1. 用户层do_iptunnel
   1. IPPROTO_IPV6:sit,isatap=1
      1. tnl_add_ioctl(SIOCADDTUNNEL, "sit0", p.name, &p);
   2. 参数
      1. remote
      2. local
      3. dev
      4. ttl/hoplimit
      5. name
2. ip6_tunnel_init
   1. xfrm6_tunnel_register
      1. ip4ip6_handler
         1. ip4ip6_rcv
      2. ip6ip6_handler
         1. ip6ip6_rcv
   2. ip6_tnl_init_net
      1. 注册设备：ip6tnl0
      2. ip6_tnl_dev_setup
         1. ip6_tnl_xmit
         2. ip6_tnl_ioctl
3. sit_init
   1. xfrm4_tunnel_register
      1. sit_handler
         1. ipip6_rcv
   2. sit_init_net
      1. ipip6_tunnel_setup
         1. ipip6_tunnel_xmit
         2. ipip6_tunnel_ioctl
            1. SIOCADDTUNNEL
            2. ipip6_tunnel_locate
            3. alloc_netdev
            4. SIOCDELTUNNEL
      2. 注册sit0
4. sys_ioctl
   1. filp->f_op->unlocked_ioctl
      1. sock_ioctl
         1. cmd > SIOCDEVPRIVATE
            1. dev_ioctl
            2. dev->do_ioctl
            3. ipip6_tunnel_locate
         2. sock->ops->ioctl
5. sys_socket
   1. sock_map_fd
      1. sock_attach_fd
         1. 绑定socket_file_ops
   2. sock_create
      1. pf->create
6. inet6_init
   1. sock_register
      1. inet_create
         1. inetsw[sock->type]
         2. sock->ops = answer->ops
         3. sk_alloc
            1. sk->sk_prot = prot
      2. inet6_create
   2. proto_register
      1. 注册struct proto到proto_list
   3. udpv6_init
      1. inet6_add_protocol
      2. inet6_register_protosw
         1. inetsw6
   4. tcpv6_init
7. inet6_ioctl
   1. sk->sk_prot->ioctl
      1. udp_ioctl