Ok, this is an old (2006) but great doc that you should have a look at if you want to get to grips will all the relevant RFCs related to SIP.  Written by Jonathan Rosenburg and posted here  ... Download it and have a read, it will save you a lot of time by stopping you reading RFCs that are no longer relevant or have been replaced.  Though keep an eye out for new drafts on the IETF website as things are always changing.