Index: share/man/man4/carp.4
===================================================================
--- share/man/man4/carp.4
+++ share/man/man4/carp.4
@@ -26,7 +26,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd May 7, 2018
+.Dd June 4, 2018
 .Dt CARP 4
 .Os
 .Sh NAME
@@ -104,6 +104,12 @@
 When enabled, a vhid in a backup state would preempt a master that
 is announcing itself with a lower advskew.
 Disabled by default.
+.It Va net.inet.carp.dscp
+DSCP value in carp packet.
+Valid Values are 0 to 63.
+A value of 4 is equivalent to the old standard of TOS LOW_DELAY.
+TOS values were deprecated and replaced by DSCP in 1998.
+The default value is 56 (CS7/Network Control).
 .It Va net.inet.carp.log
 Determines what events relating to
 .Nm
Index: sys/netinet/ip.h
===================================================================
--- sys/netinet/ip.h
+++ sys/netinet/ip.h
@@ -93,6 +93,11 @@
 #define        IPTOS_PREC_PRIORITY             IPTOS_DSCP_CS1
 #define        IPTOS_PREC_ROUTINE              IPTOS_DSCP_CS0
 
+/*
+ * Offset of Diffserv decimal value to convert it to tos value .
+ */
+#define        IPTOS_DSCP_OFFSET       2
+
 /*
  * Definitions for DiffServ Codepoints as per RFC2474 and RFC5865.
  */
Index: sys/netinet/ip6.h
===================================================================
--- sys/netinet/ip6.h
+++ sys/netinet/ip6.h
@@ -104,6 +104,7 @@
 #define IPV6_FLOWLABEL_MASK    0xffff0f00      /* flow label (20 bits) */
 #endif /* LITTLE_ENDIAN */
 #endif
+#define IPV6_FLOWLABEL_LEN     20
 #if 1
 /* ECN bits proposed by Sally Floyd */
 #define IP6TOS_CE              0x01    /* congestion experienced */
Index: sys/netinet/ip_carp.c
===================================================================
--- sys/netinet/ip_carp.c
+++ sys/netinet/ip_carp.c
@@ -190,6 +190,10 @@
 static VNET_DEFINE(int, carp_allow) = 1;
 #define        V_carp_allow    VNET(carp_allow)
 
+/* Set DSCP in outgoing CARP packets. */
+static VNET_DEFINE(int, carp_dscp) = 56;
+#define        V_carp_dscp     VNET(carp_dscp)
+
 /* Preempt slower nodes. */
 static VNET_DEFINE(int, carp_preempt) = 0;
 #define        V_carp_preempt  VNET(carp_preempt)
@@ -217,6 +221,8 @@
 SYSCTL_PROC(_net_inet_carp, OID_AUTO, allow,
     CTLFLAG_VNET | CTLTYPE_INT | CTLFLAG_RW, 0, 0, carp_allow_sysctl, "I",
     "Accept incoming CARP packets");
+SYSCTL_INT(_net_inet_carp, OID_AUTO, dscp, CTLFLAG_VNET | CTLFLAG_RW,
+    &VNET_NAME(carp_dscp), 0, "DSCP value for carp packets");
 SYSCTL_INT(_net_inet_carp, OID_AUTO, preempt, CTLFLAG_VNET | CTLFLAG_RW,
     &VNET_NAME(carp_preempt), 0, "High-priority backup preemption mode");
 SYSCTL_INT(_net_inet_carp, OID_AUTO, log, CTLFLAG_VNET | CTLFLAG_RW,
@@ -933,7 +939,7 @@
                ip = mtod(m, struct ip *);
                ip->ip_v = IPVERSION;
                ip->ip_hl = sizeof(*ip) >> 2;
-               ip->ip_tos = IPTOS_LOWDELAY;
+                ip->ip_tos = V_carp_dscp << IPTOS_DSCP_OFFSET;
                ip->ip_len = htons(len);
                ip->ip_off = htons(IP_DF);
                ip->ip_ttl = CARP_DFLTTL;
@@ -983,6 +989,9 @@
                ip6 = mtod(m, struct ip6_hdr *);
                bzero(ip6, sizeof(*ip6));
                ip6->ip6_vfc |= IPV6_VERSION;
+               /* Traffic class isn't defined in ip6 struct instead
+                * it gets offset into flowid field */
+               ip6->ip6_flow |= htonl(V_carp_dscp << ( IPV6_FLOWLABEL_LEN + IPTOS_DSCP_OFFSET));
                ip6->ip6_hlim = CARP_DFLTTL;
                ip6->ip6_nxt = IPPROTO_CARP;