] modes of operation for GPRS and EPS use. A PGW may assume the role of a DHCP server when it needs to serve UEs and does not need to communicate with external DHCP servers in a PDN. The PGW may assume the role of a DHCP client, with or without being a DHCP server for UEs at the same time, when it needs to interact with external DHCP servers in the PDN. Finally, the PGW may also assume the role of a DHCP relay instead of being a combined DHCP server+client. All these possible modes of operation are illustrated in Figure 4.10 and described in the 3GPP stage-3 specifications [25], but not comprehensively (or clearly) in the stage-2 architecture specifications [8, 15, 16]. The different DHCP modes in a PGW have always been confusing. When the PGW is just a server or a client, its functionality is obvious. On the other hand, what is different between server-client and relay modes of the PGW? The answer lies in how the DHCPv6 protocol behaves from a client point of view when it talks to a server or a relay, and from a PGW point of view for UE initiated DHCPv6 messages. IPv6 in 3GPP Networks SGSN/SGW GGSN/PGW AAA Server Create PDP Context Request or Create Session Request or Access-Request or Only for the Primary PDP Context or Default Bearer. IPv6 prefix/address Access-Accept or allocation takes place. Accounting-Request (start) or May be sent later in Create PDP Context Response or case of deferred IPv4 Create Session Response or Accounting-Response (start) or address allocation. DHCPv4/Deferred address allocation completes or DHCPv4 lease extension Access-Request (Authorize Only) or Optional for DHCPv4 (e.g. AAR (Authorize Only) when new lease is greater Access-Accept (Authorize Only) or than session timeout), AAA (Authorize Only) or deferred IPv4 address allocation Accounting-Request (Interim) or ACR (Interim) Accounting-Response (Interim) or ACA (Interim) Update PDP Context Request or Modify Bearer Request or PBU Accounting-Request (Interim) or Conditional Update PDP Context Response or ACR (Interim) for Context o