## 6 Results
### DTN Architecture  
In terms of architecture, 20 papers discussed different elements of DNT, including the BP, topology, and how it differs from TCP/IP. In general, these works are intended to illustrate and describe how DTN works and its interaction with other layers, as well as to demonstrate that it can be used with different transport and data link protocols. From our point of view, here is not where the issues related to the performance of DTN lay, and so there are no works trying to make changes in terms of architecture. DTN is a network architecture mainly designed for transmissions in harsh environments where there is no infrastructure for network communication, as it has been destroyed or is overloaded. Some of the characteristics of such a network are intermittent connectivity (never fully connected), limited resources (bandwidth, storage, energy supply), and variable delays. DTN is considered an opportunistic network because connections are intermittent and there is no guaranteed routing path between the origin and the destination. Scenarios for this topology include military connections, disaster areas, interplanetary transmissions, and rural communities with limited communication infrastructure [10]. The BP provides transport services for applications in larger packets than IP. Any layer, such as TCP, UDP, or LTP, can be used for transferring a bundle over a single hop. Bundles have a limited lifetime, TTL, after which it expires. Retransmission can happen in the origin or intermediate nodes [15]. The bundle layer and its BP are located between the application layer and the transport layer. It offers a message-oriented, store-carry-forward overlay network, and end-to-end connectivity. DTNs implement convergence layer adapters (CLA) to be able to provide functions for any protocol from different networks. BP data units are of variable sizes, so bundles are created depending on the situation. BP PDU consists of one or more blocks that contain metadata or application data. There is no error correction or detection [16]. The BP can use either TCP or UDP to transmit over a single hop given its feature of hop-to-hop reliability instead of end-to-end communication. After expiration, bundles can be retransmitted either by the origin or an intermediate node that keeps a copy of it. Too many copies can create congestion and lower performance [17]. BP can be implemented in  
Figure 3: Paper categories. a single process running on a general-purpose computer, a thread as a background process, an object on an object-oriented OS, or a special-purpose device [18]. BP interfaces with lower-layer protocols through CLA and provides features for custody transfer so that an intermediate node takes care of bundles, proactive and reactive bundle fragmentation to handle intermittent connectivity, and late binding so that the final hop resolves the DNS name. In TCP/IP, routers perform non-persistent (short-term) storage as continuous connectivity and short delays are assumed, so information is stored only at end nodes, whereas in DTN, persistent (long-term) storage is performed at intermediate nodes [19]. The BP in DTN defines how data are encoded and sent over the wire and might include mechanisms for reliability. As many DTN applications work with opportunistic contacts, a Discovery Protocol might take place so that neighbors might discover each other. Finally, the routing protocol itself has an impact on performance. Morgenroth refers to the DTN2 protocol stack, including Ethernet-based CL for wired communication between neighboring nodes without IP and/or TCP. uDTN is a BP implementation for wireless sensor networks (WSNs) running in 8-bit microcontrollers. PDUs are wrapped in IEEE 802.15.4 radio data frames and transported between nodes [20]. Unlike TCP/IP where there must be one or more links between the source and destination, DTN nodes receive messages and store them until another node is available to eventually reach the destination.