Why only link-local-address ??

Hi All,

        As IPV6CP negotiates _only_ the interface identifier
        for link local address, we should have some other
        mechanisms to assign global addresses to PPP ends.
        Examples of such mechanisms are: sending a router
        advertisement (RA) of certain prefix or manual allotment
        of address etc.

        Is there a strong reason for the decision of having only
        the interface-identifier in IPV6CP negotiation ?? Why
        shouldn't we have a prefix option in IPV6CP ?? Sorry, if
        this topic is discussed already. If so, please direct
        me to locate the answer.

        Thanks in advance for your response.

Regards,
Anil