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RFC 7315

Private Header (P-Header) Extensions to the Session Initiation Protocol (SIP) for the 3GPP

Pages: 43
Informational
Errata
Obsoletes:  3455
Updated by:  79137976
Part 1 of 2 – Pages 1 to 28
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Top   ToC   RFC7315 - Page 1
Internet Engineering Task Force (IETF)                         R. Jesske
Request for Comments: 7315                              Deutsche Telekom
Obsoletes: 3455                                                 K. Drage
Category: Informational                                   Alcatel-Lucent
ISSN: 2070-1721                                              C. Holmberg
                                                                Ericsson
                                                               July 2014


                  Private Header (P-Header) Extensions
         to the Session Initiation Protocol (SIP) for the 3GPP

Abstract

This document describes a set of private header (P-header) Session Initiation Protocol (SIP) fields used by the 3GPP, along with their applicability, which is limited to particular environments. The P-header fields are used for a variety of purposes within the networks that the partners implement, including charging and information about the networks a call traverses. This document obsoletes RFC 3455. 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 a candidate for any level of Internet Standard; see Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7315.
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Copyright Notice

   Copyright (c) 2014 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
   (http://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.

Table of Contents

1. Overall Applicability ...........................................3 2. Conventions .....................................................3 3. Overview ........................................................3 4. SIP Private Header Fields .......................................4 4.1. The P-Associated-URI Header Field ..........................4 4.1.1. Applicability Statement for the P-Associated-URI Header Field .......................5 4.1.2. Usage of the P-Associated-URI Header Field ..........5 4.2. The P-Called-Party-ID Header Field .........................6 4.2.1. Applicability Statement for the P-Called-Party-ID Header Field .....................10 4.2.2. Usage of the P-Called-Party-ID Header Field ........11 4.3. The P-Visited-Network-ID Header Field .....................12 4.3.1. Applicability Statement for the P-Visited-Network-ID Header Field ..................12 4.3.2. Usage of the P-Visited-Network-ID Header Field .....13 4.4. The P-Access-Network-Info Header Field ....................17 4.4.1. Applicability Statement for the P-Access-Network-Info Header Field .................18 4.4.2. Usage of the P-Access-Network-Info Header ..........18 4.5. The P-Charging-Function-Addresses Header Field ............19 4.5.1. Applicability Statement for the P-Charging-Function-Addresses Header Field .........20 4.5.2. Usage of the P-Charging-Function-Addresses Header Field .......................................21 4.6. The P-Charging-Vector Header Field ........................23 4.6.1. Applicability Statement for the P-Charging-Vector Header Field .....................25 4.6.2. Usage of the P-Charging-Vector Header Field ........25 4.6.3. Usage of the transit-ioi ...........................27 4.6.4. Usage of the related-icid ..........................28
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   5. Formal Syntax ..................................................28
      5.1. P-Associated-URI Header Syntax ............................29
      5.2. P-Called-Party-ID Header Syntax ...........................29
      5.3. P-Visited-Network-ID Header Syntax ........................29
      5.4. P-Access-Network-Info Header Syntax .......................29
      5.5. P-Charging-Function-Addresses Header Syntax ...............31
      5.6. P-Charging-Vector Header Syntax ...........................32
      5.7. New Headers ...............................................33
   6. Security Considerations ........................................33
      6.1. P-Associated-URI Header Field .............................33
      6.2. P-Called-Party-ID Header Field ............................34
      6.3. P-Visited-Network-ID Header Field .........................34
      6.4. P-Access-Network-Info Header Field ........................35
      6.5. P-Charging-Function-Addresses Header Field ................36
      6.6. P-Charging-Vector Header Field ............................36
   7. IANA Considerations ............................................37
   8. Contributors and Acknowledgements ..............................38
   9. References .....................................................39
      9.1. Normative References ......................................39
      9.2. Informative References ....................................39
   Appendix A. Changes from RFC 3455 .................................41

1. Overall Applicability

The SIP extensions specified in this document make certain assumptions regarding network topology, linkage between SIP and lower layers, and the availability of transitive trust. These assumptions apply only to private networks and are not appropriate for use in an Internet environment. The mechanisms specified here were designed to satisfy the requirements specified in the 3GPP Release 5 requirements on SIP [RFC4083] for which either no general-purpose solution was planned (where insufficient operational experience was available to understand if a general solution would be needed) or for which a more general solution is not yet mature. For more details about the assumptions made about these extensions, consult the Applicability subsection for each extension.

2. Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

3. Overview

The 3GPP uses SIP as the protocol to establish and tear down multimedia sessions in the context of its IP Multimedia Subsystem (IMS), as described in the 3GPP TS 23.228 [TS23.228] and 3GPP TS
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   24.229 [TS24.229].  RFC 3455 [RFC3455] defines SIP private header
   extensions (referred to as P-headers) that are required by the 3GPP
   specification.  Note that the requirements for these extensions are
   documented in RFC 4083 [RFC4083].  This document obsoletes RFC 3455
   [RFC3455].  This document updates existing P-header descriptions to
   address additional requirements that are needed for 3GPP Release 11.
   Each of the P-headers is described in the sections below.

4. SIP Private Header Fields

4.1. The P-Associated-URI Header Field

This extension allows a registrar to return a set of associated URIs for a registered SIP address-of-record. We define the P-Associated- URI header field, used in the 200 (OK) response to a REGISTER request. The P-Associated-URI header field contains the set of associated URIs that are associated with the registered address-of- record. In addition to the address-of-record, an associated URI is a URI that the service provider has allocated to a user. A registrar contains information that allows zero or more URIs to be associated with an address-of-record. Usually, all these URIs (the address-of-record and the associated URIs) are allocated for the usage of a particular user. This extension to SIP allows the User Agent Client (UAC) to know, upon a successful authenticated registration, which other URIs, if any, the service provider has associated with an address-of-record URI. Note that, in standard SIP usage [RFC3261], the registrar does not register the associated URIs on behalf of the user. Only the address-of-record that is present in the To header field of the REGISTER is registered and bound to the contact address. The only information conveyed is that the registrar is aware of other URIs that can be used by the same user. A situation may be possible, however, in which an application server (or even the registrar itself) registers any of the associated URIs on behalf of the user by means of a third-party registration. However, this third-party registration is out of the scope of this document. A UAC MUST NOT assume that the associated URIs are registered. If a UAC wants to check whether any of the associated URIs is registered, it can do so by mechanisms specified outside this document, e.g., the UA MAY send a REGISTER request with the To header field value set to any of the associated URIs and without a Contact header field. The 200 (OK) response will include a Contact header
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   field with the list of addresses-of-record that have been registered
   with contact addresses.  If the associated URI is not registered, the
   UA MAY register it prior to its utilization.

