routers, exchange information about their state and the state of their neighbours in the network, and then use this information to automatically pick the best path and create the routing table. If something changes, such as a router failing or a new router being put into service, then the dynamic routing protocols make adjustments to the routing table. The system of packet exchanges and decision making is known as a routing protocol. There are many routing protocols that are used in the Internet today, including OSPF, BGP, RIP, and EIGRP.

Wireless networks are like wired networks in that they need dynamic routing protocols, but they are also different enough from wired networks that they need different routing protocols. In particular, wired network connections typically work well or don't work at all (eg., either an Ethernet cable is plugged in, or it isn't). Things are not so clear when working with wireless networks. Wireless communication can be affected by objects moving into the path of the signal, or by interfering signals. Consequently, links may work well, or poorly, or vary between the two extremes. Since existing network protocols don't take the quality of a link into account when making routing decisions, the IEEE 802.11 committees and the IETF are working on standardizing protocols for wireless networks. It is currently unclear when a single standard for dealing with variable link quality will emerge.

In the meantime, there are many ongoing ad-hoc programming attempts to address the problem. Some examples include Hazy Sighted Link State (HSLS), Ad-hoc On-demand Distance Vector (AODV), and Optimized Link State Routing (OLSR). Another is SrcRR, a combination of DSR and ETX implemented by the M.I.T. Roofnet project. Later in this chapter we will see an example of how to implement a network using OLSR to make routing decisions.