Sun RPC
ONC RPC - short for Open Network Computing Remote Procedure Call, sometimes also called Sun RPC based on its origin, is a widely deployed (possibly most widely deployed) remote procedure call system. Initialy developed for the Network File System, ONC RPC supports procedure calls over both UDP and TCP. The interface description language for ONC RPC is XDR (eXternal Data Representation) which was initialy designed for description and encoding of data for cross-platform communication.
Access to rpc services on a machine are provided via port mapper that listens for queries on a well-known port.
ONC RPC is described in RFC_1831.