A datagram connection communicates with individual packets rather
than streams of data. Each call to process-send sends one
datagram packet (see Sending Input to Processes), and each datagram
received results in one call to the filter function.
The datagram connection doesn’t have to talk with the same remote
peer all the time. It has a remote peer address which specifies
where to send datagrams to. Each time an incoming datagram is passed
to the filter function, the peer address is set to the address that
datagram came from; that way, if the filter function sends a datagram,
it will go back to that place. You can specify the remote peer
address when you create the datagram connection using the
:remote keyword. You can change it later on by calling
set-process-datagram-address.
If process is a datagram connection or server, this function returns its remote peer address.
If process is a datagram connection or server, this function sets its remote peer address to address.