l2tp_v2_session_key
u32 session_key = l2tp_v2_session_key(tunnel->tunnel_id,
u32 session_key = l2tp_v2_session_key(tunnel_id, session_id);
*key = l2tp_v2_session_key(tid, 0);
session_key = l2tp_v2_session_key(tunnel->tunnel_id,