ecrit B. Rosen
Internet-Draft NeuStar
Intended status: Standards Track H. Schulzrinne
Expires: May 22, 2008 Columbia U.
J. Polk
Cisco Systems
A. Newton
TranTech/MediaSolv
November 19, 2007
Framework for Emergency Calling using Internet Multimedia
draft-ietf-ecrit-framework-04
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
The IETF has several efforts targeted at standardizing various
aspects of placing emergency calls. This document describes how all
of those component parts are used to support emergency calls from
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citizens and visitors to authorities.
Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Overview of how emergency calls are placed . . . . . . . . . . 7
4. Which devices and services should support emergency calls . . 10
5. Identifying an emergency call . . . . . . . . . . . . . . . . 11
6. Location and its role in an emergency call . . . . . . . . . . 13
6.1. Types of location information . . . . . . . . . . . . . . 14
6.2. Location Determination . . . . . . . . . . . . . . . . . . 15
6.2.1. User-entered location information . . . . . . . . . . 16
6.2.2. Access network "wire database" location information . 17
6.2.3. End-system measured location information . . . . . . . 17
6.2.4. Network measured location information . . . . . . . . 18
6.3. Who adds location, endpoint or proxy . . . . . . . . . . . 18
6.4. Location and references to location . . . . . . . . . . . 19
6.5. End system location configuration . . . . . . . . . . . . 19
6.6. When location should be configured . . . . . . . . . . . . 21
6.7. Conveying location in SIP . . . . . . . . . . . . . . . . 22
6.8. Location updates . . . . . . . . . . . . . . . . . . . . . 22
6.9. Multiple locations . . . . . . . . . . . . . . . . . . . . 23
6.10. Location validation . . . . . . . . . . . . . . . . . . . 23
6.11. Default location . . . . . . . . . . . . . . . . . . . . . 24
6.12. Other location considerations . . . . . . . . . . . . . . 24
6.13. LIS and LoST Discovery . . . . . . . . . . . . . . . . . . 24
7. Uninitialized devices . . . . . . . . . . . . . . . . . . . . 24
8. Routing the call to the PSAP . . . . . . . . . . . . . . . . . 25
9. Signaling of emergency calls . . . . . . . . . . . . . . . . . 27
9.1. Use of TLS . . . . . . . . . . . . . . . . . . . . . . . . 27
9.2. SIP signaling requirements for User Agents . . . . . . . 27
9.3. SIP signaling requirements for proxy servers . . . . . . . 27
10. Call backs . . . . . . . . . . . . . . . . . . . . . . . . . . 28
11. Mid-call behavior . . . . . . . . . . . . . . . . . . . . . . 28
12. Call termination . . . . . . . . . . . . . . . . . . . . . . . 29
13. Disabling of features . . . . . . . . . . . . . . . . . . . . 29
14. Media . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
15. Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
16. Security Considerations . . . . . . . . . . . . . . . . . . . 30
17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 30
18. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
18.1. Normative References . . . . . . . . . . . . . . . . . . . 31
18.2. Informative References . . . . . . . . . . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 35
Intellectual Property and Copyright Statements . . . . . . . . . . 37
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1. 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 [RFC2119].
This document uses terms from [RFC3261] and
[I-D.ietf-ecrit-requirements]. In addition the following terms are
used:
Access network: The network that supplies IP packet service to an
endpoint. In a residential or small business environment, this
might be a DSL or cable modem or WiMax service. In a large
enterprise environment, this would be the enterprise network. In
a mobile environment, this might be a mobile (cellular) data
network or a WiFi network
(Emergency) Call taker: The person who answers an emergency call at
the PSAP
Confidence The mathematically derived statistical estimate
indicating how sure the measuring system is that the location data
estimate is accurate, within the bounds defined by the Uncertainty
value. This is expressed as a percentage, such as 90%, or 45%
etc.
Dispatch Location Location used for dispatching responders to the
person in need of assistance. Must be precise as opposed to that
needed for Routing Location.
Emergency services routing proxy (ESRP): A proxy server that
provides routing services for a group of PSAPs
Location configuration: The process where an endpoint learns its
physical location
Location conveyance: The process of sending location to another
element
Location determination: The process of finding where an endpoint is
physically. For example, the endpoint may contain a GPS receiver
used to measure its own location or location may be determined by
administration using a wiremap database or similar
Location Information Server (LIS): An element that stores location
information for retrieval by an authorized entity
Mobile device: User agent that changes geographic location and
possibly its network attachment point during an emergency call
NENA (National Emergency Number Association): A North American
organization of public safety focused individuals defining
emergency calling specifications and procedures
Nomadic device (user): User agent that is connected to the network
temporarily, for relatively short durations, but does not move
significantly during the lifetime of a network connection or
during the emergency call. Examples include a laptop using an
IEEE 802.11 hotspot or a desk IP phone that is moved from one
cubicle to another
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Routing Location: The location of an endpoint that is used for
routing an emergency call. May not be as precise as the Dispatch
Location.
Stationary device: An immobile user agent that is connected to the
network at a fixed, long-term-stable geographic location.
Examples include a home PC or a pay phone
Uncertainty The mathematically derived statistical estimate,
expressed in meters, indicating the size of the area used in the
calculation of Confidence.
2. Introduction
Requesting help in an emergency using a communications device such as
a telephone or mobile is an accepted practice in most of the world.
As communications devices increasingly utilize the Internet to
interconnect and communicate, users will continue to expect to use
such devices to request help, regardless of whether or not they
communicate using IP. This document describes establishment of a
communications session by a user to a "Public Safety Answering Point"
(PSAP) that is a call center established by response agencies to
accept emergency calls. Such citizen/visitor-to-authority calls can
be distinguished from those that are created by responders
(authority-to-authority) using public communications infrastructure
often involving some kind of priority access as defined in Emergency
Telecommunications Service (ETS) in IP Telephony [RFC4190]. They
also can be distinguished from emergency warning systems that are
authority-to-citizen.
Supporting emergency calling requires cooperation by a number of
elements, their vendors and service providers. It discusses how end
device and applications create emergency calls, how access networks
supply location for some of these devices, how service providers
assist the establishment and routing, and how PSAPs receive calls
from the Internet.
The emergency response community will have to upgrade their
facilities to support the wider range of communications services, but
cannot be expected to handle wide variation in device and service
capability. New devices and services are being made available that
could be used to make a request for help that are not traditional
telephones, and users are increasingly expecting them to be used to
place emergency calls. However, many of the technical advantages of
Internet multimedia require re-thinking of the traditional emergency
calling architecture. This challenge also offers an opportunity to
improve the operation of emergency calling technology, while
potentially lowering its cost and complexity.
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It is beyond the scope of this document to enumerate and discuss all
the differences between traditional (Public Switched Telephone
Network) and IP based telephony, but calling on the Internet is
characterized by:
o the interleaving over the same infrastructure of a wider variety
of services;
o the separation of the access provider from the application
provider;
o the plethora of different media that can be accommodated;
o potential mobility of all end systems, including endpoints
nominally thought of as fixed systems and not just those using
radio access technology. For example, a wired phone connected to
a router using a mobile data network such as EV-DO as an uplink.
