netdev - [PATCH] tcp-illinois: incorrect beta usage

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Date:	Wed, 28 Nov 2007 15:47:25 -0800
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	"Julian Shao Liu" <shaoliu@...c.edu>,
	"David S. Miller" <davem@...emloft.net>,
	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	"'Lachlan Andrew'" <lachlan.andrew@...il.com>,
	<shaoliu@...nceton.EDU>, "'Douglas Leith'" <doug.leith@...m.ie>,
	"'Robert Shorten'" <robert.shorten@...m.ie>, netdev@...r.kernel.org
Subject: [PATCH] tcp-illinois: incorrect beta usage

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@...ux-foundation.org>

--- a/net/ipv4/tcp_illinois.c	2007-08-18 07:50:15.000000000 -0700
+++ b/net/ipv4/tcp_illinois.c	2007-11-28 15:39:04.000000000 -0800
@@ -298,7 +298,7 @@ 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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