Index: netinet/in_pcb.c
===================================================================
RCS file: /cvs/src/sys/netinet/in_pcb.c,v
retrieving revision 1.249
diff -u -p -r1.249 in_pcb.c
--- netinet/in_pcb.c	27 May 2020 20:44:07 -0000	1.249
+++ netinet/in_pcb.c	25 Jun 2020 10:17:49 -0000
@@ -85,6 +85,7 @@
 #include <net/if_var.h>
 #include <net/pfvar.h>
 #include <net/route.h>
+#include <net/toeplitz.h>
 
 #include <netinet/in.h>
 #include <netinet/in_var.h>
@@ -515,6 +516,8 @@ in_pcbconnect(struct inpcb *inp, struct 
 	}
 	inp->inp_faddr = sin->sin_addr;
 	inp->inp_fport = sin->sin_port;
+	inp->inp_flowid = stoeplitz_ip4port(inp->inp_laddr.s_addr,
+	    inp->inp_faddr.s_addr, inp->inp_lport, inp->inp_fport);
 	in_pcbrehash(inp);
 #ifdef IPSEC
 	{
@@ -549,6 +552,7 @@ in_pcbdisconnect(struct inpcb *inp)
 	}
 
 	inp->inp_fport = 0;
+	inp->inp_flowid = 0;
 	in_pcbrehash(inp);
 	if (inp->inp_socket->so_state & SS_NOFDREF)
 		in_pcbdetach(inp);
Index: netinet/in_pcb.h
===================================================================
RCS file: /cvs/src/sys/netinet/in_pcb.h,v
retrieving revision 1.120
diff -u -p -r1.120 in_pcb.h
--- netinet/in_pcb.h	21 Jun 2020 05:14:04 -0000	1.120
+++ netinet/in_pcb.h	25 Jun 2020 10:17:49 -0000
@@ -148,6 +148,7 @@ struct inpcb {
 	void	*inp_upcall_arg;
 	u_int	inp_rtableid;
 	int	inp_pipex;		/* pipex indication */
+	uint16_t inp_flowid;
 };
 
 LIST_HEAD(inpcbhead, inpcb);
Index: netinet/tcp_output.c
===================================================================
RCS file: /cvs/src/sys/netinet/tcp_output.c,v
retrieving revision 1.128
diff -u -p -r1.128 tcp_output.c
--- netinet/tcp_output.c	10 Nov 2018 18:40:34 -0000	1.128
+++ netinet/tcp_output.c	25 Jun 2020 10:17:49 -0000
@@ -1037,6 +1037,8 @@ send:
 				ip->ip_tos |= IPTOS_ECN_ECT0;
 #endif
 		}
+		SET(m->m_pkthdr.csum_flags, M_FLOWID);
+		m->m_pkthdr.ph_flowid = tp->t_inpcb->inp_flowid;
 		error = ip_output(m, tp->t_inpcb->inp_options,
 			&tp->t_inpcb->inp_route,
 			(ip_mtudisc ? IP_MTUDISC : 0), NULL, tp->t_inpcb, 0);