Effects of Receive Buffer Size and Timer Granularity on TCP Performance over Erroneous Links in a LEO Satellite Network Effects of Receive Buffer Size and Timer Granularity on TCP Performance over Erroneous Links in a LEO Satellite Network

This master thesis presents a preliminary investigation into the impact of TCP’s advertised receive buffer size and timer granularity on TCP performance over an erroneous link in a low earth orbit (LEO) satellite environment. Conducted simulations include over 200 different combinations of TCP flavor, advertised receive buffer size, timer granularity, and bit error rates. Results show that TCP can only approximate the variable propagation delay of the LEO satellite link for unsaturated channels and that the minimum timer granularity which prevents a premature expiration of the retransmission timeout (RTO) depends on the advertised receive buffer. Low bit error rates (BERs) do not influence TCP’s capability to track the variable propagation delay in contrast to high BERs. Final results indicate that the relative performance degradation of TCP over erroneous LEO satellite links does not depend on the ability to estimate the variable propagation delay but rather on the employed TCP flavor. [35] The presented work includes basic principles of TCP with respect to its congestion avoidance and retransmission mechanisms. Additionally, an introduction to satellite constellations is given and further detailed to highlight the characteristics of the analyzed satellite network. Based upon this analysis, this thesis will mathematically show that only the upand downlink delay dominate within the end-to-end delay’s variability. Thus, the developed simulation model might neglect any propagation delay associated with inter-satellite links (ISLs). In order to simplify the simulation of erroneous satellite links, some aspect of up-to-day data encoding and forward error correction (FEC) schemes are presented. This work shows that on a transport layer level, the satellite link reveals either a uniformly distributed BER or is not available at all. [38,39] As the entire work was conducted within a BMBF-founded research project at Fraunhofer FOKUS, references to project constrains and system parameters are included whenever necessary. [6]