Advanced Program to Program Communications

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APPC ( Advanced Program to Program Communications ) is a protocol that computer programs can use to communicate over a network.

APPC was developed as part of IBM's Systems Network Architecture (SNA). This resulted in several APIs , which for the most part could also be operated without problems from programming languages ​​such as COBOL or REXX .

APPC is linked to the term LU 6.2 ( Logical Unit Type 6.2 )

APPC is largely limited to the IBM computer world: mainframe operating systems, AS / 400 , OS / 2 , AIX ; for Windows there is an SNA server product from Microsoft. Within the IBM system world, APPC is supported by every system software, e.g. B. on the mainframe CICS , DB2 , IMS and also the MVS itself.

In contrast to TCP / IP , where both communication partners always have a unique role (one is always a server, the other is always a client), with APPC the communication partners have equal rights, ie everyone can be both a server and a client. The role as well as the number of parallel sessions between the partners is negotiated via so-called CNOS sessions (change number of sessions) with a special log mode (e.g. snasvcmg at IBM). The data itself is then communicated via so-called data sessions, the log modes of which can be determined in detail by the VTAM administrator (e.g. length of the data blocks, encryption, etc.).

With the triumphant advance of TCP / IP, APPC is on the decline.