4.1.1. Applicability Statement for the P-Associated-URI Header Field

The P-Associated-URI header field is applicable in SIP networks where the SIP provider allows a set of identities that a user can claim (in header fields like the From header field) in requests that the UA generates. Furthermore, it assumes that the provider knows the entire set of identities that a user can legitimately claim and that the user is willing to restrict its claimed identities to that set. This is in contrast to normal SIP usage, where the From header field is explicitly an end-user-specified field.

4.1.2. Usage of the P-Associated-URI Header Field

The registrar inserts the P-Associated-URI header field into the 200 (OK) response to a REGISTER request. The header field value is populated with a list of URIs that are associated to the address-of- record. If the registrar supports the P-Associated-URI header field extension and there is at least one associated URI, then the registrar MUST insert the P-Associated-URI header field in all the 200 (OK) responses to a REGISTER request. The absence of a P-Associated-URI header field indicates that there are no associated URIs for the registered address-of-record.
4.1.2.1. Procedures at the UA
A UAC may receive a P-Associated-URI header field in the 200 (OK) response for a REGISTER request. The presence of a header field in the 200 (OK) response for a REGISTER request implies that the extension is supported at the registrar. The header field value contains a list of one or more associated URIs to the address-of-record. The UAC MAY use any of the associated URIs to populate the From header field value, or any other SIP header field value that provides information of the identity of the calling party, in a subsequent request. The UAC MAY check whether or not the associated URI is registered. This check can be done, e.g., by populating the To header field value in a REGISTER request sent to the registrar and without a Contact header field. The 200 (OK) response will include a Contact header field with the list of address-of-record that have been registered
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   with contact addresses.  As described in SIP [RFC3261], the 200 (OK)
   response may contain a Contact header field with zero or more values
   (zero meaning the address-of-record is not registered).

4.1.2.2. Procedures at the Registrar
A registrar that receives and authorizes a REGISTER request MAY associate zero or more URIs with the registered address-of-record. If the address-of-record under registration does not have any associated URIs, the P-Associated-URI header field SHALL NOT be included. Otherwise, a registrar that supports this specification MUST include a P-Associated-URI header field in the 200 (OK) response to a REGISTER request that contains a contact header. The header field MUST be populated with a comma-separated list of URIs that are associated to the address-of-record under registration.
4.1.2.3. Procedures at the Proxy
This header is not intended to be used by proxies -- a proxy does not add, read, modify, or delete the header field; therefore, any proxy MUST relay this header field unchanged.

4.2. The P-Called-Party-ID Header Field

A proxy server inserts a P-Called-Party-ID header field, typically in an INVITE request, en route to its destination. The header is populated with the Request-URI received by the proxy in the request. The User Agent Server (UAS) identifies to which address-of-record, out of several registered addresses-of-record, the invitation was sent (for example, the user may be simultaneously using one personal SIP URI and one business SIP URI to receive invitation to sessions). The UAS can use the information to render different distinctive audiovisual alerting tones, depending on the URI used to receive the invitation to the session. Users in the 3GPP IP Multimedia Subsystem (IMS) may get one or several SIP URIs (address-of-record) to identify the user. For example, a user may get one business SIP URI and one personal SIP URI. As an example of utilization, the user may make available the business SIP URI to coworkers and may make available the personal SIP URI to members of the family.
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   At a certain point in time, both the business SIP URI and the
   personal SIP URI are registered in the SIP registrar, so both URIs
   can receive invitations to new sessions.  When the user receives an
   invitation to join a session, he/she should be aware of which of the
   registered SIP URIs this session was sent to.

   This requirement is stated in the 3GPP Release 5 requirements on SIP
   [RFC4083].

   The problem arises during the terminating side of a session
   establishment.  At that time, the SIP proxy that is serving a UA gets
   an INVITE request, and the SIP server retargets the SIP URI that is
   present in the Request-URI, and replaces that SIP URI with the SIP
   URI published by the user in the Contact header field of the REGISTER
   request at registration time.

   One can argue that the To header field conveys the semantics of the
   called user, and therefore, this extension to SIP is not needed.
   Although the To header field in SIP may convey the called party ID in
   most situations, there are two particular cases when the above
   assumption is not correct:

   1.  The session has been forwarded, redirected, etc., by previous SIP
       proxies, before arriving to the proxy that is serving the called
       user.

   2.  The UAC builds an INVITE request and the To header field is not
       the same as the Request-URI.

   The problem of using the To header field is that this field is
   populated by the UAC and not modified by proxies in the path.  If the
   UAC, for any reason, did not populate the To header field with the
   address-of-record of the destination user, then the destination user
   is not able to distinguish to which address-of-record the session was
   destined.

   Another possible solution to the problem is built upon the
   differentiation of the Contact header field value between different
   address-of-record at registration time.  The UA can differentiate
   each address-of-record it registers by assigning a different Contact
   header field value.  For example, when the UA registers the address-
   of-record sip:id1, the Contact header field value can be sip:id1@ua,
   while the registration of the address-of-record sip:id2 can be bound
   to the Contact header field value sip:id2@ua.

   The solution described above assumes that the UA explicitly registers
   each of its addresses-of-record, and therefore, it has full control
   over the contact address values assigned to each registration.
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   However, if the UA does not have full control of its registered
   addresses-of-record, because of, e.g., a third-party registration,
   the solution does not work.  This may be the case of the 3GPP
   registration, where the UA may have previously indicated to the
   network, by means outside of SIP, that some other addresses-of-record
   may be automatically registered when the UA registers a particular
   address-of-record.  The requirement is covered in the 3GPP Release 5
   requirements on SIP [RFC4083].

