Subject: Re: PMTUD blackhole detection
To: Kevin Lahey <>
From: None <>
List: tech-net
Date: 05/02/2001 09:10:25
>>	on PR 12790, I posted a simple patch for TCP PMTUD blackhole detection.
>>	if there's no ack for a while, the patch will do the following:
>>	- turn off path MTU discovery for IPv4
>>	- uses minimum MTU (1280) for IPv6
>>	the behavior will remedy path MTU discovery blackhole, if our side
>>	is the sender of bigger packet.  it does not solve problems when
>>	the other side is sending bigger packet and choking with the lack
>>	of icmp too big messages.
>>	see RFC2923 section 2.1 for the exact description of the issue.
>Cool.  Is there some way that we could set a flag in the route, 
>rather than just having a per-connection flag?  That way we only have
>to pay the cost of the two timeouts once per black hole, rather
>than once per connection...

	if you know that PMTUD is failing for particular paths, you should
	be able to set a lower path MTU value by using route(8) (use -mtu