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Network Working Group H. Schulzrinne
Request for Comments: 5012 Columbia U.
Category: Informational R. Marshall, Ed.
TCS
January 2008
Requirements for Emergency Context Resolution with
Internet Technologies
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Abstract
This document defines terminology and enumerates requirements for the
context resolution of emergency calls placed by the public using
voice-over-IP (VoIP) and general Internet multimedia systems, where
Internet protocols are used end to end.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Terminology . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Emergency Services . . . . . . . . . . . . . . . . . . . . 3
3.2. Service Providers . . . . . . . . . . . . . . . . . . . . 3
3.3. Actors . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.4. Call Routing Entities . . . . . . . . . . . . . . . . . . 5
3.5. Location . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.6. Identifiers, Numbers, and Dial Strings . . . . . . . . . . 6
3.7. Mapping . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Basic Actors . . . . . . . . . . . . . . . . . . . . . . . . . 8
5. High-Level Requirements . . . . . . . . . . . . . . . . . . . 10
6. Identifying the Caller's Location . . . . . . . . . . . . . . 12
7. Emergency Service Identifier . . . . . . . . . . . . . . . . . 14
8. Mapping Protocol . . . . . . . . . . . . . . . . . . . . . . . 16
9. Security Considerations . . . . . . . . . . . . . . . . . . . 20
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 20
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
12.1. Normative References . . . . . . . . . . . . . . . . . . . 21
12.2. Informative References . . . . . . . . . . . . . . . . . . 21
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1. Introduction
Users of both voice-centric (telephone-like) and non-voice services,
such as text communication for hearing-disabled users (see [RFC3351]
and [toip]), expect to be able to initiate a request for help in case
of an emergency.
Unfortunately, the existing mechanisms to support emergency calls
that have evolved within the public circuit-switched telephone
network (PSTN) are not appropriate to handle evolving IP-based voice,
text, and real-time multimedia communications. This document
outlines the key requirements that IP-based end systems and network
elements, such as Session Initiation Protocol (SIP) [RFC3261]
proxies, need to satisfy in order to provide emergency call services,
which at a minimum, offer the same functionality as existing PSTN
services, with the additional overall goal of making emergency
calling more robust, less costly to implement, and multimedia-
capable.
This document only focuses on end-to-end IP-based calls, i.e., where
the emergency call originates from an IP end system and terminates in
an IP-capable public safety answering point (PSAP), conveyed entirely
over an IP network.
We first define terminology in Section 3. The document then outlines
various functional issues that relate to placing an IP-based
emergency call, including a description of baseline requirements
(Section 5), identification of the emergency caller's location
(Section 6), use of a service identifier to declare a call to be an
emergency call (Section 7), and finally, the mapping function
required to route the call to the appropriate PSAP (Section 8).
The primary purpose of the mapping protocol is to produce a PSAP URI
drawn from a preferred set of URI schemes such as SIP or SIPS URIs,
based on both location information [RFC4119] and a service identifier
in order to facilitate the IP end-to-end completion of an emergency
call.
Aside from obtaining a PSAP URI, the mapping protocol is useful for
obtaining other information as well. There may be a case, for
example, where an appropriate emergency number is not known, only the
location. The mapping protocol can then return a geographically
appropriate emergency number based on the input.
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Since some PSAPs may not immediately support IP, or because some user
equipment (UE) may not initially support emergency service
identifiers, it may be necessary to also support emergency service
identifiers that utilize less-preferred URI schemes, such as a tel
URI in order to complete an emergency call via the PSTN.
Identification of the caller, while not incompatible with the
requirements for messaging outlined within this document, is
considered to be outside the scope of this document.
Location is required for two separate purposes: first, to support the
routing of the emergency call to the appropriate PSAP and second, to
display the caller's location to the call taker to help in
dispatching emergency assistance to the appropriate location.
This latter use, the display of location information to the PSAP, is
orthogonal to the mapping protocol, and is outside the scope of this
document.
2. Requirements Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119],
with the important qualification that, unless otherwise stated, these
terms apply to the design of the mapping protocol, not its
implementation or application.
3. Terminology
3.1. Emergency Services
Basic emergency service: Basic emergency service allows a caller to
reach a PSAP serving its current location, but the PSAP may not be
able to determine the identity or geographic location of the
caller, except by the call taker asking the caller.
Enhanced emergency service: In enhanced emergency service, the PSAP
call taker can determine the caller's current location.
