Network Working Group Shrawan Kumar Khatri
Internet-Draft Vikram Singh
Cherng-Shung Hsu
Intended status: Standard Track Qualcomm Incorporated
Expires: January 2024 July 22, 2023
A SIP Response Code (497) for Call Transfer Failure
draft-khatri-sipcore-call-transfer-fail-response-05.txt
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Abstract
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This document defines the 497 (Call Transfer Failure) SIP
response code, allowing Call Pull and Call Push parties to
indicate that the operation was rejected. Optional warning codes
are defined to carry granular information. SIP entities may use
this information to adjust how subsequent calls can be handled
intelligently.
Table of Contents
1. Introduction................................................. 2
2. Normative Language........................................... 4
3. Motivation................................................... 4
4. Theory of Operation.......................................... 5
5. IANA Considerations.......................................... 8
5.1. SIP Response Code....................................... 8
5.2. Warning codes........................................... 8
6. Security Considerations...................................... 9
7. References................................................... 9
7.1. Normative References.................................... 9
8. Acknowledgments............................................. 10
1. Introduction
Packet switch calls for voice, video and text applications using
IP Multimedia Subsystems (IMS) are anchored in the IMS core
network. The signaling plane and media plane of IMS calls
established on one device can be pushed ("Call Push") to another
device. Similarly, IMS calls established on one device can be
pulled ("Call Pull") by another device using SIP signaling.
The call status during the SIP transaction can be conveyed through
SIP response codes. RFC 3261 has defined many SIP response codes.
The IMS core network MAY define a policy to allow or reject the
Call Pull or Call Push operation. There are numerous reasons why
the Call Pull or Call Push operation can fail. In case of call
transfer failure due to policy defined by the IMS core network,
the IMS core network MAY want to convey the failure to the user
agent (UA) through a response code. Based on the response code,
the UA MAY determine whether and how to establish a subsequent
call.
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The existing response codes in RFC 3261 are not sufficient to
convey the information about the call transfer failure to the UA.
Overloading an existing response code could lead to unnecessary
subsequent signaling which could burden the IMS core network. To
avoid possible signaling overload in the IMS core network and to
accurately convey the call transfer failure to the UA, a new
response code along with associated optional warning codes to be
included in a Warning header field are proposed in this RFC.
The following call flows illustrate the successful Call Pull.
UA Core Network
| |
| |
| 1. INVITE: |
| Contact-Header: +g.3gpp.iut-focus |
|-------------------------------------------------------->|
| |
| 2. 200 OK |
|<--------------------------------------------------------|
| |
Figure 1: Call Pull Success
1. The UA sends an INVITE to Pull the call from another device.
Feature Tag: +g.3gpp.iut-focus [RFC6809] is added in the
Contact Header field.
2. The call transfer request satisfied. The Core Network sends
200 OK.
The following call flows illustrate the successful Call Push.
UA Core Network
| |
| 1. REFER: |
| Contact-Header: +g.3gpp.iut-focus |
|-------------------------------------------------------->|
| |
| 2. 202 Accepted |
|<--------------------------------------------------------|
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| |
Figure 2: Call Push Success
1. The UA sends a REFER to Push the call to another device.
Feature Tag: +g.3gpp.iut-focus is added in the Contact Header
field.
2. The call transfer request satisfied. The Core Network sends
202 Accepted.
2. Normative Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",
"MAY", and "OPTIONAL" in this document are to be interpreted as
described in BCP 14 [RFC2119] [RFC8174] when, and only when, they
appear in all capitals, as shown here.
3. Motivation
Seamless transfer of the media plane of an on-going voice call
between devices is a legacy behavior. The signaling plane in the
legacy behavior resides on the originating device.
If the two devices can run IMS over SIP signaling, the signaling
plane and media plane can be transferred between these devices
with minimal media flow interruption. The control plane and media
plane transfer procedures are beyond the scope of this RFC.
