Subject: [SIP] Fwd: SIP transfer call flows

From: Rohan Mahy <[email-masked]>
Date: Fri, 23 Mar 2001 18:19:57 -0800

Hi Folks,

As promised on Tuesday, below are some sample flows illustrating some uses 
of REFER (some with Replaces).

thanks,
-rohan

>X-Sender: [email-masked]
>X-Mailer: QUALCOMM Windows Eudora Pro Version 4.1
>Date: Mon, 19 Mar 2001 17:01:27 -0600
>To: [email-masked]
>From: Rohan Mahy <[email-masked]>
>Subject: SIP transfer call flows
>
>Sample Call Flows
>Flow A: Blind Transfer
>                       Blind Transfer
>
>
>       Transferor        Transferee       Transfer Target
>           A                  B                   C
>
>           |                  |                   |
>           |    INVITE   A1   |                   |
>           |----------------->|                   |
>           |    180      A2   |                   |
>           |<-----------------|                   |
>           |                  |                   |
>           |    200      A3   |                   |
>           |<-----------------|                   |
>           |    ACK      A4   |                   |
>           |----------------->|                   |
>           |                  |                   |
>           |   2-way RTP      |                   |
>           |<================>|                   |
>           |                  |                   |
>           |    REFER    A5   |                   |
>           |----------------->|                   |
>           |    202      A6   |                   |
>           |<-----------------|       INVITE  A7  |
>           |    BYE      A8   |------------------>|
>           |----------------->|       180     A9  |
>           |    200      A10  |<------------------|
>           |------------------|                   |
>           |                  |                   |
>           |                  |                   |
>           |                  |                   |
>           |     time passes .......              |
>           |                  |                   |
>           |                  |                   |
>           |                  |       200     A11 |
>           |(   NOTIFY  A12  )|<------------------|
>           |(<---------------)|       ACK     A13 |
>           |(   200     A14  )|------------------>|
>           |(--------------->)|    2-way RTP      |
>           |                  |<=================>|
>           |                  |                   |
>
>A1: A -> B
>INVITE sip:B SIP/2.0
>To: <sip:B>
>From: <sip:A>;tag=89289
>Call-Id: X
>
>A5: A-> B
>REFER sip:B SIP/2.0
>To: <sip:B>;tag=12314
>From: <sip:A>;tag=89289
>Call-Id: X
>Refer-To: <sip:C>
>Referred-By: <sip:A>;ref=<sip:C>
>
>A7: B -> C
>INVITE sip:C SIP/2.0
>To: <sip:C>
>From: <sip:B>;tag=12314
>Call-Id: Y
>Referred-By: <sip:A>;ref=<sip:C>
>
>(optional) A12: B -> A
>NOTIFY sip:A SIP/2.0
>To: <sip:A>;tag=89289
>From: <sip:B>;tag=12314
>Event: refer
>Content-Type: application/sip
>
>SIP/2.0 200 OK
>
>Flow B: Attended Transfer
>                      Attended Transfer
>
>
>       Transferor        Transferee       Transfer Target
>           A                  B                   C
>
>           |                  |                   |
>           |    INVITE   B1   |                   |
>           |----------------->|                   |
>           |    180      B2   |                   |
>           |<-----------------|                   |
>           |                  |                   |
>           |    200      B3   |                   |
>           |<-----------------|                   |
>           |    ACK      B4   |                   |
>           |----------------->|                   |
>           |                  |                   |
>           |   2-way RTP      |                   |
>           |<================>|                   |
>           |                  | begin attended    |
>           |                  | transfer          |
>           |    INVITE   B5   |                   |
>           |----------------->|                   |
>           |    200      B6   |                   |
>           |<-----------------|                   |
>           |    ACK      B7   |                   |
>           |----------------->|                   |
>           |                  |                   |
>           |   held  RTP      |                   |
>           |<::::::::::::::::>|                   |
>           |                  |                   |
>           |                  |  INVITE     B8    |
>           |------------------------------------->|
>           |                  |  180        B9    |
>           |<-------------------------------------|
>           |                  |                   |
>           |                  |  200        B10   |
>           |<-------------------------------------|
