Methods, systems, and apparatus, including computer program products, for exchanging information in accordance with a protocol. In one aspect, signals reporting user actions related to resources and ranking data ranking the resources based on the signals are exchanged in accordance with a protocol. The protocol defines types of user actions that can be reported by the signals, a format for specifying values for one or more of a set of predefined parameters for qualifying the user actions, a format for specifying one or more subscriber-specific parameters relating to the resources or the user actions, and a format for identifying resources to which the user actions are related using a namespace defined by the subscriber.