approach actually mimics end host multi-interfacing. However, using multiple default bearers for configuring more than one IPv4 address and/or IPv6 prefix on a UE has undesired signaling and resource consumption side effects as mentioned in Section 4.1.1. We will return to this topic and possible future enhancements concerning multiple bearers and the IP addressing models in Section 6.2.2. Deploying IPv6 in 3GPP Networks 4.1.3 Link Models in 3GPP Link models have a significant role from the IP functionality point of view, especially when it comes to IPv6. Originally the 3GPP GPRS architecture had a single link model: a point-to-point link between a UE and a GGSN, which had been modeled after a PPP link. The same link model still holds for the 3GPP EPS architecture, when GTP-based S5/S8-interfaces are used and where the point-to-point link is between a UE and a PGW. Actually, since 3GPP Release-8 there are now multiple link models due to the additional supported IP mobility and tunneling technologies in EPC [5, 18-20]. How- ever, we only concentrate on the two IP mobility protocols and link models that have commercial deployments: a GTP-based and a PMIP-based solution for 3GPP access technologies. The Link Model for 3GPP Access using GTP-based Interfaces Figure 4.3 illustrates the user plane for the GTP-based S5/S8-interfaces. The Gn/Gp- based interfaces would be almost the same for the user plane except for different gateway naming. The link model for 3GPP access using GTP-based Gn-, Gp-, and S5/S8-interfaces has the following generic characteristics on the user plane: The 3GPP link is a point-to-point resembling link between a UE and a PGW. There can only be two nodes on the link: the UE and the first-hop router. The PGW is the first-hop router for the UE. The IPv4 address and/or IPv6 prefix are topologically anchored at the PGW. The link has no link-layer addresses. The PGW never configures an IPv4 address on its interface facing the UE. The lifetime of IP addresses on the link share fate of the PDN Connection 