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Introduce a lower bound of 2 MSS to TCP Cubic.

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Running TCP Cubic together with ECN could end up reducing cwnd down to 1 byte, if the
receiver continously sets the ECE flag, resulting in very poor transmission speeds.

In line with RFC6582 App. B, a lower bound of 2 MSS is introduced, as well as a typecast
to prevent any potential integer overflows during intermediate calculation steps of the
adjusted cwnd.

Reported by:	Cheng Cui
Reviewed by:	tuexen (mentor)
Approved by:	tuexen (mentor)
MFC after:	2 weeks
Sponsored by:	NetApp, Inc.
Differential Revision:	https://reviews.freebsd.org/D23353
This commit is contained in:
rscheff 2020-04-30 11:11:28 +00:00
parent 04816a15fb
commit b76a10884a
1 changed files with 7 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

11
sys/netinet/cc/cc_cubic.c
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@ -366,8 +366,9 @@ cubic_post_recovery(struct cc_var *ccv)
CCV(ccv, t_maxseg);
else
/* Update cwnd based on beta and adjusted max_cwnd. */
CCV(ccv, snd_cwnd) = max(1, ((CUBIC_BETA *
cubic_data->max_cwnd) >> CUBIC_SHIFT));
CCV(ccv, snd_cwnd) = max(((uint64_t)cubic_data->max_cwnd *
CUBIC_BETA) >> CUBIC_SHIFT,
2 * CCV(ccv, t_maxseg));
}
cubic_data->t_last_cong = ticks;
@ -433,6 +434,7 @@ static void
cubic_ssthresh_update(struct cc_var *ccv)
{
struct cubic *cubic_data;
uint32_t ssthresh;
cubic_data = ccv->cc_data;
@ -441,10 +443,11 @@ cubic_ssthresh_update(struct cc_var *ccv)
* subsequent congestion events, set it to cwnd * beta.
*/
if (cubic_data->num_cong_events == 0)
CCV(ccv, snd_ssthresh) = CCV(ccv, snd_cwnd) >> 1;
ssthresh = CCV(ccv, snd_cwnd) >> 1;
else
CCV(ccv, snd_ssthresh) = ((u_long)CCV(ccv, snd_cwnd) *
ssthresh = ((uint64_t)CCV(ccv, snd_cwnd) *
CUBIC_BETA) >> CUBIC_SHIFT;
CCV(ccv, snd_ssthresh) = max(ssthresh, 2 * CCV(ccv, t_maxseg));
}