[Preston] MTU's and Linux

[Jerry Nicholls]

On Sat, 2004-07-31 at 03:22, Gareth Llewellyn wrote:
> Get this people.
> 
> Bulldog wants an MTU (maximum transmission unit) of 1458 in order to
> communicate correctly. Currently our NIC's are set to 1500 which means we
> are only getting around 1103kb instead of our 2272kb that we are supposed to
> be getting.
> 
> Ok now setting the MTU on our routers external interface to 1458 results in
> absoloutly no 'net access. Setting the internal NIC to 1458 as well means we
> can get out to the net.
> 
> But it gets worse, now although we can get to the net, we can only reach
> SOME of the net. Several websites result in "Destination Net Unreachable"
> and to make things worse there is no perceivable increase in bandwidth.
> 
> Any ideas as to what the problem may be?

The 1458 MTU ought not to be required anymore, but it does appear to
still have an effect.

It's related to the fact that, in general, an Ethernet frame is 1500
byte, which most NICs and other network hardware will assume. However,
where ADSL is involved there can be many varied encapsulations involved,
eg L2TP (at 42 octet). This could lead to larger packet sizes than this
default 1500 octet which would therefore require fragmentation. If a
packet was sent with the DF bit set but a firewall/whatever in the path
wasn't allowing the relevant ICMP "fragmentation needed" packet through,
then the communication channel would never work - hence the lack of
connectivity to various sites. The value of 1458 was suggested by BT
some time back when they didn't have jumbo frames (1500+ octet) enabled
within their core network, as this would allow for the 42 octect L2TP
overhead.

However, this 1458 still might not give the best throughput. You're
better of enforcing the reduced MTU on the NICs rather than the router,
and trying various values. Where I work (Zen Internet) we've tried
various values from 1430 - 1478 and they all give better performance at
different times of the day. At home I run Linux with the MTU set to 1458
and my wife's 2K PC goes via a Linux bridging firewall that forces the
MTU/MSS down by 42 - this way I don't have to pfaff with getting 2K to
honour a lower MTU, and I can also plug other people's PCs in on my
network without having to alter their settings.

Hope this made sense,
Jerry.
-- 
Jerry Nicholls <jerry at nightwatch.org.uk>