ToRapture's Blog

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UDP learn by Python3

Posted on Edited on Disqus:

Find max packet size

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# coding: utf-8

import socket


def find_max_udp_packet_size():
    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    max_sz = 0
    l, r = 0, 65535 - 8 - 20
    while l <= r:
        mid = int((l + r) / 2)
        ok = True

        try:
            sock.sendto(b'A' * mid, ('localhost', 9))
        except socket.error:
            ok = False

        if ok:
            max_sz = mid
            l = mid + 1
        else:
            r = mid - 1

    return max_sz


def main():
    max_sz = find_max_udp_packet_size()
    print('max_sz = %s' % max_sz)


if __name__ == '__main__':
    main()

I used the code above to find the max size of a UDP packet size by default configuration. The result on my computer is: 

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$ uname -v
Darwin Kernel Version 19.2.0: Sat Nov  9 03:47:04 PST 2019; root:xnu-6153.61.1~20/RELEASE_X86_64
$ python3 --version
Python 3.6.4
$ python3 udp_packet_size.py
max_sz = 9216

More test

Use the code below to test. 

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# coding: utf-8

import socket

from IPython import embed


def main():
    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    embed()
    sock.close()


if __name__ == '__main__':
    main()

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In [1]: sock.bind(('localhost', 10001))

In [2]: receiver = ('localhost', 10002)

In [3]: sock.sendto(b'01234567', receiver)
Out[3]: 8

In [4]: sock.sendto(b'01234567', receiver)
Out[4]: 8

In [5]: sock.sendto(b'01234567', receiver)
Out[5]: 8

In [6]: sock.sendto(b'01234567', receiver)
Out[6]: 8
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In [1]: sock.bind(('localhost', 10002))

In [2]: receiver = ('localhost', 10001)

In [3]: print(sock.recvfrom(1024))
(b'01234567', ('127.0.0.1', 10001))

In [4]: print(sock.recvfrom(4))
(b'0123', ('127.0.0.1', 10001))

In [5]: print(sock.recvfrom(4))
(b'0123', ('127.0.0.1', 10001))

In [6]: print(sock.recvfrom(4))
(b'0123', ('127.0.0.1', 10001))

In [7]: print(sock.recvfrom(4))

We can see that if the bufsize is smaller the length of the packet received, will only read bufsize bytes of the received data, and the rest of that packet will not be returned in subsequent reads.