PPPOE, MTU, and boom.

Wed Jul 18 06:53:09 UTC 2001

>I have confirmed that when I block all ICMP to/from a website, we cannot
>browse that site -- which is somewhat obivious. The question is, how, as
>an internet community as a whole, do we fix this?
>
>Seems to me that most people using PPPOE would have a problem here. Or, am
>I alone?
>
>My testing has been limited to Win2k, but I've heard similar reports on
>WinME, 98, etc.

We've come across this too, and spent quite a while diagnosing. The 
problem exists wherever there's an MTU reduction, and is caused by a 
combination of ICMP filtering (breaks PMTUD), and Microsoft's attempt at 
PMTUD (they just set the DF bit on all packets and expect to get an ICMP 
reply back if the packet is too large).

Simon
-- 
Simon Lockhart                       |   Tel: +44 (0)1737 839676 
Internet Engineering Manager         |   Fax: +44 (0)1737 839516 
BBC Internet Services                | Email: Simon.Lockhart at bbc.co.uk 
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