Bad UDP checksum has no effect: why?

问题:

I'm trying to test a UDP program on what happens if it receives data with a bad UDP checksum. Strangely, it seems to have no effect, and the payload is received successfully, at least on OS X via the loopback interface.

An example is below, where data is sent using SOCK_RAW + IPPROTO_UDP, specifying the checksum manually to something incorrect, and received using SOCK_DGRAM.

import asyncio
import socket
import struct

async def server():
    with \
        socket.socket(
                socket.AF_INET, socket.SOCK_DGRAM
    ) as sock:
        sock.setblocking(False)
        sock.bind(('', 4567))
        payload = await loop.sock_recv(sock, 512)
        print(payload)  # Prints b'somedata'

async def main():
    asyncio.ensure_future(server())
    await asyncio.sleep(0)

    with socket.socket(
            family=socket.AF_INET, type=socket.SOCK_RAW, proto=socket.IPPROTO_UDP,
    ) as sock:

        local_ip = '127.0.0.1'
        src_port = 4566
        dest_port = 4567

        payload = b'somedata'
        length = 8 + len(payload)
        checksum = 3  # 3 is not the right checksum
        header_bad_checksum = struct.pack('!4H', src_port, dest_port, length, checksum)
        sock.sendto(header_bad_checksum + payload, (local_ip, 4567))

        await asyncio.sleep(1)

loop = asyncio.get_event_loop()
loop.run_until_complete(main())
loop.close()

Why is this? I would have expected the payload to be ignored.

---

The Wireshark dump of this UDP message is below, showing that (in this case) the checksum is 3.