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Copyright Notice

Copyright (C) The Internet Society (2004). All Rights Reserved.

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
4.1  SIP URIs for Emergency Calls
4.2  Tel URIs for Emergency Calls
5.  Request Handling
6.  Identifying the Local Emergency Numbers
7.  Alternative Identifiers Considered
8.  IANA Considerations
9.  Security Considerations
10.  Acknowledgements
§  Normative References
§  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], 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 [schulzrinne-sipping-emergency-arch]. 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.

2. Terminology

In this document, the key words "MUST", "MUSTNOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119[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 must be able to use their familiar "home" emergency identifier. Users should 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], 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

A single, global (set of) identifiers 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 emergency call identifier in the Request-URI. The Request-URI MUST be either an emergency SIP URI defined in Section Section 4.1 or an emergency tel URI defined in Section Section 4.2.

4.1 SIP URIs for Emergency Calls

It is RECOMMENDED that SIP-based [RFC3261] end systems and proxy servers support a uniform emergency call identifier, namely 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. 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 addition, we reserve user addresses beginning with the string "sos." for specific emergency services:

sos.fire      fire brigade
sos.rescue    ambulance (rescue)
sos.marine    marine guard
sos.police    police (law enforcement)
sos.mountain  mountain rescue

The sub-addresses are also case-insensitive. Additional subaddresses can be registered with IANA (Section Section 8).

(In some areas, these emergency services use different numbers.)

The SIP URI user name "sos" and user names starting with "sos." MUSTNOT be assigned to any regular user.

4.2 Tel URIs for Emergency Calls

User agents SHOULD determine the local emergency numbers, either by consulting their manual configuration for devices that do not move across national borders, by DHCP, DNS NAPTR or some other configuration mechanism [schulzrinne-sipping-emergency-arch]. If a user agent has no knowledge of local emergency numbers, it MUST also recognize the digit strings 000, 08, 112, 110, 118, 119, 911 and 999 as emergency numbers.

(SIP user agents, such as Ethernet deskphones, that are unlikely to move frequently across national borders can easily implement a local dialing plan that recognizes local emergency numbers. Mobile devices, including PDAs and laptops, may not have a reliable way of determining their current location. Using automatic configuration avoids collisions with extensions that equal one of the eight numbers above. If a local network does not have an outbound proxy server, local dial plans also do not apply, so the problem of number collision does not arise. Collisions with non-emergency service numbers are still possible, albeit less likely. For example, 118 is used for directory assistance in Finland.)

If the user dials any of these digit strings, the UAC SHOULD generate a request with the "sos" URI described in Section Section 4.1 unless it has discovered a local outbound proxy. In that case, a UAC MAY use a "tel" URI [RFC2806] without 'phone-context', such as

tel:911
tel:112

Outbound proxy servers MUST be configurable to recognize additional local emergency numbers 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. Request Handling

Once identified, a user agent can either determine the appropriate ECC locally or delegate this task to an outbound proxy. Details are in [schulzrinne-sipping-emergency-arch].

Outbound proxy servers MUST recognize all local emergency numbers as well as the tel URIs enumerated in Section Section 4.2. The proxy MAY use any additional information contained in the call request, such as Mobile Country Code and the Mobile Network Code for 3GPP devices, to recognize additional numbers as 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.

OPTIONS requests to the user "sos" and the "sos.*" addresses (sos.fire, etc.) can be used to test if the "sos" addresses are valid. As in standard SIP, a 200 (OK) response indicates that the address was recognized and a 404 (Not found) that it was not. Such request cause no further action. It is RECOMMENDED that user agents periodically automatically check for the availability of the "sos" identifier and alert the user if the check fails. The period of such automated checks SHOULDNOT be less than once per day and MUST be randomly placed over the testing interval.

6. 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 [schulzrinne-sipping-emergency-arch.] Given the different trade-offs in user agent implementation complexity and deployment difficulty, it appears likely that multiple such mechanisms will co-exist.

7. Alternative Identifiers Considered

The "sos" SIP URI reserved user name proposed here follows the convention of RFC 2142[RFC2142] and the "postmaster" convention documented in RFC 2822[RFC2822]. One drawback is that it may conflict with locally assigned addresses of the form "sos@somewhere".

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.

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. 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 alphanumeric characters only and is case-insensitive.
• Descriptive text that describes the emergency service.

9. Security Considerations

The SIP specification [RFC3261] 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.

Additional security considerations related to call routing, destination authentication and other issues are detailed in [schulzrinne-sipping-emergency-arch].

10. Acknowledgements

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

Normative References

Informative References

Author's Address

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