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Date:      Fri, 21 May 1999 20:27:23 +0300
From:      Ruslan Ermilov <ru@ucb.crimea.ua>
To:        Anand Buddhdev <arb@anand.org>
Cc:        freebsd-questions@FreeBSD.ORG
Subject:   Re: Path MTU discovery on FBSD
Message-ID:  <19990521202723.A48193@relay.ucb.crimea.ua>
In-Reply-To: <19990521173815.B1722@africaonline.co.ke>; from Anand Buddhdev on Fri, May 21, 1999 at 05:38:15PM %2B0300
References:  <19990521173815.B1722@africaonline.co.ke>

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On Fri, May 21, 1999 at 05:38:15PM +0300, Anand Buddhdev wrote:
> Today one of the firewalls at my upstream provider started to block ICMP. I
> noticed that a lot of my email started slowing down. I got a lot of
> deferrals (I'm using qmail). I have heard that this can happen when Path
> MTU discovery fails becuase of ICMP blocks.
> 
This is true.

> Does FreeBSD 3.1-RELEASE do this?
> 
In FreeBSD, MTU discovery is performed on a per route basis and is ``on'' by
default.  To make your FreeBSD host not to do P-MTU discovery, you should lock
MTU along a route.  This could be done with ``-lock -mtu'' option of ``route''
command.  For example, in /etc/rc.conf, set

defaultrouter="<your_default_router> -lock -mtu 1500"

> How could I actually diagnose that this is indeed happening. I know
> about tcpdump, but I'm not very familiar with the details of TCP/IP.
> 
When P-MTU discovery takes place, packets come with DF bit set.  If the
packet (with DF bit set) can't be further forwarded by some interim router,
this router sends back an ICMP message type 3 code 4 (destination unreachable:
fragmentation needed but don't-fragment bit set).  When your host receives
such a message, it corrects its routing table by lowering down MTU.

If your ISP blocks incoming ICMP, you'll never receive such messages, thus
your host will re-transmit packet until timeout.

> any pointers will be appreciated.
> 
1. RFC-1191 - Path MTU discovery
2. http://www.worldgate.com/~marcs/mtu/ - Path MTU Discovery and Filtering ICMP


Cheers,
-- 
Ruslan Ermilov		Sysadmin and DBA of the
ru@ucb.crimea.ua	United Commercial Bank
+380.652.247.647	Simferopol, Ukraine

http://www.FreeBSD.org	The Power To Serve
http://www.oracle.com	Enabling The Information Age


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