ZyXEL Communications ZyWall USG20-VPN User Manual page 364

• Instead of using the pre-shared key, the USG and remote IPSec router check the signatures on
each other's certificates. Unlike pre-shared keys, the signatures do not have to match.
• The local and peer ID type and content come from the certificates.
Note: You must set up the certificates for the USG and remote IPSec router first.
IPSec SA Overview
Once the USG 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 USG, 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 USG 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
USG and remote IPSec router (for example, for remote management), not between computers on
the local and remote networks.
Note: The USG and remote IPSec router must use the same encapsulation.
These modes are illustrated below.
Figure 237 VPN: Transport and Tunnel Mode Encapsulation
Original Packet
Transport Mode Packet
Chapter 21 IPSec VPN
IP Header TCP
Header
IP Header AH/ESP
Header
USG20(W)-VPN Series User's Guide
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Data
TCP Data
Header