    Internet Engineering Task Force (IETF)                         R. Jesske
    Request for Comments: 8606                              Deutsche Telekom
    Updates: 3326                                                  June 2019
    Category: Standards Track
    ISSN: 2070-1721

                 ISDN User Part (ISUP) Cause Location Parameter
                        for the SIP Reason Header Field

    Abstract

       The SIP Reason header field is defined to carry ISUP (ISDN User Part)
       cause values as well as SIP response codes.  Some services in SIP
       networks may need to know the ISUP location where the call was
       released in the PSTN (Public Switched Telephone Network) to correctly
       interpret the reason of release.  This document updates RFC 3326 by
       adding a location parameter for this purpose.

    Status of This Memo

       This is an Internet Standards Track document.

       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).  Further information on
       Internet Standards is available in 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/rfc8606.

    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.

    Jesske                       Standards Track                    [Page 1]

    RFC 8606             ISUP Release Location Parameter           June 2019

    Table of Contents

       1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
       2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
       3.  Rationale . . . . . . . . . . . . . . . . . . . . . . . . . .   3
       4.  Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . .   3
       5.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
       6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   5
       7.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
       8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
         8.1.  Registration of the Location Parameter for the Reason
               Header Field  . . . . . . . . . . . . . . . . . . . . . .   6
       9.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
       Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   7
       Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

    1.  Introduction

       Section 3.4 of [RFC3326] describes a SIP message flow for canceling
       an INVITE request when a REL (release) message is received from the
       ISUP side.  That document specifies the SIP Reason header field
       [RFC3326] that is used to indicate the reason of release.  The reason
       of release indicates why a SIP Dialog or a PSTN call, in cases where
       the call was interworked to the PSTN, was terminated.  The
       termination may be normal, based on a failure within an entity (e.g.
       temporary failure) or caused by other factors (e.g., congestion).
       The reason may be a SIP response or an ISUP release cause as
       specified within [Q.850].  [RFC6432] specifies that an ISUP [Q.850]
       cause code can be carried within a SIP response, but not the Q.850
       location information.  The [Q.850] location information identifies
       the part of the ISUP network where the call was released.

       This document adds a location value parameter to the reason-extension
       parameter defined in [RFC3326] so that the [Q.850] location value can
       be interworked from the PSTN.  The interworking from the PSTN needs
       only to include the location received by the interworking gateway.
       [Q.850] describes the definitions of the cause code values and the
       locations used in ISDN and DSS1 (Digital Subscriber Signalling System
       No. 1) environments.  The cause code is used for identifying the
       reason of release of a call, and the location identifies where the
       call was released.

    Jesske                       Standards Track                    [Page 2]

    RFC 8606             ISUP Release Location Parameter           June 2019

    2.  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.

    3.  Rationale

       The primary intent of the parameter defined in this specification is
       for use in IMS (IP Multimedia Subsystem) networks defined by 3GPP,
       but it is also open to be used by any other networks that include
       ISUP interworking gateways and use Q.850 reason codes.  The purpose
       of this parameter is to hold the location of the call release so that
       it can be transported from the originating PSTN entity to the SIP
       entity via a response or BYE message.  The ISDN location is defined
       in [Q.850].

    4.  Mechanism

       As defined by [RFC6432], any SIP Response message, with the exception
       of 100 (Trying), MAY contain a Reason header field with a Q.850
       [Q.850] cause code.

       This specification adds a parameter with the ISUP location value
       defined in [Q.850] to the Reason header field that identifies the
       location of the call release in ISUP.  The location is a 4-bit value
       that reflects the possible locations where an ISUP call is released.
       Some values are spare or reserved for national use.  The Augmented
       BNF (ABNF) [RFC5234] for this parameter is shown in Figure 1.

    Jesske                       Standards Track                    [Page 3]

    RFC 8606             ISUP Release Location Parameter           June 2019

         reason-extension    =/ isup-cause-location
         isup-cause-location =  "location" EQUAL isup-location-value

         isup-location-value =
            "U" /      ; for 0 0 0 0 user
            "LPN" /    ; for 0 0 0 1 private network serving the local user
            "LN" /     ; for 0 0 1 0 public network serving the local user
            "TN" /     ; for 0 0 1 1 transit network
            "RLN" /    ; for 0 1 0 0 public network serving the remote user
            "RPN" /    ; for 0 1 0 1 private network serving the remote user
            "LOC-6" /  ; for 0 1 1 0 spare
            "INTL" /   ; for 0 1 1 1 international network
            "LOC-8" /  ; for 1 0 0 0 spare
            "LOC-9" /  ; for 1 0 0 1 spare
            "BI" /     ; for 1 0 1 0 network beyond interworking point
            "LOC-11" / ; for 1 0 1 1 spare
            "LOC-12" / ; for 1 1 0 0 reserved for national use
            "LOC-13" / ; for 1 1 0 1 reserved for national use
            "LOC-14" / ; for 1 1 1 0 reserved for national use
            "LOC-15"   ; for 1 1 1 1 reserved for national use

                      Figure 1: ABNF for isup-cause-location

       Note: These are the location values defined within [Q.850].  The
       'LOC-*' names are the wire codepoints for the values currently left
       as 'spare' or 'reserved' in [Q.850]; these will continue to be the
       wire codepoints in the case of future allocation or national usage of
       the such values.

