ANTECEDENTES

Temporally Ordered Routing Algorithm (TORA) is a distributed routing protocol based on a link reversal algorithm. It is designed to discover routes on-demand, provide multiple routes to a destination, establish routes quickly, and minimize communication overhead by localizing the reaction to topological changes when possible. Route optimality (the shortest-path routing) is considered of secondary importance, and longer routes are often used to avoid the overhead of discovering newer routes. It is also not necessary (nor desirable) to maintain routes between every source/destination pair at all times.

The actions taken by TORA can be described in terms of water flowing downhill toward a destination node through a network of tubes that model the routing state of the network. The tubes represent links between nodes in the network, the junctions of the tubes represent the nodes, and the water in the tubes represents the packets flowing toward the destination. Each node has a height with respect to the destination that is computed by the routing protocol. If a tube between two nodes becomes blocked such that water can no longer flow through it, the height of the nodes is set to a height greater than that of any neighboring nodes, such that water will now flow back out of the blocked tube and find an alternate path to the destination.

At each node in the network, a logically separate copy of TORA is run for each destination. When a node needs a route to a particular destination, it broadcasts a route query packet containing the address of the destination. This packet propagates through the network until it reaches either the destination or an intermediate node having a route to the destination. The recipient of the query packet then broadcasts an update packet listing its height with respect to the destination (if the recipient is the destination, this height is 0). As this packet propagates back through the network, each node that receives the update sets its height to a value greater than the height of the neighbor from which the update was received. This has the effect of creating a series of directed links from the original sender of the query to the node that initially generated the update.

When a node discovers that a route to a destination is no longer valid, it adjusts its height so that it is at a local maximum with respect to its neighbors and transmits an update packet.

When a node detects a network partition, where a part of the network is physically separated from the destination, the node generates a clear packet that resets the routing state and removes invalid routes from the network.

TORA is a routing layer above network level protocol called theInternet Mobile Ad hoc Networking(MANET)Encapsulation Protocol (IMEP). IMEP is designed to support the operation of many routing algorithms, network control protocols, or other upper layer pro- tocols intended for use in mobilead hocnetworks. The protocol incorporates mechanisms for supporting link status and neighbor connectivity sensing, control packet aggregation and encapsulation, one-hop neighbor broadcast reliability, multipoint relaying, network- layer address resolution, and provides hooks for interrouter authentication procedures.

In TORA, each node must maintain a structure describing the node’s height as well as the status of all connected links per connection supported by the network. Each node must also be in constant coordination with neighboring nodes in order to detect topology changes and converge. As was found with DSDV, routing loops can occur while the network is reacting to a change in topology.

TORA is designed to carry IP traffic over wireless links in anad hoc network. On the basis of simulation results by Park and Corson, it is best suited to large, densely packed arrays of nodes with very low node mobility. Brochet al. simulated node mobility and found TORA to be encumbered by its layering on top of IMEP and that IMEP caused considerable congestion when TORA was trying to converge in response to node mobility. This resulted in TORA requiring between one to two orders of magnitude more routing overhead than otherad hoc routing protocols investigated by Broch.

TORA is a highly adaptive, efficient, and scalable distributed routing algorithm based on the concept of link reversal. TORA is proposed for highly dynamic mobile, multihop wireless networks. It is a source-initiated on-demand routing protocol. It finds multiple routes from a source node to a destination node. The main feature of TORA is that the control messages are localized to a very small set of nodes near the occurrence of a topological change. To achieve this, the nodes maintain routing information about adjacent nodes. The protocol has three basic functions: route creation, route maintenance, and route erasure.

Each node has a quintuple associated with it: 1. Logical time of a link failure

2. The unique ID of the node that defined the new reference level 3. A reflection indicator bit

4. A propagation ordering parameter 5. The unique ID of the node.

The first three elements collectively represent the reference level. A new reference level is defined each time a node loses its last downstream link due to a link failure. The last two values define a delta with respect to the reference level.

Route creation is done using Query (QRY) and Update (UPD) packets. The route creation algorithm starts with the height (propagation ordering parameter in the quintuple)

ON-DEMAND ROUTING PROTOCOLS 171

of destination set to 0 and all other node’s height set to NULL (i.e., undefined). The source broadcasts a QRY packet with the destination node’s identifier in it. A node with a non- NULL height responds with a UPD packet that has its height in it. A node receiving a UPD packet sets its height to one more than that of the node that generated the UPD. A node with higher height is considered upstream and a node with lower height is considered downstream. In this way, a Directed Acyclic Graph (DAG) is constructed from the source to the destination.

When a node moves, the DAG route is broken, and route maintenance is needed to reestablish a DAG for the same destination. When the last downstream link of a node fails, it generates a new reference level. This results in the propagation of that reference level by neighboring nodes. Links are reversed to reflect the change in adapting to the new reference level. This has the same effect as reversing the direction of one or more links when a node has no downstream links.

In the route erasure phase, TORA floods a broadcast clear packet (CLR) throughout the network to erase invalid routes.

In TORA, there is a potential for oscillations to occur, especially when multiple sets of coordinating nodes are concurrently detecting partitions, erasing routes, and building new routes based on each other. Because TORA uses internodal coordination, its instability problem is similar to the count-to-infinity problem in distance-vector routing protocols, except that such oscillations are temporary and route convergence will ultimately occur.