, since the used IP address space does not topologically belong to the operator/used PLMN. Figure 4.8 shows an example GTP-based 3GPP EPC deployment with an external to the PGW 'APN router', which should also serve as the first-hop router to the UE. There are multiple tunneling technologies that are not limited to those listed below: Generic Routing Encapsulation (GRE) [88] Layer 2 Tunneling Protocol version 3 (L2TPv3) [89] IPsec tunneling [73, 77] IP in IP tunneling (IPIP) [90]. All of the listed tunneling protocols are capable of transporting IPv6 traffic within the tunnel, if the support is implemented by both tunnel endpoints. There are some non-trivial IPv6 link model specific considerations with the external router deployments to take into account. There have been implementations that do not have entirely correct link model in all cases. According to the IPv6 standard RFC 4861 [24], the first-hop router on the link is the node that can also originate the Router Advertisement. However, this is not always understood, especially when the PGW is made to behave like a bridge (or a relay) and the external router that terminates the tunneling protocols is supposed to be the first-hop router. There are deployed implementations where the PGW terminates all NDP messages (including originating the Router Advertisement) but from the IPv6 hop-limit point of view the external router is the first-hop router. This obviously is not correct. Whether it has any practical negative impact is an another issue. It could have, for example, if the PGW does not allow on-link scoped IPv6 traffic destined to multicast addresses to go through it. Regarding PMIP on S5-interface, having an external first-hop router and still getting the IPv6 link-model correct is not trivial. This is due the fact that the SGW is supposed to be the first-hop router when PMIPv6 protocol is used instead of the PGW. Therefore, the tunnel to the external router should originate from the SGW instead of the PGW. However, this is not the case. Whether this detai