Introduction Connectivity is the basic stuff from which the Internet is made. Therefore@ metrics determining whether pairs of hosts (IP addresses) can reach each other must form the base of a measurement suite. We define several such metrics@ some of which serve mainly as building blocks for the others. This memo defines a series of metrics for connectivity between a pair of Internet hosts. It builds on notions introduced and discussed in RFC 2330@ the IPPM framework document. The reader is assumed to be familiar with that document.