rfc:rfc8498
Internet Engineering Task Force (IETF) M. Mohali Request for Comments: 8498 Orange Updates: 5502 February 2019 Category: Informational ISSN: 2070-1721
A P-Served-User Header Field Parameter for an
Originating Call Diversion (CDIV) Session Case in the
Session Initiation Protocol (SIP)
Abstract
The P-Served-User header field was defined based on a requirement
from the 3rd Generation Partnership Project (3GPP) IMS (IP Multimedia
Subsystem) in order to convey the identity of the served user, his/
her registration state, and the session case that applies to that
particular communication session and application invocation. A
session case is metadata that captures the status of the session of a
served user regardless of whether or not the served user is
registered or the session originates or terminates with the served
user. This document updates RFC 5502 by defining a new P-Served-User
header field parameter, "orig-cdiv". The parameter conveys the
session case used by a proxy when handling an originating session
after Call Diversion (CDIV) services have been invoked for the served
user. This document also fixes the ABNF in RFC 5502 and provides
more guidance for using the P-Served-User header field in IP
networks.
Status of This Memo
This document is not an Internet Standards Track specification; it is published for informational purposes.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are candidates for any level of Internet Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8498.
Mohali Informational [Page 1] RFC 8498 P-Served-User Parameter for CDIV in SIP February 2019
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2. Basic Use Case . . . . . . . . . . . . . . . . . . . . . 4 1.3. Problem Statement . . . . . . . . . . . . . . . . . . . . 5 2. Conventions and Terminology . . . . . . . . . . . . . . . . . 5 3. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 6 4. Proxy Behavior and Parameter Handling . . . . . . . . . . . . 6 5. Clarification of RFC 5502 Procedures . . . . . . . . . . . . 7 6. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 6.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 7 6.2. ABNF . . . . . . . . . . . . . . . . . . . . . . . . . . 8 7. Call Flow Examples . . . . . . . . . . . . . . . . . . . . . 9 7.1. Call Diversion Case . . . . . . . . . . . . . . . . . . . 9 7.2. Call Diversion and Privacy . . . . . . . . . . . . . . . 11 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 9. Security Considerations . . . . . . . . . . . . . . . . . . . 13 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 14 10.1. Normative References . . . . . . . . . . . . . . . . . . 14 10.2. Informative References . . . . . . . . . . . . . . . . . 15 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 15 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction
1.1. General
The P-Served-User header field [RFC5502] was defined based on a requirement from the 3rd Generation Partnership Project (3GPP) IMS (IP Multimedia Subsystem) in order to convey the identity of the served user, his/her registration state, and the session case between a Serving Call Session Control Function (S-CSCF) and an Application Server (AS) on the IMS Service Control (ISC) interface. A session case is metadata that captures the status of the session of a served user regardless of whether or not the served user is registered or the session originates or terminates with the served user. For more information on session cases and the IMS, a detailed description can be found in [TS.3GPP.24.229].
[RFC5502] defines the originating and terminating session cases for a registered or unregistered user. This document extends the P-Served- User header field to include the session case for a forwarded leg when both a CDIV service has been invoked and an originating service of the diverting user has to be triggered.
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The sessioncase-param parameter of the P-Served-User header field is
extended with the "orig-cdiv" parameter for this originating-after-
CDIV session case.
The following section defines usage of the "orig-cdiv" parameter of the P-Served-User header field, Section 3 discusses the applicability and scope of this new header field parameter, and Section 4 specifies the proxy behavior for handling the new header field parameter. Section 5 clarifies some of the [RFC5502] procedures, Section 6 describes the extended syntax and corrects the syntax of [RFC5502], Section 7 gives some call flow examples, Section 8 registers the P-Served-User header field parameters with IANA, and Section 9 discusses the security properties of the environment where this new header field parameter is intended to be used.
1.2. Basic Use Case
In the 3GPP IMS (IP Multimedia Subsystem), the S-CSCF (Serving CSCF) is a SIP proxy that serves as a registrar and handles originating and terminating session states for users assigned to it. This means that any call that is originated by a specific user or any call that is terminated to that specific user will pass through the S-CSCF that is assigned to that user.
At the moment that an S-CSCF is assigned to a specific user, the user profile is downloaded from the Home Subscriber Server (HSS) to that S-CSCF; see [TS.3GPP.29.228]. The user profile contains the list of actions to be taken by the S-CSCF for the served user depending on the session direction (originating or terminating) and the user state (registered or not) in the IMS network. With this user profile, the S-CSCF determines the current case and applies the corresponding actions such as forwarding the request to an AS. The AS then goes through a similar process of determining who is the current served user, what is his/her "registration state", and what is the "session case" of the session. [RFC5502] defines all those parameters and in particular the originating and terminating session cases.
