Hello,
During exploring the mailing list archive I noticed that there are many messages asking for some info on SIP itself.
I prepared a small (20 pages) SIP introduction describing the basics. It is in no way exhaustive, but I hope it helps. Let me know if anything important is missing.
The introduction will be included into the ser documentation (docbook master source can be found in sip_router/doc/sip)
HTML, PS, PDF and plaintext renderings are here: http://iptel.org/ser/sipintro.html
(also accessible from the main ser page, see menu documentation).
any feedback is greatly appreciated, Jan.