net: ipv6_fragment: Add PMTU support

If PMTU is enabled, then use the MTU value from it instead of always
using network interface MTU.

Signed-off-by: Jukka Rissanen <[email protected]>

Author: jukkar

subsys/net/ip/ipv6_fragment.c

@@ -686,7 +686,7 @@ static int send_ipv6_fragment(struct net_pkt *pkt,
 }
 
 int net_ipv6_send_fragmented_pkt(struct net_if *iface, struct net_pkt *pkt,
-				 uint16_t pkt_len)
+				 uint16_t pkt_len, uint16_t mtu)
 {
 	uint16_t next_hdr_off;
 	uint16_t last_hdr_off;
@@ -713,12 +713,12 @@ int net_ipv6_send_fragmented_pkt(struct net_if *iface, struct net_pkt *pkt,
 	/* The Maximum payload can fit into each packet after IPv6 header,
 	 * Extension headers and Fragmentation header.
 	 */
-	fit_len = NET_IPV6_MTU - NET_IPV6_FRAGH_LEN -
+	fit_len = (int)mtu - NET_IPV6_FRAGH_LEN -
 		(net_pkt_ip_hdr_len(pkt) + net_pkt_ipv6_ext_len(pkt));
 	if (fit_len <= 0) {
 		/* Must be invalid extension headers length */
 		NET_DBG("No room for IPv6 payload MTU %d hdrs_len %d",
-			NET_IPV6_MTU, NET_IPV6_FRAGH_LEN +
+			mtu, NET_IPV6_FRAGH_LEN +
 			net_pkt_ip_hdr_len(pkt) + net_pkt_ipv6_ext_len(pkt));
 		return -EINVAL;
 	}

subsys/net/ip/ipv6_nbr.c

@@ -810,13 +810,31 @@ enum net_verdict net_ipv6_prepare_for_send(struct net_pkt *pkt)
 	 * contain a proper value and we can skip other checks.
 	 */
 	if (net_pkt_ipv6_fragment_id(pkt) == 0U) {
-		uint16_t mtu = net_if_get_mtu(net_pkt_iface(pkt));
 		size_t pkt_len = net_pkt_get_len(pkt);
+		uint16_t mtu;
+
+		if (IS_ENABLED(CONFIG_NET_IPV6_PMTU)) {
+			struct sockaddr_in6 dst = {
+				.sin6_family = AF_INET6,
+			};
+
+			net_ipv6_addr_copy_raw((uint8_t *)&dst.sin6_addr, ip_hdr->dst);
+
+			ret = net_pmtu_get_mtu((struct sockaddr *)&dst);
+			if (ret <= 0) {
+				goto use_interface_mtu;
+			}
+
+			mtu = ret;
+		} else {
+use_interface_mtu:
+			mtu = net_if_get_mtu(net_pkt_iface(pkt));
+			mtu = MAX(NET_IPV6_MTU, mtu);
+		}
 
-		mtu = MAX(NET_IPV6_MTU, mtu);
 		if (mtu < pkt_len) {
 			ret = net_ipv6_send_fragmented_pkt(net_pkt_iface(pkt),
-							   pkt, pkt_len);
+							   pkt, pkt_len, mtu);
 			if (ret < 0) {
 				NET_DBG("Cannot fragment IPv6 pkt (%d)", ret);
 				return NET_DROP;

subsys/net/ip/net_private.h

@@ -245,7 +245,7 @@ int net_ipv4_send_fragmented_pkt(struct net_if *iface, struct net_pkt *pkt,
 
 #if defined(CONFIG_NET_IPV6_FRAGMENT)
 int net_ipv6_send_fragmented_pkt(struct net_if *iface, struct net_pkt *pkt,
-				 uint16_t pkt_len);
+				 uint16_t pkt_len, uint16_t mtu);
 #endif
 
 extern const char *net_verdict2str(enum net_verdict verdict);