In general, techniques are described for defining an interface to a network router software infrastructure that allows developers to dynamically extend a routing protocol executed by the network router to distribute data throughout the routing domain for use with custom applications. In some examples, a routing protocol process executing on a control plane of a network device may expose an interface, such as an Application Programming Interface (API), that defines methods and parameters for extending the operation of a routing protocol executed by the routing protocol process. Third-party developers may then build custom applications that invoke the interface of the routing protocol process to define new routing protocol messages or message payloads, inject application-specific data into the routing protocol process for distribution within the newly defined messages/payloads, and extract application-specific data that is carried by newly defined messages/payloads that are originated by another network device in the routing domain.