svn commit: r362580 - head/sys/netinet/cc

[Richard Scheffenegger]

Author: rscheff
Date: Wed Jun 24 13:52:53 2020
New Revision: 362580
URL: https://svnweb.freebsd.org/changeset/base/362580

Log:
  TCP: fix cubic RTO reaction.
  
  Proper TCP Cubic operation requires the knowledge
  of the maximum congestion window prior to the
  last congestion event.
  
  This restores and improves a bugfix previously added
  by jtl@ but subsequently removed due to a revert.
  
  Reported by:	chengc_netapp.com
  Reviewed by:	chengc_netapp.com, tuexen (mentor)
  Approved by:	tuexen (mentor), rgrimes (mentor)
  MFC after:	2 weeks
  Sponsored by:	NetApp, Inc.
  Differential Revision:	https://reviews.freebsd.org/D25133

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

Modified: head/sys/netinet/cc/cc_cubic.c
==============================================================================
--- head/sys/netinet/cc/cc_cubic.c	Wed Jun 24 13:49:30 2020	(r362579)
+++ head/sys/netinet/cc/cc_cubic.c	Wed Jun 24 13:52:53 2020	(r362580)
@@ -313,10 +313,15 @@ cubic_cong_signal(struct cc_var *ccv, uint32_t type)
 		 * timeout has fired more than once, as there is a reasonable
 		 * chance the first one is a false alarm and may not indicate
 		 * congestion.
+		 * This will put Cubic firmly into the concave / TCP friendly
+		 * region, for a slower ramp-up after two consecutive RTOs.
 		 */
 		if (CCV(ccv, t_rxtshift) >= 2) {
 			cubic_data->flags |= CUBICFLAG_CONG_EVENT;
 			cubic_data->t_last_cong = ticks;
+			cubic_data->max_cwnd = CCV(ccv, snd_cwnd_prev);
+			cubic_data->K = cubic_k(cubic_data->max_cwnd /
+						CCV(ccv, t_maxseg));
 		}
 		break;
 	}