Internet Engineering Task Force (IETF) A. Allen, Ed.

Internet Engineering Task Force (IETF) A. Allen, Ed.
Request for Comments: 7255 Blackberry
Category: Informational May 2014
ISSN: 2070-1721

Using the International Mobile station Equipment Identity (IMEI)
Uniform Resource Name (URN) as an Instance ID

Abstract

This specification defines how the Uniform Resource Name (URN)
reserved for the Global System for Mobile Communications Association
(GSMA) identities and its sub-namespace for the International Mobile
station Equipment Identity (IMEI) can be used as an instance-id. Its
purpose is to fulfill the requirements for defining how a specific
URN needs to be constructed and used in the '+sip.instance' Contact
header field parameter for outbound behavior.

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/rfc7255.

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Copyright Notice

Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.

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described in the Simplified BSD License.

Table of Contents

1. Introduction ....................................................2
2. Terminology .....................................................3
3. Background ......................................................3
4. 3GPP Use Cases ..................................................5
5. User Agent Client Procedures ....................................5
6. User Agent Server Procedures ....................................6
7. 3GPP SIP Registrar Procedures ...................................6
8. Security Considerations .........................................7
9. Acknowledgements ................................................7
10. References .....................................................8
10.1. Normative References ......................................8
10.2. Informative References ....................................8

1. Introduction

This specification defines how the Uniform Resource Name (URN)
reserved for the Global System for Mobile Communications Association
(GSMA) identities and its sub-namespace for the International Mobile
station Equipment Identity (IMEI) as specified in RFC 7254 [1] can be
used as an instance-id as specified in RFC 5626 [2] and also as used
by RFC 5627 [3].

RFC 5626 [2] specifies the '+sip.instance' Contact header field
parameter that contains a URN as specified in RFC 2141 [4]. The
instance-id uniquely identifies a specific User Agent (UA) instance.
This instance-id is used as specified in RFC 5626 [2] so that the
Session Initiation Protocol (SIP) registrar (as specified in RFC 3261
[9]) can recognize that the contacts from multiple registrations
correspond to the same UA. The instance-id is also used as specified

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by RFC 5627 [3] to create Globally Routable User Agent URIs (GRUUs)
that can be used to uniquely address a UA when multiple UAs are
registered with the same Address of Record (AoR).

RFC 5626 [2] requires that a UA SHOULD create a Universally Unique
Identifier (UUID) URN as specified in RFC 4122 [6] as its instance-id
but allows for the possibility to use other URN schemes. Per
RFC 5626, "If a URN scheme other than UUID is used, the UA MUST only
use URNs for which an RFC (from the IETF stream) defines how the
specific URN needs to be constructed and used in the "+sip.instance"
Contact header field parameter for outbound behavior". This
specification meets this requirement by specifying how the GSMA IMEI
URN is used in the '+sip.instance' Contact header field parameter for
outbound behavior, and RFC 7254 [1] specifies how the GSMA IMEI URN
is constructed.

The GSMA IMEI is a URN for the IMEI -- a globally unique identifier
that identifies mobile devices used in the GSM, Universal Mobile
Telecommunications System (UMTS), and 3rd Generation Partnership
Project (3GPP) Long Term Evolution (LTE) networks. The IMEI
allocation is managed by the GSMA to ensure that the IMEI values are
globally unique. Details of the formatting of the IMEI as a URN are
specified in RFC 7254 [1], and the definition of the IMEI is
contained in 3GPP TS 23.003 [10]. Further details about the GSMA's
role in allocating the IMEI, and the IMEI allocation guidelines, can
be found in GSMA PRD TS.06 [11].

2. Terminology

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 RFC 2119 [7].

3. Background

GSM, UMTS, and LTE capable mobile devices represent 90% of the mobile
devices in use worldwide. Every manufactured GSM, UMTS, or LTE
mobile device has an allocated IMEI that uniquely identifies this
specific mobile device. Among other things, in some regulatory
jurisdictions the IMEI is used to identify that a stolen mobile
device is being used, to help to identify the subscription that is
using it, and to prevent use of the mobile device. While GSM was
originally a circuit switched system, enhancements such as the
General Packet Radio Service (GPRS) and UMTS have added IP data
capabilities that, along with the definition of the IP Multimedia
Subsystem (IMS), have made SIP-based calls and IP multimedia sessions
from mobile devices possible.

