Richard,

you are probably right. Are you able to set it to all
sizes smaller than 1500? Maybe they have to be a
modulo of something? Earlier I think the sizes was
750?

And yes, you can not set it higher than max size. I
mostly use atm cards for my test purposes, and there I
can set MTU's to whatever I want under 9180. 

I have done alot of testing to find out which MTU that
fits best, especially with tcp, udp, with or without
nodelay. I have found that some sizes are crap, and I
have tested alot of them. My guess is that this is due
to the fact that the IP packet does not fit very well
inside some frame sizes.

Sorry that I probably was mistaken regarding the MTU's
under ethernet. Does it improve latency and throughput
when you resized the MTU for your purposes? 

Cheers!

--- Richard Rauch <rauch@rice.edu> wrote:
> Um, *I* can set the MTU on my ethernet interfaces
> (except an ne2 PCIMCIA
> card in my laptop).  On the ne2, I can do a ``route
> change default -mtu
> ...'' however, which seems to work just about as
> well.  (You might not be
> able to set them larger than 1500, but you should be
> able to set them
> smaller.)  As I understand it, ethernet is *not*
> using fixed-sized frames,
> so setting a lower MTU is entirely possible.  It
> might not work if the
> driver, or card, don't choose to support it---though
> the route command
> should still always work to force a lower effective
> MTU.
> 
> (I needed to drop the MTU on Internet communications
> through ipnat on my
> gateway.  With default MTU's, connections sending
> large blocks tend to
> stall.)
> 
> 
>   ``I probably don't know what I'm talking about.''
> --rauch@math.rice.edu
> 


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