Re: [sipcore] Motivation for submission of RFC

[Christer Holmberg <[e-mail]>]

Hi,
A couple of additional comments:
Q5: Please note that RFC 3261 does not define usage for 4xx warning codes. It says "The first digit of warning codes beginning with "3" indicates warnings specific to SIP. ".
Q6: I think you should use a media feature tag (RFC 3840), not a feature capability indicator.
Regards,
Christer

From: sipcore <[e-mail]> On Behalf Of Christer Holmberg
Sent: perjantai 26. maaliskuuta 2021 10.23
To: Shrawan Khatri <[e-mail]>; [e-mail]; [e-mail]
Cc: Lena Chaponniere <[e-mail]>; Simon Hsu <[e-mail]>; Marcelo Pazos <[e-mail]>; Yong Xie <[e-mail]>; Waqar Zia <[e-mail]>; Vikram Singh (IMS) <[e-mail]>
Subject: Re: [sipcore] Motivation for submission of RFC

Hi Shrawan,
A couple of initial comments:
Q1: Please add references to the 3GPP specifications that define the procedure you are referring to.
Q2: It would help people not familiar with the 3GPP procedures if you could include a couple of simple call flows showing the call push and call pull procedures.
Q3: I don't understand why you include the 497 feature-tag in a SUBSCRIBE. Since it is a response code to an INVITE request, you should include the feature-tag in the INVITE request.
Q4: 5 listed editors on a 10 page draft looks a little strange :) While I personally don't care, you may get some feedback on that.
Regards,
Christer

From: sipcore <[e-mail]<mailto:[e-mail]>> On Behalf Of Shrawan Khatri
Sent: torstai 11. maaliskuuta 2021 20.11
To: [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>
Cc: Waqar Zia <[e-mail]<mailto:[e-mail]>>; Simon Hsu <[e-mail]<mailto:[e-mail]>>; Marcelo Pazos <[e-mail]<mailto:[e-mail]>>; Yong Xie <[e-mail]<mailto:[e-mail]>>; Lena Chaponniere <[e-mail]<mailto:[e-mail]>>; Vikram Singh (IMS) <[e-mail]<mailto:[e-mail]>>
Subject: [sipcore] Motivation for submission of RFC

Hello SIPCORE Chair and SIPCORE team,
Please find the motivation behind this RFC draft.

Regards,
Shrawan Khatri and team

The purpose of the draft
Signaling plane and Media plane of an IMS calls can be transferred between devices using IMS Signaling. 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 user agent (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. This is because handling of these existing response codes 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 codes, 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.

A method is defined in this draft such that a call transfer failure SIP response code is defined 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.

Intended target audience
The following is the intended audience of this RFC:
*             IMS Core Network Planners that support IMS call transfer across multiple devices.
*             IMS System Designer and Developers of IMS Core Network and User Agent that support call transfer using IMS Signaling.

Benefit
*             In the absence of this new mechanism,
o             The IMS Core network has to rely on existing SIP response codes, for which a specific device behavior has already been defined. For instance, upon receiving some of these response codes, such as 403 (Forbidden), the device may initiate IMS re-registration procedure, which is not needed in case of call transfer failure and will result in unnecessary SIP signaling.
o             The existing SIP response codes do not accurately convey the reason for the call transfer failure.

*             With the new mechanism
o             A specific behavior can be defined for the new SIP response code.
o             Unnecessary SIP signaling may be avoided.
o             A User Agent may develop intelligent algorithms for subsequent call transfer procedure based on the warning code received in the Warning header field along with the new SIP response code.



Name:                  draft-khatri-sipcore-call-transfer-fail-response

Revision:              00

Title:                      A SIP Response Code (497) for Call Transfer Failure

Document date:               2021-03-11

Group:                  Individual Submission

Pages:                   10

URL:            https://www.ietf.org/archive/id/draft-khatri-sipcore-call-transfer-fail-response-00.txt

Status:         https://datatracker.ietf.org/doc/draft-khatri-sipcore-call-transfer-fail-response/

Htmlized:       https://datatracker.ietf.org/doc/html/draft-khatri-sipcore-call-transfer-fail-response

Htmlized:       https://tools.ietf.org/html/draft-khatri-sipcore-call-transfer-fail-response-00