Henning Schulzrinne’s site
Henning Schulzrinne of Columbia University is one of the key authors of the core SIP protocol, and many of the supporting RFCs and drafts, as well as a key researcher in the area of multimedia IP. His SIP web pages contain a wealth of additional information for developers and other technically-oriented readers.
The IETF SIP Working Group, and the SIPPING Working Group pages. The IETF is the standards body responsible for defining the SIP protocol, and the supporting RFCs and drafts.
The SIP School Wiki. A community-maintained Wiki of information about SIP.
Tech-Invite. A Portal devoted to SIP and surrounding technologies.
