Asio is good but can frustrate people unaccustomed to the heavily templated boost approach. High learning barrier. cpp-netlib aims to wrap and simplify Asio for

Asio is good but can frustrate people unaccustomed to the heavily templated boost approach. High learning barrier. cpp-netlib aims to wrap and simplify Asio for people doing the typical stuff, but I've no experience. POCO, Qt, and libevent are all good approaches. Having followed Carmack a bit over the years my guess is he built something from scratch himself in C++.