on handling related signaling, which is rather lightweight compared to conventional EPS Bearer setup. Impact on the UE Implementation The UE modifications for the evolved bearer type will concentrate around the interface between the IP stack and the cellular modem, be the IP stack and cellular modem inside the same or separate devices. Essentially, after the initial setup of the new type of bearer, the modem has to be able to map uplink packets arriving from the IP stack to correct PDCP PDU types, and differentiate the downlink packets arriving with different PDCP PDU types to the IP stack. The IP stack itself will see multiple routers and will select among them using standard procedures already used on multi-router links, and hence the IP stack itself does not need protocol changes. Of course the IP stack should have the standard features required for multi-router links, such as support for RFC 4191. However, UE mobility may cause changes on eNodeBs and/or SGWs, and hence the information Future of IPv6 in 3GPP Networks that the IP stack has learned through those routers, with ICMPv6 or DHCPv6, can become obsolete. Hence changes in eNodeB and SGW need to cause 'link-up' events from the cellular modem to the IP stack SO that the IP stack can perform information refresh procedures. There could be several approaches for presenting different PDCP PDU types to the IP stack. One viable approach, which follows the NBMA link design, is to present the IP stack with one network interface with multiple routers, each having different link-layer addresses (see Table 6.1). In this approach the cellular modem would use the specified link- layer addresses as source addresses when encapsulating downlink IPv6 packets to Ethernet frames of the UE's internal interface. The above implementation approach has the benefit of being considerably transparent to the applications, especially if the UE architecture style binds applications to use a single network interface at a time. Furthermore, if the IP stack is on a separate device, this 