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TCP/IP Guide by Charles M. Kozierok

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IPsec Encapsulating Security Payload (ESP)

The IPsec AH provides integrity authentication services to IPsec-capable devices so that they can verify that messages are received intact from other devices. For many applications, however, this is only one piece of the puzzle. We want to not only protect against intermediate devices changing the datagrams, but also to protect against them examining their contents as well. For this level of private communication, AH is not enough; we need to use the ESP protocol.

The main job of ESP is to provide the privacy we seek for IP datagrams by encrypting them. An encryption algorithm combines the data in the datagram with a key to transform it into an encrypted form. This is then repackaged using a special format ...

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