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ECRIT Working Group                                             M. Patel
Internet-Draft                               InterDigital Communications
Intended status: Standards Track                        November 9, 2010
Expires: May 13, 2011

 SOS Uniform Resource Identifier (URI) Parameter for Marking of Session
    Initiation Protocol (SIP) Requests related to Emergency Services


   This document defines a new Session Initiation Protocol (SIP) Uniform
   Resource Identifier (URI) parameter intended for marking SIP
   registration requests related to emergency calls and allow admission
   control to ensure successful initiation of emergency calls.  The
   usage of this new URI parameter complements the usage of the Service
   Uniform Resource Name (URN) and is not intended to replace it.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on May 13, 2011.

Copyright Notice

   Copyright (c) 2010 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
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   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   3.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . . . 4
   4.  The "sos" URI Parameter . . . . . . . . . . . . . . . . . . . . 4
     4.1.  REGISTER Request  . . . . . . . . . . . . . . . . . . . . . 4
     4.2.  2xx Response to REGISTER Request  . . . . . . . . . . . . . 5
     4.3.  Backwards compatibility issues  . . . . . . . . . . . . . . 5
   5.  Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     9.2.  Informative References  . . . . . . . . . . . . . . . . . . 8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8

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1.  Introduction

   One way to differentiate a SIP-based emergency call from an ordinary
   call is by the presence of the Service URN as defined in RFC 5031
   [RFC5031] (and used in the IETF emergency services architecture
   described in PhoneBCP[I-D.ietf-ecrit-phonebcp]).  The 3GPP IP
   Multimedia Subsystem (IMS) emergency services architecture,
   illustrated in 3GPP TS 23.167 [3GPP.23.167], specifies that the User
   Equipment (UE) needs to perform emergency registration prior to or
   during the initiation of an emergency call.

   In some countries, it is a regulatory requirement that devices be
   able to place emergency calls in circumstances where other calls may
   not be permitted.  When a UAC issues an emergency marked REGISTER
   request it indicates to the registrar that roaming and barring
   restrictions should not be applied for the registered address-of-
   record in order to successfully initiate an emergency session.
   Furthermore, distinguishing emergency registration from non-emergency
   registration allows the registrar to ensure that the contact address
   associated with previous registration of the address-of-record
   included in the emergency REGISTER request is not replaced.

   Emergency registration is possible only when the UE has sufficient
   credentials to register with its home network and can detect that an
   emergency session is initiated.  Unfortunately, marking of the
   emergency registration cannot be fulfilled by the use of the Service
   URN.  The circumstances where such an emergency registration is
   beneficial are listed below:

   - the UE is not registered with its home network;

   - the UE is currently registered but roaming (to ensure that the
   emergency call is handled in the visited network, as required by some

   This document concentrates on a use case defined by 3GPP as described
   above.  However, the solution proposed does not preclude other
   systems that require emergency registration to occur prior to placing
   an emergency call, to ensure that any subscription related
   restrictions are removed to allow successful initiation of emergency

   This document proposes a way to mark a REGISTER request as an
   emergency registration.

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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119]

3.  Requirements

   Req: Where emergency registration is required prior to placing an
   emergency call, it shall be possible to distinguish emergency
   registration from non-emergency registration.

4.  The "sos" URI Parameter

   This section provides an overview of the proposed new URI parameter
   to be used for marking REGISTER requests related to emergency

   A new URI parameter "sos" is defined in this document.  The "sos"
   parameter is appended to a URI consistent with RFC 3261 [RFC3261].
   It is proposed that use of this URI parameter is restricted to the
   Contact header included in the REGISTER request (and the 2xx response
   to the REGISTER request) related to an emergency call only.

   Inclusion of the "sos" URI parameter in a REGISTER request SHALL
   indicate that the REGISTER request pertains to emergency
   registration.  The "sos" URI parameter MUST NOT be considered as a
   replacement for the Service URN for emergency calls originated by a

4.1.  REGISTER Request

   In networks where the UA sends a REGISTER request for emergency
   registration prior to placing an emergency call, the "sos" URI
   parameter MUST be appended to the URI in the Contact header.  This
   serves as an indication to the registrar that the request is for
   emergency registration thus requesting the registrar to not apply any
   restrictions to the user's service which might prevent emergency
   calls from successfully being initiated.


   Contact: "Alice" <sip:alice@example.com;sos> ;q=0.7; expires=3600

   In the event that more than one Contact header field is included in
   the REGISTER request, only the contact addresses that include the

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   "sos" URI parameter shall be considered as emergency registered
   contact addresses.

   The "sos" URI parameter MUST NOT be included in non-REGISTER
   requests, and MUST NOT be included in REGISTER requests that do not
   pertain to emergency calls.

