Layering costs time since all those packet headers have to be created at the sender and decoded at the receiver. However, they make it easy to mix and match different technologies and to keep track of where the packets are and where they are going. Not all computers use Ethernet, but may use token ring or several other technologies, but the IP packets are the same on all. Further, UDP is used for some applications instead of TCP. For instance, the Unix talk program, which is a primitive two-person chat facility or typewritten telephone, uses UDP because it is not imperative that every packet be delivered with 100% reliability.