On Android, how are Wi-Fi and 3G Internet traffic stored in memory, and when are they updated or cleared by the system?

I need to know about Wi-Fi and 3G internet traffic. I have found that traffic data are stored on the way /statistics/rx_bytes, /statistics/tx_bytes, /sys/class/net/.
Question: If I get data from these paths, what does this data mean? When did this data begin to be collected: from Wi-Fi activation or from turning on the phone?
And when does the system clear or update Wi-Fi traffic data? (My application should save daily traffic)