How does Linux consider network resource consumption to be a process?

Question

System administration

How can I use either only built-in utilities (cat, ls, objdump, od etc) to understand which process consumes network traffic and how much?
Where to look and how to count?
Ideally, you need to write a shell utility that shows which process is eating up network resources.
Not all systems have pre-installed all sorts of systems that monitor bandwidth per process, it gets here and you need to make a bike.

Answer 1

https://github.com/raboof/nethogs/tree/master/src see the source, nethogs gets everything from /proc.