someone asked Beej how sockets work in C. he got tired of explaining it. so in 1995 he put it all online.
it's been the definitive socket programming guide for 30 years.
it covers everything: TCP, UDP, IPv4, IPv6, non-blocking I/O, select(), poll().
graduate OS courses worldwide assign it. it's funnier than any technical book has a right to be.
it's free and always will be.