>           |                  |  ACK        B11   |
>           |------------------------------------->|
>           |                  |                   |
>           |              2-way RTP               |
>           |<====================================>|
>           |                  |                   |
>           |     time passes .......              |
>           |                  |                   |
>           |                  |                   |
>           |                  |                   |
>           |                  | complete transfer |
>           |    REFER    B12  |                   |
>           |----------------->|                   |
>           |    202      B13  |                   |
>           |<-----------------|   INVITE    B14   |
>           |                  |------------------>|
>           |                  |   200       B15   |
>           |    NOTIFY   B16  |<------------------|
>           |<-----------------|   ACK       B17   |
>           |    200      B18  |------------------>|
>           |----------------->|    2-way RTP      |
>           |                  |<=================>|
>           |                  |  BYE        B19   |
>           |<-------------------------------------|
>           |                  |  200        B20   |
>           |------------------------------------->|
>           |    BYE      B21  |                   |
>           |----------------->|                   |
>           |    200      B22  |                   |
>           |<-----------------|                   |
>           |                  |                   |
>
>B1: A -> B
>INVITE sip:B SIP/2.0
>To: <sip:B>
>From: <sip:A>;tag=89289
>Call-Id: X
>
>B5: A -> B
>INVITE sip:B SIP/2.0
>To: <sip:B>;tag=12314
>From: <sip:A>;tag=89289
>Call-Id: X
>
>c=0.0.0.0
>
>B8: A -> C
>INVITE sip:C SIP/2.0
>To: <sip:C>
>From: <sip:A>;tag=89289
>Call-Id: Z
>
>B12: A-> B
>REFER sip:B SIP/2.0
>To: <sip:B>;tag=12314
>From: <sip:A>;tag=89289
>Call-Id: X
>Refer-To: <sip:C?Replaces=Z;to-tag=7465;from-tag=89289>
>Referred-By: <sip:A>;ref=<sip:C?Replaces=Z;to-tag=7465;from-tag=89289>
>
>B14: B -> C
>INVITE sip:C SIP/2.0
>To: <sip:C>
>From: <sip:B>;tag=12314
>Call-Id: Y
>Replaces: Z;to-tag=7465;from-tag=89289
>Referred-By: <sip:A>;ref=<sip:C?Z;to-tag=7465;from-tag=89289>
>
>B16: B -> A
>NOTIFY sip:A SIP/2.0
>
>SIP/2.0 200 OK
>
>B19: C -> A
>BYE sip:A SIP/2.0
>To: <sip:A>;tag=89289
>From: <sip:C>;tag=7465
>Call-Id: Z
>
>B21: A -> B
>BYE sip:B SIP/2.0
>To: <sip:B>;tag=12314
>From: <sip:A>;tag=89289
>Call-Id: X
>
>Flow C: Attended Transfer completed early
>
>               Attended Transfer completed early
>
>
>       Transferor        Transferee       Transfer Target
>           A                  B                   C
>
>           |                  |                   |
>           |    INVITE   C1   |                   |
>           |----------------->|                   |
>           |    180      C2   |                   |
>           |<-----------------|                   |
>           |                  |                   |
>           |    200      C3   |                   |
>           |<-----------------|                   |
>           |    ACK      C4   |                   |
>           |----------------->|                   |
>           |                  |                   |
>           |   2-way RTP      |                   |
>           |<================>|                   |
>           |                  | begin attended    |
>           |                  | transfer          |
>           |    INVITE   C5   |                   |
>           |----------------->|                   |
>           |    200      C6   |                   |
>           |<-----------------|                   |
>           |    ACK      C7   |                   |
>           |----------------->|                   |
>           |                  |                   |
>           |   held  RTP      |                   |
>           |<::::::::::::::::>|                   |
>           |                  |                   |
>           |                  |  INVITE     C8    |
>           |------------------------------------->|
>           |                  |  183        C9    |
>           |<-------------------------------------|
>           |<========= early media ===============|
>           |                  |                   |
>           |     time passes .......              |
>           |                  |                   |
>           |                  | complete transfer |
>           |                  | early             |
>           |    REFER    C10  |                   |
>           |----------------->|                   |
>           |    202      C11  |                   |
>           |<-----------------|  INVITE     C12   |
>           |    BYE      C13  |------------------>|
