Since it was originally proposed by Ahlswede et al. (2000), network coding (NC) has been the focus of many research efforts and proposals. In particular, NC has been put forward as a means to increase the capacity of wireless mesh networks, by exploiting the broadcast nature of wireless links. However, in spite of the fairly large number of papers on the subject of network coding, the impact of NC on TCP performance in wireless mesh networks is still not fully understood. By means of a thorough simulation study, this paper aims thus at providing insights about the interaction between TCP's congestion control mechanisms and the underlying NC done by wireless relays. In particular, we focus on the influence of the main parameters related to the coding buffer used by such relays to encode packets. Our results suggest the feasibility of devising engineering rules for setting such parameters. Another main conclusion, in line with previous preliminary findings by the authors (Hassayoun et al., 2010) is that TCP performance can indeed be improved by the use of NC, but only as long as the wireless channel quality is above a certain level.

The traditional problem of end-hosts efficiently and fairly utilizing end-to-end paths becomes significantly harder when the end-hosts are multihomed. Such is the case, for instance, when an end-host has simultaneous connectivity through several service providers, or when a mobile device is simultaneously connected via both a wireless LAN and a cellular network. A multihoming-aware transport protocol, such as MPTCP or SCTP, that sends data over the multiple resulting end-to-end paths must be fair to other flows in the network while being able to maximize its own throughput. In this paper, we present Dynamic Window Coupling (DWC), a multipath congestion control mechanism that seeks to achieve both these goals. DWC uses loss and delay signals to detect shared bottlenecks, explicitly grouping and sharing congestion control across subflows on paths that have a common bottleneck, while separating congestion control for subflows on paths with distinct bottlenecks. DWC detects shifting bottlenecks in the network and responds by dynamically regrouping subflows. Simulations demonstrate that DWC detects shared bottlenecks under most network topologies and conditions that we considered, regroups subflows correctly as bottlenecks shift, aggregates throughput across distinct bottlenecks, and is fair to other TCP flows at all bottlenecks.

Network coding (NC) is a promising technique to improve throughput in wireless mesh networks. However, some previous studies have found that the actual performance improvements offered by NC may be much lower than the gains predicted by theory. This is especially so when the bulk of the traffic carried by the mesh network is composed of TCP flows. By means of both mathematical modeling and ns-2 simulations, we explore some issues due to random packet loss in coded mesh networks. Our results illustrate how the use of network coding may induce synchronization between TCP flows; they also suggest that, under particular conditions of random packet loss, the aggregate throughput may actually be lower when NC is used.

Adapting router buffer sizing rules and TCP's congestion control to Gbit/s link speeds has been the subject of many studies and proposals. In a previous work (Hassayoun and Ros, 2008), we found that high-speed versions of TCP may be prone to strong packet-loss synchronization between flows. Given that synchronized losses may lead to oscillatory behavior in router queues, a relatively large amount of buffering may be needed to sustain an adequate TCP performance. In this paper, we are interested in evaluating the potential impact of the Random Early Detection (RED) queue management algorithm, in the context of high-speed TCP versions. In principle, under mild congestion RED should tend to reduce loss synchronization between flows. Hence, we wanted to test in particular whether RED would allow to use moderately-sized buffers, while ensuring good performance. Simulation results are used to assess the effect of RED on goodput, link utilization and fairness among high-speed flows, for a wide range of buffer sizes and several high-speed TCP variants.