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The latest enhancement, known as LTE, introduces even higher data
rates and dispenses with the circuit switched infrastructure
completely. This means that with LTE networks, voice calls will need
to be conducted using IP and IMS. However, the transition to all IP
SIP-based IMS networks worldwide will take a great many years, and
mobile devices, being mobile, will need to operate in both IP/SIP/IMS
mode and circuit switched mode. This means that calls and sessions
will need to be handed over between IP/SIP/IMS mode and circuit
switched mode mid-call or mid-session. Also, since many existing GSM
and UMTS radio access networks are unable to support IP/SIP/IMS-based
voice services in a commercially acceptable manner, some sessions
could have some media types delivered via IP/IMS simultaneously with
voice media delivered via the circuit switched domain to the same
mobile device. To achieve this, the mobile device needs to be
simultaneously attached via both the IP/SIP/IMS domain and the
circuit switched domain.

To meet this need, the 3GPP has specified how to maintain session
continuity between the IP/SIP/IMS domain and the circuit switched
domain in 3GPP TS 24.237 [12], and in 3GPP TS 24.292 [13] has
specified how to access IMS hosted services via both the IP/SIP/IMS
domain and the circuit switched domain.

In order for the mobile device to access SIP/IMS services via the
circuit switched domain, the 3GPP has specified a Mobile Switching
Center (MSC) server enhanced for IMS Centralized Services (ICS) and a
MSC server enhanced for Single Radio Voice Call Continuity (SR-VCC)
that control mobile voice call setup over the circuit switched radio
access while establishing the corresponding voice session in the core
network using SIP/IMS. To enable this, the MSC server enhanced for
ICS or the MSC server enhanced for SR-VCC performs SIP registration
on behalf of the mobile device, which is also simultaneously directly
registered with the IP/SIP/IMS domain. The only mobile device
identifier that is transportable using GSM/UMTS/LTE signaling is the
IMEI; therefore, the instance-id included by the MSC server enhanced
for ICS or the MSC server enhanced for SR-VCC when acting on behalf
of the mobile device, and the instance-id directly included by the
mobile device, both need to be based on the IMEI.

Additionally, in order to meet the above requirements, the same IMEI
that is obtained from the circuit switched signaling by the MSC
server needs to be obtainable from SIP signaling so that it can be
determined that both the SIP signaling and circuit switched signaling
originate from the same mobile device.

For these reasons, 3GPP TS 24.237 [12] and 3GPP TS 24.292 [13]
already specify the use of the URN namespace for the GSMA IMEI URN as
specified in RFC 7254 [1] as the instance-id used by GSM/UMTS/LTE

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mobile devices, the MSC server enhanced for SR-VCC, and the MSC
server enhanced for ICS, for SIP/IMS registrations and emergency-
related SIP requests.

4. 3GPP Use Cases

1. The mobile device includes its IMEI in the SIP REGISTER request
so that the SIP registrar can perform a check of the Equipment
Identity Register (EIR) to verify whether this mobile device is
allowed to access the network for non-emergency services or is
barred from doing so (e.g., because the device has been stolen).
If the mobile device is not allowed to access the network for
non-emergency services, the SIP registrar can reject the
registration and thus prevent a barred mobile device from
accessing the network for non-emergency services.

2. The mobile device includes its IMEI in SIP INVITE requests used
to establish emergency sessions. This is so that the Public
Safety Answering Point (PSAP) can obtain the IMEI of the mobile
device for identification purposes if required by regulations.

3. The IMEI that is included in SIP INVITE requests by the mobile
device and used to establish emergency sessions is also used in
cases of unauthenticated emergency sessions to enable the network
to identify the mobile device. This is especially important if
the unauthenticated emergency session is handed over from the
packet switched domain to the circuit switched domain. In this
scenario, the IMEI is the only identifier that is common to both
domains, so the Emergency Access Transfer Function (EATF) in the
network, which in such cases coordinates the transfer between
domains, can use the IMEI to determine that the circuit switched
call is from the same mobile device that was in the emergency
session in the packet switched domain.

