Index: sys/netinet/tcp_syncache.c
===================================================================
--- sys/netinet/tcp_syncache.c
+++ sys/netinet/tcp_syncache.c
@@ -1505,6 +1505,10 @@
 	return (rv);
 }
 
+/*
+ * Send SYN|ACK to the peer.  Either in response to the peer's SYN,
+ * i.e. m0 != NULL, or upon 3WHS ACK timeout, i.e. m0 == NULL.
+ */
 static int
 syncache_respond(struct syncache *sc, struct syncache_head *sch, int locked,
     const struct mbuf *m0)
@@ -1688,6 +1692,11 @@
 
 	M_SETFIB(m, sc->sc_inc.inc_fibnum);
 	m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
+	/*
+	 * If we have peer's SYN and it has a flowid, then let's assign it to
+	 * our SYN|ACK.  ip6_output() and ip_output() will not assign flowid
+	 * to SYN|ACK due to lack of inp here.
+	 */
 	if (m0 != NULL && M_HASHTYPE_GET(m0) != M_HASHTYPE_NONE) {
 		m->m_pkthdr.flowid = m0->m_pkthdr.flowid;
 		M_HASHTYPE_SET(m, M_HASHTYPE_GET(m0));