[abalashov]

It's a good exercise to go through.

However, out in industry, a good rule of thumb is that if it hasn't been around for at least ten years, it's not a usable SIP stack. SIP interoperability is incredibly fragile.

The basics look easy. Here are some minimally required headers, looks a lot like HTTP, send a message, get a reply, bam, done. I remember when I first started working with SIP at age 19 or 20 (ten years ago), I had the conceit that one could just read RFC 3261 and whip up a little SIP stack real quick. "Looks pretty easy".

Little. Did. I. Know. It took me several more jobs and three or four years of consulting beyond that to get to the point where I would say I had anything remotely resembling commercially valuable SIP expertise to offer. These days I have a consulting business around that premise, but that's ten years and a whole specialist career down the road. All for a little HTTP-like protocol with human-readable headers.

There are only a handful of viable commercial and FOSS SIP stacks out there. All painstakingly evolved to get there. For any serious endeavour you should use one of them. :-)