Caching strategies in on-demand routing protocols for wireless ad hoc networks

Abstract

A b s t r a c t An on-demand routing protocol for wireless ad hoc networks is one that searches for and attempts to discover a route to some destination node only when a sending node originates a data packet addressed to that node. In order to avoid the need for such a route discovery to be performed before each data packet is sent, such routing protocols must cache routes previously discovered. This paper presents an analysis of the effects of different design choices for this caching in on-demand routing protocols in wireless ad hoc networks, dividing the problem into choices of cache structure, cache capacity, and cache timeout. Our analysis is based on the Dynamic Source Routing protocol (DSR), which operates entirely on-demand. Using detailed simulations of wireless ad hoc networks of 50 mobile nodes, we studied a large number of different caching algorithms that utilize a range of design choices, and simulated each cache primarily over a set of 50 different movement scenarios drawn from 5 different types of mobility models. We also define a set of new mobility metrics that allow accurate characterization of the relative difficulty that a given movement scenario presents to an ad hoc network routing protocol, and we analyze each mobility metric's ability to predict the actual difficulty in terms of routing overhead experienced by the routing protocol across the scenarios in our study. I n t r o d u c t i o nCaching is an important part of any on-demand routing protocol for wireless ad hoc networks. In an ad hoc network [10, 6], all nodes cooperate in order to dynamically establish and maintain routing in the network, forwarding packets for each other to allow communication between nodes not directly within wireless transmission range. Rather than using the periodic or background exchange of routing information common in most routing protocols, an on-demand routing protocol is one that searches for and attempts to discover a route to some destination node only when a sending node originates a data packet addressed to that node. In order to avoid the need for such a route discovery to be performed before each data packet is sent, an on-demand routing protocol must cache routes previously discovered. Such caching then introduces the problem of proper strategies for managing the structure and contents of this cache as nodes in the network move in and out of wireless transmission range of one another, possibly invalidating some cached routing information.Several routing protocols for wireless ad hoc networks have used on-demand mechanisms, including TORA [14], DSR [9], AODV [15], ZRP [4], and LAR [11]. For example, in the Dynamic Source Routing protocol (DSR) [l, 8, 9] in its simplest form, when some node S originates a data packet destined for a node D to which S does not currently know a route, S initiates a new Route Discovery by beginning a controlled flood of a request packet through the network. When a copy of this request packet reaches either D or another node that has a cached route to D, this node then...

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