SIP P-headers update: Updates to Private Header (P-Header) Extension Usage in Session Initiation Protocol (SIP) Requests and Responses

Internet Engineering Task Force (IETF)                       C. Holmberg
Request for Comments: 9878                                      Ericsson
Obsoletes: 7976                                               N. Biondic
Updates: 7315                                                       ETSI
Category: Informational                                     G. Salgueiro
ISSN: 2070-1721                                      Cisco Systems, Inc.
                                                               R. Jesske
                                                        Deutsche Telekom
                                                           November 2025


    Updates to Private Header (P-Header) Extension Usage in Session
            Initiation Protocol (SIP) Requests and Responses

Abstract

   The Third Generation Partnership Project (3GPP) has identified cases
   where different SIP private header extensions referred to as "P-"
   header fields, and defined in RFC 7315, need to be included in SIP
   requests and responses where they were not allowed according to RFC
   7315.  This document updates RFC 7315, in order to allow inclusion of
   the affected "P-" header fields in such requests and responses.  This
   document obsoletes RFC 7976.  The changes related to RFC 7976 involve
   the inclusion of the P-Visited-Network-ID header field in SIP
   responses.

   This document also makes updates to RFC 7315 in order to fix
   misalignments that occurred when RFC 3455 was obsoleted by RFC 7315.

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/rfc9878.

Copyright Notice

   Copyright (c) 2025 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 Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Misalignments and 3GPP Use Cases
     2.1.  General
     2.2.  Misalignments
     2.3.  3GPP Use Cases
       2.3.1.  General
       2.3.2.  P-Access-Network-Info
       2.3.3.  P-Charging-Vector
   3.  Updates to RFC 7315
   4.  IANA Considerations
   5.  Security Considerations
   6.  Operational considerations
   7.  Changes since RFC 7976
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Acknowledgments
   Authors' Addresses

1.  Introduction

   The Third Generation Partnership Project (3GPP) has identified cases
   where different Session Initiation Protocol (SIP) [RFC3261] private
   header extensions referred to as "P-" header fields, and defined in
   [RFC7315], need to be included in SIP requests and responses where
   they were not allowed according to [RFC7315].  This document updates
   [RFC7315], in order to allow inclusion of the affected "P-" header
   fields in such requests and responses.

   This document also makes updates to [RFC7315] in order to fix
   misalignments that occurred when [RFC3455] was updated and
   subsequently obsoleted by the publication of [RFC7315].

2.  Misalignments and 3GPP Use Cases

2.1.  General

   [RFC7315] contains contradictory statements regarding the usage of
   SIP "P-" header fields in SIP requests and responses, which leave the
   presence of the SIP "P-" header fields in the SIP requests and
   responses open to interpretation and different implementations.
   Statements in Section 5.7 of [RFC7315] are not aligned with the
   definitions and usage of the SIP "P-" header fields specified in
   Section 4 of [RFC7315].  This section describes the misalignments
   that occurred when [RFC3455] was updated and subsequently obsoleted
   by the publication of [RFC7315], and how they are fixed.

      |  NOTE: In the case of the P-Called-Party-ID header field,
      |  allowing it in PUBLISH requests was done deliberately in
      |  [RFC7315].  Therefore, it is not considered a misalignment.

   Since [RFC7315] was published, 3GPP defined new use cases that
   require the RFC to be updated.  This section describes the 3GPP use
   cases behind the updates, and the updates needed to [RFC7315] in
   order to support the use cases.

   Section 3 updates [RFC7315], based on the misalignments and 3GPP use
   cases.

2.2.  Misalignments

   The following updates are needed in order to fix the misalignments
   that occurred when [RFC3455] was obsoleted by [RFC7315]:

   *  P-Associated-URI: Remove the statement that the header field can
      appear in the SIP REGISTER method.

   *  P-Called-Party-ID: Delete the statement that the P-Called-Party-ID
      header field can appear in SIP responses.  Add a statement that
      the P-Called-Party-ID header field can appear in the SIP REFER
      method.

   *  P-Access-Network-Info: Add a statement that the P-Access-Network-
      Info header field can appear in SIP responses.

   *  P-Charging-Vector: Add a statement that the P-Charging-Vector
      header field can appear in SIP responses.  Add a statement that
      the P-Charging-Vector header field cannot appear in the SIP ACK
      method.

   *  P-Charging-Function-Addresses: Add a statement that the P-
      Charging-Function-Addresses header field can appear in SIP
      responses.

   The following update is needed in order to clarify the usage of the
   header compared to [RFC7315]:

   *  P-Visited-Network-ID: Add a statement that the P-Visited-Network-
      ID header field cannot appear in the SIP NOTIFY, PRACK, INFO, and
      UPDATE methods.  Add statement that the P-Visited-Network-ID
      header field can appear in all non-100 (Trying) responses.

