Limiting the number of SIP dialogs accepted by a security policy
In addition to limiting the rates for receiving SIP messages, you can use the following command to limit the number of SIP dialogs (or SIP calls) that the FortiGate accepts.
config voip profile
set max-dialogs 2000
This command sets the maximum number of SIP dialogs that can be open for SIP sessions accepted by any security policy that you add the VoIP profile to. The default setting of 0 does not limit the number of dialogs. You can add a limit to control the number of open dialogs and raise and lower it as required. You might want to limit the number of open dialogs for protection against SIP-based attackers opening large numbers of SIP dialogs. Every dialog takes memory and FortiGate CPU resources to process. Limiting the number of dialogs may improve the overall performance of the FortiGate. Limiting the number of dialogs will not drop calls in progress but may prevent new calls from connecting.