Note

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UDP Multicast Example Explanation

Example Location

Development board send/receive example:

• Development board firmware path: /sdcard/examples/14-Socket/udp_multicast.py

• SDK source path: src/canmv/resources/examples/14-Socket/udp_multicast.py

The companion scripts for the PC are also provided with the firmware, but they should run in the PC’s Python environment:

• Sender firmware path: /sdcard/examples/14-Socket/udp_multicast_sender_pc.py

• Sender source path: src/canmv/resources/examples/14-Socket/udp_multicast_sender_pc.py

• Receiver firmware path: /sdcard/examples/14-Socket/udp_multicast_receiver_pc.py

• Receiver source path: src/canmv/resources/examples/14-Socket/udp_multicast_receiver_pc.py

Multicast allows a single sender to deliver the same datagram to multiple receivers in the same multicast group. The development board script switches between sender and receiver modes via IS_SENDER, and the PC-side scripts serve as the other end for verification.

Parameter Configuration

MULTICAST_GROUP = "239.255.0.1"
MULTICAST_PORT = 5007
IS_SENDER = False

NETWORK_TYPE = "wifi_sta"
WLAN_DEVICE = "auto"
WIFI_SSID = "TEST"
WIFI_PASSWORD = "12345678"
NETWORK_TIMEOUT = 20

The development board and the PC must be on the same LAN that supports multicast forwarding. If a switch, wireless router, or hotspot has client isolation, multicast filtering, or IGMP restrictions enabled, the data may not reach its destination.

Common Flow

netif, ip = connect_network(...)

if IS_SENDER:
    multicast_sender(ip)
else:
    multicast_receiver()

The script first obtains a valid IP, then selects the role based on IS_SENDER. The default value False means the development board acts as the receiver.

Development Board Sender Mode

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
ttl = struct.pack("b", 1)
sock.setsockopt(0, 33, ttl)
sock.sendto(message.encode(), (MULTICAST_GROUP, MULTICAST_PORT))

Here, the options 0 and 33 correspond to IPPROTO_IP and IP_MULTICAST_TTL respectively. TTL is set to 1, so that the data only propagates on the local network. The script sends once every 2 seconds, and includes the development board’s IP and an incrementing counter in the message.

Test steps:

1. Run udp_multicast_receiver_pc.py on the PC.

2. Change IS_SENDER in the development board script to True and run it.

3. The PC side should continuously display multicast messages from the development board.

Development Board Receiver Mode

sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
sock.bind(("0.0.0.0", MULTICAST_PORT))

mreq = struct.pack(
    "4s4s",
    inet_aton(MULTICAST_GROUP),
    inet_aton("0.0.0.0"),
)
sock.setsockopt(0, 35, mreq)

The receiver first listens on port 5007 across all interfaces, then joins 239.255.0.1 via a membership request. Option 35 corresponds to IP_ADD_MEMBERSHIP. After receiving data, the script prints the sender’s address and the decoded payload; data that cannot be decoded as UTF-8 will be flagged as binary content.

Test steps:

1. Keep IS_SENDER = False in the development board script and run it.

2. Run udp_multicast_sender_pc.py on the PC.

3. The development board’s serial port should display Hello from PC multicast sender! once per second.

Troubleshooting Points

• Confirm that the multicast address and port on both ends match exactly.

• Confirm that the system firewall allows UDP 5007.

• On PCs with multiple network cards, confirm that the multicast traffic is sent out from the interface that can communicate with the development board.

• When regular UDP unicast works but multicast has no data, focus on checking the access point’s client isolation, IGMP Snooping, and multicast filtering settings.