2.3.  3GPP Use Cases

2.3.1.  General

   The following updates are needed in order to implement the 3GPP use
   cases:

   *  P-Access-Network-Info: Add statement that the P-Access-Network-
      Info header field can appear in the SIP ACK method when triggered
      by a SIP 2xx response.

   *  P-Charging-Vector: Add statement that the P-Charging-Vector header
      field can appear in the SIP ACK method when triggered by a SIP 2xx
      response.

   This following sections describe, for individual "P-" header fields,
   the 3GPP use cases that are the basis for the updates.  The use cases
   are based on the procedures defined in [TS24.229].

2.3.2.  P-Access-Network-Info

   The P-Access-Network-Info header field may contain the Network
   Provided Location Information (NPLI).  The NPLI is described in
   [TS23.228].

   A proxy in possession of appropriate information about the access
   technology might insert a P-Access-Network-Info header field with its
   own values.  Such values are identified by the string "network-
   provided" defined in [RFC7315].  Based on operator policy, roaming
   agreement, or both, the local time of the visited network may be
   included.

   The Call Data Records (CDRs) generated within the IP Multimedia
   Subsystem (IMS) have to contain the NPLI in order to guarantee
   correct billing.  When an IMS session is modified, the NPLI also
   needs to be stored, as the location of the user at the time when the
   session is modified may generate a charging event.  In case of a
   session modification event at IMS, the NPLI needs to be provided:

   *  when the bearer establishment is triggered, or

   *  at session release when the bearer deactivation is triggered, or

   *  when the bearer modification is triggered, e.g., a QoS
      modification for the use of a newly negotiated codec.

   In some scenarios, the bearer modification may be triggered by the
   proxy upon reception of a Session Description Protocol (SDP) answer
   within the SIP 2xx response to the SIP INVITE request.  In such case,
   the NPLI needs to be provided within the SIP ACK request.  However,
   [RFC7315] does not allow the usage of the P-Access-Network-Info
   header field in a SIP ACK request.

   Upon reception of the SDP answer within a SIP 2xx response on the SIP
   INVITE request, a proxy may initiate procedures to obtain the NPLI
   and may include the P-Access-Network-Info header field with the NPLI
   in the SIP ACK request.

   The P-Access-Network-Info header field shall not be included in SIP
   ACK requests triggered by non-2xx responses.

2.3.3.  P-Charging-Vector

   [RFC7315] defines an Inter Operator Identifier (IOI) to enable
   different operators involved in a SIP dialog or a transaction outside
   a dialog to identify each other by exchanging operator identification
   information within the P-Charging-Vector header field.

   In the interconnection scenarios in multi-operator environments,
   where one or more transit operators are between the originating and
   terminating operator, the identities of the involved transit
   operators are represented by a transit-ioi parameter of the P-
   Charging-Vector header field.

   Transit operators can be selected independently for each SIP method
   and direction of request.  A transit network will only have knowledge
   of an individual SIP request, and transit network selection will be
   an independent decision for each request and could be made based on
   load, cost, percentage, time of day, and other factors.  For this
   reason, it is necessary that the P-Charging-Vector header field,
   which carries the transit IOI information, is included in each SIP
   request and response.  However, [RFC7315] does not allow the usage of
   the P-Charging-Vector header field in the SIP ACK request.

   A SIP proxy that supports this extension and receives the SIP ACK
   request may include a P-Charging-Vector header field in the SIP ACK
   request.

   The P-Charging-Vector header field shall not be included in SIP ACK
   requests triggered by non-2xx responses.

3.  Updates to RFC 7315

   This section contains the update to Section 5.7 of [RFC7315], in
   order to implement the misalignment fixes and the 3GPP requirements
   described in Section 2.  In the old text, blank lines have been added
   for readability.

   Old text:

   |  The P-Associated-URI header field can appear in SIP REGISTER
   |  method and 2xx resonses [sic].
   |
   |  The P-Called-Party-ID header field can appear in SIP INVITE,
   |  OPTIONS, PUBLISH, SUBSCRIBE, and MESSAGE methods and all
   |  responses.
   |
   |  The P-Visited-Network-ID header field can appear in all SIP
   |  methods except ACK, BYE, and CANCEL and all responses.
   |
   |  The P-Access-Network-Info header field can appear in all SIP
   |  methods except ACK and CANCEL.
   |
   |  The P-Charging-Vector header field can appear in all SIP methods
   |  except CANCEL.
   |
   |  The P-Charging-Function-Addresses header field can appear in all
   |  SIP methods except ACK and CANCEL.

