Connectivity is the basic stuff from which the Internet is made.
We define several such metrics, some of which serve mainly as building blocks for the others. This metric is probably not directly useful, because it is instantaneous and unidirectional. Most applications also require connectivity over an interval. Finally, one might not have instantaneous connectivity due to a transient event such as a full queue at a router, even if at nearby instants in time one does have connectivity. These points are addressed below, with this metric serving as a building block.