This document focuses on how devices using the Internet can place
emergency calls and how PSAPs can handle Internet multimedia
emergency calls natively, rather than describing how circuit-switched
PSAPs can handle VoIP calls. In many cases, PSAPs making the
transition from circuit-switched interfaces to packet-switched
interfaces may be able to use some of the mechanisms described here,
in combination with gateways that translate packet-switched calls
into legacy interfaces, e.g., to continue to be able to use existing
call taker equipment. There are many legacy telephone networks that
will persist long after most systems have been upgraded to IP
origination and termination of emergency calls. There will be PSAPs
that require new systems to terminate to existing mechanisms for some
time. Many of these legacy systems use telephone number based
routing. Gateways and conversions between existing systems and newer
systems defined by this document will be required. Since existing
systems are governed primarily by local government regulations and
national standards, the gateway and conversion details will be
governed by national standards and thus are out of scope for this
document.
Existing emergency call systems are organized locally or nationally;
there are currently no international standards. However, the
Internet crosses national boundaries, and thus international
standards for equipment and software are required. To further
complicate matters, VoIP endpoints can be connected through tunneling
mechanisms such as virtual private networks (VPNs). Tunnels can
obscure the identity of the actual access network that knows the
location. This significantly complicates emergency calling, because
the location of the caller and the first element that routes
emergency calls can be on different continents, with different
conventions and processes for handling of emergency calls.
The IETF has historically refused to create national variants of its
standards. Thus, this document attempts to take into account best
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practices that have evolved for circuit switched PSAPs, but makes no
assumptions on particular operating practices currently in use,
numbering schemes or organizational structures.
This document discusses the use of the Session Initiation Protocol
(SIP) [RFC3261] by PSAPs and calling parties. While other inter-
domain call signaling protocols may be used for emergency calling,
SIP is ubiquitous and possesses the proper support of this use case.
Only protocols such as H.323, XMPP/Jingle, ISUP and SIP are suitable
for inter-domain communications, ruling out MGC protocols such as
MGCP or H.248/Megaco. The latter protocols can naturally be used by
the enterprise or carrier placing the call, but any such call would
reach the PSAP through a media gateway controller, similar to how
inter-domain VoIP calls would be placed. Other signaling protocols
may also use protocol translation to communicate with a SIP-enabled
PSAP.
Existing emergency services rely exclusively on voice and
conventional text telephony ("TTY") media streams. However, more
choices of media offer additional ways to communicate and evaluate
the situation as well as to assist callers and call takers in
handling emergency calls. For example, instant messaging and video
could improve the ability to communicate and evaluate the situation
and to provide appropriate instruction prior to arrival of emergency
crews. Thus, the architecture described here supports the creation
of sessions of any media type, negotiated between the caller and PSAP
using existing SIP protocol mechanisms [RFC3264].
As a framework document, no normative behavior is contained herein.
A companion document, [I-D.ietf-ecrit-phonebcp] describes Best
Current Practice for this subject and contains normative language for
devices, access and calling network elements.
Supporting emergency calling does not require any specialized SIP
header fields, request methods, status codes, message bodies, or
event packages, but does require that existing mechanisms be used in
certain specific ways, as described below. User agents unaware of
the recommendations in this draft may be able to place emergency
calls, but functionality may be impaired. For example, if the UA
does not implement the location mechanisms described, an emergency
call may not be routed to the correct PSAP, and if the caller is
unable to supply his exact location, dispatch of emergency responders
may be delayed. Suggested behavior for both endpoints and servers is
provided.
From the point of view of the PSAP three essential elements
characterize an emergency call:
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o The call is routed to the most appropriate PSAP, selected
principally by the location of the caller.
o The PSAP must be able to automatically obtain the location of the
caller sufficiently accurate to dispatch a responder to help the
caller.
o The PSAP must be able to re-establish a session to the caller if
for any reason the original session is lost.
3. Overview of how emergency calls are placed
An emergency call can be distinguished (Section 5) from any other
call by a unique Service URN [I-D.ietf-ecrit-service-urn], that is
placed in the call set-up signaling when a home or visited emergency
dial string is detected. Because emergency services are local to
specific geographic regions, a caller must obtain his location
(Section Section 6) prior to making emergency calls. To get this
location, either a form of measuring (e.g., GPS) (Section 6.2.3)
device location in the endpoint is deployed, or the endpoint is
configured (Section 6.5) with its location from the access network's
Location Information Server (LIS). The location is conveyed
(Section 6.7) in the SIP signaling with the call. The call is routed
(Section 8) based on location using the LoST protocol
[I-D.ietf-ecrit-lost], that maps a location to a set of PSAP or URIs.
Each URI resolves to a PSAP or an Emergency Services Routing Proxy
(ESRP) that serves an incoming proxy for group of PSAPs. The call
arrives at the PSAP with the location included in the INVITE request.
The following is a quick overview for a typical Ethernet connected
telephone using SIP signaling. It illustrates one set of choices for
various options presented later in this document.
o The phone "boots" and connects to its access network
o The phone gets location from the DHCP server [RFC4676] or
[RFC3825], a HELD server [I-D.ietf-geopriv-http-location-delivery]
or the first level switch's LLDP server [LLDP].
o The phone obtains the local emergency dial string(s) from the
[I-D.ietf-ecrit-lost] server for its current location. It also
receives and caches the PSAP URI obtained from LoST.
o It recognizes an emergency call from the dial strings and uses
"urn:service:sos" [I-D.ietf-ecrit-service-urn] to mark an
emergency call.
o It determines the PSAP's URI by querying the LoST mapping server
with its location.
o It puts its location in the SIP INVITE in a Geolocation header
[I-D.ietf-sip-location-conveyance] and forwards the call using its
normal outbound call processing, that commonly involves an
outgoing proxy.
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o The proxy recognizes the call as an emergency call and routes the
call using normal SIP routing mechanisms to the URI specified.
o The call routing commonly traverses an incoming proxy server
(ESRP) in the emergency services network. That proxy would route
to the PSAP.
o The call is established with the PSAP and common media streams are
created.
o The location of the caller is displayed to the call taker.
Configuration Servers
. . . . . . . . . . . . . . . . .
. .
. +--------+ +----------+ .
. +--------+ | +----------+ | .
. | LIS | | | SIP | | .
. | |-+ | Registrar|-+ .
. +--------+ +----------+ .
. ^ ^ .
. . | . . . . . . . | . . . . . .
| |
|[M1][M4] |[M2]
| | +--------+
|+--------------+ +--------+ |
|| | LoST | |
||+-------------------->| Servers|-+
||| [M3][M5] +--------+ +-------+
||| | PSAP2 |
||| +-------+
|||
||| [M6] +-------+ [M7]+------+ [M8]+-------+
Alice ------>| Proxy |---->| ESRP |---->| PSAP1 |-----> Call-Taker
+-------+ +------+ +-------+
+-------+
| PSAP3 |
+-------+
Figure 1: Emergency Call Component Topology
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The typical message flow would be:
[M1] Alice -> LIS LCP Request(s) (ask for location)
LIS -> ALICE LCP Reply(s) (replies with location)
[M2] Alice -> Registrar SIP REGISTER
Registrar -> Alice SIP 200 OK (REGISTER)
[M3] Alice -> LoST Server Initial LoST Query (contains location)
Lost Server -> Alice Initial LoST Response (contains
PSAP-URI and dial string)
*** Some time later, Alice dials/initiates emergency call ***
[M4] Alice -> LIS LCP Request (update location)
LIS -> ALICE LCP Reply (replies with location)
[M5] Alice -> LoST Server Update LoST Query (contains location)
Lost Server -> Alice LoST Response (contains PSAP-URI)
[M6] Alice to Outgoing Proxy INVITE (service URN,
Location and PSAP URI)
Outgoing Proxy to ESRP INVITE (service URN,
Location and PSAP URI)
ESRP to PSAP INVITE (service URN, Location and PSAP URI)
*** 200 OK and ACK propogated back from PSAP to Alice ***
Figure 2: Emergency Call Message Flow
Figure 1 shows emergency call component topology and the text above
shows call establishment. These include the following:
o Alice - who makes the emergency call.
