The user enters a hostname at the Horizon Client and this starts the primary Horizon protocol. This is a control protocol for authentication, authorization and session management. It uses XML-structured messages over HTTPS (HTTP over SSL). This protocol is sometimes known as the Horizon XML-API control protocol. In a load-balanced environment as shown in the Architecture section, the load balancer routes this connection to one of the Unified Access Gateway appliances. The load balancer usually selects the appliance based first on availability, and then out of the available appliances that will route traffic based on the least number of current sessions. This has the effect of evenly distributing the traffic from different clients across the available set of Unified Access Gateway appliances.