17.1.5.3 TORA

Tora is a distributed routing protocol based on "link reversal" algorithm. At every node a separate copy of TORA is run for every destination. When a node needs a route to a given destination it broadcasts a QUERY message containing the address of the destination for which it requires a route. This packet travels through the network until it reaches the destination or an intermediate node that has a route to the destination node. This recepient node node then broadcasts an UPDATE packet listing its height wrt the destination. As this node propagates through the network each node updates its height to a value greater than the height of the neighbour from which it receives the UPDATE. This results in a series of directed links from the node that originated the QUERY to the destination node. If a node discovers a particular destination to be unreachable it sets a local maximum value of height for that destination. Incase the node cannot find any neighbour having finite height wrt this destination it attempts to find a new route. In case of network partition, the node broadcasts a CLEAR message that resets all routing states and removes invalid routes from the network.

TORA operates on top of IMEP (Internet MANET Encapsulation Protocol) that provides reliable delivery of route-messages and informs the routing protocol of any changes of the links to its neighbours. IMEP tries to aggregate IMEP and TORA messages into a single packet (called block) in order to reduce overhead. For link-status sensing and maintaining a list of neighbour nodes, IMEP sends out periodic BEACON messages which is answered by each node that hears it by a HELLO reply message. See ns/tora directory and ns/tcl/mobility/tora.tcl for implementation of tora in ns.