o Configuration Servers - Servers providing Alice's UA its IP
address and other configuration information, perhaps including
location by-value or by-reference. In this flow, DHCP is used as
an example location configuration protocol (LCP). Configuration
servers also may include a SIP registrar for Alice's UA. Most SIP
UAs will register, so it will be a common scenario for UAs that
make emergency calls to be registered with such a server in the
originating calling network. Registration would be required for
the PSAP to be able to call back after an emergency call is
completed. All the configuration messages are labeled M1 through
M3, but could easily require more than 3 messages to complete.
o ESRP - The emergency services routing proxy server that is the
incoming call proxy in the emergency services domain. The ESRP
makes further routing decisions (e.g. based on PSAP state and the
location of the caller) to choose the actual PSAP that handles the
call. In some jurisdictions, this may involve another LoST query.
o LoST server - Processes the LoST request for Location + Service
URN to PSAP-URI Mapping function, either for an initial request
from a UA, or an in-call routing by the Proxy server in the
originating network, or possibly by an ESRP.
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o PSAP - Call center where emergency calls are destined for.
Generally, Alice's UA either has location configured manually, has an
integral location measurement mechanism, or it runs a LCP [M1] to
obtain location from the access (broadband) network. For most
devices, a LCP will be used, for example a DHCPREQUEST message or
another location acquisition mechanism. Alice's UA then will most
likely register [M2] with a SIP domain. This allows her to be
contacted by other SIP entities. Next, her UA will perform an
initial LoST query [M3] to learn a URI for use if the LoST query
fails during an emergency call, or to use to test the emergency call
mechanism. The LoST response may contain the dial string for
emergency calls appropriate for the location provided.
At some time after her device has booted, Alice initiates an
emergency call. She may do this by dialing an emergency dial string
valid for her current ("local") location, or for her "home" location.
The UA recognizes the dial string. The UA attempts to refresh its
location [M4], and with that location, to refresh the LoST mapping
[M5], in order to get the most accurate information to use for
routing the call. If the location request or the LoST request fails,
or takes too long, the UA uses values it has cached.
The UA creates a SIP INVITE [M6] request that includes the location.
[I-D.ietf-sip-location-conveyance] defines a SIP Geolocation header
that contains either a location-by-reference URI or a [RFC2396] "cid"
URL indicating where in the message body the location-by-value is.
The INVITE message is routed to the ESRP [M7], that is the first
inbound proxy for the emergency services domain. This message is
then routed by the ESRP towards the most appropriate PSAP for Alice's
location [M8], as determined by PSAP state, location and other
information.
A proxy in the PSAP chooses an available call taker and extends the
call to its UA.
The 200 OK response to the INVITE request traverses the path in
reverse, from call taker UA to PSAP proxy to ESRP to originating
network proxy to Alice's UA. The ACK completes the call set-up and
the emergency call is established, allowing the PSAP call-taker to
talk to Alice about Alice's emergency.
4. Which devices and services should support emergency calls
Support for voice calls and real-time text calls placed through PSTN
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facilities or systems connected to the PSTN is found in present
PSAPs. Future PSAPs will however support Internet connectivity and a
wider range of media types and provide higher functionality. In
general, if a user could reasonably expect to be able to place a call
for help with the device, then the device or service should support
emergency calling. Certainly, any device or service that looks like
and works like a telephone (wired or mobile) should support emergency
calling, but increasingly, users have expectations that other devices
and services should work.
Certainly, any device or service that looks like and works like a
telephone (wired or mobile) should support emergency calling, but
increasingly, users have expectations that other devices and services
should work.
Using current (evolving) standards, devices that create media
sessions and exchange audio, video and/or text, and have the
capability to establish sessions to a wide variety of addresses, and
communicate over private IP networks or the Internet, should support
emergency calls.
5. Identifying an emergency call
Using the PSTN, emergency help can often be summoned by dialing a
nationally designated, widely known number, regardless of where the
telephone was purchased. The appropriate number is determined by the
infrastructure the telephone is connected to. However, this number
differs between localities, even though it is often the same for a
country or region, as it is in many countries in the European Union.
In some countries, there is a single digit sequence that is used for
all types of emergencies. In others, there are several sequences
that are specific to the type of responder needed, e.g., one for
police, another for fire. For end systems, on the other hand, it is
desirable to have a universal identifier, independent of location, to
allow the automated inclusion of location information and to allow
the device and other entities in the call path to perform appropriate
processing within the signaling protocol in an emergency call set-up.
Since there is no such universal identifier, as part of the overall
emergency calling architecture, common emergency call URNs are
defined in [I-D.ietf-ecrit-service-urn]. An example, for a single
number environment is "urn:service:sos". Users are not expected to
"dial" an emergency URN. Rather, appropriate emergency dial strings
is translated to corresponding service URNs, carried in the Request-
URI of the INVITE. Such translation is best done by the endpoint,
because emergency calls convey location in the signaling, but non
emergency calls do not normally do that. If the device recognizes
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the emergency call, it can include location. Dial string recognition
could be performed in a signaling intermediary (proxy server) if for
some reason, the endpoint does not recognize it. For devices that
are mobile or nomadic, an issue arises of whether the home or visited
dialing strings should be used. Many users would prefer that their
home dialing sequences work no matter where they are. Local laws and
regulations may require the visited dialing sequence(s) always work.
Having the home dial string work is optional.
The mechanism for obtaining the dialing sequences for a given
location is provided by LoST [I-D.ietf-ecrit-lost]. If the endpoint
does not support the translation of dial strings to telephone
numbers, the dialing sequence would be represented as a dial string
[RFC4967] and the outgoing proxy would recognize the dial string and
translate to the service URN. To determine the local dial string,
the proxy needs the location of the endpoint. This may be difficult
in situations where the user can roam or be nomadic. Endpoint
recognition of emergency dial strings is therefore preferred, and in
fact if a service provider is unable to guarantee that it can
correctly determine local emergency dialstrings then it is required
that the endpoint do the recognition.
Note: It is undesirable to have a single "button" emergency call user
interface element. These mechanisms tend to result in a very high
rate of false or accidental emergency calls. In order to minimize
this rate, devices SHOULD only initiate emergency calls based on
entry of specific emergency call dial strings.
While in some countries there is a single 3 digit dial string that is
used for all emergency calls (i.e. 9-1-1 in North America), in some
countries there are several 3 digit numbers used for different types
of calls. For example, in Switzerland, 1-1-7 is used to call police,
1-1-8 is used to call the fire brigade, and 1-4-4 is used for
emergency medical assistance. In other countries, there are no
"short codes" or "service codes" for 3 digit dialing of emergency
services and local (PSTN) numbers are used.