   In the next paragraphs, we show an example of the problem, in the
   case in which there has been some sort of call forwarding in the
   session, so that the UAC is not aware of the intended destination URI
   in the current INVITE request.

   We assume that a UA is registering to its proxy (P1).

     Scenario                      UA --- P1

         F1 Register UA -> P1
              REGISTER sip:example.com SIP/2.0
              Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
              To: sip:user1-business@example.com
              From: sip:user1-business@example.com;tag=456248
              Call-ID: 843817637684230998sdasdh09
              CSeq: 1826 REGISTER
              Contact: <sip:user1@192.0.2.4>

   The user also registers his personal URI to his/her registrar.

         F2 Register UA -> P1
              REGISTER sip:example.com SIP/2.0
              Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashdt8
              To: sip:user1-personal@example.com
              From: sip:user1-personal@example.com;tag=346249
              Call-ID: 2Q3817637684230998sdasdh10
              CSeq: 1827 REGISTER
              Contact: <sip:user1@192.0.2.4>

   Later, the proxy/registrar (P1) receives an INVITE request from
   another proxy (P2) destined to the user's business SIP address-of-
   record.  We assume that this INVITE request has undergone some sort
   of forwarding in the past, and as such, the To header field is not
   populated with the SIP URI of the user.  In this case, we assume that
   the session was initially addressed to
   sip:other-user@othernetwork.com.  The SIP server at othernetwork.com
   has forwarded this session to sip:user1-business@example.com.
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            Scenario                      UA --- P1 --- P2

         F3 Invite P2 -> P1
              INVITE sip:user1-business@example.com SIP/2.0
              Via: SIP/2.0/UDP 192.0.2.20:5060;branch=z9hG4bK03djaoe1
              To: sip:other-user@othernetwork.com
              From: sip:another-user@anothernetwork.com;tag=938s0
              Call-ID: 843817637684230998sdasdh09
              CSeq: 101 INVITE

   The proxy P1 retargets the user and replaces the Request-URI with the
   SIP URI published during registration time in the Contact header
   field value.

         F4 Invite P1 -> UA
              INVITE sip:user1@192.0.2.4 SIP/2.0
              Via: SIP/2.0/UDP 192.0.2.10:5060;branch=z9hG4bKg48sh128
              Via: SIP/2.0/UDP 192.0.2.20:5060;branch=z9hG4bK03djaoe1
              To: sip:other-user@othernetwork.com
              From: sip:another-user@anothernetwork.com;tag=938s0
              Call-ID: 843817637684230998sdasdh09
              CSeq: 101 INVITE

   When the UAS receives the INVITE request, it cannot determine whether
   it got the session invitation due to his registration of the business
   or the personal address-of-record.  Neither the UAS nor proxies /
   application servers can provide this user a service based on the
   destination address-of-record of the session.

   We solve this problem by allowing the proxy that is responsible for
   the home domain (as defined in SIP) of the user to insert a P-Called-
   Party-ID header field that identifies the address-of-record to which
   this session is destined.

   If this SIP extension is used, the proxy serving the called user will
   get the message flow F5, it will populate the P-Called-Party-ID
   header field in message flow F6 with the contents of the Request-URI
   in F4.  This is show in flows F5 and F6 below:
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         F5 Invite P2 -> P1
              INVITE sip:user1-business@example.com SIP/2.0
              Via: SIP/2.0/UDP 192.0.2.20:5060;branch=z9hG4bK03djaoe1
              To: sip:other-user@othernetwork.com
              From: sip:another-user@anothernetwork.com;tag=938s0
              Call-ID: 843817637684230998sdasdh09
              CSeq: 101 INVITE

         F6 Invite P1 -> UA
              INVITE sip:user1@192.0.2.4 SIP/2.0
              Via: SIP/2.0/UDP 192.0.2.10:5060;branch=z9hG4bKg48sh128
              Via: SIP/2.0/UDP 192.0.2.20:5060;branch=z9hG4bK03djaoe1
              To: sip:other-user@othernetwork.com
              From: sip:another-user@anothernetwork.com;tag=938s0
              Call-ID: 843817637684230998sdasdh09
              P-Called-Party-ID: <sip:user1-business@example.com>
              CSeq: 101 INVITE

   When the UA receives the INVITE request F6, it can determine the
   intended address-of-record of the session and apply whatever service
   is needed for that address-of-record.

4.2.1. Applicability Statement for the P-Called-Party-ID Header Field

The P-Called-Party-ID header field is applicable when the UAS needs to be aware of the intended address-of-record that was present in the Request-URI of the request, before the proxy retargets to the contact address. The UAS may be interested in applying different audiovisual alerting effects or other filtering services, depending on the intended destination of the request. It is especially valuable when the UAS has registered several addresses-of-record to his registrar, and therefore, the UAS is not aware of the address-of-record that was present in the INVITE request when it hit his proxy/registrar, unless this extension is used. P-Called-Party-ID header field and the History-Info header field: At the time RFC 3455 [RFC3455] was written, the History-Info header field was a long way from specification. This header has now been specified and approved in RFC 7044 [RFC7044]. It is acknowledged that the History-Info header field will provide equivalent coverage to that of the P-Called-Party-ID header field. However, the P-Called-Party-ID header field is used entirely within the 3GPP system and does not appear to SIP entities outside that of a single 3GPP operator.
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4.2.2. Usage of the P-Called-Party-ID Header Field

The P-Called-Party-ID header field provides proxies and the UAS with the address-of-record that was present in the Request-URI of the request, before a proxy retargets the request. This information is intended to be used by subsequent proxies in the path or by the UAS. Typically, a SIP proxy inserts the P-Called-Party-ID header field prior to retargetting the Request-URI in the SIP request. The header field value is populated with the contents of the Request-URI, prior to replacing it with the contact address.
4.2.2.1. Procedures at the UA
A UAC MUST NOT insert a P-Called-Party-ID header field in any SIP request or response. A UAS may receive a SIP request that contains a P-Called-Party-ID header field. The header field will be populated with the address- of-record received by the proxy in the Request-URI of the request, prior to its forwarding to the UAS. The UAS MAY use the value in the P-Called-Party-ID header field to provide services based on the called party URI, such as, e.g., filtering of calls depending on the date and time, distinctive presentation services, distinctive alerting tones, etc.
4.2.2.2. Procedures at the Proxy
A proxy that has access to the contact information of the user can insert a P-Called-Party-ID header field in any of the requests indicated in Section 5.7. When included, the proxy MUST populate the header field value with the contents of the Request-URI present in the SIP request that the proxy received. It is necessary that the proxy that inserts the P-Called-Party-ID header field has information about the user, in order to prevent a wrong delivery of the called party ID. This information may, for example, have been learned through a registration process. A proxy or application server that receives a request containing a P-Called-Party-ID header field MAY use the contents of the header field to provide a service to the user based on the URI of that header field value. A SIP proxy MUST NOT insert a P-Called-Party-ID header field in REGISTER requests.
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4.3. The P-Visited-Network-ID Header Field

