p1 contains programs and files that implements the SLIP protocol (RFC 1055).
p2 contains programs and files used to create a simple interpreter that configures a described virtual network of switches, routers, and hosts, and then requests that specified operations be executed on those systems.
p3 contains programs and files that add ARP to p2.
p4 contains programs and files that add the IP layer to p3 and implements ICMP.
p5 contains programs and files that add TCP and APP layers to p4.