5. User Agent Client Procedures

A User Agent Client (UAC) that has an IMEI as specified in 3GPP TS
23.003 [10] and that is registering with a 3GPP IMS network MUST
include in the "sip.instance" media feature tag the GSMA IMEI URN
according to the syntax specified in RFC 7254 [1] when performing the
registration procedures specified in RFC 5626 [2] or RFC 5627 [3], or
any other procedure requiring the inclusion of the "sip.instance"
media feature tag. The UAC SHOULD NOT include the optional 'svn'
parameter in the GSMA IMEI URN in the "sip.instance" media feature
tag, since the software version can change as a result of upgrades to
the device firmware that would create a new instance-id. Any future
non-zero values of the 'vers' parameter, or the future definition of
additional parameters for the GSMA IMEI URN that are intended to be

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used as part of an instance-id, will require that an update be made
to this RFC. The UAC MUST provide character-by-character identical
URNs in each registration according to RFC 5626 [2]. Hence, any
optional or variable components of the URN (e.g., the 'vers'
parameter) MUST be presented with the same values and in the same
order in every registration as in the first registration.

A UAC MUST NOT use the GSMA IMEI URN as an instance-id, except when
registering with a 3GPP IMS network. When a UAC is operating in IMS
mode, it will obtain from the Universal Integrated Circuit Card
(UICC) (commonly known as the SIM card) the domain of the network
with which to register. This is a carrier's IMS network domain. The
UAC will also obtain the address of the IMS edge proxy to send the
REGISTER request containing the IMEI using information elements in
the Attach response when it attempts to connect to the carrier's
packet data network. When registering with a non-3GPP IMS network, a
UAC SHOULD use a UUID as an instance-id as specified in RFC 5626 [2].

A UAC MUST NOT include the "sip.instance" media feature tag
containing the GSMA IMEI URN in the Contact header field of non-
REGISTER requests, except when the request is related to an emergency
session. Regulatory requirements can require that the IMEI be
provided to the PSAP. Any future exceptions to this prohibition will
require the publication of an RFC that addresses how privacy is not
violated by such usage.

6. User Agent Server Procedures

A User Agent Server (UAS) MUST NOT include its "sip.instance" media
feature tag containing the GSMA IMEI URN in the Contact header field
of responses, except when the response is related to an emergency
session. Regulatory requirements can require that the IMEI be
provided to the PSAP. Any future exceptions to this prohibition will
require the publication of an RFC that addresses how privacy is not
violated by such usage.

7. 3GPP SIP Registrar Procedures

In 3GPP IMS, when the SIP registrar receives in the Contact header
field a "sip.instance" media feature tag containing the GSMA IMEI URN
according to the syntax specified in RFC 7254 [1] the SIP registrar
follows the procedures specified in RFC 5626 [2]. The IMEI URN MAY
be validated as described in RFC 7254 [1]. If the UA indicates that
it supports the extension in RFC 5627 [3] and the SIP registrar
allocates a public GRUU according to the procedures specified in
RFC 5627 [3], the instance-id MUST be obfuscated when creating the
'gr' parameter in order not to reveal the IMEI to other UAs when the

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public GRUU is included in non-REGISTER requests and responses. 3GPP
TS 24.229 [8] subclause 5.4.7A.2 specifies the mechanism for
obfuscating the IMEI when creating the 'gr' parameter.

