3.2. Multicast Forwarding

Multicast Forwarding allows reaching devices on the Thread and Wi-Fi network in the same multicast group from both sides.

Hardware Prerequisites

To perform Multicast Forwarding, the following devices are required:

An ESP Thread Border Router
A Thread CLI device
A Linux Host machine

The Border Router and the Linux Host machine shall be connected to the same Wi-Fi network and the Thread device shall join the Thread network formed by the Border Router.

Limits on Multicast Groups

Note that to forward packets between the Thread and the Wi-Fi network, the multicast group scope shall be at least admin-local(ff04). Link-local and realm-local multicast packets will not be forwarded.

Reaching the Multicast Group from the Wi-Fi Network via ICMP

First, join the multicast group on the Thread CLI device:

mcast join ff04::123

Now you can ping Thread CLI device on your Linux Host:

ping -I wlan0 -t 64 ff04::123

The output similar to shown below may indicate that the device can be reached on the Wi-Fi network via the multicast group:

$ ping -I wlan0 -t 64 ff04::123
PING ff04::123(ff04::123) from fdde:ad00:beef:cafe:2eea:7fff:fe37:b4fb wlan0: 56 data bytes
64 bytes from fd66:afad:575f:1:744d:573e:6e60:188a: icmp_seq=1 ttl=254 time=132 ms

Reaching the Multicast Group from the Wi-Fi Network via UDP

First, join the multicast group and create a UDP socket on the Thread CLI device:

> mcast join ff04::123
Done
> udp open
Done
> udp bind :: 5083
Done

Use the following python script, which is named multicast_udp_client.py, to send UDP messages to the Thread CLI device via the multicast group:

import socket
import time

data = b'hello'
group = 'ff04::123'
REMOTE_PORT = 5083
network_interface = 'wlan0' # Change to the actual interface name

sock = socket.socket(socket.AF_INET6, socket.SOCK_DGRAM)
sock.setsockopt(socket.SOL_SOCKET, socket.SO_BINDTODEVICE, network_interface.encode())
sock.setsockopt(socket.IPPROTO_IPV6, socket.IPV6_MULTICAST_HOPS, 32)
sock.sendto(data, (group, REMOTE_PORT))
sock.close()

On the Linux Host machine, you need to start a UDP client by running this script:

python3 multicast_udp_client.py

On the Thread CLI device, the message will be printed:

> mcast join ff04::123
Done
> udp open
Done
> udp bind :: 5083
Done
5 bytes from fd11:1111:1122:2222:4a9e:272e:6a50:cf61 56024 hello

Reaching the Multicast Group from the Thread Network

Use the following python script, which is named multicast_udp_server.py, to join an admin-local group and set up a UDP server on the Linux machine:

import socket
import struct

if_index = socket.if_nametoindex('wlan0')
sock = socket.socket(socket.AF_INET6, socket.SOCK_DGRAM)
sock.bind(('::', 5083))
sock.setsockopt(
        socket.IPPROTO_IPV6, socket.IPV6_JOIN_GROUP,
        struct.pack('16si', socket.inet_pton(socket.AF_INET6, 'ff04::123'),
                                if_index))
while True:
        data, sender = sock.recvfrom(1024)
        print(data, sender)

On the Linux Host machine, you need to start a UDP server by running this script:

python3 multicast_udp_server.py

After launching the script and the Thread CLI device will be able to send UDP messages to the Linux Host via the multicast group:

udp open

udp send ff04::123 5083 hello

You will get a result Done after executing each of the commands, the expected output is below:

> udp open
Done
> udp send ff04::123 5083 hello
Done

On the Linux Host machine, the message will be printed:

$ python3 multicast_udp_server.py
b'hello' ('fd66:afad:575f:1:744d:573e:6e60:188a', 49154, 0, 0)