[Mimedefang] Strange activity

[Yizhar Hurwitz]

HI.

> Date: Fri, 6 Jan 2006 12:58:22 -0500
> From: WBrown at e1b.org
> Subject: Re: [Mimedefang] Re:  Strange activity

> "Yizhar Hurwitz" <yizhar at mail.com> wrote on 01/06/2006 11:58:27 AM:
>> However, regardless if this is related or not, I suggest that mail
>> servers will use by default a low MTU value,
>> for example 1300 .
>> As far as I understand, it can only improve performance and avoid
>> some problems with no negative side effects.
>
> What is the benefit of this change?  How does it improve performance?

Well, as you know, many systems nowdays use xDSL lines, that some of them have lower MTU because of tunneling protocols 
(such as PPPoE).
And also, many firewalls drop ICMP packets required for PMTU, so you cannot trust PMTU to find the best packet size.
Some firewalls might also drop fragment packets.

Therefore, manual tuning of MTU on Internet facing servers, can avoid dropped or fragmented packets.
I did not make a statistical research, but the logic is:
Lowering MTU from 1500 (Ethernet default) to something in the range 1300-1450 will have no negative impact (or 
neglectiable one) when connecting with hosts that can support MTU 1500 all the way, BUT will have a positive impact when 
connecting with hosts over lines that do need lower one.

Or the logic again rephrased:
Better be on the safe side -
don't use MTU of 1500 when you know that it can cause problems with some hosts, even if your connection does support it.

This article can help understanding part of the problem(s):
Path MTU Discovery:
http://www.sendmail.org/tips/pathmtu.html

Yizhar Hurwitz
http://yizhar.mvps.org