8. Security Considerations

Because IMEIs, like other formats of instance-ids, can be correlated
to a user, they are personally identifiable information and therefore
MUST be treated in the same way as any other personally identifiable
information. In particular, the "sip.instance" media feature tag
containing the GSMA IMEI URN MUST NOT be included in requests or
responses intended to convey any level of anonymity, as this could
violate the user's privacy. RFC 5626 [2] states that "One case where
a UA could prefer to omit the "sip.instance" media feature tag is
when it is making an anonymous request or some other privacy concern
requires that the UA not reveal its identity". The same concerns
apply when using the GSMA IMEI URN as an instance-id. Publication of
the GSMA IMEI URN to networks to which the UA is not attached, or
with which the UA does not have a service relationship, is a security
breach, and the "sip.instance" media feature tag MUST NOT be
forwarded by the service provider's network elements when forwarding
requests or responses towards the destination UA. Additionally, an
instance-id containing the GSMA IMEI URN identifies a mobile device
and not a user. The instance-id containing the GSMA IMEI URN MUST
NOT be used alone as an address for a user or as an identification
credential for a user. The GRUU mechanism specified in RFC 5627 [3]
provides a means to create URIs that address the user at a specific
device or User Agent.

Entities that log the instance-id need to protect them as personally
identifiable information. Regulatory requirements can require that
carriers log SIP IMEIs.

In order to protect the "sip.instance" media feature tag containing
the GSMA IMEI URN from being tampered with, those REGISTER requests
containing the GSMA IMEI URN MUST be sent using a security mechanism
such as Transport Layer Security (TLS) (RFC 5246 [5]) or another
security mechanism that provides equivalent levels of protection such
as hop-by-hop security based upon IPsec.

9. Acknowledgements

The author would like to thank Paul Kyzivat, Dale Worley, Cullen
Jennings, Adam Roach, Keith Drage, Mary Barnes, Peter Leis, James Yu,
S. Moonesamy, Roni Even, and Tim Bray for reviewing this document and
providing their comments.

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10. References

10.1. Normative References

[1] Montemurro, M., Ed., Allen, A., McDonald, D., and P. Gosden, "A
Uniform Resource Name Namespace for the Global System for Mobile
Communications Association (GSMA) and the International Mobile
station Equipment Identity (IMEI)", RFC 7254, May 2014.

[2] Jennings, C., Mahy, R., and F. Audet, "Managing Client-
Initiated Connections in the Session Initiation Protocol (SIP)",
RFC 5626, October 2009.

[3] Rosenberg, J., "Obtaining and Using Globally Routable User Agent
URIs (GRUUs) in the Session Initiation Protocol (SIP)", RFC
5627, October 2009.

[4] Moats, R., "URN Syntax", RFC 2141, May 1997.

[5] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS)
Protocol Version 1.2", RFC 5246, August 2008.

[6] Leach, P., Mealling, M., and R. Salz, "A Universally Unique
IDentifier (UUID) URN Namespace", RFC 4122, July 2005.

[7] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.

[8] 3GPP, "IP multimedia call control protocol based on Session
Initiation Protocol (SIP) and Session Description Protocol
(SDP); Stage 3", 3GPP TS 24.229 (Release 8), March 2014,
<ftp://ftp.3gpp.org/Specs/archive/24_series/ 24.229/>.

[9] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.

10.2. Informative References

[10] 3GPP, "Numbering, addressing and identification", 3GPP TS 23.003
(Release 8), March 2014, <ftp://ftp.3gpp.org/Specs/
archive/23_series/23.003/>.

[11] GSM Association, "IMEI Allocation and Approval Guidelines", PRD
TS.06 (DG06) Version 6.0, July 2011,
<http://www.gsma.com/newsroom/wp-content/uploads/2012/06/
ts0660tacallocationprocessapproved.pdf>.

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[12] 3GPP, "Mobile radio interface Layer 3 specification; Core
network protocols; Stage 3", 3GPP TS 24.237 (Release 8),
September 2013, <ftp://ftp.3gpp.org/Specs/archive/
24_series/24.237/>.

[13] 3GPP, "IP Multimedia (IM) Core Network (CN) subsystem
Centralized Services (ICS); Stage 3", 3GPP TS 24.292 (Release
8), December 2013, <ftp://ftp.3gpp.org/Specs/
archive/24_series/24.292/>.

Author's Address

Andrew Allen (editor)
Blackberry
1200 Sawgrass Corporate Parkway
Sunrise, Florida 33323
USA

EMail: [email protected]

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