Session level load balancing

This particular application consists of a front end tier of VoIP servers. Each server opens up a connection to the VS on the LTM and the LTM makes an LB decision to 1 of 4 application servers. OneConnect is enabled on the VS on the LTM. The LB method of the pool is set to Least Connections because the requirement is even load balancing across the web servers (IIS) even though 1 of them is 1.5-2.5 times faster than the others. This makes Observed, not a viable alternative

 

 

The traffic going through these connections is HTTP1.1. When the first session starts, an lb decision is made and a tcp connection is opened up to the backend server. The next session reuses the same clientside tcp connection and the lb decision is made. Since client 1 is on server1, client 2 goes to server2. Client 3 comes in and lb's to server1, client 4 comes in and lb's to server 2. what is happening with this app is least connections is lbing based on tcp connections as it should when we need it to lb based on sessions. Since there is no way to split the client side into a 1 session (phone call) per tcp connection and since Oneconnect is reusing tcp connections, we're seeing unequal lbing.

 

 

There is a specific get request start_page.asp that marks the start of each new session and a stop page stop_page.asp that marks the end of it. How can we configure the LTM in this situation to lb based on sessions rather than tcp connections ?