[I-D.ietf-ecrit-service-urn] introduces a universal emergency service
URN scheme. On the wire, emergency calls include this type of URI in
the Request-URI [RFC3261]. The scheme includes a single emergency
URN (urn:service:sos) for use in countries with a single emergency
dial string, and responder-specific ones (urn:service:sos.police) for
countries where the user dials each service with separate numbers.
Using the service:sos URN scheme, emergency calls can be recognized
as such throughout the Internet.
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6. Location and its role in an emergency call
Location is central to the operation of emergency services. It is
frequently the case that the user in an emergency is unable to
provide a unique, valid location themselves. For this reason,
location provided by the endpoint or the access network is needed.
For practical reasons, each PSAP generally handles only calls for a
certain geographic area, with overload arrangements between PSAPs to
handle each others calls. Other calls that reach it by accident must
be manually re-routed (transferred) to the most appropriate PSAP,
increasing call handling delay and the chance for errors. The area
covered by each PSAP differs by jurisdiction, where some countries
have only a small number of PSAPs, while others decentralize PSAP
responsibilities to the level of counties or municipalities.
In most cases, PSAPs cover at least a city or town, but there are
some areas where PSAP coverage areas follow old telephone rate center
boundaries and may straddle more than one city. Irregular boundaries
are common, often for historical reasons. Routing must be done based
on PSAP service boundaries, the closest PSAP, or the PSAP that serves
the nominal city name provided in the location may not be the correct
PSAP.
Accuracy of routing location is a complex subject. Calls must be
routed quickly, but accurately, and location determination is often a
time/accuracy tradeoff, especially with mobile devices or self
measuring mechanisms. It is considered acceptable to base a routing
decision on an accuracy equal to the area of one sector of a mobile
cell site if no more accurate routing location is available.
Routing to the most appropriate PSAP is always calculated on the
location of the caller, despite the fact that some emergency calls
are placed on behalf of someone else, and the location of the
incident is sometimes not the location of the caller. In some cases,
there are other factors that enter into the choice of the PSAP that
gets the call, which may include factors other than location (such as
caller media and language preference, PSAP state, etc.). However,
location of the caller is the primary input to the routing decision.
Routing is but one of two uses for location in an emergency call.
The other is for dispatch of a responder. Many mechanisms used to
locate a caller have a relatively long "cold start" time. To get a
location accurate enough for dispatch may take as much as 30 seconds.
This is too long to wait for emergencies. Accordingly, it is common,
especially in mobile systems to use a coarse location, for example,
the cell site and sector serving the call, for routing purposes, and
then to update the location when a more precise value is known prior
to dispatch. In this document we use "routing location" and
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"dispatch location" when the distinction matters.
Accuracy of dispatch location is sometimes determined by local
regulation, and is constrained by available technology. The actual
requirement exceeds available technology. It is required that a
device making an emergency call close to the "demising" or separation
wall between two apartments in a high rise apartment building report
location with sufficient accuracy to determine on what side of the
wall it is on. This implies perhaps a 3 cm accuracy requirement. As
of the date of this memo, typical assisted GPS uncertainty with 95%
confidence is 100 m. As technology advances, the accuracy
requirements for location will need to be increased. Wired systems
using wire tracing mechanisms can provide location to a wall jack in
specific room on a floor in a building, and may even specify a
cubicle or even smaller resolution. As this discussion illustrates,
emergency call systems demand the most stringent location accuracy
available.
Location usually involves several steps to process and multiple
elements are involved. In Internet emergency calling, where the
endpoint is located is "Determined" using a variety of measurement or
wire-tracing methods. Endpoints may be "Configured" with their own
location by the access network. In some circumstances, a proxy
server may insert location into the signaling on behalf of the
endpoint. The location is "Mapped" to the URI to send the call to,
and the location is "Conveyed" to the PSAP (and other elements) in
the signaling. Likewise, we employ Location Configuration Protocols,
Location Mapping Protocols, and Location Conveyance Protocols for
these functions. The Location-to-Service Translation protocol
[I-D.ietf-ecrit-lost] is the Location Mapping Protocol defined by the
IETF.
6.1. Types of location information
There are several ways location can be specified:
Civic Civic location information describes the location of a person
or object by a street address that corresponds to a building or
other structure. Civic location may include more finely grained
location information such as floor, room and cubicle. Civic
information comes in two forms:
Jurisdictional This refers to a civic location using actual
political subdivisions, especially for the community name.
Postal This refers to a civic location for mail delivery. The
name of the post office sometimes does not correspond to the
community name and a postal address may contain post office
boxes or street addresses that do not correspond to an actual
building. Postal addresses are generally unsuitable for
emergency call dispatch because the post office conventions
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(for community name, for example) do not match those known by
the responders. The fact that they are unique can sometimes be
exploited to provide a mapping between a postal address and a
civic address suitable to dispatch a responder to. In IETF
location protocols, there is a element (Postal Community Name)
that can be included in a location to provide the post office
name as well as the actual jurisdictional community name.
There is no other accommodation for postal addresses in these
protocols.
Geospatial (geo): Geospatial addresses contain longitude, latitude
and altitude information based on an understood datum and earth
shape model. While there have been many datums developed over
time, most modern systems are using or moving towards the
WGS84[WGS84] datum.
Cell tower/sector: Cell tower/sector is often used for identifying
the location of a mobile handset, especially for routing of
emergency calls. Cell tower and sectors identify the cell tower
and the antenna sector that a mobile device is currently using.
Traditionally, the tower location is represented as a point chosen
to be within a certain PSAP service boundary who agrees to take
calls originating from that tower/sector, and routing decisions
are made on that point. Cell/sector information could also be
represented as an irregularly shaped polygon of geospatial
coordinates reflecting the likely geospatial location of the
mobile device. Whatever representation is used must route
correctly in the LoST database, where "correct" is determined by
local PSAP management.
In IETF protocols, civic and geospatial forms are both supported.
The civic forms include both postal and jurisdictional fields. A
cell tower/sector can be represented as a point (geo or civic) or
polygon. Other forms of location representation must be mapped into
either a geo or civic for use in emergency calls.
For emergency call purposes, conversion of location information from
civic to geo or vice versa prior to conveyance is not desirable. The
location should be sent in the form it was determined. Conversion
between geo and civic requires a database. PSAPs may need to convert
from whatever form they receive to another for responder purposes.
They have a suitable database. However, if a conversion is done
before the PSAP, and the database used is not exactly the one the
PSAP uses, the double conversion has a high probability of
introducing an error.
6.2. Location Determination
Location information can be entered by the user or installer of a
device ("manual configuration"), measured by the end system, can be
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delivered to the end system by some protocol or measured by a third
party and inserted into the call signaling. Choice of location
determination mechanisms and their properties are out of scope for
this document.
In some cases, an entity may have multiple sources of location
information, possibly partially contradictory. This is particularly
likely if the location information is determined both by the end
system and a third party. Although self measured location (e.g.
GPS) is attractive, access network provided location could be much
more accurate, and more reliable in some environments (indoor high
rise in dense urban areas for example). In general, the closer an
entity is to the source of location, the more it is in the best
position to determine which location is "best" for a particular
purpose. In emergency calling, the PSAP is the least likely to be
able to appropriately choose which location when multiple conflicting
locations are presented to it.
