Added the following text at the end of the introduction:

   Because of the layered structure of the NSIS protocol suite, protocol
   extensions to cover a new signalling requirement could be carried out
   either within GIST, or within the signalling application layer, or
   even both.  General guidelines on how to extend different layers of
   the protocol suite, and in particular when and how it is appropriate
   to extend GIST, are contained in a separate document, [14].  In this
   document, Section 9 gives the formal IANA considerations for the
   registries already defined by the GIST specification.