mtu

Tue Mar 26 19:35:42 EST 2002

You should not increase the MTU above 1500, you are just causing other
devices further upstream to have to segment your packets, to say nothing of
the fact, that you can't send out a packet with more than 1500 bytes inside
an ethernet frame; 1500 is the maximum you can put into an ethernet packet;
though, as Kerry said 1524 is the ethernet frame size, and it can hole 1500
bytes of whatever you put into it.  If you increase the MTU you will
actually slow things down.  If you want to play with other parameters that
could effect transfer efficiency depending upon the speed of your link,
propagation-delay ETC, then fiddle with things like TCP winow-size, MSS,
(though, I don't know why you'd want to play with that either), and other
things like that.

You'd only ever want to change MTU if you're on a link using PPPOE (which I
use), and then the MTU is 1456 bytes or something like that; that's a bit
more overhead per packet, but that's the price you pay.  If you're on a
slow modem link, MTU might be set lower, to speed up response when you're
doing other things on the link, but those are some of the reasons you'd
want to adjust MTU.

Brian Borowski

On Wed, 27 Mar 2002, Kerry Hoath wrote:

> The ethernet frame size is 1524 bytes. You can't
> raise the mtu higher than that you just end up with a pile of back to back
> full-sized ethernet frames and loose performance due to the
> fragmentation.
> What transferrs are you trying to speed up and what sort of card is this? There may
> be other options you can tweak that don't involve fooling with the
> framesize on ethernet.
> Increasing mtu can also cause problems with cards that are not capable of
> receiving giant frames. You're also violating
> the ethernet standards if you exceed the maximum framesize.
> I doubt you will gain any performance by reducing wire overhead, your
> need for speed can be solved in other ways. How fast is this machine and is the
> card isa/pci?
> If it isn't a pci busmaster and is an rtl8029, it's a cheap
> card and is a connectivity solution. Snap it in half and go buy a real card
> if speed and system performance is important to you or tolerate
> the slower speeds to save money.
> 3com 905 cards cost a lot of money for a good reason, they are fast
> efficient and easy on cpu.
> The rtl8029 and rtl8139 are mass market pci ethernet chips that aren't known
> for speed. Linux usually drives them as best it can but
> there are all sorts of hardware limitations to work around and
> bugs that bite.
>
> Regards, Kerry.
> On Tue, Mar 26, 2002 at 02:28:26PM -0500, Igor Gueths wrote:
> > Hi Kerry. Wouldn't increasing the mtu value decrease over head? Because I am trying to raise the value to 10000 or something, not lower it.
> > ----- Original Message -----
> > From: Kerry Hoath <kerry at gotss.net>
> > To: <speakup at braille.uwo.ca>
> > Sent: Tuesday, March 26, 2002 6:46 AM
> > Subject: Re: mtu
> >
> >
> > > I can't see why you'd want to do this on an ethernet
> > > interface anyway since ethernet is tuned to handle mtu of 1500 and reducing it
> > > is just going to increase wire overhead and do nothing more.
> > > MTU discovery means that if something else has a lower mtu this will be communicated back
> > > to the kernel and th e networking stack will take care of it.
> > > Well just because I don't know why you want to do this doesn't mean it can't be done.
> > > Once the route to the interface is in place you need to del the route and re-add it
> > > with the mtu value so for example:
> > > route del 192.168.1.0
> > > route add -net 192.168.1.0 dev eth0 mtu 768
> > > and I think the ip tools can do this too
> > > try ip route help
> > > and
> > > ip nei help
> > >
> > > Regards, Kerry.
> > > On Mon, Mar 25, 2002 at 08:19:13PM -0500, Igor Gueths wrote:
> > > > Hi. Has anyone successfully changed their mtu value on their network interface? I am unable to do this via /etc/network/interfaces. I put mtu 1000 after the line where it says iface eth0 inet dhcp. I restarted inetd and init, and the value hasn't changed. Does anyone know how to go about doing this? Thanks!
> > > >
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