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Re: coverage

To: Andrew Morton <andrewm@xxxxxxxxxx>
Subject: Re: coverage
From: Donald Becker <becker@xxxxxxxxx>
Date: Mon, 3 Apr 2000 07:59:05 -0400 (EDT)
Cc: kuznet@xxxxxxxxxxxxx, netdev@xxxxxxxxxxx
In-reply-to: <38E8695B.E99D0D6A@xxxxxxxxxx>
Sender: owner-netdev@xxxxxxxxxxx
On Mon, 3 Apr 2000, Andrew Morton wrote:
> kuznet@xxxxxxxxxxxxx wrote:
> > 
> > It is easy. "ping -f" does not create any real load on the network,
> > it is pure latency test, when there is always only one packet in flight.
> I use 'ping -l 100000'
>      -l preload

And, as you found out, most of the preloaded packets are discarded, at least
with most versions of 'ping' and the kernel.  The discard is done by the
queue layer, before the device driver gets to see the packets.

> > >   - Tx stuck, invoke tx_timeout
> > 
> > I think any man working with network has in his table some broken hub,
> > which generates only collisions. 8) Mmm... what will occur if to plug
> > crossed pair to two hub slots? Will it simulate broken hub then?
> > (I did not advise you to do this! 8))

This won't accomplish what you expect.
Most chips won't transmit on twisted pair without having link beat,
producing a different type of error.

Coax transceivers always get a collision when no cable is connected,
resulting in a 16 collision error, not a Tx timeout.  You can sometimes
simulate the same effect on 10baseT with a mispaired cable, although not
with 100baseTx.

Donald Becker
Scyld Computing Corporation, becker@xxxxxxxxx

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