1. In a content delivery network (CDN) in which participating content providers offload given content for delivery from content servers managed by a content delivery network service provider, a method comprising: tracking a client machine user agent across a plurality of content provider domains being managed by the content delivery network service provider; and with permission of a first participating content provider, providing a second participating content provider with information obtained during the tracking step, wherein the information tracks the client machine user agent across one or more content domains associated with the first participating content provider.