A TSO Submit command is used to start the server. Once started, it executes the TCP/IP connection sequence for an iterative server (INITAPI, SOCKET, BIND, LISTEN, SELECT, and ACCEPT) and then waits for the client to request connection.

Note that the BIND call returns a socket descriptor which is then used to listen for a connection request. The ACCEPT call also returns a socket descriptor, which is used for the application data connection. Meanwhile, the original listener socket is available to receive additional connection requests.

The client is started by calling an IMS™ transaction which, in turn, executes the TCP/IP connection sequence for a client (INITAPI, SOCKET, and CONNECT).

Upon receiving the connection request from the client, the server issues a READ and waits for the client to WRITE the initial message. The server contains a READ/WRITE loop which echoes client transmissions until an "END" message is received. When this message is received, it sets a 'last record' switch, echoes the end message to the client, and terminates.

Note that in order for the server to terminate, it must close two sockets: one -- the socket on which it listens for connection requests; the other -- the socket on which the data transfers took place.

The client and server both include Write To Operator macros, which allow you to monitor progress through the application logic flow. At the end of this appendix you will find a sample of the WTO output from the client and the server.