In basic call scenarios, there is no particular issue for the S-CSCF and AS to know which scenario needs to be realized, but in case of CDIV services for which the session is re-targeted, the session cases defined in [RFC5502] pose some limitations as described in the following section.
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1.3. Problem Statement
To illustrate the problem statement, let's imagine Alice trying to call Bob and Bob having a CDIV service activated towards Carol's address. In the case of a CDIV service, the received request is first treated as a terminating session case (at Bob's side), and the terminating filter criteria configured in the S-CSCF is performed. A filter criteria is information in the user profile that determines whether an initial request is sent to a particular AS. When the AS receives the call initiation request, the AS is able to determine the served user (Bob) and the session case (here "term") from the received P-Served-User header field content and is able to execute terminating services. When the CDIV service is executed (as a terminating service of Bob), the AS changes the target (Request-URI) of the session (toward Carol's address) and a new call leg is created. The served user becomes the diverting user. This new call leg could be considered as an originating call leg from the diverting user (Bob), but this is not the case. Indeed, the originating user remains the same (Alice), and some of the diverting user's originating services should not be triggered as if it was an originating call. For instance, the originating user identity (Alice) should not be restricted because the diverting user (Bob) has a privacy service for his own identity. The privacy of the diverting user should apply to information related to this user only (e.g., in the History-Info header field). In the same manner, some specific services will need to be triggered on the outgoing leg after a CDIV. Without a dedicated session case for originating-after-CDIV, the S-CSCF cannot trigger an originating service for the diverting user, nor can an AS execute the procedures for this particular session case.
For this use case, this document creates a new parameter
("orig-cdiv") for the originating-after-CDIV session case to be
embedded in the P-Served-User header field.
2. Conventions and Terminology
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.
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3. Applicability
The use of the P-Served-User header field extensions is only applicable inside a Trust Domain [RFC3324] for the P-Served-User header field. Nodes in such a Trust Domain explicitly trust each other to convey the served user and to be responsible for withholding that information outside of the Trust Domain. The means by which the network determines the served user and the policies that are executed for a specific served user is outside the scope of this document.
4. Proxy Behavior and Parameter Handling
The following section illustrates how this header field parameter can be used in a 3GPP network.
For a terminating call, the following steps will be followed:
1. The S-CSCF receives the initial INVITE request for a terminating
call and determines that the session case is for a terminating
user as described in [RFC5502].
2. The S-CSCF determines who is the served user by looking at the
Request-URI and saves the current Request-URI.
3. The S-CSCF analyzes the filter criteria. It then sends the
request to the AS of the served user as an INVITE that includes
the P-Served-User header field with the "sescase" parameter set
to "term" and the "regstate" set to the corresponding value in
order to trigger execution of terminating services.
4. Based on some criteria, the AS concludes that the request has to
be diverted to another target user or application. The AS
replaces the received Request-URI with the new diverted-to
address and stores the successive Request-URI(s) values by adding
one or two History-Info header field entry(ies) [RFC7044] in the
outgoing INVITE. In the History-Info header field, the served
user address is tagged by using the mp-param header field
parameter added in the newly created entry that contains the
diverted-to address. The AS forwards the INVITE request back to
the S-CSCF.
5. When receiving back the INVITE request, the S-CSCF can see that
the topmost Route header field contains its own hostname, but the
Request-URI does not match the saved Request-URI. In this case,
the S-CSCF updates the P-Served-User header field content by
replacing the "sescase" parameter with the "orig-cdiv" parameter.
The P-Served-User header field value remains unchanged.
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6. The S-CSCF forwards the INVITE request to an AS that hosts the
served user's (diverting user's) originating services, which need
to be executed on the forwarded leg after a CDIV service.
7. When the AS receives the INVITE request, it determines that the
session case is for the "orig-cdiv" session case and performs the
originating services to be executed after retargeting for the
diverting user (i.e., served user).
