[Sipping] rfc5359 vs. sipping-cc-transfer: Blind/Unattended Transfer Question

[Luca Scheuring <[e-mail]>]

Hi,

In the call flow for unattended transfer shown in rfc5359 section 2.4
the transferor (Alice) sends the BYE (F9) right after receiving the
NOTIFY (100 Trying) (F7) from the transferee (Bob).

Statement from draft-ietf-sipping-cc-transfer-11 section 6 / page 7:

   [...]
   Each of the flows below shows the dialog between the Transferor and
   the Transferee remaining connected (on hold) during the REFER
   process.  While this provides the greatest flexibility for recovery
   from failure, it is not necessary.  If the Transferor's agent does
   not wish to participate in the remainder of the REFER process and has
   no intention of assisting with recovery from transfer failure, it
   could emit a BYE to the Transferee as soon as the REFER transaction
   completes.  This flow is sometimes known as "unattended transfer" or
   "blind transfer".
   [...]

The last sentence implies that if the transferor _wants_ to
participate in the remainder of the REFER process (and therefore waits
with sending the BYE until another NOTIFY with "200 OK" has been
received) this is called attended transfer. But both rfc5359 and
draft-ietf-sipping-cc-transfer-11 define "attended transfer" as a
transfer where the transferor establishes a call with the transfer
target to alert it to the impending transfer. Can somebody clarify
this issue?

This question came up when analyzing problems that occur if
Alice/transferor is an agent that remains in the transfer process
until NOTIFY with "200 OK" is received (in order to offer the
possibility to un-hold call to Bob/transferee again in case transfer
target doesn't reply / declines the call) while Bob/transferee is an
agent who expects a call flow as described in rfc5359 section 2.4.
Observed problem: Alice is on a call with Bob and then wants to
attempt an unattended/blind transfer to Carol. Carol's phone is
ringing, but no answer yet. Alice thinks Carol won't answer and
therefore un-holds the call to Bob in order to recover. Alice and Bob
are in communication again, but Bob is still calling Carol. If Carol
finally answers, Bob doesn't know how to handle this session. Call
from Bob to Carol ends up as a "phantom call" at Bob that cannot be
terminated by Bob.

Thanks,
Luca