## 1 Introduction  
The National Aeronautics and Space Administration (NASA) is currently in an experimental phase of implementing space communications using computer networking protocols [1]. Transmission Control Protocol (TCP) [2] has severe performance degradation in a space-communications scenario. This is due mainly to significant operating differences between the terrestrial environment and space environment [3, 4, 5]. Many alternative protocols have been developed for reliable data transport in a space environment [5, 6, 7, 8]; for a comparative discussion of them, refer to the tutorial presented in [9]. Delay/disruption tolerant networking (DTN) [10, 11] was developed to combat the long propagation latency and link outages that are typical for the space environment. It is the only candidate networking technology for future heterogeneous space communication networks. Future 7G/8G networks are expected to integrate both terrestrial Internet and space-based networks, including inter-planetary Internet such as cislunar and deep-space networks. In other words, space networks will become an integral part of future 7G/8G networks. The internet of things (IoTs), internet of vehicles (IoVs), vehicle-to-everything (V2X) networks and deep reinforcement learning systems will be of paramount significance to future 7G/8G networks [12; 13; 14; 15]. Furthermore, space networks will eventually integrate with IoTs, IoVs, and V2X communication networks, with both space vehicles (or spacecrafts) and terrestrial vehicles being incorporated. DTN operates based on a \"store and forward\" mechanism with an optional custody transfer service [16; 17]. DTN's Bundle Protocol (BP) [18] operates to build a \"store and forward\" overlay network so that DTN separately implements the \"store and forward\" mechanism and the optional custody transfer service. The \"store and forward\" mechanism allows DTN to suspend and resume data transfers when links are interrupted, which are common over space data links. BP has the capability to cope with random and frequent link outages and take advantage of opportunistic connectivity. BP works by utilizing the services of a selected underlying \"convergence layer adapter\" (CLA) [18; 19] and the protocol stack. DTN is designed to operate in this manner because different protocols may be needed for the best performance over different link segments of the end-to-end DTN path. Several CLAs have been developed for BP, and the following three are widely considered for space missions: TCP CLA (i.e., TCPCL) [19], User Datagram Protocol (UDP) CLA (i.e., UDPCL) [20], and Licklider Transmission Protocol (LTP) [21; 22; 23] CLA (i.e., LTPCL). For a general overview of DTN operation infrastructure and CLAs, refer to [16]. Interplanetary communication generally uses relaying to compensate for the limited direct connectivity [24; 25]. Almost all the data from the Mars Exploration Rovers (MERs) were sent to Earth via the relaying service of the Mars Global Surveyor and Mars Odyssey orbiter [26]. With the relaying of Mars Odyssey orbiter, the MERs increased their data return from around 30 Mbytes/sol achievable with the direct-to-Earth (DTE) link to around 100-200 Mbytes/sol [27]. The relaying also decreased the power consumption required for data return from roughly 5 Watt hours per Mbyte with the DTE links to around 0.1 Watt hour per Mbyte. For flight missions to the Moon and other planets, similar benefits are also expected. In a typical cislunar and other interplanetary Internet (IPN) [28] communication scenarios, relay-based communication can be operated using two different routing configurations. One configuration is through a relay spacecraft orbiting the planet, and another is through an Earth-orbiting spacecraft, as illustrated in Figure 1. The tracking and data relay satellite system (TDRSS) [29; 30] is considered a possible candidate for the relay of cislunar communication, according to [24].