Redback attributes

Hello, I have redback NASes for bringing up DSL connections. I would like to make the RedBack boxes bring connection down for a user once he has reached his total amount of traffic allowed (UP+DOWN). Does anyone using redback devices (or not!) knows how to achive this? I tried sending the Session-Traffic-Limit to the boxes, but it didn't seemed to change anything! Does anyone knows what this attribute is for? regards