3GPP networks are composed of a collection of so-called home networks, visited networks, and subscribers. A particular home network may have roaming agreements with one or more visited networks. The effect of this is that when a mobile terminal is roaming, it can use resources provided by the visited network in a transparent fashion. One of the conditions for a home network to accept the registration of a UA roaming to a particular visited network, is the existence of a roaming agreement between the home and the visited network. There is a need to indicate to the home network which network is the visited network that is providing services to the roaming UA. 3GPP user agents always register to the home network. The REGISTER request is proxied by one or more proxies located in the visited network towards the home network. For the sake of a simple approach, it seems sensible that the visited network includes an identification that is known to the home network. This identification should be globally unique, and it takes the form of a quoted-text string or a token. The home network may use this identification to verify the existence of a roaming agreement with the visited network, and to authorize the registration through that visited network. Note that P-Visited-Network-ID information reveals the location of the user, to the level of the coverage area of the visited network. For a national network, for example, P-Visited-Network-ID would reveal that the user is in the country in question.

4.3.1. Applicability Statement for the P-Visited-Network-ID Header Field

The P-Visited-Network-ID header field is applicable whenever the following circumstances are met: 1. There is transitive trust in intermediate proxies between the UA and the home network proxy via established relationships between the home network and the visited network, supported by the use of standard security mechanisms, e.g., IPsec, Authentication and Key Agreement (AKA), or Transport Layer Security (TLS). 2. An endpoint is using resources provided by one or more visited networks (a network to which the user does not have a direct business relationship). 3. A proxy that is located in one of the visited networks wants to be identified at the user's home network.
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   4.  There is no requirement that every visited network need be
       identified at the home network.  Those networks that want to be
       identified make use of this extension.  Those networks that do
       not want to be identified do nothing.

   5.  A commonly pre-agreed text string or token identifies the visited
       network at the home network.

   6.  The UAC sends a REGISTER request or dialog-initiating request
       (e.g., INVITE request) or a standalone request outside a dialog
       (e.g., OPTIONS request) to a proxy in a visited network.

   7.  The request traverses, en route to its destination, a first proxy
       located in the visited network and a second proxy located in the
       home network or its destination is the registrar in the home
       network.

   8.  The registrar or home proxy verifies and authorizes the usage of
       resources (e.g., proxies) in the visited network.

   The P-Visited-Network-ID header field assumes that there is trust
   relationship between a home network and one or more transited visited
   networks.  It is possible for other proxies between the proxy in the
   visited network that inserts the header, and the registrar or the
   home proxy, to modify the value of P-Visited-Network-ID header field.
   Therefore, intermediaries participating in this mechanism MUST apply
   a hop-by-hop integrity-protection mechanism such as IPsec or other
   available mechanisms in order to prevent such attacks.

4.3.2. Usage of the P-Visited-Network-ID Header Field

The P-Visited-Network-ID header field is used to convey to the registrar or home proxy in the home network the identifier of a visited network. The identifier is a text string or token that is known by both the registrar or the home proxy at the home network and the proxies in the visited network. Typically, the home network authorizes the UA to roam to a particular visited network. This action requires an existing roaming agreement between the home and the visited network. While it is possible for a home network to identify one or more visited networks by inspecting the domain name in the Via header fields, this approach has a heavy dependency on DNS. It is an option for a proxy to populate the Via header field with an IP address, for example, and in the absence of a reverse DNS entry, the IP address will not convey the desired information.
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   Any SIP proxy in the visited network that receives any of the
   requests indicated in Section 5.7 MAY insert a P-Visited-Network-ID
   header field when it forwards the request.  In case a REGISTER
   request or other request is traversing different administrative
   domains (e.g., different visited networks), a SIP proxy MAY insert a
   new P-Visited-Network-ID header field if the request does not contain
   a P-Visited-Network-ID header field with the same network identifier
   as its own network identifier (e.g., if the request has traversed
   other different administrative domains).

   Note also that, there is no requirement for this header field value
   to be readable in the proxies.  Therefore, a first proxy MAY insert
   an encrypted header field that only the registrar can decrypt.  If
   the request traverses a second proxy located in the same
   administrative domain as the first proxy, the second proxy may not be
   able to read the contents of the P-Visited-Network-ID header field.
   In this situation, the second proxy will consider that its visited
   network identifier is not already present in the value of the header
   field, and therefore, it will insert a new P-Visited-Network-ID
   header field value (hopefully with the same identifier that the first
   proxy inserted, although perhaps, not encrypted).  When the request
   arrives at the registrar or proxy in the home network, it will notice
   that the header field value is repeated (both the first and the
   second proxy inserted it).  The decrypted values should be the same,
   because both proxies where part of the same administrative domain.
   While this situation is not desirable, it does not create any harm at
   the registrar or proxy in the home network.

   The P-Visited-Network-ID header field is normally used at
   registration.  However, this extension does not preclude other
   usages.  For example, a proxy located in a visited network that does
   not maintain registration state MAY insert a P-Visited-Network-ID
   header field into any standalone request outside a dialog or a
   request that creates a dialog.  At the time of writing this document,
   the only requests that create dialogs are INVITE requests [RFC3261],
   SUBSCRIBE requests [RFC6665], and REFER requests [RFC3515].

