[Sipping] Question on draft-anil-sipping-bla-03

[Fredrik Thulin <ft at it.su.se>]

Hi

I have a question on bridged line appearance with a state agent.

When the state agent sends it's SUBSCRIBE to a newly registered UA, that 
UA answers the SUBSCRIBE and then sends a SUBSCRIBE in the other 
direction. I have only read the draft semi-carefully, but I haven't 
understood what it is that makes the state agent know what Bridged Line 
Appearance that SUBSCRIBE is intended for. I'm talking about this 
moment in the establishment of the dual-SUBSCRIBEs :

   F9 Alice ----> State Agent 
    
   SUBSCRIBE sip:sa at stateagent.example.com SIP/2.0  
   Via: SIP/2.0/UDP ua1.example.com;branch=z9hG4bKf10fac97E7A76D6A  
   From: <sip:alice at example.com>;tag=925A3CAD-CEBB276E  
   To: <sip:sa at stateagent.example.com>  

How does the state agent know that this SUBSCRIBE is about 
sip:alice at example.com, and not sip:some-other-bla at example.com?
Is it the Contact the state agent used in F5 that is really unique to 
this particular BLA? If so, I would suggest showing this more clearly 
by letting the state agent use Contact: 
<sip:alice-at-example.com at stateagent.example.com> or similar, instead 
of <sip:sa at stateagent.examle.com>.


Also, a couple of comments :

* Maybe the reg-event NOTIFY from the registrar to the state agent 
should be shown in the section 6.1.1 and 6.1.2 diagrams? Now, one may 
wonder how the state agent knows it should send a SUBSCRIBE.

* You have many SIP headers where long headers are wrapped. That's OK 
but you have to start the next line with whitespace, for example :

   Change

      Via: SIP/2.0/UDP 
      stateagent.example.com;branch=z9hG4bK1846984327225734  

   into

      Via: SIP/2.0/UDP 
        stateagent.example.com;branch=z9hG4bK1846984327225734

* In message "F13 Bob ----> State Agent" (section 6.1.2), the To: is 
<sip:alice at stateagent.example.com>. Where does that address come from? 
I suspect a typo. alice at stateagent then appears in lots of other 
messages too.

/Fredrik

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