There are a number of content distribution networks (CDNs) deployed and/or planned. Consistent with the general history of internetworking, people are starting to think about ways to interconnect these different CDNs, and talking about "peering at the content level", "CDN peering," or "contentpeering." Regardless of name, the goal is to allow different CDNs -- implemented using differing technology or under different administrative domains -- to cooperate with each other, forming a larger "virtual CDN." Outline of proposed solution: Recent discussions have proposed treating a CDN as containing a direction system, a distribution system, and an accounting system. The proposed approach to CDN interoperation is through supporting the transfer of information between the corresponding systems of two peered networks. We propose the development of three protocols or protocol families, one for each of the kinds of systems that can be peered.