--- sys/netinet6/ip6_output.c-orig	2020-01-28 18:02:49.000000000 +0100
+++ sys/netinet6/ip6_output.c	2020-01-28 18:03:05.000000000 +0100
@@ -998,14 +998,15 @@
 	 * with ext. hdrs.
 	 */
 	if (sw_csum & CSUM_DELAY_DATA_IPV6) {
-		sw_csum &= ~CSUM_DELAY_DATA_IPV6;
+		m->m_pkthdr.csum_flags &= ~CSUM_DELAY_DATA_IPV6;
 		m = mb_unmapped_to_ext(m);
 		if (m == NULL) {
 			error = ENOBUFS;
 			IP6STAT_INC(ip6s_odropped);
 			goto bad;
 		}
-		in6_delayed_cksum(m, plen, sizeof(struct ip6_hdr));
+		in6_delayed_cksum(m, plen - optlen,
+		    sizeof(struct ip6_hdr) + optlen);
 	} else if ((ifp->if_capenable & IFCAP_NOMAP) == 0) {
 		m = mb_unmapped_to_ext(m);
 		if (m == NULL) {
@@ -1016,14 +1017,14 @@
 	}
 #ifdef SCTP
 	if (sw_csum & CSUM_SCTP_IPV6) {
-		sw_csum &= ~CSUM_SCTP_IPV6;
+		m->m_pkthdr.csum_flags &= ~CSUM_SCTP_IPV6;
 		m = mb_unmapped_to_ext(m);
 		if (m == NULL) {
 			error = ENOBUFS;
 			IP6STAT_INC(ip6s_odropped);
 			goto bad;
 		}
-		sctp_delayed_cksum(m, sizeof(struct ip6_hdr));
+		sctp_delayed_cksum(m, sizeof(struct ip6_hdr) + optlen);
 	}
 #endif
 	m->m_pkthdr.csum_flags &= ifp->if_hwassist;
@@ -1116,7 +1117,9 @@
 				error = ENOBUFS;
 				goto bad;
 			}
-			in6_delayed_cksum(m, plen, hlen);
+			in6_delayed_cksum(m, plen - optlen,
+			    sizeof(struct ip6_hdr) + optlen);
+
 			m->m_pkthdr.csum_flags &= ~CSUM_DELAY_DATA_IPV6;
 		}
 #ifdef SCTP
@@ -1127,7 +1130,7 @@
 				error = ENOBUFS;
 				goto bad;
 			}
-			sctp_delayed_cksum(m, hlen);
+			sctp_delayed_cksum(m, sizeof(struct ip6_hdr) + optlen);
 			m->m_pkthdr.csum_flags &= ~CSUM_SCTP_IPV6;
 		}
 #endif