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The emerging converged IP multimedia subsystem (IMS) allows the use of unlicensed, nondedicated and nondeterministic computer access networks for delivering IP multimedia services. To provide end-to-end quality-of-service (QoS) over such networks combined with the IMS core network, for resource demanding real-time services, such as live multimedia services, is a serious challenge. The main problem is the variation of resource availability, both in the network and end devices, during a single multimedia session. One solution to this problem is adaptation of content and quality of the media according to availability of resources in both the end devices and network. However, this solution requires accurate and timely signalling of resource availability between communicating parties during a multimedia session. In this paper, we suggest how signalling can be done using the Session Initiation Protocol (SIP) which is the call session control protocol for IMS. The real-time availability of the network resources (e.g. bandwidth, buffer space) and the end-device resources (e.g. battery, CPU, memory, storage) during a session is monitored and communicated over the network by the resource managers implemented in the end- devices. Our solution does not require any modifications of the IMS architecture and can be implemented as a plug-in.