ZyXEL P-2812HNU-51c User Manual download pdf (Page 270)

Chapter 12 IPSec VPN

P-2812HNU-51c User’s Guide

270

• Choose a Diffie-Hellman public-key cryptography key group (DH1 or DH2).

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

should stay up before it times out. An IKE SA times out when the IKE SA lifetime

period expires. If an IKE SA times out when an IPSec SA is already established,

the IPSec SA stays connected.

In phase 2 you must:

• Choose an encryption algorithm.

• Choose an authentication algorithm

• Choose a Diffie-Hellman public-key cryptography key group.

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

SA should stay up before it times out. The P-2812HNU-51c automatically

renegotiates the IPSec SA if there is traffic when the IPSec SA lifetime period

expires. If an IPSec SA times out, then the IPSec router must renegotiate the

SA the next time someone attempts to send traffic.

12.5.4 Negotiation Mode

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.

12.5.5 IPSec and NAT

Read this section if you are running IPSec on a host computer behind the P-

2812HNU-51c.

NAT is incompatible with the AH protocol in both Transport and Tunnel mode.

An IPSec VPN using the AH protocol digitally signs the outbound packet, both data

payload and headers, with a hash value appended to the packet. When using AH

protocol, packet contents (the data payload) are not encrypted.

A NAT device in between the IPSec endpoints will rewrite either the source or

destination address with one of its own choosing. The VPN device at the receiving

end will verify the integrity of the incoming packet by computing its own hash

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ZyXEL P-2812HNU-51c User Manual Page 270