Implement the IPv4 Header Checksum Algorithm

One's Complement Checksum

The IPv4 header checksum is a 16-bit field for error detection on the header only. Every router recomputes it because the TTL byte changes at each hop. For example, a packet from 192.168.1.1 to 192.168.0.199 with TTL 64 has header words like 0x4500, 0x003C, 0xC0A8, 0x0001.

Algorithm

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Verification sums the header including the stored checksum. A valid header yields 0xFFFF.

Carry folding example: 0xFFFF + 0x0001 = 0x10000, fold to 0x0001.

Important Rules

• Covers only the IP header, never the payload
• Pad with a zero byte if the header has an odd byte count
• Recompute after any header field change (TTL decrement, NAT rewrite)

Why This Exercise

This task requires you to implement ipv4_checksum() and verify it on a simulated header. The same one's-complement algorithm appears in TCP and UDP checksums, and NIC offload hardware recomputes it in silicon on every outgoing packet. Linux's csum_partial() and BSD's in_cksum() exist because naive 16-bit summation silently drops carry overflow without the fold step.