Not able to send data to a UDP IPv6 socket until I ping

Question

I am writing a program to send/receive data to a device using an IPv6 UDP port. My python code is running on a Win7 platform and the device is a piece of custom hardware. The PC and device are directly connected and do not get routed through a LAN.

The program work... sort of. When I initially attempt to send a message to the device, the send fails. To be more specific, as viewed by wireshark, I never see the message get sent to the device. When I ping the device using ping <ipv6_addr> -t on the command line, the first 1 or 2 attempts fail, after which, the pings start being received. I kill the ping and attempt to send the message just as before, but now it succeeds. Wireshark now shows all the messages. If I keep sending messages over a relatively short duration (somewhere between 10 sec to a 2 minute), the program works as expected. If I wait longer, my message will once again not be sent. If I keep the ping program continuously pinging, my messages also get sent regardless of the time between message sending activity. Here is my code:

import socket

def connect(port):
    if socket.has_ipv6:  
        client = socket.socket(socket.AF_INET6, socket.SOCK_DGRAM)   
        client.bind(('', port))
        client.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
        client.settimeout(0.25)

def send_data(addr, port, data):
    client.sendto(data, (addr, port))

def main_app():
    connect(10000)
    bin_data = binascii.a2b_hex('deadbeef')

    # Do some processing
    send_data(<ipv6_address>, 10000, bin_data)

Some googling did not yield any good leads on whether the problem is Python or something else, so I figured I would start with the Python as see where it takes me.

Any assistance would be greatly appreciated!

Answer 1

After some digging around I believe I found my problem. I was not including the IP address of the PC running the python in my bind statement. So changing my connect() function above to

def connect(self):
    # ... 
    client.bind(('fe80::', port))
    # ...

allowed my messages to be sent consistently. Im pretty new to socket programming, so can anyone provide an explanation why this fix works?

Thanks

Answer 2

Couple of hints:

• Call client.setsockopt() before you call client.bind().

• You need to call socket.close() when you are done with the socket.

• What i suspect to be the reason behind the weird issues with ping and timing issues could be the combination of a low timeout value and the NDP caching. The timeout of 0.25 is probably for some reason too low for NDP resolution + data transmission to complete (maybe your embedded device stack is slow). When you do the ping, it does NDP for you, and creates a cached info that you can use from your app, without breaking the timeout limit.

  I would suggest trying to up the limit: client.settimeout(2)