diff --git a/share/man/man4/cc_dctcp.4 b/share/man/man4/cc_dctcp.4
index c242ccd392de..5c5be4fed96e 100644
--- a/share/man/man4/cc_dctcp.4
+++ b/share/man/man4/cc_dctcp.4
@@ -28,7 +28,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd January 12, 2015
+.Dd July 29, 2019
 .Dt CC_DCTCP 4
 .Os
 .Sh NAME
@@ -60,7 +60,7 @@ In addition, when classic ECN is used as sender and DCTCP is used as
 receiver, DCTCP avoids to mirror back ACKs only when the CWR flag is
 set in the incoming packet.
 .Pp
-The other specifications are based on the paper and Internet Draft referenced
+The other specifications are based on the paper and the RFC referenced
 in the
 .Sx SEE ALSO
 section below.
@@ -70,16 +70,27 @@ The algorithm exposes the following tunable variables in the
 branch of the
 .Xr sysctl 3
 MIB:
-.Bl -tag -width ".Va alpha"
+.Bl -tag -width ".Va slowstart"
 .It Va alpha
-An initial estimator of the congestion on the link.
-Default is 0.
-.It Va dctcp_shift_g
-An estimation gain in the alpha calculation.
-Default is 16.
+The initial value to estimate the congestion on the link.
+The valid range is from 0 to 1024, where 1024 reduces the congestion
+window to half, if a CE is observed in the first window and
+.Va alpha
+could not yet adjust to the congestion level on that path.
+Default is 1024.
+.It Va shift_g
+An estimation gain in the
+.Va alpha
+calculation.
+This influences the responsiveness when adjusting alpha
+to the most recent observed window.
+Valid range from 0 to 10, the default is 4, resulting in an effective
+gain of 1 / ( 2 ^
+.Va shift_g
+), or 1/16th.
 .It Va slowstart
-A trigger to halve congestion window after slow start.
-Default does nothing to halve window.
+A flag if the congestion window should be reduced by one half after slow start.
+Valid settings 0 and 1, default 0.
 .El
 .Sh SEE ALSO
 .Xr cc_chd 4 ,
@@ -108,10 +119,12 @@ Default does nothing to halve window.
 .Re
 .Rs
 .%A "Stephen Bensley"
-.%A "Lars Eggert"
 .%A "Dave Thaler"
-.%T "Microsoft's Datacenter TCP (DCTCP): TCP Congestion Control for Datacenters"
-.%U "http://tools.ietf.org/html/draft-bensley-tcpm-dctcp-01"
+.%A "Praveen Balasubramanian"
+.%A "Lars Eggert"
+.%A "Glenn Judd"
+.%T "Data Center TCP (DCTCP): TCP Congestion Control for Data Centers"
+.%U "https://tools.ietf.org/html/rfc8257"
 .Re
 .Sh HISTORY
 The
diff --git a/sys/netinet/cc/cc_dctcp.c b/sys/netinet/cc/cc_dctcp.c
index 9affd0da2b32..2e11253fb1d2 100644
--- a/sys/netinet/cc/cc_dctcp.c
+++ b/sys/netinet/cc/cc_dctcp.c
@@ -56,8 +56,9 @@ __FBSDID("$FreeBSD$");
 #include <netinet/cc/cc.h>
 #include <netinet/cc/cc_module.h>
 
-#define MAX_ALPHA_VALUE 1024
-VNET_DEFINE_STATIC(uint32_t, dctcp_alpha) = 0;
+#define DCTCP_SHIFT 10
+#define MAX_ALPHA_VALUE (1<<DCTCP_SHIFT)
+VNET_DEFINE_STATIC(uint32_t, dctcp_alpha) = MAX_ALPHA_VALUE;
 #define V_dctcp_alpha	    VNET(dctcp_alpha)
 VNET_DEFINE_STATIC(uint32_t, dctcp_shift_g) = 4;
 #define	V_dctcp_shift_g	    VNET(dctcp_shift_g)
@@ -65,14 +66,14 @@ VNET_DEFINE_STATIC(uint32_t, dctcp_slowstart) = 0;
 #define	V_dctcp_slowstart   VNET(dctcp_slowstart)
 
