v4-only, then PDN Type IPv4 PDN Connections are requested. If the UE is an IPv6-only, then PDN Type IPv6 PDN Connections are requested. If the UE has a dual-stack, then PDN Type IPv4v6 PDN Connection should be requested. If the UE does not know what PDN Types the radio modem supports, then PDN Type IPv4v6 PDN Connection should be requested. Depending on the network side support and configuration, the activated PDN Con- nection might have a different PDN Type than was requested. In reality, UEs do not IPv6 in 3GPP Networks (need to) follow these guidelines and there are typically per-APN settings provided by the UE vendor or the operator that override the 3GPP specified default behavior. There are, for example, deployment cases where it makes sense for an operator to favor single IP version PDN Connections for dual-stack capable UEs and provide dual-stack connec- tivity using one of the available IPv6 transition mechanisms. We discuss IPv6 transition in detail in Chapter 5. 4.5.2 Backward Compatibility with Earlier Releases There are a few PDN Type handling rules that are important to remember. Prior to 3GPP Release-9 an SGSN treats a PDP Type IPv4v6 as a PDP Type IPv4 (like any PDP Type it does not understand) [12]. On the other hand, an SGSN starting from a Release-9 SGSN and an S4-SGSN/MME starting from Release-8 treat an unknown PDN Type as a PDN Type IPv6 [13]. The unknown PDN Type may arrive at the MME either from a UE or an HLR/HSS. The treatment should be the same. Starting from late 3GPP Release-10, the PDN Type IPv4v6 in an HLR/HSS subscription profile implicitly means that both PDN Types IPv4 and IPv6 are supported for the given APN configuration, and a SGSN/MME can then act accordingly. Prior to Release-10 the SGSN/MME might have rejected the UE initiated request for the PDN Type IPv4 or IPv6 even if the PDN Type IPv4v6 was subscribed, but not IPv4 or IPv6. This was the case in some early deployments, for example, when an HLR had a PDP Type IPv4v6 provisioned for an APN and the subscription profile got