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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.
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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.
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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.
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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.
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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.
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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>.
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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
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