Internet Engineering Task Force
Internet Draft H. Schulzrinne
Columbia U.
draft-schulzrinne-sipping-sos-04.txt
January 8, 2003
Expires: June 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 describes how Session Initiation Protocol (SIP) user
agents and proxies can set up emergency calls and, more generally,
reach emergency assistance via SIP requests. For that purpose, it
defines a universal emergency SIP URI, sip:sos@domain and
sips:sos@domain, that allows SIP user agents to contact the local
emergency number. 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.
1 Introduction
Using the PSTN, emergency help can often be summoned at a designated,
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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) [1], 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.
We also describe how emergency 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 6 discusses one possible approach.
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.
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 BCP 14, RFC 2119
[2] and indicate requirement levels for compliant SIP
implementations.
2 Requirements
o SIP-based end systems must be able to reach emergency call
centers. These emergency call centers may have SIP
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capabilities or may be reachable only via a SIP-to-PSTN
gateway. Since each ECC serves only a limited geographic area,
often defined by jurisdictional boundaries such as state,
province or county, SIP-based emergency requests must
o While current emergency call centers are limited to voice and
TDD (telecommunication device for the deaf) communications,
future SIP-based ECCs should handle all relevant means of
interaction, including multimedia and instant messaging [8].
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 [1], 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.
o Emergency call mechanisms should not require a specific
technology for determining the location of the caller.
3 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.
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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 3.1 or an emergency tel URI defined in Section 3.2.
3.1 SIP URIs for Emergency Calls
It is RECOMMENDED that SIP-based [1] 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 11).
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In some areas, these emergency services use different
numbers.
The SIP URI user name "sos" and user names starting with "sos." MUST
NOT be assigned to any regular user.
3.2 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 (Section 9) or some other
configuration mechanism. 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 the United Kingdom.
If the user dials any of these digit strings, the UAC SHOULD generate
a request with the "sos" URI described in Section 3.1 unless it has
discovered a local outbound proxy as described in Section 9. In that
case, a UAC MAY use a "tel" URI [3,4] 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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4 Request Handling
Once identified, a user agent SHOULD direct an emergency call request
to an outbound proxy server or use the discovery mechanism described
in Section 9 to find a local PSTN gateway that can connect the caller
to a local emergency call center.
Outbound proxy servers MUST recognize all local emergency numbers as
well as the tel URIs enumerated in Section 3.2. 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.
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.
5 Determining User Agent Location
Proper handling of emergency calls requires knowledge of the caller
location to route the call to the appropriate ECC and to help the ECC
in locating the caller when rendering emergency assistance. We
describe the routing details in Section 6.
The SIP UA determines its location, preferably ahead of the emergency
call. It MAY use DHCP [9], retrieving the the location one or more
of: geospatial coordinates (longitude, latitude and altitude) [10],
civil address (street and community) [11] or network access
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information such as the port and switch number or the wireless cell
identity.
The UA needs to inform the DHCP server about its network attachment
point. There are several possibilities, including use of the RFC 3046
[12] 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.
The UAC inserts this location into a SIP header field. For geographic
information, this might look something like the following:
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 [13].
(The SIP header field format is fictitious and is defined in TBD.)
For 3GPP networks, the P-Access-Network-Info header field [14] can
convey the cell information, as defined in 3GPP TS 24.229.
Alternatively, an 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.
6 Request Routing
Any proxy, outbound or otherwise, that receives such a request MUST
forward (proxy) or redirect the request to the appropriate emergency
number local to the caller, using the location information described
in Section 5.
Note that in some limited cases, the proxy may be able to determine
that the requestor is in the same local network even without explicit
location information. This may be the case, for example, if the IP
address of the request indicates a local device and the network
offers no VPN services. Even under these restricted circumstances,
back-to-back UAs may mislead the proxy in this estimation.
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We distinguish two cases, depending on whether the proxy has access
to a location-to-ECC mapping service or not. A special, but
important, case is that the caller is known to be local to a PSTN
gateway.
6.1 Known to be Local
In some cases, the proxy server can reliably determine that the
caller and a local PSTN gateway are in the same emergency service
area. In that case, the proxy forwards the call request to the
gateway, translating the emergency URI into the local emergency
number.
