Abstract
The last decade has seen the clear dominance of the Internet Protocol in the data communication networks. Although the killer applications have changed significantly during this time, from file transfer and email to Web browsing and more recently to file sharing, TCP has been consistently responsible for the overwhelming majority of Internet traffic. More significantly, recent protocol design efforts within the Internet community adopt fundamental features of the protocol, strongly suggesting that TCP and TCP-like traffic will continue contributing the majority of Internet data traffic, at least in the short-term future. Given the clear IP dominance, broadband satellite networks may be viewed as yet another subnet over which the TCP/IP suite, in general, and TCP traffic in particular, have to be efficiently supported. In this Thesis, we investigate issues relevant to the support of elastic TCP traffic over satellite networks. We focus our attention on geostationary satellite networks, which have almost monopolized the interest of satcom community the last decade, in light of the high risks and investment involved in the development of satellite constellations. We investigate mechanisms available at the satellite network for the provision of service differentiation to TCP flows. We demonstrate that fundamental satellite access network capabilities provide enough flexibility for the provision of qualitative service differentiation for TCP flows over these networks without necessitating per-flow state at the MAC layer and/or the computational overhead of prediction methods that are not straightforward for TCP traffic. Moreover, the split-TCP mechanism, despite the security-related and reliability-related concerns it raises, provides the network operator with significant additional flexibility in the treatment of TCP traffic. The mechanism forms a transport-layer differentiation mechanism that, when combined with lower-layer capabilities, can give rise to separate bearer services over the satellite network. We make two contributions to the study of the split-TCP concept. The first contribution is an analytical model for the estimation of split-TCP latency and the buffer requirements at the intermediate node that hosts the split-TCP agent. The second contribution is a split-TCP scheme, called split-Delayed Duplicate Acknowledgments (split-DDA), which draws heavily on DDA, a TCP variant presented and evaluated earlier as an end-to-end scheme in the context of terrestrial wireless networks. The provision of quantitative Quality-of-Service guarantees to TCP flows necessitates some form of access control to the satellite network and the presence of TCP performance enhancing proxy agents can assist this task significantly. We present and evaluate a heuristic implicit admission control algorithm for TCP flows over split-TCP satellite networks that can preserve the target requirements in terms of TCP steady-state throughput and TCP latency. We describe generic fixed-point approximations for the performance of TCP flows in geostationary satellite networks. We provide examples for the method applicability to various satellite network configurations evaluate the method against simulation results in the context of MAC-shared satellite links with dynamic bandwidth allocation mechanisms and discuss its strong and weak points. The utility of these approximations is further demonstrated in the case of the algorithm we introduce for the dynamic control of the TCP maximum receive window variable in. split-TCP satellite networks. The algorithm accelerates TCP transfers at low load without overloading unnecessarily the MAC buffers at high load. Finally, as an addendum, we provide a case study of Web browsing over bandwidth on demand satellite links. Spanning three layers, namely application, transport and access layers, the study demonstrates the impact of the radio interface mechanisms upon the performance perceived at the application layer.