Handler of new connection requests on bgp server port.

        Checks if new connection request is valid and starts new instance of
        protocol.

def start_protocol(self, socket):
        """Handler of new connection requests on bgp server port.

        Checks if new connection request is valid and starts new instance of
        protocol.
        """
        assert socket
        peer_addr, peer_port = self.get_remotename(socket)
        peer = self._peer_manager.get_by_addr(peer_addr)
        bgp_proto = self.build_protocol(socket)

        # We reject this connection request from peer:
        # 1) If we have connection initiated by a peer that is not in our
        #     configuration.
        # 2) If this neighbor is not enabled according to configuration.
        if not peer or not peer.enabled:
            LOG.debug('Closed connection %s %s:%s as it is not a recognized'
                      ' peer.', 'from' if bgp_proto.is_reactive else 'to',
                      peer_addr, peer_port)
            # Send connection rejected notification as per RFC
            code = BGP_ERROR_CEASE
            subcode = BGP_ERROR_SUB_CONNECTION_RESET
            bgp_proto.send_notification(code, subcode)
        elif bgp_proto.is_reactive and \
                peer.connect_mode is CONNECT_MODE_ACTIVE:
            LOG.debug('Closed connection from %s:%s as connect_mode is'
                      ' configured ACTIVE.', peer_addr, peer_port)
            # Send connection rejected notification as per RFC
            code = BGP_ERROR_CEASE
            subcode = BGP_ERROR_SUB_CONNECTION_RESET
            bgp_proto.send_notification(code, subcode)
        elif not (peer.in_idle() or peer.in_active() or peer.in_connect()):
            LOG.debug('Closing connection to %s:%s as we have connection'
                      ' in state other than IDLE or ACTIVE,'
                      ' i.e. connection resolution',
                      peer_addr, peer_port)
            # Send Connection Collision Resolution notification as per RFC.
            code = BGP_ERROR_CEASE
            subcode = BGP_ERROR_SUB_CONNECTION_COLLISION_RESOLUTION
            bgp_proto.send_notification(code, subcode)
        else:
            bind_ip, bind_port = self.get_localname(socket)
            peer._host_bind_ip = bind_ip
            peer._host_bind_port = bind_port
            self._spawn_activity(bgp_proto, peer)