diff --git a/sys/netinet/cc/cc_cubic.h b/sys/netinet/cc/cc_cubic.h
--- a/sys/netinet/cc/cc_cubic.h
+++ b/sys/netinet/cc/cc_cubic.h
@@ -52,6 +52,12 @@
 /* ~0.7 << CUBIC_SHIFT. */
 #define	CUBIC_BETA		179
 
+/* ~0.53 << CUBIC_SHIFT: 3 * (1-beta_cubic)/(1+beta_cubic). */
+#define	CUBIC_ALPHA		135
+
+/* 1 << CUBIC_SHIFT. */
+#define	CUBIC_ALPHA_ONE		256
+
 /* ~0.3 << CUBIC_SHIFT. */
 #define	ONE_SUB_CUBIC_BETA	77
 
@@ -142,31 +148,36 @@
 
 /*
  * Implementation based on the formulae found in the CUBIC Internet Draft
- * "draft-ietf-tcpm-cubic-04".
+ * "draft-ietf-tcpm-rfc8312bis-15".
  *
  */
 
 static __inline float
-theoretical_cubic_k(double wmax_pkts)
+theoretical_cubic_k(double wmax_pkts, double cwnd_epoch_pkts)
 {
 	double C;
 
 	C = 0.4;
 
-	return (pow((wmax_pkts * 0.3) / C, (1.0 / 3.0)) * pow(2, CUBIC_SHIFT));
+	if (wmax_pkts <= cwnd_epoch_pkts)
+		return 0.0;
+
+	return (pow((wmax_pkts - cwnd_epoch_pkts) / C, (1.0 / 3.0)) * pow(2, CUBIC_SHIFT));
 }
 
 static __inline unsigned long
-theoretical_cubic_cwnd(int ticks_since_epoch, unsigned long wmax, uint32_t smss)
+theoretical_cubic_cwnd(int ticks_since_epoch, unsigned long wmax,
+    unsigned long cwnd_epoch, uint32_t smss)
 {
-	double C, wmax_pkts;
+	double C, wmax_pkts, cwnd_epoch_pkts;
 
 	C = 0.4;
 	wmax_pkts = wmax / (double)smss;
+	cwnd_epoch_pkts = cwnd_epoch / (double)smss;
 
 	return (smss * (wmax_pkts +
 	    (C * pow(ticks_since_epoch / (double)hz -
-	    theoretical_cubic_k(wmax_pkts) / pow(2, CUBIC_SHIFT), 3.0))));
+	    theoretical_cubic_k(wmax_pkts, cwnd_epoch_pkts) / pow(2, CUBIC_SHIFT), 3.0))));
 }
 
 static __inline unsigned long
@@ -178,23 +189,30 @@
 }
 
 static __inline unsigned long
-theoretical_tf_cwnd(int ticks_since_epoch, int rtt_ticks, unsigned long wmax,
-    uint32_t smss)
+theoretical_tf_cwnd(unsigned long west, unsigned long bytes_acked, unsigned long cwnd,
+    unsigned long cwnd_prior, unsigned long smss)
 {
 
-	return ((wmax * 0.7) + ((3 * 0.3) / (2 - 0.3) *
-	    (ticks_since_epoch / (float)rtt_ticks) * smss));
+	float cubic_alpha;
+
+	if (west >= cwnd_prior)
+		cubic_alpha = 1.0;
+	else
+		cubic_alpha = (3.0 * (1.0 - 0.7)) / (1.0 + 0.7);
+
+	return (west + (cubic_alpha * ((bytes_acked * smss) / (float)cwnd)));
 }
 
 #endif /* !_KERNEL */
 
 /*
  * Compute the CUBIC K value used in the cwnd calculation, using an
- * implementation of eqn 2 in the I-D. The method used
- * here is adapted from Apple Computer Technical Report #KT-32.
+ * implementation mentioned in Fig. 2 of draft-ietf-tcpm-rfc8312bis-15.
+ * The method used here is adapted from Apple Computer Technical
+ * Report #KT-32.
  */
 static __inline int64_t
-cubic_k(unsigned long wmax_pkts)
+cubic_k(unsigned long wmax_pkts, unsigned long cwnd_epoch_pkts)
 {
 	int64_t s, K;
 	uint16_t p;
@@ -202,8 +220,13 @@
 	K = s = 0;
 	p = 0;
 
