Subject: Re: Solaris 2.2 defaults to "do not fragme

From: [email-masked] (Dave Johnson)
Date: 24 Mar 1994 10:58:57 -0500

In article [email-masked], [email-masked] (W. Richard Stevens) writes:

(quoting from a previous article...)
>> I noticed recently when telneting to a Solaris 2.2 host
>> that things worked for a bit and then the connection
>> would hang and timeout.  I traced it to the IP packets
>> coming from the Sun having the "do not fragment" flag
>> turned on.  This stopped the packets from reaching me
>> because an MTU along the way was too small for the packet
>> and the router dropped the packet.  Is this the default?
>> How do I change it?
>
>It's called "Path MTU Discovery" and is described in RFC 1191.
.
.
.
>To turn it off use ndd(8) and set ip_path_mtu_discovery to 0.
.
.
.
>	Rich Stevens  ([email-masked])

We've been suffering with this problem under Solaris2.3 for
some months now, and contrary to the suggestions by Sun and
others to set ip_path_mtu_discovery to 0, this does not help
a bit.  Only setting the interface to an mtu of 850 will allow
us to ftp large files or receive large email messages from
crimelab.com; this is terribly undesirable.

Perhaps crimelab's ftp machine is also running solaris, and
transfer will continue to fail unless they also turn off
ip_path_mtu_discovery, but I think Sun's code needs to do a
better job of detecting when the packets are getting lost and
fall back to smaller packets or packets without the DF bit.

	Dave Johnson
	Gradient Technologies, Inc.
	[email-masked]