Another approach to accommodate different protocols and protocol stacks within a single network node is the IEEE P1520 proposal for active networks. This standardization effort aims to define a set of APIs for networks. The APIs are defined to interpret protocol tags that are attached to the single messages passed throughout heterogeneous network environments.The introduction of open control and signaling interfaces for the various functional levels of network nodes has its rationale in the functional distribution between these layers, whereby a set of standardized APIs facilitates the access to the various functional levels and thereby enables the interpretation of protocol information contained within arriving packets. As the Microsoft foundation classes (MFC) and the Java developers kit (JDK) have shown for the area of high level application development, the open provision of standardized APIs can enable software developers to write control and signaling software so that even nonstandardized networking protocols can be used within such (active) networking environments.The protocol stack reference model for active network nodes, defines four different levels for application programming interfaces that implement the various protocol stack features (services and functionalities) necessary to provide internetworking functionality.

The interfaces between these functional levels denote the separation between end user applications, value added services, the network service provide, and the underlying network resources. The P1520 reference model identifies four interfaces:

• V-interface: a user level interface providing APIs to write personalized end user software ; it also provides access to the value added services level.
• U-interface: this interfaces allows users to access the generic network services such as requesting or terminating connections or bandwidth. It furthermore allows the configuration of connections, depending on user demands and physical availability of resources.
• L-interface: provides a set of APIs that enables the direct access to, and manipulation of, the states of the local network nodes and resources. It also allows the implementation of any communication service.
• CCM-interface (Connection Control and Management): consists of various protocols that enable the exchange of state and control information between the physical hardware elements.

Altogether, the IEEE P1520 framework offers the potential to customize communication channels and traffic types across networks; the technology delivers a viable solution for bridging between different legacy access schemes.