29.0613GPPInterworking between the Public Land Mobile Network (PLMN) supporting packet based services and Packet Data Networks (PDN)Release 17TS
The Packet Domain can interwork with IP networks and networks handling Non-IP data services. When interworking with the IP networks, the Packet Domain can operate IPv4 and/or IPv6. The interworking point with the IP networks or networks handling Non-IP data services is at the Gi and Sgi reference point. Additionally, the interworking point with network handling Non-IP data services may also be the T6a (T6) reference point for Control Plane CIoT EPS Optimizations (see 3GPP TS 29.128 ) . These interworking points are shown in figure 7.
Figure 7: IP network interworking
The GGSN/P-GW for interworking with the IP network is the access point of the Packet Domain (see figure 8). In this case the Packet Domain network will look like any other IP network or subnetwork.
Figure 8: The protocol stacks of GGSN and P-GW for the IP network interworking
Typically in the IP networks, the interworking with subnetworks is done via IP routers. The Gi reference point is between the GGSN and the external IP network; and the Sgi reference point is between the P-GW and the external IP network. From the external IP network’s point of view, the GGSN/P-GW is seen as a normal IP router. The L2 and L1 layers are operator specific.
It is out of the scope of the present document to standardise the router functions and the used protocols in the Gi/Sgi reference point.
Interworking with user defined ISPs and private/public IP networks is subject to interconnect agreements between the network operators.
No user data or header compression is done in the GGSN/P-GW.
Both the GGSN/P-GW (for both Control Plane and User Plane CIoT EPS Optimizations) and the SCEF (for Control Plane CIoT EPS Optimization) for interworking with the network handling Non-IP data services are the access points of the Packet Domain. See 3GPP TS 23.401  and 3GPP TS 23.060  for further details.