967,295) bytes [29]. This option is still waiting for the future, as there are very few link types that can transport packets larger than 65535 bytes and through the Internet it is impossible to find paths that would allow Maximum Transmission Units (MTUs) larger than 65535 bytes (see Section 3.2.3 for Path MTU discussion). In practice the Path MTU is at maximum 1500 bytes and is often less. Tunnel Encapsulation Limit Option: This option, defined only for the Destination options header, is inserted by a node performing tunneling to indicate to following nodes how many additional encapsulations are permitted for this packet. In the special case of 'zero' additional allowed encapsulations a tunneling node can prevent further Introduction to IPv6 tunneling taking place. If a node that would like to encapsulate the packet it receives finds it disallowed by this option, it will generate an ICMPv6 Parameter Problem message. Detailed description of this option's use is described in RFC 2473 [30]. Router Alert Option: The Router Alert option, which is only defined for the Hop-by-hop Options header, can be used to alert routers on the packet's path to examine the packet more closely [31]. The reason for alerting routers can be, for example, to require special processing such as for quality of service reasons or to get a router to pay attention to multicast messages that the router otherwise would have no interest in. The Router Alert option is specifically needed for the Multicast Listener Discovery (MLD) feature described in Section 3.2.4. Quick-Start Option (experimental): RFC 4782 defines an experimental option for the Hop-by-hop Options header that allows transport protocols, especially TCP, to deter- mine the allowed sending rate at the beginning of a transport session and after idle periods of time [32]. We will not discussing this option any further in this book. Common Architecture Label IPv6 Security Option (CALIPSO) (informational): A Hop-by-hop option for including explicit sensitivity labels for IPv6 packets i