Rewrote the start and end of Section 4.3.2 to clarify that the scope of the
discussion is the interaction with the NSLP, and that only the description of
the GIST internal processing is deferred to a later section.

Revised start:

   Once a message has been received, it is processed locally within the
   GIST layer.  Further processing depends on the message type and
   payloads carried; most of the GIST payloads are associated with
   internal state maintenance, and details are covered in Section 4.4.
   This section concentrates on the interaction with the signalling
   application, in particular the decision to peer and how data is
   delivered to the NSLP.

Revised end:

   For all GIST message types other than Queries, if the message
   includes an NSLP payload, this MUST be delivered locally to the
   signalling application identified by the NSLPID.  The format of the
   payload is not constrained by GIST, and the content is not
   interpreted.  Delivery is subject to the following validation checks
   which MUST be applied in the sequence given:

(... and then the bullets as before.)