Modeling RTT for DTN Protocol Over Asymmetric Cislunar Space Channels

Extensive work has been done in the performance evaluation of delay-/disruption-tolerant networking (DTN) technologies in cislunar and deep-space communications using simulation or emulation file transfer experiments. However, little work has been seen for studying in an analytical manner the performance of DTN protocols for space communications characterized by asymmetric channel rates. Accurate estimate of file delivery latency [i.e., round trip time (RTT)] is essential for efficient transmission control, reliable delivery, and bandwidth usage optimization of a protocol. In this paper, we present a performance analysis of the main DTN convergence layer adapter (CLA), i.e., Licklider transmission protocol (LTP) CLA (LTPCL adapter), over cislunar space channels, focusing on RTT estimates, in the presence of asymmetric channel rates. Analytical models are built for the RTT estimate of LTPCL transmissions with the effect of delay caused by space channel-rate asymmetry, channel impairment, and link disruption taken into consideration. The models are validated by file transfer experiments using a PC-based test bed. It is found that a smaller LTP data block size results in a longer delay in acknowledgement (ACK) transmission and thus a longer RTT. With a block size equal to or larger than a calculated threshold size, the ACK delay is avoided; thus, their RTT is consistently very low, leading to significantly shorter file delivery time and goodput advantage.