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[TCP] illinois: Incorrect beta usage

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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>
This commit is contained in:
Stephen Hemminger 2007-11-30 01:10:55 +11:00 • committed by Herbert Xu
commit a357dde9df
1 changed files with 1 additions and 1 deletions
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2
net/ipv4/tcp_illinois.c
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@ -298,7 +298,7 @@ static u32 tcp_illinois_ssthresh(struct sock *sk)
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);
}