https://evan-moon.github.io/2019/11/17/tcp-handshake/
https://evan-moon.github.io/2019/11/10/header-of-tcp/
https://velog.io/@ahsy92/%EA%B8%B0%EC%88%A0%EB%A9%B4%EC%A0%91-TCP%EC%99%80-UDP - TCP와 UDP