Licklider Transmission Protocol is a point to point protocol for the use in deep space links. LTP is designed to run directly over a data link layer, but it can also run on the User Datagram Protocol (UDP), e.g. if used in a sensor network, in a private network (where bandwidth contestation isn’t an issue), and for software debug purposes.

The LTP engine will store the data in a queue, check whether the link towards the destination is available and if so, start the transmission. If the link is not available, it will keep the data in its queue and start transmission as soon as a so-called “link state cue” arrives, either from an underlying link layer or from a Management information base.

LTP is commonly seen as the standard underlying convergence layer protocol for the Bundle Protocol, supporting a wide selection of networks. LTP’s main features are:

  • reliable data transport for important data (such as a file header)
  • unreliable data transport for less important data (such as image pixels)
  • no negotiation exchange due to potentially higher round trip times and to avoid link underutilisation
  • energy efficiency, as it will only send if a link is available and distinguishes between important and unimportant data
  • timers work together with communication schedules and can be suspended whenever a scheduled link outage occurs
  • needs to be informed about link layer availability, round-trip time and communication schedule
  • unidirectional sessions, circumvents high round-trip times or inability of peer to transmit.