The remote host is missing an update to the system as announced in the referenced advisory FreeBSD-SA-08:09.icmp6.asc

Upgrade your system to the appropriate stable release or security branch dated after the correction date https://secure1.securityspace.com/smysecure/catid.html?in=FreeBSD-SA-08:09.icmp6.asc

IPv6 nodes use ICMPv6 amongst other things to report errors encountered while processing packets. The 'Packet Too Big Message' is sent in case a node cannot forward a packet because the size of the packet is larger than the MTU of next-hop link. In case of an incoming ICMPv6 'Packet Too Big Message', there is an insufficient check on the proposed new MTU for a path to the destination.