## 3 Overview of DTN and Its Data Transport Protocols for Space  
As mentioned, a store-and-forward transmission mechanism with optional custody transfer is adopted by DTN to deal with frequent link outages and long data delivery latency [18]. Similar to the operation of traditional postal systems, the mechanism moves each data fragment hop by hop until the entirety of the source data are delivered to the destination [16]. To implement the store-and-forward transmission service, BP [18] operates on top of underlying heterogeneous CLP stacks (together with various data transport and Internet protocols) to build an overlay network.  
The DTN protocol stack (including the specific protocols) compared to the seven-layer OSI model [57] is illustrated in Figure 2. As observed, the DTN protocol architecture is similar to the one of the Internet. As IP interconnects physical networks such as the Ethernet or SONET, BP interconnects \"internets\", including both Internet-based terrestrial networks (e.g., IEEE 802.11 and Ethernet) and space-based networks built over space links such as the Advanced Orbiting Systems (AOS) and Proximity-1.  
As introduced, convergence-layer adapters for a variety of transport protocols and implementations have been developed to support DTN operation in heterogeneous networks, including TCPCL, UDPCL and LTPCL. As shown in Figure 2, these CLAs and their associated data transport protocols locate between BP and the network/data link layers.  
For TCPCL and UDPCL of DTN, as the names imply, they are designed to operate over TCP or UDP, respectively. In other words, they use TCP or UDP to provide reliable or unreliable communication services between DTN nodes [19].  
Compared to TCP and UDP, LTP for LTPCL was developed as the primary transport protocol of space DTN. LTP was designed to provide selectable transmission service according to mission requirements. In other words, LTP can provide reliable transmission service (as TCP does) and unreliable service (as UDP does) [22]. Both transmission services can be carried out for a single data block by specifying the \"red\" portion (for reliable service) of data or the \"green\" portion (for unreliable service) of data. Unlike the basic  
Figure 2: DTN protocol stack vs. OSI stack [57].  
UDPCL, LTP supports the segmentation of large DTN bundles for transmission in multiple UDP datagrams. To accommodate the highly asymmetric data rates of the down link and forward link, some protocol designers try to reduce the amount of ACK traffic over the constrained ACK channel. For LTP, this is realized by aggregating multiple BP bundles into a single LTP block that is acknowledged at block granularity [57]. The protocol design and operation of LTP are well discussed in previous studies [22]; therefore, they are not discussed in detail in this paper.  
TCP and UDP show high performance for data/file transfer over terrestrial links--as they have consistent link connectivity and long latency--but not over space links. In comparison, it is expected that LTP works effectively over a space link characterized by intermittent connectivity, long link outage and long propagation delay. Therefore, it is reasonable to concatenate two or three protocols for optimal data/file transfer in cislunar and deep-space communications. For example, for the end-to-end transfer from the Moon to an Earth ground station, TCPCL or UDPCL should likely be considered for use over the link between the data node on the surface of Moon to its orbiter, which has negligible delay. In contrast, as the link between the Moon orbiter and the data destination node on the Earth ground station has a long propagation delay and intermittent connectivity, LTPCL should be adopted.  
In next section, we discuss each of the DTN CLA configurations, including their concatenations and their operational architecture, in cislunar communications that we adopted for the investigation in this work.