Network Working Group H. Schulzrinne
Internet-Draft Columbia U.
Expires: April 26, 2006 October 23, 2005
Emergency Services URI for the Session Initiation Protocol
draft-ietf-sipping-sos-01
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Copyright (C) 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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Emergency URIs . . . . . . . . . . . . . . . . . . . . . . . 4
5. Identifying the Local Emergency Numbers . . . . . . . . . . 5
6. Request Handling . . . . . . . . . . . . . . . . . . . . . . 6
7. Alternative Approaches Considered . . . . . . . . . . . . . 6
8. Media Feature Tag Registration: Service . . . . . . . . . . 7
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . 8
10. Security Considerations . . . . . . . . . . . . . . . . . . 8
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
12.1 Normative References . . . . . . . . . . . . . . . . . . 9
12.2 Informative References . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . 11
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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 [I-D.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.
The solution described here is not as general as the alternative
approach, service URNs [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
[RFC2119] and indicate requirement levels for compliant
implementations.
3. Requirements
o 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.
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o 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.
o 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.
o 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.
o Given incremental deployment, emergency call functionality should
be testable by the user without causing an emergency response.
o 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] 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.
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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] extension
and define a new media feature tag, service, in Section 8.
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]. 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].
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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] 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 [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@domain".
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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 [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] 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.
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Additional service types can be registered with IANA (Section
Section 9).
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.
o 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.
o Descriptive text that describes the emergency service.
10. Security Considerations
The SIP specification [RFC3261] details security considerations that
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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].
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
[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.
[RFC3361] Schulzrinne, H., "Dynamic Host Configuration Protocol
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(DHCP-for-IPv4) Option for Session Initiation Protocol
(SIP) Servers", RFC 3361, August 2002.
[RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
"Indicating User Agent Capabilities in the Session
Initiation Protocol (SIP)", RFC 3840, August 2004.
[RFC3841] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
Preferences for the Session Initiation Protocol (SIP)",
RFC 3841, August 2004.
12.2 Informative References
[I-D.schulzrinne-sipping-emergency-arch]
Schulzrinne, H. and B. Rosen, "Emergency Services for
Internet Telephony Systems",
draft-schulzrinne-sipping-emergency-arch-02 (work in
progress), October 2004.
[I-D.schulzrinne-sipping-service]
Schulzrinne, H., "A Uniform Resource Name (URN) for
Services", draft-schulzrinne-sipping-service-00 (work in
progress), July 2005.
[RFC2142] Crocker, D., "MAILBOX NAMES FOR COMMON SERVICES, ROLES AND
FUNCTIONS", RFC 2142, May 1997.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.
Author's Address
Henning Schulzrinne
Columbia University
Department of Computer Science
450 Computer Science Building
New York, NY 10027
US
Phone: +1 212 939 7004
Email: hgs@cs.columbia.edu
URI: http://www.cs.columbia.edu
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