There are various reasons why an on-going call cannot be
transferred between the devices. Some of these reasons are policy
driven, for instance: the call to be transferred is in the circuit
switched (CS) domain and the operator's policy does not allow
transfer of a CS call, the call is an emergency call and the
operator's policy does not allow transfer of an emergency call,
the call is a mobile-terminated call and the operator's policy
does not allow transfer of a mobile-terminated call, or the call
is a video call and the operator's policy does not allow transfer
of a video call.
The UA initiating the call transfer procedure will be notified of
any failure through a SIP response code. However the existing SIP
response codes are not suitable to adequately convey the
information regarding why the call transfer request is not
accepted by the network: handling of these existing response codes
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has already been implemented by various devices, with an
associated device behavior defined for a specific purpose not
related to call transfer. For instance, upon receiving some of
these response code, such as 403 (Forbidden), the device MAY
initiate IMS re-registration procedure, which is not needed in
case of Call Pull/Call Push failure and will result in unnecessary
SIP signaling.
Consequently, to accurately convey the information about the call
transfer failure to the UA, a new response code is required along
with an optional warning code in a Warning header field to convey
the exact reason why the call could not be transferred, so that
the UA can determine the subsequent steps (e.g. call termination)
and optionally provide an indication of the reason for the failure
to the user.
Backward compatibility is maintained in both the UE and Network
side. The Network component that has not implemented this RFC
shall use the existing response code. The UE that has not
implemented this RFC shall handle this new response code as an
unknown response code.
4. Theory of Operation
Response code:
A new SIP response code 497 is defined.
Description: Call Transfer Failure
The server understood the call transfer request but is refusing
the service. The SIP response with SIP response code 497 MAY
include a Warning header field [RFC3261] with a warning code set
to one of the values listed below and the associated warning text
conveying granular information about the reason for the call
transfer failure, so as to enable the UA to develop extra logic
for subsequent call transfer procedure.
Warning header:
An optional Warning header will carry more granular failure
information as follows:
380. Inactive state
381. Local Receive-only state
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382. Local Transmit-only state
383. Remote Receive-only state
384. Remote Transmit-only state
385. Hold state
386. Mortal state
387. Conference call
388. Emergency call
389. Video call
390. Real Time Text call
391. Circuit Switch call
392. Non existing call
393. Single Radio Voice Call Continuity in progress
Feature-tag:
media feature-tag:+g.3gpp.iut-focus [3GPP TS 24.337]
In Call Pull, INVITE method is used and media feature tag
"+g.3gpp.iut-focus" is included in the Contact Header field.
In Call Push, REFER method is used and media feature tag
"+g.3gpp.iut-focus" is included in the Contact Header field.
Call Transfer failure
If call transfer using INVITE or REFER method fails and
response code 497 is supported by the network, the SIP response
SHALL include SIP response code 497.
If a call transfer fails and response code 497 is not supported
by the network, the SIP response code should be chosen from
existing
SIP response codes defined in RFC 3261
Example:
Warning: 388 proxy.example.com "Call is an emergency call"
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The following call flow illustrates the usage of SIP response
code 497 and the associated warning codes:
UA Core Network
| |
| |
| 1. INVITE: |
| Contact-Header: +g.3gpp.iut-focus |
|-------------------------------------------------------->|
| |
| 2. 497 Call Transfer Failure |
| Warning: 388 "Call is an emergency call" |
|<--------------------------------------------------------|
| |
Figure 3: Usage of SIP response code 497 Call Pull
1. The UA sends an INVITE to Pull the call from another device.
Feature Tag: +g.3gpp.iut-focus is added in the Contact Header
field.
