Index: sys/net/pfvar.h
===================================================================
--- sys/net/pfvar.h
+++ sys/net/pfvar.h
@@ -330,6 +330,8 @@
 		(neg)							\
 	)
 
+#define PF_ALGNMNT(off) (((off) % 2) == 0)
+
 struct pf_rule_uid {
 	uid_t		 uid[2];
 	u_int8_t	 op;
@@ -1735,6 +1737,10 @@
 void	pf_change_proto_a(struct mbuf *, void *, u_int16_t *, u_int32_t,
 	    u_int8_t);
 void	pf_change_tcp_a(struct mbuf *, void *, u_int16_t *, u_int32_t);
+void	pf_patch_16_unaligned(struct mbuf *, u_int16_t *, void *, u_int16_t,
+	    bool, u_int8_t);
+void	pf_patch_32_unaligned(struct mbuf *, u_int16_t *, void *, u_int32_t,
+    bool, u_int8_t);
 void	pf_send_deferred_syn(struct pf_state *);
 int	pf_match_addr(u_int8_t, struct pf_addr *, struct pf_addr *,
 	    struct pf_addr *, sa_family_t);
Index: sys/netpfil/pf/pf.c
===================================================================
--- sys/netpfil/pf/pf.c
+++ sys/netpfil/pf/pf.c
@@ -295,6 +295,8 @@
 			    struct pf_state_key *, struct pf_state_key *);
 static int		 pf_addr_wrap_neq(struct pf_addr_wrap *,
 			    struct pf_addr_wrap *);
+static void		 pf_patch_8(struct mbuf *, u_int16_t *, u_int8_t *, u_int8_t,
+			    bool, u_int8_t);
 static struct pf_state	*pf_find_state(struct pfi_kif *,
 			    struct pf_state_key_cmp *, u_int);
 static int		 pf_src_connlimit(struct pf_state **);
@@ -2091,16 +2093,60 @@
 u_int16_t
 pf_cksum_fixup(u_int16_t cksum, u_int16_t old, u_int16_t new, u_int8_t udp)
 {
-	u_int32_t	l;
-
-	if (udp && !cksum)
-		return (0x0000);
-	l = cksum + old - new;
-	l = (l >> 16) + (l & 65535);
-	l = l & 65535;
-	if (udp && !l)
-		return (0xFFFF);
-	return (l);
+	u_int32_t x;
+
+	x = cksum + old - new;
+	x = (x + (x >> 16)) & 0xffff;
+
+	/* optimise: eliminate a branch when not udp */
+	if (udp && cksum == 0x0000)
+		return cksum;
+	if (udp && x == 0x0000)
+		x = 0xffff;
+
+	return (u_int16_t)(x);
+}
+
+static void
+pf_patch_8(struct mbuf *m, u_int16_t *cksum, u_int8_t *f, u_int8_t v, bool hi,
+    u_int8_t udp)
+{
+	u_int16_t old = htons(hi ? (*f << 8) : *f);
+	u_int16_t new = htons(hi ? ( v << 8) :  v);
+
+	if (*f == v)
+		return;
+
+	*f = v;
+
+	if (m->m_pkthdr.csum_flags & (CSUM_DELAY_DATA | CSUM_DELAY_DATA_IPV6))
+		return;
+
+	*cksum = pf_cksum_fixup(*cksum, old, new, udp);
+}
+
+void
+pf_patch_16_unaligned(struct mbuf *m, u_int16_t *cksum, void *f, u_int16_t v,
+    bool hi, u_int8_t udp)
+{
+	u_int8_t *fb = (u_int8_t *)f;
+	u_int8_t *vb = (u_int8_t *)&v;
+
+	pf_patch_8(m, cksum, fb++, *vb++, hi, udp);
+	pf_patch_8(m, cksum, fb++, *vb++, !hi, udp);
+}
+
+void
+pf_patch_32_unaligned(struct mbuf *m, u_int16_t *cksum, void *f, u_int32_t v,
+    bool hi, u_int8_t udp)
+{
+	u_int8_t *fb = (u_int8_t *)f;
+	u_int8_t *vb = (u_int8_t *)&v;
+
+	pf_patch_8(m, cksum, fb++, *vb++, hi, udp);
+	pf_patch_8(m, cksum, fb++, *vb++, !hi, udp);
+	pf_patch_8(m, cksum, fb++, *vb++, hi, udp);
+	pf_patch_8(m, cksum, fb++, *vb++, !hi, udp);
 }
 
 u_int16_t
@@ -2326,6 +2372,7 @@
 		return 0;
 
