Stale sessions and simultaneous use issue

Hi all! I had configured MySQL to store sessions of the users and I have a lots of records with wrong acctsessiontime value because NAS are sent Accounting On/Off packets and the database is update with the actual timestamp. I change deletestalesessions = no in sql.conf but the database is update too. What is the function of this parameter? There are some attribute to avoid stale sessions? I remember Idle-Timeout attribute but if the NAS isn't live the server never kill that session right? Thanks for all. Kings regards. Fernando.