This "out vs in" comparison in order to determine who pays is a meme that won't die. I've never known agreements to be formed in such a way. Maybe before my day in the dial up era when "out vs in" could serve as rule of thumb of sorts to determin who was an access ISP and who was an "Upstream" or "Tier 1" ISP. Can someone provide a source for this?
Historically, ratios were used for peering between ISPs of the same type. When one backbone peers with another backbone, 1:1 ratio can be a simple proxy for equal value. (ISPs liked simplicity in the old days because it made the Internet cheaper than the complex telco X.25/ATM/SONET networks with their complex tariffs and settlement.) When a content provider peers with a broadband ISP, they can exchange equal value without exchanging equal traffic.