-	/* (wmax * beta)/C with CUBIC_SHIFT worth of precision. */
-	s = ((wmax_pkts * ONE_SUB_CUBIC_BETA) << CUBIC_SHIFT) / CUBIC_C_FACTOR;
+	/* Handle the corner case where W_max <= cwnd_epoch */
+	if (wmax_pkts <= cwnd_epoch_pkts) {
+		return 0;
+	}
+
+	/* (wmax - cwnd_epoch) / C with CUBIC_SHIFT worth of precision. */
+	s = ((wmax_pkts - cwnd_epoch_pkts) << (2 * CUBIC_SHIFT)) / CUBIC_C_FACTOR;
 
 	/* Rebase s to be between 1 and 1/8 with a shift of CUBIC_SHIFT. */
 	while (s >= 256) {
diff --git a/sys/netinet/cc/cc_cubic.c b/sys/netinet/cc/cc_cubic.c
--- a/sys/netinet/cc/cc_cubic.c
+++ b/sys/netinet/cc/cc_cubic.c
@@ -278,6 +278,8 @@
 						       CUBICFLAG_IN_APPLIMIT);
 				cubic_data->t_epoch = ticks;
 				cubic_data->K = cubic_k(cubic_data->W_max /
+							CCV(ccv, t_maxseg),
+							cubic_data->cwnd_epoch /
 							CCV(ccv, t_maxseg));
 			}
 			usecs_since_epoch = (ticks - cubic_data->t_epoch) * tick;
@@ -333,6 +335,7 @@
 			    cubic_data->W_max < CCV(ccv, snd_cwnd)) {
 				cubic_data->W_max = CCV(ccv, snd_cwnd);
 				cubic_data->K = cubic_k(cubic_data->W_max /
+				    CCV(ccv, t_maxseg), cubic_data->cwnd_epoch /
 				    CCV(ccv, t_maxseg));
 			}
 		}
@@ -354,8 +357,9 @@
 
 	cubic_data = ccv->cc_data;
 
-	cubic_data->W_max = ulmax(cubic_data->W_max, CCV(ccv, snd_cwnd));
-	cubic_data->K = cubic_k(cubic_data->W_max / CCV(ccv, t_maxseg));
+	cubic_data->cwnd_prior = ulmax(cubic_data->cwnd_prior, CCV(ccv, snd_cwnd));
+	cubic_data->W_max = cubic_data->cwnd_prior;
+	cubic_data->t_epoch = 0;	/* This will recalculate K */
 	if ((cubic_data->flags & CUBICFLAG_HYSTART_ENABLED) == 0) {
 		/*
 		 * Re-enable hystart if we have been idle.
@@ -437,9 +441,8 @@
 		if (!IN_FASTRECOVERY(CCV(ccv, t_flags))) {
 			if (!IN_CONGRECOVERY(CCV(ccv, t_flags))) {
 				cubic_ssthresh_update(ccv, mss);
-				cubic_data->flags |= CUBICFLAG_CONG_EVENT;
-				cubic_data->t_epoch = ticks;
-				cubic_data->K = cubic_k(cubic_data->W_max / mss);
+				cubic_data->num_cong_events++;
+				cubic_data->t_epoch = 0; /* This will recalculate K */
 			}
 			ENTER_RECOVERY(CCV(ccv, t_flags));
 		}
@@ -454,9 +457,8 @@
 		}
 		if (!IN_CONGRECOVERY(CCV(ccv, t_flags))) {
 			cubic_ssthresh_update(ccv, mss);
-			cubic_data->flags |= CUBICFLAG_CONG_EVENT;
-			cubic_data->t_epoch = ticks;
-			cubic_data->K = cubic_k(cubic_data->W_max / mss);
+			cubic_data->num_cong_events++;
+			cubic_data->t_epoch = 0; /* This will recalculate K */
 			CCV(ccv, snd_cwnd) = CCV(ccv, snd_ssthresh);
 			ENTER_CONGRECOVERY(CCV(ccv, t_flags));
 		}