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Copyright Notice

Copyright © The Internet Society (2005).

Abstract

As part of an overall architecture for supporting emergency calling for the Session Initiation Protocol (SIP), this document defines universal emergency SIP URIs, sip:sos@domain and sips:sos@domain, that allows SIP user agents to contact the local emergency call center. It also defines conventions that increase the high probability of reaching the appropriate emergency call center. The document does not define any SIP protocol extensions.

Table of Contents

1.  Introduction
2.  Terminology
3.  Requirements
4.  Emergency URIs
5.  Identifying the Local Emergency Numbers
6.  Request Handling
7.  Alternative Approaches Considered
8.  Media Feature Tag Registration: Service
9.  IANA Considerations
10.  Security Considerations
11.  Acknowledgements
12.  References
    12.1  Normative References
    12.2  Informative References
§  Author's Address
§  Intellectual Property and Copyright Statements

1. Introduction

Using the public switched telephone network (PSTN), emergency help can often be summoned at a designated, widely known number, regardless of where the telephone was purchased. However, this number differs between localities, even though it is often the same for a country or continent-size region (such as many countries in the European Union or North America). For end systems based on the Session Initiation Protocol (SIP) [RFC3261] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.), it is desirable to have a universal identifier, independent of location, to simplify the user experience and to allow the device to perform appropriate processing. Here, we define a common user identifier, "sos", as the contact mechanism for emergency assistance. This identifier is meant to be used in addition to any local emergency numbers.

This document specifies only a small part of a comprehensive set of recommendations for operating emergency services. The overall architecture is described in [I-D.schulzrinne-sipping-emergency-arch] (Schulzrinne, H. and B. Rosen, “Emergency Services for Internet Telephony Systems,” October 2004.). That document describes, for example, how a device that identifies a call as an emergency call can route it to the appropriate emergency call center (ECC).

This document does not introduce any new SIP header fields, request methods, status codes, message bodies, or events. User agents unaware of the recommendations in this draft can place emergency calls, but may not be able to provide the same user interface functionality. The document suggests behavior for proxy servers, in particular outbound proxy servers.

The solution described here is not as general as the alternative approach, service URNs (Schulzrinne, H., “A Uniform Resource Name (URN) for Services,” July 2005.)[I-D.schulzrinne-sipping-service], but requires no changes to end systems or proxies.

2. Terminology

In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.)[RFC2119] and indicate requirement levels for compliant implementations.

3. Requirements

• It should be possible for devices to provide user interfaces that can directly cause an emergency call, without the user having to "dial" or type a specific address.
• Even as each country is likely to operate their emergency calling infrastructure differently, SIP devices should be able to reach emergency help and, if possible, be located in any country.
• While traveling, users should be able to use their familiar "home" emergency number. Users must also be able to dial the local emergency number in the country they are visiting.
• Any mechanism must be deployable incrementally and work even if not all SIP entities support emergency calling. User agents conforming to the SIP specification [RFC3261] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.), but unaware of this document, must be able to place emergency calls, possibly with restricted functionality.
• Given incremental deployment, emergency call functionality should be testable by the user without causing an emergency response.
• Emergency calling mechanisms must support existing emergency call centers based on circuit-switched technology as well as future ECC that are SIP-capable.

4. Emergency URIs

Having a single, global identifier for emergency services is highly desirable, as it allows end system and network devices to be built that recognize such services and can act appropriately. Such actions may include restricting the functionality of the end system, providing special features, overriding user service constraints or routing session setup messages.

SIP user agents (UAs) that determine that a dialog or transaction relates to an emergency MUST use an an emergency SIP URI defined below as the Request-URI and "To" header field.

It is RECOMMENDED that SIP-based [RFC3261] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) end systems and proxy servers support a uniform emergency call identifier, namely the SIP and SIPS URIs with the reserved user name "sos" within any domain, e.g.,

  sip:sos@example.com
  sips:sos@example.com

The reserved name is case-insensitive.

The host part of the emergency URI SHOULD be the host portion of the address-of-record of the caller. The "sips" form SHOULD be used to ensure integrity and confidentiality; the "sip" form MAY be used if a "sips" call fails with status code 416 (Unsupported URI Scheme). All SIP requests with URIs of this form are assumed to be emergency calls.

The domain-of-record was chosen since a SIP user agent may not be able to determine the local domain it is visiting. This also allows each user to test this facility, as the user can ensure that such services are operational in his home domain. An outbound proxy in the visited domain can handle the call if it believes to be in a position to provide appropriate emergency services.