 struct dctcp {
-	int     bytes_ecn;	/* # of marked bytes during a RTT */
-	int     bytes_total;	/* # of acked bytes during a RTT */
-	int     alpha;		/* the fraction of marked bytes */
-	int     ce_prev;	/* CE state of the last segment */
-	int     save_sndnxt;	/* end sequence number of the current window */
-	int	ece_curr;	/* ECE flag in this segment */
-	int	ece_prev;	/* ECE flag in the last segment */
-	uint32_t    num_cong_events; /* # of congestion events */
+	uint32_t bytes_ecn;	  /* # of marked bytes during a RTT */
+	uint32_t bytes_total;	  /* # of acked bytes during a RTT */
+	int      alpha;		  /* the fraction of marked bytes */
+	int      ce_prev;	  /* CE state of the last segment */
+	tcp_seq  save_sndnxt;	  /* end sequence number of the current window */
+	int      ece_curr;	  /* ECE flag in this segment */
+	int      ece_prev;	  /* ECE flag in the last segment */
+	uint32_t num_cong_events; /* # of congestion events */
 };
 
 static MALLOC_DEFINE(M_dctcp, "dctcp data",
@@ -369,18 +370,18 @@ dctcp_update_alpha(struct cc_var *ccv)
 	dctcp_data->bytes_total = max(dctcp_data->bytes_total, 1);
 
 	/*
-	 * Update alpha: alpha = (1 - g) * alpha + g * F.
+	 * Update alpha: alpha = (1 - g) * alpha + g * M.
 	 * Here:
 	 * g is weight factor
 	 *	recommaded to be set to 1/16
 	 *	small g = slow convergence between competitive DCTCP flows
 	 *	large g = impacts low utilization of bandwidth at switches
-	 * F is fraction of marked segments in last RTT
+	 * M is fraction of marked segments in last RTT
 	 *	updated every RTT
 	 * Alpha must be round to 0 - MAX_ALPHA_VALUE.
 	 */
-	dctcp_data->alpha = min(alpha_prev - (alpha_prev >> V_dctcp_shift_g) +
-	    (dctcp_data->bytes_ecn << (10 - V_dctcp_shift_g)) /
+	dctcp_data->alpha = ulmin(alpha_prev - (alpha_prev >> V_dctcp_shift_g) +
+	    ((uint64_t)dctcp_data->bytes_ecn << (DCTCP_SHIFT - V_dctcp_shift_g)) /
 	    dctcp_data->bytes_total, MAX_ALPHA_VALUE);
 
 	/* Initialize internal parameters for next alpha calculation */
@@ -398,14 +399,10 @@ dctcp_alpha_handler(SYSCTL_HANDLER_ARGS)
 	new = V_dctcp_alpha;
 	error = sysctl_handle_int(oidp, &new, 0, req);
 	if (error == 0 && req->newptr != NULL) {
-		if (new > 1)
+		if (new > MAX_ALPHA_VALUE)
 			error = EINVAL;
-		else {
-			if (new > MAX_ALPHA_VALUE)
-				V_dctcp_alpha = MAX_ALPHA_VALUE;
-			else
-				V_dctcp_alpha = new;
-		}
+		else
+			V_dctcp_alpha = new;
 	}
 
 	return (error);
@@ -420,7 +417,7 @@ dctcp_shift_g_handler(SYSCTL_HANDLER_ARGS)
 	new = V_dctcp_shift_g;
 	error = sysctl_handle_int(oidp, &new, 0, req);
 	if (error == 0 && req->newptr != NULL) {
-		if (new > 1)
+		if (new > DCTCP_SHIFT)
 			error = EINVAL;
 		else
 			V_dctcp_shift_g = new;
@@ -454,7 +451,7 @@ SYSCTL_NODE(_net_inet_tcp_cc, OID_AUTO, dctcp, CTLFLAG_RW, NULL,
 SYSCTL_PROC(_net_inet_tcp_cc_dctcp, OID_AUTO, alpha,
     CTLFLAG_VNET|CTLTYPE_UINT|CTLFLAG_RW, &VNET_NAME(dctcp_alpha), 0,
     &dctcp_alpha_handler,
-    "IU", "dctcp alpha parameter");
+    "IU", "dctcp alpha parameter at start of session");
 
 SYSCTL_PROC(_net_inet_tcp_cc_dctcp, OID_AUTO, shift_g,
     CTLFLAG_VNET|CTLTYPE_UINT|CTLFLAG_RW, &VNET_NAME(dctcp_shift_g), 4,