A classification is a definition comprising a system of definitions.
— FRIEDRICH VON SCHLEGEL
Traditionally, the post office forwards messages based on the destination address in each letter. Thus all letters to Timbuctoo were forwarded in exactly the same way at each post office. However, to gain additional revenue, the post office introduced service differentiation between ordinary mail, priority mail, and express mail. Thus forwarding at the post office is now a function of the destination address and the traffic class. Further, with the spectre of terrorist threats and criminal activity, forwarding could even be based on the source address, with special screening for suspicious sources.
In exactly the same way, ...