SuSE Linux S/390: Version SLES7
When trying to connect from a suSE Linux Enterprise Server for S/390 and zSeries (both, 31bit-mode and 64bit-mode) to an OS/390 installation an error message "connection timeout" occurs.
The connection timeout should only occur in the second case, but due to OS/390 not handling the TCP option ECN (Explicit Congestion Notification) correctly, it also occurs for the first case. Turn off ECN with the following command to get the correct message "connection refused":
echo 0 >/proc/sys/net/ipv4/tcp_ecn