System Administration Guide: IP Services
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IPsec Packet Flow

Figure 19-1 shows how an IP addressed packet, as part of an IP datagram, proceeds when IPsec has been invoked on an outbound packet. The flow diagram illustrates where authentication header (AH) and encapsulating security payload (ESP) entities can be applied to the packet. How to apply these entities, as well as how to choose the algorithms, are described in subsequent sections.

Figure 19-2 shows the IPsec inbound process.

Figure 19-1 IPsec Applied to Outbound Packet Process
Flow diagram shows that the outbound packet is first protected by ESP, and then by AH. The packet then goes to a tunnel or a physical interface.
Figure 19-2 IPsec Applied to Inbound Packet Process
Flow diagram shows that IPsec first processes the AH header, then the ESP header on inbound packets. A packet that is not protected enough is dropped.
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