3 files changed, 31 insertions, 0 deletions
diff --git a/src/net/ipv4.c b/src/net/ipv4.c
index 8bda5c86f..c5cfd134d 100644
--- a/src/net/ipv4.c
+++ b/src/net/ipv4.c
@@ -621,6 +621,7 @@ struct net_protocol ipv4_protocol __net_protocol = {
 struct tcpip_net_protocol ipv4_tcpip_protocol __tcpip_net_protocol = {
 	.name = "IPv4",
 	.sa_family = AF_INET,
+	.header_len = sizeof ( struct iphdr ),
 	.tx = ipv4_tx,
 	.netdev = ipv4_netdev,
 };
diff --git a/src/net/ipv6.c b/src/net/ipv6.c
index 6f2e94776..f753751df 100644
--- a/src/net/ipv6.c
+++ b/src/net/ipv6.c
@@ -988,6 +988,7 @@ struct net_protocol ipv6_protocol __net_protocol = {
 struct tcpip_net_protocol ipv6_tcpip_protocol __tcpip_net_protocol = {
 	.name = "IPv6",
 	.sa_family = AF_INET6,
+	.header_len = sizeof ( struct ipv6_header ),
 	.tx = ipv6_tx,
 	.netdev = ipv6_netdev,
 };
diff --git a/src/net/tcpip.c b/src/net/tcpip.c
index 6fac8c52c..4bcbe64bb 100644
--- a/src/net/tcpip.c
+++ b/src/net/tcpip.c
@@ -6,6 +6,7 @@
 #include <ipxe/iobuf.h>
 #include <ipxe/tables.h>
 #include <ipxe/ipstat.h>
+#include <ipxe/netdevice.h>
 #include <ipxe/tcpip.h>
 
 /** @file
@@ -123,6 +124,34 @@ struct net_device * tcpip_netdev ( struct sockaddr_tcpip *st_dest ) {
 }
 
 /**
+ * Determine maximum transmission unit
+ *
+ * @v st_dest		Destination address
+ * @ret mtu		Maximum transmission unit
+ */
+size_t tcpip_mtu ( struct sockaddr_tcpip *st_dest ) {
+	struct tcpip_net_protocol *tcpip_net;
+	struct net_device *netdev;
+	size_t mtu;
+
+	/* Find appropriate network-layer protocol */
+	tcpip_net = tcpip_net_protocol ( st_dest );
+	if ( ! tcpip_net )
+		return 0;
+
+	/* Find transmitting network device */
+	netdev = tcpip_net->netdev ( st_dest );
+	if ( ! netdev )
+		return 0;
+
+	/* Calculate MTU */
+	mtu = ( netdev->max_pkt_len - netdev->ll_protocol->ll_header_len -
+		tcpip_net->header_len );
+
+	return mtu;
+}
+
+/**
  * Calculate continued TCP/IP checkum
  *
  * @v partial		Checksum of already-summed data, in network byte order