This Recommendation International Standard specifies a protocol which is used to provide a connectionless-mode service as defined in the network service definition and associated management functions. The specification consists of the protocol mechanisms and PDU structure for the exchange of data between end systems and intermediate systems using connectionless data transfers. The protocol defines: data transfer, error reporting and echo function. This Recommendation International Standard includes a description of the mapping from the protocol operation onto a generalized OSI data link service which can be used in conjunction with real subnetworks.