netdev - Re: [PATCH] Fix tcp_hybla zero congestion window growth with small rho and large cwnd

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Date:	Tue, 07 Oct 2008 15:58:59 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	root@...ielinux.net
Cc:	netdev@...r.kernel.org, ccaini@...es.unibo.it,
	rfirrincieli@...es.unibo.it
Subject: Re: [PATCH] Fix tcp_hybla zero congestion window growth with small
 rho and large cwnd

From: Daniele Lacamera <root@...ielinux.net>
Date: Sat, 04 Oct 2008 17:34:57 +0200

> Because of rounding, in certain conditions, i.e. when in congestion
> avoidance state rho is smaller than 1/128 of the current cwnd, TCP
> Hybla congestion control starves and the cwnd is kept constant
> forever.
>
> This patch forces an increment by one segment after #send_cwnd calls
> without increments(newreno behavior).
>
> Signed-off-by: Daniele Lacamera <root@...ielinux.net>

I mad a slight coding style fix and then applied
your patch to net-2.6 as follows.

tcp: Fix tcp_hybla zero congestion window growth with small rho and large cwnd.

Because of rounding, in certain conditions, i.e. when in congestion
avoidance state rho is smaller than 1/128 of the current cwnd, TCP
Hybla congestion control starves and the cwnd is kept constant
forever.

This patch forces an increment by one segment after #send_cwnd calls
without increments(newreno behavior).

Signed-off-by: Daniele Lacamera <root@...ielinux.net>
Signed-off-by: David S. Miller <davem@...emloft.net>

diff --git a/net/ipv4/tcp_hybla.c b/net/ipv4/tcp_hybla.c
index bfcbd14..c209e05 100644
--- a/net/ipv4/tcp_hybla.c
+++ b/net/ipv4/tcp_hybla.c
@@ -150,7 +150,11 @@ static void hybla_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
 		ca->snd_cwnd_cents -= 128;
 		tp->snd_cwnd_cnt = 0;
 	}
-
+	/* check when cwnd has not been incremented for a while */
+	if (increment == 0 && odd == 0 && tp->snd_cwnd_cnt >= tp->snd_cwnd) {
+		tp->snd_cwnd++;
+		tp->snd_cwnd_cnt = 0;
+	}
 	/* clamp down slowstart cwnd to ssthresh value. */
 	if (is_slowstart)
 		tp->snd_cwnd = min(tp->snd_cwnd, tp->snd_ssthresh);
--
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