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Implement the IPv4 Header Checksum Algorithm

hard7/9

Parse an IPv4 Packet Manually Parse a TCP Segment Manually

Language

Compiler

2.0s Limit 256MB

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Implementation

Write a C program that implements the IPv4 header checksum algorithm. Compute the checksum on a simulated header, then verify it.

Requirements:

Implement a function ipv4_checksum() that takes header bytes and length
Use one's complement sum of all 16-bit words
Fold carry bits back into the result
Return the computed checksum as a 16-bit unsigned value
Print the computed checksum in hexadecimal (0xXXXX format)
Verify the checksum by computing again with checksum field included
Print whether verification succeeded
Include the keywords: checksum, 0x, verify

Why This Matters

This exact one's-complement algorithm is defined in RFC 1071 and reused for TCP and UDP checksums, plus it's what NIC checksum-offload hardware recomputes in silicon on every outgoing packet. Getting the carry-fold step wrong is a classic bug: Linux's csum_partial() and BSD's in_cksum() both exist because naive 16-bit summation silently drops overflow.

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Validation Suite

main.c

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