6.2.1. User-entered location information
Location information can be maintained by the end user or the
installer of an endpoint in the endpoint itself, or in a database.
Location information provided by end users is almost always less
reliable than measured or wire database information, as users may
mistype location information or may enter civic address information
that does not correspond to a recognized (i.e. valid, see Section
Section 6.10) address. Users can neglect to change the data when the
location of a device changes during or after movement.
All that said, there are always a small number of cases where the
automated mechanisms used by the access network to determine location
fail to accurately reflect the actual location of the endpoint. For
example, the user may deploy his own WAN behind an access network,
effectively removing an endpoint some distance from the access
network's notion of its location. There must be some mechanism
provided to provision a location for an endpoint by the user or by
the access network on behalf of a user. The use of the mechanism
introduces the possibility of users falsely declaring themselves to
be somewhere they are not. As an aside, normally, if an emergency
caller insists that he is at a location different from what any
automatic location determination system reports he is, responders
will always be sent to the user's self-declared location. However
this is a matter of local policy and is outside the scope of this
document.
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6.2.2. Access network "wire database" location information
Location information can be maintained by the access network,
relating some form of identifier for the end subscriber or device to
a location database ("wire database"). In enterprise LANs, wiremap
databases map Ethernet switch ports to building locations. In DSL
installations, the local telephone carrier maintains a mapping of
wire-pairs to subscriber addresses.
Accuracy of location historically has been to a street address level.
However, this is not sufficient for larger structures. The PIDF-LO
[RFC4119] with a recent extension [I-D.ietf-geopriv-revised-civic-lo]
permits interior building/floor/room and even finer specification of
location within a street address. When possible, interior location
should be supported.
The threshold for when interior location is needed is approximately
650 m2 (that is derived from fire brigade recommendations of spacing
of alarm pull stations) should have, but interior space layout,
construction materials and other factors should be considered. The
ultimate goal is to be able to find the person in need quickly if
responders arrive at the location given.
Even for IEEE 802.11 wireless access points, wire databases may
provide sufficient location resolution. The location of the access
point as determined by the wiremap may be supplied as the location
for each of the clients of the access point. However, this may not
be true for larger-scale systems such as IEEE 802.16 (WiMAX) and IEEE
802.22 that typically have larger cells than those of IEEE 802.11.
The civic location of an IEEE 802.16 base station may be of little
use to emergency personnel, since the endpoint could be several
kilometers away from the base station.
Wire databases to the home are likely to be the most promising
solution for residential users where a service provider knows the
customer's service address. The service provider can then perform
address validation (see Section 6.10), similar to the current system
in some jurisdictions.
6.2.3. End-system measured location information
Global Positioning System (GPS) and similar satellite based (e.g.
Galileo) receivers may be embedded directly in the end device. GPS
produces relatively high precision location fixes in open-sky
conditions, but the technology still faces several challenges in
terms of performance (time-to-fix and time-to-first-fix), as well as
obtaining successful location fixes within shielded structures, or
underground. It also requires all devices to be equipped with the
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appropriate GPS capability. GPS-derived locations are currently
accurate to tens of meters. Many mobile devices require using some
kind of "assist", that may be operated by the access network (A-GPS)
or by a government (WAAS).
GPS systems may be always on; where location will always be available
accurately (assuming the device can "see" enough satellites). Mobile
devices may not be able to sustain the power levels required to keep
the measuring system active. This means that when location is
needed, the device has to start up the measurement mechanism. This
typically takes tens of seconds, far too long to wait to be able to
route an emergency call. For this reason, devices that don't have
end-system measured location mechanisms always on need another way to
get a routing location. Typically this would be a location
associated with a radio link (cell site/sector).
6.2.4. Network measured location information
The access network may locate end devices. Techniques include:
Wireless triangulation: Elements in the network infrastructure
triangulate end systems based on signal strength, angle of arrival
or time of arrival. Common mechanisms deployed include:
1. Time Difference Of Arrival - TDOA
2. Uplink Time Difference Of Arrival - U-TDOA
3. Angle of Arrival - AOA
4. RF-Fingerprinting
5. Advanced Forward Link Trilateration - AFLT
6. Enhanced Forward Link Trilateration - EFLT
Sometimes multiple mechanisms are combined, for example A-GPS with
AFLT
Location beacons: A short range wireless beacon, e.g., using
Bluetooth or infrared, announces its location to mobile devices in
the vicinity. This allows devices to get location from the beacon
source's location.
6.3. Who adds location, endpoint or proxy
The IETF emergency call architecture prefers endpoints to learn their
location and supply it on the call. Outbound proxies that support
devices that do not support location may have to add location to
emergency calls at a proxy server. Some calling networks have
relationships with all access networks the device may be connected
to, and that may allow the proxy to accurately determine location of
the endpoint. However NATs and other middleboxes often make it
impossible to determine a reference identifier the access network
could use to determine the location. Systems designers are
discouraged from relying on proxies to add location. The technique
may be useful in some limited circumstances as devices are upgraded
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to meet the requirements of this document, or where relationships
between access networks and calling networks are feasible and can be
relied upon to get accurate location.
Proxy insertion of location complicates dial string recognition. As
noted in Section Section 6, local dial strings depend on the location
of the caller. If the device does not know its own location, it
cannot use the LoST service to learn the local emergency dial
strings. The calling network must provide another way for the device
to learn the local dial string (and update it when the user moves to
a location where the dial string(s) change) or do the dial string
determination itself.
6.4. Location and references to location
Location information may be expressed as the actual civic or
geospatial value but can be transmitted as by value (wholly contained
within the signaling message) or by reference (a URI pointing to the
value residing on a remote node waiting to be dereferenced). Each
form is better suited to some applications than others.
When location is transmitted by value, the location information is
available to each device; on the other hand, location objects can be
large, and only represent a single snapshot of the device's location.
Location references are small and can be used to represent a time-
varying location, but the added complexity of the dereference step
introduces a risk that location will not be available to parties that
need it.
6.5. End system location configuration
Unless a user agent has access to provisioned or locally measured
location information, it must obtain it from the access network.
There are several location configuration protocols (LCPs) that can be
used for this purpose such as:
DHCP DHCP can deliver civic [RFC4676] or geospatial [RFC3825]
information. User agents need to support both formats. Note that
a user agent can use DHCP, via the DHCP REQUEST or INFORM
messages, even if it uses other means to acquire its IP address.
HELD HELD [I-D.ietf-geopriv-http-location-delivery] can deliver a
civic or geo, by value or by reference, as a layer 7 protocol.
The query typically uses the IP address of the requestor as an
identifier and returns the location value or reference associated
with that identifier. HELD is typically transported on HTTP.
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Link-Layer Discovery Protocol Layer Discovery Protocol [LLDP] with
Media Endpoint Device extensions [LLDP-MED] can be used to deliver
location information directly from the Layer 2 network
infrastructure, and also supports both civic and geospatial
formats identical in format to DHCP methods.
Each LCP has limitations in the kinds of networks that can reasonably
support it. For this reason, it is not possible to choose a single
mandatory-to-deploy LCP. For endpoints with common network
connections (such as an Ethernet jack or a WiFi connection) serious
incompatibilities would ensue unless every network supported every
protocol, or alternatively, every device supported every protocol.
