netdev - netem: the reorder discussion

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Date:	Fri, 23 Aug 2013 16:16:43 +0200
From:	Teco Boot <teco@...-net.nl>
To:	Eric Dumazet <eric.dumazet@...il.com>,
	Ferry Huberts <mailings@...ie.com>
Cc:	Netem <netem@...ts.linux-foundation.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"core-users@...itd.nrl.navy.mil" <core-users@...itd.nrl.navy.mil>
Subject: netem: the reorder discussion

I had reorder problems with netem qdisc. I make use of the NRL CORE network emulator, which sets up virtual routers and links, build with network namespaces and netem. Typical CORE config for a n1<-->n2 link with rate (1024Kbps), delay (50ms) and no jitter:
  qdisc tbf 1: dev n1.eth0.222 root refcnt 2 rate 1024Kbit burst 2999b lat 488.6ms 
  qdisc netem 10: dev n1.eth0.222 parent 1:1 limit 1000 delay 50.0ms
  qdisc tbf 1: dev n2.eth0.222 root refcnt 2 rate 1024Kbit burst 2999b lat 488.6ms 
  qdisc netem 10: dev n2.eth0.222 parent 1:1 limit 1000 delay 50.0ms

I added jitter to this bi-directional link, e.g. 20ms. Now the delay for each packet is 50ms +/- 20ms is 30ms to 70ms.
However, this has some unexpected results: packets may be reordered. That is because the actual delay is calculated for each packet. Some packets have a larger delay, some have a smaller. If the inter-packet spacing is smaller than the time differences set by netem (up to 2x configured jitter), packets are reordered. In this example with +/20ms, this is the case with packet rate larger than 25pps. Reordering has bad effects on transport protocol throughputs. Reordering is less common on the Internet, so I don't want to emulate such. I don't say there is no reordering, I just say I don't want this netem behavior.

The netem guide mentions this unexpected results. It was caused by a change in version 1.1 (2.6.15). For people like me that do not want this, there is a work-around mentioned. However, this doesn't work anymore since somewhere before 2.6.31.
http://www.linuxfoundation.org/collaborate/workgroups/networking/netem
https://lists.linux-foundation.org/pipermail/netem/2011-April/001507.html

So what to do? The recent patch from Eric Dumazet eliminates jitter_reordering when netem rate is configured. Maybe not the clean approach, but acceptable. Ferry his patch eliminates jitter_reordering. Some people may use reorder by jitter, so this "feature" should not get removed. So Ferry his patch is not accepted. On the other hand, people like me are very confused by current behavior.

What we could do is keep existing features and describe what netem currently does. That is:
 - netem with delay and jitter may reorder packets, if inter packet spacing is smaller than jitter
 - reordering caused by delay and jitter can be turned off by using netem rate. rate can be set to very high value is no shaping is wanted.

Are we OK with this approach?
Adjust the linux-foundation & man page info? Who can do this?

Teco

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