netdev - Re: [PATCH net-next] netem: fix classful handling

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Date:	Thu, 29 Dec 2011 19:36:20 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Hagen Paul Pfeifer <hagen@...u.net>
Cc:	Stephen Hemminger <shemminger@...tta.com>,
	David Miller <davem@...emloft.net>,
	Dave Taht <dave.taht@...il.com>,
	"John A. Sullivan III" <jsullivan@...nsourcedevel.com>,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next] netem: fix classful handling

Le jeudi 29 décembre 2011 à 19:25 +0100, Hagen Paul Pfeifer a écrit :
> * Eric Dumazet | 2011-12-29 19:10:36 [+0100]:
> 
> >I dont quite understand the question. The patch I posted is supposed to
> >fix the problem. What do you want to tell to the user ?
> 
> I assumed that the patch makes it possible to replace standard tfifo?! Tfifo
> provides strict ordering, other qdisc's do not. So I thought if someone use
> netem rate|jitter with e.g. SFQ then this should be mentioned somewhere. E.g.
> "rate|jitter can only be used with tfifo qdisc". Correct me if I am wrong.
> 

Current netem uses a single queue, default tfifo.

Then if you change this tfifo by SFQ, you lose tfifo, and netem doesnt
work at all.

I claim we cannot remove tfifo.

After my patch, you now have :

1) An internal mandatory tfifo queue, to fulfill time_to_send
requirements.

2) An optional qdisc (SFQ in your example), where packets are queued
once dequeued from tfifo at the right time (after netem delay/rate
respected), eventually a packet can finaly be delivered to device with a
_bigger_ delay than the one predicted in tfifo, because of trafic
shaping happening in this optional qdisc.

Full netem block :

              +---------+                   +--------+
-->>-enqueue--| tfifo   |---->>>>--enqueue--|  SFQ   |-dequeue--->>>>> 
              +---------+                   +--------+

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