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Old 05-23-2006, 10:15 PM   #1
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Fine tuning my MTU

Currently my MTU is set at the default 1500.

What is the best way of determining my optimal MTU? Is there a program that can assist? Is there a decent guide out there for accomplishing this in Linux? (99% of the guides out there are for Windows)

Thanks in advance
Old 05-24-2006, 06:11 AM   #2
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There is no "optimal Maximum Transfer Unit" for all destinations.

On one path you may cross a link layer that is using cells of 500 while on others, frames of 1500 will not get fragmented.
It is dynamic. To resolve this, there is a protocol called Path MTU discovery.

Unfortunatly P-MTUD is not really working everywhere . Sometimes people put their firewall to a paranoid level which result in poor network performance. It gets even more complex when you cross a vpn or a private adress space where NAT doesn't do its job of Nating icmp packets.

When I was on PPPoE I had set my MTU to 1400 and I had a very good network performance.

MTU is a per-path parameter. Its not even a per-destination one as packets can take different paths to reach the same destination.

There is a way of determining the MTU to one destination:
#ping -M do -s <icmp_packet_size> <server>
Start with icmp packet size from 1400 and go up using dichotomy. Until you find the optimal "path to <server> maximum transfer unit".
The MTU for this server is icmp_packet_size+header(ip)+header(icmp)=icmp_packet_size+28

You can configure it like this:
#this disable path mtu discovery
sysctl -w net.ipv4.ip_no_pmtu_disc=1
#this sets interface mtu
ifconfig eth0 blablah.. mtu 1400

A website which was interesting for improving performance (have not checked lately)
A reference guide with some ideas:

Last edited by nx5000; 05-24-2006 at 06:12 AM.
Old 05-24-2006, 10:40 AM   #3
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I just want my mtu optimized between me and my isp gateway....Thats the only constant, and I know on this connection, it is less than 1500.

Thanks for the link, and the 'mini-guide', both usable info.


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