The cost for the use of a particular Underlay Connectivity Service can be flat rate (i.e., based on units of time such as $500/month) or usage-based (i.e., based on how much data is sent across it such as $10/TB). The BILLING-METHOD Policy Criterion provides control over the charge type of the network that can be used to forward an Application Flow. It can have the value Flat-Rate-Only or Either.

[R30] If Policy Criterion BILLING-METHOD=Flat-Rate-Only is applied to an Application Flow, then the Application Flow MUST be forwarded over an Underlay Connectivity Service with flat-rate (i.e., time-based) charging.

[R31] If Policy Criterion BILLING-METHOD=Either is applied to an Application Flow, then this Policy Criterion MUST NOT be considered in the forwarding decision for the Application Flow.

This Policy Criterion does not include a Usage-Only option. Since flat-rate services are paid for whether they are used or not, it seems unlikely that a Subscriber would want to avoid using a flat-rate service if one were available and met the constraints imposed by the other Policy Criteria.

Also, this Policy Criterion provides a simplified model for how Underlay Connectivity Service charge models can be used. There are other possible models such as a threshold-based model where one type would be used up to a particular level of bandwidth and the other would be used beyond that. Alternative models and situations are beyond the scope of this document. Refer to MEF 74 for examples of a broader range of billing options.