svn commit: r360491 - head/sys/netinet/cc

[Richard Scheffenegger]

Author: rscheff
Date: Thu Apr 30 11:11:28 2020
New Revision: 360491
URL: https://svnweb.freebsd.org/changeset/base/360491

Log:
  Introduce a lower bound of 2 MSS to TCP Cubic.
  
  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

Modified:
  head/sys/netinet/cc/cc_cubic.c

Modified: head/sys/netinet/cc/cc_cubic.c
==============================================================================
--- head/sys/netinet/cc/cc_cubic.c	Thu Apr 30 06:34:34 2020	(r360490)
+++ head/sys/netinet/cc/cc_cubic.c	Thu Apr 30 11:11:28 2020	(r360491)
@@ -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));
 }