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;
        }
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