From 1984 (RFC 0909):

Data is passed from the application program to the LDP user
    process in the form of commands. In the case of an LDP server
    process, command responses originate in LDP itself. Below LDP is
    the transport protocol. The Reliable Data Protocol (RDP --
    RFC 908) is the recommended transport procotol. Data is passed
    across the LDP/RDP interface in the form of messages. (TCP may
    be used in place of RDP, but it will be less efficient and it
    will require more resources to implement.) An internet layer
    (IP) normally comes between RDP and the network layer, but RDP
    may exchange data packets directly with the network layer.

(For something somewhat less dull and unimaginative, you might try RFC 1149.)