3.2. Service Providers
Internet Access Provider (IAP): An organization that provides
physical and data link (layer 2) network connectivity to its
customers or users, e.g., through digital subscriber lines, cable
TV plants, Ethernet, leased lines, or radio frequencies. Examples
of such organizations include telecommunication carriers,
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municipal utilities, larger enterprises with their own network
infrastructure, and government organizations, such as the
military.
Internet Service Provider (ISP): An organization that provides IP
network-layer services to its customers or users. This entity may
or may not provide the physical-layer and data link (layer-2)
connectivity, such as fiber or Ethernet, i.e., it may or may not
play the role of an IAP.
Application Service Provider (ASP): The organization or entity that
provides application-layer services, which may include voice (see
"Voice Service Provider"). This entity can be a private
individual, an enterprise, a government, or a service provider.
An ASP is more general than a Voice Service Provider, since
emergency calls may use other media beyond voice, including text
and video. For a particular user, the ASP may or may not be the
same organization as his IAP or ISP.
Voice Service Provider (VSP): A specific type of Application Service
Provider that provides voice related services based on IP, such as
call routing, a SIP URI, or PSTN termination. In this document,
unless noted otherwise, any reference to "Voice Service Provider"
or "VSP" may be used interchangeably with "Application/Voice
Service Provider" or "ASP/VSP".
3.3. Actors
(Emergency) caller: The term "caller" or "emergency caller" refers
to the person placing an emergency call or sending an emergency
instant message (IM).
User Equipment (UE): User equipment is the device or software
operated by the caller to place an emergency call. A SIP user
agent (UA) is an example of user equipment.
Call taker: A call taker is an agent at the PSAP that accepts calls
and may dispatch emergency help. Sometimes the functions of call
taking and dispatching are handled by different groups of people,
but these divisions of labor are not generally visible to the
caller and thus do not concern us here.
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3.4. Call Routing Entities
Emergency Service Routing Proxy (ESRP): An ESRP is an emergency call
routing support entity that invokes the location-to-PSAP URI
mapping function, to return an appropriate PSAP URI, or the URI
for another ESRP. Client mapping requests could also be performed
by a number of entities, including entities that instantiate the
SIP proxy role and the SIP user agent client role.
Public Safety Answering Point (PSAP): A PSAP is a facility where
emergency calls are received under the responsibility of a public
authority. (This terminology is used by both the European
Telecommunications Standards Institute (ETSI), in ETSI SR 002 180,
and the National Emergency Number Association (NENA).) In the
United Kingdom, PSAPs are called Operator Assistance Centres; in
New Zealand, Communications Centres. Within this document, it is
assumed, unless stated otherwise, that PSAPs support the receipt
of emergency calls over IP, using appropriate application layer
protocols, such as SIP for call signaling and RTP for media.
3.5. Location
Location: A geographic identification assigned to a region or
feature based on a specific coordinate system, or by other precise
information such as a street number and name. It can be either a
civic or geographic location.
Civic location: A described location based on some reference system,
such as jurisdictional region or postal delivery grid. A street
address is a common example of a civic location.
Geographic location: A reference to a point that is able to be
located, as described by a set of defined coordinates within a
geographic coordinate system, such as latitude and longitude
within the WGS-84 datum. For example, a 2-D geographic location
is defined as an (x,y) coordinate value pair according to the
distance north or south of the equator and east or west of the
prime meridian.
Location validation: A caller location is considered valid if the
civic or geographic location is recognizable within an acceptable
location reference system (e.g., United States Postal Address or
the WGS-84 datum) and can be mapped to one or more PSAPs. While
it is desirable to determine that a location exists, validation
may not ensure that such a location exists, but rather may only
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ensure that the location falls within some range of known values.
Location validation ensures that a location is able to be
referenced for mapping, but makes no assumption about the
association between the caller and the caller's location.
3.6. Identifiers, Numbers, and Dial Strings
(Emergency) service number: The (emergency) service number is a
string of digits used to reach the (emergency) service. The
emergency service number is often just called the emergency
number. It is the number typically dialed on devices directly
connected to the PSTN and the number reserved for emergency calls
by national or regional numbering authorities. It only contains
the digits 0 through 9, #, and *. The service number may depend
on the location of the caller. For example, the general emergency
service number in the United States is 911 and the poison control
service number is 18002221222. In most cases, the service number
and dial string are the same; they may differ in some private
phone networks. A service number may be carried in tel URLs
[RFC3966], along with a context identifier. In the North American
numbering plan, some service numbers are three-digit N11 or
service codes, but not all emergency numbers have three digits. A
caller may have to dial a service dial string (below) that differs
from the service number when using a PBX.