   In order to avoid conflicts with identifiers, especially when the
   number of roaming agreements between networks increase, care must be
   taken when selecting the value of the P-Visited-Network-ID header
   field.  The identifier MUST be globally unique to avoid duplications.
   Although there are many mechanisms to create globally unique
   identifiers across networks, one such mechanism is already in
   operation, and that is DNS.  The P-Visited-Network-ID header field
   does not have any connection to DNS, but the values in the header
   field can be chosen from the DNS entry representing the domain name
   of the network.  This guarantees the uniqueness of the value.
Top   ToC   RFC7315 - Page 15
4.3.2.1. Procedures at the UA
In the context of the network to which the header fields defined in this document apply, a User Agent has no knowledge of the P-Visited- Network-ID when sending the REGISTER request. Therefore, UACs MUST NOT insert a P-Visited-Network-ID header field in any SIP message.
4.3.2.2. Procedures at the Registrar and Proxy
A SIP proxy that is located in a visited network MAY insert a P-Visited-Network-ID header field in any of the requests indicated in Section 5.7. The header field MUST be populated with the contents of a text string or a token that identifies the administrative domain of the network where the proxy is operating towards the user's home network. A SIP proxy or registrar which is located in the home network can use the contents of the P-Visited-Network-ID header field as an identifier of one or more visited networks that the request traversed. The proxy or registrar in the home network may take local-policy-driven actions based on the existence (or nonexistence) of a roaming agreement between the home and the visited networks. This means, for instance, the authorization of the actions of the request is based on the contents of the P-Visited-Network-ID header field. A SIP proxy that is located in the home network MUST delete this header field when forwarding the message outside the home network administrative domain, in order to retain the user's privacy. A SIP proxy that is located in the home network SHOULD delete this header field when the home proxy has used the contents of the header field or the request is routed based on the called party's identification, even when the request is not forwarded outside the home network administrative domain. Note that a received P-Visited-Network-ID from a UA is not allowed and MUST be deleted when the request is forwarded.
4.3.2.3. Examples of Usage
We present an example in the context of the scenario shown in the following network diagram: Scenario UA --- P1 --- P2 --- REGISTRAR
Top   ToC   RFC7315 - Page 16
   This example shows the message sequence for a REGISTER transaction
   originating from UA eventually arriving at the REGISTRAR.  P1 is an
   outbound proxy in the visited network for UA.  In this case, P1
   inserts the P-Visited-Network-ID header field.  Then, P1 routes the
   REGISTER request to REGISTRAR via P2.

   Message sequence for REGISTER using P-Visited-Network-ID header
   field:

         F1 Register UA -> P1
              REGISTER sip:example.com SIP/2.0
              Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
              To: sip:user1-business@example.com
              From: sip:user1-business@example.com;tag=456248
              Call-ID: 843817637684230998sdasdh09
              CSeq: 1826 REGISTER
              Contact: <sip:user1@192.0.2.4>

   In flow F2, proxy P1 adds its own identifier in a quoted string to
   the P-Visited-Network-ID header field.

         F2 Register P1 -> P2
              REGISTER sip:example.com SIP/2.0
              Via: SIP/2.0/UDP p1@visited.net;branch=z9hG4bK203igld
              Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashd8
              To: sip:user1-personal@example.com
              From: sip:user1-personal@example.com;tag=346249
              Call-ID: 2Q3817637684230998sdasdh10
              CSeq: 1826 REGISTER
              Contact: <sip:user1@192.0.2.4>
              P-Visited-Network-ID: "Visited network number 1"

   Finally, in flow F3, proxy P2 decides to insert its own identifier,
   derived from its own domain name to the P-Visited-Network-ID header
   field.

         F3 Register P2 -> REGISTRAR
              REGISTER sip:example.com SIP/2.0
              Via: SIP/2.0/UDP p2@other.net;branch=z9hG4bK2bndnvk
              Via: SIP/2.0/UDP p1@visited.net;branch=z9hG4bK203igld
              Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashd8
              To: sip:user1-personal@example.com
              From: sip:user1-personal@example.com;tag=346249
              Call-ID: 2Q3817637684230998sdasdh10
              CSeq: 1826 REGISTER
              Contact: <sip:user1@192.0.2.4>
              P-Visited-Network-ID: other.net,"Visited network number 1"
Top   ToC   RFC7315 - Page 17

4.4. The P-Access-Network-Info Header Field

This section describes the P-Access-Network-Info header field. This header field is useful in SIP-based networks that also provide Layer 2 (L2) / Layer 3 (L3) connectivity through different access technologies. SIP UAs may use this header field to relay information about the access technology to proxies that are providing services. The serving proxy may then use this information to optimize services for the UA. For example, a 3GPP UA may use this header field to pass information about the access network such as radio access technology and radio cell identity to its home service provider. For the purpose of this extension, we define an access network as the network providing the L2/L3 IP connectivity, which, in turn, provides a user with access to the SIP capabilities and services provided. In some cases, the SIP server that provides the user with services may wish to know information about the type of access network that the UA is currently using. Some services are more suitable or less suitable depending on the access type, and some services are of more value to subscribers if the access network details are known by the SIP proxy that provides the user with services. In other cases, the SIP server that provides the user with services may simply wish to know crude location information in order to provide certain services to the user. For example, many of the location-based services available in wireless networks today require the home network to know the identity of the cell the user is being served by. Some regulatory requirements exist mandating that for cellular radio systems, the identity of the cell where an emergency call is established is made available to the emergency authorities. The SIP server that provides services to the user may desire to have knowledge about the access network. This is achieved by defining a new private SIP extension header field, P-Access-Network-Info header field. This header field carries information relating to the access network between the UAC and its serving proxy in the home network. A proxy providing services based on the P-Access-Network-Info header field must consider the trust relationship to the UA or outbound proxy including the P-Access-Network-Info header field.
Top   ToC   RFC7315 - Page 18

4.4.1. Applicability Statement for the P-Access-Network-Info Header Field

This mechanism is appropriate in environments where SIP services are dependent on SIP elements knowing details about the IP and lower- layer technologies used by a UA to connect to the SIP network. Specifically, the extension requires that the UA know the access technology it is using, and that a proxy desires such information to provide services. Generally, SIP is built on the everything over IP and IP over everything principle, where the access technology is not relevant for the operation of SIP. Since SIP systems generally should not care or even know about the access technology, this SIP extension is not for general SIP usage. The information revealed in the P-Access-Network-Info header field is potentially very sensitive. Proper protection of this information depends on the existence of specific business and security relationships amongst the proxies that will see SIP messages containing this header field. It also depends on explicit knowledge of the UA of the existence of those relationships. Therefore, this mechanism is only suitable in environments where the appropriate relationships are in place, and the UA has explicit knowledge that they exist.

