inally corrected the link models for all non-3GPP accesses SO that they follow IETF definitions on link models, including PMIP-based S5/S8-interface - now the SGW officially decreases Hop Limit/TTL. 4.2 End User IPv6 Service Impact on the 3GPP System 4.2.1 User, Control and Transport Planes 3GPP made a wise decision to separate all interfaces that are transported over IP into control plane, a user plane, and a transport plane. The control plane includes signaling protocols such as Diameter [29] and GTP Control Plane (GTP-C) [3] among others. Since the IP version used to transport a control plane is independent of the actual content, the Information Element (IE) such as Diameter Attribute Value Pairs (AVPs), any control plane interface can be migrated to IPv6 from the information content point of view, while still transporting it over an IPv4-only transport plane in the packet core. This allows for a phased migration of the control plane to IPv6 - that is, keeping the actual transport plane in IPv4 while the control plane itself is migrated into IPv6. Figures 4.5a and 4.5b illustrate a typical common construction of the control plane and the transport plane in the 3GPP GPRS/EPC. All end user traffic is tunneled within a packet core, typically inside a GTP User Plane (GTP-U) [4] or a PMIP [6] tunnel. Similarly to the control plane, the actual content - that is, the user plane tunneled inner IP traffic and addressing that the end user and the UE AVPs / IEs / PMIPv6 MH + MOs Diameter / GTP-C / Control Control Plane UDP (for IPv4 only) Plane SCTP or UDP Transport Plane Transport Plane (a) A generic signaling control plane (b) A control plane for PMIP. UDP is only required when using IPv4 transport Figure 4.5 A control plane versus a transport plane. IPv6 in 3GPP Networks Payload IPv4 / IPv6 traffic Payload IPv4 / IPv6 GTP-U header traffic Plane Plane Transport Transport Plane Plane (a) A GTP user plane (b) A PMIP user plane Figure 4.6 A user plane versus a transport plane. see - is separated from the tunnel's outer IP