5. Clarification of RFC 5502 Procedures
This document provides the following guidance for the handling of the P-Served-User header field that is missing in [RFC5502]:
o The P-Served-User header field MUST NOT be repeated within a
request for a particular session at a particular time for the
reason that session cases are mutually exclusive. This document
updates [RFC5502] to clearly state that the P-Served-User header
field MUST NOT contain multiple values either comma-separated or
header-separated. This document also updates the syntax of the
header from [RFC5502] to reflect this uniqueness of parameter
values.
o [RFC5502] does not clearly state what to do with the received
P-Served-User header field when a call is diverted to another
destination. This document highlights that there are several ways
of handling the P-Served-User header field: the S-CSCF could store
the previous "regstate" value and decide that the same value
applies, the "regstate" may no longer be relevant after a
diverting service so the S-CSCF removes it, or the "regstate"
could be combined with the "orig-cdiv" session case to provide
different services depending on whether the served user is
registered or unregistered. These choices are implementation
dependent.
6. Syntax
6.1. General
[RFC5502] defines the P-Served-User header field with the sessioncase-param parameter "sescase", which is specified as having "orig" and "term" as predefined values. This document defines an additional parameter, "orig-cdiv", for the sessioncase-param.
Because this document extends the existing sessioncase-param parameter, and because errors have been identified in the syntax, this document corrects and extends the P-Served-User header field.
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The extension of the sessioncase-param parameter to add the "orig-cdiv" session case is done in a way that fits the parameter format introduced in Release 11 of the 3GPP [TS.3GPP.24.229] and maintains backward compatibility.
"EQUAL", "HCOLON", "SEMI", "name-addr", "addr-spec", and "generic-param" are defined in [RFC3261].
If the "addr-spec" contains a comma, question mark, or semicolon, the "name-addr" form MUST be used. The "name-addr" form requires the use of angle brackets (< and >).
6.2. ABNF
The Augmented Backus-Naur Form (ABNF) [RFC5234] syntax of the P-Served-User header field is described in [RFC5502].
This document updates [RFC5502] to correct the P-Served-User header field ABNF syntax and extend it as the following:
P-Served-User = "P-Served-User" HCOLON PServedUser-value
*(SEMI served-user-param)
served-user-param = sessioncase-param
/ registration-state-param
/ generic-param
PServedUser-value = name-addr / addr-spec
sessioncase-param = "sescase" EQUAL ("orig"/"term")/ orig-cdiv
registration-state-param = "regstate" EQUAL ("unreg" / "reg")
orig-cdiv = "orig-cdiv"
Examples of possible P-Served-User header fields:
P-Served-User: <sip:user@example.com>; orig-cdiv; regstate=reg or P-Served-User: <sip:user@example.com>; orig-cdiv or P-Served-User: <sip:user@example.com>; sescase=term; regstate=unreg
This document allows choosing between "addr-spec" and "name-addr" when constructing the header field value. As specified in RFC 8217, the "addr-spec" form MUST NOT be used if its value would contain a comma, semicolon, or question mark [RFC8217].
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7. Call Flow Examples
7.1. Call Diversion Case
The following call flow shows a session establishment when Alice calls Bob, who has a CDIV service that diverts to Carol when Bob is busy.
proxy server UA
Alice Bob's…S-CSCF-B……….AS-B………….Bob Carol
| | | | | | INVITE F1 | | | | |--------------->| INVITE F2 | | | | |--------------->| | | | | INVITE F3 | | | | |<---------------| INVITE F4 | | | |-------------------------------->| | | | 486 F5 | | | |<--------------------------------| | | | 486 F6 | | | | |--------------->| | | | | INVITE F7 | | | | |<---------------| | | | | INVITE F8 | | | | |--------------->| | | | | INVITE F9 | | | | |<---------------| INVITE F10 | | |------------------------------------------------->| | | | | | | | | | 180 F11 | | | | 180 F12 |<---------------| | | 180 F13 |<---------------| | | 180 F14 |<---------------| | | |<---------------| | | | | | | | |
[Alice calls Bob]
F1 INVITE Alice -> S-CSCF-B
INVITE sip:bob@example.com SIP/2.0
From: Alice <sip:alice@domaina.com>;tag=1928301774
To: Bob <sip:bob@example.com>
F2 INVITE S-CSCF-B -> AS-B
INVITE sip:bob@example.com SIP/2.0
From: Alice <sip:alice@domaina.com>;tag=1928301774
To: Bob <sip:bob@example.com>
