ICMPv6

IPv6 requires a control protocol for exchanging and processing error and informational messages, just as IPv4 does. And like IPv4, it uses ICMP to do this. But the ICMP used by IPv6 is not the same ICMP as used by IPv4. Although ICMP for IPv4 has a Protocol Number of 1, ICMPv6 for IPv6 has a Next Header value of 58.

ICMPv6 is specified in RFC 2463. Many of the functions defined in this RFC are the same ones defined for ICMP for IPv4; but there are many ICMP messages, such as Source Quench and Timestamp, that have no equivalent in ICMPv6.

Comparing the ICMPv6 header shown in Figure 2-8 to the ICMP header shown in Figure 1.28, you can see that they are identical. And like ICMP, ICMPv6 uses a combination of type and code values to identify general types and then subtypes under them. The values defined in RFC 1885 are listed in Table 2-5.

Figure 2-8. The ICMPv6 header format.

Table 2-5. ICMPv6 Message Type and Code fields.

Type	Code	Message
1		DESTINATION UNREACHABLE
	0	No route to destination
	1	Communication with destination administratively prohibited
	2	Not a neighbor
	3	Address unreachable
	4	Port unreachable
2	0	PACKET TOO BIG
3		TIME EXCEEDED
	0	Hop limit exceeded in transit
	1	Fragment reassembly time exceeded
4		PARAMETER PROBLEM
	0	Erroneous header field encountered
	1	Unrecognized Next Header type encountered
	2	Unrecognized IPv6 option encountered
128	0	ECHO REQUEST
129	0	ECHO REPLY
130	0	GROUP MEMBERSHIP QUERY
131	0	GROUP MEMBERSHIP REPORT
132	0	GROUP MEMBERSHIP REDUCTION

In addition to the basic error and informational functions of ICMPv6, there are mechanisms that use the ICMPv6 messages. For example, the Path MTU Discovery mechanism mentioned in the previous section sends packets of increasing size to a destination. When the smallest MTU of the links on the path to the destination is exceeded by a given packet size, the packet is dropped and a Packet Too Big message is sent to the source address; the source then knows the smallest MTU on the path. And, as with IPv4, Echo and Echo Reply messages are used by the Ping function.

But in addition to basic error and information messages, there is a separate set of ICMPv6 messages defined that are used by an essential IPv6 protocol: the Neighbor Discovery Protocol, described in the next section.