P-334U/P-335U User’s Guide

If you do not enable PFS, the ZyXEL Device and remote IPSec router use the same root key that was generated when the IKE SA was established to generate encryption keys.

The DH key exchange is time-consuming and may be unnecessary for data that does not require such security.

13.1.4 Additional IPSec VPN Topics

This section discusses other IPSec VPN topics that apply to either IKE SAs or IPSec SAs or both. Relationships between the topics are also highlighted.

13.1.4.1 SA Life Time

SAs have a lifetime that specifies how long the SA lasts until it times out. When an SA times out, the ZyXEL Device automatically renegotiates the SA in the following situations:

•There is traffic when the SA life time expires

•The IPSec SA is configured on the ZyXEL Device as nailed up (see below)

Otherwise, the ZyXEL Device must re-negotiate the SA the next time someone wants to send traffic.

Note: If the IKE SA times out while an IPSec SA is connected, the IPSec SA stays connected.

An IPSec SA can be set to keep alive Normally, the ZyXEL Device drops the IPSec SA when the life time expires or after two minutes of outbound traffic with no inbound traffic. If you set the IPSec SA to keep alive , the ZyXEL Device automatically renegotiates the IPSec SA when the SA life time expires, and it does not drop the IPSec SA if there is no inbound traffic.

Note: The SA life time and keep alive settings only apply if the rule identifies the remote IPSec router by a static IP address or a domain name. If the Secure Gateway Address field is set to 0.0.0.0, the ZyXEL Device cannot initiate the tunnel (and cannot renegotiate the SA).

13.1.4.2 Encryption and Authentication Algorithms

In most ZyXEL Devices, you can select one of the following encryption algorithms for each proposal. The encryption algorithms are listed here in order from weakest to strongest.

•Data Encryption Standard (DES) is a widely used (but breakable) method of data encryption. It applies a 56-bit key to each 64-bit block of data.

•Triple DES (3DES) is a variant of DES. It iterates three times with three separate keys, effectively tripling the strength of DES.

You can select one of the following authentication algorithms for each proposal. The algorithms are listed here in order from weakest to strongest.

•MD5 (Message Digest 5) produces a 128-bit digest to authenticate packet data.

•SHA1 (Secure Hash Algorithm) produces a 160-bit digest to authenticate packet data.

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Chapter 13 IPSec VPN