推荐两篇学习SIP的文章

http://arstechnica.com/business/news/2010/01/voip-in-depth-an-introduction-to-the-sip-protocol-part-1.ars

http://arstechnica.com/telecom/guides/2010/03/voip-in-depth-an-introduction-to-the-sip-protocol-part-2.ars/4