Which event typically triggers the recalculation of the IPv4 header checksum by routers?

• A. TTL is decremented
• B. The payload is encrypted
• C. The destination address changes
• D. The protocol value changes

Answer: (A)

IPv4 header checksums are calculated over the header fields, so any change to the header at a router requires updating the checksum. The typical change that happens at every hop is the TTL (time-to-live) field being decremented by 1. When TTL is reduced, the header content changes, and the checksum must be adjusted to reflect that new header.

Payload encryption doesn’t alter the IPv4 header fields, so it doesn’t trigger a checksum update. The destination address usually remains the same during standard routing (it would only change in special cases like NAT or tunneling). The protocol field isn’t changed in normal forwarding either; it would only change with certain encapsulation or header rewrites. Therefore, the decrement of TTL is the event that necessitates recalculating the header checksum.