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Handbook

Security Association

6.0.0
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Security Association

The establishment of a Security Association (SA) is the successful outcome of Phase 1 negotiations. Each peer maintains a database of information about VPN connections. The information in each SA can include cryptographic algorithms and keys, keylife, and the current packet sequence number. This information is kept synchronized as the VPN operates. Each SA has a Security Parameter Index (SPI) that is provided to the remote peer at the time the SA is established. Subsequent IPsec packets from the peer always reference the relevant SPI. It is possible for peers to have multiple VPNs active simultaneously, and correspondingly multiple SPIs.

The IPsec SA connect message generated is used to install dynamic selectors. These selectors can be installed via the auto-negotiate mechanism. When phase 2 has auto-negotiate enabled, and phase 1 has mesh selector-type set to subnet, a new dynamic selector will be installed for each combination of source and destination subnets. Each dynamic selector will inherit the auto-negotiate option from the template selector and begin SA negotiation. Phase 2 selector sources from dial-up clients will all establish SAs without traffic being initiated from the client subnets to the hub.

Remote IP address change detection

SAs are stored in a hash table when keyed off the IPsec SA SPI value. This enables the FortiGate, for each inbound ESP packet received, to immediately look up the SA and compare the stored IP address against the one in the incoming packet. If the incoming and stored IP addresses differ, an IP address change can be made in the kernel SA, and an update event can be triggered for IKE.

Security Association

The establishment of a Security Association (SA) is the successful outcome of Phase 1 negotiations. Each peer maintains a database of information about VPN connections. The information in each SA can include cryptographic algorithms and keys, keylife, and the current packet sequence number. This information is kept synchronized as the VPN operates. Each SA has a Security Parameter Index (SPI) that is provided to the remote peer at the time the SA is established. Subsequent IPsec packets from the peer always reference the relevant SPI. It is possible for peers to have multiple VPNs active simultaneously, and correspondingly multiple SPIs.

The IPsec SA connect message generated is used to install dynamic selectors. These selectors can be installed via the auto-negotiate mechanism. When phase 2 has auto-negotiate enabled, and phase 1 has mesh selector-type set to subnet, a new dynamic selector will be installed for each combination of source and destination subnets. Each dynamic selector will inherit the auto-negotiate option from the template selector and begin SA negotiation. Phase 2 selector sources from dial-up clients will all establish SAs without traffic being initiated from the client subnets to the hub.

Remote IP address change detection

SAs are stored in a hash table when keyed off the IPsec SA SPI value. This enables the FortiGate, for each inbound ESP packet received, to immediately look up the SA and compare the stored IP address against the one in the incoming packet. If the incoming and stored IP addresses differ, an IP address change can be made in the kernel SA, and an update event can be triggered for IKE.