The final major directory that we'll highlight is labelled vio, for "virtual I/O".
The vio routines are wrappers for the various network I/O calls that happen with different protocols. The idea is that in the main modules one won't have to write separate bits of code for each protocol. Thus vio's purpose is somewhat like the purpose of Microsoft's winsock library.
That wraps up our quick look at the seven major directories. Just one summary chart remains to do.