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I have a question related to computer networks, the question is in which of the following case the TCP checksum will not find an error:

1) a single bit flip occurs in the 10th byte (i.e., one bit in the 10th byte goes from 0 to 1, or from 1 to 0)

2) the first byte of the payload that was originally 00000001 becomes 00000000 and the third byte of the payload that was originally 00000000 becomes 00000001

3) the third bit of the first byte of the payload flips from 1 to 0, AND the third bit of the second byte of the payload flips from 0 to 1

4) the first byte of the payload that was originally 00000001 becomes 00000000, and the second byte of the payload that was originally 00000000 becomes 00000001

Ortomala Lokni
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Andre Liberty
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    This is not in the scope of SO. – zubergu Aug 03 '15 at 09:27
  • @zubergu Computing the TCP checksum is a network programming task. – Ortomala Lokni Aug 03 '15 at 09:29
  • @OrtomalaLokni I find it disputable. Flagged, see what happens. No point in argument over such long time dead topic anyway. – zubergu Aug 03 '15 at 09:33
  • Possible duplicate of [Can a TCP checksum fail to detect an error? If yes, how is this dealt with?](https://stackoverflow.com/questions/3830206/can-a-tcp-checksum-fail-to-detect-an-error-if-yes-how-is-this-dealt-with) – j_kubik Aug 05 '19 at 15:15

1 Answers1

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RFC 793 says:

The checksum field is the 16 bit one's complement of the one's complement sum of all 16 bit words in the header and text.

1) A single bit flip change the checksum.

2) As the sum is on 16 bit words, this will leave the checksum unchanged.

3) The two bit changes will not sum up and will change the checksum.

4) Same as 3)

Only the second case will leave the checksum unchanged.

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Ortomala Lokni
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