Optimizing route-cache lifetime in ad hoc networks

On-demand routing reduces the control overhead in mobile ad hoc networks, but it has the major drawback of introducing latency between route-request arrival and the determination of a valid route. This paper addresses the issue of minimizing the delay in on-demand routing protocols through optimizing the Time-to-Live (TTL) interval for route caching. An analytical framework is introduced to compute the expected routing delay when a source node or an intermediate node has a cached route with any given TTL value. Furthermore, numerical methods are proposed to determine the optimal TTL of a newly cached route. We present simulation results that support the validity of our analysis. Using the proposed analytical framework, we study how the routing delay is affected by route length, route-request frequency, and the frequency of topology variation. We show that the proposed optimal route-cache TTL strategy can significantly reduce the routing delay over systems that either does not use route-cache or keeps route-cache indefinitely long. We further show that the performance gain of optimizing the route-cache TTL increases with increasing traffic pattern localization.