An SNA protocol that establishes a session between two programs. It allows peer-to-peer communications as well as interaction between programs running in the host with PCs, Macs and midrange computers. Before LU 6.2, processing was done only in the mainframe. LU 6.2 allows processing to take place at both ends of the communications, necessary for today's distributed computing and client/server environment. See APPC and CPI-C.