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The IP Multimedia Subsystem (IMS) is a converged service enabler that facilitates the rapid deployment of multimedia services in IP networks. The IMS defines a Policy Based Management model to ensure QoS-enabled connectivity by providing mediation and interaction between applications and transport layer resources. This paper reviews the current state of the art regarding the necessary mediation between QoS control elements and transport layer resources in the IMS. An enhancement to discover end to end signaling and media routes is proposed. This mechanism uses SIP routing information to discover origin, destination and transit administrative domains, and allows an application to effectively issue resource requests from its home network and enable QoS-enabled connectivity across all traversed transport segments. We present a prototype implementation that demonstrates the service initiated QoS model, in addition to the proposed algorithm that allows home network based resource reservation across administrative domains. The testbed and proposed mechanism are subjected to validation and performance tests and results are presented; in particular the effects the proposed enhancements have on session setup delay and signaling overhead are examined.