24.3273GPPGeneral Packet Radio System (GPRS) and 3GPP I-WLAN aspectsMobility between 3GPP Wireless Local Area Network (WLAN) interworking (I-WLAN) and 3GPP systemsRelease 12Stage 3TS
3GPP TS 23.327  describes how to handle the mobility between 3GPP I-WLAN and GPRS systems based on DSMIPv6 protocol. Each system, GPRS or I-WLAN, can be the UE’s home link or foreign link depending on the IP prefix associated to the systems.
The GPRS can be the UE’s home link if the HNP is assigned in the router advertisement to the UE after PDP context establishment. Otherwise, the GPRS system will be considered as a foreign link.
The I-WLAN can be the UE’s home link if the HNP is assigned to the UE by PDG with an INTERNAL_IP6_SUBNET attribute. Otherwise, the I-WLAN will be considered as a foreign link.
In both cases, the HA function is the anchor point of the UE mobility between 3GPP I-WLAN and GPRS systems.
This annex describes possible deployment cases where one of the two systems acts as the home link.