[aklug] Re: Mike B router issue

On Sat, 05 Sep 2009 13:09:01 -0800
Jim Courtney <courtney@ieee.org> wrote:

PPPoE 1454 byte Frame:
         
TCP/IP Payload 1454
PPP Headers +2
PPPoE Headers +6
Ethernet Headers +18
           
Total Frame Size 1480

1480 / 48 bytes= 30 cells plus a 40 byte remainder

Thus, the first 30 cells will contain 48 bytes of user data, and the last cell will 40 bytes of data, plus the 8 byte SAR Trailer. Since the frame and associated ATM overhead fit exactly in to 31 cells, a 1454 byte MTU eliminates all ATM cell padding overhead.........

frame size when using a 1492 byte MTU, is actually 1518 bytes.

Stolen from the internet somewhere

> I don't know how ACS does things, but with MTA it is usually best to have
> the router accept the settings from the network during pppoe negotiation.
> This should be the factory default in a consumer router. Hopefully I'm
> remembering the details correctly here, it has been a few years. At one
> time,Microsoft web servers such as microsoft.com and hotmail had certain
> ICMPcapabilities disabled, and as a result TCP/IP sessions were unable to
> negotiate the Maximum Segment Size of the TCP payload for the connection.
> Microsoft servers would send only 1480-byte ethernet packets, so if you had
> your router MTU set too low you couldn't connect to these servers. PPPoE
> adds8 bytes of overhead so the network usually uses 1492 for PPPoE MTU,
> sincethe maximum ethernet packet size is supposed to be 1500.
> There are two numbers here - PPPoE MTU and TCP payload size. A TCP packet
> has40 bytes of overhead. 1492 - 40 = 1452. So with the common 1492-byte
> PPPoEpayload (MTU), a full TCP payload (MSS) would be negotiated as 1452
> bytes. I'm not sure how it got started, but I suspect there was some
> confusion between PPPoE MTU and TCP MSS, and 1452 started to be used as the
> value for PPPoE MTU. I can't think of any other reason for why 1452 became
> sopopular. When website operators started disabling ICMP as some sort of
> security measure, things broke.
>
>
> Joshua J. Kugler wrote: On Saturday 05 September 2009, Greg Madden said
> something like: On Saturday 05 September 2009, Royce Williams wrote: Greg
> Madden wrote, on 9/5/2009 12:19 AM: Thanks for the tip on poorly behaving
> routers. It turns out my Linksys WRT54 GL is my conection problem. I plugged
> the ACS modem, in bridge mode, directly into my box and use pppoe from
> there.No issues. This is the second router that doesn't do well, with ACS
> pppoe stuff ? Could it be an MTU issue? i don't think so, tried 1400 -1460
> inincrements of 8. Tried quite a few things... Did you try 1452? That seems
> to be a PPPOE favorite, but it's not divisible by 8. j
>
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-- 
Jeff  <jeff@arcticpc.com>
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