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Chapter 21 IPSec
VSG1435-B101 Series User’s Guide
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21.4.4 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.
21.4.5 IPSec and NAT
Read this section if you are running IPSec on a host computer behind the ZyXEL
Device.
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.