Use of ASP identifier in M3UA

Hai all please clarify following:

Q.(1)
What is the actual use of ASP identifier in M3UA

As per "draft-ietf-sigtran-rfc3332bis-05.txt"
in section:
3.5.1 ASP Up

      The optional ASP Identifier parameter contains a unique value that
      is locally significant among the ASPs that support an AS.  The SGP
      should save the ASP Identifier to be used, if necessary, with the
      Notify message (see Section 3.8.2).

in section:
4.3.4.1 ASP Up Procedures

   If the ASP Up message contains an ASP Identifier, the SGP
   should save the ASP Identifier for that ASP. The SGP MUST send an ASP
   Up Ack message in response to a received ASP Up message even if the
   ASP is already marked as ASP-INACTIVE at the SGP.

Q.(2)
If  an SGP receives an ASP-UP message in other than down state(INACTIVE 
/ ACTIVE)
Should the SGP has to validate the ASP-Identifier received before 
sending UP-ACK..?
If so, if the received ASP Identifier value is same as the one which is 
saved earlier in down state
should the SGP has to send error message with "Invalid ASP 
Identifier"....? or
Should the SGP has to consider and save the new ASP Identifier value 
received...?

Thanks
Prakash.

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