TCP/IP connection limits

The number of outstanding connection requests that can be queued at a single TCP/IP port depends on the platform. An error occurs if the limit is reached.

This connection limit is not the same as the maximum number of clients we can attach to an IBM MQ server. We can connect more clients to a server, up to the level determined by the server system resources. The backlog values for connection requests are shown in the following table:

If the connection limit is reached, the client receives a return code of MQRC_HOST_NOT_AVAILABLE from the MQCONN call, and an AMQ9202 error in the client error log ( /var/mqm/errors/AMQERROn.LOG on UNIX and Linux systems or amqerr0n.log in the errors subdirectory of the IBM MQ client installation on Windows ). If the client retries the MQCONN request, it might be successful.

To increase the number of connection requests we can make, and avoid error messages being generated by this limitation, we can have multiple listeners each listening on a different port, or have more than one queue manager.