   New text:

   |  The P-Associated-URI header field can appear in SIP REGISTER 2xx
   |  responses.
   |
   |  The P-Called-Party-ID header field can appear in the SIP INVITE,
   |  OPTIONS, PUBLISH, REFER, SUBSCRIBE, and MESSAGE requests.
   |
   |  The P-Visited-Network-ID header field can appear in all SIP
   |  requests and the associated non-100 response message except in
   |  ACK, BYE, CANCEL, NOTIFY, PRACK, INFO, UPDATE.
   |
   |  The P-Access-Network-Info header field can appear in all SIP
   |  requests and the associated non-100 responses, except in CANCEL
   |  requests, CANCEL responses, and ACK methods triggered by non-2xx
   |  responses.
   |
   |  The P-Charging-Vector header field can appear in all SIP requests
   |  and the associated non-100 responses, except in CANCEL requests,
   |  CANCEL responses, and ACK requests triggered by non-2xx responses.
   |
   |  The P-Charging-Function-Addresses header field can appear in all
   |  SIP methods and the associated non-100 responses, except in CANCEL
   |  requests, CANCEL responses, and ACK requests.

4.  IANA Considerations

   This document has no IANA actions.

5.  Security Considerations

   The security considerations for these "P-" header fields are defined
   in [RFC7315].  This specification allows some header fields to be
   present in messages where they were previously not allowed, and the
   security considerations and assumptions described in [RFC7315] (e.g.,
   regarding only sending information to trusted entities) also apply to
   those messages.  In addition, this specification also disallows some
   header fields to be present in messages where they were previously
   allowed.  That does not cause any security issues, but implementors
   need to be aware that implementations may not have been updated
   according to this document, and take proper actions if a header field
   occurs, or does not occur, in a message where it should occur (or
   occurs in a message where it should not occur).  This document adds
   the ability to include P-Access-Network-Info in ACK requests.  As
   documented in [RFC7315], P-Access-Network-Info may include privacy
   sensitive information, including the user's location.  The security
   and privacy considerations for P-Access-Network-Info in ACK requests
   are similar to those for the other SIP requests discussed in
   Section 6.4 of [RFC7315].  The security and privacy considerations
   for the P-Visited-Network-ID header field are similar to those for
   the other SIP responses discussed in Section 6.3 of [RFC7315].

6.  Operational considerations

   As the "P-" header fields are mainly used in (and in most cases, only
   defined for) networks defined by the 3GPP, where the updates defined
   in this document are already defined [TS24.229], the updates are not
   seen to cause backward-compatibility concerns.

7.  Changes since RFC 7976

   The changes related to RFC 7976 involve the inclusion of the P-
   Visited-Network-ID header field in SIP responses.  Specifically, any
   SIP response message, with the exception of a 100 (Trying) response,
   may include a P-Visited-Network-ID header field.

8.  References

8.1.  Normative References

   [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>.

   [RFC7315]  Jesske, R., Drage, K., and C. Holmberg, "Private Header
              (P-Header) Extensions to the Session Initiation Protocol
              (SIP) for the 3GPP", RFC 7315, DOI 10.17487/RFC7315, July
              2014, <https://www.rfc-editor.org/info/rfc7315>.

   [TS23.228] 3GPP, "IP Multimedia Subsystem (IMS); Stage 2", 3GPP
              TS 23.228,
              <https://portal.3gpp.org/desktopmodules/Specifications/
              SpecificationDetails.aspx?specificationId=821>.

   [TS24.229] 3GPP, "IP multimedia call control protocol based on
              Session Initiation Protocol (SIP) and Session Description
              Protocol (SDP); Stage 3", 3GPP TS 24.229,
              <https://portal.3gpp.org/desktopmodules/Specifications/
              SpecificationDetails.aspx?specificationId=1055>.

8.2.  Informative References

   [RFC3455]  Garcia-Martin, M., Henrikson, E., and D. Mills, "Private
              Header (P-Header) Extensions to the Session Initiation
              Protocol (SIP) for the 3rd-Generation Partnership Project
              (3GPP)", RFC 3455, DOI 10.17487/RFC3455, January 2003,
              <https://www.rfc-editor.org/info/rfc3455>.

Acknowledgments

   The author would like to acknowledge the constructive feedback
   provided by Michael Kreipl, Charles Eckel, and Paul Kyzivat.

   Thanks to Paul Kyzivat, Jean Mahoney, Ben Campbell, and Adam Roach
   for providing comments on an earlier draft version of this document.

Authors' Addresses

   Christer Holmberg
   Ericsson
   Hirsalantie 11
   FI-02420 Jorvas
   Finland
   Email: christer.holmberg@ericsson.com


   Nevenka Biondic
   ETSI
   650 route des Lucioles
   06921 Sophia Antipolis cedex
   France
   Email: Nevenka.Biondic@etsi.org


   Gonzalo Salgueiro
   Cisco Systems, Inc.
   7200-12 Kit Creek Road
   Research Triangle Park, NC 27709
   United States of America
   Email: gsalguei@cisco.com


   Roland Jesske
   Deutsche Telekom
   Deutsche-Telekom-Allee 9
   64295 Darmstadt
   Germany
   Email: r.jesske@telekom.de
   URI:   www.telekom.com