Re: 2 cents re: MTU

Alan Jarvi (Alan.Jarvi@noaa.gov)
25 Jun 1998 07:00:07 -0400



I can personally vouch for the fact that if you get an incorrect setting for
RWIN or the other settings, it will affect your connection. I played around
with RWIN and other settings, when I was using a dial connection to my former
ISP, and forgot about changing it again when I got connected via a cable modem
(this is for my home system)...The cable connection was dog slow until I changed
it back to the Windows default value of 8192 (I believe that is the correct
number). I now download a 20 Meg file from Netscape (for testing the
connection) in 2-3 minutes versus 30 minutes or more with the incorrect setting.

Bottom line...make sure you keep track of any and all changes you make...:-)

Al

----
Al Jarvi
Computer Specialist (Systems Analyst)
NOAA, National Weather Service
Office: 301.713.0386 Ext: 118
Email: Alan.Jarvi@noaa.gov
______________________________ Reply Separator _________________________________
Subject: 2 cents re: MTU
Author: jamie@erbes.com at EXTERNAL
Date: 6/25/1998 3:14 AM

MTU - Max transmission unit - default from windows is 1500 which is
good for LAN but typically not so good for analog dial-up. The objective is to
match the setting of your ISP, which is usually 576. You may need to call your
ISP to see what their MTU is for your access type.

MSS - Max Segment Size - Windows adjusts this automatically according to your
MTU.

RWIN - Receive window - I've seen Windows default to 8192, which, again, is OK
for LAN but not so good for analog dial-up. A good rule of thumb is to set
RWIN at 4X your MSS. Then round it to a number divisible by 32 for further
optimization.

So if your ISP sets MTU at 576, you should change yours to match. Then subtract
40 for your MSS value of 536 (but don't set it because Windows will take care
of it for you). Multiply 536 by 4 for 2144 and round it to 2048 for your RWIN
setting.

General guidelines - as someone commonly writes in this list: "Mileage will
vary"


Jamie Erbes - jamie@erbes.com
Erbes Development Group, Inc.
http://www.erbes.com