I would like to start one discussion regarding Distributed Application- Business Services and SLA which are running on Linux servers. One server is App server and other is DB server both runs on Linux. Let's say that Network port or URL down is critical for application working because APP server can't access DB.

It depends upon the criticality and SLA defined with the customer. What is the criticality of SUP Upstream server goes down (if it's not client facing) ? You can configure child primary server's SUP to get synced with Microsoft. See, similar scenario is discussed in the below thread