[patch 25/36] TCP: illinois: Incorrect beta usage

[Greg KH <gregkh@suse.de>]

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2.6.22-stable review patch.  If anyone has any objections, please let us
know.

------------------
From: Stephen Hemminger <shemminger@linux-foundation.org>

[TCP] illinois: Incorrect beta usage

[ Upstream commit: a357dde9df33f28611e6a3d4f88265e39bcc8880 ]

Lachlan Andrew observed that my TCP-Illinois implementation uses the
beta value incorrectly:
The parameter  beta  in the paper specifies the amount to decrease
*by*:  that is, on loss,
 W <-  W -  beta*W
but in   tcp_illinois_ssthresh() uses  beta  as the amount
to decrease  *to*: W <- beta*W

This bug makes the Linux TCP-Illinois get less-aggressive on uncongested network,
hurting performance. Note: since the base beta value is .5, it has no
impact on a congested network.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
 net/ipv4/tcp_illinois.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/net/ipv4/tcp_illinois.c
+++ b/net/ipv4/tcp_illinois.c
@@ -300,7 +300,7 @@ static u32 tcp_illinois_ssthresh(struct 
 	struct illinois *ca = inet_csk_ca(sk);
 
 	/* Multiplicative decrease */
-	return max((tp->snd_cwnd * ca->beta) >> BETA_SHIFT, 2U);
+	return max(tp->snd_cwnd - ((tp->snd_cwnd * ca->beta) >> BETA_SHIFT), 2U);
 }
 
 

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