Under IPv4, when is UDP checksum optional and when is it mandatory under IPv6?

• A. In IPv4, UDP checksum is mandatory; in IPv6, it is optional.
• B. In IPv4, UDP checksum is optional; in IPv6, it is mandatory.
• C. UDP checksum is optional in both IPv4 and IPv6.
• D. UDP checksum is mandatory in both IPv4 and IPv6.

Answer: (B)

The key idea is how integrity protection is provided at the IP layer versus the transport layer. UDP checksums are used to detect corruption of the UDP payload, and they include a pseudo-header that binds the datagram to the IP addressing and length.

For IPv4, the IP layer has its own header checksum, which provides some error detection, and UDP’s checksum is optional. If the UDP checksum field is zero, the datagram is considered unprotected by UDP; if a checksum is present, it protects the UDP payload and the relevant pseudo-header. This optionality exists because some networks historically relied on the IP header checksum and for performance or compatibility reasons chose not to require UDP checksums.

For IPv6, there is no IP header checksum at all. To ensure data integrity end-to-end, UDP must provide its own checksum for every datagram, and the standard requires this field to be computed. The UDP checksum in IPv6 covers the UDP header, payload, and a larger IPv6 pseudo-header, tying the integrity directly to the correct addresses and length.

So, the UDP checksum is optional in IPv4 but mandatory in IPv6.