Protohackers is a programming challenge website, in the style of Advent of Code, but centred around network programming instead of generic programming challenges. It's a great little set of challenges to either train new people how to write networking code, keep your skills sharp if you don't write networking code all that often or if you want to learn how to write networking code in a new language
Smoke Test is a challenge that simply requires one to write a TCP echo server (noting that it must be able to handle binary files).