This paper presents a framework for the provision of dynamic Quality of Service (QoS) support in Virtual Private Networks (VPNs) running over an MPLS-enabled public network infrastructure. A Dynamic Bandwidth Allocation scheme that includes traffic estimators and the development of a resource reservation algorithm, capable of modifying the resource allocation in real-time, is used. Three traffic estimation algorithms are implemented and tested. This system can automatically adjust to the bandwidth size of a VPN tunnel. The technique is beneficial to Internet Service Providers (ISPs) and corporate users alike. The superior resource management achieved through the examined approach can produce lower costs for the users and higher profits to the ISP. Implementation and experimental evaluation of the technique, using our MPLS and Diffserv enabled Linux test-bed, confirmed its ability to provide better resource utilization.