[H-GEN] Telstra ADSL

Bruce Campbell bc at humbug.org.au
Wed Jun 19 08:13:58 EDT 2002


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On Tue, 18 Jun 2002, Robert Brockway wrote:

> I've setup a Linux (RH 7.3) box to connect to Bigpond ADSL using the
> rp-pppoe package (it even came with directions for Bigpond :)
>
> to leave them (30 minutes or more).  I'm quite certain that it locks the
> session for any amount of data greater than 1 packet (evidence later).
> MTU is 1492 (default).

Well, not quite.  Its freezing due to the wonders of Path MTU discovery,
and the breakage that ensures when this is blocked somewhere on the path
between you and the remote site.  Abstractly, the problem is one of
ensuring that you do not send any packets that are larger than the
smallest MTU of any link between you and the site.  ( Then they'll be
fragmented, and the 2nd fragment will (obviously) die. )

I infer that this box is being a router (and that you're not doing any
proxying of TCP streams), thus this box is doing initial fragmentation
from ethernet (windows box to unix) MTU of 1500 to the pppoe mtu of 1412.

_A_ solution is to drop the MTU on the ethernet to less than this (say,
1000 bytes), _or_ to stop the windows network from getting directly out
(run them through squid and other semi-transparent proxies).

This problem is much more fun when it suddenly happens to your whole ISP,
or when someone of importance in another country suddenly cannot get their
mail from you because of their network stopping PMTU discovery from
working.

--==--
Bruce.



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