WorldOfVoIP

• H Schulzrinne, J Rosenberg, Internet Telephony: architecture and protocols - an IETF perspective , IEEE Computer Network, Feb 1999
• H Schulzrinne, J Rosenberg, A Comparison of SIP and H.323 for Internet Telephony , July 1998, NOSSDAV'98
• Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, SIP: Session Initiation Protocol , RFC 3261, June 2002.
• A primer on the H.323 series standard
• RFC for Real Time Streaming Protocol (RTSP) , April 1998.
• RFC for RTP: A Transport Protocol for Real-time applications , Jan 1996

Skype

• An Experimental Study of the Skype Peer-to-Peer VoIP System , by Saikat Guha (Cornell), Neil Daswani, Ravi Jain (Google). IPTPS 2006
• An Analysis of the Skype Peer-to-Peer Internet Telephony Protocol by Salman A. Baset and Henning Schulzrinne (about Skype version 1.4)