2. The call transfer request cannot be satisfied due to the call
requested to be transferred being an emergency call. Since
the core network supports SIP response code 497, the core
network sends a 497 Call Transfer Failure with a Warning
header field set to: 388 "Call is an emergency call"
UA Core Network
| |
| 1. REFER: |
| Contact-Header: +g.3gpp.iut-focus |
|-------------------------------------------------------->|
| |
| 2. 497 Call Transfer Failure |
| Warning: 388 "Call is an emergency call" |
|<--------------------------------------------------------|
| |
Figure 4: Usage of SIP response code 497 Call Push
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1. The UA sends a REFER to Push the call to another device.
Feature Tag: +g.3gpp.iut-focus is added in the Contact Header
field.
2. The call transfer request cannot be satisfied due to the call
requested to be transferred being an emergency call. Since
the core network supports SIP response code 497, the core
network sends a 497 Call Transfer Failure with a Warning
header field set to: 388 "Call is an emergency call"
5. IANA Considerations
5.1. SIP Response Code
This document registers a new SIP response code. This response
code is defined by the following information, which has been added
to the "Response Codes" sub-registry under the "Session Initiation
Protocol (SIP) Parameters" registry
<http://www.iana.org/assignments/sip-parameters>.
Response Code: 497
Description: CALL TRANSFER FAILURE
The server understood the request to transfer the call but is
refusing the service. This response MAY include a Warning header
field [RFC3261] with a warning code set to one of the values
listed in section 5.2 and the associated warning text conveying
granular information about the reason for the call transfer
failure.
Reference: [RFCxxxx]
5.2. Warning codes
This document registers new warning codes. These warning codes are
defined by the following information, which has been added to the
"Warning Codes (warn-codes)" sub-registry under the "Session
Initiation Protocol (SIP) Parameters" registry
<http://www.iana.org/assignments/sip-parameters>.
380. Inactive state
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381. Local Receive-only state
382. Local Transmit-only state
383. Remote Receive-only state
384. Remote Transmit-only state
385. Hold state
386. Mortal state
387. Conference call
388. Emergency call
389. Video call
390. Real Time Text call
391. Circuit Switch call
392. Non existing call
393. Single Radio Voice Call Continuity in progress
6. Security Considerations
The general discussion in [RFC3261] applies.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G.,
Johnston, A., Peterson, J., Sparks, R., Handley, M.,
and E. Schooler, "SIP: Session Initiation Protocol",
RFC 3261, DOI 10.17487/RFC3261, June 2002,
<http://www.rfc-editor.org/info/rfc3261>.
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[RFC5407] Hasebe, M., Koshiko, J., Suzuki, Y., Yoshikawa, T.,
Kyzivat, P., "Example Call Flows of Race Conditions in
the Session Initiation Protocol (SIP)", BCP 147, RFC
5407, DOI 10.17487/RFC5407, December 2008,
<http://www.rfc-editor.org/info/rfc5407>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, FC 8174, DOI 10.17487/RFC8174,
May 2017, <http://www.rfc-editor.org/info/rfc8174>.
[RFC6809] Holmberg, C., Sedlacek, I., and H. Kaplan, "Mechanism
to Indicate Support of Features and Capabilities in the
Session Initiation Protocol (SIP)", RFC 6809, DOI
10.17487/RFC6809, November 2012, <https://www.rfc-
editor.org/info/rfc6809>.
[3GPP TS 24.337] "Universal Mobile Telecommunications System
(UMTS); LTE; IP Multimedia (IM) Core Network
(CN)subsystem IP Multimedia Subsystem (IMS) inter-UE
transfer
8. Acknowledgments
Lena Chaponniere, Qualcomm Inc. and Waqar Zia, Qualcomm Inc. for
the technical guidance.
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Authors' Addresses
Shrawan Khatri
5775 Morehouse Drive
San Diego, CA 92121
United States of America
Email: skhatri@qualcomm.com
Vikram Singh
5775 Morehouse Drive
San Diego, CA 92121
United States of America
viksingh@qualcomm.com
Cherng-Shung Hsu
5775 Morehouse Drive
San Diego, CA 92121
United States of America
simonh@qualcomm.com
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