message_alloc
msg = message_alloc(t, 0, ISAKMP_HDR_SZ);
msg = message_alloc(isakmp_sa->transport, 0, ISAKMP_HDR_SZ);
reply = message_alloc(msg->transport, 0, ISAKMP_HDR_SZ);
extern struct message *message_alloc(struct transport *, u_int8_t *, size_t);
msg = message_alloc(t, buf, n);
msg = message_alloc(t, buf + sizeof (u_int32_t), n);