MTU selection closer to weave script

Note that if initFastdp sets the MTU for the datapath then the same
value is used for the bridge also.

net/bridge.go

@@ -225,6 +225,9 @@ func initBridgePrep(config *BridgeConfig) error {
 	linkAttrs := netlink.NewLinkAttrs()
 	linkAttrs.Name = config.WeaveBridgeName
 	linkAttrs.HardwareAddr = mac
+	if config.MTU == 0 {
+		config.MTU = 65535
+	}
 	linkAttrs.MTU = config.MTU // TODO this probably doesn't work - see weave script
 	if err = netlink.LinkAdd(&netlink.Bridge{LinkAttrs: linkAttrs}); err != nil {
 		return err
@@ -251,6 +254,15 @@ func initFastdp(config *BridgeConfig) error {
 	if err != nil {
 		return err
 	}
+	if config.MTU == 0 {
+		/* GCE has the lowest underlay network MTU we're likely to encounter on
+		   a local network, at 1460 bytes.  To get the overlay MTU from that we
+		   subtract 20 bytes for the outer IPv4 header, 8 bytes for the outer
+		   UDP header, 8 bytes for the vxlan header, and 14 bytes for the inner
+		   ethernet header.  In addition, we subtract 34 bytes for the ESP overhead
+		   which is needed for the vxlan encryption. */
+		config.MTU = 1376
+	}
 	return netlink.LinkSetMTU(datapath, config.MTU)
 }