Summary: Multi-Hop Wireless Ad Hoc Network Routing Protocols

On the surface, Broch et al. merely present a comparison of four ad-hoc multi-hop wireless routing protocols in terms of routing overhead, delivery ratio of packets and path optimality under a simulation of a mobile network.  Pulling this off required a huge quantity of work and amounts to an impressive piece of research.  First the ns event simulator had to be extended to accurately model wireless link conditions including signal attenuation, restricted packet capture conditions, and a MACA/MACAW link layer.  Then each of the competing routing protocols had to be implemented from the spec, and in the process, optimizations were added.

DSDV resembles traditional distance vector routing protocols, but it augmented with a sequence number to allow the rapid replacement of broken routes: a best path sort sorts first by sequence, and among the highest sequences routes choses the one with lowest cost.  TORA is an on-demand protocol that focuses on route correction and low disruptiveness instead of optimality: small route adjustments are made as locally as possible to reduce route discovery overhead.  DSR is a straightforward source routing protocol, and as such is significantly different from the other approaches: packets are routed not via next-hop but following the complete ordered list of hops provided by the client.  Clients learn of routes through route requests, and nodes reply to these requests based on fragments of source routes that they have promiscuously overheard from other senders.  The newcomer AODV is a hybrid: hop-wise routing tables are maintained via on-demand mechanisms.

TORA crumbles under load and DSDV performs predictably OK, as we might have predicted.  DSR does really well almost across the board.  Its overhead (in terms of messages) is very low because learning routes promiscuously is basically free, but overhead in terms of stored state is likely to be high.