Currently, Networks based on TCP/IP; work on the principle of providing end-to-end inter-process communication using a concatenation of dissimilar link-layer technologies. The standardization of the IP protocols and its mapping into network-specific link-layer data frames as required supports the concept of interoperability in a packet-switched domain. Thought not specifically mentioned, a number of assumptions are made about the performance characteristics of the underlying link, to achieve smooth operation. For example, an end to end path exists between the sender and the receiver of data, the round trip delay time between them is not excessive and the probability of end-to-end dropped packets is small. However, there are a number of challenging scenarios where these assumptions are violated and thus the use of Internet Protocols results in poor or no service at all (Fall, 2003).
Restriction of TCP/IP in Challenging Scenarios
The challenging scenarios are characterized by frequent disconnection, long delays in transfers of data and low data rate. The example of such networks include
Military Ad-hoc Networks which are expected to operate in hostile environments where mobility, environmental factors or intentional jamming may cause frequent disconnections. In addition, data traffic on these networks competes for bandwidth with other services. As a result of this, the data traffic delay spans several seconds while high priority traffic is carried on the same links.
Deep Space Communications which involves very long distance communication links with propagation delays in the seconds or minutes). These systems are subjected to high latencies due to scheduled interruption, outages due to environmental conditions (weather) or occasionally available store-and-forward network service (Fall, 2003).
Disruption and Delay Tolerant Network
Delay and Disruption Tolerant Networks (DTNs), are special networks, which are designed to operate in environment where no end-to-end connection is guaranteed, there are high latencies in data transmission and frequent disruptions. The DTNs overcome all the shortcomings of TCP based networks which results in their poor performance in unpredictable link conditions (Farrell, Geraghty, Cahill, Humphreys, Mcdonald, 2006).
The technology of Disruption and Delay Tolerant Networks has its history in research project funded by US Government grants. The purpose of these projects was to develop such technologies which could sustain significant delay and packet corruptions of space travel. Initially, the scope of these projects was only limited to short-range communication between manned missions to the moon and the ground station, but it was rapidly expanded in the following years resulting in a whole new sub field of Delay Tolerant Networks that made possible the idea of InterPlanetary Internet (IPN) (Farrell et al, 2006).