Intermediaries are software entities, deployed on hosts of the wireline and wireless network, that mediate the interaction between clients and servers of the World Wide Web. In this paper we present a survey of intermediaries, focusing on systems beyond simple caching proxies. We classify different intermediary systems into three categories, based on their functionality and focus: First, we investigate notification intermediaries, which are driven by end-user profiles and operate even in the absence of end-user connection. Then, we study intermediaries developed to support wireless connectivity, mobility, and ubiquity. Finally, we examine intermediary infrastructures designed to extend the support of the core network for the development and deployment of new services. Based on this survey, we propose a detailed taxonomy of intermediaries and identify key features of emerging intermediary infrastructures. Taking into account recent advances and trends in wireless and pervasive Internet technologies, we present a number of research challenges, which need to be addressed in order to integrate intermediary systems in next-generation Internet infrastructures. (C) 2004 Elsevier B.V. All rights reserved.
