Networking forces us to adopt more and more a protocol-centric view of pro- gramming that is, to stop developing monolithic applications but, rather, glue components exchanging messages together. One of the key problem then is to design how components interact. Interaction design is not limited to the concep- tion of an API (Application Programmer’s Interface) stating the functionalities and the nature of exchanged data but includes how to make these components behave appropriately along all their lifetime.