4.4.2. Usage of the P-Access-Network-Info Header

When a UA generates a SIP request or response that it knows is going to be securely sent to its SIP proxy that is providing services, the UA inserts a P-Access-Network-Info header field into field the SIP message. This header contains information on the access network that the UA is using to get IP connectivity. The header is typically ignored by intermediate proxies between the UA and the SIP proxy that is providing services. The proxy providing services can inspect the header and make use of the information contained there to provide appropriate services, depending on the value of the header. Before proxying the request onwards to an untrusted administrative network domain, this proxy strips the header from the message. Additionally, the first outbound proxy, if in possession of appropriate information, can also add a P-Access-Network-Info header field with its own information.
4.4.2.1. UA Behavior
A UA that supports this extension and is willing to disclose the related parameters MAY insert the P-Access-Network-Info header field in any SIP request or response.
Top   ToC   RFC7315 - Page 19
   The UA inserting this information MUST have a trust relationship with
   the proxy that is providing services to protect its privacy by
   deleting the header before forwarding the message outside of the
   proxy's domain.  This proxy is typically located in the home network.

   In order to avoid the deletion of the header, there MUST also be a
   transitive trust in intermediate proxies between the UA and the proxy
   that provides the services.  This trust is established by business
   agreements between the home network and the access network, and
   generally supported by the use of standard security mechanisms, e.g.,
   IPsec, AKA, and TLS.

4.4.2.2. Proxy Behavior
A proxy MUST NOT modify the value of the P-Access-Network-Info header field. A proxy in possession of appropriate information about the access technology MAY insert a P-Access-Network-Info header field with its own values. A proxy sending towards an untrusted entity MUST remove any P-Access-Network-Info header field containing a "network- provided" value. A proxy that is providing services to the UA, can act upon any information present in the P-Access-Network-Info header field value, if is present, to provide a different service depending on the network or the location through which the UA is accessing the server. For example, for cellular radio access networks, the SIP proxy located in the home network MAY use the cell ID to provide basic localized services. A proxy that provides services to the user is typically located in the home network and is therefore trusted. It MUST delete the header when the SIP signaling is forwarded to a SIP server located in an untrusted administrative network domain. The SIP server providing services to the UA uses the access network information that is of no interest to other proxies located in different administrative domains.

4.5. The P-Charging-Function-Addresses Header Field

3GPP has defined a distributed architecture that results in multiple network entities becoming involved in providing access and services. There is a need to inform each SIP proxy involved in a transaction about the common charging functional entities to receive the generated charging records or charging events.
Top   ToC   RFC7315 - Page 20
   The solution provided by 3GPP is to define two types of charging
   functional entities: Charging Collection Function (CCF) and Event
   Charging Function (ECF).  CCF is used for offline charging (e.g., for
   postpaid account charging).  ECF is used for online charging (e.g.,
   for pre-paid account charging).  There may be more than a single
   instance of CCF and ECF in a network, in order to provide redundancy
   in the network.  In case there are more than a single instance of
   either the CCF or the ECF addresses, implementations SHOULD attempt
   sending the charging data to the ECF or CCF address, starting with
   the first address of the sequence (if any) in the P-Charging-
   Function-Addresses header field.  If the first address of the
   sequence is not available, then the next address (ccf-2 or ecf-2)
   MUST be used if available.  The CCF and ECF addresses MAY be passed
   during the establishment of a dialog or in a standalone transaction.
   More detailed information about charging can be found in 3GPP TS
   32.240 [TS32.240] and 3GPP TS 32.260 [TS32.260].

   We define the SIP private header field P-Charging-Function-Addresses
   header field.  A proxy MAY include this header field, if not already
   present, in either the initial request or response for a dialog or in
   the request and response of a standalone transaction outside a
   dialog.  When present, only one instance of the header MUST be
   present in a particular request or response.

   The mechanisms by which a SIP proxy collects the values to populate
   the P-Charging-Function-Addresses header field values are outside the
   scope of this document.  However, as an example, a SIP proxy may have
   preconfigured these addresses or may obtain them from a subscriber
   database.

4.5.1. Applicability Statement for the P-Charging-Function-Addresses Header Field

The P-Charging-Function-Addresses header field is applicable within a single private administrative domain where coordination of charging is required, for example, according to the architecture specified in 3GPP TS 32.240 [TS32.240]. The P-Charging-Function-Addresses header field is not included in a SIP message sent outside of the own administrative domain. The header is not applicable if the administrative domain does not provide a charging function.
Top   ToC   RFC7315 - Page 21
   The P-Charging-Function-Addresses header field is applicable whenever
   the following circumstances are met:

   1.  A UA sends a REGISTER or dialog-initiating request (e.g., INVITE
       request) or a standalone transaction request outside a dialog to
       a proxy located in the administrative domain of a private
       network.

   2.  A registrar, proxy, or UA that is located in the administrative
       domain of the private network wants to generate charging records.

   3.  A registrar, proxy, or UA that is located in the private network
       has access to the addresses of the charging function entities for
       that network.

   4.  There are other proxies that are located in the same
       administrative domain of the private network and that generate
       charging records or charging events.  The proxies want to send,
       by means outside SIP, the charging information to the same
       charging collecting entities than the first proxy.

