CRIPTOGRAFIA - FESP

end-to-end conversation between two hosts. This protection is either
authentication or encryption (or both), but it is not a tunneling protocol. It has
nothing to do with a traditional VPN: it's simply a secured IP connection.
In AH Transport Mode, the IP packet is modified only slightly to include the new
AH header between the IP header and the protocol payload (TCP, UDP, etc.), and
there is a shuffling of the protocol code that links the various headers together.
This protocol shuffling is required to allow the original IP packet to be
reconstituted at the other end: after the IPsec headers have been validated upon
receipt, they're stripped off, and the original protocol type (TCP, UDP, etc.) is
stored back in the IP header. We'll see this chain of next header fields again and
again as we examine IPsec.
When the packet arrives at its destination and passes the authentication check,
the AH header is removed and the Proto=AH field in the IP header is replaced
with the saved "Next Protocol". This puts the IP datagram back to its original
state, and it can be delivered to the waiting process.