APPC is a peer-oriented program-to-program API where either Transaction Program (TP) can initiate a conversation. However, APPC / LU6.2 traditionally is half-duplex, as defined by IBM in the LU6.2 "base option set". Under half-duplex operation, once the converation is established, only one TP has direction to send on a conversation at a time. The state of the conversation, as viewed by a particular TP, governs which APPC verbs can be issued by the TP at a particular time.

IBM now supports "full duplex" APPC where either program can issue both send and receive verbs at the same time, as well as non-blocking APPC operation, which are relatively new APPC option sets defined by IBM.

These option sets are:

112 -- Full-duplex conversations and expedited data
113 -- Non-blocking support

IBM defines these option sets in the IBM Transaction Programmer's Reference Manual for LU Type 6.2 (IBM document number GC30-3084).

SNA Server does not support option set 112 or 113. SNA Server only supports half-duplex operation, though either end can initiate the conversation. SNA Server implements non-blocking support for APPC and CPIC API's for 16- bit and 32-bit Windows per the Windows APPC and CPIC specifications, though these specifications do not follow the IBM-defined option set 113.

See Chapter 1 of the SNA Server Reference Guide for a list of LU6.2 option sets supported by SNA Server.