Juwei Shi Yao Wang Lanzhi Gu Lichun Li Wenjie Lin Yinong Li Yang Ji Ping Zhang A Hierarchical Peer-to-Peer SIP System for Heterogeneous Overlays Interworking. 93-97 2007 GLOBECOM http://dx.doi.org/10.1109/GLOCOM.2007.25 conf/globecom/2007 db/conf/globecom/globecom2007.html#ShiWGLLLJZ07