Licklider Transmission Protocol (LTP) is used to provide retransmission of network data when the route is very long or when there are frequent interruptions in network connectivity .Typically this is used in interplanetary and deep space networking. Hence the name ltp-deepspace. An overview of LTP gives a brief summary of LTP. The specification on LTP gives lots of details about the protocol. Motivation for LTP explains lots of details behind why LTP is necessary. NASA has a challenge for a DTN and that explains the details overview of a DTN.
The Internet Engineering Task Force (IETF) has an interesting memo on Delay-Tolerant Networking Architecture as well as the Bundle Protocol Specification (which is the core feature of a DTN). The Bundle Protocol (BP) can work over Transmission Control Protocol (TCP). If TCP is not used at the Transmission layer of the OSI model, then Licklider Transmission Protocol (LTP) which goes over User Datagram Protocol (UDP) is used to Transmit the Bundles. Either BP/TCP or BP/LTP/UDP is used for BP transmission.