Is SIP simple?

first 25 related RFCs (for SIP and SDP) - total of 823 pages (SIP alone: 269 pages)
RFC 3261 was longest RFC ever (based on byte count; 663,043 bytes)
There are claims that one can still build a simple user agent in a (long) evening, but there is substantial work required with respect to security (due to TLS, S/MIME, AAA, Denial of Service issues, …)

SIP timeline - showing a simple version of Alice inviting Bob to a SIP session:

Transcript

[slide119] Well, the first 25 related RFCs for SIP and SDP total 823 pages. SIP alone is 269 pages. It was the longest at that time, RFC ever produced. And there are some who claim that you can build a simple SIP user agent in a long evening. You can do it. It's not that hard. The hard problem is to actually handle the other matters like providing security. So, with TLS, S-MIME, AAA, denial of service, those are the things that really make it complex. The basic SIP is really simple. We see the diagram on the right. Alice sends an invite to Bob. Bob sends back a message, OK, 200. Alice says, OK. And now they have their session. And then at some point, either Alice or Bob say goodbye. Boom. We can go home now, right? That's SIP.