 use ICMPv6, such as Neighbor Discovery Protocol (NDP), do implement retransmissions when packet delivery is important. However, in some cases, such as with ICMPv6 Packet Too Big or Time Exceeded, there are no retransmissions. Therefore, for example, firewalls should not be as limiting as in IPv4 and should not filter ICMPv6. Filtering of ICMPv6 can cause unexpected issues for IPv6. For example, IPv6 Path MTU Discovery procedure that depends on the ICMPv6, and will not work if ICMPv6 packets are dropped along the way (see Section 3.2.3). Table 3.2 summarizes ICMPv6 messages that have been standardized this far. The table contains section references for the majority of the messages that are described in more detail in dedicated sections within this book. After the design of the ICMPv6, need arose to have ways to extend the already defined messages. RFC 4884 introduces a concept of multi-part operation as a way to extend ICMPv6 messages with additional information [57]. The enhancement brought by RFC 4884 is essentially introduction of a length-field to several ICMPv6 messages and an extension header appended at the end of an ICMPv6 message. This extension header can then contain extension objects, such as those defined in 'ICMP Extensions for Multiprotocol Label Switching' [58] and 'Extending ICMP for Interface and Next-Hop Identification' [59] RFCs. Roughly the ICMPv6 messages can be divided into two categories - ICMPv6 error, and ICMPv6 information messages. We will look at these two categories in more detail in the following. 3.3.1 Error Messages A set of ICMPv6 error messages has been defined that can be used by network routers and destination nodes for informing sending nodes about different error conditions. The four widely used messages are described below. Destination Unreachable A router on a network or a destination node may generate an ICMPv6 Destination Unreachable message. A router generates the Destination Unreachable when it has no route to a destination, communication with a destination is administ