sed REQUEST DHCPACK or Create PDP Context Response or REPLY Create Session Response or DHCPREQUEST or RENEW DHCPACK or Delete PDP Context Request or REPLY Delete Bearer Request Delete PDP Context Response or Delete Bearer Response Update PDP Context Request or UE requests DHCPv4-based Modify Bearer Request or PBU address allocation or just Update PDP Context Response or wants to configure additional Modify Bearer Response or PBA parameters UE starts DHCPv4 Gateway as a DHCPv4 Relay or combined Server + Client relays messages to the server DHCPDISCOVER DHCPOFFER Not used if Rapid DHCPREQUEST Commit is used DHCPACK UE starts DHCPv6 Gateway as a DHCPv6 Relay or combined Server + Client relays messages to the server INFORMATION-REQUEST INFORMATION-REQUEST or RELAY-FORW REPLY REPLY or RELAY-REPL Figure 4.11 Possible DHCPv4 and DHCPv6 interactions over the (S)Gi with an external DHCP server during the lifetime of the PDP Context/PDN Connection. 4.2.5 Roaming Challenges While there is interest in offering a roaming [98, 99] service for IPv6-enabled UEs and subscriptions, not all visited networks, such as Visited PLMNs (VPLMNs), are prepared for outbound roamers requesting IPv6 services: The visited network SGSN may not support the IPv6 PDP Context or IPv4v6 PDP Context types. These should mostly concern pre-Release-9 2G/3G networks without an Deploying IPv6 in 3GPP Networks S4-SGSN, but there is no definitive rule, as the deployed feature sets vary depending on implementations and product licenses. The visited network might not be commercially ready for IPv6 outbound roamers, while everything works technically at the user plane level. This could lead to 'revenue leakage', especially from the visited operator's point of view (note that the use of a visited network PGW does not really exist today in commercial deployments for data roaming). The visited network might not be technically ready for billing outbound roamers using IPv6; that is, the visited operator is just not able to collect relevant and required billing and 