Calculate average time latency between client and server

1. Server send time tick S(T1) to client.
2. Client ack receiving time C(T1) and latency from server to client L(S2C) back to server.
3. Server get L(S2C) and calculate L(C2S)
4. The average latency is ( L(S2C) + L(C2S) )/2

Note.
L(S2C) = "latency form server to client" + "Time offset between server and client".
L(C2S) = "latency form client to server" - "Time offset between server and client".
So we can ignore "Time offset between server and client".