[Gllug] What's that network congestion setting called?

[Ian Northeast]

John Hearns wrote:
> On Mon, 2005-02-07 at 09:01 +0000, John Winters wrote:
> 
>>I'm investigating a Linux box (running 2.6.8) which is getting very bad
>>network performance some of the time.  I recall a network negotiation
>>field which some routers don't implement correctly and can IIRC cause
>>symptoms similar to what I'm seeing, but after sleeping on it overnight I
>>still can't remember what it's called.
>>
> 
> path MTU discovery if I'm not wrong?

That too doesn't cause poor performance as such. The effect of a PMTUD 
SNAFU (black hole) is that small packets get through but large ones don't.

So you see things like mail connections failing with a "timeout sending 
message body" type error after a successful exchange of headers, or web 
pages failing to load after a successful connection to the server - or 
sometimes one web page loading, if it's small enough, then the next failing.

But the big packets never get through at all, they don't get through 
slowly. The source just keeps on resending them at the same size, 
because it didn't receive the "fragmentation required", until it gives up.

This can be a bugger to diagnose.

While this probably isn't the place to praise AIX, one thing I'll say 
for it is the way IBM implemented PMTUD is fail safe - if the necessary 
packets get firewalled, it just drops down to 576. It does so sometimes 
when it's unnecessary, but it never gets blackholed.

Regards, Ian


-- 
Gllug mailing list  -  Gllug at gllug.org.uk
http://lists.gllug.org.uk/mailman/listinfo/gllug