# All about Delay-Tolerant Networking (DTN) Contributions to Future Internet
## 2 Concept and Challenges  
DTN was originally developed, as defined in RFC 4838 [2], to ensure reliable message delivery in highly dynamic and challenging Interplanetary Network (IPN) topologies, which are environments in which the conventional networking paradigms pose significant challenges. As a result, the fundamental concept of DTN, which distinguishes itself from those paradigms, is the acknowledgment of communication disruptions as an inherent feature rather than an abnormality. Therefore, DTN revolves around enabling communication in scenarios characterized by intermittent or disruptive connectivity, prolonged latency, and other challenging conditions. In order to understand the operational principles of DTN, mechanisms such as the Store-Carry-and-Forward (SCF) and protocols like the Bundle Protocol (BP) serve as foundational paradigms in the DTN suite. The SCF mechanism enables communication in environments with intermittent connectivity, thus transforming intermediate (mobile) nodes into relays or \"data mules\" to store, carry, and forward messages toward the destina  
Figure 1: Graphical paper overview. tion nodes until connectivity becomes available again in an asynchronous \"hop-by-hop\" manner. Complementing the SCF approach, the BP, presented in RFC 4838 and 5050 [2; 8] as BP6, serves as a cornerstone protocol in DTN deployments. A bundle is defined as a series of contiguous encapsulated data blocks, which are independently routed and forwarded through the network. Its robustness lies in its ability to abstract underlying network complexities, thus providing a common interface for communication regardless of the underlying transport mechanisms or network topologies. Some notable features are the inherent support for store-and-forward operations through the custody transfer mechanism and Endpoint Identifiers (EIDs) for flexible addressing. The BP has been extended in RFC 9171 [9] (referred to as BP7), while several protocols and implementations have been built upon it. In summary, DTN operates by storing and forwarding messages (bundles) between nodes via the BP, i.e., Bundle Protocol Agents (BPAs), thus allowing communication in environments with intermittent connectivity. Bundles are relayed through the network opportunistically using custody transfer to ensure reliability. However, the applicability of DTN principles has found relevance in terrestrial concepts as well. This transition from extraterrestrial to terrestrial environments resulted in various DTN-related protocols and solutions tailored to address the unique challenges encountered in real-world deployments. Some of the major challenges DTN tackles are listed below:  
* Intermittent/Disruptive Connectivity and Link Disruptions: Deals with occasional breaks or interruptions in network connectivity, e.g., space missions, remote areas, or disaster zones with limited connectivity. Mechanisms are utilized to store and forward messages until connectivity is restored or alternative routes become available. * High Latencies/Round Trip Time (RTT) and Low Throughput: Encounters long delays in message transmission and acknowledgment due to long-distance communication or congestion in the network. Also, issues achieving high data transfer rates due to limited bandwidth, intermittent connectivity, or congestion. * Data Losses, Message Fragmentation, and Reassembly: Occur due to network congestion, link disruptions, or node failures. Also, in cases of limited bandwidth or size restrictions on transmitted data, large messages need to be fragmented into smaller pieces for transmission and afterward reassembled to accurately reconstruct the original message. * Storage and Energy Constraints: Limitations regarding the availability of storage space for storing and forwarding messages or the energy consumption, especially in resource-constrained devices or networks. Storage management techniques, as well as prioritization, scheduling, or energy-efficient protocols and algorithms are employed.