What we had in mind when writting this was the equivalent to the Q.931 
Redirecting Number information element (and whatever it's ISUP equivalent is, 
or even Diversion header for that matter).
 
The target maps to the Number digits (which is always there), and the cause 
maps to the "Reason for redirection,octet 3b", which is optional. Absence means 
the reason is unknown. I would say that if cause is
not there, then it means the same as cause=404.
 
Also, if target is not there, I guess it means that the number is not known. 
 
But you are right, I don't believe the text in 4458 explains it properly.


________________________________

        From: DRAGE, Keith (Keith) [mailto:[email protected]] 
        Sent: Friday, April 10, 2009 17:45
        To: Hans Erik van Elburg
        Cc: [email protected]; Cullen Jennings; Audet, Francois (SC100:3055); 
Elwell, John
        Subject: RE: [Sip] Clarification on RFC 4458: SIP URIs for Applications 
suchas Voicemail and Interactive Voice Response (IVR)
        
        
        And which part of the wording in RFC 4458 do you use to justify that?
         
        The whole point of the question is that we are trying to ascertain what 
is a legitimate conformance with RFC 4458.
         
        regards
         
        Keith


________________________________

                From: [email protected] [mailto:[email protected]] On 
Behalf Of Hans Erik van Elburg
                Sent: Friday, April 10, 2009 9:06 AM
                To: DRAGE, Keith (Keith)
                Cc: [email protected]; Cullen Jennings; Francois Audet; Elwell, John
                Subject: Re: [Sip] Clarification on RFC 4458: SIP URIs for 
Applications such as Voicemail and Interactive Voice Response (IVR)
                
                
                Specifications that only include the cause value mappings as 
specified in RFC4458 reuse only that part of RFC4458. 
                
                Such reuse is of course fine, but carefull wording is required 
not to imply full conformance with RFC4458 as that would lead to the wrong 
implications/expectations.
                
                /Hans Erik van Elburg
                
                
                
                On Wed, Apr 8, 2009 at 6:06 PM, DRAGE, Keith (Keith) 
<[email protected]> wrote:
                

                        RFC 4458 defines two SIP URI parameters.
                        
                        I can find no specific language in the document that 
indicates whether these two URI parameters can be used independently of each 
other, or whether there is an expectation that both URI parameters should only 
exist in the presence of each other. Specifically does an implementation that 
only includes the "cause" SIP URI parameter and not the "target" sip URI 
parameter conform to this specification?
                        
                        All the text and examples seem to carry the implication 
that where one exists, the other one will also.
                        
                        Could one of the authors or some other interested party 
clarify what they believe the intention is?
                        
                        regards
                        
                        Keith
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