 	while (hlen >= TCPOLEN_SACKLEN) {
+		size_t startoff = opt - opts;
 		olen = opt[1];
 		switch (*opt) {
 		case TCPOPT_EOL:	/* FALLTHROUGH */
@@ -2340,10 +2387,16 @@
 				for (i = 2; i + TCPOLEN_SACK <= olen;
 				    i += TCPOLEN_SACK) {
 					memcpy(&sack, &opt[i], sizeof(sack));
-					pf_change_proto_a(m, &sack.start, &th->th_sum,
-					    htonl(ntohl(sack.start) - dst->seqdiff), 0);
-					pf_change_proto_a(m, &sack.end, &th->th_sum,
-					    htonl(ntohl(sack.end) - dst->seqdiff), 0);
+					pf_patch_32_unaligned(m,
+					    &th->th_sum, &sack.start,
+					    htonl(ntohl(sack.start) - dst->seqdiff),
+					    PF_ALGNMNT(startoff),
+					    0);
+					pf_patch_32_unaligned(m, &th->th_sum,
+					    &sack.end,
+					    htonl(ntohl(sack.end) - dst->seqdiff),
+					    PF_ALGNMNT(startoff),
+					    0);
 					memcpy(&opt[i], &sack, sizeof(sack));
 				}
 				copyback = 1;
Index: sys/netpfil/pf/pf_norm.c
===================================================================
--- sys/netpfil/pf/pf_norm.c
+++ sys/netpfil/pf/pf_norm.c
@@ -1517,6 +1517,7 @@
 	u_int8_t *opt;
 	int copyback = 0;
 	int got_ts = 0;
+	size_t startoff;
 
 	KASSERT((src->scrub || dst->scrub),
 	    ("%s: src->scrub && dst->scrub!", __func__));
@@ -1560,6 +1561,7 @@
 		opt = hdr + sizeof(struct tcphdr);
 		hlen = (th->th_off << 2) - sizeof(struct tcphdr);
 		while (hlen >= TCPOLEN_TIMESTAMP) {
+			startoff = opt - (hdr + sizeof(struct tcphdr));
 			switch (*opt) {
 			case TCPOPT_EOL:	/* FALLTHROUGH */
 			case TCPOPT_NOP:
@@ -1589,10 +1591,12 @@
 					    (src->scrub->pfss_flags &
 					    PFSS_TIMESTAMP)) {
 						tsval = ntohl(tsval);
-						pf_change_proto_a(m, &opt[2],
+						pf_patch_32_unaligned(m,
 						    &th->th_sum,
+						    &opt[2],
 						    htonl(tsval +
 						    src->scrub->pfss_ts_mod),
+						    PF_ALGNMNT(startoff),
 						    0);
 						copyback = 1;
 					}
@@ -1605,8 +1609,11 @@
 					    PFSS_TIMESTAMP)) {
 						tsecr = ntohl(tsecr)
 						    - dst->scrub->pfss_ts_mod;
-						pf_change_proto_a(m, &opt[6],
-						    &th->th_sum, htonl(tsecr),
+						pf_patch_32_unaligned(m,
+						    &th->th_sum,
+						    &opt[6],
+						    htonl(tsecr),
+						    PF_ALGNMNT(startoff),
 						    0);
 						copyback = 1;
 					}
@@ -1903,6 +1910,7 @@
 	int		 rewrite = 0;
 	u_char		 opts[TCP_MAXOLEN];
 	u_char		*optp = opts;
+	size_t		 startoff;
 
 	thoff = th->th_off << 2;
 	cnt = thoff - sizeof(struct tcphdr);
@@ -1912,6 +1920,7 @@
 		return (rewrite);
 
 	for (; cnt > 0; cnt -= optlen, optp += optlen) {
+		startoff = optp - opts;
 		opt = optp[0];
 		if (opt == TCPOPT_EOL)
 			break;
@@ -1928,9 +1937,11 @@
 		case TCPOPT_MAXSEG:
 			mss = (u_int16_t *)(optp + 2);
 			if ((ntohs(*mss)) > r->max_mss) {
-				th->th_sum = pf_proto_cksum_fixup(m,
-				    th->th_sum, *mss, htons(r->max_mss), 0);
-				*mss = htons(r->max_mss);
+				pf_patch_16_unaligned(m,
+				    &th->th_sum,
+				    mss, htons(r->max_mss),
+				    PF_ALGNMNT(startoff),
+				    0);
 				rewrite = 1;
 			}
 			break;