orged Source Link-Layer Address option into the Neighbor Advertisement message). All the above are still hacks/tricks to mostly alleviate issues caused by broken UE implementations. The UE and the PGW implementations should just understand the 3GPP link model properly and then implement the NDP state changes accordingly. 4.9.2 PDN Connection Model and Multiple IPv6 Prefixes When 3GPP was adding IPv6 into their specifications, IETF recommended that the 3GPP link should allow multiple IPv6 prefixes on the link - see Section 2 of [168]. As we know, this recommendation never materialized in 3GPP. Instead when multiple prefixes are desired then multiple PDN Connections (default bearers) get activated. There are certain benefits to this approach such as the possibility for a UE to be connected to different PGWs simultaneously that may be dedicated to different purposes. On the other hand, using multiple PDN Connections just to enable multi-addressing sounds a bit wasteful. Each connection requires a handful of resources on the network side. The connection activation signaling is not the most lightweight one and involves multiple core network elements. On the UE side, multiple connections imply multiple Radio Access Bearers (RABs) over the radio interface, which are also considered as an expensive resource. However, a single /64 prefix on a PDN Connection is such a fundamental part of the 3GPP link model that changing it is not straightforward. We still consider the current model too restrictive. Changes to the 3GPP link model and the PDN Connection address- ing assumptions are definitely something worth rethinking for future 3GPP releases. We will offer some initial discussions about the possible evolution of the 3GPP architecture in Chapter 6. 4.10 IPv6 Specific Security Considerations IPv6 has been advocated as a more secure protocol than IPv4. Unfortunately, this not true and was most likely inspired by the early IPv6 node requirements [169] stating that every Deploying IPv6 in 3GPP Networks IPv6 stack must support 