[Gllug] OT: MTU sizes, w/ Broadband

[Murray]

so I've been reading about how BT are suggesting MTU size is changed to
1458, rather than (the common default?) of 1500.  (this isn't new, I think
it surfaced dec 2002)

http://www.adslguide.org.uk/newsarchive.asp?item=929

It also refrences another article saying BT are going to fix it at their
end.  Either way, I'm less concerned here about whether BT is giving good
advice or not, and more about the concept of MTU & it's implementation.

Anyway...Linux Networking Howto Glossary states:

MTU - ... determines the largest datagram than can be transmitted by an IP
interface without it needing to be broken down into smaller units. ....
Note, this only prevents fragmentation locally, some other link in the path
may have a smaller MTU and the datagram will be fragmented there. Typical
values are 1500 bytes for an ethernet interface, or 576 bytes for a SLIP
interface.

So, if all my local boxen are all set to 1500, then

- within my local network, things should be just dandy
- internet access *might* be a little slower

If I change to the suggested 1458, then

- I have to change *every* box on my network, otherwise a box with 1500
might get *marginally* slower locally.

But unless the whole internet uses similar values (and I assume MTU is
relative to the connection speed), there will always be fragmentation
occuring somewhere.

How much of the following holds?

- Within one local network/subnet (that I have control over), all machines
should aim to have a consistant MTU?
- If MTU fragmentation is unavoidable, where is the best place for it to
occur? On the boundary of my local network/subnet? (Aka, adsl modem and/or
gateway box)
- Am I pissing in the bitstream?



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