When a host in headquarters sends an IP datagram to a salesperson
in a hotel, the gateway router in headquarters converts the vanilla IPv4 datagram
into an IPsec datagram and then forwards this IPsec datagram into the Internet.
This IPsec datagram actually has a traditional IPv4 header, so that the routers in the
public Internet process the datagram as if it were an ordinary IPv4 datagram—to
them, the datagram is a perfectly ordinary datagram