- Traffic That Must Not Be Dropped
- Traffic That Normally Should Not Be Dropped
- Traffic That Will Be Dropped Anyway -- No Special Attention Needed
- Traffic for Which a Policy Should Be Defined
- Traffic That Should Be Dropped Unless a Good Case Can Be Made
This is a nice reference for those who wish to implement some degree of control over ICMPv6, which is an integral part of IPv6 and not something one can blindly block.