GTPv0
There have been three versions of GTP to date. The original version of GTP (version 0) has the following differences from version GTPv1.
• the tunnel identification is not random
• there are options for transporting X.25
• the fixed port number 3386 is used for all functions, not just charging
• optionally TCP is allowed as a transport instead of UDP
• not all message types are supported in version 0