Aggressive and proactive LTP control signal handling for minimal session delivery time: RTT rules the world

Several elements characterize deep space communications, including weak signal-to-noise-ratio (SNR), high bit error rate (BER), asymmetric channel bandwidth, and long propagation delay. In deep space missions, one-way light time (OWLT) is relatively much longer than in near-Earth missions. OWLT dominates data delivery completion time during a Licklider transmission protocol (LTP) transaction, making other communication elements relatively insignificant. As delay-/disruption-tolerant networking (DTN) technology plays a major role in communication for space exploration missions, especially Artemis missions in a cislunar environment, the performance of the LTP "convergence layer" protocol grows more important; reducing the time required to close an LTP transmission session will be increasingly critical. LTP session completion is crucial for mission operation because it must be bounded to support realtime operation. This study found that the LTP session closing time can be unacceptably long when link performance is in the BER range of 10⁻⁵ to 10⁻⁶, which is commonly experienced in space exploration communications. This paper presents an aggressive and proactive LTP control signal handling mechanism, conforming to the published LTP standard, that can reduce the latency of LTP session closing time at the cost of somewhat diminished goodput ratio. By applying this scheme in tests configured for segment size 2000 and BER 10⁻⁶, 99.67% of LTP sessions closed within 5 OWLTs, while similar tests in which this scheme was omitted which has 8.39% of shorter session closing time and only 4% chances of exceeding 5 OWLTs against a case non-applying it. Through numerical models and simulations, we show that the overhead is marginally acceptable and can contribute to better QoS over DTN operation in cislunar or deep space missions by bounding the LTP session closing time.