The following ICMP6 types must be allowed for PMTU
Destination Unreachable (Type 1) - All codes
Packet Too Big (Type 2)
Time Exceeded (Type 3) - Code 0 only
Parameter Problem (Type 4) - Codes 1 and 2 only
See section 4.3 http://www.ietf.org/rfc/rfc4890.txt
