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I am thinking the same problem. TCP checksum includes data field. Is it means AT&T flip bits and re-calculate checksum?

It looks like someone on the twitter thread checked and the checksums are wrong. Im not sure why these packets aren't discarded by your computer.

edit: apparently osx doesnt drop these packets according to the twitter thread.

"It depends" - Your NIC could be doing some tcp offload for you, at which point any checksum you'd observe with tcpdump/wireshark would be flat out wrong.

Generally - both the tcp checksum and the 802.11/Ethernet CRC isn't all that strong, so it's entirely within the realm of possibility of having bitflip patterns that fool both.

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