The Solipsis network is probably suited to UDP best : one-way short notification messages, no critical content (at least not in the layer that handles topology discovery and maintenance). Also, Solipsis is meant to be usable on ad-hoc mobile networks, where TCP may not be a very good choice (?).

Obviously, UDP has the drawback of doing very little stuff compared to TCP. One of the issues is fragmentation ; as UDP is a datagram protocole, fragmentation has to be dealt with at whatever level.

UDP fragmentation is commonly handled by the IP layer ; that is, UDP datagrams are fragmented into several IP packets and then reassembled at the end point of the connection. This means fragmentation is theoretically handled by the OS and transparent to the application. There are several drawbacks :

• All implementations have limit to UDP packet size anyway. It seems 8192 is a common limit.
• Fragment reassembly is an all-or-nothing process ; this means if one fragment is lost (or arrives too late - it seems a common value for this timeout is around 20-30s.), the whole UDP datagram is discarded.
• Certain routers/firewalls block path MTU discovery, so that fragmentation cannot properly be done on the emitting side ; I don't know if it affects UDP as well as TCP (could UDP datagrams be fragmented later in the path ?).

Thus UDP fragmentation leads to more unreliable communications, and should probably be avoided. This means using, if possible, small enough UDP datagrams that fragmentation will not occur on common networks. Less than 1500 bytes is ok for Ethernet-based networks, and 500 bytes is said to be small enough for every network out there.

Simplistic file transfer over UDP : the TFTP (http://www.faqs.org/rfcs/rfc1350.html) protocol.

The SIP protocol can use UDP : see the SIP RFC (http://www.faqs.org/rfcs/rfc3261.html).