Secondary Content Delivery

Described herein are systems, methods, and apparatuses for secondary content delivery via a network. A data stream carrying, for example, a primary content may also carry a network address of a schedule that indicates one or more times at which secondary content should be presented in coordination with the primary content. A user device may receive this data stream, and use it to obtain the secondary content and to present it with the primary content. The user device may also determine whether the schedule has changed by comparing a received network address with a previously-received network address.