The error message given if the DDM TCP/IP server is not started is CPE3425.
The message text is:
A remote host refused an attempted connect operation.
You can also get this message if you specify the wrong port on the Create DDM file (CRTDDMF) or Change DDM File (CHGDDMF) command. For a DB2® UDB for iSeries™ server, not using the secure sockets protocol, the port should always be 446 or 447. It is recommended that the 446 always be used for clear text transmissions, and 447 be used for Internet Protocol Security Architecture (IPSec). To start the DDM server on the remote server, run the STRTCPSVR *DDM command. You can request that it be started whenever TCP/IP is started by running the CHGDDMTCPA AUTOSTART(*YES) command.