Symptoms

An installation might exhibit the behavior that KNXnet/IP devices are connected after NETx Server start and occasionally go offline after a few minutes. Shortly after, they go online. This switching is repeated throughout the runtime of the NETx Server.

Cause

KNXnet/IP uses UDP for network transport, which is a connection-less protocol. Upon sending a packet to the destination, a firewall keeps the port open for a specified time and after that closes the port. Consequently, packets received after that time are considered unrelated to the previously outgoing packet and are therefore dropped.

The KNXnet/IP protocol requires a client (like the NETx Server) to send a heartbeat signal to the device, and the device shall answer immediately.

However, if a firewall decides that the incoming packet is unrelated to previous traffic then it will be dropped and the device appears to be silent.

Resolution

Check firewalls and other active network components:

• Ensure that the respective UDP ports (default 3671, 20147) are open in both ways.
• Make sure that the UDP keep alive time is longer than 60 seconds – many firewalls allow a configuration of this setting.