TCP support is built in to the core of the Tcl interpreter. To be able to use the UDP protocol, you have to use an external package. The default choice is usually the
TclUDP extension, which is available from http://sourceforge.net/projects/tcludp/ (it also comes as a part of ActiveTcl bundle; if you don't have it, install it with
teacup install udp).
In contrast to TCP, which is a connection-oriented protocol, UDP is connection-less. This means that every data package (datagram) travels from one peer to another on its own, without a return acknowledgement or retransmission in the case of lost packets. What is more, one of the peers may send packages that are never received (for example if the second peer is not listening at ...