Diameter Credit-Control Application - Message Flows

Message Flows

The message flows are in general driven by the control-point asking for units and the server granting them. The message may also be generated by other diameter applications, such as NASREQ (RFC4005) for sessions that are time/usage-limited.

The following diagram shows a simplified message flow for a session using quota grants.

The client starts by requesting 10 units from the server. The server verifies that the user/subscriber has enough balance for it. In this example the server grants the client all the units it requested. if the subscriber had had insufficient balance it could have granted less units or rejected it completely.

When or before the subscriber session has used the granted units the client sends an update to the server telling it how many units have been used and how many it would like granted this time. The client is allowed to request units before the previous grant is completely used, in order to avoid suspending the subscriber session while talking to the server. In this example the client sends the request when 7 units of the 10 previously granted units have been used; and ask for 10 more units, which the server grants. The server can use the used-units count for debiting the subscriber balance (granting units does not indicate that they will be used. The Used-Units AVP contains the actual usage). It is also possible for the server to tell the client how long the grant is valid, in which case the client is expected to send an update when the grant timer expires.

There can be many update messages during a session.

Finally, the subscriber has ended the session, and the client sends a termination message to the server containing the last Used-Units. The server can use the termination message to clear any related reservations made in the back-end balance management system. If the subscriber did not terminate the session himself but instead depleted his balance then the server would have responded earlier with reject to an update message, possibly telling the client/control-point to redirect traffic (this normally only makes sense for HTTP/WAP traffic).