In some cases, end users or, more likely, emergency service routing proxies may want to request specific emergency services. We support this feature by leveraging the caller preferences [RFC3841] (Rosenberg, J., Schulzrinne, H., and P. Kyzivat, “Caller Preferences for the Session Initiation Protocol (SIP),” August 2004.) extension and define a new media feature tag, service, in Section 8 (Media Feature Tag Registration: Service).

The SIP URI user name "sos" MUST NOT be assigned to any regular user.

Outbound proxy servers MUST be configurable to recognize additional emergency numbers valid in their geographic service area in "tel" URIs.

There are about 60 service numbers for emergency services in the world; including them all is not practical, as that would interfere with existing local two, three and four-digit dialing plans.

5. Identifying the Local Emergency Numbers

There are many ways that a user agent can configure emergency numbers for use in analyzing calls made with telephony-type user input. Such numbers become part of the device dialplan. Mechanisms include configuration tokens such as SIM cards in mobile devices, network-specific solutions (e.g., for 3GPP networks) or protocol-based solutions. Protocol-based solutions, using XCAP and DNS, are discussed in [I-D.schulzrinne-sipping-emergency-arch] (Schulzrinne, H. and B. Rosen, “Emergency Services for Internet Telephony Systems,” October 2004.). Given the different trade-offs in user agent implementation complexity and deployment difficulty, it appears likely that multiple such mechanisms will co-exist.

Regardless of the mechanism chosen and whether any of the configuration mechanisms succeed, the digit strings "112" and "911" MUST always be recognized as emergency numbers.

User agents SHOULD allow users and/or administrators to configure additional emergency numbers.

User agents SHOULD attempt to ascertain the set of emergency numbers that are valid in the geographic region that the user agent is currently located in. Two such mechanisms included XCAP and DNS, discussed in [I-D.schulzrinne-sipping-emergency-arch] (Schulzrinne, H. and B. Rosen, “Emergency Services for Internet Telephony Systems,” October 2004.).

If and only if an end system is unable to determine the emergency numbers valid in its current geographical location, it SHOULD recognize any of the number 000, 08, 110, 999, 118 and 119 as emergency numbers. Since these numbers are also used for non-emergency purposes, their automatic transformation incurs the risk of accidentally dialing an emergency number when, for example, directory assistance was desired. To minimize such mistakes, end systems SHOULD alert users that they are dialing a recognized emergency number.

This behavior corresponds to the guidelines in 3GPP TS 22.101. Since general SIP end points cannot be assumed to have SIMs or USIMs, this document uses the default list (000, 08, 110, 999, 118 and 119) only if no other configuration information about geographically local emergency numbers is available.

6. Request Handling

Outbound proxy servers SHOULD check whether a tel URIs or a SIP URIs containing a dial string represents an emergency number within its geographic service area, but only if they can be reasonably certain that the call originated from within that area, e.g., if the call contained location information or the network is known to only be reachable from a restricted geographic area. Typically, these service areas encompass whole countries since many countries now have nationwide emergency numbers. Once they recognize an emergency number, they translate the Request-URI to an "sos" URI as described above.

The proxy MAY use any additional information contained in the call request to recognize additional numbers as emergency numbers. Such information includes the Mobile Country Code and the Mobile Network Code for 3GPP devices or country information in location information available about the call, The [I-D.schulzrinne-sipping-emergency-arch] (Schulzrinne, H. and B. Rosen, “Emergency Services for Internet Telephony Systems,” October 2004.) document describes a possible DNS-based mechanism to obtain country-specific emergency numbers.

It is RECOMMENDED that gateway SIP MESSAGE requests are directed to a TTY-for-the-deaf translator or a short-message service (SMS) if the emergency call center cannot handle SIP instant messaging.

7. Alternative Approaches Considered

The "sos" SIP URI reserved user name proposed here follows the convention of RFC 2142 (Crocker, D., “MAILBOX NAMES FOR COMMON SERVICES, ROLES AND FUNCTIONS,” May 1997.)[RFC2142] and the "postmaster" convention documented in RFC 2822 (Resnick, P., “Internet Message Format,” April 2001.)[RFC2822]. One drawback is that it may conflict with locally assigned addresses of the form "sos@domain".

There are a number of possible alternatives, each with their own set of advantages and problems:

tel:sos

This solution avoids name conflicts, but is not a valid "tel" URI. It also only works if every outbound proxy knows how to route requests to a proxy that can reach emergency services. The SIP URI proposed here only requires a user's home domain to be appropriately configured.

urn:service:sos

A related document (Schulzrinne, H., “A Uniform Resource Name (URN) for Services,” July 2005.)[I-D.schulzrinne-sipping-service] defines a URN for identifying services, such as emergency calling. This solution fits most cleanly into the overall URI architecture and avoids dependencies on the home domain, but, like the tel URI solution above, also requires that every outbound proxy can resolve this URN and can route calls accordingly. Alternatively, the end system has to be configured with a suitable URN-resolving proxy, e.g., in its home domain.