(Emergency) service dial string: The service dial string identifies
the string of digits that a caller must dial to reach a particular
(emergency) service. In devices directly connected to the PSTN,
the service dial string is the same as the service number and may
thus depend on the location of the caller. However, in private
phone networks, such as in PBXs, the service dial string consists
of a dialing prefix to reach an outside line, followed by the
emergency number. For example, in a hotel, the dial string for
emergency services in the United States might be 9911. Dial
strings may contain indications of pauses or wait-for-secondary-
dial-tone indications. Service dial strings are outside the scope
of this document.
(Emergency) service identifier: The (emergency) service identifier
describes the emergency service, independent of the user interface
mechanism, the signaling protocol that is used to reach the
service, or the caller's geographic location. It is a protocol
constant and used within the mapping and signaling protocols. An
example is the service URN [RFC5031].
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(Emergency) service URL: The service URL is a protocol-specific
(e.g., SIP) or protocol-agnostic (e.g., im: [RFC3860]) identifier
that contains the address of the PSAP or other emergency service.
It depends on the specific signaling or data transport protocol
used to reach the emergency service.
Service URN: A service URN is an implementation of a service
identifier, which can be applied to both emergency and non-
emergency contexts, e.g., urn:service:sos or
urn:service:counseling. Within this document, service URNs are
referred to as 'emergency service URNs' [RFC5031].
Home emergency number: A home emergency number is the emergency
number valid at the caller's customary home location, e.g., his
permanent residence. The home location may or may not coincide
with the service area of the caller's VSP.
Home emergency dial string: A home dial string is the dial string
valid at the caller's customary home location, e.g., his permanent
residence.
Visited emergency number: A visited emergency number is the
emergency number valid at the caller's current physical location.
We distinguish the visited emergency number if the caller is
traveling outside his home region.
Visited emergency dial string: A visited emergency dial string is
the dial string number valid at the caller's current physical
location.
3.7. Mapping
Mapping: Mapping is the process of resolving a location to one or
more PSAP URIs that directly identify a PSAP, or point to an
intermediary that knows about a PSAP and that is designated as
responsible for serving that location.
Mapping client: A mapping client interacts with the mapping server
to learn one or more PSAP URIs for a given location.
Mapping protocol: A protocol used to convey the mapping request and
response.
Mapping server: The mapping server holds information about the
location-to-PSAP URI mapping.
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Mapping service: A network service that uses a distributed mapping
protocol to perform a mapping between a location and a PSAP, or
intermediary that knows about the PSAP, and is used to assist in
routing an emergency call.
4. Basic Actors
In order to support emergency services covering a large physical
area, various infrastructure elements are necessary, including
Internet Access Providers (IAPs), Application/Voice Service Providers
(ASP/VSPs), Emergency Service Routing Proxy (ESRP) providers, mapping
service providers, and PSAPs.
This section outlines which entities will be considered in the
routing scenarios discussed.
Location
Information +-----------------+
|(1) |Internet | +-----------+
v |Access | | |
+-----------+ |Provider | | Mapping |
| | | (3) | | Service |
| Emergency |<---+-----------------+-->| |
| Caller | | (2) | +-----------+
| |<---+-------+ | ^
+-----------+ | +----|---------+------+ |
^ | | Location | | |
| | | Information<-+ | |
| +--+--------------+ |(5) | | (6)
| | | | |
| | +-----------v+ | |
| (4) | | | | |
+--------------+--->| ESRP |<--+---+
| | | | |
| | +------------+ |
| | ^ |
| | (7) | | +----+--+
| (8) | +------------>| |
+--------------+----------------------->| PSAP |
| | | |
|Application/ | +----+--+
|Voice |
|Service |
|Provider |
+---------------------+
Figure 1: Framework for Emergency Call Routing
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Figure 1 shows the interaction between the entities involved in the
call. There are a number of different deployment choices, as can be
easily seen from the figure.
Is the Internet Access Provider also the Application/Voice Service
Provider? In the Internet today, the roles of Internet access
provider and application/voice service provider are typically
provided by different entities. As a consequence, the Application/
Voice Service Provider is typically not able to directly determine
the physical location of the emergency caller.
