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Licklider Transmission Protocol (LTP)

by Yee Wei Law - Friday, 8 March 2024, 11:03 AM
 

CCSDS has adopted the Licklider Transmission Protocol (LTP) as specified in the IETF RFC 5326 [BFR08b] and the associated security extensions specified in IETF RFC 5327 [BFR08a] to provide reliability and authentication mechanisms on top of an underlying (usually data link) communication service [CCS15b, Sec. 1.1].

In an Interplanetary Internet setting deploying the Bundle Protocol [BFR08b], LTP is intended to serve as a reliable “convergence layer” protocol operating in pairwise fashion between adjacent in-range Interplanetary Internet nodes.

LTP aggregates multiple layer-(N+1) PDUs into a single layer-N PDU for reliable delivery — this allows the system to reduce the acknowledgement-channel bandwidth in the case that the layer-(N+1) (and higher) protocols transmit many small PDUs, each of which might otherwise require independent acknowledgement; see Fig. 1.

In CCSDS settings [CCS15b, Sec. 1.2], LTP is intended for use over package delivery services including packet telecommand and packet telemetry.

For space links, LTP is typically deployed over a CCSDS data link that supports CCSDS Encapsulation Packets so that one LPT segment can be encapsulated in a single Encapsulation Packet.

LTP can also operate over ground-network services, in which case it is usually deployed over UDP; see Fig. 2.

Fig. 1: To protocols above LTP (e.g., Bundle Protocol), LTP enables reliable delivery of layer-(N+1) PDUs across a link [CCS15b, Figure 1-1].

For LTP, the interface to the data link is via either direct encapsulation in CCSDS Space Packets or the CCSDS Encapsulation Service.

Fig. 2: A simplified end-to-end view of the system elements involved in a typical Solar System Internetwork (SSI) scenario: a Space User Node, an SSI “cloud”, and an Earth User Node, where the mission operations centre (MOC) is located [CCS13, Figure 3-4].

References

[BFR08a] S. C. Burleigh, S. Farrell, and M. Ramadas, Licklider Transmission Protocol - Security Extensions, RFC 5327, September 2008. https://doi.org/10.17487/RFC5327.
[BFR08b] S. C. Burleigh, S. Farrell, and M. Ramadas, Licklider Transmission Protocol - Specification, RFC 5326, September 2008. https://doi.org/10.17487/RFC5326.
[CCS13] CCSDS, Space Communications Cross Support—Architecture Description Document, Informational Report CCSDS 901.0-G-1, The Consultative Committee for Space Data Systems, November 2013. Available at https://public.ccsds.org/Pubs/901x0g1.pdf.
[CCS15b] CCSDS, Licklider Transmission Protocol (LTP) FOR CCSDS, Recommended Standard CCSDS 734.1-B-1, The Consultative Committee for Space Data Systems, May 2015. Available at https://public.ccsds.org/pubs/734x1b1.pdf.