Dear SIP gurus,

My UA application requires "consultative call transfer" (transferor 
converses with the destination first). Obviously I need to implement 
RFC3515 / REFER method. 

Another requirement of this application is that the destination must 
accept the transferee's INVITE directly(without user interaction), but 
ONLY if it contains certain credentials.

Consider the following scenario where epA (the transferor) has a session 
with epB(the transferee) and epC(the destination):

epA                   epB                   epC
 |                     |                     |
 |<===================>|                     | * epA has a session 
established with the transferee
 |<=========================================>| * epA has a session established with 
the destination
 |                     |                     |
 |                     |                     |
 |-----REFER/200------>|                     | * epA REFERs epB to epC ### 
REFER contains credentials ###
 |<-NOTIFY/200(trying)-|                     |
 |                     |---INVITE/200/ACK--->| * epB INVITEs epC  ### epC 
verifies credentials ###
 |<-NOTIFY/200(succes)-|<===================>| * epB and epC establish a 
session
 |                     |                     |
 |-----BYE/200-------->|                     | * epA ends session with epB
 |------------------------BYE/200----------->| * epA ends session with epC
 |                     |                     |
 |                     |                     |

In some way, epA has to provide epB credentials which epB must use to 
INVITE epC. I think these credentials e.g. could be something like the 
epA-epC call ID and from/to tags.

What headers/bodies etc... are normally used to carry these credentials to 
be interoperable with other UAs other than my own application.

Other suggestions and comments are more than welcome.

Martin van den Berg
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