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Lectures notes for 2010 - KTH

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Header Checksum<br />

• Ensures integrity of header fields<br />

• Hop-by-hop (not end-to-end)<br />

• Header fields must be correct <strong>for</strong> proper and safe processing of IP!<br />

• Payload is not covered<br />

• Other checksums<br />

• Hop-by-hop: using link-layer CRC<br />

– IP assumes a strong link layer checksum/CRC - as the IP checksum is weak<br />

• End-to-end: Transport layer checksums, e.g., TCP & UDP checksums, cover payload<br />

• Internet Checksum Algorithm, RFC 1071<br />

• Treat headers as sequence of 16-bit integers<br />

• Add them together<br />

• Take the one’s complement of the result<br />

Note that recent work concerning IP over wireless links assume that the payload<br />

can have errors and will still be received (see work concerning selective coverage<br />

of UDP checksum).<br />

Maguire Header Checksum 3: 19 of 33<br />

maguire@kth.se <strong>2010</strong>.03.21 Internetworking/Internetteknik

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