[OpenAFS] Re: mtu problem

Andrew Deason adeason@sinenomine.net
Thu, 7 Feb 2013 11:37:38 -0600

On Thu, 7 Feb 2013 16:36:18 +0000 (GMT)
Antony Mayi <antonymayi@yahoo.com> wrote:

> modern tcp/ip stack is setting Don'tFragment flag by default so
> oversized packets are always dropped (relevant ICMP should be sent
> back for PMTU discovery to kick in though which is not happening in my
> case).

Okay, but there is no tcp/ip traffic here. In 1.6, OpenAFS does not
(yet) interpret the ICMP errors necessary for standard PMTU discovery,
and so we explicitly turn it off on Linux. UDP PMTU must be handled by
the application; it doesn't just happen transparently like it does for
TCP communication, and we haven't done it properly yet. (We have a
different mechanism for trying to figure out the path mtu, which I
thought worked well enough for common scenarios...)

Are you actually seeing the DF bit set on the packets? If it is... well
first of all, "why?", but even if so, are you sure you're not seeing the
ICMP errors on the wire? (they could very well appear on the wire, and
we're not doing anything with them)

What version of OpenAFS is running at the two ends?

> >By "decreasing it on the client side", so you mean you adjusted the
> >actual interface MTU? You can try forcing the max packet size Rx uses
> >with the -rxmaxmtu option to afsd. Say, 'afsd -rxmaxmtu 1200'.
> yes, I meant adjusting client interface MTU. I already tried the
> -rxmaxmtu without any success.

I can try looking at this... I thought this was addressed after the
network problems at the illinois conference; but if someone remembers
more issues or something, chime in.

Andrew Deason