Python sockets steal each other's packages

Question

I am trying to write a program for parallel checking the data rate for packets of various sizes. However, I noticed something strange that the packet size did not seem to affect the transfer time according to my program, while Unix binary pingcould disable some of the packet sizes that I use. I sent 4 packets containing the string testquest, and one that was only 2000 bytes equal to 0. However, when I printed the results, they all contained "testquest" (and were much shorter than 2000 bytes). The only thing I can conclude is that these sockets somehow get the same packet, which explains how they all have the same rtt.

I did this MCVE to illustrate the problem (you can ignore the "checksum" function, it is included for completeness, but I know from experience that it works):

#!/usr/bin/env python3
import socket
import struct
import time
from multiprocessing.pool import ThreadPool as Pool
from sys import argv, byteorder

def calculate_checksum(pkt):
    """
    Implementation of the "Internet Checksum" specified in RFC 1071 (https://tools.ieft.org/html/rfc1071)
    Ideally this would act on the string as a series of 16-bit ints (host
    packed), but this works.
    Network data is big-endian, hosts are typically little-endian,
    which makes this much more tedious than it needs to be.
    """

    countTo = len(pkt) // 2 * 2
    total, count = 0, 0

    # Handle bytes in pairs (decoding as short ints)
    loByte, hiByte = 0, 0
    while count < countTo:
        if (byteorder == "little"):
            loByte = pkt[count]
            hiByte = pkt[count + 1]
        else:
            loByte = pkt[count + 1]
            hiByte = pkt[count]
        total += hiByte * 256 + loByte
        count += 2

    # Handle last byte if applicable (odd-number of bytes)
    # Endianness should be irrelevant in this case
    if countTo < len(pkt): # Check for odd length
        total += pkt[len(pkt) - 1]

    total &= 0xffffffff # Truncate sum to 32 bits (a variance from ping.c, which
                        # uses signed ints, but overflow is unlikely in ping)

    total = (total >> 16) + (total & 0xffff)    # Add high 16 bits to low 16 bits
    total += (total >> 16)                      # Add carry from above (if any)

    return socket.htons((~total) & 0xffff)

def ping(args):
    sock, payload = args[0], args[1]
    header = struct.pack("!BBH", 8, 0, 0)
    checksum = calculate_checksum(header+payload)
    header = struct.pack("!BBH", 8, 0, checksum)

    timestamp = time.time()
    sock.send(header+payload)

    try:
        response = sock.recv(20+len(payload))
    except socket.timeout:
        return 0

    return (len(response), (time.time() - timestamp) * 1000)


host = argv[1] # A host that doesn't respond to ping packets > 1500B

# 1 is ICMP protocol number
sockets = [socket.socket(socket.AF_INET, socket.SOCK_RAW, proto=1) for i in range(12)]

for i, sock in enumerate(sockets):
    sock.settimeout(0.1)
    sock.bind(("0.0.0.0", i))
    sock.connect((host, 1)) # Port number should never matter for ICMP

args = [(sockets[i], bytes(2**i)) for i in range(12)]

for arg in args:
    print(ping(arg))
    arg[0].close()

This actually shows me something more disturbing - it looks like rtt really decreases with increasing packet size! Calling this program (as root, to obtain socket permissions):

0
0
(24, 15.784025192260742)
(28, 0.04601478576660156)
(28, 0.025033950805664062)
(28, 0.033855438232421875)
(28, 0.03528594970703125)
(28, 0.04887580871582031)
(28, 0.05316734313964844)
(28, 0.03790855407714844)
(28, 0.0209808349609375)
(28, 0.024080276489257812)

but now notice what happens when I try to send a 2048 packet with ping:

user@mycomputer ~/src/connvitals $ time ping -c1 -s2048 $box
PING <hostname redacted> (<IP address redacted>): 2048 data bytes

--- <hostname redacted> ping statistics ---
1 packets transmitted, 0 packets received, 100.0% packet loss

real  0m11.018s
user  0m0.005s
sys   0m0.008s

The package fell not only, but it takes 11 seconds! So why - if my timeout is set to 100 ms - does this package receive a "successful" response from my python script in just ~ 0.04ms ??

Thanks in advance for any help you can provide.

Update:

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Answer 1

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