6.2 Mapping Service Available
We refer to the location-to-ECC mapping service as a jurisdictional
directory service (JDS) since it maps geographic and/or civil
locations to emergency response jurisdictions. [TBD: better term?]
The JDS can be considered a special kind of SIP location service. The
protocol between the proxy and the JDS is beyond the scope of this
document.
One plausible solution simply proxies the SIP request
itself to the JDS.
Conceptually, the JDS is provided with a geographic location and
possibly the type of emergency service requested (for
"sip:sos.service" URIs) and returns SIP or tel URIs for one or more
ECCs serving the caller. If the JDS does not recognize the service
specification, it treats the mapping request like a general emergency
service request.
The tel URI returned by the JDS will contain a globally reachable
(E.164) number, i.e., a global-number according to [4]. The proxy
routes the call accordingly, using a local mechanism to determine the
appropriate gateway, e.g., TRIP [5].
Ideally, the chosen gateway should 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).
6.3 No Mapping Service Available
If the proxy does not have access to a JDS, it attempts to pick the
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closest PSTN gateway, translates the Request-URI to a locally valid
emergency number and proxies the call to that gateway.
If a proxy receives a service-specific request of the form
"sip:sos.*@domain" (such as "sos.fire@example.com"), the proxy
forwards it to the local appropriate specific 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 "sip:sos@domain".
7 Caller Identification
When using a PSTN gateway, the gateway 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.
8 Call Behavior
The user agent SHOULD not issue a REFER during an emergency call.
The user agent SHOULD NOT issue a BYE request during an emergency
call. If the user "hangs up", it is RECOMMENDED that the end system
generate an alert tone until the user reconnects.
The UA SHOULD automatically accept an incoming call from the same
entity that accepted the previous emergency call.
This allows the ECC to call back should the call be
interrupted accidentally.
9 Identifying the Local Emergency Numbers and Gateway
There are many ways that a user agent can configure emergency numbers
for use in analyzing calls made with telephony-type user input. These
include configuration tokens such as SIM cards in mobile devices, or
protocol-based solutions. We describe one such protocol-based
mechanism, based on DHCP, but this does not imply a requirement for
devices.
We propose a new DHCP option that enumerates the valid local
emergency identifiers, as a list of "tel", "sip" or "sips" URIs.
These identifiers can be used by the UA to trigger special behavior
when the user dials those numbers, or they can identify a local PSTN
gateway that can provide local emergency service. A DHCP server
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SHOULD advertise its local emergency number as well as those numbers
among the eight digit strings enumerated in Section 3.2 that do not
collide with local non-emergency services or extensions.
This DHCP option MUST NOT be used if DHCP does not announce the local
SIP server [6].
Unlike an outbound proxy server, the DHCP server is very
likely to be located within the same country as the user
agent. However, since the user agent needs to perform the
call routing, it makes little sense to have the DHCP
information identify a set of numbers that mean nothing
special to the outbound proxy server. Thus, server
identification and emergency number identification belong
together.
If the local network supports the location of gateways via SLP [7],
the user agent can discover such gateways. The SLP service
description needs to be enhanced with a list of valid emergency
numbers.
Details are described in TBD.
[This is for discussion only and, if suitable, will move to a
different draft.]
10 Alternative Identifiers Considered
The "sos" SIP URI reserved user name proposed here follows the
convention of RFC 2142 [15] and the "postmaster" convention
documented in RFC 2822 [16]. 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.
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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.
11 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.
12 Security Considerations
The SIP specification [1] 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.
Given the sensitive nature of many emergency calls, it is 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.
Outbound proxies may need to adjust their authentication requirements
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for such emergency calls.
