FortiOS Handbook V2, Chapter 19: Load Balancing

FortiOS firewall server load balancing includes the features you would expect of a server load balancing solution. Traffic can be balanced across multiple backend servers based on multiple methods including static (failover), round robin, weighted to account for different sized servers, or based on the health and performance of the server including round trip time, number of connections. The load balancer supports HTTP, HTTPS, SSL or generic TCP/UDP or IP protocols. Session persistence is supported based on the SSL session ID or based on an injected HTTP cookie.