The overlapping squares in the figure indicate that some functions
can be collapsed into a single entity. As an example, the
Application/Voice Service Provider might be the same entity as the
Internet Access Provider. There is, however, no requirement that
this must be the case. Additionally, we consider that end systems
might act as their own ASP/VSP, e.g., either for enterprises or for
residential users.
Various potential interactions between the entities depicted in
Figure 1 are described below:
1. Location information might be available to the end host itself.
2. Location information might, however, also be obtained from the
Internet Access Provider.
3. The emergency caller might need to consult a mapping service to
determine the PSAP (or other relevant information) that is
appropriate for the physical location of the emergency caller,
possibly considering other attributes, such as appropriate
language support by the emergency call taker.
4. The emergency caller might get assistance for emergency call
routing by infrastructure elements that are emergency call
routing support entities, such as an Emergency Service Routing
Proxy (ESRP) in SIP.
5. Location information is used by emergency call routing support
entities for subsequent mapping requests.
6. Emergency call routing support entities might need to consult a
mapping service to determine where to route the emergency call.
7. For infrastructure-based emergency call routing (in contrast to
UE-based emergency call routing), the emergency call routing
support entity needs to forward the call to the PSAP.
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8. The emergency caller may interact directly with the PSAP, where
the UE invokes mapping, and initiates a connection, without
relying on any intermediary emergency call routing support
entities.
5. High-Level Requirements
Below, we summarize high-level architectural requirements that guide
some of the component requirements detailed later in the document.
Re1. Application/Voice service provider existence: The initiation
of an IP-based emergency call SHOULD NOT assume the existence of
an Application/Voice Service Provider (ASP/VSP).
Motivation: The caller may not have an application/voice service
provider. For example, a residence may have its own DNS domain
and run its own SIP proxy server for that domain. On a larger
scale, a university might provide voice services to its students
and staff, but might not be a telecommunication provider.
Re2. International applicability: Regional, political, and
organizational aspects MUST be considered during the design of
protocols and protocol extensions that support IP-based emergency
calls.
Motivation: It must be possible for a device or software developed
or purchased in one country to place emergency calls in another
country. System components should not be biased towards a
particular set of emergency numbers or languages. Also, different
countries have evolved different ways of organizing emergency
services, e.g., either centralizing them or having smaller
regional subdivisions, such as the United States or
municipalities, handle emergency calls within their jurisdiction.
Re3. Distributed administration: Deployment of IP-based emergency
services MUST NOT depend on a single central administrative
authority.
Motivation: The design of the mapping protocol must make it
possible to deploy and administer emergency calling features on a
regional or national basis without requiring coordination with
other regions or nations. The system cannot assume, for example,
that there is a single global entity issuing certificates for
PSAPs, ASP/VSPs, IAPs, or other participants.
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Re4. Multi-mode communication: IP-based emergency calls MUST
support multiple communication modes, including, for example,
audio, video, and text.
Motivation: Within the PSTN, voice and text telephony (often
called TTY or text-phone in North America) are the only commonly
supported media. Emergency calling must support a variety of
media. Such media should include voice, conversational text (RFC
4103 [RFC4103]), instant messaging, and video.
Re5. Mapping result usability: The mapping protocol MUST return one
or more URIs that are usable within a standard signaling protocol
(i.e., without special emergency extensions).
Motivation: For example, a SIP URI that is returned by the mapping
protocol needs to be usable by any SIP-capable phone within a SIP-
initiated emergency call. This is in contrast to a "special
purpose" URI, which may not be recognizable by a legacy SIP
device.
Re6. PSAP URI accessibility: The mapping protocol MUST support
interaction between the client and server where no enrollment to a
mapping service exists or is required.
Motivation: The mapping server may well be operated by a service
provider, but access to the server offering the mapping must not
require use of a specific ISP or ASP/VSP.
Re7. Common data structures and formats: The mapping protocol
SHOULD support common formats (e.g., PIDF-LO) for location data.
Motivation: Location databases should not need to be transformed
or modified in any unusual or unreasonable way in order for the
mapping protocol to use the data. For example, a database that
contains civic addresses used by location servers may be used for
multiple purposes and applications beyond emergency service
location-to-PSAP URI mapping.
Re8. Anonymous mapping: The mapping protocol MUST NOT require the
true identity of the target for which the location information is
attributed.
Motivation: Ideally, no identity information is provided via the
mapping protocol. Where identity information is provided, it may
be in the form of an unlinked pseudonym (RFC 3693 [RFC3693]).
