telnet maxmtu slow

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From - Tue Aug 15 15:16:01 2000
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From: Jeff Liebermann <jeffl@comix.santa-cruz.ca.us>
Newsgroups: comp.unix.sco.misc
Subject: Re: problem with telnet and SCO 5.0.5
Date: Tue, 15 Aug 2000 11:43:57 -0700
Organization: Committee To Maintain an Independent Xenix
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On Tue, 15 Aug 2000 17:54:25 GMT, "jjsmd" <jjsmd@bellatlantic.net> wrote:

>I am slightly confused about your last response:
>
>at the SCO box I am connected to a Ugate3200 router which is connected to a
>ADSL modem using PPOE.
>
>at the Windoz end I am connected to a Linksys router which is connected to a
>cable modem.
>
>when you mentioned adjusting the MTU on the windoz machine was that even
>though the SCO box is the one connected to PPOE while the windoz is
>connected to a cable modem.?

Naw.  I blew it.  I was doing 3 other things while responding (like I'm
doing right now) over a period of about an hour.  I got everything
backwards.

>If that would be the case why would it work if I eliminate the Linksys? It
>seems that whaever is wrong is in the connection between my windoz and the
>Linksys including the Linksys.

Some box is doing Path MTU discovery and coming up with the wrong answer.
Allow me to change my mind (again).  Even though taking the Linksys out
of the puzzle "solved" the problem, methinks the source of the problem is
at the SCO end.  MaxMTU is a negotiated value and methinks the Linksys is
screwing up.  This was solved in Linksys firmware 1.23 where nobody could
connect to www.pcconnection.com because of Path MTU negotiation screwups.
It's the PPPoE part of the puzzle that's causing the problem.  Fire up
your web browser and disclose your Linksys version.

Try this on the OSR5 box.  Edit /etc/tcp and change the ifconfig line
from something like this (all on one line):

ifconfig -p net0 192.168.111.5  netmask 255.255.255.0 broadcast \
     192.168.111.255 perf 4096 8192 1

to something like this (all on one line):

ifconfig -p net0 192.168.111.5  netmask 255.255.255.0 broadcast \
     192.168.111.255 mtu 1492 perf 32768 24576 1

You can probably just run this from the command line and without a reboot
(not sure).  This does 3 things:
   Changes the maximum receive window from whatever to 32768 for DSL rx.
   Leave maximum send window at 24576 for both 10baseT or 100baseTX.
   Changes the Maximum MTU from 1500 to 1492 for PPPoE.
   Leaves the "don't fragment" flag turned on.
See man page at:
 http://stage.caldera.com/cgi-bin/ssl_getmanpage?ifconfig+ADMN+OS5+ifconfig
Please do NOT run "inconfig" as the instructions indicate as it breaks a
symlink causing side effects.  See:
        /etc/default/inet
for current values.  Edit manually as required and reboot when you're
convinced it works.  Check for the busted symlink:
  http://aplawrence.com/cgi-bin/ta.pl?105698

I don't think you're going to notice any performance changes as you're
limited in speed by the *OUTGOING* performance of the cable modem and DSL
lines which methinks Bell Altantic throttles to 128KB.

I'm not sure if you have to change your MaxMTU value on your Windoze
boxes (on the cable modem end).  Probably not.  If the server limits
packets to 1492, and everyone negotiates the Path MTU correctly, then it
should not be a problem.  You can sorta tell (i.e. guess) if there's a
problem by running:
        netstat -s
on the Windoze end and looking at the IP section for "datagrams failing
fragrmentation."

Also, you should re-check that the office LAN Windoze boxes still work
reliably.  Dropping the MaxMTU from 1500 to 1492 will require them to
negotiate the value.  If anything in the path blocks ICMP echo (like
through a router), the MTU negotiation may fail as previously described.


-- 
Jeff Liebermann   jeffl@comix.santa-cruz.ca.us
150 Felker St #D  Santa Cruz CA  95060
831-421-6491 pager   831-429-1240 fax
http://www.cruzio.com/~jeffl/sco/   SCO stuff

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