Internet Engineering Task Force
Internet Draft H. Schulzrinne
Columbia U.
draft-schulzrinne-sipping-sos-03.txt
December 6, 2002
Expires: May 2003
Emergency Services for Internet Telephony based on the Session
Initiation Protocol (SIP)
STATUS OF THIS MEMO
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
This document defines a universal emergency SIP URI, sip:sos@domain,
that allows SIP user agents to contact the local emergency number. It
also describes how such calls are routed for both PSTN and IP-based
emergency call centers.
1 Introduction
Using the 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 region (such as many countries in the
European Union). For end systems based on the Session Initiation
Protocol (SIP) [2], it is desirable to have a universal identifier,
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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.
We also describe how such calls are routed to the appropriate
emergency call center (ECC). (In the United States and Canada,
emergency call centers are referred to as Public Safety Answering
Points (PSAPs).) Since each emergency call center is generally only
responsible for a specific geographic area, it is important that
calls are routed to the correct ECC. Regardless of whether the ECC is
connected to the PSTN or is directly reachable via SIP, the network
location of the caller has little relationship to its physical
location. If the call is routed through a PSTN gateway, the
originating number is likely either associated with the gateway or is
permanently assigned to the IP phone, regardless of where it is
currently located. For SIP-based ECCs, the IP address or Contact
header information in the call only provides crude approximation as
to the geographic location of the caller and may well be completely
wrong if virtual private networks are used. Thus, the SIP request
needs to convey the location of the caller so that the call can be
routed appropriately. Section 5 discusses one possible approach.
1.1 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 RFC 2119 [1] and
indicate requirement levels for compliant SIP implementations.
2 Requirements
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. The details of the emergency service
and the associated end system and network server policies can be
specific to jurisdictions and are beyond the scope of this document.
3 Emergency URI
It is RECOMMENDED that SIP-based [2] end systems and proxy servers
support a uniform emergency call identifier, namely the user name
"sos" at any domain, e.g.,
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sip:sos@example.com
The host part of the emergency URI SHOULD be the host portion of the
address-of-record of the caller.
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, user agents and proxies SHOULD also recognize the
telephone numbers 911 and 112, expressed as a "tel" URI [3,4] such as
tel:911;phone-context=+1
tel:112;phone-context=+1
for this purpose. Where feasible, user agents SHOULD recognize
additional, local emergency numbers. Outbound proxy servers MUST be
configurable to recognize additional local emergency numbers.
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.
In addition, we define subaddresses of 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
In some areas, these emergency services use different
numbers.
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The "sos" user name and user names starting with "sos." MUST NOT be
assigned to any regular user. It is RECOMMENDED that 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
messaging.
4 Request Handling
A user agent SHOULD direct an "sos" request to an outbound proxy
server.
Using a proxy server that is local to the user agent is
more likely to reach a geographically local server,
although that is not guaranteed if virtual private networks
are being used.
User agent servers and proxy servers MUST NOT require that the user
agent client be registered or authenticated in order to place an
emergency call.
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 SHOULD NOT be less than once per day and MUST be
randomly placed over the testing interval.
Any proxy, outbound or otherwise, that receives such a request MUST
forward (proxy) or redirect the request to the appropriate local
emergency number (e.g., 911 in the United States or 112 in Europe).
Typically, the proxy server routes the call to an appropriate PSTN
gateway, translating the request URI to the local emergency number.
Any SIP PSTN gateway shall translate this emergency identifier to the
locally supported emergency number. Determining the appropriate
number is discussed in Section 5.
If a proxy receives a "sos.*" request (such as sos.fire), the proxy
forwards it to the appropriate emergency service. If it does not
recognize the suffix (e.g., fire), it MUST forward the request to the
appropriate general emergency contact, handling it as if the address
was "sos".
5 Request Routing
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Each SIP request directed to the emergency URI SHOULD contain
information about the caller's location. We outline a mechanism
below.
1. The SIP UA determines its location, preferably ahead of the
emergency call. It MAY use DHCP [5], retrieving the the
location either as geospatial coordinates (longitude,
latitude and altitude) [6] or as a civil address (street
and community)
The UA needs to inform the DHCP server about its network
attachment point. There are several possibilities,
including use of the RFC 3046 [7] Agent-Circuit-ID or
Remote-ID sub-options. This approach will only work if the
DHCP relay agent is colocated with the LAN device close to
the SIP UA. Another option, not yet fully supported, is to
have the UA determine the device and port information and
then include this in the DHCP request. There currently is
no DHCP option for doing so, however.