P-Served-User: <sip:bob@example.com>; term; regstate=reg
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F3 INVITE AS-B -> S-CSCF-B
INVITE sip:bob@example.com SIP/2.0
From: Alice <sip:alice@domaina.com>;tag=1928301774
To: Bob <sip:bob@example.com>
P-Served-User: <sip:bob@example.com>; term; regstate=reg
F4 INVITE S-CSCF-B -> Bob
INVITE sip:bob@192.0.2.4 SIP/2.0
From: Alice <sip:alice@domaina.com>;tag=1928301774
To: Bob <sip:bob@example.com>
P-Served-User: <sip:bob@example.com>; term; regstate=reg
[Bob is busy. His CDIV when busy is invoked towards Carol]
F5-F6 486 BUSY Bob -> S-CSCF-B -> AS-B
486 BUSY
From: Alice <sip:alice@domaina.com>;tag=1928301774
To: Bob <sip:bob@example.com>;tag=es43sd
[Alice's call is diverted to Carol]
F7 INVITE AS-B -> S-CSCF-B
INVITE sip:carol@domainc.com SIP/2.0
From: Alice <sip:alice@domaina.com>;tag=1928301774
To: Bob <sip:bob@example.com>
P-Served-User: <sip:bob@example.com>; term; regstate=reg
[The forwarded leg to Carol is identified as an originating call after CDIV, which should not trigger all of Bob's originating services]
F8 INVITE S-CSCF-B -> AS-B
INVITE sip:carol@domainc.com SIP/2.0
From: Alice <sip:alice@domaina.com>;tag=1928301774
To: Bob <sip:bob@example.com>
P-Served-User: <sip:bob@example.com>; orig-cdiv; regstate=reg
F9 INVITE AS-B -> S-CSCF-B
INVITE sip:carol@domainc.com SIP/2.0
From: Alice <sip:alice@domaina.com>;tag=1928301774
To: Bob <sip:bob@example.com>
P-Served-User: <sip:bob@example.com>; orig-cdiv; regstate=reg
F10 INVITE S-CSCF-B -> Carol
INVITE sip:carol@192.0.2.7 SIP/2.0
From: Alice <sip:alice@domaina.com>;tag=1928301774
To: Bob <sip:bob@example.com>
Figure 1. P-Served-User During CDIV Service
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7.2. Call Diversion and Privacy
The following call flow shows a CDIV use case for which Alice has no identity restriction service and Bob has an unconditional CDIV service towards Carol and an identity presentation restriction service.
proxy server UA
Alice Bob's…S-CSCF-B……….AS-B………….Bob Carol
| | | | | | INVITE F1 | | | | |--------------->| INVITE F2 | | | | |--------------->| | | | | INVITE F3 | | | | |<---------------| | | | | INVITE F4 | | | | |--------------->| | | | | INVITE F5 | | | | |<---------------| INVITE F6 | | | |------------------------------------------------->| | | | | | | | | | 180 F7 | | | | 180 F8 |<---------------| | | 180 F9 |<---------------| | | 180 F10 |<---------------| | | |<---------------| | | | | | | | |
[Alice calls Bob]
F1 INVITE Alice -> S-CSCF-B
INVITE sip:bob@example.com SIP/2.0
From: Alice <sip:alice@domaina.com>;tag=1928301774
To: Bob <sip:bob@example.com>
Supported: histinfo
F2 INVITE S-CSCF-B -> AS-B
INVITE sip:bob@example.com SIP/2.0
From: Alice <sip:alice@domaina.com>;tag=1928301774
To: Bob <sip:bob@example.com>
P-Served-User: <sip:bob@example.com>; term; regstate=reg
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[Bob's unconditional CDIV to Carol is triggered]
F3 INVITE AS-B -> S-CSCF-B
INVITE sip:carol@domainc.com SIP/2.0
From: Alice <sip:alice@domaina.com>;tag=1928301774
To: Carol <sip:carol@domainc.com>
P-Served-User: <sip:bob@example.com>; term; regstate=reg
History-Info:
<sip:bob@example.com>;index=1,
<sip:carol@domainc.com;cause=302>;index=1.1;mp=1
[Alice's call is diverted to Carol]
F4 INVITE S-CSCF-B -> AS-B
INVITE sip:carol@domainc.com SIP/2.0
From: Alice <sip:alice@domaina.com>;tag=1928301774
To: Carol <sip:carol@domainc.com>
P-Served-User: <sip:bob@example.com>; orig-cdiv; regstate=reg
History-Info:
<sip:bob@example.com>;index=1,
<sip:carol@domainc.com;cause=302>;index=1.1;mp=1
F5 INVITE AS-B -> S-CSCF-B
INVITE sip:carol@domainc.com SIP/2.0
From: Alice <sip:alice@domaina.com>;tag=1928301774
To: Carol <sip:carol@domainc.com>
P-Served-User: <sip:bob@example.com>; orig-cdiv; regstate=reg
History-Info:
<sip:bob@example.com?privacy=history>;index=1,
<sip:carol@domainc.com;cause=302>;index=1.1;mp=1
[Forwarded leg to Carol is identified as an originating call after CDIV that allows Bob's privacy service to be applied to his identity within the History-Info header field]
F6 INVITE S-CSCF-B -> Carol
INVITE sip:carol@192.0.2.7 SIP/2.0
From: Alice <sip:alice@domaina.com>;tag=1928301774
To: Carol <sip:carol@domainc.com>
History-Info:
<sip:bob@example.com?privacy=history>;index=1,
<sip:carol@domainc.com;cause=302>;index=1.1;mp=1
<sip:carol@192.0.2.7>;index=1.1.1;rc=1.1
Figure 2. P-Served-User When Privacy Requested
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8. IANA Considerations
The syntax of the P-Served-User header field [RFC5502] is updated in Section 4 of this document.
