ZyXEL P-2602HW Benutzerhandbuch

 Chapter 18 VPN Screens

P-2602H(W)(L)-DxA User’s Guide

• Set the IPSec SA lifetime. This field allows you to determine how long the IPSec SA 

should stay up before it times out. The ZyXEL Device automatically renegotiates the 
IPSec SA if there is traffic when the IPSec SA lifetime period expires. The ZyXEL Device 
also automatically renegotiates the IPSec SA if both IPSec routers have keep alive 
enabled, even if there is no traffic. If an IPSec SA times out, then the IPSec router must 
renegotiate the SA the next time someone attempts to send traffic.

The phase 1 Negotiation Mode you select determines how the Security Association (SA) will 
be established for each connection through IKE negotiations. 

• Main Mode ensures the highest level of security when the communicating parties are 

negotiating authentication (phase 1). It uses 6 messages in three round trips: SA 
negotiation, Diffie-Hellman exchange and an exchange of nonces (a nonce is a random 
number). This mode features identity protection (your identity is not revealed in the 
negotiation). 

• Aggressive Mode is quicker than Main Mode because it eliminates several steps when 

the communicating parties are negotiating authentication (phase 1). However the trade-off 
is that faster speed limits its negotiating power and it also does not provide identity 
protection. It is useful in remote access situations where the address of the initiator is not 
know by the responder and both parties want to use pre-shared key authentication.

Diffie-Hellman (DH) is a public-key cryptography protocol that allows two parties to establish 
a shared secret over an unsecured communications channel. Diffie-Hellman is used within 
IKE SA setup to establish session keys. 768-bit (Group 1 - DH1) and 1024-bit (Group 2 – 
DH2) Diffie-Hellman groups are supported. Upon completion of the Diffie-Hellman 
exchange, the two peers have a shared secret, but the IKE SA is not authenticated. For 
authentication, use pre-shared keys.

Enabling PFS means that the key is transient. The key is thrown away and replaced by a brand 
new key using a new Diffie-Hellman exchange for each new IPSec SA setup. With PFS 
enabled, if one key is compromised, previous and subsequent keys are not compromised, 
because subsequent keys are not derived from previous keys. The (time-consuming) Diffie-
Hellman exchange is the trade-off for this extra security.
This may be unnecessary for data that does not require such security, so PFS is disabled 
(None) by default in the ZyXEL Device. Disabling PFS means new authentication and 
encryption keys are derived from the same root secret (which may have security implications 
in the long run) but allows faster SA setup (by bypassing the Diffie-Hellman key exchange).

Click Advanced Setup in the 

 screen to open this screen.