Mario Baldi Luca De Marco Fulvio Risso Livio Torrero Providing End-to-End Connectivity to SIP User Agents Behind NATs. 5902-5908 2008 ICC http://dx.doi.org/10.1109/ICC.2008.1103 conf/icc/2008 db/conf/icc/icc2008.html#BaldiMRT08