diff --git a/share/man/man4/ip.4 b/share/man/man4/ip.4
--- a/share/man/man4/ip.4
+++ b/share/man/man4/ip.4
@@ -90,10 +90,13 @@
 .Ed
 .Pp
 .Dv IP_TOS
-and
+may be used to set the differential service codepoint (DSCP) and the
+explicit congestion notfication (ECN) codepoint.
+Setting the ECN codepoint - the two least significant bits - on a
+socket using a transport protocol implementing ECN has no effect.
+.Pp
 .Dv IP_TTL
-may be used to set the type-of-service and time-to-live
-fields in the
+configures the time-to-live (TTL) field in the
 .Tn IP
 header for
 .Dv SOCK_STREAM , SOCK_DGRAM ,
@@ -102,7 +105,7 @@
 sockets.
 For example,
 .Bd -literal
-int tos = IPTOS_LOWDELAY;       /* see <netinet/ip.h> */
+int tos = IPTOS_DSCP_EF;       /* see <netinet/ip.h> */
 setsockopt(s, IPPROTO_IP, IP_TOS, &tos, sizeof(tos));
 
 int ttl = 60;                   /* max = 255 */
diff --git a/sys/netinet/ip.h b/sys/netinet/ip.h
--- a/sys/netinet/ip.h
+++ b/sys/netinet/ip.h
@@ -79,7 +79,7 @@
 #define	IPTOS_LOWDELAY		0x10
 #define	IPTOS_THROUGHPUT	0x08
 #define	IPTOS_RELIABILITY	0x04
-#define	IPTOS_MINCOST		0x02
+#define	IPTOS_MINCOST		IPTOS_DSCP_CS0
 
 /*
  * Definitions for IP precedence (also in ip_tos) (deprecated).
diff --git a/sys/netinet/tcp_usrreq.c b/sys/netinet/tcp_usrreq.c
--- a/sys/netinet/tcp_usrreq.c
+++ b/sys/netinet/tcp_usrreq.c
@@ -82,6 +82,7 @@
 #include <netinet/in_pcb.h>
 #include <netinet/in_systm.h>
 #include <netinet/in_var.h>
+#include <netinet/ip.h>
 #include <netinet/ip_var.h>
 #ifdef INET6
 #include <netinet/ip6.h>
@@ -1759,6 +1760,8 @@
 		case IPPROTO_IP:
 			switch (sopt->sopt_name) {
 			case IP_TOS:
+				inp->inp_ip_tos &= ~IPTOS_ECN_MASK;
+				break;
 			case IP_TTL:
 				/* Notify tcp stacks that care (e.g. RACK). */
 				break;