Generate BFD packet with Ethernet/IPv4/UDP encapsulated.

def bfd_packet(src_mac, dst_mac, src_ip, dst_ip, ipv4_id,
                   src_port, dst_port,
                   diag=0, state=0, flags=0, detect_mult=0,
                   my_discr=0, your_discr=0, desired_min_tx_interval=0,
                   required_min_rx_interval=0,
                   required_min_echo_rx_interval=0,
                   auth_cls=None):
        """
        Generate BFD packet with Ethernet/IPv4/UDP encapsulated.
        """
        # Generate ethernet header first.
        pkt = packet.Packet()
        eth_pkt = ethernet.ethernet(dst_mac, src_mac, ETH_TYPE_IP)
        pkt.add_protocol(eth_pkt)

        # IPv4 encapsulation
        # set ToS to 192 (Network control/CS6)
        # set TTL to 255 (RFC5881 Section 5.)
        ipv4_pkt = ipv4.ipv4(proto=inet.IPPROTO_UDP, src=src_ip, dst=dst_ip,
                             tos=192, identification=ipv4_id, ttl=255)
        pkt.add_protocol(ipv4_pkt)

        # UDP encapsulation
        udp_pkt = udp.udp(src_port=src_port, dst_port=dst_port)
        pkt.add_protocol(udp_pkt)

        # BFD payload
        bfd_pkt = bfd.bfd(
            ver=1, diag=diag, state=state, flags=flags,
            detect_mult=detect_mult,
            my_discr=my_discr, your_discr=your_discr,
            desired_min_tx_interval=desired_min_tx_interval,
            required_min_rx_interval=required_min_rx_interval,
            required_min_echo_rx_interval=required_min_echo_rx_interval,
            auth_cls=auth_cls)
        pkt.add_protocol(bfd_pkt)

        pkt.serialize()
        return pkt.data