Chapter 9 High Availability for FortiOS 5.0 : HA and failover protection : Session failover (session pick-up) : SSL offloading and HTTP multiplexing session failover
  
SSL offloading and HTTP multiplexing session failover
SSL offloading and HTTP multiplexing are both enabled from firewall virtual IPs and firewall load balancing. Similar to the features applied by UTM, SSL offloading and HTTP multiplexing requires the FortiGate unit to maintain very large amounts of internal state information for each session. Sessions accepted by security policies containing virtual IPs or virtual servers with SSL offloading or HTTP multiplexing enabled do not resume after a failover.