netdev - Re: [PATCH 2/2] net: netem: always adjust now/delay when not reordering

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Date:	Wed, 21 Aug 2013 08:39:35 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Johannes Naab <jn@...sta.de>
Cc:	mailings@...ie.com, netdev@...r.kernel.org
Subject: Re: [PATCH 2/2] net: netem: always adjust now/delay when not
 reordering

On Wed, 2013-08-21 at 17:17 +0200, Johannes Naab wrote:
> On 08/20/2013 05:11 PM, Ferry Huberts wrote:
> > From: Ferry Huberts <ferry.huberts@...agic.nl>
> > 
> > Not doing this (current behaviour) introduces reordering.
> > 
> > The packet_len_2_sched_time call is the only thing that logically
> > depends on q->rate, so move the now/delay adjustment out of the if.
> > 
> > Signed-off-by: Ferry Huberts <ferry.huberts@...agic.nl>
> > ---
> 
> Hi,
> 
> The documentation for netem does explicitly mention the reordering with
> jitter, and gives instructions on how to avoid it. (I have not tested if
> it works as intended).


Yes.

The user specifically adds a random delay of 0 to 510 ms to packets,
and expect netem to not reorder packets sent every 100ms.

They see netem as a single medium between two endpoints with a guarantee
of no reordering, and cumulative delays.

So if you send a burst of 100 packets, they might expect the last packet
will be send after ~50 seconds.

This clearly needs a new option to netem, because this is not the
default behavior we want.



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