Formal metrics have a number of appealing properties.
Foremost of these is that they include the possibility of developing an analytic framework for understanding different aspects of Internet behavior. Formal metrics are also easier to define than empirical metrics. A key problem with formal metrics, however, relates to exactly this easier form of definition. At first blush it might seem straight forward to define a formal metric, but often the underlying notions prove slippery. Then the transmission time for B bits depends on the bits being sent.