place before the user plane IP starts and conceptually shares similarities with PPP Network Control Protocol (NCP). Actually, in places the NAS protocol encapsulates various Internet Protocol Control Protocol (IPCP) options. The reason why we put effort into opening PCOs and NAS protocol at this level of detail is their popularity and intensive use in 3GPP networks. There is always a temptation to extend NAS protocol to every IP configuration need at the expense of IETF protocols for the same purpose. Since the introduction of non-3GPP accesses, several attempts have been made to standardize PCO delivery over DHCP [119] or PMIPv6 [120]. In the case of PMIPv6, 3GPP has already standardized a 3GPP vendor specific mobility option for encapsulating PCO [121]. Figure 4.22 shows the Packet Data Protocol Address information element [12] in detail. We have left some irrelevant (and never deployed) details out of the figure, such as the Octets Packet data protocol address IEI = 0x2B Length (2 for none, 6 for IPv4, 18 for IPv6, 22 for IPv4v6) Spare '0000' PDP Type Organization 1 for IETF PDP Type 0x21 for IPv4, 0x57 for IPv6, 0x8D for IPv4v6 5-n (24) PDP Address (IPv4, full IPv6 or IPv4 followed by full IPv6) Figure 4.22 PDP Address Information Element for GPRS. Deploying IPv6 in 3GPP Networks additional PDP Type organization choices and their respective PDP Type companions. Prior to Release-9 only PDP Types IPv4 and IPv6 were supported. If for any reason a UE or an SGSN does not understand the PDP Type content, it must be treated as PDP Type IPv4 (i.e. 0x21 in hexadecimal). The Packet Data Protocol Address information element is used in GPRS, and may appear in the Activate PDP Context Request, the Activate PDP Context Accept, the Request PDP Context Activation, and the Modify PDP Context request NAS protocol messages. Unlike in EPS, the PDP Address information element may also be included in NAS messages originated by a UE (i.e. the Activate PDP Context Request message). The information element also contains the requested