The LSS and IS use HTTP or HTTPS traffic to communicate. The HTTP payload of the HTTP request contains all the traffic needed for the IS and the LSS to exchange information, instruct the LSS to add scans, send the results back to the IS for report generation, etc.
In either case, port 80 (HTTP) or port 443 (HTTPS) is used. If required, the LSS can use a Proxy (authenticated, or anonymous) server. You can switch the role of who initiates the connection so that either the LSS or the IS initiates the connection. On large deployments, it is recommended to let the LSS initiate the connection.