What is ICMPv6?

ICMPv6 (Internet Control Message Protocol for IPv6) is IPv6's control protocol, carrying error reports, ping, and the messages hosts use to find each other.

ICMPv6 is defined in RFC 4443 and is carried directly in IPv6 as next header 58. Its messages fall into two groups. Error messages (types 0–127) include Destination Unreachable (1), Packet Too Big (2), Time Exceeded (3), which is what traceroute relies on, and Parameter Problem (4). Informational messages (types 128–255) include Echo Request (128) and Echo Reply (129), used by ping. Unlike IPv4, IPv6 also moved its local plumbing into ICMPv6: Neighbor Discovery uses types 133–137 for Router Solicitations, Router Advertisements, Neighbor Solicitations and Advertisements, and Redirects, and Multicast Listener Discovery uses ICMPv6 too.

That's why the old IPv4 habit of blocking ICMP doesn't carry over. Drop ICMPv6 on a router's own input and its neighbours can't resolve its link-layer address or hear its Router Advertisements, so SLAAC and basic connectivity break. Drop Packet Too Big messages in transit and you create path MTU black holes, because IPv6 routers never fragment packets; senders must hear Packet Too Big to shrink their packets (see PMTUD). A sensible firewall allows the error messages and Neighbor Discovery, and rate-limits or restricts Echo if needed; RFC 4890 gives detailed filtering recommendations. ICMPv6 is a separate protocol from ICMP for IPv4, with different type numbers, so rules written for one don't match the other. The IPv6 on VyOS lesson shows the firewall rules that keep Neighbor Discovery and SLAAC working.