Re: Linux Networking MTU Setting

From: Floyd Davidson (floyd_at_barrow.com)
Date: 08/21/03 

Date: 20 Aug 2003 19:37:08 -0800

sally_cmp@msn.com (sallie) wrote:
>Ian Northeast <ian@house-from-hell.demon.co.uk> wrote:
>> sallie wrote:
>> >
>> > I am a newbie of Linux networking. But my company would like me to
>> > reset the MTU of the Linux server. So, I need to reset the Linux MTU
>> > in order to have a faster speed and better performance of the network.
>> >
>> > Does anyone know what optium value for MTU is? And how to change it?
>> >
>> > I really have no idea what value it is...Thx Thx!!!
>>
>> The optimum MTU value is normally the default, e.g. 1500 for 10/100Mb
>> ethernet. If there are routers on your LAN which cannot cope with that
>> then path MTU discovery should take care of it.
>>
>> If someone wants you to change it then presumably they have some idea as
>> to why and what to. Ask them.
>>
>> Regards, Ian
>
>Thanks for your suggestion. Actually, the current setting is 1500, but
>the speed is kind of slow. So, they would like to reset the MTU.

So just how is changing the MTU going to speed things up???

Depending on what you are running (PPPoE, VPN, whatever) there
are varying amounts of overhead per ethernet frame. For example,
a PPP header is 40 bytes. Hence you might have, if you've tunneled
several protocols, say 200 bytes of headers on top of your data.
The significance is the percentage of each ethernet frame that is
overhead. With an MTU of 1500, if you have 200 bytes of header,
your data is 1300 bytes out of 1500, or 87 percent. To get just
a ten percent increase in efficiency, you'd need to have an MTU
of over 5000.

Two problems. One is buffer sizing, which was designed for
MTU's of 1500 for 10Mbps ethernet, and latency, which will be
potentially 3 times the delay with frames that are 3 times as
long.

All of which is to say that, on its face, changing the MTU does
*not* equate to speeding up a link.

That is unless you have some specific problem being caused by
the size of your MTU. But you need to specifically determine
what that might be rather than just blindly "reset the MTU"
because some ignorant suit behind a desk is screaming "Do
something!".

With that said... let me show you an example that is specific.
There are implementations of PPPoE where the PPPoE overhead of 8
bytes causes a problem. Some xDSL modems apparently do not
properly do link MTU discovery accurately, and hang as a result.
The cure is to reduce the connected ethernet interface MTU by 8
bytes to 1492 v. 1500.

Note that this is exactly the opposite direction of "more
efficient". 

-- 
Floyd L. Davidson           <http://web.newsguy.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska)                         floyd@barrow.com

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