## 7 Discussions and Clarification of Comparison Results
### Cislunar (Space) Segment of the End-to-End Path  
For all four configurations, the long-delay cislunar segment is involved, and it always has the same configurations:  
* Always running LTP over UDP/IP/PPP;
* Always 115,200 bps;
* Varying signal propagation delay (1280 to 5000 ms each way);
* Varying BER (0 to \(10^{-5}\)). As discussed in Section 4, LTPCL/UDP/IP/PPP is always configured for the long-delay cislunar segment (either Moon orbiter \(\rightarrow\) Earth ground station or Moon surface \(\rightarrow\) Earth orbiter) with the maximum and default sender/receiver buffer size of 16 Mbytes for all three stations, so this segment behaves in the same way in each configuration. End-to-end goodput is reduced as signal propagation delay increases. The root cause of this effect is that the number of block transmission sessions this LTP implementation can conduct concurrently is limited to a fixed maximum \(N\) established at system initialization (This is to prevent LTP from exceeding available storage and memory resources). This limit means that as soon as \(N\) sessions have been initiated, no new transmission session can be begun until one of the existing sessions completes. As signal propagation delay increases the length of time required to complete a session (i.e., receive a positive ACK from the \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Performance** & **BER = 0** & **BER = 10\({}^{-6}\)** & **BER = 10\({}^{-5}\)** \\ \hline
1 & LTP LTP PRIM & LTP LTP PRIM & LTP LTP PRIM \\ \hline
2 & LTP TCP SEC & LTP TCP SEC &
\begin{tabular}{l} LTP TCP SEC \\ (Delay \(<\) 2500 ms) \\ UDP LTP PRIM \\ (Delay \(\geq\) 2500 ms) \\ \end{tabular} \\ \hline
3 & \begin{tabular}{l} LTP UDP SEC \\ (Delay \(<\) 3500 ms) \\ UDP LTP PRIM \\ (Delay \(>\) 4000 ms) \\ \end{tabular} & \
\begin{tabular}{l} LTP UDP SEC \\ (Delay \(<\) 3500 ms) \\ UDP LTP PRIM \\ (Delay \(>\) 3500 ms) \\ \end{tabular} &
\begin{tabular}{l} UDP LTP PRIM \\ (Delay \(<\) 2500 ms) \\ UDP LTP PRIM \\ (Delay \(\geq\) 2500 ms) \\ \end{tabular} \\ \hline
4 & \begin{tabular}{l} UDP LTP PRIM \\ (Delay \(<\) 3500 ms) \\ LTP UDP SEC \\ (Delay \(\geq\) 3500 ms) \\ \end{tabular} & \
\begin{tabular}{l} UDP LTP PRIM \\ (Delay \(<\) 3500 ms) \\ UDP LTP PRIM \\ (Delay \(>\) 3500 ms) \\ \end{tabular} &
\begin{tabular}{l} UDP LTP PRIM \\ (Delay \(\geq\) 2500 ms) \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Rough performance ranking of investigated DTN protocols. receiver) increases, and therefore, the time at which new transmission can be initiated is postponed, overall latency for the exercise increases, and goodput drops. The mitigation for this effect is to increase \(N\) so that by the time the \(N\)th session is being initiated the first session has already completed, so that there is never any delay in initiating a new session. However, this strategy solves the problem only when there is never any data loss: data loss will delay the completion of a session by one or more round-trips, delaying the initiation of new sessions regardless of the session limit, thereby increasing overall latency and reducing goodput. Therefore,  
Whenever the rate of data loss on the cislunar segment is greater than zero, end-to-end goodput drops as signal propagation delay on the cislunar segment increases. Because this data loss is random, the drop in end-to-end goodput due to this effect is not deterministic, but the trend is clear. This drop in goodput with increasing delay is especially pronounced when commanded data loss, due to changes in the commanded BER, is introduced. However, the effect is noticeable even when the commanded BER is zero, because one additional source of data loss remains: since LTP is running over UDP/IP/PPP, which lacks congestion control, there can always be UDP congestion loss on the cislunar segment. This is data loss that is an artifact of the testbed rather than a controlled variable.