When should I use UDP instead of TCP?
UDP is good for sending messages from one system to another when the
order isn't important and you don't need all of the messages to get to
the other machine. This is why I've only used UDP once to write the
example code for the faq. Usually TCP is a better solution. It saves
you having to write code to ensure that messages make it to the
desired destination, or to ensure the message ordering. Keep in mind
that every additional line of code you add to your project in another
line that could contain a potentially expensive bug.
If you find that TCP is too slow for your needs you may be able to get
better performance with UDP so long as you are willing to sacrifice
message order and/or reliability.
UDP must be used to multicast messages to more than one other machine
at the same time. With TCP an application would have to open separate
connections to each of the destination machines and send the message
once to each target machine. This limits your application to only
communicate with machines that it already knows about.