UDP is a simple protocol. Its official specification, RFC 768 [Postel 1980], requires only three pages. The services it provides to a user process, above and beyond IP, are port numbers and an optional checksum. We used UDP to examine this checksum and to see how fragmentation is performed.
We then examined the ICMP unreachable error that is part of the new path MTU discovery feature (Section 2.9). We watched path MTU discovery using Traceroute and UDP. We also looked at the interaction between UDP and ARP whereby most ARP implementations only retain the most recently transmitted datagram to a given destination, while waiting for an ARP reply.
The ICMP source quench error can be sent by a system that is receiving IP datagrams faster than they can be processed . It is easy to generate these ICMP errors using UDP.