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6. Identifying the Caller's Location
Location can either be provided directly (by value), or via a pointer
(by reference), and represents either a civic location, or a
geographic location. An important question is how and when to attach
location information to the VoIP emergency signaling messages. In
general, we can distinguish three modes of operation of how a
location is associated with an emergency call:
UA-inserted: The caller's user agent inserts the location
information into the call-signaling message.
UA-referenced: The caller's user agent provides a pointer (i.e., a
location reference), via a permanent or temporary identifier, to
the location information, which is stored by a location server
somewhere else and then retrieved by the PSAP, ESRP, or other
authorized entity.
Proxy-inserted: A proxy along the call path inserts the location or
location reference.
The following requirements apply:
Lo1. Reference datum: The mapping protocol MUST support the WGS-84
coordinate reference system and MAY support other coordinate
reference systems.
Motivation: Though many different datums exist around the world,
this document recommends the WGS-84 datum since it is designed to
describe the whole earth, rather than a single continent or other
region, and is commonly used to represent Global Positioning
System coordinates.
Lo2. Location delivery by-value: The mapping protocol MUST support
the delivery of location information using a by-value method,
though it MAY also support de-referencing a URL that references a
location object.
Motivation: The mapping protocol is not required to support the
ability to de-reference specific location references.
Lo3. Alternate community names: The mapping protocol MUST support
both the jurisdictional community name and the postal community
name fields within the PIDF-LO [RFC4119] data.
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Motivation: The mapping protocol must accept queries with either a
postal or jurisdictional community name field, or both, and
provide appropriate responses. If a mapping query contains only
one community name and the database contains both jurisdictional
and postal community names, the mapping protocol response SHOULD
return both community names.
Lo4. Validation of civic location: The mapping protocol MUST be
able to report the results of validating civic locations (street
addresses).
Motivation: Location validation provides an opportunity to help
ascertain ahead of time whether or not a successful mapping to the
appropriate PSAP will likely occur when it is required.
Validation may also help to avoid delays during emergency call
setup due to invalid location data.
Lo5. Information about location data used for mapping: The mapping
protocol MUST support the ability to provide ancillary information
about the resolution of location data used to retrieve a PSAP URI.
Motivation: The mapping server may not use all the data elements
in the provided location information to determine a match, or may
be able to find a match based on all of the information except for
some specific data elements. The uniqueness of this information
set may be used to differentiate among emergency jurisdictions.
Precision or resolution in the context of this requirement might
mean, for example, explicit identification of the data elements
that were used successfully in the mapping.
Lo6. Contact for location problems: The mapping protocol MUST
support a mechanism to contact an appropriate authority to resolve
mapping-related issues for the queried location. For example, the
querier may want to report problems with the response values or
indicate that the mapping database is mistaken on declaring a
civic location as non-existent.
Motivation: Initially, authorities may provide URLs where a human
user can report problems with an address or location. In
addition, web services may be defined to automate such reporting.
For example, the querier may wish to report that the mapping
database may be missing a newly built or renamed street or house
number.
Lo7. Limits to validation: Successful validation of a civic
location MUST NOT be required to place an emergency call.
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Motivation: In some cases, a civic location may not be considered
valid. This fact should not result in the call being dropped or
rejected by any entity along the call setup signaling path to the
PSAP.
Lo8. 3D sensitive mapping: The mapping protocol MUST implement
support for both 2D and 3D location information, and MAY accept
either a 2D or 3D mapping request as input.
Motivation: It is expected that queriers may provide either 2D or
3D data. When a 3D request is presented within an area only
defined by 2D data within the mapping server, the mapping result
would be the same as if the height or altitude coordinate had been
omitted from the mapping request.
Lo9. Database type indicator: The mapping protocol MAY support a
mechanism that provides an indication describing a specific type
of location database used.
Motivation: It is useful to know the source of the data stored in
the database used for location validation, either for civic or
geographic location matching. In the United States, sources of
data could include the United States Postal Service, the Master
Street Address Guide (MSAG), or commercial map data providers.
7. Emergency Service Identifier
Emergency service identifiers are protocol constants that allow
protocol entities, such as SIP proxy servers, to distinguish
emergency calls from non-emergency calls and to identify the specific
emergency service desired. Emergency service identifiers are a
subclass of service identifiers that more generally identify services
reachable by callers. An example of a service identifier is the
service URN [RFC5031], but other identifiers, such as tel URIs
[RFC3966], may also serve this role during a transition period.
