native would be for UEs to support Packetization Layer Path MTU Discovery [72]. An enlightened reader would now be slightly confused since 3GPP specifications [8] state that the maximum safe MTU value is 1358 octets. Where does this value originate? Let us examine one of the worst case scenarios where the GTP packets are tunnel mode IPsec [73] protected between a RAN node and the core network, and IPv6 is deployed even in the transport layer. In that case the user plane packet is first encapsulated in a GTP tunnel, which results in the following overhead: 40 octets of IPv6 header 8 octets of UDP header 16 octets of extended GTP-U header. This comes to 64 octets of GTP overhead. The GTP packet is further encapsulated into an IPsec tunnel. The final IPsec tunnel overhead depends on the used encryption and Deploying IPv6 in 3GPP Networks integrity protection algorithms. The 3GPP security specification [74] mandates the support for AES-CBC [75] with a key length of 128 bits and the use of HMAC_SHA-1 [76] for integrity protection. Therefore the overhead for IPsec with those algorithms can be calculated: 40 octets of IPv6 header 4+4 octets of Encapsulating Security Payload (ESP) [77] Security Parameters Index (SPI) and Sequence Number overhead 16 octets of Initialization Vector for the encryption algorithm Padding to make the size of the encrypted payload a multiple of 16 Padding Length and Next Header octets (2 octets) 12 octets of Integrity Check Value In order to make the user plane packet size as large as possible, a padding of 0 octets is assumed. With this zero padding assumption, the total overhead for ESP is 78 octets. In total, with 64 octets of GTP overheard, that comes to 142 octets of header overhead, which leads to 1500 - 142 = 1358 octets for the final user plane packet. Policy and Charging Control System There is not much to add about PCC [59] that has not already been said earlier regarding the IPv6 support for user plane IP traffic. PCC and its Diameter based interfaces have been IPv6 compliant at the 