Subject: Re: IP DF problems (again)
To: Wolfgang Rupprecht <>
From: Jason Thorpe <>
List: current-users
Date: 02/02/1998 11:05:18
On 2 Feb 1998 09:24:49 -0800 
 Wolfgang Rupprecht <> wrote:

 > BTW, NetBSD could do path-MTU discovery, but it is off by default.
 > Avoiding problems with misconfigured firewalls is no doubt one reason
 > its still off.  (Routers that don't send back icmp's for packets that
 > they need to fragment but aren't allowed to might be another more
 > minor problem.  Are there still any out there that have this
 > lossage???)

There are two reasons it is turned off by default:

	(1) No Black Hole Detection (i.e. detect the case where you
	    don't get ACKs, but you're not getting ICMP errors either,
	    so try a lower MTU).  This is something the MS stack SHOULD
	    be doing, in order to avoid the problem Jukka noticed.

	(2) Currently, the routes added by the PMTU engine don't expire.
	    That needs to be fixed.

I believe Kevin has both of these on his List :-)

Jason R. Thorpe                             
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