They are generally suitable only for low value transactions where accountability after the fact is sufficient to deter abuse. Agora allows its payment protocol to piggyback on existing HTTP GetRequest messages without additional communication overhead. Agora also adds non repudiation and an online arbiter that functions much like NetCents . Usage metrics may be, e.g. elapsed or usage time, or number of bytes or packets transmitted. The ISP architecture de fines a carefully designed protocol that supports these and other options.
Some proposals are proprietary and no proposal has achieved wide use.
Embedding specific online payment methods into ISP implementations would therefore involve considerable licensing and maintenance difficulties. World Wide Web Conference, W3C, Santa Clara, CA, 1997. New classes of agents will be designed specifically to serve the needs of other agents. Charging per click on the web . In Proceedings of the Sixth International World Wide Web Conference, April 1997.