es as if the appropri- ate link-layer addressing options were always present. It should also be understood that if NUD fails, the 3GPP specifications are silent about what should happen next and this would specifically be a concern for the router-host NUD. Link Model for 3GPP Access using PMIP-based Interfaces PMIP-based S5/S8-interfaces for the 3GPP access and S2a/S2b interfaces for trusted non-3GPP access were introduced to the 3GPP architecture starting from Release-8 [16]. Figure 4.4 illustrates the user plane for the PMIP-based S5/S8-interfaces. The link model for 3GPP access using PMIP-based S5/S8-interfaces has the following generic character- istics on the user plane: The link is a point-to-point resembling link between a UE and an SGW. There can only be two nodes on the link: the UE and the first-hop router. The SGW is the first-hop router for the UE. Note that this is a fundamental difference between GTP-based and PMIP-based interfaces, and there are certain known anomalies regarding the specification of the link model in the 3GPP specifications. We will discuss the PMIP-based S5/S8-interface link model further later in this section. The IPv4 address and/or IPv6 prefix is still topologically anchored at the PGW, in spite of the first-hop router being at the SGW. The 3GPP link has no link-layer addresses. The SGW may configure an IPv4 address on its interface facing the UE as the default gateway address. However, the NAS signaling, as of Release-11, is not able to convey the default gateway address to the UE. The lifetime of IP addresses on the link share fate of the PDN Connection lifetime. One IP hop One IP hop RELAY 'relay' GTP-U GTP-U UDP/IP UDP/IP LTE-Uu S5/S8 eNodeB (MAG) (LMA) Figure 4.4 The link-model and the user plane protocol stack for EPS PMIP-based interfaces. IPv6 in 3GPP Networks The 3GPP link model has the following IPv6 specific properties and characteristics: The SGW is the only peer node the UE exchanges NDP messages with. The SGW has to configure a link-local IPv6 address to its UE fa