[tcpip] Provide tcpip_mtu() to determine the maximum transmission unit

Provide the function tcpip_mtu() to allow external code to determine
the (transport-layer) maximum transmission unit for a given socket
address.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

1 files changed, 29 insertions, 0 deletions

diff --git a/src/net/tcpip.c b/src/net/tcpip.c
index 6fac8c52..4bcbe64b 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