Presentation on theme: "The Wireless Routing Protocol (WRP) Invented by J.J.Garcia Luna Aceves and S. Murthy Belongs to a general class of algorithms called Path Finding Algorithms."— Presentation transcript:
The Wireless Routing Protocol (WRP) Invented by J.J.Garcia Luna Aceves and S. Murthy Belongs to a general class of algorithms called Path Finding Algorithms (PFA). Use information with respect to the length of and the second-to-last-hop on the shortest path.
Path Finding Algorithm Each node maintains the shortest path spanning tree maintained by each of its neighbors. Using this, it creates its own shortest path spanning tree. Note: Shortest path Shortest Weighted Path Periodically, each node broadcasts an update (could be null: why?) that reports updates to its spanning tree. Each update contains : The destination address Distance to that destination An identifier of the node that is at the second-to-last-hop on the shortest path to that destination.
WRP : Nitty Gritty Details Uses the generic ideas in PFA Reduces the number of temporary routing loops. Let us call second-to-last-hop node as the predecessor node. When a node receives an update from a neighbor it checks the consistency of the predecessor information reported by all its neighbors. The spanning tree also gives info with regards to the next hop neighbor towards that destination. We call this next hop neighbor the successor node.
In WRP, each node maintains four tables: Distance Table Routing Table Link-Cost Table Message Retransmission List (MRL)
Distance Table Let us consider a particular node A. In this table, for each destination K, node A maintains the distance to K via each of its neighbors (say B) D A KB, and the predecessor node P A KB, along that path. This table is used in the construction of the minimum spanning tree.
Routing Table Aided by the spanning tree formed: thanks to the distance table. For each destination Node K, it contains + The address of Node K. + The distance to Node K. + The successor on the shortest path to Node K. + The predecessor on this path to Node K. May also contain a flag to indicate whether the path is correct, or erroneous (no path to destination).
Link-Cost Table Tabulates the cost of transmitting through each neighbor. Note : Cost depends on link characteristics – need not be hop count. Also contains for each neighbor B, the time that has elapsed since last receiving an error free message from B.
Message Re-transmission List Contains info with regards to each transmitted update by the node (say node A). Each update message has a seq. no. that is tabulated in this table. Also contains a retransmission counter and ACK required flag vector. The flag vector indicates the neighbors that responded to the update. Help eliminate redundant update transmissions.
WRP: An Example Let cost of Link 2 be 10. DestCostPredSucc A0AA B1AB C2BB D3CB E3CB X2BB ROUTING TABLE AT A
Now Let Link 1 fail. Node A will notice this failure. Let us consider a single destination – say Node X. Node A will set the distance to X to infinity and its predecessor and successor values to null. This is broadcast to Node C. Node C computes the alternate route to A which is by means of Link 2. This info is then transmitted to A. A then realizes (by means of Cs new spanning tree) that it can reach other nodes via C.
Node As new routing table would now look like: DestCostPredSucc A0AA B11CC C10AC D11CC E CC X12BC
Advantages/ Limitations Advantages: No Loops Lower number of updates upon link failure – reports sent only to neighbors. Overhead grows as O(n) – n is the number of nodes. Disadvantages Messages may be large. Maintenance of four tables. Hello packets required – cannot go into sleep mode – overhead. Scalability still an issue !
Why all this ? Wireless is tough !!!
Reference E.M. Royer and C-K.Toh, A Review of Current Routing Protocols for Ad Hoc Wireless Networks, IEEE Personal Communications Magazine, pp 46-55, April S.Murthy and J.J.Garcia-Luna-Aceves, An Efficient Routing Protocol for Wireless Networks, ACM Mobile Networks and Applications Journal, pp , October
Link State Routing Distance Vector Routing Protocols are attractive because of their simplicity. Another class of protocols that are proactive (exchange control messages to form and maintain routing tables even before they are required) are link state routing protocols. Popular Link State Routing Protocol -- OSPF Require each and every node in the network to have a knowledge of the complete topology of the network. A local computation of the best path to each destination is made.
A E B C D Link 1 Link 6 Link 2 Link 4 Link 3 Link 5 FrmToLinkCost AB11 AC21 BA11 BC31 CA21 CB31 CD41 CE51 DC41 DE61 EC51 ED61 The Network is now depicted by the Map shown as
A particular node is responsible for each entry of this table. It has to update this periodically. Update is to be relayed to all nodes in the network. The entry termed as cost is also often referred to as state. Once each node has the entire topology map, it tries to compute the shortest path to each destination. Can you think of one popular algorithm that is used for achieving this ?
YES! It is the Dijkstras Algorithm. Each node computes shortest weighted path tree using this topology information; it places itself at the root of this tree. Reference : Introduction to Algorithms by Cormen, Leiserson and Rivest.
ADVANTAGES: Each node can do a local loop free computation. Can reflect appropriate metrics to reflect the state of a link. DISADVANTAGES Not scalable Each link state update has to be flood to every node in the network – flooding is expensive – especially in an ad hoc network where link-state changes are frequent. Note: Sequence numbers are required to ensure that flooding does not continue for ever !