2. It then inserts this location into a SIP header field, for
example:
Location: ;lat=38.89868 ;long=-77.03723 ;alt=15 ;alt-unit=m
;lares=0.000122 ;lores=0.000122
;hno=600 ;lmk="White House" ;mcn="Washington"
;stn="Pennsylvania" ;sts="Ave" ;sta="DC"
;privacy=dnf
Here, we assume that the DHCP option provided a resolution
of 22 bits. The example is taken from [8].
(The SIP header field format is fictitious and is defined
in TBD.)
Alternatively, the outbound proxy may map the UA's device
address to a physical location, e.g., based on a traceback
within an Ethernet switched LAN. Such mechanisms are beyond
the scope of this document.
3. The outbound proxy or recipient of the SIP request uses the
location information to determine the address of the
emergency call center. We call this entity the emergency
call router (ECR). The ECR needs to make a two-step
determination. First, it determines if the caller is local,
i.e., guaranteed to be served by the same ECC ("emergency
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service area"). For example, this may be the case for an
ECR located on a corporate or university campus or a DSL
provider which precisely knows that a subset of its lines
are local to a ECC service area. If the caller is not
local, it needs to look up the serving ECC in a database.
The protocol for doing this lookup is currently not
standardized.
There are two basic decisions:
1. The ECC uses SIP-based technology, regardless of
location. In that case, the ECR simply proxies or
redirects the request to the SIP-capable ECC. Note
that the ECC can also be a front-end for a regional or
national call routing service that in turn finds the
correct emergency dispatch center.
2. The ECC uses traditional technology. Here, we have two
sub-cases:
1. The caller and ECR are known to be served by the
same ECC. In that case, the ECR places a normal
emergency call.
2. The caller and ECR are not in the same emergency
service area. In that case, a database lookup
determines the PSTN number of the ECC. The ECR
then routes the call to an appropriate PSTN
gateway that can place the call. Ideally, the
gateway may be local to the ECC, but that may not
be achievable, as it would require a gateway in
every community.
In the United States, for example, there are
about 5,000 primary emergency call centers,
called Public Safety Answering Points
(PSAPs).
In both of these cases, the ECR causes the calling
number to be a telephone number that is mapped by the
ECC to the location of the caller. (The process for
creating such mappings is beyond the scope of this
document. The process has been demonstrated in some
jurisdictions for multi-line telephone systems.) It is
not clear whether all circuit-switched trunk
technologies allow potentially arbitrary, out-of-area
calling numbers.
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6 Alternative Identifiers Considered
The scheme proposed here follows the convention of RFC 2142 [9]. 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.
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.
7 Security Considerations
The SIP specification [2] details a number of security
considerations. 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.
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.
8 Normative References
[1] S. Bradner, "Key words for use in RFCs to indicate requirement
levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.
[2] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, J.
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Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: session
initiation protocol," RFC 3261, Internet Engineering Task Force, June
2002.
9 Informative References
[3] A. Vaha-Sipila, "URLs for telephone calls," RFC 2806, Internet
Engineering Task Force, Apr. 2000.
[4] H. Schulzrinne and A. Vaha-Sipila, "The tel URI for telephone
calls," Internet Draft, Internet Engineering Task Force, Oct. 2002.
Work in progress.
[5] R. Droms, "Dynamic host configuration protocol," RFC 2131,
Internet Engineering Task Force, Mar. 1997.
[6] J. Polk et al., "DHCP option for geographic location," Internet
Draft, Internet Engineering Task Force, Oct. 2002. Work in progress.
[7] M. Patrick, "DHCP relay agent information option," RFC 3046,
Internet Engineering Task Force, Jan. 2001.
[8] J. Polk et al., "Semantics for DHC location object within
GEOPRIV," Internet Draft, Internet Engineering Task Force, Oct.
2002. Work in progress.
[9] D. Crocker, "Mailbox names for common services, roles and
functions," RFC 2142, Internet Engineering Task Force, May 1997.
10 Acknowledgements
Andrew Allen, James Polk, Brian Rosen and John Schnizlein contributed
helpful comments.
11 Authors' Addresses
Henning Schulzrinne
Dept. of Computer Science
Columbia University
1214 Amsterdam Avenue
New York, NY 10027
USA
electronic mail: schulzrinne@cs.columbia.edu
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