4.5.2. Usage of the P-Charging-Function-Addresses Header Field

A SIP proxy that receives a SIP request MAY insert a P-Charging- Function-Addresses header field prior to forwarding the request, if the header was not already present in the SIP request. The header filed contains one or more parameters that contain the hostnames or IP addresses of the nodes that are willing to receive charging information. A SIP proxy that receives a SIP request that includes a P-Charging- Function-Addresses header field can use the hostnames or IP addresses included in the value, as the destination of charging information or charging events. The means to send those charging information or events are outside the scope of this document, and usually, do not use SIP for that purpose.
4.5.2.1. Procedures at the UA
This document does not specify any procedure at the UA located outside the administrative domain of a private network, with regard to the P-Charging-Function-Addresses header field. Such UAs need not understand this header. However, it might be possible that a UA is located within the administrative domain of a private network (e.g., a Public Switched Telephone Network (PSTN) gateway, or conference mixer), and it may have access to the addresses of the charging entities. In this case,
Top   ToC   RFC7315 - Page 22
   a UA MAY insert the P-Charging-Function-Addresses header field in a
   SIP request or response when the next hop for the message is a proxy
   or UA located in the same administrative domain.  Similarly, such a
   UA MAY use the contents of the P-Charging-Function-Addresses header
   field in communicating with the charging entities.

4.5.2.2. Procedures at the Proxy
A SIP proxy that supports this extension and receives a request or response without the P-Charging-Function-Addresses header field MAY insert a P-Charging-Function-Addresses header field prior to forwarding the message. The header is populated with a list of the addresses of one or more charging entities where the proxy should send charging-related information. If a proxy that supports this extension receives a request or response with the P-Charging-Function-Addresses header field, it MAY retrieve the information from the header field to use with application-specific logic, i.e., charging. If the next hop for the message is within the administrative domain of the proxy, then the proxy SHOULD include the P-Charging-Function-Addresses header field in the outbound message. However, if the next hop for the message is outside the administrative domain of the proxy, then the proxy MUST remove the P-Charging-Function-Addresses header field.
4.5.2.3. Examples of Usage
We present an example in the context of the scenario shown in the following network diagram: Scenario UA1 --- P1 --- P2 --- UA2 In this scenario, we assume that P1 and P2 belong to the same administrative domain. The example below shows the message sequence for an INVITE transaction originating from UA1 and eventually arriving at UA2. P1 is an outbound proxy for UA1. In this case, P1 inserts charging information. Then, P1 routes the request via P2 to UA2.
Top   ToC   RFC7315 - Page 23
   Message sequence for INVITE using P-Charging-Function-Addresses
   header field:

         F1 Invite UA1 -> P1
            INVITE sip:ua2@home1.net SIP/2.0
            Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
            To: sip:ua2@home1.net
            From: sip:ua1@home1.net;tag=456248
            Call-ID: 843817637684230998sdasdh09
            CSeq: 18 INVITE
            Contact: sip:ua1@192.0.2.4

         F2 Invite P1 -> P2
            INVITE sip:ua2@home1.net SIP/2.0
            Via: SIP/2.0/UDP p1@home1.net:5060;branch=z9hG4bK34ghi7ab04
            Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
            To: sip:ua2@home1.net
            From: sip:ua1@home1.net;tag=456248
            Call-ID: 843817637684230998sdasdh09
            CSeq: 18 INVITE
            Contact: sip:ua1@192.0.2.4
            P-Charging-Function-Addresses:
                                     ccf=192.0.8.1; ecf=192.0.8.3,
                                     ccf-2=192.0.8.2; ecf-2=192.0.8.4

   Now both P1 and P2 are aware of the IP addresses of the entities that
   collect charging record or charging events.  Both proxies can send
   the charging information to the same entities.

4.6. The P-Charging-Vector Header Field

3GPP has defined a distributed architecture that results in multiple network entities becoming involved in providing access and services. Operators need the ability and flexibility to charge for the access and services as they see fit. This requires coordination among the network entities (e.g., SIP proxies), which includes correlating charging records generated from different entities that are related to the same session. The correlation information includes, but is not limited to, a globally unique charging identifier that makes the billing effort easy. A charging vector is defined as a collection of charging information. The charging vector MAY be filled in during the establishment of a dialog or standalone transaction outside a dialog. The information inside the charging vector MAY be filled in by multiple network entities (including SIP proxies) and retrieved by multiple network
Top   ToC   RFC7315 - Page 24
   entities.  There are three types of correlation information to be
   transferred: the IMS Charging Identity (ICID) value, the address of
   the SIP proxy that creates the ICID value, and the Inter Operator
   Identifier (IOI).

   ICID is a charging value that identifies a dialog or a transaction
   outside a dialog.  It is used to correlate charging records.  ICID
   MUST be a globally unique value.  One way to achieve globally
   uniqueness is to generate the ICID using two components: a locally
   unique value and the hostname or IP address of the SIP proxy that
   generated the locally unique value.

   The IOI identifies both the originating and terminating networks
   involved in a SIP dialog or transaction outside a dialog.  There MAY
   be an IOI generated from each side of the dialog to identify the
   network associated with each side.

   Additionally, in a multi-network environment, one or more transit IOI
   identifiers MAY be included along the path of the SIP dialog or
   transaction outside a dialog.  Due to network policy, a void value
   MAY be included instead of the transit network name.  The void value
   is used to indicate that a transit network appeared but due to
   operator policy the network name is not shown.

   Furthermore, in a multi-service provider environment, one or more
   transit IOIs MAY be included along the path of the SIP dialog or
   transaction outside a dialog.  Due to service provider policy, a void
   value MAY be included instead of the transit service provider.  The
   void value is used to indicate that a transit appeared but due to
   service provider policy the service provider name is not shown.

   There is also expected to be access network charging information,
   which consists of network-specific identifiers for the access level
   (e.g., Universal Mobile Telecommunications System (UMTS) radio access
   network or IEEE 802.11b).  The details of the information for each
   type of network are not described in this memo.

   We define the SIP private header P-Charging-Vector header field.  A
   proxy MAY include this header, if not already present, in either the
   initial request or response for a dialog, or in the request and
   response of a standalone transaction outside a dialog.  When present,
   only one instance of the header MUST be present in a particular
   request or response.

