matching on ICMP responses

[Dirk Morris <dmorris@xxxxxxxxxxxx>]

Is there a way to match only certain ICMP responses that relate to 
certain other sessions.

example: is there a rule/match I can use to specify that I want to block 
all ICMP port unreachable errors, but _only_
in response to UDP packets (not TCP). Or maybe both TCP and UDP, but 
only port 123. etc etc.

Does this functionality exist? (possibly in some patch?)