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Tcp checksum field

As explained in Acing the CCNA Exam: Volume 1 Fundamentals and Protocols

The TCP checksum field is a TCP-header field containing a calculated value used to detect corruption in a TCP segment.

The TCP checksum field stores a calculated value in the TCP header. It helps the receiver detect whether a TCP segment was corrupted during transmission. section 22.2.1

A receiver uses the field as follows:

  • Calculates its own checksum from the received segment. section 22.2.1
  • Compares that value with the checksum carried in the TCP header. section 22.2.1
  • Discards the segment and sends no acknowledgment when the values differ, causing the sender to retransmit it. section 22.2.1

How Acing the CCNA Exam: Volume 1 Fundamentals and Protocols covers it

How deep it goes
Defines checksum calculation and validation, but does not explain the algorithm’s arithmetic.
Examples
Uses a faulty TCP segment as the example: the receiver discards it, sends no acknowledgment, and prompts retransmission.
What you can do after
Diagnose corrupted TCP delivery by connecting checksum mismatches with discarded segments and retransmissions.
How it connects
Links TCP checksum validation with Ethernet Frame Check Sequence, IPv4 header checksum, acknowledgments, and retransmission.
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