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TCP/IP Guide by Charles M. Kozierok

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IPsec Authentication Header (AH)

As I mentioned earlier in this chapter, AH is one of the two core security protocols in IPsec. This is another protocol whose name has been well chosen. It provides authentication of either all or part of the contents of a datagram through the addition of a header that is calculated based on the values in the datagram. The parts of the datagram that are used for the calculation, and the placement of the header, depend on the mode (tunnel or transport) and the version of IP (IPv4 or IPv6).

The operation of AH is surprisingly simple, especially for any protocol that has anything to do with network security. The simplicity is analogous to the algorithms used to calculate checksums or perform cyclic redundancy (CRC) ...

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