GTP-C contains the networking layer messages. These address routing, versioning, and other similar low level issues.
For GTPv0 and GTPv1, when a subscriber requests a Packet Data Protocol (PDP) context, the SGSN will send a create PDP context request GTP-C message to the GGSN giving details of the subscriber's request. The GGSN will then respond with a create PDP context response GTP-C message which will either give details of the PDP context actually activated or will indicate a failure and give a reason for that failure. This is a UDP message on port 2123.
GTP-C message types include Path Management Messages, Location Management Messages, and Mobility Management Messages.