Weird network behaviors with UDP sockets and python

Question

I made some python script and ran it both on my computer and some distant shell (some website that provides shell access).

I'm using threads, pipes and UDP sockets to transfer data in a P2P fashion, so each script can both receive and send through the same socket. To test if this works, I open one terminal on my computer and one other terminal with ssh, connected to my shell. I make sure the script is the same on both machines, and feed it with an ip address.

Here is the script: http://codepad.org/V9Q1KcDT

(I don't know if I should paste it directly here or not)

My problem is this: strings I send seems to land something 20% of the time, sometimes often, sometimes not, and it seems to be random...

What am I doing wrong? Are UDP so unreliable? Are python thread+pipe+socket too slow? Could it be some kind of network problem with my shell provider? Is my program flawed? Are pipes a good solution to communicate with threads?

I have no problem not using a shell and I have not tried, but it's useful for testing purposes.

BTW if i'm behing a router, how does the router knows where to send the packet if I'm not the only computer connected? (I tried when I was the only one, it behaved identically).

Answer 1

User Datagram Protocol(UDP) could easily stand for Unreliable Datagram Protocol. UDP provides no guarantees regarding delivery so if you need reliability you have to implement it yourself by resending messages.

That said, threading and multiprocessing and remote machines and networks are a lot of variables to diagnose at once. I'd advise you to step back and try to boil the problem down to something simpler.

For instance:

• first try TCP instead of UDP
• instead of threads, run 2 processes (one doing nit_send, one doing nit_recv)
• run them both on your local machine (use localhost: 127.0.0.1 )

Once you have this basic test working then add features back in. For instance, switch to UDP. Once you've got your UDP issues sorted out locally try introducing remote machines or threads, etc.

Regarding the router question all machines connected to your router are most likely NAT'd so they all appear to have the same IP address (the IP address of your router) to machines on the internet. Look into NAT and port-forwarding if you want to route traffic to a specific machine on your local subnet.