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IMPROVE TCP PERFORMANCE IN AD HOC NETWORKS , WITH DELAY ROUTING AND PATH RESERVATION

In the present work we examine the impact of average path-delay routing (APDR) to TCP, in ad hoc networks. The route discovery algorithm is based on the lowest average-delay path, and is OLSR based. Although such an approach might seem evident for the performance of TCP, its simulation study on realistic MANET scenarios allows to draw interesting conclusions. First, the route selected by OLSR is not always the best one from the viewpoint of TCP round-trip time (RTT), so the shortest path does not necessarily guarantee the lowest delay. Therefore, a decrease on the end-to-end average is often achieved at the expense of a longer path. Even though the collision probability does not increase as a result of the increased hop count, APDR results in reducing the overall throughput. As shown, events of regular route oscillations cause packet reordering, and in the context of TCP, evoke duplicate acknowledgements (ACKs) and retransmissions. The route oscillations happen because a node cannot tell apart the delay it induces to the channel, from the total delay. Second, only the delay as a routing metric is not sufficient, without the invocation of a path reservation scheme, to avoid the route oscillations. In ad hoc networks with no mobility, we apply APDR using source routing, without changing the path during the TCP flow. With the path reservation, APDR outperforms standard OLSR and improves the throughput. In networks incorporating mobility, because the path changes due to route disconnections, packet reordering cannot be avoided. The APDR study helps to deeper comprehend the TCP behavior with respect to density and mobility, and yields for source routing, to avoid the multiple paths.