ESP
| We use ESP when we want confidentiality in our sessions. Confidentiality is established via encryption algorithms. Optionally, ESP can perform integrity checks on our peer and the data it is sending. IPSec implements ESP by creating a shim header between Layer 2 and Layer 3. The Layer 2.5 header is identified as in use by Layer 2, identifying the next layer protocol as protocol number 50. The ESP header has a next protocol field, which identifies the next Layer 4 transport protocol in use, usually TCP or UDP. ESP also has a trailer, which identifies IPSec information as well as ESP integrity-check information.
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