Use of congestion-aware routing to spatially separate TCP connections in wireless ad hoc networks

TCP sessions in ad hoc networks compete with each other for bandwidth. The use of shortest path routing can result in multiple TCP sessions being channeled via a few congested areas or hotspots. As a consequence, most of these multiple TCP sessions interfere with each other and hence, experience significant performance degradations. Spatially separating the TCP sessions such that they inflict much lower interference effects on each other may provide gains in performance. In this paper we first investigate the possibilities of achieving such gains by considering a centralized, ideal, and unrealistic congestion-aware routing approach. We find that spatial separation benefits are possible with the considered approach, and can especially help long (in terms of hop count) TCP connections. We then consider the implementation of a distributed routing protocol to achieve the aforementioned spatial separation benefits. We find that due to practicalities such as the need for the exchange of congestion state, the existence of stale congestion information and the creation of sub-optimal paths, the benefits due to spatial separation are considerably undermined. We perform both macroscopic simulations and microscopic studies of specific constructed examples to understand the reasons and quantify the various effects with both the centralized and the distributed approaches. Our studies suggest that achieving noteworthy performance gains by spatially separating TCP sessions may be extremely difficult if not impossible in ad hoc networks.