IPv4 header checksum

The IPv4 header checksum is a checksum used in version 4 of the Internet Protocol (IPv4) to detect corruption in the header of IPv4 packets. It is carried in the IP packet header, and represents the 16-bit result of summation of the header words.[1]

The IPv6 protocol does not use header checksums. Its designers considered that the whole-packet link layer checksumming provided in protocols, such as PPP and Ethernet, combined with the use of checksums in upper layer protocols such as TCP and UDP, are sufficient.[2] Thus, IPv6 routers are relieved of the task of recomputing the checksum whenever the packet changes, for instance by the lowering of the Hop limit counter on every hop.

Computation

The checksum calculation is defined in RFC 791:[3]

The checksum field is the 16-bit ones' complement of the ones' complement sum of all 16-bit words in the header. For purposes of computing the checksum, the value of the checksum field is zero.

If there is no corruption, the result of summing the entire IP header, including checksum, should be zero. At each hop, the checksum is verified. Packets with checksum mismatch are discarded. The router must adjust the checksum if it changes the IP header (such as when decrementing the TTL).[4]

Examples

Calculating the IPv4 header checksum

Take the following truncated excerpt of an IPv4 packet. The header is shown in bold and the checksum is underlined.
4500 0073 0000 4000 4011 b861 c0a8 0001
c0a8 00c7 0035 e97c 005f 279f 1e4b 8180

For ones' complement addition, each time a carry occurs, we must add a 1 to the sum.[5] A carry check and correction can be performed with each addition or as a post-process after all additions. If another carry is generated by the correction, another 1 is added to the sum.

To calculate the checksum, we can first calculate the sum of each 16 bit value within the header, skipping only the checksum field itself. Note that these values are in hexadecimal notation.
4500 + 0073 + 0000 + 4000 + 4011 + c0a8 + 0001 + c0a8 + 00c7 = 2479C
The first digit is the carry count and is added to the sum:
2 + 479C = 479E (if another carry is generated by this addition, another 1 must be added to the sum)
To obtain the checksum we take the ones' complement of this result: B861 (as shown underlined in the original IP packet header).

Verifying the IPv4 header checksum

When verifying a checksum, the same procedure is used as above, except that the original header checksum is not omitted.
4500 + 0073 + 0000 + 4000 + 4011 + b861 + c0a8 + 0001 + c0a8 + 00c7 = 2fffd
Add the carry bits:
fffd + 2 = ffff
Taking the ones' complement (flipping every bit) yields 0000, which indicates that no error is detected. IP header checksum does not check for the correct order of 16 bit values within the header.

See also

References
  1. IP Datagram General Format
  2. Iljitsch van Beijnum (September 2006). "IPv6 Internals". Cisco.
  3. "Internet Header Format". Internet Protocol DARPA Internet program protocol specification. IETF. September 1981. p. 14. doi:10.17487/RFC0791. STD 5. RFC 791. Retrieved 2009-10-27.
  4. Baker, Fred, ed. (June 1995). "Header Checksum". Requirements for IP Version 4 Routers. IETF. p. 44. sec. 4.2.2.5. doi:10.17487/RFC1812. RFC 1812. Retrieved 2009-10-27.
  5. "Compute 16-bit One's Complement Sum". Retrieved 2019-02-07.
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