[aklug] Re: Mike B router issue

From: Jim Courtney <courtney@ieee.org>
Date: Sat Sep 05 2009 - 13:09:01 AKDT

 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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Received on Sat Sep 5 13:08:32 2009

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