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What is a virtual private network (VPN)? This paper analyzes the various definitions that exist and explains the different viewpoints that have led to their emergence. Furthermore, it describes an evolution from today's Internet‐based VPNs to emerging solutions by which quality of service (QoS) for a variety of service classes can be guaranteed. Specifically, this paper discusses two methods of implementing QoS‐capable VPNs—one defined over a pure asynchronous transfer mode (ATM) infrastructure and another based on multiprotocol label switching (MPLS), a new protocol that offers a connection‐oriented approach to Internet protocol (IP) transport over a variety of infrastructures. In the near term, it might make sense for a service provider to offer VPN services over an existing ATM infrastructure. However, in the long term, as this paper proposes, evolution toward the MPLS‐based approach is the course to follow.

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