[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>

src/include/ipxe/tcpip.h

 	const char *name;
 	/** Network address family */
 	sa_family_t sa_family;
+	/** Fixed header length */
+	size_t header_len;
 	/**
 	 * Transmit packet
 	 *
 		      struct net_device *netdev,
 		      uint16_t *trans_csum );
 extern struct net_device * tcpip_netdev ( struct sockaddr_tcpip *st_dest );
+extern size_t tcpip_mtu ( struct sockaddr_tcpip *st_dest );
 extern uint16_t generic_tcpip_continue_chksum ( uint16_t partial,
 						const void *data, size_t len );
 extern uint16_t tcpip_chksum ( const void *data, size_t len );

src/net/ipv4.c

 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,
 };

src/net/ipv6.c

 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,
 };

src/net/tcpip.c

 #include <ipxe/iobuf.h>
 #include <ipxe/tables.h>
 #include <ipxe/ipstat.h>
+#include <ipxe/netdevice.h>
 #include <ipxe/tcpip.h>
 
 /** @file
 	return NULL;
 }
 
+/**
+ * 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
  *