4.2.  2xx Response to REGISTER Request

   If the registrar receives a REGISTER request that includes the "sos"
   URI parameter in the Contact header field, the registrar MUST include
   the "sos" URI parameter in the Contact header field in the 200 (OK)
   response sent by the registrar upon successful registration.  The
   "sos" URI parameter is appended to the URI included in the Contact

4.3.  Backwards compatibility issues

   The backwards compatibility scenario considered in this document is
   where a legacy registrar does not support the "sos" URI parameter.
   In this case, if the registrar receives a REGISTER request that
   includes the "sos" URI parameter in the Contact header field, the
   registrar proceeds with registration procedures and silently ignores
   the URI-parameter in accordance with RFC 3261[RFC3261].  This ensures
   the user is registered and thus can successfully initiate an
   emergency call.

   The drawback of proceeding with registration is if the address-of-
   record is for example barred or has roaming restrictions applied,
   then these restrictions will not be lifted and thus registration will
   be unsuccessful.  This can limit the UAC's ability to successfully
   place an emergency call.

   If registration is successful, the 200 (OK) response from a legacy
   registrar includes the "sos" URI parameter in the Contact header
   field.  Thus the UA is unaware that the registrar does not support
   the "sos" URI parameter.  Providing the registration was successful,
   the UA's ability to place an emergency call is not compromised.  The
   UA need not know that the registrar does not support the URI

   The consequence of the registrar not supporting the "sos" URI
   parameter, in addition to the drawback pertaining to restrictions
   applied to the address-of-record, are as follows:

   - the risk of the registrar overwriting previous registrations of the
   registered address-of-record, and thus disrupting any on-going non-
   emergency sessions associated with the UA, its address-of-record and

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   previously registered contact address.

   - incoming calls, such as a PSAP call back (to a previously made
   emergency call) to the registered address-of-record might not be
   routed correctly to the UA that placed the emergency call, due to not
   suppressing any network based services such as call forwarding, or UA
   based services which can divert the call elsewhere, or if the
   address-of-record is associated to more than one contact address.

5.  Formal Syntax

   The following syntax specification uses the augmented Backus-Naur
   Form (BNF) as described in RFC 5234 [RFC5234].

   The "sos" URI parameter is a "uri-parameter", as defined by RFC

   uri-parameter =/ sos-param

   sos-param = "sos"

6.  IANA Considerations

   This specification defines one new SIP URI parameter, as per the
   registry created by RFC 3969 [RFC3969]

   Parameter Name: sos

   Predefined Values: none

   Reference: [RFCXXXX]

   [NOTE TO IANA: Please replace XXXX with the RFC number of this

7.  Security Considerations

   As an identifier, the "sos" parameter itself does not raise any
   particular security issues.  The semantics described by the "sos"
   parameter are meant to be well-known so privacy considerations do not
   apply to the URI parameter.  The main possibility of attack involves
   use of the "sos" parameter to bypass the normal procedures in order
   to achieve fraudulent use of services or to bypass security
   procedures.  The usage of this parameter as described in this
   document is purely for the purpose of the REGISTER request and hence

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   in presence of user authentication it is ensured that the respective
   user can be held accountable.

   It is RECOMMENDED to log events of misuse of the "sos" URI parameter,
   for example by including it in a request or response not related to
   an emergency call.

   Emergency registration can result in removing restrictions for
   roaming and/or barring of services.  Misuse of the emergency
   registered AoR and contact address can be identified within the
   network and thus requests for unauthorized service will be rejected.
   Thus, no security considerations related to hijacking of services are
   foreseen as a result of applying a marking of emergency registrations
   through the use of a SIP URI parameter.

8.  Acknowledgements

   The author would like to thank Keith Drage, Milo Orsic, Deb Barclay,
   John-Luc Bakker, Andrew Allen, Hiroshi Ishikawa, Sean Schneyer, Peter
   Leis, Georg Mayer, Marvin Bienn, Ricky Kaura, Steve Norreys, Laura
   Liess, AC Mahendran, Roozbeh Atarius, Ramachandran Subramanian and
   Sandeep Sharma, Brian Rosen, Hannes Tschofenig, Christer Holmberg and
   Henning Schulzrinne for the discussions and contributions that led to
   this work.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3261]  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.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC3969]  Camarillo, G., "The Internet Assigned Number Authority
              (IANA) Uniform Resource Identifier (URI) Parameter
              Registry for the Session Initiation Protocol (SIP)",
              BCP 99, RFC 3969, December 2004.

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9.2.  Informative References

              Rosen, B. and J. Polk, "Best Current Practice for
              Communications Services in support of Emergency Calling",
              draft-ietf-ecrit-phonebcp-16 (work in progress),
              October 2010.

   [RFC5031]  Schulzrinne, H., "A Uniform Resource Name (URN) for
              Emergency and Other Well-Known Services", RFC 5031,
              January 2008.

              3GPP, "IP Multimedia Subsystem (IMS) emergency sessions",
              3GPP TS 23.167 10.1.0, September 2010.

Author's Address

   Milan Patel
   InterDigital Communications

   Email: Milan.Patel@interdigital.com

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