From: "Elwell, John" <[EMAIL PROTECTED]>

   I still don't get this call-ID stuff. If there is an intermediate B2BUA
   that maps call-IDs between the upstream side and the downstream side and
   it is not CC-stateful, how can it ensure that it performs the same
   mapping on a call-ID in the Event header field of a SUBSCRIBE request
   that it carried out on the original INVITE request? Without such
   mapping, the callee's monitor will receive a different call-ID and will
   be unable to match.

   Even with a CC-stateful B2BUA there, what if the SUBSCRIBE request gets
   routed differently and passes through a different B2BUA?

I haven't given it much thought, but one choice would be to hash the
incoming call-ID with a 64-bit key (shared among the constellation of
B2BUAs/gateways) to generate the outgoing 'identifier'.  (As the
protocol is defined now, the B2BUA can choose the Call-Id freely, and
apply the generated 'identifier'.)

Dale
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