VoIP in-depth: An introduction to the SIP protocol

http://arstechnica.com/business/2009/12/wired-for-sound-how-sip-won-the-voip-protocol-wars/

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

http://arstechnica.com/tech-policy/2010/03/voip-in-depth-an-introduction-to-the-sip-protocol-part-2/

 

http://www.networksorcery.com/enp/protocol/sip.htm

 

http://java.sun.com/products/jain/JAIN-SIP-Tutorial.pdf

 

http://www.konnetic.com/documents/KonneticSIPIntroduction.pdf
http://www.konnetic.com/Documents/KonneticSIPIntroduction.pdf
http://www.konnetic.com/documents/KonneticSDPTechnicalOverview.pdf