Since this document only addresses emergency services, we use the
terms "emergency service identifier" and "service identifier"
interchangeably. Requirements for these identifiers include:
Id1. Multiple emergency services: The mapping protocol MUST be able
to support different emergency services distinguished by different
service identifiers.
Motivation: Some jurisdictions may offer multiple types of
emergency services that operate independently and can be contacted
directly; for example, fire, police, and ambulance services.
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Id2. Extensible emergency service identifiers: The mapping protocol
MUST support an extensible list of emergency identifiers, though
it is not required to provide mappings for every possible service.
Motivation: Extensibility is required since new emergency services
may be introduced over time, either globally or in some
jurisdictions. The availability of emergency services depends on
the locations. For example, the Netherlands are unlikely to offer
a mountain rescue service.
Id3. Discovery of emergency number: The mapping protocol MUST be
able to return the location-dependent emergency number for the
location indicated in the query.
Motivation: Users are trained to dial the appropriate emergency
number to reach emergency services. There needs to be a way to
figure out the emergency number at the current location of the
caller.
Id4. Home emergency number recognition: User equipment MUST be able
to translate a home emergency number into an emergency service
identifier.
Motivation: The UE could be pre-provisioned with the appropriate
information in order to perform such a translation or could
discover the emergency number by querying the mapping protocol
with its home location.
Id5. Emergency number replacement: There SHOULD be support for
replacement of the emergency number with the appropriate emergency
service identifier for each signaling protocol used for an
emergency call, based on local conventions, regulations, or
preference (e.g., as in the case of an enterprise).
Motivation: Any signaling protocol requires the use of some
identifier to indicate the called party, and the user equipment
may lack the capability to determine the actual service URL (PSAP
URI). The use of local conventions may be required as a
transition mechanism. Since relying on recognizing local
numbering conventions makes it difficult for devices to be used
outside their home context and for external devices to be
introduced into a network, protocols should use standardized
emergency service identifiers.
Id6. Emergency service identifier marking: Signaling protocols MUST
support emergency service identifiers to mark a call as an
emergency call.
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Motivation: Marking ensures proper handling as an emergency call
by downstream elements that may not recognize, for example, a
local variant of a logical emergency address. This marking
mechanism is related to, but independent of, marking calls for
prioritized call handling [RFC4412].
Id7. Handling unrecognized emergency service identifiers: There
MUST be support for calls that are initiated as emergency calls
even if the specific emergency service requested is not recognized
by the ESRP. Such calls will then be routed to a generic
emergency service.
Motivation: Fallback routing allows new emergency services to be
introduced incrementally, while avoiding non-routable emergency
calls. For example, a call for marine rescue services would be
routed to a general PSAP if the caller's location does not offer
marine rescue services yet.
Id8. Return fallback service identifier: The mapping protocol MUST
be able to report back the actual service mapped if the mapping
protocol substitutes another service for the one requested.
Motivation: A mapping server may be configured to automatically
look up the PSAP for another service if the user-requested service
is not available for that location. For example, if there is no
marine rescue service, the mapping protocol might return the PSAP
URL for general emergencies and include the "urn:service.sos"
identifier in the response to alert the querier to that fact.
Id9. Discovery of visited emergency numbers: The mapping protocol
MUST support a mechanism to allow the end device to learn visited
emergency numbers.
Motivation: Travelers visiting a foreign country may observe the
local emergency number, e.g., seeing it painted on the side of a
fire truck, and then rightfully expect to be able to dial that
emergency number. Similarly, a local "good Samaritan" may use a
tourist's cell phone to summon help.
8. Mapping Protocol
There are two basic approaches to invoke the mapping protocol. We
refer to these as caller-based and mediated. In each case, the
mapping client initiates a request to a mapping server via a mapping
protocol. A proposed mapping protocol, LoST, is outlined in [lost].
For caller-based resolution, the caller's user agent invokes the
mapping protocol to determine the appropriate PSAP based on the
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location provided. The resolution may take place well before the
actual emergency call is placed, or at the time of the call.
For mediated resolution, an emergency call routing support entity,
such as a SIP (outbound) proxy or redirect server, invokes the
mapping service.
Since servers may be used as outbound proxy servers by clients that
are not in the same geographic area as the proxy server, any proxy
server has to be able to translate any caller location to the
appropriate PSAP. (A traveler may, for example, accidentally or
intentionally configure its home proxy server as its outbound proxy
server, even while far away from home.)
Ma1. Baseline query protocol: A mandatory-to-implement protocol
MUST be specified.
Motivation: An over-abundance of similarly capable choices appears
undesirable for interoperability.
