An ad hoc routing protocol is a method for sending communications over a network with mobile elements, where access occurs on an ad hoc or “as needed” basis. The fixed routing protocol suitable for a wired network doesn't work for a mobile network, because it can rapidly reconfigure and change. If the network doesn't remain stable as nodes appear and disappear, participants on the network can experience data loss and other problems. For this reason, a better method for communication is necessary.
There are two broad approaches to ad hoc routing protocol design. One is table-driven. The network maintains a table of active nodes which it continuously updates in response to new information. As nodes appear and disappear, the network can calculate the shortest paths for data. As users interact with the network, it consults the table to determine how and where to send their information.
Maintenance of a table-driven network requires data storage capability, and the network sometimes reacts slowly to changes. As nodes drop in and out, the tables may not update in time, and may not be sent out as quickly as nodes need the information. Consequently, sometimes a table-driven ad hoc routing protocol lags, especially when multiple nodes failure or other problems arise. The tables cannot always keep up with changes to the network.
The other option for configuring an ad hoc routing protocol is an on demand system which periodically sends out a wave of route request packets to the nodes in the network. It uses the response to make routing decisions. Such networks can be slow to set up, because the network needs to communicate with the nodes before it can work. A phenomenon known as excess flood can also occur, where the route request packets choke out other data and slow or shut down the network.
Sometimes an ad hoc routing protocol may contain mixtures of both approaches to best suit the needs of the network and users. Such protocols allow networks to reconfigure as nodes discover each other, communicate, and assign priorities on the basis of the responses. A network can quickly set up and break down with this protocol to create access in a variety of locations. Mobile networks can be vulnerable to some types of interference, and the routing protocol may need to include security measures to protect the network and the users. These can include access locks to keep unauthorized devices off the network.