>           |----------------->|  183        C14   |
>           |    200      C15  |<------------------|
>           |<-----------------|<== early media ===|
>           |                  |                   |
>           |                  |  487        C16   |
>           |<-------------------------------------|
>           |                  |  ACK        C17   |
>           |------------------------------------->|
>           |                  |                   |
>           |                  |                   |
>           |                  |                   |
>           |     time passes .......              |
>           |                  |                   |
>           |                  |                   |
>           |                  |  200        C18   |
>           |(   NOTIFY  C19  )|<------------------|
>           |(<---------------)|  ACK        C20   |
>           |(   200     C21  )|------------------>|
>           |(--------------->)|    2-way RTP      |
>           |                  |<=================>|
>           |                  |                   |
>           |                  |                   |
>
>
>C1: A -> B
>INVITE sip:B SIP/2.0
>To: <sip:B>
>From: <sip:A>;tag=89289
>Call-Id: X
>
>C5: A -> B
>INVITE sip:B SIP/2.0
>To: <sip:B>;tag=12314
>From: <sip:A>;tag=89289
>Call-Id: X
>
>c=0.0.0.0
>
>C8: A -> C
>INVITE sip:C SIP/2.0
>To: <sip:C>
>From: <sip:A>;tag=89289
>Call-Id: Z
>
>C10: A-> B
>REFER sip:B SIP/2.0
>To: <sip:B>;tag=12314
>From: <sip:A>;tag=89289
>Call-Id: X
>Refer-To: <sip:C?Replaces=Z;to-tag=7465;from-tag=89289>
>Referred-By: <sip:A>;ref=<sip:C?Replaces=Z;to-tag=7465;from-tag=89289>
>
>C12: B -> C
>INVITE sip:C SIP/2.0
>To: <sip:C>
>From: <sip:B>;tag=12314
>Call-Id: Y
>Replaces: Z;to-tag=7465;from-tag=89289
>Referred-By: <sip:A>;ref=<sip:C?Replaces=Z;to-tag=7465;from-tag=89289>
>
>C13: A -> B
>BYE sip:B SIP/2.0
>To: <sip:B>;tag=12314
>From: <sip:A>;tag=89289
>Call-Id: X
>
>C16: C -> A
>SIP/2.0 487 Request Cancelled
>To: <sip:A>;tag=89289
>From: <sip:C>;tag=7465
>Call-Id: Z
>
>(optional) C19: B -> A
>NOTIFY sip:A SIP/2.0
>
>SIP/2.0 200 OK
>
>Flow D: Failed Blind Transfer
>
>                     Failed Blind Transfer
>
>
>       Transferor        Transferee       Transfer Target
>           A                  B                   C
>
>           |                  |                   |
>           |    INVITE   D1   |                   |
>           |----------------->|                   |
>           |    180      D2   |                   |
>           |<-----------------|                   |
>           |                  |                   |
>           |    200      D3   |                   |
>           |<-----------------|                   |
>           |    ACK      D4   |                   |
>           |----------------->|                   |
>           |                  |                   |
>           |   2-way RTP      |                   |
>           |<================>|                   |
>           |                  |                   |
>           |    REFER    D5   |                   |
>           |----------------->|                   |
>           |    202      D6   |                   |
>           |<-----------------|       INVITE  D7  |
>           |    BYE      D8   |------------------>|
>           |----------------->|       180     D9  |
>           |    200      D10  |<------------------|
>           |------------------|                   |
>           |                  |                   |
>           |                  |                   |
>           |                  |                   |
>           |     time passes .......              |
>           |                  |                   |
>           |                  |                   |
>           |                  |       404     D11 |
>           |(   NOTIFY  D12  )|<------------------|
>           |(<---------------)|       ACK     D13 |
>           |(   200     D14  )|------------------>|
>           |(--------------->)|                   |
>           |                  |                   |
>           |                  |                   |
>
>(optional) D12: B -> A
>NOTIFY sip:A SIP/2.0
>To: <sip:A>;tag=89289
>From: <sip:B>;tag=12314
>Event: refer
>Content-Type: application/sip
>
>SIP/2.0 501 OK
>
>Flow E: Failed Attended Transfer #1
>
>                Failed Attended Transfer #1
>
>
>       Transferor        Transferee       Transfer Target
>           A                  B                   C
>
>           |                  |                   |
>           |    INVITE   E1   |                   |
>           |----------------->|                   |
>           |    180      E2   |                   |
>           |<-----------------|                   |
>           |                  |                   |
>           |    200      E3   |                   |