       The UAC or UAS SHALL include the location parameter in a request or
       response when setting up the Reason header field with a [Q.850] cause
       when the ISUP [Q.850] location is available.

       The use of the location parameter is restricted to Q.850 cause
       values.  Other values MUST be ignored if present.

    5.  Example

       The following example shows a SIP 404 response message containing a
       Reason header field with a [Q.850] cause value and an isup-cause-
       location value.  The 404 Response will be sent when a gateway
       receives an ISUP release with a [Q.850] cause set to 1, meaning
       Unallocated (unassigned) number, i.e., the number is not known in the
       PSTN.

    Jesske                       Standards Track                    [Page 4]

    RFC 8606             ISUP Release Location Parameter           June 2019

             SIP/2.0 404 Not Found
             Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4bKx5st
             Via: SIP/2.0/TCP 192.0.2.3:5060;branch=z9hG4bK4321
             From: Alice <sips:alice@atlanta.example.com>;tag=1234567
             To: Bob <sips:bob@biloxi.example.com>;tag=765432
             Call-ID: 12345600@atlanta.example.com
             CSeq: 1 INVITE
             Reason: Q.850;cause=1;text="Unallocated (unassigned) number";
                  location=LN
             Content-Length: 0

            Figure 2: Example of a Location in the Reason Header Field

    6.  Privacy Considerations

       While the addition of the location parameter provides an indicator of
       the entity that added the location in the signaling path, it provides
       little more exposure than the [Q.850] cause itself.  The ISUP
       location value itself will not reveal the identity of the originating
       or terminating party of the call.  It shows only the ISUP network
       location of the device that released the call.  The ISUP location
       does not show the physical location of the caller or callee.

    7.  Security Considerations

       This document doesn't change any of the security considerations
       described in [RFC3326].  The addition of the location parameter
       provides an indicator of the [Q.850] location where the call was
       released within the PSTN.  This information may be used for specific
       location-driven services but does not create any additional security
       constraints.  Because the [Q.850] location is very imprecise, the
       [Q.850] location value itself will not add any major security
       constraints.  The use of this parameter is not restricted to a
       specific architecture.

       [RFC3398] describes detailed security considerations due to
       interworking between ISUP and SIP.  Beyond these considerations, the
       addition of the location does not introduce new security concerns.
       The location shows the network part where the call was released.
       Knowing this does not increase the possibilities of extended fraud
       scenarios.

    Jesske                       Standards Track                    [Page 5]

    RFC 8606             ISUP Release Location Parameter           June 2019

    8.  IANA Considerations

    8.1.  Registration of the Location Parameter for the Reason Header Field

       IANA has registered a new SIP header parameter in the "Header Field
       Parameters and Parameter Values" subregistry of the "Session
       Initiation Protocol (SIP) Parameters" registry
       <https://www.iana.org/assignments/sip-parameters>, per the guidelines
       in [RFC3968]:

       Header Field:  Reason

       Parameter Name:  location

       Predefined Values:  Yes

       Reference:  RFC 8606

    9.  Normative References

       [Q.850]    ITU-T, "Usage of cause and location in the Digital
                  Subscriber Signalling System No. 1 and the Signalling
                  System No. 7 ISDN user part", Recommendation ITU-T Q.850,
                  October 2018, <https://www.itu.int/rec/T-REC-Q.850>.

       [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>.

       [RFC3326]  Schulzrinne, H., Oran, D., and G. Camarillo, "The Reason
                  Header Field for the Session Initiation Protocol (SIP)",
                  RFC 3326, DOI 10.17487/RFC3326, December 2002,
                  <https://www.rfc-editor.org/info/rfc3326>.

       [RFC3398]  Camarillo, G., Roach, A., Peterson, J., and L. Ong,
                  "Integrated Services Digital Network (ISDN) User Part
                  (ISUP) to Session Initiation Protocol (SIP) Mapping",
                  RFC 3398, DOI 10.17487/RFC3398, December 2002,
                  <https://www.rfc-editor.org/info/rfc3398>.

       [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>.

    Jesske                       Standards Track                    [Page 6]

    RFC 8606             ISUP Release Location Parameter           June 2019

       [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>.

       [RFC6432]  Jesske, R. and L. Liess, "Carrying Q.850 Codes in Reason
                  Header Fields in SIP (Session Initiation Protocol)
                  Responses", RFC 6432, DOI 10.17487/RFC6432, November 2011,
                  <https://www.rfc-editor.org/info/rfc6432>.

       [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>.

    Acknowledgments

       Thanks to Michael Kreipl, Thomas Belling, Marianne Mohali, Peter
       Daws, Paul Kyzivat, Dale Worley, Yehoshua Gev, and Keith Drage for
       the comments and review.

    Author's Address

       Roland Jesske
       Deutsche Telekom
       Heinrich-Hertz Str, 3-7
       Darmstadt  64295
       Germany

       Email: r.jesske@telekom.de
       URI:   www.telekom.de

    Jesske                       Standards Track                    [Page 7]