Methods, systems, and apparatus, including computer programs encoded on computer storage media, for providing to user devices identifiers of edge servers from which to obtain content. One example system includes a traffic control service. The traffic control service is configured to receive polling requests from user devices. In response to a particular polling request, the traffic control service obtains aggregated data about the user devices and edge servers. The traffic control service selects, based on the aggregated data, a best edge server for the user device that sent the particular polling request. The traffic control service sends an identifier of the selected best edge server to the user device.