Patch "tcp_yeah: don't set ssthresh below 2" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

tcp_yeah: don't set ssthresh below 2

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
tcp_yeah-don-t-set-ssthresh-below-2.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Neal Cardwell <[email protected]>
Date: Mon, 11 Jan 2016 13:42:43 -0500
Subject: tcp_yeah: don't set ssthresh below 2

From: Neal Cardwell <[email protected]>

[ Upstream commit 83d15e70c4d8909d722c0d64747d8fb42e38a48f ]

For tcp_yeah, use an ssthresh floor of 2, the same floor used by Reno and CUBIC, per RFC 5681 (equation 4).

tcp_yeah_ssthresh() was sometimes returning a 0 or negative ssthresh value if the intended reduction is as big or bigger than the current cwnd. Congestion control modules should never return a zero or negative ssthresh. A zero ssthresh generally results in a zero cwnd, causing the connection to stall. A negative ssthresh value will be interpreted as a u32 and will set a target cwnd for PRR near 4 billion.

Oleksandr Natalenko reported that a system using tcp_yeah with ECN could see a warning about a prior_cwnd of 0 in tcp_cwnd_reduction(). Testing verified that this was due to tcp_yeah_ssthresh() misbehaving in this way.

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

@@ -219,7 +219,7 @@ static u32 tcp_yeah_ssthresh(struct sock
 	yeah->fast_count = 0;
 	yeah->reno_count = max(yeah->reno_count>>1, 2U);
 
-	return tp->snd_cwnd - reduction;
+	return max_t(int, tp->snd_cwnd - reduction, 2);
 }
 
 static struct tcp_congestion_ops tcp_yeah __read_mostly = {

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/tcp_yeah-don-t-set-ssthresh-below-2.patch