Network Working Group H. Schulzrinne
Internet-Draft Columbia U.
Expires: August 8, 2004 February 8, 2004
Emergency Services URI for the Session Initiation Protocol
draft-ietf-sipping-sos-00
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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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Emergency URIs . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1 SIP URIs for Emergency Calls . . . . . . . . . . . . . . . . . 6
4.2 Tel URIs for Emergency Calls . . . . . . . . . . . . . . . . . 7
5. Request Handling . . . . . . . . . . . . . . . . . . . . . . . 8
6. Identifying the Local Emergency Numbers . . . . . . . . . . . 9
7. Alternative Identifiers Considered . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
9. Security Considerations . . . . . . . . . . . . . . . . . . . 12
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
Normative References . . . . . . . . . . . . . . . . . . . . . 14
Informative References . . . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . 16
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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 [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.
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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.
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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.
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 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.
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.
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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
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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.
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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.
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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.
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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.
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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.
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
alphanumeric characters only and is case-insensitive.
o Descriptive text that describes the emergency service.
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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].
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10. Acknowledgements
Andrew Allen, Keith Drage, Mike Pierce, James Polk, Brian Rosen and
John Schnizlein contributed helpful comments.
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Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2806] Vaha-Sipila, A., "URLs for Telephone Calls", RFC 2806,
April 2000.
[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
(DHCP-for-IPv4) Option for Session Initiation Protocol
(SIP) Servers", RFC 3361, August 2002.
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Informative References
[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 7042
EMail: hgs@cs.columbia.edu
URI: http://www.cs.columbia.edu
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