What is UDP/IP and is there an DOS X server that uses it?

[Jeffrey J. Hallman]

The latest issue of Unix Today! has an article "The Lowdown  on
DOS/Unix Links" by Roy Lee which compares several methods of
networking DOS and Unix machines.  In his discussion of PC-Interface,
Lee says "PC-Interface actually runs on top of UDP/IP instead of
TCP/IP to further reduce protocol overhead."  This raises the two
questions in my Subject line above, namely, what is UDP/IP and can I
run an X server on a DOS machine using it rather than TCP/IP?  All of
the DOS X servers I've read about use TCP/IP to communicate with host
systems, but according to its' creators, X-Windows can run on any
reliable network.  Assuming UDP/IP is reliable, (is it?) it should be
possible to run X over it and save the expense of buying a TCP/IP
package for each DOS machine on the network.  Am I nuts, or what?