epoll/iocp is the most visible difference, however, the amount of time spent porting is more due to subtle differences in every POSIX-like function. File discriptor is int? Nope, it's SOCKET. -1 means it is invalid? Nope. Use INVALID_SOCKET instead. error codes are stored in errno? No. Use WSAGetLastError() to retrieve the error. Still, EAGAIN should be EAGAIN, even on Windows? No. Use WSAEAGAIN instead. And so on and on.