[PATCH net-next] netem: markov loss model transition fix

[PATCH net-next] netem: markov loss model transition fix

[Hagen Paul Pfeifer]

The transition from markov state "3 => lost packets within a burst
period" to "1 => successfully transmitted packets within a gap period"
has no *additional* loss event. The loss already happen for transition
from 1 -> 3, this additional loss will make things go wild.

E.g. transition probabilities:

p13:   10%
p31:  100%

Expected:

Ploss = p13 / (p13 + p31)
Ploss = ~9.09%

... but it isn't. Even worse: we get a double loss - each time.
So simple don't return true to indicate loss, rather break and return
false.

Signed-off-by: Hagen Paul Pfeifer <hagen@jauu.net>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Stefano Salsano <stefano.salsano@uniroma2.it>
Cc: Fabio Ludovici <fabio.ludovici@yahoo.it>
Signed-off-by: Hagen Paul Pfeifer <hagen@jauu.net>
---
 net/sched/sch_netem.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index a6d788d..6bf9088 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -235,7 +235,6 @@ static bool loss_4state(struct netem_sched_data *q)
 			clg->state = 2;
 		else if (clg->a3 < rnd && rnd < clg->a2 + clg->a3) {
 			clg->state = 1;
-			return true;
 		} else if (clg->a2 + clg->a3 < rnd) {
 			clg->state = 3;
 			return true;
-- 
1.8.4.rc3

Re: [PATCH net-next] netem: markov loss model transition fix

[David Miller]

From: Hagen Paul Pfeifer <hagen@jauu.net>
Date: Tue, 22 Oct 2013 23:27:06 +0200

> The transition from markov state "3 => lost packets within a burst
> period" to "1 => successfully transmitted packets within a gap period"
> has no *additional* loss event. The loss already happen for transition
> from 1 -> 3, this additional loss will make things go wild.
> 
> E.g. transition probabilities:
> 
> p13:   10%
> p31:  100%
> 
> Expected:
> 
> Ploss = p13 / (p13 + p31)
> Ploss = ~9.09%
> 
> ... but it isn't. Even worse: we get a double loss - each time.
> So simple don't return true to indicate loss, rather break and return
> false.
> 
> Signed-off-by: Hagen Paul Pfeifer <hagen@jauu.net>
> Cc: Stephen Hemminger <stephen@networkplumber.org>
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Stefano Salsano <stefano.salsano@uniroma2.it>
> Cc: Fabio Ludovici <fabio.ludovici@yahoo.it>
> Signed-off-by: Hagen Paul Pfeifer <hagen@jauu.net>

Applied.