IANA has updated the existing row for the P-Served-User header field in the "Header Fields" subregistry within the "Session Initiation Protocol (SIP) Parameters" registry:
Header Name Compact Form Reference
------------- ------------ ------------------
P-Served-User none [RFC5502][RFC8498]
IANA has added new rows for the P-Served-User header field parameters in the "Header Field Parameters and Parameter Values" subregistry within the "Session Initiation Protocol (SIP) Parameters" registry (as per the registry created by [RFC3968]):
Header Field Parameter Name Predefined Values Reference -------------- ---------------- ----------------- ------------- P-Served-User sescase Yes [RFC5502] P-Served-User regstate Yes [RFC5502] P-Served-User orig-cdiv No [RFC8498]
9. Security Considerations
The security considerations in [RFC5502] apply.
As the "orig-cdiv" parameter of the P-Served-User header field can be used to trigger applications when a call is diverted, it is important to ensure that the parameter has not been added to the SIP message by an unauthorized SIP entity. Thus, the P-Served-User header field is to be used in a trusted environment, and proxies MUST NOT insert the header unless they have sufficient knowledge that the route set includes another trusted proxy.
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10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3324] Watson, M., "Short Term Requirements for Network Asserted
Identity", RFC 3324, DOI 10.17487/RFC3324, November 2002,
<https://www.rfc-editor.org/info/rfc3324>.
[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,
<https://www.rfc-editor.org/info/rfc3261>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8217] Sparks, R., "Clarifications for When to Use the name-addr
Production in SIP Messages", RFC 8217,
DOI 10.17487/RFC8217, August 2017,
<https://www.rfc-editor.org/info/rfc8217>.
[RFC3968] Camarillo, G., "The Internet Assigned Number Authority
(IANA) Header Field Parameter Registry for the Session
Initiation Protocol (SIP)", BCP 98, RFC 3968,
DOI 10.17487/RFC3968, December 2004,
<https://www.rfc-editor.org/info/rfc3968>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC7044] Barnes, M., Audet, F., Schubert, S., van Elburg, J., and
C. Holmberg, "An Extension to the Session Initiation
Protocol (SIP) for Request History Information", RFC 7044,
DOI 10.17487/RFC7044, February 2014,
<https://www.rfc-editor.org/info/rfc7044>.
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[RFC5502] van Elburg, J., "The SIP P-Served-User Private-Header
(P-Header) for the 3GPP IP Multimedia (IM) Core Network
(CN) Subsystem", RFC 5502, DOI 10.17487/RFC5502, April
2009, <https://www.rfc-editor.org/info/rfc5502>.
10.2. Informative References
[TS.3GPP.24.229]
3GPP, "IP multimedia call control protocol based on
Session Initiation Protocol (SIP) and Session Description
Protocol (SDP);Stage 3", 3GPP TS 24.229 11.28.0, December
2018.
[TS.3GPP.29.228]
3GPP, "IP Multimedia (IM) Subsystem Cx and Dx interfaces;
Signalling flows and message contents", 3GPP TS 29.228
15.1.0, September 2018.
Acknowledgments
The author wishes to thank the 3GPP community for providing guidance, input, and comments on the document. Thanks to Dale Worley, Jean Mahoney, and Ben Campbell for their careful review of the document. Thanks to Paul Kyzivat and Adam Roach. A special thanks to Christer Holmberg.
Author's Address
Marianne Mohali Orange Orange Gardens, 44 avenue de la Republique Chatillon 92326 France
Email: marianne.mohali@orange.com
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