Chapter 4 FortiOS Carrier : GTP message type filtering : Common message types on carrier networks
  
Common message types on carrier networks
Carrier networks include many types of messages — some concern the network itself, others are content moving across the network, and still others deal with handshaking, billing, or other administration based issues.
GTP contains two major parts GTP for the control plane (GTP-C) and GTP for user data tunnelling (GTP-U). Outside of those areas there are only unknown message types.