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

Greg KH
2007-12-13T06:46:35Z

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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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