Modeling the RTT of Bundle Protocol over Asymmetric Deep-Space Channels

Delay/disruption-tolerant networking communications rely heavily on BP (Bundle Protocol), which uses the well-known approach of store-and-forward with optional custody transfer to deal with stressed communication environments. The use of BP and its performance in deep-space communication has been the subject of debate. The accurate estimate of file delivery latency (i.e., RTT (Round Trip Time)) is essential for efficient transmission control, reliable delivery, and bandwidth usage optimization of a protocol. In this paper, we present a performance analysis of BP running over UDPCL/UDP over deep-space channels, focusing on the RTT estimate, in the presence of highly asymmetric channel rates. Analytical models are built for the RTT estimate of the BP/UDPCL transmissions considering the effect of delay caused by space channel-rate asymmetry, and, channel impairment. The models are validated by file transfer experiments using a PC-based testbed. It is found that a smaller bundle size (if smaller than a calculated threshold) results in a longer delay in custody acknowledgment transmission, and thus, a longer RTT.