This article introduces a novel approach for service composition on active and programmable routers. The proposed composition framework enables flexible programmability of a router’s data path through dynamically loadable software components, called ‘active components’. The composition framework promotes transparent, dynamic and incremental deployment of network-side functionality which renders active network technologies capable of fully replacing the existing routing infrastructure by supporting all the legacy network protocols and services, and the integration of novel ones. On the other hand, the composition framework provides the necessary structures and mechanisms to deal with services composed by independent users potentially unaware of each other. The latter is particularly important as network routers are regarded as shared resources. In order to evaluate the proposed service composition framework, we have developed an implementation for the LARA++ active router architecture [S. Schmid, A component-based active router architecture, Ph.D. Thesis, Lancaster University, UK, November 2002] and deployed several of these active routers in a real-world service network. The evaluation demonstrates the flexibility of the composition framework by enabling the alteration of service composites at run-time and provides scalability in the generation of such composites by users or administrators. This work also reports on lessons learned while deploying several active services in a production network. Besides a qualitative evaluation of the service composition framework and the LARA++ architecture, we provide a number of quantitative results, which show that the overhead of our composition model does not adversely affect the performance of the router, despite the increased flexibility achieved.