On 01/04/06, Craig Skinner <[EMAIL PROTECTED]> wrote:
>
> On Sat, Apr 01, 2006 at 09:16:33AM +0100, tony sarendal wrote:
> > On 01/04/06, Craig Skinner <[EMAIL PROTECTED]> wrote:
> > >
> > >
> > > Set the MTU and MRU to 1453, not 1500.
> >
> >
> > 1453 ? Explain please.
> >
>
> Typo, should have been 1458:
>
> http://www.adslnation.com/support/knowledgebase/ht003.php
>
> http://www.adslguide.org.uk/guide/mtu.asp
> http://www.adslguide.org.uk/newsarchive.asp?item=899
>
>
In my case (aslo on crappy UK broadband) 1454 is actually optimal.
On the dsl part of the link my connection runs the Ethernet frames over ATM,
so I get this nice pancake when crossing the pvc:
ATM/AAL5/Ethernet/PPPoE/PPP/IP
Unless the IP packet is smaller than 38 bytes I have 34 bytes of overhead
before
splitting up into ATM cells.
If I were to use MTU 1458 that would make that 1458+34=1492 bytes.
1492 bytes will require 32 atm cells,32*53=1696 bytes.
1696/1458 = 16.3% overhead.
Now if I would use MTU 1454.
1454+34=1488
1488 bytes require 31 atm cells=1643 bytes
1643/1488=10.4% overhead.
Note that this is of course overhead on top of IP, not application.
On a side note I modifed the traffic shaper in PF to understand the real
overhead
of my dsl link, so I can now set my shaper to 280kbps (ATM PVC 288kbps) and
my QoS config works great no matter what the IP packet size is.
Before I could get packet loss on the pvc even if I had the shaper set to
160kbps
simply due to awesome overhead at smaller packet sizes.
Example: TCP ACK=40 bytes IP
That will require 2 ATM cells for me.
106/40 = a whopping 165% overhead on top of IP while crossing the ATM link.
I think I'm going to add GFP (EoSDH) and a few others here.
Enough ranting, time to feed the kids.
--
Tony Sarendal - [EMAIL PROTECTED]
IP/Unix
-= The scorpion replied,
"I couldn't help it, it's my nature" =-