>           |<-----------------|                   |
>           |    ACK      E4   |                   |
>           |----------------->|                   |
>           |                  |                   |
>           |   2-way RTP      |                   |
>           |<================>|                   |
>           |                  | begin attended    |
>           |                  | transfer          |
>           |    INVITE   E5   |                   |
>           |----------------->|                   |
>           |    200      E6   |                   |
>           |<-----------------|                   |
>           |    ACK      E7   |                   |
>           |----------------->|                   |
>           |                  |                   |
>           |   held  RTP      |                   |
>           |<::::::::::::::::>|                   |
>           |                  |                   |
>           |                  |  INVITE     E8    |
>           |------------------------------------->|
>           |                  |  180        E9    |
>           |<-------------------------------------|
>           |                  |                   |
>           |     time passes .......              |
>           |                  |                   |
>           |                  |  200        E10   |
>           |<-------------------------------------|
>           |                  |  ACK        E11   |
>           |------------------------------------->|
>           |                  |                   |
>           |              2-way RTP               |
>           |<====================================>|
>           |                  |                   |
>           |     time passes .......              |
>           |                  |                   |
>           |                  |                   |
>           |                  |                   |
>           |                  | complete transfer |
>           |    REFER    E12  |                   |
>           |----------------->|                   |
>           |    202      E13  |                   |
>           |<-----------------|   INVITE    E14   |
>           |                  |------------------>|
>           |                  |   606       E15   |
>           |    NOTIFY   E16  |<------------------|
>           |<-----------------|   ACK       E17   |
>           |    200      E18  |------------------>|
>           |----------------->|                   |
>           |                  |                   |
>           |                  |                   |
>           |         2-way RTP still active       |
>           |<====================================>|
>           |    held RTP      |                   |
>           |<::::::::::::::::>|                   |
>           |                  |                   |
>
>Flow F: Failed Attended Transfer #2
>
>                  Failed Attended Transfer #2
>
>
>       Transferor        Transferee       Transfer Target
>           A                  B                   C
>
>           |                  |                   |
>           |    INVITE   F1   |                   |
>           |----------------->|                   |
>           |    180      F2   |                   |
>           |<-----------------|                   |
>           |                  |                   |
>           |    200      F3   |                   |
>           |<-----------------|                   |
>           |    ACK      F4   |                   |
>           |----------------->|                   |
>           |                  |                   |
>           |   2-way RTP      |                   |
>           |<================>|                   |
>           |                  | begin attended    |
>           |                  | transfer          |
>           |    INVITE   F5   |                   |
>           |----------------->|                   |
>           |    200      F6   |                   |
>           |<-----------------|                   |
>           |    ACK      F7   |                   |
>           |----------------->|                   |
>           |                  |                   |
>           |   held  RTP      |                   |
>           |<::::::::::::::::>|                   |
>           |                  |                   |
>           |                  |  INVITE     F8    |
>           |------------------------------------->|
>           |                  |  100        F9    |
>           |<-------------------------------------|
>           |                  |                   |
>           |                  |  486        F10   |
>           |<-------------------------------------|
>           |                  |  ACK        F11   |
>           |------------------------------------->|
