Checksum

Covers the header, the data, and a pseudo-header built from the addresses one lane up. Zero means not computed.

Reading a capture

The field whose rules change depending on the lane above it: optional over IPv4, where zero legitimately means "not computed", and mandatory over IPv6, where zero is malformed. One field, two contracts, decided by something outside this header.

Where it sits

UDP Datagram > Checksum

• Open in the explorer — UDP over IPv6, in a QoS Data frame
• Open in the explorer — UDP over IPv4, in a QoS Data frame

Accuracy

Clause

RFC 768

Byte order

big-endian — network byte order

Verified

Not verified against the standard. Treat this as a claim, not a fact.