> how the sendmsg and recvmsg calls will know which kernel module to use (SCTP, RTP etc.) internally(kernel mapping: how kernel handle socket call)
Depends on the type of socket that is created.
For isntance:
ret = sock_create_kern(PF_INET, SOCK_STREAM, IPPROTO_TCP, &sock);
Take a look at the stuff in kernel source /net/socket.c