1. A method for content insertion, the method comprising: receiving, at a content server, a content request from a requesting computing device; modifying, at the content server, the content request by adding data to the content request, the data comprising one or more headers, attributes, or both, based on the content request; transmitting, by the content server, the modified content request to a second server based on one or more routing policies; and receiving, at the content server, a reply message responsive to the modified content request from the second server, the reply comprising a content insertion instruction comprising data indicative of content to transmit to the requesting computing device.