   The mechanisms by which a SIP proxy collects the values to populate
   the P-Charging-Vector header field are outside the scope of this
   document.
Top   ToC   RFC7315 - Page 25

4.6.1. Applicability Statement for the P-Charging-Vector Header Field

The P-Charging-Vector header field is applicable within a single private administrative domain or between different administrative domains where there is a trust relationship between the domains. The P-Charging-Vector header field is not included in a SIP message sent to another network if there is no trust relationship. The header is not applicable if the administrative domain manages charging in a way that does not require correlation of records from multiple network entities (e.g., SIP proxies). The P-Charging-Vector header field is applicable whenever the following circumstances are met: 1. A UA sends a REGISTER or dialog-initiating request (e.g., INVITE) or mid-dialog request (e.g., UPDATE) or a standalone transaction request outside a dialog to a proxy located in the administrative domain of a private network. 2. A registrar, proxy, or UA that is located in the administrative domain of the private network wants to generate charging records. 3. A proxy or UA that is located in the administrative domain of the private network has access to the charging correlation information for that network. 4. Optionally, a registrar, proxy, or UA that is part of a second administrative domain in another private network, whose SIP requests and responses are traversed through, en route to/from the first private network, wants to generate charging records and correlate those records with those of the first private network. This assumes that there is a trust relationship between both private networks.

4.6.2. Usage of the P-Charging-Vector Header Field

The P-Charging-Vector header field is used to convey charging-related information, such as the globally unique IMS Charging Identity (ICID) value. Typically, a SIP proxy that receives a SIP request that does not contain a P-Charging-Vector header field MAY insert it, with those parameters that are available at the SIP proxy. A SIP proxy that receives a SIP request that contains a P-Charging- Vector header field can use the values, such as the globally unique ICID, to produce charging records.
Top   ToC   RFC7315 - Page 26
4.6.2.1. Procedures at the UA
This document does not specify any procedure at a UA located outside the administrative domain of a private network (e.g., PSTN gateway or conference mixer), with regard to the P-Charging-Vector header field. UAs need not understand this header. However, it might be possible that a UA be located within the administrative domain of a private network (e.g., a PSTN gateway, or conference mixer), and it may interact with the charging entities. In this case, a UA MAY insert the P-Charging-Vector header field in a SIP request or response when the next hop for the message is a proxy or UA located in the same administrative domain. Similarly, such a UA MAY use the contents of the P-Charging-Vector header field in communicating with the charging entities.
4.6.2.2. Procedures at the Proxy
A SIP proxy that supports this extension and receives a request or response without the P-Charging-Vector header field MAY insert a P-Charging-Vector header field prior to forwarding the message. The header is populated with one or more parameters, as described in the syntax, including but not limited to, a globally unique charging identifier. If a proxy that supports this extension receives a request or response with the P-Charging-Vector header field, it MAY retrieve the information from the header value to use with application-specific logic, i.e., charging. If the next hop for the message is within the trusted domain, then the proxy SHOULD include the P-Charging-Vector header field in the outbound message. If the next hop for the message is outside the trusted domain, then the proxy MAY remove the P-Charging-Function-Addresses header field. Per local application-specific logic, the proxy MAY modify the contents of the P-Charging-Vector header field prior to sending the message.
4.6.2.3. Examples of Usage
We present an example in the context of the scenario shown in the following network diagram: Scenario UA1 --- P1 --- P2 --- UA2
Top   ToC   RFC7315 - Page 27
   This example shows the message sequence for an INVITE transaction
   originating from UA1 and eventually arriving at UA2.  P1 is an
   outbound proxy for UA1.  In this case, P1 inserts charging
   information.  Then, P1 routes the call via P2 to UA2.

   Message sequence for INVITE using P-Charging-Vector header field:

         F1 Invite UA1 -> P1
              INVITE sip:joe@example.com SIP/2.0
              Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
              To: sip:joe@example.com
              From: sip:ua1@home1.net;tag=456248
              Call-ID: 843817637684230998sdasdh09
              CSeq: 18 INVITE
              Contact: sip:ua1@192.0.2.4

         F2 Invite P1 -> P2
              INVITE sip:joe@example.com SIP/2.0
              Via: SIP/2.0/UDP P1@home1.net:5060;branch=z9hG4bK34ghi7a
              Via: SIP/2.0/UDP 192.0.2.4:5060;branch=z9hG4bKnashds7
              To: sip:joe@example.com
              From: sip:ua1@home1.net;tag=456248
              Call-ID: 843817637684230998sdasdh09
              CSeq: 18 INVITE
              Contact: sip:ua1@192.0.2.4
              P-Charging-Vector: icid-value=1234bc9876e;
                                 icid-generated-at=192.0.6.8;
                                 orig-ioi=home1.net

4.6.3. Usage of the transit-ioi

The transit-ioi is added to the P-Charging-Vector header field when traversing transit networks. It is allowed to have multiple transit-ioi values within one SIP message or response. The values within the response are independent from the values set up within the request. The element could be added either by a transit network itself or by the succeeding network at the entry point where the preceding network is known. Based on network policy, a void value can be used. Depending on the call scenario, each transit network can add either a transit network name or a void value. However, it cannot be guaranteed that all the values that are added will appear within the P-Charging-Vector header field.
Top   ToC   RFC7315 - Page 28
   Some networks can screen the P-Charging-Vector header field and
   delete transit-ioi values, e.g., networks not supporting this value.
   There are scenarios where the appearance of the transit-ioi values of
   all networks is needed to have a correct end-to-end view.

   The policies of adding, modifying, and deleting transit-ioi values
   are out of the scope of this document.

   The transit-ioi contains an indexed value that MUST be incremented
   with each value added to the P-Charging-Vector header field.

   A void value has no index.  By adding the next value, the index has
   to be incremented by the number of void entries +1.

4.6.3.1. Procedures at the Proxy
Procedures described within Section 4.5.2.2 apply. A transit-ioi MAY be added or modified by a proxy. A deletion of the transit-ioi or a entry within the tranist-ioi could appear depending on the network policy and trust rules. This is also valid by replacing the transit-ioi with a void value.

4.6.4. Usage of the related-icid

4.6.4.1. Procedures at the UA
The UAS acting as a B2BUA MAY add the related-icid into the P-Charging-Vector header field into SIP request or SIP responses. For example, the UAS can include the related-icid in a response to an INVITE request when the received INVITE request creates a new call leg towards the same remote end. The value of the related-icid is the icid value of the original dialog towards the remote end.
4.6.4.2. Procedures at the Proxy
Procedures described within Section 4.5.2.2 apply. A related-icid and "related-icid-generated-at" MAY be added or modified by a proxy. A deletion of the elements could appear depending on the network policy and trust rules.


(page 28 continued on part 2)

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