Re: MTU

From: Arthur Hagen (art_at_broomstick.com)
Date: 05/06/05 

Date: Fri, 6 May 2005 01:04:26 -0400

Greg <gtreanto@comcast.com> wrote:
> I switched my internet provider from Comcast to Verizon-Fios.
> After the switch I ran some test available on www.dslreports.com
> One of the tests is called tweek test. The results of tweek test
> was my linux box is sending 1480 byte packets instead of 1492
> byte packets. I adjusted the MTU size of the adapter to 1492 which
> is verrified by ifconfig -a eth0 |grep MTU. The tweek test runs fine
> on my XP box.
>
> My question is why is my SuSE 9.2 sending 1480 byte packets
> after adjusting the MTU size ? Does pmtu come into play here ?
> Thanks for any insites.

This is because 1480 is a more optimal number for PPPoE over ADSL when the
provider uses ATM, even though 1492 is the largest possible frame size. A
breakdown of *why* follows:

1480 bytes equals 30 ATM cells of 48 bytes, plus one cell with 40 bytes and
a 8 byte ATM "SAR" trailer (which ATM uses to reassemble the packets).

If using an MTU of 1492, you get 31 ATM cells of 48 bytes, plus one cell
with 40 bytes of blank padding and an 8 byte SAR. The 40 bytes of blank
padding is because ATM packets have a fixed size.

Since each ATM frame has a 5 byte packet overhead, the total overhead when
for sending an infinitely large amount of data can be calculated:

When using an MTU of 1480, the overhead is 31*5, + 8 bytes SAR, + 26 bytes
IP/ethernet/PPPoE headers, or 189 bytes per (1480 - 26) or 1454 bytes of
data, i.e. 13%

When using an MTU of 1492, the overhead is 32*5 + 40 blank padding + 8 bytes
SAR + 26 bytes IP/ethernet/PPPoE headers, or 234 bytes per 1466 (1492 - 26)
bytes of data, i.e. ~16%

Of course, even better is to switch to a provider or plan that *doesn't* use
PPPoE, but instead bridged ethernet with a MTU of 1500. Then the overhead
is 18 bytes per 1500-18 bytes of data, or ~1.2% Yes, there's that much of a
difference, and it makes sense to pay 10% extra to avoid PPPoE if possible
(there's other benefits too, like the line not having to reauthenticate if
the line goes "idle").

Many "speed tweak" guides recommend that ADSL users with PPPoE should set it
to 1492 and slams Microsoft for making it 1480 in Windows 2000/XP PPPoE.
This is just plain wrong -- for once, Microsoft got it right.
Take claims that it helped someone's speed to increase it with a *large*
grain of salt. It's more likely to be a placebo effect. 

-- 
*Art

Relevant Pages

  • Re: Cisco 5002 question
    ... 1010 ATM switch installed in the bottom slots of the Catalyst ... Modules that support EtherChannel. ... switch requires a hardware upgrade. ...
    (comp.dcom.sys.cisco)
  • Re: ATM Switch Vulnerabilities
    ... Subject: ATM Switch Vulnerabilities ... > vulnerabilities or risks, especially anything that works below the IP ... This list is provided by the SecurityFocus Security Intelligence Alert ...
    (Pen-Test)
  • Re: [fw-wiz] Firewalling between T-1s, an ATM switch and a switched office
    ... > cisco switch which connects another office. ... > Can this work through ACL's at the ATM switch? ... > ACL's on source and destination IP's and ports? ... On that you emulate your "normal" Ethernet network. ...
    (Firewall-Wizards)
  • Re: PPPoE Server Publishing
    ... Agreed, PPPoE does, but at the time it was my only option. ... get a different provider. ... OK, I'll articulate, it isn't being blocked by my ISP or from my ISA server ... Delete and recreate the Publishing Rules from scratch. ...
    (microsoft.public.isa.configuration)
  • Re: PPPoE Server Publishing
    ... I'm moving to a non PPPoE provider that should be installed today. ... should fix my problems and allow me to continue using ISA server. ... How to configure a PPPoE connection in ISA Server 2006 or in ISA Server 2004 ...
    (microsoft.public.isa.configuration)