For this reason, a list of LCPs is established in
[I-D.ietf-ecrit-phonebcp]. Every endpoint that could be used to
place emergency calls must implement all of the protocols on the
list. Every access network must deploy at least one of them. It is
recognized that this is an onerous requirement, that it would be
desirable to eliminate. However, since it is the variability of the
networks that prevent a single protocol from being acceptable, it
must be the endpoints that implement all of them, and to accommodate
a wide range of devices, networks must deploy at least one of them.
Often, network operators and device designers believe that they have
a simpler environment and some other network specific mechanism can
be used to provide location. Unfortunately, it is very rare to
actually be able to limit the range of devices that may be connected
to a network.
For example, existing mobile networks are being used to support
routers and LANs behind a wireless data network WAN connection, with
Ethernet connected phones connected to that. It is possible that the
access network could support a protocol not on the list, and require
every handset in that network to use that protocol for emergency
calls. However, the Ethernet connected phone won't be able to
acquire location, and the user of the phone is unlikely to be
dissuaded from placing an emergency call on that phone. The
widespread availability of gateways, routers and other network-
broadening devices means that indirectly connected endpoints are
possible on nearly every network. Network operators and vendors are
cautioned that shortcuts to meeting this requirement are seldom
successful.
Location for non-mobile devices is normally expected to be acquired
at network attachment time and retained by the device. It should be
refreshed when the cached value becomes invalid. For example, if
DHCP is the acquisition protocol, refresh of location may occur when
the IP address lease is renewed. At the time of an emergency call,
the location should be refreshed, with the retained location used if
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the location acquisition does not immediately return a value. Mobile
devices may determine location at network attachment time and
periodically thereafter as a backup in case location determination at
the time of call does not work. Mobile device location may be
refreshed when a TTL expires, the device moves beyond some boundaries
(as provided by [I-D.ietf-ecrit-lost]). Normally, mobile devices
will acquire its location at call time for use in an emergency call
routing. See Section Section 6.8 for a further discussion on
location updates for dispatch location.
There are many examples of end devices which are applications running
on a more general purpose device, such as a personal computer. In
some circumstances, it is not possible for application programs to
access the network device at a level necessary to implement the LLDP-
MED protocol, and in other cases, obtaining location via DHCP may be
impossible. In any case it is desirable for an operating system
which could be used for any application which could make emergency
calls to have an API which provides the location of the device for
use by any application.
6.6. When location should be configured
Devices should get routing location immediately after obtaining local
network configuration information. The presence of NAT and VPN
tunnels (that assign new IP addresses to communications) can obscure
identifiers used by LCPs to determine location, especially using
HELD. In some cases, such as residential NAT devices, the NAT is
before the access network demarcation point and thus the IP address
seen by the access network is the right identifier for location of
the residence. In many enterprise environments, VPN tunnels can
obscure the actual IP address. Some VPN mechanisms can be bypassed
(a query to the LCP can be designated to go through the direct IP
path, using the correct IP address, and not through the tunnel). In
other cases, no bypass is possible. Of course, LCPs that use Layer 2
mechanisms (DHCP Location options and LLDP-MED) are usually immune
from such problems because they do not use the IP address as the
identifier for the device seeking location.
It is desirable that routing location information be periodically
refreshed. A LIS supporting a million subscribers each refreshing
once per day would need to support a query rate of 1,000,00 / (24 *
60 * 60) = 12 queries per second.
It is desirable for routing location information to be requested
immediately before placing an emergency call. However, if there is
any significant delay in getting more recent location, the call
should be placed with the most recent location information the device
has. In mobile handsets, routing is often accomplished with the cell
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site and sector of the tower serving the call, because it can take
many seconds to start up the location determination mechanism and
obtain an accurate location.
There is a tradeoff between the time it takes to get a routing
location and the accuracy (technically, confidence and uncertainty)
obtained. Routing an emergency call quickly is required. However,
if location can be substantially improved by waiting a short time
(e.g. for some sort of "quick fix"), it's preferable to wait. 3
seconds, that is the current nominal time for a quick fix, is a very
long time to wait for help, and systems designers should attempt to
provide accurate routing location in much less time.
NENA recommends IP based systems complete calls in two seconds (last
dial press to ring at PSAP).
6.7. Conveying location in SIP
When an emergency call is placed, the endpoint should put location in
the signaling with the call. That is referred to as "conveyance" to
distinguish it from "configuration". In SIP, the location
information is conveyed following the procedures in
[I-D.ietf-sip-location-conveyance]. The form of the location
information obtained by the acquisition protocol may not be the same
as the conveyance protocol uses (PIDF-LO [RFC4119]). Mapping by the
endpoint to PIDF may be required.
6.8. Location updates
As discussed above, it make take some time for some measurement
mechanisms to get a location accurate enough for dispatch, and a
routing location with less accuracy may be provided to get the call
established early. The PSAP needs the dispatch location before it
sends the call to the responder. This requires an update of the
location.
In addition, the location of a mobile caller, e.g., in a vehicle or
aircraft, can change significantly during the emergency call. While
most often this change is not significant, the PSAP must be able to
get updated location information while it is processing the call.
Subscription is preferred so that the LIS notifies the PSAP when
accurate location is updated rather than requiring a poll operation
from the PSAP to the LIS.
A PSAP has no way to request an update of a location-by-value. If
the UAC gets new location, it must reINVITE or UPDATE to supply the
new location.
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Generally, the PSAP can wait for an accurate location for dispatch.
However, there is no fixed limit known in advance; it depends on the
nature of the emergency. At some point the PSAP must dispatch. In a
subscription environment, the PSAP could update the parameters in the
filter (immediate response required). In a HELD dereference, there
is no way to cancel and the PSAP will have to choose a ResponseTime
that it will wait for even if it wants to dispatch sooner than that.
(Change as the discussion on ResponseTime evolves).
6.9. Multiple locations
Handling multiple locations is discussed in
[I-D.ietf-geopriv-pdif-lo-profile]. Conflicting location information
is particularly harmful if different routes (PSAPs) result from LoST
queries for the multiple locations. Guidelines for dealing with
multiple locations are also given in [I-D.ietf-ecrit-lost].
Generally, if a UA gets multiple locations, it must choose the one to
use. If a proxy is inserting location and has multiple locations, it
must choose the one to use.
The ability of the UA or proxy to understand how and from whom it
learned its location, and include this information element in the
location object that is sent to the PSAP, provides the call-taker
with many pieces of information to make decisions upon, and guidance
for what to ask the caller and what to tell the responders.
The call should indicate the location information that has been used
for routing, so that the same location information is used for all
call routing decisions. The location conveyance mechanism
[I-D.ietf-sip-location-conveyance] contains a parameter for this
purpose.
6.10. Location validation
It is recommended that location must be validated prior to a device
placing an actual emergency call; some jurisdictions require that
this be done. Validation in this context means both that there is a
mapping from the address to a PSAP and that the PSAP understands how
to direct responders to the location. Determining the addresses that
are valid can be difficult. There are, for example, many cases of
two names for the same street, or two streets with the same name in a
city. In some countries, the current system provides validation.
For example, in the United States, the Master Street Address Guide
(MSAG) records all valid street addresses and is used to ensure that
the service addresses in phone billing records correspond to valid
emergency service street addresses. Validation is normally a concern
for civic addresses, although there could be a concern that a given
geo is within at least one PSAP service boundary; that is, a "valid"
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geo is one where there is a mapping.
