Section: 13.3.1.1 IPv4 Address allocation and IPv4 parameter configuration via DHCPv4
Content: The UE may obtain the IPv4 address and/or its configuration parameters at or after the initial access signalling (e.g. default bearer establishment) to the packet domain. The request for IPv4 address and/or configuration parameters from the UE may trigger the P-GW acting as DHCPv4 client to request the IPv4 address and/or configuration parameters from an external DHCPv4 server and deliver them to the UE. See subclause 11.2.1.2.2 for details. The DHCPv4 functions in the P-GW, the UE and the external DHCPv4 server shall be compliant to RFC 2131 [26], RFC 1542 [27] and RFC 4039 [79]. The following bullet items describe the successful IPv4 address allocation and parameter configuration signalling flow between the P-GW and the external DHCPv4 server as depicted Figure 16h. For a detailed description of the DHCPv4 messages, refer to RFC 2131 [26], RFC 1542 [27] and RFC 4039 [79]. Figure 16h is an example signalling flow for IPv4 address allocation and parameter configuration at the default bearer setup using DHCPv4 as specified in RFC 2131 [26]. If the optimized signalling (Rapid Commit Option) is used as per RFC 4039 [79], the messages 2-3 can be eliminated. 1) The DHCPv4 client function in the P-GW sends a DHCPDISCOVER as an IP limited broadcast message, i.e. the destination address 255.255.255.255, towards the external PDN. If the P-GW has the DHCPv4 server IP addresses configured for the APN, the DHCPDISCOVER shall be send as unicast (or even multicast) to the external DHCPv4 servers. 2) Upon receiving the DHCPDISCOVER request message, the external DHCPv4 servers reply by sending a DHCPOFFER message including an offered IP address. Several DHCPOFFER messages may be received by the P-GW if multiple DHCPv4 servers respond to the DHCPDISCOVER. 3) The DHCPv4 client function in the P-GW processes the messages and sends a DHCPREQUEST towards the selected external DHCPv4 server. 4) Upon receiving the DHCPREQUEST message, the selected external DHCPv4 server acknowledges the address allocation by sending a DHCPACK containing the lease period (T1), the time-out time (T2) and the configuration information requested in DHCPREQUEST. The P-GW stores the allocated IPv4 address, the lease timers and the configuration parameters. The IPv4 address and the configuration parameters shall be delivered to the UE through P-GW to UE messages. Figure 16h: The signalling flow for IPv4 address allocation and parameter configuration using DHCPv4 Figure 16i is an example signalling flow for IPv4 address lease renew by using DHCPv4 protocol as specified in RFC 2131 [26]. 1) The DHCPv4 client function in the P-GW sends a unicast DHCPREQUEST towards the external DHCPv4 server to extend the lease period of the allocated IPv4 address. 2) The external DHCPv4 server replies with a DHCPACK message confirming the renewed lease and the T1 and T2 timers are restarted. Figure 16i: The signalling flow for IPv4 address lease renew using DHCPv4