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Chapter 25 IPSec VPN • The local and peer ID type and content come from the certificates. Note: You must set up the certificates for the ZyWALL and remote IPSec router first. IPSec SA Overview Once the ZyWALL and remote IPSec router have established the IKE SA, they can securely negotiate an IPSec SA through which to send data between computers on the networks. Note: The IPSec SA stays connected even if the underlying IKE SA is not available anymore. This section introduces the key components of an IPSec SA. Local Network and Remote Network In an IPSec SA, the local network, the one(s) connected to the ZyWALL, may be called the local policy. Similarly, the remote network, the one(s) connected to the remote IPSec router, may be called the remote policy. Active Protocol The active protocol controls the format of each packet. It also specifies how much of each packet is protected by the encryption and authentication algorithms. IPSec VPN includes two active protocols, AH (Authentication Header, RFC 2402) and ESP (Encapsulating Security Payload, RFC 2406). Note: The ZyWALL and remote IPSec router must use the same active protocol. Usually, you should select ESP. AH does not support encryption, and ESP is more suitable with NAT. Encapsulation There are two ways to encapsulate packets. Usually, you should use tunnel mode because it is more secure. Transport mode is only used when the IPSec SA is used for communication between the ZyWALL and remote IPSec router (for example, for remote management), not between computers on the local and remote networks. Note: The ZyWALL and remote IPSec router must use the same encapsulation. 476 ZyWALL USG 2000 User's Guide