LoST [I-D.ietf-ecrit-lost] includes a location validation function.
Validation should ideally be performed when a location is entered
into a Location Information Server. It should be confirmed
periodically, because the mapping database undergoes slow change; new
streets are added or removed, community names change, postal codes
change, etc. Endpoints may wish to validate locations they receive
from the access network, and will need to validate manually entered
locations. Proxies that insert location may wish to validate
locations they receive from a LIS. Test functions (Section 15)
should also re-validate.
6.11. Default location
Occasionally, the access network cannot determine the actual location
of the caller. In these cases, it must supply a default location.
The default location should be as accurate as the network can
determine. For example, in a cable network, a default location for
each Cable Modem Termination System (CMTS), with a representative
location for all cable modems served by that CMTS could be provided
if the network is unable to resolve the subscriber to any unit less
than the CMTS. Default locations must be marked as such so that the
PSAP knows that the location is not accurate.
6.12. Other location considerations
The endpoint is responsible for mapping any form of location it
receives from an LCP into PIDF-LO form if the LCP did not directly
return a PIDF.
6.13. LIS and LoST Discovery
If endpoints are to get their location and determine the routing of
emergency calls, they must be able to discover a LIS (if the HELD
protocol is used), and a LoST server. DHCP options are defined for
this purpose: [I-D.thomson-geopriv-lis-discovery] and
[I-D.thomson-geopriv-lis-discovery]
In some cases, it may be necessary for the service provider to
provision a LoST server address in the device.
7. Uninitialized devices
Support of devices that are not registered, or that don't have valid
call back identifiers is complex. In some jurisdictions, for some
services, support of emergency calls from so-called "uninitialized"
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devices is required. For example, cellular providers in the United
States must support calls to 9-1-1 from a mobile phone that does not
have an active service contract. It is attractive for such devices
to be able to be used in an emergency. However, the requirement to
do so has caused a huge number of prank calls to the emergency
service. In some countries, it is common to attempt to place an
emergency call from an unitialized device in the local bazaars to
prove to a would-be purchaser that the phone works. For this reason,
PSAP authorities discourage support for unititialized devices.
An unitialized device that can place an emergency call must supply
location the same as a fully enabled device, must carry a call back
URI that can be used to call the device back, and should have
identifiers in the signaling that can be used to identify the device.
8. Routing the call to the PSAP
Emergency calls are routed based on one or more of the following
criteria expressed in the call setup request (INVITE):
Location: Since each PSAP serves a limited geographic region and
transferring existing calls delays the emergency response, calls
need to be routed to the most appropriate PSAP. In this
architecture, emergency call setup requests contain location
information, expressed in civic or geospatial coordinates, that
allows such routing. If there is no or imprecise (e.g., cell
tower and sector) information at call setup time, an on-going
emergency call may also be transferred to another PSAP based on
location information that becomes available in mid-call.
Type of emergency service: In some jurisdictions, emergency calls
for fire, police, ambulance or mountain rescue are directed to
just those emergency-specific PSAPs. This mechanism is supported
by marking emergency calls with the proper service identifier
[I-D.ietf-ecrit-service-urn].
Media capabilities of caller: In some cases, emergency call centers
for specific caller media preferences, such as typed text or
video, are separate from PSAPs serving voice calls. Routing based
on media would be accomplished at an ESRP. Also, even if media
capability does not affect the selection of the PSAP, there may be
call takers within the PSAP that are specifically trained, e.g.,
in interactive text or sign language communications, where routing
within the PSAP based on the media offer would be provided.
Routing for calls by location and by service is the primary function
LoST [I-D.ietf-ecrit-lost] provides. LoST accepts a query with
location (by-value) in either civic or geospatial form, plus a
service identifier, and returns a URI (or set of URIs) to route the
call to. Normal SIP [RFC3261] routing functions are used to resolve
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the URI to a next hop destination.
The endpoint can complete the LoST mapping from its location at boot
time, and periodically thereafter. It should attempt to obtain a
"fresh" location, and from that a current mapping when it places an
emergency call. If accessing either its location acquisition or
mapping functions fail, it should use this cached value. The call
would follow its normal outbound call processing.
Determining when the device leaves the area provided by the LoST
service can tax small mobile devices. For this reason, the LoST
server should return a simple (small number of points) polygon for
geo reported location [I-D.ietf-geopriv-pdif-lo-profile]. This can
be an enclosing subset of the area when the reported point is not
near an edge or a smaller edge section when the reported location is
near an edge. Civic location is uncommon for mobile devices, but
reporting that the same mapping is good within a community name, or
even a street, may be very helpful for WiFi connected devices that
roam and obtain civic location from the AP they are connected to.
Networks that support devices that do not implement LoST mapping
themselves would have the outbound proxy do the mapping. The proxy
must have the location of the endpoint, that is often difficult for
the calling network to accurately determine. The endpoint may have
its location, but would not normally include it on the call
signaling. There is no mechanism provided in
[I-D.ietf-sip-location-conveyance] to allow a proxy to require the
endpoint supply location, because that would open the endpoint to an
attack by any proxy on the path to get it to reveal location. The
Proxy can redirect a call to the service URN that, if the device
recognized the significance, would include location in the redirected
call. All networks should detect emergency calls and supply default
location and/or routing if it is not already performed.
With the URI obtained from mapping, whether by the endpoint or the
proxy, the proxy routes the call. Normal SIP [RFC3261] and [RFC3263]
mechanisms are used to route calls to the URI obtained from the LoST
query.
Often, the SIP routing of an emergency call will first route to an
incoming call proxy in the domain operated by the emergency service.
That proxy is called an "Emergency Services Routing Proxy" (ESRP).
The ESRP, which is a normal SIP proxy server, may use a variety of
PSAP state information, the location of the caller, and other
criteria to onward route the call to the PSAP. In order for the ESRP
to route on media choice, the initial INVITE has to supply an SDP
Offer.
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9. Signaling of emergency calls
9.1. Use of TLS
As discussed above, location is carried in all emergency calls in the
call signaling. Since emergency calls carry privacy-sensitive
information, they are subject to the requirements for geospatial
protocols [RFC3693]. In particular, signaling information should be
carried in TLS, i.e., in 'sips' mode. Although there are exceptions
in [RFC3693] for emergency calls (for example, local policy may
dictate that location is sent with an emergency call even if the
user's policy would otherwise prohibit that), it is unacceptable to
have an emergency call fail to complete because a TLS connection was
not created for any reason. Thus the call should be attempted with
TLS, but if the TLS session establishment fails, the call should be
automatically retried without TLS. [I-D.ietf-sip-sips] recommends
that to achieve this effect the target request a sip URI, but use TLS
on the outbound connection. An element that recieves a request over
a TLS connection should attempt to create a TLS connection to the
next hop.
Location may be used for routing by multiple proxy servers on the
path. Confidentiality mechanisms such as S/MIME encryption of SIP
signaling [RFC3261] cannot be used because they obscure location.
Only hop-by-hop mechanisms such as TLS should be used. Many SIP
devices do not support TLS. Implementing location conveyance in SIP
mandates inclusion of TLS support.
In many cases, persistent TLS connections can be maintained between
elements to minimize the time needed to establish them
[I-D.ietf-sip-outbound]. In other circumstances, use of session
resumption [RFC4507] is recommended. IPSEC [RFC2401] is an
acceptable alternative to TLS.
