UDP packages appear in wireshark, but are not received by program

Question

Hello everybody, I am trying to read UDP packages sent by an FPGA with my computer. They are sent to port 21844 and to the IP 192.168.1.2 (which is my computer's IP). I can see the package in wireshark, they have no errors. When I run however this little python script, then only a very very small fraction of all packages are received by it, also depending if wireshark is running or not.

import socket
import sys


HOST, PORT = "192.168.1.2", 21844
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.bind((HOST,PORT)) 
received ,address= sock.recvfrom(2048)
print address

I use windows 7 with Norton Internet Security, where I allow all traffic in the firewall for the FPGA IP and also for python. The same program on a Windows XP computer does not receive anything either...

Thanks for any help!

Answer 1

The TCP/IP stack of your OS doesn't hold those packets for you for eternity. Your script looks like something that very much depends on when it is run. Try to recvfrom in a loop, and run the script in the background. Then, start sending packets from your FPGA.

For extra convenience, explore the SocketServer module from Python's stdlib.

Answer 2

Ok, I found the problem: The UDP checksum in the FPGA was computed wrongly. Wireshark shows every package, but by default it does not check if the checksum is correct. When I set the checksum to 0x0000, then the packages arrive in python! Thanks for your help again!