plane. The transport plane can be migrated to IPv6 later. However, the IPv6 packet header (minimum 40 octets) is considerably larger than the IPv4 (minimum 20 octets). Since all IP traffic is tunneled in the packet core, and the transport layer Maximum Transmission Units (MTUs) may have been optimized for typical IPv4 traffic, the undesired outcome can be increased IPv6 packet fragmentation [65] or in the worst case packets not going through at all (e.g. due the filtering of Internet Control Message Protocol (ICMP) messages). Possible introduction of IPv6 transition mechanisms is not going to make the situation any better, quite the contrary (see Section 5.2.2 for detailed discussion). Typical tunneling overhead in 3GPP networks is (assuming an IPv4-based transport plane) a minimum of 36 octets originating from the outer IPv4 header (20 octets), User Datagram Protocol (UDP) [66] header (8 octets), and GTP-U header (minimum 8 octets). Assuming a 1500-octets MTU in the packet core transport plane, this would lead to a maximum working MTU of 1464 octets on the user or control planes. It is a common practice that 3GPP-aware UE stacks either optimize their 3GPP link default MTU value, or Transport Control Protocol (TCP) Maximum Segment Size (MSS) [67], in order to avoid unnecessary fragmentation in the packet core [68]. Again assuming 1500 octets MTU in the packet core transport plane, the maximum TCP MSS for the IPv6 traffic would be 1404 (i.e. 1500-36-40-20) octets where, as for IPv4, the maximum MSS would be 1424 (i.e. 1500-36-20-20) octets. Some Packet Data Network Gateways also try to alleviate IPv6 originated MTU issues by advertising an MTU in Router Advertisements that take the underlying GTP tunneling overhead into account or alternatively do on-wire MSS clamping. An IPv6 enabled end host [33] is recommended to perform a Path MTU Discovery procedure [69]. However, it is known that many networks unnecessarily filter Packet Too Big Internet Control Message Protocol version 6 (ICMPv6) messages [70, 71]. An alter