fortigate-ipsec-40-mr3

Advanced phase 2 settings Phase 2 parameters
P2 Proposals
Replay detection
Figure 4: Advanced phase 2 settings
In phase 2, the VPN peer or client and the FortiGate unit exchange keys again to
establish a secure communication channel. The P2 Proposal parameters select the
encryption and authentication algorithms needed to generate keys for protecting the
implementation details of Security Associations (SAs). The keys are generated
automatically using a Diffie-Hellman algorithm.
IPsec tunnels can be vulnerable to replay attacks. Replay detection enables the FortiGate
unit to check all IPsec packets to see if they have been received before. If any encrypted
packets arrive out of order, the FortiGate unit discards them.
Perfect forward secrecy (PFS)(
Keylife
Auto-negotiate
By default, phase 2 keys are derived from the session key created in phase 1. Perfect
forward secrecy forces a new Diffie-Hellman exchange when the tunnel starts and
whenever the phase 2 keylife expires, causing a new key to be generated each time. This
exchange ensures that the keys created in phase 2 are unrelated to the phase 1 keys or
any other keys generated automatically in phase 2.
The Keylife setting sets a limit on the length of time that a phase 2 key can be used. The
default units are seconds. Alternatively, you can set a limit on the number of kilobytes
(KB) of processed data, or both. If you select both, the key expires when either the time
has passed or the number of KB have been processed. When the phase 2 key expires, a
new key is generated without interrupting service.
By default, the phase 2 security association (SA) is not negotiated until a peer attempts to
send data. The triggering packet and some subsequent packets are dropped until the SA
is established. Applications normally resend this data, so there is no loss, but there might
be a noticeable delay in response to the user.
Automatically establishing the SA can also be important for a dialup peer. This ensures
that the VPN tunnel is available for peers at the server end to initiate traffic to the dialup
peer. Otherwise, the VPN tunnel does not exist until the dialup peer initiates traffic.
IPsec VPNs for FortiOS 4.0 MR3
58 01-434-112804-20120111
http://docs.fortinet.com/