GTP Layer for the Control Plane

7.3 GTP Layer for the Control Plane

The GTP layer for the control plane (GTP-C) tunnels signaling messages between GSNs in the GPRS backbone network. The GTP-C enables several procedures to be performed through the GPRS backbone network, including path management, tunnel management, location management, and mobility management.

The path management is used by a GSN A to detect if a GSN B, with which the GSN A is in contact, is alive , or if a GSN has restarted after a failure. The tunnel management procedures are used to create, update, and delete GTP tunnels in order to route IP PDUs between an MS and an external PDN via the GSNs.

The location-management procedure is performed during the network- requested PDP context activation procedure if the GGSN does not have an SS7 MAP interface (i.e., Gc interface). It is used to transfer location messages between the GGSN and a GTP-MAP protocol-converting GSN in the GPRS backbone network.

The MM procedures are used by a new SGSN in order to retrieve the IMSI and the authentication information or MM and PDP context information in an old SGSN. They are performed during the GPRS attach and the inter-SGSN routing update procedures.

7.3.1 Path Management Procedure

The path management procedure checks if a given GSN is alive or has been restarted after a failure. In case of SGSN restart, all MM and PDP contexts are deleted in the SGSN, since the associated data is stored in a volatile memory. In the case of GGSN restart, all PDP contexts are deleted in the GGSN. A GSN A stores a GSN A restart counter in nonvolatile memory and a GSN B restart counter in volatile memory. After a GSN A restart, the GSN A restart counter is incremented, while the GSN-B restart counter is cleared. If a GGSN detects a restart in an SGSN, the GGSN deactivates all PDP contexts related to the SGSN. If an SGSN detects a restart in a GGSN, the GGSN deactivates all PDP contexts related to the GGSN before requesting the MS to reactivate them.

The path management procedure is activated by a GSN A toward a GSN B with which the GSN A is in contact. A GSN A sends an ECHO REQUEST message for each active UDP/IP path. This is considered as active when it is used to multiplex GTP tunnels between two GSNs for activated PDP contexts. If a GSN B receives an ECHO REQUEST message from a GSN A, it sends back an ECHO RESPONSE message with a recovery field that specifies its GSN restart counter. The GSN A compares its old GSN B restart counter with the one returned by the GSN B. If the two values do not match, the GSN A considers the GSN B to have been restarted. In this case, the GSN A considers that all PDP contexts used by the GSN B are inactive and therefore deletes these related PDP contexts.

Note that:

1. The event triggering the sending of an ECHO REQUEST message is implementation specific.

2. The GSN restart counter values are also exchanged between an SGSN and a GGSN during the PDP context procedures if the GSN A is in contact with the GSN B for the first time or if the GSN A has restarted and its GSN restart counter has not yet been sent to the GSN B.

Figure 7.28 illustrates the path management procedure.

Figure 7.28: Path management procedure.

7.3.2 Tunnel Management Procedures

Tunnel management procedures are defined to create, update, and delete tunnels within the GPRS backbone network. A GTP tunnel is used to deliver packets between an SGSN and a GGSN. A GTP tunnel is identified in each GSN node by a TEID, an IP address, and a UDP port number.

A GTP tunnel is created during the PDP context creation subprocedure, modified during the PDP context update subprocedure, and deleted during the PDP context removal subprocedure. All these subprocedures are executed within the GSNs during the PDP context procedures.

During these procedures, a GSN provides two IP addresses to the peer GSN: one for the control plane and another one for the user plane. These GSN addresses are used by the peer GSN for the sending of messages in the control plane and for the sending of packets in the user plane.

7.3.3 Location Management

Location management subprocedures are used between a GGSN that does not support an SS7 MAP interface (i.e., Gc interface) and a GTP-MAP protocol-conversing GSN. This GSN supports both Gn and Gc interfaces and is able to perform a protocol conversing between GTP and MAP. This is illustrated in Figure 7.29.

Figure 7.29: GGSN-HLR signaling via a GTP-MAP protocol-converter in a GSN. (From-[1].)

The location-management procedures are related to the network-requested PDP context activation procedure, and are used to determine both the SGSN IP address where the MS is located and whether the MS is reachable through this SGSN.

7.3.4 Mobility Management Between SGSNs

The MM procedures are used between SGSNs at the GPRS-attach and inter-SGSN routing update procedures.

An identity procedure has been defined to retrieve the IMSI and the authentication information in an old SGSN. This procedure may be performed at the GPRS attach.

A recovery procedure enables information related to MM and PDP contexts in an old SGSN to be retrieved. This procedure is started by a new SGSN during an inter-SGSN RA update procedure.