>           |                  |                   |
>           |   held  RTP      |                   |
>           |<::::::::::::::::>|                   |
>           |                  |                   |
>
>Flow G: Attended Transfer with early completion failure #1
>
>       Attended Transfer with early completion failure #1
>
>
>       Transferor        Transferee       Transfer Target
>           A                  B                   C
>
>           |                  |                   |
>           |    INVITE   G1   |                   |
>           |----------------->|                   |
>           |    180      G2   |                   |
>           |<-----------------|                   |
>           |                  |                   |
>           |    200      G3   |                   |
>           |<-----------------|                   |
>           |    ACK      G4   |                   |
>           |----------------->|                   |
>           |                  |                   |
>           |   2-way RTP      |                   |
>           |<================>|                   |
>           |                  | begin attended    |
>           |                  | transfer          |
>           |    INVITE   G5   |                   |
>           |----------------->|                   |
>           |    200      G6   |                   |
>           |<-----------------|                   |
>           |    ACK      G7   |                   |
>           |----------------->|                   |
>           |                  |                   |
>           |   held  RTP      |                   |
>           |<::::::::::::::::>|                   |
>           |                  |                   |
>           |                  |  INVITE     G8    |
>           |------------------------------------->|
>           |                  |  180        G9    |
>           |<-------------------------------------|
>           |                  |                   |
>           |     time passes .......              |
>           |                  |                   |
>           |                  | complete transfer |
>           |                  | early             |
>           |    REFER    G10  |                   |
>           |----------------->|                   |
>           |    202      G11  |                   |
>           |<-----------------|  INVITE     G12   |
>           |    BYE      G13  |------------------>|
>           |----------------->|  180        G14   |
>           |    200      G15  |<------------------|
>           |<-----------------|                   |
>           |                  |  487        G16   |
>           |<-------------------------------------|
>           |                  |  ACK        G17   |
>           |------------------------------------->|
>           |                  |                   |
>           |     time passes .......              |
>           |                  |                   |
>           |                  |                   |
>           |                  |  408        G18   |
>           |(   NOTIFY  G19  )|<------------------|
>           |(<---------------)|  ACK        G20   |
>           |(   200     G21  )|------------------>|
>           |(--------------->)|                   |
>           |                  |                   |
>           |                  |                   |
>
>Flow H: Attended Transfer with early completion failure #2
>
>       Attended Transfer with early completion failure #2
>
>
>       Transferor        Transferee       Transfer Target
>           A                  B                   C
>
>           |                  |                   |
>           |    INVITE   H1   |                   |
>           |----------------->|                   |
>           |    180      H2   |                   |
>           |<-----------------|                   |
>           |                  |                   |
>           |    200      H3   |                   |
>           |<-----------------|                   |
>           |    ACK      H4   |                   |
>           |----------------->|                   |
>           |                  |                   |
>           |   2-way RTP      |                   |
>           |<================>|                   |
>           |                  | begin attended    |
>           |                  | transfer          |
>           |    INVITE   H5   |                   |
>           |----------------->|                   |
>           |    200      H6   |                   |
>           |<-----------------|                   |
>           |    ACK      H7   |                   |
>           |----------------->|                   |
>           |                  |                   |
>           |   held  RTP      |                   |
