The IMS™ Listener uses two queues for the management of connection requests:

1. The backlog queue (managed by MVS™ TCP/IP) contains client connection requests that have not yet been accepted by the Listener. If a client requests a connection while the backlog queue is full, TCP/IP rejects the connection request. The number of requests allowed in the backlog queue is specified in the LISTENER startup configuration statement (BACKLOG parameter), see LISTENER statement.
2. The active sockets queue contains the sockets that are held by the Listener while they wait for assignment to a server program. After the Listener has accepted the connection, the connection belongs to the Listener until it is accepted by the server. If the Listener uses all of its sockets and cannot accept any more connections, subsequent requests go into the backlog queue. The maximum number of sockets available is specified in the LISTENER startup configuration statement, (MAXACTSKT parameter), see LISTENER statement.
   Tip: The backlog value specified on the listen call cannot be larger than the value configured by the SOMAXCONN statement in the stack's TCPIP PROFILE (the default value is 10). No error is returned if a larger backlog is requested. If you want a larger backlog, update the SOMAXCONN statement. See the z/OS Communications Server: IP Configuration Reference for details.