Failure of TCP congestion control under diversity routing

TCP does not perform well in networks with stochastic channels, with links that randomly drop packets or have long outages. Diversity routing has been proposed to improve TCP's performance in these networks. In diversity routing, a sublayer between TCP and the network replicates each transmitted packet and sends the multiple copies along parallel paths. As long as at least one of the copies reaches the receiver, TCP considers the transmission successful and maintains high throughput. Previous investigations of diversity routing have analyzed TCP's performance when there is a single flow and no congestion. In this paper, we analyze the performance of multiple flows in a congested network. The contribution of this work is the discovery of three adverse effects that occur when TCP is combined with diversity routing. The effects are: link capacity overflow, rate unfairness, and lock-out of late-arriving flows. Simulation is used to analyze the root cause of these effects. Our conclusion is that naïve diversity routing fundamentally breaks the TCP congestion control mechanism and cannot be used for TCP performance improvement in networks with stochastic channels, such as many wireless and satellite networks. We propose an improvement to diversity routing that may overcome these problems, enabling use of unmodified TCP.