>           |<::::::::::::::::>|                   |
>           |                  |                   |
>           |                  |  INVITE     H8    |
>           |------------------------------------->|
>           |                  |  180        H9    |
>           |<-------------------------------------|
>           |                  |                   |
>           |     time passes .......              |
>           |                  |                   |
>           |                  | complete transfer |
>           |                  | early             |
>           |    REFER    H10  |                   |
>           |----------------->|                   |
>           |    202      H11  |                   |
>           |<-----------------|  INVITE     H12   |
>           |    BYE      H13  |------------------>|
>           |----------------->|  606        H14   |
>           |    200      H15  |<------------------|
>           |<-----------------|  ACK        H16   |
>           |(   NOTIFY  H17  )|------------------>|
>           |(<---------------)|                   |
>           |(   200     H18  )|                   |
>           |(--------------->)|                   |
>           |                  |                   |
>           |                  |                   |
>
>Flow I: Failed transfer with auto-callback
>
>              Failed transfer with auto-callback
>
>
>       Transferor        Transferee       Transfer Target
>           A                  B                   C
>
>           |                  |                   |
>           |    INVITE   I1   |                   |
>           |----------------->|                   |
>           |    180      I2   |                   |
>           |<-----------------|                   |
>           |                  |                   |
>           |    200      I3   |                   |
>           |<-----------------|                   |
>           |    ACK      I4   |                   |
>           |----------------->|                   |
>           |                  |                   |
>           |   2-way RTP      |                   |
>           |<================>|                   |
>           |                  | begin attended    |
>           |                  | transfer          |
>           |  reINVITE   I5   |                   |
>           |----------------->|                   |
>           |    200      I6   |                   |
>           |<-----------------|                   |
>           |    ACK      I7   |                   |
>           |----------------->|                   |
>           |                  |                   |
>           |   held  RTP      |                   |
>           |<::::::::::::::::>|                   |
>           |                  |                   |
>           |                  |  INVITE     I8    |
>           |------------------------------------->|
>           |                  |  181        I9    |
>           |<-------------------------------------|
>           |                  |                   |
>           |     time passes .......              |
>           |                  |                   |
>           |                  | complete transfer |
>           |                  | early             |
>           |    REFER    I10  |                   |
>           |----------------->|                   |
>           |    202      I11  |                   |
>           |<-----------------|  INVITE     I12   |
>           |                  |------------------>|
>           |                  |  181        I13   |
>           |                  |<------------------|
>           |                  |                   |
>           |                  |                   |
>           |     time passes .......              |
>           |                  |                   |
>           |                  |                   |
>           |                  |  486        I14   |
>           |    NOTIFY   I15  |<------------------|
>           |<-----------------|  ACK        I16   |
>           |    200      I17  |------------------>|
>           |----------------->|                   |
>           |                  |                   |
>           | alert A until A answers ...          |
>           |                  |                   |
>           |  reINVITE   I18  |                   |
>           |----------------->|                   |
>           |    200      I19  |                   |
>           |<-----------------|                   |
>           |    ACK      I20  |                   |
>           |----------------->|                   |
>           |                  |                   |
>           |   RTP resumed    |                   |
>           |<================>|                   |
>           |                  |                   |
>

