Multicast

In simple terms, I can do a broadcast on a network. This results in every client on the network receiving and processing the packet. That's a great way to cause congestion on a network! There are cases where the output of process needs to be seen by many other processes on many other nodes.

Imagine I have power monitoring equipment in my data centre, and I have distributed control to take action based on the power state. For example, if the power is out, and the temperature is OK, I might shut down ventilation fans to reduce the load on the Uninterruptible Power Supply (UPS).

One way to do this is to use multicast. Make all the data available, any device which wants to consume the data can do so. A modern architecture for this might use MQTT.

First, I create Exercises_11\settings\mc.py

MCSERVER = {
    "MCAST_GROUP": '239.1.1.1',
    "IP_ADDRESS": '127.0.0.1',
    "PORT": 5001
}

MCCLIENT = {
    "MCAST_GROUP": '239.1.1.1',
    "IP_ADDRESS": '127.0.0.1',
    "PORT": 5001
}

Then I create Exercises_11\mc_client.py

'''
Multicast client by: JOR
Sends multicast packets to a particular address and port.
Alpha: 13FEB22
'''
import socket
import time
from datetime import datetime
import settings.mc as settings

# Set multicast information
MCAST_GRP = settings.MCCLIENT["MCAST_GROUP"]
MCAST_PORT = settings.MCCLIENT["PORT"]
MCAST_IF_IP = settings.MCCLIENT["IP_ADDRESS"]

print(f'This is the client, make sure its IP address matches {MCAST_IF_IP} in settings.')
print(f'This selects which interface is used for multicast to {MCAST_GRP}.')
print('This script has no error handling, by design')

while True:
    with socket.socket(socket.AF_INET, socket.SOCK_DGRAM, socket.IPPROTO_UDP) as s:
        # inet_aton converts IPv4 from the a dotted decimal string to 32 bit packed binary format
        s.setsockopt(socket.IPPROTO_IP, socket.IP_ADD_MEMBERSHIP, socket.inet_aton(MCAST_GRP) + socket.inet_aton(MCAST_IF_IP))
        message_text = f"ATU {datetime.now()}"
        message = message_text.encode('utf-8')
        s.sendto(message, (MCAST_GRP, MCAST_PORT))
        print(f'Sent {message_text} to {MCAST_GRP}:{MCAST_PORT}')
        time.sleep(1)

Then I create Exercises_11\mc_server.py

'''
Multicast Server by: JOR
Reads multicast packets from a particular address and port.
Alpha: 13FEB22
'''
import socket
import struct
import settings.mc as settings

# Set multicast information
MCAST_GRP = settings.MCSERVER["MCAST_GROUP"]
SERVER_ADDRESS = ('', settings.MCSERVER["PORT"])
MCAST_IF_IP = settings.MCSERVER["IP_ADDRESS"]

print('This is the server.')
print(f'Make sure its IP address matches {MCAST_IF_IP} in settings.')
print(f'This selects which interface is used to listen for multicast as {MCAST_GRP}.')
print('This script has no error handling, by design.')

with socket.socket(socket.AF_INET, socket.SOCK_DGRAM) as s:
    s.bind(SERVER_ADDRESS)
    # inet_aton converts IPv4 from the a dotted decimal string to 32 bit packed binary format
    s.setsockopt(socket.IPPROTO_IP, socket.IP_ADD_MEMBERSHIP, socket.inet_aton(MCAST_GRP) + socket.inet_aton(MCAST_IF_IP))

    while True:
        print('Waiting to receive message')
        data, address = s.recvfrom(1024)
        print(f'received {len(data)} bytes from {address}')
        print(data)