From constant traffic matrices to hose workload model for VPN tree design

Virtual Private Networks (VPNs) are overlay networks established on top of a public network backbone with the goal of providing a low cost but secure network solution. The allocation of bandwidth for VPN tunnels to meet the requirements specified by customers is an important traffic engineering research issue. This paper addresses the general problem of computing a constrained VPN with a tree topology, and having optimum bandwidth allocation. This is a hard combinatorial optimization problem [1]. This issue was studied first by different resource allocation methods [2] and next was studied by general optimization methods [3]. In this paper, we present integer programming formulations (IPFs) assuming a hose as well as a pipe workload models. Earlier work [2] showed that the hose model results in VPN trees over provisioned by a factor of 2-to-3. Extensive simulations using test networks and their corresponding hose workloads, generated with Brite [4] show that our exact methods produce VPN tree solutions with a bandwidth over-provisioning factor as low as 1.6, and never exceeds a factor of 2, compared to solutions obtained using a pipe workload.