Mechanism of selecting gateway for downlink data of NS server

[Z-J-X]

For loranwan communication，The node sends a signal，multiple gateways will receive this message and pass it to ns，However, the downlink signal to this node will only be transmitted through one gateway,What is the mechanism for ns to select gateway.We can get the signal strength information related to the gateway from mqtt. I understand that we can calculate the best gateway according to this information. However, I don't know what the calculation method of NS server is.
Our purpose is to know in advance which gateway the downlink data passes through, so as to reduce the pressure on the gateway