Posted in

UDP multicasting with Python

Welcome to the next pikoTutorial!

The minimal receiver

Python
import struct
from argparse import ArgumentParser
from socket import socket, AF_INET, SOCK_DGRAM, inet_aton, IPPROTO_IP, \
    IP_ADD_MEMBERSHIP, INADDR_ANY, SOL_SOCKET, SO_REUSEADDR
# define command line interface
parser = ArgumentParser(description='UDP Multicast Receiver')
parser.add_argument('-mc_address', help='Multicast address')
# parse command line options
args = parser.parse_args()
# create a UDP socket
with socket(AF_INET, SOCK_DGRAM) as receiver_socket:
    # on the socket level, set the ability to reuse address
    receiver_socket.setsockopt(SOL_SOCKET, SO_REUSEADDR, 1)
    # bind the socket to the receiver's own address and port
    multicast_ip = args.mc_address.split(':')[0]
    multicast_port = args.mc_address.split(':')[1]
    receiver_socket.bind((multicast_ip, int(multicast_port)))
    # set up the multicast group membership by converting addresses to
    # 4sl (4 byte string and 4 byte long integer)
    multicast_request = struct.pack("4sl", inet_aton(multicast_ip), INADDR_ANY)
    # join the multicast group
    receiver_socket.setsockopt(IPPROTO_IP, IP_ADD_MEMBERSHIP, multicast_request)
    print(f'Receiver listening on {multicast_ip}:{multicast_port}')
    # receive messages (in this case, indefinitely until interrupted)
    data, sender_address = receiver_socket.recvfrom(1024)  # Buffer size 1024 bytes
    print(f'Received message from {sender_address[0]}:{sender_address[1]} : {data.decode()}')

Note for beginners: do not remove line 14 from the receiver implementation! Without the ability to re-use the same address, you will not be able to run more than 1 multicast receiver and you’ll end up with a simple UDP client/server setup.

The minimal sender

Python
import time
from argparse import ArgumentParser
from socket import socket, AF_INET, SOCK_DGRAM, IPPROTO_UDP
# define command line interface
parser = ArgumentParser()
parser.add_argument('-sender_address',
                    help='Sender\'s source address')
parser.add_argument('-mc_address', nargs='+',
                    help='List of multicast addresses separated by space')
# parse command line options
args = parser.parse_args()
# create a UDP socket
with socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP) as sender_socket:
    # assign sender specific source port
    sender_ip: str = args.sender_address.split(':')[0]
    sender_port: int = int(args.sender_address.split(':')[1])
    sender_socket.bind((sender_ip, sender_port))
    # send the message to each multicast address in the list
    for address in args.mc_address:
        multicast_ip: str = address.split(':')[0]
        multicast_port: int = int(address.split(':')[1])
        sender_socket.sendto(f'This message is for {multicast_ip}:{multicast_port} receivers'.encode(),
                             (multicast_ip, multicast_port))

Multicast on a single IP and a single port

The simplest scenario assumes 1 sender sending from 127.0.0.1:3000 address to 2 receivers listening on the shared 224.0.0.1:5000 multicast address:

UDP multicast single port receivers

To achieve that, run as many receivers as you want with command:

Bash
python3 receiver.py -mc_address 224.0.0.1:5000

And the sender with command:

Bash
python3 sender.py -sender_address 127.0.0.1:3000 -mc_address 224.0.0.1:5000

Note for beginners: multicast IPs can range from 224.0.0.0 to 239.255.255.255.

Multicast on a single IP and multiple ports

In this setup 1 sender sends message from 127.0.0.1:3000 address to a single multicast IP and two multicast ports: 224.0.0.1:5000 and 224.0.0.1:5001.

UDP multicasting multiple port

Run first group of receivers with command:

Bash
python3 receiver.py -mc_address 224.0.0.1:5000

And the second group of receivers with command:

Bash
python3 receiver.py -mc_address 224.0.0.1:5001

After that you can send a multicast messages to all of them running sender with the following command:

Bash
python3 sender.py -sender_address 127.0.0.1:3000 -mc_address 224.0.0.1:5000 224.0.0.1:5001
Read also on dev.to: 5 Python good practices that make life easier

Multicast on multiple IPs

Groups of receivers can be of course differentiated also by multicast IP.

UDP multicast on multiple IPs

Note for beginners: in this example I decided to not specify the sender’s IP. In the previous examples, I used loopback interface on which 127.0.0.1 IP is always available, but multiple multicast IPs on that interface may not work on your machine without additional configuration. I don’t know what are real network interfaces available on your machine and what IPs are assigned on them, so I let the operating system choose it automatically by leaving the sender’s IP empty.

For that, run the first group of receivers with command:

Bash
python3 receiver.py -mc_address 224.0.0.1:5000

And the second group of receivers with command:

Bash
python3 receiver.py -mc_address 224.0.0.2:5000

Now the only thing left to do is to launch the multicast sender (leaving the IP empty – see the note above):

Bash
python3 sender.py -sender_address :3000 -mc_address 224.0.0.1:5000 224.0.0.2:5000

Read also: