Add physical MTU recommendation hint to network config via API.

include/ZeroTierOne.h

@@ -977,6 +977,11 @@ typedef struct
 	 */
 	unsigned int mtu;
 
+	/**
+	 * Recommended MTU to avoid fragmentation at the physical layer (hint)
+	 */
+	unsigned int physicalMtu;
+
 	/**
 	 * If nonzero, the network this port belongs to indicates DHCP availability
 	 *
@@ -1604,6 +1609,9 @@ typedef int (*ZT_PathCheckFunction)(
  * Note that this can take a few seconds the first time it's called, as it
  * will generate an identity.
  *
+ * TODO: should consolidate function pointers into versioned structure for
+ * better API stability.
+ *
  * @param node Result: pointer is set to new node instance on success
  * @param uptr User pointer to pass to functions/callbacks
  * @param now Current clock in milliseconds

node/Network.cpp

@@ -1224,6 +1224,7 @@ void Network::_externalConfig(ZT_VirtualNetworkConfig *ec) const
 	ec->status = _status();
 	ec->type = (_config) ? (_config.isPrivate() ? ZT_NETWORK_TYPE_PRIVATE : ZT_NETWORK_TYPE_PUBLIC) : ZT_NETWORK_TYPE_PRIVATE;
 	ec->mtu = ZT_IF_MTU;
+	ec->physicalMtu = ZT_UDP_DEFAULT_PAYLOAD_MTU - (ZT_PACKET_IDX_PAYLOAD + 16);
 	ec->dhcp = 0;
 	std::vector<Address> ab(_config.activeBridges());
 	ec->bridge = ((_config.allowPassiveBridging())||(std::find(ab.begin(),ab.end(),RR->identity.address()) != ab.end())) ? 1 : 0;