ration/translation solution has to be provided. If that is a stateful NAT64 (see Section 5.3.3), then the state has to be in the mobile operator core network, which implies investment in a powerful NAT farm. On the other hand, the mobile operator may seek to deploy dual-stack access that solves the issue of accessing the IPv4 Internet. However, dual-stack does not make the IPv4 address shortage to go away. On a contrary, every dual-stack UE must eventually be configured with an IPv4 address, which typically increases the pressure for deploying even bigger NAT44 farms within the mobile operator packet core. Since these NAT solutions are usually stateful, scalability concerns are real. 5.2.2 Encapsulation The approach of encapsulating packets belonging to one address family to packets of the other address family comes with a set of benefits and drawbacks. The main benefit is that the encapsulated packets do not need to be modified, and hence can be forwarded as is. This provides the best compatibility and great transparency of the tunneling technologies for the nodes whose packets are tunneled. Due to the strong benefits, the IETF community generally prefers the tunneling based approaches when native dual-stack is not available. However, in particular for 3GPP networks, a set of significant drawbacks for host-based tunneling approaches has been identified. We discuss these drawbacks below. GPRS Tunneling Protocol (GTP) is Already There The GTP [16] used in 3GPP already is a tunneling protocol that provides mobiles with IP connectivity over various types of network infrastructures - it can tunnel over IPv4 as well as IPv6, and hence any kind of tunneling protocol on top of GTP is essentially just overhead. It has been shown from practical experience that, excluding rare Serving Gateway Support Nodes (SGSNs) that do not support the IPv6 type of Packet Data Protocol (PDP) context, the IPv6 PDP context can be set up almost anywhere on this planet where 3GPP packet access is provided. Quality of Service (QoS) Cannot Be 