Rob wrote:
The next router upstream will send an ICMP "destination unreachable" when it cannot resolve the IP address into a hardware address, which normally happens when it gets no reply to its ARP requests.
Only once the ARP cache times out, and only if the link level protocol uses ARP.
However, those ICMP "destination unreachable" are often ignored by applications, including ntpd. Some people even block them at their firewalls.
They are difficult to handle for UDP based protocols, as the application has to specifically receive them and to correlate them with the specific request.
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