Subject: Re: IP DF problems (again)
To: Jason Thorpe <>
From: Dave Huang <>
List: current-users
Date: 02/02/1998 17:34:15
On Mon, 2 Feb 1998, Jason Thorpe wrote:
> 	(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.

MS' stack (at least the WinNT one) _can_ do black hole detection, but
it's off by default for some reason... for those of you who have NT
machines and want to turn black hole detection on:

Registry path:

EnablePMTUBHDetect     REG_DWORD

Range: 0 (false) or 1 (true)

Default: 0 

If the value of this entry is 1, TCP tries to detect black hole routers
while doing Path MTU Discovery. TCP will try to send segments without
the Don't Fragment bit set if several retransmissions of a segment go
unacknowledged. If the segment is acknowledged as a result, the MSS will
be decreased and the Don't Fragment bit will be set in future packets on
the connection. 

EnablePMTUDiscovery     REG_DWORD

Range: 0 (false) or 1 (true)

Default: 1

Determines whether TCP uses a fixed, default maximum transmission unit
(MTU) or attempts to find the actual MTU. If the value of this entry is
0, TCP uses an MTU of 576 bytes for all connections to computers outside
of the local subnet. If the value of this entry is 1, TCP attempts to
discover the MTU (largest packet size) over the path to a remote host.
Name: Dave Huang     |   Mammal, mammal / their names are called /
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