An RPC server offers its procedures as parts of a "program". Here, the notion of programs is introduced. A program is identified by a program number and a version number. The service of a program is a set of procedures that are described by procedure numbers and signatures. The signature of a procedure is a pair (s,u) of XDR types that should be interpreted as function s -> u. To make work easier, programs are encapsulated as an opaque type t. This data structure can store procedure names in addition to the plain numbers, so you can refer to procedures by their names. But note that these names are only local identifiers that are not transferred in the RPC protocol. The signature can be formulated upon a type system (a set of definitions of type names).
Type of RPC programs
signature p name returns the triple
(proc_nr, in_t, out_t)
for the procedure
proc_nr is the procedure number,
in_t the argument type and
out_t the result type.