URI parameter:

One could create a special URI, such as "aor-domain;user=sos". This avoids the name conflict problem, but requires mechanism-aware user agents that are capable of emitting this special URI.

Special domain:

A special domain, such as "sip:fire@sos.int" could be used to identify emergency calls. This has similar properties as the "tel:sos" URI, except that it is indeed a valid URI.

8. Media Feature Tag Registration: Service

This specification defines an additional media feature tag, extending the SIP tree entries described in [RFC3840] (Rosenberg, J., Schulzrinne, H., and P. Kyzivat, “Indicating User Agent Capabilities in the Session Initiation Protocol (SIP),” August 2004.) and following the registration process in Section 12.1 of that document. This section serves as the IANA registration for the service feature tags, which are made into the SIP media feature tag tree.

This facility is not meant to encourage end users to select emergency services where a uniform ECC for all such services exist. Rather, these identifiers reflect current practice in jurisdictions that already have different numbers for the different emergency services. For example, in Germany, ambulance and fire use 112, while police uses 110.

We expect that users will rarely invoke specific emergency services directly. Rather, they might be generated by outbound proxy servers translating dial strings or be generated when pressing icon-bearing speed dial buttons.

Using feature tags has the advantage that they are not affected by entities that translate URIs, e.g., to route emergency calls to a specific ECC.

The service types for this feature tag are case-insensitive. Additional service types can be registered with IANA (Section Section 9 (IANA Considerations)).

Media feature tag name:

sip.emergency-service

ASN.1 Identifier:

New assignment by IANA.

Summary of the media feature indicated by this tag:

Each feature tag indicates the type of communication service requested.

Values appropriate for use with this feature tag:

Token with an equality relationship. Initial values include a number of emergency services:

sos:

general emergency service

sos.fire:

fire brigade

sos.marine:

marine guard

sos.mountain:

mountain rescue

sos.police:

police (law enforcement)

sos.rescue:

ambulance, emergency medical service

sos.test:

testing, not a real emergency call

The feature tag is intended primarily for use in the following applications, protocols, services, or negotiation mechanisms:

This feature tag is most useful in a communications application, for describing the capabilities of a user agent providing a particular type of communication service.

Examples of typical use:

Allowing an emergency service proxy to select the desired emergency service, such as police or ambulance.

Related standards or documents:

RFC3840.

Security Considerations:

Security considerations for this media feature tag are discussed in Section 11.1 of RFC3840.

9. IANA Considerations

Subaddresses of the "sos" address are registered with IANA This specification establishes the "sos" subaddres sub-registry under http://www.iana.org/assignments/sip-parameters.

Subaddresses are registered by the IANA when they are published in standards track RFCs. The IANA Considerations section of the RFC must include the following information, which appears in the IANA registry along with the RFC number of the publication.

• Name of the subaddress. The name MAY be of any length, but SHOULD be no more than twenty characters long. The name MUST consist of NVT alphanumeric characters only and is case-insensitive.
• Descriptive text that describes the emergency service.

10. Security Considerations

The SIP specification [RFC3261] (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) details security considerations that apply to emergency calls as well. Security for emergency calls has conflicting goals, namely to make it as easy and reliable as possible to reach emergency services, while discouraging and possibly tracing prank calls. It appears unlikely that classical authentication mechanisms can be required by emergency call centers, but SIP proxy servers may be able to add identifying information.

Given the sensitive nature of many emergency calls, it is highly desirable to use the "sips" URI to ensure transport-level confidentiality and integrity. However, this may cause the call to fail in some environments.

Allowing the user agent to clearly and unambiguously identify emergency calls makes it possible for the user agent to make appropriate policy decisions. For example, a user agent policy may reveal a different amount of information to the callee when making an emergency call. Local laws may affect what information network servers or service providers may be allowed or be required to release to emergency call centers. They may also base their decision on the user-declared destination of the call.

Recognizing only "sos" in the user's home domain, i.e., the domain of the user's AOR, prevents spoofing where a link points to a fake emergency calling number and leads the user to, for example, include location information in the request.

Additional security considerations related to call routing, destination authentication and other issues are detailed in [I-D.schulzrinne-sipping-emergency-arch] (Schulzrinne, H. and B. Rosen, “Emergency Services for Internet Telephony Systems,” October 2004.).

11. Acknowledgements

Andrew Allen, Keith Drage, Cullen Jennings, Mike Pierce, James Polk, Brian Rosen and John Schnizlein contributed helpful comments.

12. References

12.1 Normative References

12.2 Informative References

Author's Address

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