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Networking Fundamentals · Foundational

Checksum

A value calculated over data so the receiver can detect corruption in transit.

Definition

The IPv4 header, TCP and UDP each carry a 16-bit ones-complement checksum; Ethernet uses a stronger CRC-32 in the FCS. A checksum mismatch causes the receiving layer to discard the data. IPv6 removed the header checksum, relying on Layer 2 and Layer 4 checks, and made the UDP checksum mandatory.

Example

A corrupted TCP segment fails its checksum and is discarded, triggering retransmission.

Exam tip

Checksums detect errors but do not correct them, and they are not a security control.

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