1. A method of controlling the shape of a peer-to-peer content distribution cloud, the method comprising: under control of at least one tracker node in communication with a plurality of peer nodes, the method comprising the acts of, at the tracker node: (i) receiving a request from one of the peer nodes requesting information about one or more other peer nodes to connect to, the request comprising: details of peer nodes that the requesting peer node does not want to connect to; and opaque metadata that is semantically unknown to the tracker node and including items of information about a behaviour at the requesting peer node; (ii) accessing information about a plurality of available peer nodes associated with the tracker node, the accessed information comprising: details of the peer nodes that the requesting peer node does not want to connect to; and one or more of: quality of service information, upload speed information, download speed information, number of existing peer connections, and opaque metadata that is semantically unknown to the tracker node, and including items of information about a behaviour at at least one of the plurality of available peer nodes, (iii) selecting one or more of the available peer nodes on the basis of one or more criteria, the criteria including quality of service levels, relative upload ability, relative download ability, and/or relative number of existing peer connections; and on the basis of the accessed information; wherein the selecting comprises determining, at the tracker node, how many of the opaque metadata items at the requesting peer node match opaque metadata items provided by peer nodes during registration; and (iv) sending information about the selected available peer nodes to the requesting peer node.