---

From: Paul Kyzivat <[email-masked]>
Date: Mon, 26 Mar 2001 12:06:00 -0500

Rohan,

I don't see a clear algorithm for caller A to decide
when to terminate its call to B. In some cases this
is done immediately upon receiving successful status
from the REFER, and in other cases it is done later.
The one rule I can see that almost fits the examples
is:
  if A has a successful call to C
  then terminate A-B call when A-C call is terminated
  else terminate A-B call when refer succeeds.

But this rule is violated in call flow I. That call flow
seems to depend on the NOTIFY to decide handling of
the A-B call. This requires knowing if B will send
a NOTIFY.

It seems to me that in all cases it is preferable to 
condition the disposition of the A-B call on the ultimate 
outcome of the REFER, learned via NOTIFY. The other
ways of handling this seem like less desirable fallbacks
if no NOTIFY will be delivered. But of course this
requires knowing if B will deliver NOTIFYs. Perhaps this
needs to be communicated with PROXY-REQUIRE and SUPPORTS.

On another subject, I like the early attended transfer.
Flow C would be especially interesting if C9 were 182 
rather than 183. Then the question arises: Does the new 
call in C12 assume the queue position previously held by the 
prior call? (This might be important if this sort of call 
queuing becomes common and the caller needs to switch to another 
device.) My assumption is that the decision about whether
the new call takes the queue position of the replaced call
is for C to decide.

	Paul Kyzivat
	Cisco Systems  

Rohan Mahy wrote:
> 
> Hi Folks,
> 
> As promised on Tuesday, below are some sample flows illustrating some
> uses of REFER (some with Replaces).
> 
> thanks,
> -rohan
> 
> > Referred-By:
> > <sip:A>;ref=<sip:C?Replaces=Z;to-tag=7465;from-tag=89289>
> >
> > B14: B -> C
> > INVITE sip:C SIP/2.0
> > To: <sip:C>
> > From: <sip:B>;tag=12314
> > Call-Id: Y
> > Referred-By:
> > <sip:A>;ref=<sip:C?Replaces=Z;to-tag=7465;from-tag=89289>
> >
> > C12: B -> C
> > INVITE sip:C SIP/2.0
> > To: <sip:C>
> > From: <sip:B>;tag=12314
> > Call-Id: Y
> > Referred-By:
> > <sip:A>;ref=<sip:C?Replaces=Z;to-tag=7465;from-tag=89289>
> >
> > C13: A -> B
> > BYE sip:B SIP/2.0
> > To: <sip:B>;tag=12314
> > From: <sip:A>;tag=89289
> > Call-Id: X
> >
> > C16: C -> A

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---

From: Rohan Mahy <[email-masked]>
Date: Mon, 26 Mar 2001 11:36:17 -0800

Hi,

Comments inline.

At 09:06 AM 3/26/01 , Paul Kyzivat wrote:
>when to terminate its call to B.

That's correct.  B (the transferee) cannot predict when the transferor will 
terminate the call.  If A is an IP phone, this is done at the request of a 
human and therefore at his/her whim.  If A is an application, the app will 
typically expose one API that terminates the call early (blind transfer), 
and an async call that attempts the transfer, and then gets an event with 
the transfer status.

>In some cases this
>prior call?

yes, absolutely!  and in the example with 183, if early media is sent from 
C to B, it should be the continuation of the early media sent from C to A.

>(This might be important if this sort of call

I think this is always implied by the Replaces.  If C doesn't want to honor 
the Replaces, I think it should decline the INVITE.

thanks,
-rohan

---

From: Paul Kyzivat <[email-masked]>
Date: Mon, 26 Mar 2001 15:33:28 -0500

I'm not worried about B predicting this.
I am worried about how to program a UA
to carry out one of these logical functions.

I guess your point is that most of the steps
in the flow are individually controlled by
the human associated with the UA. In this case,
the human is guided partially by the content of 
the media streams, in addition to the signalling.

I am thinking about the issues of combining this
with third party call control. In that case,
only the sip signalling is available to guide the
call flow. There needs to be some way to figure
out when to terminate the various legs using only
the sip messages.

But perhaps I need to come up with my own
scenario to illustrate this.

> >needs to be communicated with PROXY-REQUIRE and SUPPORTS.

As I understand it, once the invite corresponding to the refer
succeeds, there is no point in retaining on hold the A-B 
call that has been referred. I would like UA-A to 
automatically drop that call for me at that point unless
I have manually done it earlier. But it can't accept
responsibility for that unless it knows it will receive a 
NOTIFY.

	Paul