## 2 Introduction to IPv6  
IPv6 is the successor of IPv4 and will replace it in the long run as the main protocol of the network layer. IPv6 is aimed at providing end-to-end communication between network interfaces even when the number of Internet participants and corresponding demand for address space keep on increasing massively, for example caused by the growing demand for Internet-enabled mobile devices. Security, Quality of Service (QoS), and reduced load for routers are further goals of IPv6 [5]. IPv6 is not downward compatible, therefore a simple switch of protocols is not possible. This is also due to various old network devices that are optimized for the use with IPv4 and hence do not support new version. The development of the next generation Internet protocol began in 1993 when people realized that the address space would not suffice forever. IPv6 was first published in 1995 in the RFC 1883 [1], which was deprecated in 1998 by RFC 2460 [23]. IPv6 quadruples the address length of IPv4 to 128 bit. This extension leads to an exponentially growth of the address-space size to \(2^{128}\approx 340\) undecillion. This would in theory correspond to 6.65 \(\times 10^{23}\) addresses per square meter of the earth. Such a tremendous amount of addresses makes it possible togive a unique address to every device connected to the Internet for a practically indefinite amount of time and enables a true end-to-end communication among them. The effectively available address space is certainly smaller than theoretically possible, since large blocks are reserved for special purposes such as multicast, or for purposes yet unknown. The smallest allocation possible is furthermore a/64 prefix. This leaves 64 bit to be assigned to network devices. While this will also lead to a lot of waste of addresses, this decision was made to improve manageability and routability of networks [23]. Moreover, there are also further standards published around IPv6 that, for example, define interoperability with other protocols or compatibility with IPv4. Basically, IPv6 serves the same purpose as IPv4 does, namely the packet-oriented connection of host systems. The following are the main features introduced with IPv6: a simplified IP header structure, Extension Headers, Stateless Address Autoconfiguration (RFC 4862) [24], IP Security Extensions (IPsec), Mobile IPv6 (MIPv6), QoS, route aggregation, and Path Maximum Transmission Unit (PMTU) Discovery. Although IPv6 has already been specified in 1995, IPv4 is still the most popular protocol in networks of all sizes including the Internet as has been shown by several studies. With CAIDA, kc claffy investigated the global IPv6 peering of AS's in 2010. Only 307 thousand paths to networks were sufficient to cover 99% of all routed prefixes for IPv6, while 170 million paths where used for IPv4, covering 96% of all routed IPv4 prefixes [25]. According to Dell Inc., there were only 44 ISPs worldwide who offered native IPv6 connectivity in 2010 [26]. Why does it take so long for IPv6 to replace IPv4? It is true that the IPv4 address space is very small and would have been exhausted for a long time if the principle of end-to-end connectivity had been upheld. However, techniques such as Network Address Translation (NAT) were developed that are virtually extending the address space, making it possible to use a single address for multiple sites by utilizing formerly unused transport layer ports [27]. Moreover, some of the features introduced with IPv6, such as IPsec [28] and QoS, were made available for IPv4 as well. Another problem is the unclear business case for IPv6 [29]. So far, there are only very few applications leveraging the features of IPv6, and there is barely any noticeable advantage for end customers. Hence, it is difficult for ISPs to sell IPv6 as a feature to customers and charge for it. Until now, most ISPs have postponed the migration of IPv6 to the point of time when it will be indispensable because IPv4 addresses will not be available anymore.