* Nathan Ollerenshaw [2004-05-14 11:22]:
> >I'm having a problem with bandwidth for SMTP traffic to and from
> >certain hosts. It manifests itself as a sharp drop in transfer rate
> >after about 200kB, down from the regular maximum transfer rate of
> >that connection to about 4kB/s.
>
> This is exactly the kind of behaviour you will see when there is an
> PMTU blackhole.
>
> [...]
>
> For example, my home ADSL router negotiates an MTU of 1492 to my ISP.
> If I set MTU on any of my machines to 1500, small transfers will work
> fine, however anything over a certain size (1.5k, roughtly ;) will
> start to stall because the larger TCP packets will start getting
> dropped. [...]
>
> My suggestion would be to tune the MTU on your server down, which will
> lower overall performance but should work around the problem that you
> are describing. The easiest way to set it is to do:
>
> ifconfig eth0 mtu 1492
>
> You can then lower the number in steps of 8 until it starts working.
> If you want to just test it to see if this is the problem, set it to
> 1400 (far too low) and then test.
Daniel, what you're describing and a solution that Nathan provides is
exactly what happened to me during my BBF -> NGT migration. Same
symptoms, same solution. I just couldn't remember that I've "been there,
done that". When I set MTU to 1496 and the problem was gone.
--
Kirill Miazine
mailto:km@krot.org
http://km.krot.org/