It is desirable to be able to verify that the call is reaching a true
emergency call center. The caller is unlikely to know or be able to
obtain the public key of the destination ECC since it does not even
know the ECC identity. The responding ECC could sign the response,
via standard SIP S/MIME mechanisms. However, the principal in the
certificate would appear as a random-looking domain name, such as
admin.fayette.co.ga.us, which cannot be reliably identified as an
ECC. Here, an attribute certificate (AC) [17] could be used to
associate the attribute "ECC" with the SIP URI. However, it appears
unlikely that such an Attribute Authority will emerge anytime soon,
particularly across national borders. Alternatively, ICANN could
create a new restricted top-level domain, such as .sos, that is open
only to accredited emergency response entities. Clearly, this is also
not likely in the short term.
The caller has little choice other than to trust the outbound proxy
server acting as an ERC or to act as its own ERC. In the former, more
likely case, the ERC will obtain the ECC identity from a database
source it trusts. The ERC then only has to ensure that the call
reaches the appropriate domain, which standard TLS server
authentication accomplishes, regardless of the domain name used by
the ECC and without the notion of attribute certificates. Since a
caller cannot assume that all ECCs will have valid ACs, the absence
of such a certificate is unlikely to cause the caller to abandon the
call in an emergency. Thus, the transitive trust model, which is
easier to implement, appears to be a more pragmatic approach.
13 Normative References
[1] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. R. Johnston, J.
Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: session
initiation protocol," RFC 3261, Internet Engineering Task Force, June
2002.
[2] S. Bradner, "Key words for use in rfcs to indicate requirement
levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.
[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, Dec. 2002.
Work in progress.
[5] J. Rosenberg, H. F. Salama, and M. Squire, "Telephony routing
over IP (TRIP)," RFC 3219, Internet Engineering Task Force, Jan.
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2002.
[6] H. Schulzrinne, "Dynamic host configuration protocol (dhcp-for-
ipv4) option for session initiation protocol (SIP) servers," RFC
3361, Internet Engineering Task Force, Aug. 2002.
[7] W. Zhao and H. Schulzrinne, "Locating ip-to-public switched
telephone network (PSTN) telephony gateways via SLP," internet draft,
Internet Engineering Task Force, Aug. 2002. Work in progress.
14 Informative References
[8] B. Campbell, J. Rosenberg, and H. Schulzrinne, eds., "Session
initiation protocol (SIP) extension for instant messaging," RFC 3428,
Internet Engineering Task Force, Dec. 2002.
[9] R. E. Droms, "Dynamic host configuration protocol," RFC 2131,
Internet Engineering Task Force, Mar. 1997.
[10] J. Polk et al., "DHCP option for geographic location," internet
draft, Internet Engineering Task Force, Oct. 2002. Work in progress.
[11] H. Schulzrinne, "DHCP option for civil location," internet
draft, Internet Engineering Task Force, Dec. 2002. Work in progress.
[12] M. Patrick, "DHCP relay agent information option," RFC 3046,
Internet Engineering Task Force, Jan. 2001.
[13] J. Polk et al., "Semantics for DHC location object within
GEOPRIV," internet draft, Internet Engineering Task Force, Oct.
2002. Work in progress.
[14] M. Garcia, E. Henrikson, and D. L. Mills, "Private extensions
(p-header) extensions to the session initiation protocol (SIP) for
the 3rd-generation partnership project (3GPP)," internet draft,
Internet Engineering Task Force, Nov. 2002. Work in progress.
[15] D. H. Crocker, "Mailbox names for common services, roles and
functions," RFC 2142, Internet Engineering Task Force, May 1997.
[16] P. Resnick, ed., "Internet message format," RFC 2822, Internet
Engineering Task Force, Apr. 2001.
[17] S. Farrell and R. Housley, "An Internet attribute certificate
profile for authorization," RFC 3281, Internet Engineering Task
Force, Apr. 2002.
15 Change History
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15.1 Changes since -03
o Added description of local discovery mechanism for finding a
local gateway.
o Noted that 'sos' is case-insensitive and only applies to SIP
URIs, not other URIs.
o Described the ECC verification options available to a caller.
o Added local gateway discovery.
o Added outline of how to use DHCP for configuring additional
local emergency numbers.
o Added 3GPP emergency numbers beyond 112 and 911.
16 Acknowledgements
Andrew Allen, Keith Drage, Mike Pierce, James Polk, Brian Rosen and
John Schnizlein contributed helpful comments.
17 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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