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A major tool used for evaluating routing protocols in ad hoc and delay-tolerant networks is simulation. Whereas the results from simulations give good insights, they are limited to the specific scenario set-up that is used. If the scenario changes, new and often time-consuming simulations have to be run. Moreover, the simulation time in packet-level simulators with fairly realistic physical layer implementation, such as ns-2, generally grows rapidly in the number of nodes. This practically limits the number of nodes in a simulation, even if the limit can be extended by the use of simulation federations. Larger scenarios can also be facilitated by the use of more abstraction in the physical layer; abstractions that may impact the validity of the results. In this article, we present the forward-wait framework-a mathematical model describing the packet movements for opportunistic geographic delay-tolerant routing protocols. By describing packet movements as a sequence of alternating forwarding and waiting phases, the framework can accurately predict the routing performance. Key input parameters to the framework are random variables describing the forwarding and waiting phases. We show how the properties of the random variables can be derived, both via abstract modeling and small scale ns-2 simulation data. The model is then used to demonstrate the prediction capabilities of the framework in providing results that are close to the (much slower) packet-level simulations.

Abstract-Performing large scale physical experiments to evaluate the packet delivery ratio, latency, and resource utilization in mobile ad hoc networks is a costly and difficult exercise. A more cost-effective method of performing trade-off studies in connection with protocol evaluations is simulation. However, simulations are often time consuming and require a detailed description of a given scenario's mobility and topology. A more general approach for scalability and performance studies is the use of abstract models of behavior in networks of mobile nodes. This paper illustrates the power of such a model by illustrating the prediction capability of the Forward-Wait framework with respect to the routing performance of geographic delay-tolerant routing. The Forward-Wait mathematical framework describes packet movements as a sequence of alternating forwarding and waiting phases. Key input parameters to the Forward-Wait framework are distributions describing the forwarding and waiting phases, and the source destination distance. The paper shows how these distributions can be derived and used to predict the routing performance in different scenarios and a variety of network sizes.