Linchen Yu Xiaofei Liao Hai Jin Pengcheng Qin A Hierachical VoIP System Based on Peer-to-Peer SIP: A Manageable Approach. 2494-2500 2010 CIT http://doi.ieeecomputersociety.org/10.1109/CIT.2010.426 conf/IEEEcit/2010 db/conf/IEEEcit/IEEEcit2010.html#YuLJQ10