r Modify Bearer Response or PBA Accounting-Response (Interim) or bearer modifications ACA (Interim) DHCPv4/Deferred IPv4 address re-allocation/release Conditional/ Accounting-Request (Interim) or Optional ACR (Interim) Accounting-Response (Interim) or ACA (Interim) Disconnect-Request or Delete PDP Context Request or For network initiated Delete Bearer Request Disconnect-Response or context/bearer Delete PDP Context Response or deactivation Delete Bearer Response Delete PDP Context Request or Delete Session Request or PBU Accounting-Request (Stop) or Delete PDP Context Response or ACR (Stop) Delete Session Response or PBA Accounting-Response (Stop) or ACA (Stop) Figure 4.9 Possible RADIUS and Diameter interactions over the (S)Gi-interface with an external AAA server during the lifetime of the PDP Context/PDN Connection. First, a DHCPv6 client can never initiate a unicast message to a DHCPv6 server without having talked to it earlier and having received an explicit permission for unicast communication via the Server Unicast Option sent by the DHCPv6 server [26]. Having said that, a PGW that would like, for example, to select a specific DHCPv6 server based on the APN cannot rely on the normal solicitation and multicast based DHCPv6 server discovery. Sending a unicast is, on the other hand, allowed for a DHCPv6 relay. A PGW acting as a relay could accomplish the desired DHCPv6 server selection functionality, but only in the case when the UE initiated the DHCPv6 message. Deploying IPv6 in 3GPP Networks DHCPv4/v6 Server DHCPv4/v6 Client DHCPv4/v6 DHCPv4/v6 DHCPv4/v6 Relay Server Client DHCPv4/v6 Server Possible tunnel GTP-U over UDP/IP/L2/L1 GGSN/PGW (S)Gi Figure 4.10 Possible DHCPv4 and DHCPv6 modes over the (S)Gi-interface with an external DHCP server. Second, from a PGW packet processing point of view, it might be desirable that it cannot be bypassed by a UE. If the PGW has to, for any reason, inspect 'IP level control plane' such as DHCPv6, then being a relay is not optimal. If a DHCPv6 server allows unicast commun