ICMPv6 is described in several RFC's is crucial for IPv6 day-to-day operations.

ICMP uses IPv6 protokol - or next header - 58 and builds on the IPv4 ICMP implementation. Several new message Types are defined in IPv6 for example Stateless Autoconfiguration.

ICMP type field: The type of ICMP packet - for example Ping
ICMP Code field: Codefield used by ICMP Type
Checksum field: Checksum that cover all of the ICMP message.
ICMP data field: The Data that connects to the ICMP Type.

ICMP Types

Examples of ICMPv6 Types
Message	Type	Type of message	definition
Destination Unreachable	1	Error	The IP address or port is not active in the Destination host
Packet Too Big	2	Error	The Packet is larger than the MTU of the outgoing link
Time Exceeded	3	Error	Hop-Linit Reached zero. The packet is discarded
Echo Request	128	Informational	A Ping packet
Echo Reply	129	Informational	A reply on a Ping packet

More information

See IPv6 in depth (Loads PDF)