netdev - Re: [PATCH 1/2] improve netem reorder flexibility

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Date:	Sat, 19 Dec 2009 11:38:16 +0100
From:	Christian Samsel <christian.samsel@...h-aachen.de>
To:	David Miller <davem@...emloft.net>
Cc:	shemminger@...tta.com, netdev@...r.kernel.org
Subject: Re: [PATCH 1/2] improve netem reorder flexibility

Am Samstag, 19. Dezember 2009 04:59:07 schrieb David Miller:
> From: Christian Samsel <christian.samsel@...h-aachen.de>
> Date: Fri, 18 Dec 2009 19:01:28 +0100
> 
> > Am Freitag, 18. Dezember 2009 18:32:40 schrieb Stephen Hemminger:
> >> On Fri, 18 Dec 2009 14:18:37 +0100
> >>
> >> Christian Samsel <christian.samsel@...h-aachen.de> wrote:
> >> > This patch adds a new feature to netem: The newly introduced parameter
> >> > reorderdelay TIME is the time a reordered packet is delayed. It can be
> >> > used in a combination with delay TIME to enable netem to produce late
> >> > packets, which is not possible in the standard netem / iproute2. In
> >> > the standard version reordered packets are always sent immediately and
> >> > therefore are always early packets.
> >>
> >> I like the idea but is it binary compatible with older kernels/ older
> >>  iproute2 utilities?
> >
> > Thats a good point. I will test it. So far i think it should work.
> 
> It won't work.
> 
> If you change the size of the tc_netem_qopt, older kernels will
> reject because the size is different from what the kernel expects.

I just tested it both ways (vanilla kernel + changed tc, changed kernel + 
vanilla tc). The reordering behaved wierd. But i did not see something like a 
"reject". Other netem features like DROP still worked fine.
Are you sure this can't be fixed in some way? From my feeling, it might be 
just a small tweaking in parameter handling necessary.

Obviously, sending the new data to an unchanged kernel won't work, but that 
should be ok, isn't it?

> So newer iproute2 will not work with older kernels.
> 
> You can't change these structures, add new netlink attributes
> to pass the new data instead.

I will look into this, but having similiar netem data in two different places 
seems awkward to me.
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