Currently, we execute ExtendImageTicket command every 4 minutes, i.e. session expiration (5 mins) - refresh allowance (1 min). This calculation is fragile, as we poll the image transfer every 5 minutes - see bug 1619019 for an instance in which engine sent extend after 5 minutes. So we need to increase ImageTransferHostTicketRefreshAllowanceInSeconds config value to ensure extend is called more frequently (e.g. every minute).