Comment by fenesiistvan

Yes, the SIP protocol is not so simple. However a lot of well working implementations already exists (source code, libraries, ready to use software). The core messaging in SIP (RFC 3428) is extremely simple to add as it is just a single method (MESSAGE).

Adding presence and other funny things with the SIMPLE protocol family is another subject. (However the basic presence is also very simple ...the complications comes when you wish to add file transfer and other fancy things).

But these can be used as an extra so at least the for chat you are fully interoperable with all other vendors.