Receiving multicast data with socket recvfrom in Python

Question

I have a multicast server sending data that must be captured by a python client. The problem is that recvfrom does not receive any data or at least receive the first packet and sorta caches it. If I use recvfrom in a loop then my data is received correctly.

My question is why I should use recvfrom in a loop to have the expected behavior?

from socket import *

s=socket(AF_INET, SOCK_DGRAM)
s.bind(('172.30.102.141',12345))
m=s.recvfrom(1024)
print m[0]

# sleep for x seconds here

m=s.recvfrom(1024)
print m[0]
# print the exact same thing as previously...

Answer 1

One thing is for sure, multicast is basically sending UDP packages and you have to keep listening for new packages. That is true even for TCP protocol based communication.

When you use low level interfaces for network communication, like socket is, it's up on both sides to define application level protocol.

That means, you define how receiving party concludes that message is complete. This is because message could get split in multiple parts/packets that get through the network. So receiving side has to assemble them in a proper way and then check if the message is whole. After that you push it up through the pipeline of processing messages or whatever you do in receiving side.

When using UDP, receiving side doesn't know if there is any packet on its way, so it just does try to recvfrom 1024 bytes and finishes. It doesn't know and should not care if there is more data on it's way. It's up to you to take care of that.