Note

The best reference for the SIP protocol is RFC3261. To read the RFCs is a little bit boring and sleepy (it is very good when you have insomnia). You can find the RFC at: http://www.ietf.org/rfc/rfc3261.txt.

A good SIP tutorial can be found at Columbia University: http://www.cs.columbia.edu/~coms6181/slides/11/sip_long.pdf. Together with this you can find a lot of information about SIP at http://www.cs.columbia.edu/sip/.

A very good tutorial can be found at the iptel website: http://www.iptel.org/files/sip_tutorial.pdf.

There is a mailing list where you can post questions about SIP called SIP implementors: https://lists.cs.columbia.edu/mailman/listinfo/sip-implementors.