Ma2. Extensible protocol: The mapping protocol MUST be designed to
support the extensibility of location data elements, both for new
and existing fields.
Motivation: This is needed, for example, to accommodate future
extensions-to-location information that might be included in the
PIDF-LO ([RFC4119]).
Ma3. Incrementally deployable: The mapping protocol MUST be
designed to support its incremental deployment.
Motivation: It must not be necessary, for example, to have a
global street level database before deploying the system. It is
acceptable to have some misrouting of calls when the database does
not (yet) contain accurate PSAP service area information.
Ma4. Any time mapping: The mapping protocol MUST support the
ability of the mapping function to be invoked at any time,
including while an emergency call is in process and before an
emergency call is initiated.
Motivation: If the mapping query fails at call time, it may be
advantageous to be able to fall back to the result of an earlier
mapping query. This prior knowledge would be obtained by
performing a mapping query at any time prior to an emergency call.
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Ma5. Anywhere mapping: The mapping protocol MUST support the
ability to provide mapping information in response to an
individual query from any (earthly) location, regardless of where
the mapping client is located, either geographically or by network
location.
Motivation: The mapping client, such as an ESRP, may not
necessarily be anywhere close to the caller or the appropriate
PSAP, but must still be able to obtain mapping information.
Ma6. Appropriate PSAP: The mapping protocol MUST support the
routing of an emergency call to the PSAP responsible for a
particular geographic area.
Motivation: Routing to the wrong PSAP will result in delays in
handling emergencies as calls are redirected, and therefore will
also result in inefficient use of PSAP resources at the initial
point of contact. It is important that the location determination
mechanism not be fooled by the location of IP telephony gateways
or dial-in lines into a corporate LAN (and dispatch emergency help
to the gateway or campus, rather than the caller), multi-site LANs
and similar arrangements.
Ma7. Multiple PSAP URIs: The mapping protocol MUST support a method
to return multiple PSAP URIs, which cover the same geographic
area.
Motivation: Different contact protocols (e.g., PSTN via tel URIs
and IP via SIP URIs) may be routed to different PSAPs. Less
likely, two PSAPs may overlap in their coverage region.
Ma8. Single primary URI per contact protocol: Though the mapping
protocol may be able to include multiple URIs in the response, it
SHOULD return only one primary URI per contact protocol used, so
that clients are not required to select among different targets
for the same contact protocol.
Motivation: There may be two or more URIs returned when multiple
contact protocols are available (e.g., SIP and SMS). The client
may select among multiple contact protocols based on its
capabilities, preference settings, or availability.
Ma9. Non-preferred URI schemes: The mapping protocol MAY support
the return of a less-preferred URI scheme, such as a tel URI.
Motivation: In order to provide incremental support to non-IP
PSAPs, it may be necessary to be able to complete an emergency
call via the PSTN.
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Ma10. URI properties: The mapping protocol MUST support the ability
to provide ancillary information about a contact that allows the
mapping client to determine relevant properties of the PSAP URI.
Motivation: In some cases, the same geographic area is served by
several PSAPs; for example, a corporate campus might be served by
both a corporate security department and the municipal PSAP. The
mapping protocol should then return URIs for both, with
information allowing the querying entity to choose one or the
other. This determination could be made by either an ESRP, based
on local policy, or by direct user choice, in the case of caller-
based methods.
Ma11. Mapping referral: The mapping protocol MUST support a
mechanism for the mapping client to contact any mapping server and
be referred to another mapping server that is more qualified to
answer the query.
Motivation: Referrals help mitigate the impact of incorrect
configuration that directs a client to the wrong initial mapping
server.
Ma12. Split responsibility: The mapping protocol MUST support the
division of data subset handling between multiple mapping servers
within a single level of a civic location hierarchy.
Motivation: For example, two mapping servers for the same city or
county may handle different streets within that city or county.
Ma13. URL for error reporting: The mapping protocol MUST support
the ability to return a URL that can be used to report a suspected
or known error within the mapping database.
Motivation: If an error is returned, for example, there needs to
be a URL that points to a resource that can explain or potentially
help resolve the error.
Ma14. Resilience to mapping server failure: The mapping protocol
MUST support a mechanism that enables the client to fail over to
different (replica) mapping server.
Motivation: The failure of a mapping server should not preclude
the mapping client from receiving an answer to its query.
Ma15. Traceable resolution: The mapping protocol SHOULD support the
ability of the mapping client to be able to determine the entity
or entities that provided the emergency address resolution
information.
