I am sending UDP packets from one PC to another. I monitor all activities using Wire Shark. I notice that for some time there has been a smooth transfer of packets from one system to another. Then, ICMP packets unexpectedly appear with the error 'port unreachable' . Then they disappear for some time, and again smooth transmission of UDP packets occurs. Then these ICMP packets reappear with the same "unreachable port" error. Then these ICMP packets disappear again and so on. And this continues periodically.
ICMP
'port unreachable'
Someone can shed light on what might be causing this.
The reason for this is that there is no process on the receiver waiting for this port.
You must have a client with an open socket and bind () to this port.
One odd error that may be associated with a Port Unreachable message is visible when a normal normally working conversation is interrupted by a Port Unreachable message. When you check the conversation, you notice that the unreachable port worked without problems. Frames came from the port number, when, suddenly - The port is unavailable. This indicates an overload condition or a problem with setting the process priority on the report server. The process in question was replaced with a memory and could not quickly return back to avoid an unattainable indication.
Source: https://www.savvius.com/resources/compendium/tcp_ip/unreachable#port_unreachable