Howdy,
Several people asked me after the meeting about why we would need a session-id 
if people just implemented secure-call-id.  Clearly I didn't articulate it well 
enough, so here goes another try...

ISTM, there are numerous "types" of B2BUA's in the World.  Some of those 
B2BUA's replace the Call-ID for a single reason: security/privacy, because the 
Call-ID had an IP/host in it.  For example SBC's are of such a type, typically. 
 Using a secure-call-id should remove the incentive they have for replacing it, 
which will improve scenarios for dialog-matching in out-of-dialog requests.  

There are also other B2BUA types, which replace the Call-ID for different 
reasons.  IP-PBX's, App-Servers, SoftSwitches, etc., I would put in that camp.  
I don't know why they replace the Call-ID, but clearly their designers feel 
they need to.  It may just be for strict compliance with RFC-3261, which may 
well be the right thing to do for their case.  I believe the designers of those 
devices fully expect that the out-of-dialog request reaches them, and that it's 
their Call-ID that is in it.  They just didn't expect *other* B2BUA's to be in 
the path, such as SBC's.

So the secure-call-id is to provide a better path forward for B2BAU's which 
only change the call-id for the privacy reason.  

For troubleshooting purposes, however, that's not enough.  People want to 
correlate the session as it crosses as many B2BUA's as possible in their 
logs/monitoring-equipment, and thus the session-id draft.

-hadriel

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