Re: Fragment ID wrap workaround (read-only, untested).

On Thu, 15 Jul 2004 07:49:13 -0700
David Stevens [off-list ref] wrote:

quoted

Yes, running fragmentation over those is not a good idea, but
still it should not be made worse.

Delivery to the user of incorrect data is the problem, and, no, it doesn't
make that worse. :-) The scenario, to make it clear for everyone, is a]

The data corruption problem only starts to become a real issue
at Gigabit Speeds and faster. Normally such congested links are much slower

small loss rate on a fast network leads to reassembling packets with the
same IP ID that are not the same packet when the ID wraps before the frag
queue timer has expired. If you're blasting away on a gigabit network (or
faster) and you drop one fragment (or more) from a packet you've received,
that frag queue will be there 65536 packets later when you reuse the same 
ID
for a different packet. I think that works out to be 7 secs or so at full
rate-- well within the 1-4 minute typical frag queue timer on most 
systems.

[...]

I'm well aware of the Gigabit+ NFS problem. My standard suggestion to solve
it is to just get rid of NFS over UDP - it always was a bad idea.

My point was just that you're concentrating on that one only,
but you're potentially causing more problems for slow links.
The stack has to work well both for slow and fast links though.

quoted

In general handling a link where the RTT increases would seem
tricky with your scheme. Unlike TCP there is no retransmit
to save the day.

In the particular case (NFS over UDP), there is both a retransmit (done
by RPC) and significant loss rate to start with. As long as the time-out
is conservative, I don't think this has to affect other cases
significantly.

NFS over UDP is just a bad idea. Don't do it. NFS over TCP 
works fine these days and should be the prefered choice for everybody.

I don't really see the point of risking problems with slower links
just to fix a fundamentally flawed protocol.

And you cannot rely on all UDP based protocols doing this as well.

-Andi