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Typical UDP Exchange


As an example, your machine could periodically attempt to contact a remote network time server. Internet time servers are assigned UDP service port 123. The time server will return the current time of day. Your system could update it’s system clock with the information from the more accurate network time server. If the exchange is unsuccessful because the time server isn’t running on the remote machine or the remote machine is down at the time, an ICMP error message, Service or Host Not Available, is returned.

The client program on your system initiates the request. An unprivileged port is assigned to the request. An outgoing UDP request packet is constructed with the unprivileged port as the source port. The destination port is the well-known time service port, 123.
The time server responds with a UDP packet containing the current time. An incoming UDP response packet will arrive from the time server's IP address from source port 123, addressed to your IP address and the unprivileged destination port you initially provided.