https://docs.mongodb.com/v3.2/tutorial/deploy-geographically-distributed-replica-set/#connectivity

before deploy geographically distributed replica set, current I need to configure network, sometimes it's too difficult or don't have permission or cannot connect to public network directly. if replset member and client can connect to other repliset member through a proxy (for example a socket proxy or from environment), and the proxy ensure the connection is securely and efficiently, it will be more easier and convenient.