Presumably accounting detail is most useful when it traces traffic to the user. Certainly if the purpose of accounting is to charge prices as incentives, those incentives will be most effective if they affect the person actually making the usage decisions. Another layer of expensive and complex authorization and accounting software will be required on the host computer in order to track which user accounts are responsible for which packets. Imagine, for instance, trying to account for student e-mail usage at a large public computer cluster. Another tool is NetTraMet , which has been used in New Zealand for several years to do network accounting.
Accounting is more practical and less costly the higher the level of aggregation.
Whether accounting at lower levels of aggregation is worthwhile is a different question that depends importantly on cost-saving innovations in internetwork accounting methods.