Pv4. The logic from the UE point of view could be to vigorously try to establish any IP connectivity, since that most probably leads to a better end user experience than no connectivity at all. 4.5.6 Inter-RAT Handovers and Inter-SGSN Routing Area Updates The 3GPP specifications strictly mandate that during an inter-RAT handover or an inter- SGSN RAU, the PDN Types for the activated PDN Connections in the source and target nodes must match one-to-one. It is considered to be a network planning and deployment failure if that assumption does not hold. This could build up to an issue specifically when UTRAN to E-UTRAN inter-RAT handovers are desired and PDN Type IPv4v6 is used. As a reminder, GPRS had PDP Type IPv4v6 support only since Release-9, whereas EPS had that even from Release-8. The Dual Address Bearer Flag was specifically developed to alleviate issues in such deployment scenarios. Table 4.2 in Section 4.2.2 listed the 3GPP release versions of SGSNs and an MME that have IPv6 and dual-stack support. If the network deploys S4-SGSN then there are no 3GPP release issues with inter- RAT handovers between UTRAN and E-UTRAN since S4-SGSN uses EPS protocols and supported PDN Type IPv4v6 starting from Release-8. The issues arise if and when the GPRS side of the network still deploys SGSN for Gn/Gp-interfaces and the interworking takes place in a Gn/Gp-aware PGW. It could be expected that misconfigurations might happen and UEs with established dual-stack connectivity attempt to move and attach to older release network segments. IPv6 in 3GPP Networks GTPv1 specifications [9] prior 3GPP Release-9 state that when receiving an information element that has an invalid length in a request message, the receiving node sends an error response with a cause code set to 'Mandatory IE incorrect'. This would actually happen when a Release-7 SGSN receives the PDP Context information element with the Extended End User Address included for dual-stack purposes in a Forward Relocation Request message. Likewise, GTPv1 specifications prio