Comment by karaziox

Well, the specification defines two negociation procedures. You are not forced to support both, you are not even forced to support one, but then how would you make use of it? And you are also free to negociate it another way should you decide to. The spec defines the protocol, not the transport on which it is used or the way the underlying communication is established. They could have put the two negociation methods in another RFC for what it's worth, since they don't affect the inner workings of the protocol itself.