Negotiation Mode; Ipsec And Nat - ZyXEL Communications SBG3500-N000 User Manual

• 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 SBG3500-N 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.

20.7.4 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.

20.7.5 IPSec and NAT

Read this section if you are running IPSec on a host computer behind the SBG3500-N.
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 value, and complain that the hash value appended to
the received packet doesn't match. The VPN device at the receiving end doesn't know about the
NAT in the middle, so it assumes that the data has been maliciously altered.
IPSec using ESP in Tunnel mode encapsulates the entire original packet (including headers) in a
new IP packet. The new IP packet's source address is the outbound address of the sending VPN
gateway, and its destination address is the inbound address of the VPN device at the receiving end.
When using ESP protocol with authentication, the packet contents (in this case, the entire original
packet) are encrypted. The encrypted contents, but not the new headers, are signed with a hash
value appended to the packet.
Tunnel mode ESP with authentication is compatible with NAT because integrity checks are
performed over the combination of the "original header plus original payload," which is unchanged
by a NAT device.
SBG3500-N000 User's Guide
Chapter 20 IPSec VPN
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