[sipcore] About Join and Replaces Headers:

[[e-mail]]

By RFC3911 and RFC3981, the handling object of Join and Replaces headers 
is dialog:

RFC3911: The Join header is used to logically join an existing SIP dialog 
with a new SIP dialog.
RFC3981: The Replaces header is used to logically replace an existing SIP 
dialog with a new SIP dialog. 

Dialog has meaning is signalling level, not for service level. So, if we 
use Join and Replaces's semantics to build call service, the real handling 
object should be session.

For normal usage, such as:
A and B has a session;
C send a INVITE with Join header to A;
A build a basic conference for the three-party.

But if we consider some more complex use case, there might be problems. A 
and B has a audio and video call, and C just want to join to the audio 
part(part of the streams). 

Abstractly, the problem is how can we express the semantics, such as Join 
part of the streams in current session or Replaces part of the streams in 
current session? 

Thanks,

Gao

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