We present a novel protocol for IEEE 802.15.4 beacon-less sensor networks, which provides support to RSSI-based localization algorithms to estimate the position of mobile nodes. The proposed protocol is designed to allow the application to change the mobile node operation mode on demand. In this way it is possible to manage the trade-off between energy consumption and achieved position accuracy. First results related to the parameterization of the protocol and the expected mobile node energy consumption are presented.