Chapter 4 FortiOS Carrier : ­­­­Overview of FortiOS Carrier features : GTP basic concepts : Transport : GTPv0
  
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