Generating 'Fragment Needed but DF was Set' ICMP & Dummynet

[Alexander Motin]

Hi!

I observe a strange thing. When I create dummynet pipe on output router 
interface with lower MTU system stops to generate 'Fragment Needed but 
DF was Set' ICMP in cases when it must. If I create this pipe on 
incoming interface there is no problem.

I check this on many routers under 4.8 and 5.2 FreeBSD.

Is this a bug or feature? :) How pipes can be created leaving ICMP 
generation working?

-- 
Alexander Motin