Network Stack - Overview

4-Layer system
link: device drivers (OSI Layer 1 + 2)
network: IP, IPv6, ICMP (OSI Layer 3)
transport: TCP, UDP (OSI Layer 4)
application: userland apps (OSI Layers 5-7)

mostly code to validate packets and figuring out where to put them
getting the edge cases correctly is hard
robustness if the other side has bugs

Many protocols implemented
IP, IPv6, TCP and UDP the most used ones