(Focusing on "web apps" here because they have a strong identity to which meter data and policies can be attached. Other implementations are possible for the more general case, but that's not the top priority atm.) The general use cases I'd like to address here are 0. "How much of my data plan have I used?" --- overall guesstimate at number of bytes Gecko has sent/received while on various interfaces. 1. "Which of my apps are using all my network data?" --- usage data charged per app. Won't add up to 100% of total from (0) in general, but we could charge the "slop" to "System" or something. 2. "Don't let app X use data when I'm on 3g." --- enforce simple usage policies. We can expose (0) on some simple new API. (1) and (2) probably make sense tacked on to some of the existing mozApp interfaces. Will file work items as followup bugs.