## 5 Testbed Setup and Analysis  
### Testbed Setup  
In Figure 4, we show a block diagram of the Space Communication and Networking Testbed (SCNT) adopted for the experimental file transfer in this study. The SCNT was designed to emulate the relay type of IPN communication scenarios via the primary relay path and the secondary relay path illustrated in Figure 1. As presented in Figure 4, three Linux-based PCs--TX, MX and RX--simulate the data source on the plaeting udet in Figure 1. Via the primary relay ath, a planet orbiter (in Figure 4a) or an Earth orbiter (in Figure 4b), and a ground station on the Earth, respectively. The testbed was validated in our previous research [8; 37; 40]. For the details of SGLS operation and better explanation of the simulation parameters, refer to [37; 40].  
The major experimental configurations are listed in Table 1. The experimental factors and their settings are straightforward and clear. As a matter of fact, the one-way light time in cislunar communication is at least 1.25 s, while it can be 5 s at maximum [24]. Following this fact, we choose nine different cislunar long-haul delays (i.e., the delays for MX \(\rightarrow\) RX in Figure 4a and TX \(\rightarrow\) MX in Figure 4b), as listed in Table 1, for the performance evaluation of DTN. Considering the proximity link delays (i.e., the delays for Mo  
Figure 4: SCNT block diagram. (**a**) For primary cislunar relay architecture. (**b**) For secondary cislunar relay architecture.  
Earth orbiter \(\rightarrow\) Earth) are so short that they can be negligible compared to the long-haul delays, they are ignored in the simulation setup.  
The PPP link rate of 115,200 bit/s is chosen for both data and ACK channels. This rate is common for cislunar communications [58; 59]. Table 2 shows the data link rates and propagation delay experienced in a few actual lunar exploration missions. The experimental parameters in this work fall within the range of the cislunar communication parameters of actual missions in Table 2, and therefore, their selections are practical. The maximum and default sender/receiver buffer size of 16 Mbytes is configured for TX, MX and RX. This buffer size is big enough to accommodate the data transmission, with a large BDP caused mainly by the mentioned long link delays.  
\begin{table}
\begin{tabular}{l l} \hline \hline
**Experiment Factors** & **Settings** \\ \hline \multirow{3}{*}{Protocol implementation} & Interplanetary Overlay Network (ION) \\  & v4.1.1 by Jet Propulsion Laboratory (JPL) [57] \\ \hline \multirow{6}{*}{Protocol configurations} & LTP LTP PRIM \\  & UDP LTP PRIM \\  & LTP UDP SEC \\  & LTP TCP SEC (with SACK option enabled for TCP) \\  & (See Figure 3 for the details) \\ \hline LTP segment settings & 100\% red data \\ \hline LTP segment size & 1000 bytes \\ \hline MTU size & 1500 bytes \\ \hline Nominal Block Size (NBS) \(\times\) Maximum number of sessions (NOS) & 1800 (bytes) \(\times\) 32 (Selected based on tuning the protocol for the maximum goodput) \\ \hline Maximum and default sender and receiver buffer size & 16 Mbytes for TX, MX and RX \\ \hline Operating system & Fedora Linux 38 (kernel 6.2) \\ \hline BER & 0, 10\({}^{-6}\), and 10\({}^{-5}\) (to emulate different channel qualities) \\ \hline \multirow{6}{*}{One-way link latency} & Nine different delays selected in a range of 1280–5000 ms \\  & (Typical cislunar link delays chosen according to [24]) \\ \hline Channel rate on each link & 115,200 bit/s \\ \hline Link disruption & Not introduced \\ \hline File size for simulation & 1 Mbyte \\ \hline Sample size & 16 repetitive runs \\ \hline \hline \end{tabular}
\end{table}
Table 1: Experimental configuration.