Agents are a useful abstraction frequently employed as a basic building block in modeling service, information and resource sharing in global environments. The connecting of requester with provider agents requires the use of specialized agents known as middle-agents. In this paper, we propose a formal framework intended to precisely characterize types of middle-agents with a special focus on matchmakers, brokers and front-agents by formally modeling their interactions with requesters and providers. Our approach is based on capturing interaction protocols between requesters, providers and middle-agents as finite state processes represented using FSP process algebra. The resulting specifications are formally verifiable using FLTL temporal logic. The main results of this work include (i) precise specification of interaction protocols depending on the type of middle-agent (this can also be a basis for characterizing types of middle-agents), (ii) improvement of communication between designers and developers and facilitation of formal verification of agent systems, (iii) guided design and implementation of agent-based software systems that incorporate middle-agents.