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Motivation: To improve reliability and performance, it is
important to be able to trace which servers contributed to the
resolution of a query.
Ma16. Minimal additional delay: Mapping protocol execution SHOULD
minimize the amount of delay within the overall call-setup time.
Motivation: Since outbound proxies will likely be asked to resolve
the same geographic coordinates repeatedly, a suitable time-
limited caching mechanism should be supported.
Ma17. Freshness indication: The mapping protocol SHOULD support an
indicator describing how current the information provided by the
mapping source is.
Motivation: This is especially useful when an alternate mapping is
requested, and alternative sources of mapping data may not have
been created or updated with the same set of information or within
the same time frame. Differences in currency between mapping data
contained within mapping sources should be minimized.
9. Security Considerations
Threats and security requirements are discussed in a separate
document [RFC5069].
10. Contributors
The information in this document is partially derived from text
written by the following contributors:
Nadine Abbott nabbott@telcordia.com
Hideki Arai arai859@oki.com
Martin Dawson Martin.Dawson@andrew.com
Motoharu Kawanishi kawanishi381@oki.com
Brian Rosen br@brianrosen.net
Richard Stastny Richard.Stastny@oefeg.at
Martin Thomson Martin.Thomson@andrew.com
James Winterbottom James.Winterbottom@andrew.com
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11. Acknowledgments
In addition to thanking those listed above, we would like to also
thank Guy Caron, Barry Dingle, Keith Drage, Tim Dunn, Patrik
Faltstrom, Clive D.W. Feather, Raymond Forbes, Randall Gellens,
Michael Haberler, Michael Hammer, Ted Hardie, Gunnar Hellstrom,
Cullen Jennings, Marc Linsner, Rohan Mahy, Patti McCalmont, Don
Mitchell, John Morris, Andrew Newton, Steve Norreys, Jon Peterson,
James Polk, Benny Rodrig, John Rosenberg, Jonathan Rosenberg, John
Schnizlein, Shida Schubert, James Seng, Byron Smith, Barbara Stark,
Richard Stastny, Tom Taylor, Hannes Tschofenig, and Nate Wilcox for
their helpful input.
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.
12.2. Informative References
[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.
[RFC3351] Charlton, N., Gasson, M., Gybels, G., Spanner, M., and A.
van Wijk, "User Requirements for the Session Initiation
Protocol (SIP) in Support of Deaf, Hard of Hearing and
Speech-impaired Individuals", RFC 3351, August 2002.
[RFC3693] Cuellar, J., Morris, J., Mulligan, D., Peterson, J., and
J. Polk, "Geopriv Requirements", RFC 3693, February 2004.
[RFC3860] Peterson, J., "Common Profile for Instant Messaging
(CPIM)", RFC 3860, August 2004.
[RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers",
RFC 3966, December 2004.
[RFC4103] Hellstrom, G. and P. Jones, "RTP Payload for Text
Conversation", RFC 4103, June 2005.
[RFC4119] Peterson, J., "A Presence-based GEOPRIV Location Object
Format", RFC 4119, December 2005.
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[RFC4412] Schulzrinne, H. and J. Polk, "Communications Resource
Priority for the Session Initiation Protocol (SIP)",
RFC 4412, February 2006.
[RFC5031] Schulzrinne, H., "A Uniform Resource Name (URN) for
Emergency and Other Well-Known Services", RFC 5031,
January 2008.
[RFC5069] Taylor, T., Ed., Tschofenig, H., Schulzrinne, H., and M.
Shanmugam, "Security Threats and Requirements for
Emergency Call Marking and Mapping", RFC 5069,
January 2008.
[lost] Hardie, T., "LoST: A Location-to-Service Translation
Protocol", Work in Progress, August 2007.
[toip] Wijk, A. and G. Gybels, "Framework for real-time text over
IP using the Session Initiation Protocol (SIP)", Work
in Progress, August 2006.
Authors' Addresses
Henning Schulzrinne
Columbia University
Department of Computer Science
450 Computer Science Building
New York, NY 10027
US
Phone: +1 212 939 7004
EMail: hgs+ecrit@cs.columbia.edu
URI: http://www.cs.columbia.edu
Roger Marshall (editor)
TeleCommunication Systems, Inc.
2401 Elliott Avenue
2nd Floor
Seattle, WA 98121
US
Phone: +1 206 792 2424
EMail: rmarshall@telecomsys.com
URI: http://www.telecomsys.com
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