lifetime. One IP hop RELAY RELAY GTP-U GTP-U GTP-U GTP-U UDP/IP UDP/IP UDP/IP UDP/IP LTE-Uu S5/S8 eNodeB Figure 4.3 The link-model and the user plane protocol stack for EPS GTP-based interfaces. IPv6 in 3GPP Networks The 3GPP link model has the following IPv6 specific properties and characteristics: The PGW is the only peer node with which the UE exchanges Neighbor Discovery Protocol (NDP) messages. The PGW has to configure a link-local IPv6 address to its UE facing interface and no other IPv6 addresses with a different scope. Specifically, the PGW does not configure any addresses out of the IPv6 prefix it advertises on the link. The link has exactly one, per UE unique /64 IPv6 prefix (under the same routing domain, if Unique Local Addresses (ULAs) are used) and the UE configures its non-link scoped unicast IPv6 addresses based on this IPv6 prefix. The lifetime of the /64 IPv6 prefix on the link shares fate of the PDN Connection lifetime. Therefore, the /64 prefix has both preferred and valid lifetimes set to infinity. The /64 prefix advertised on the link cannot be used for on-link determination. The PGW selects the IPv6 Interface IDentifier (IID) [21] for the UE. The IID is communicated to the UE over the NAS signaling and the UE is required to configure its link-local IPv6 address using the PGW selected IID. This is to make sure there can never be a Duplicate Address Detection (DAD) [22] failure on the link. (Several modem drivers and frameworks fail to deliver the IID from the radio modem and NAS signaling to the host side. Rather they generate their own IID, which opens a theoretical possibility for a link-local address collision on the 3GPP link.) NDP address resolution and redirection functions are not needed. There are no link- layer addresses or nodes other than the UE and the PGW on the link. However, doing for example address resolution should cause no harm other than generating unneces- sary traffic. NDP DAD should not be needed. The DupAddrDetectTransmits [22] configura- tion variable in the host sho