Event for user's NAT Device Type: Tell user if the node is behind an Easy or Hard NAT (#173)

* event for NAT device type
Co-authored-by: Marten Seemann <martenseemann@gmail.com>

event/nattype.go

+package event
+
+import "github.com/libp2p/go-libp2p-core/network"
+
+// EvtNATDeviceTypeChanged is an event struct to be emitted when the type of the NAT device changes for a Transport Protocol.
+//
+// Note: This event is meaningful ONLY if the AutoNAT Reachability is Private.
+// Consumers of this event should ALSO consume the `EvtLocalReachabilityChanged` event and interpret
+// this event ONLY if the Reachability on the `EvtLocalReachabilityChanged` is Private.
+type EvtNATDeviceTypeChanged struct {
+	// TransportProtocol is the Transport Protocol for which the NAT Device Type has been determined.
+	TransportProtocol network.NATTransportProtocol
+	// NatDeviceType indicates the type of the NAT Device for the Transport Protocol.
+	// Currently, it can be either a `Cone NAT` or a `Symmetric NAT`. Please see the detailed documentation
+	// on `network.NATDeviceType` enumerations for a better understanding of what these types mean and
+	// how they impact Connectivity and Hole Punching.
+	NatDeviceType network.NATDeviceType
+}

network/nattype.go

+package network
+
+// NATDeviceType indicates the type of the NAT device.
+type NATDeviceType int
+
+const (
+	// NATDeviceTypeUnknown indicates that the type of the NAT device is unknown.
+	NATDeviceTypeUnknown NATDeviceType = iota
+
+	// NATDeviceTypeCone indicates that the NAT device is a Cone NAT.
+	// A Cone NAT is a NAT where all outgoing connections from the same source IP address and port are mapped by the NAT device
+	// to the same IP address and port irrespective of the destination address.
+	// With regards to RFC 3489, this could be either a Full Cone NAT, a Restricted Cone NAT or a
+	// Port Restricted Cone NAT. However, we do NOT differentiate between them here and simply classify all such NATs as a Cone NAT.
+	// NAT traversal with hole punching is possible with a Cone NAT ONLY if the remote peer is ALSO behind a Cone NAT.
+	// If the remote peer is behind a Symmetric NAT, hole punching will fail.
+	NATDeviceTypeCone
+
+	// NATDeviceTypeSymmetric indicates that the NAT device is a Symmetric NAT.
+	// A Symmetric NAT maps outgoing connections with different destination addresses to different IP addresses and ports,
+	// even if they originate from the same source IP address and port.
+	// NAT traversal with hole-punching is currently NOT possible in libp2p with Symmetric NATs irrespective of the remote peer's NAT type.
+	NATDeviceTypeSymmetric
+)
+
+func (r NATDeviceType) String() string {
+	switch r {
+	case 0:
+		return "Unknown"
+	case 1:
+		return "Cone"
+	case 2:
+		return "Symmetric"
+	default:
+		return "unrecognized"
+	}
+}
+
+// NATTransportProtocol is the transport protocol for which the NAT Device Type has been determined.
+type NATTransportProtocol int
+
+const (
+	// NATTransportUDP means that the NAT Device Type has been determined for the UDP Protocol.
+	NATTransportUDP NATTransportProtocol = iota
+	// NATTransportTCP means that the NAT Device Type has been determined for the TCP Protocol.
+	NATTransportTCP
+)
+
+func (n NATTransportProtocol) String() string {
+	switch n {
+	case 0:
+		return "UDP"
+	case 1:
+		return "TCP"
+	default:
+		return "unrecognized"
+	}
+}