9.2. SIP signaling requirements for User Agents
SIP UAs that do local dial string interpretation, location, and
emergency call route will create SIP INVITE messages with the Service
URN in the Request URI, the LoST-determined URI for the PSAP in a
Route header, and the location in a Geolocation header. The INVITE
must also have appropriate call back identifiers To enable media
sensitive routing, the call should include an SDP offer.
9.3. SIP signaling requirements for proxy servers
SIP Proxy servers in the path of an emergency call must be able to
assist UAs that are unable to provide any of the location based
routing steps and recognition of dial strings. They are also
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expected to provide identity information for the caller.
10. Call backs
The call-taker must be able to reach the emergency caller if the
original call is disconnected. In traditional emergency calls,
wireline and wireless emergency calls include a callback identifier
for this purpose. In SIP systems, the caller must include a Contact
header field indicating its device URI, if globally routable, or
possibly a GRUU [I-D.ietf-sip-gruu] if calls need to be routed via a
proxy. This identifier would be used to initiate call-backs
immediately by the call-taker if, for example, the call is
prematurely dropped. This is a change from [RFC3261] where Contact:
is optional.
In addition, a call-back identifier must be included either as the
URI in the From header field [RFC3261] verified by SIP Identity
[RFC4474] , or as a network asserted URI [RFC3325]. This identifier
would be used to initiate a call-back at a later time and may reach
the caller, not necessarily on the same device (and at the same
location) as the original emergency call as per normal SIP rules.
Emergency authorities generally discourage support of unitialized
devices (see Section 7. If an uninitialized device does place an
emergency call, some kind of call back URI must be provided (e.g. a
GRUU) in the Contact: header. It is useful to be able to call the
device back some time later as well by including some form of URI in
a network asserted identity.
11. Mid-call behavior
A PSAP may need to REFER [RFC3515] a call to a bridge for
conferencing. The caller should also be prepared to have the call
transferred (usually attended, but possibly blind) as per
[I-D.ietf-sipping-service-examples]. PSAPs often include
dispatchers, responders or specialists on a call. Some responder's
dispatchers are not located in the primary PSAP. The call may have
to be transferred to another PSAP. Most often this will be an
attended transfer, or a bridged transfer. Relay services for
communication with people with disabilities may be included in the
call in this way.
SIP Caller Preferences [RFC3841] can be used to signal how the PSAP
should handle the call. For example, a language preference expressed
in an Accept-Language header may be used as a hint to cause the PSAP
to route the call to a call taker who speaks the requested language.
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SIP Caller Preferences may also be used to indicate a need to invoke
a relay service for communication with people with disabilities in
the call.
12. Call termination
It is undesirable for the caller to terminate an emergency call.
PSAP call termination is accomplished with normal SIP call
termination procedures.
13. Disabling of features
Certain features that can be invoked while a normal call is active
are not permitted when the call is an emergency call. Services such
as Call Waiting, Call Transfer, Three Way Call and Flash Hold should
be disabled.
Certain features can interfere with calls from a PSAP and should be
disabled. The domain of a PSAP can be determined from the domain
answering an emergency call. A time limit after an emergency call
should be established during which any call from the same domain and
directed to the supplied Contact: or AoR should be accepted as a
call-back from the PSAP.
14. Media
PSAPs should always accept RTP media streams [RFC3550].
Traditionally, voice has been the only media stream accepted by
PSAPs. In some countries, text, in the form of BAUDOT codes or
similar tone encoded signaling within a voiceband is accepted ("TTY")
for persons who have hearing disabilities. With the Internet comes a
wider array of potential media that a PSAP should accept. Using SIP
signaling includes the capability to negotiate media. Normal SIP
offer/answer [RFC3264] negotiations should be used to agree on the
media streams to be used. PSAPs should accept real-time text
[RFC4103]. All PSAPs should accept G.711 A law (and mu Law in North
America) encoded voice as described in [RFC3551]. Newer text forms
are rapidly appearing, with Instant Messaging now very common, PSAPs
should accept IM with at least [RFC3428] as well as [RFC3920]. Video
may be important to support Video Relay Service (Sign language
interpretation) as well as modern video phones.
While it is desirable for media to be kept secure, preferably by use
of Secure RTP [RFC3711], there is not yet consensus on how best to
signal keying material for SRTP. As a consequence, no recommendation
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to support SRTP can yet be made for emergency calls.
15. Testing
Since the emergency calling architecture consists of a number of
pieces operated by independent entities, it is important to be able
to test whether an emergency call is likely to succeed without
actually occupying the human resources at a PSAP. Both signaling and
media paths need to be tested since NATs and firewalls may allow the
session setup request to reach the PSAP, while preventing the
exchange of media.
[I-D.ietf-ecrit-phonebcp] includes a description of an automated test
procedure that validates routing, signaling and media path
continuity. This test would be used at boot time, and whenever the
device location changes enough that a new PSAP mapping is returned
from LoST. A manual operation for the test should also be possible.
The PSAP needs to be able to control frequency and duration of the
test, and since the process could be overused, it may temporarily or
permanently suspend its operation.
There is a concern associated with testing during a so-called
"avalanche-restart" event where, for example a large power outage
affects a large number of endpoints, that, when power is restored,
all attempt to reboot and, possibly, test. Devices need to randomize
their initiation of a boot time test to avoid the problem.
16. Security Considerations
Security considerations for emergency calling have been documented in
[I-D.ietf-ecrit-security-threats], and [I-D.barnes-geopriv-lo-sec].
Ed. Note: go through that doc and make sure any actions needed are
captured in the BCP text.
17. Acknowledgements
This draft was created from a
draft-schulzrinne-sipping-emergency-arch-02 together with sections
from draft-polk-newton-ecrit-arch-considerations-02.
Design Team members participating in this draft creation include
Hannes Tschofenig, Ted Hardie, Martin Dolly, Marc Linsner, Roger
Marshall, Stu Goldman, Shida Schubert and Tom Taylor. Further
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comments and input was provided by Richard Barnes, Barbara Stark and
James Winterbottom.
18. References
18.1. Normative References
[I-D.barnes-geopriv-lo-sec]
Barnes, R., "Threats to GEOPRIV Location Objects",
draft-barnes-geopriv-lo-sec-00 (work in progress),
July 2007.
[I-D.ietf-ecrit-dhc-lost-discovery]
Schulzrinne, H., "A Dynamic Host Configuration Protocol
(DHCP) based Location-to-Service Translation Protocol
(LoST) Discovery Procedure",
draft-ietf-ecrit-dhc-lost-discovery-02 (work in progress),
July 2007.
[I-D.ietf-ecrit-lost]
Hardie, T., "LoST: A Location-to-Service Translation
Protocol", draft-ietf-ecrit-lost-06 (work in progress),
August 2007.
[I-D.ietf-ecrit-phonebcp]
Rosen, B. and J. Polk, "Best Current Practice for
Communications Services in support of Emergency Calling",
draft-ietf-ecrit-phonebcp-02 (work in progress),
September 2007.
[I-D.ietf-ecrit-requirements]
Schulzrinne, H. and R. Marshall, "Requirements for
Emergency Context Resolution with Internet Technologies",
draft-ietf-ecrit-requirements-13 (work in progress),
March 2007.
[I-D.ietf-ecrit-security-threats]
Taylor, T., "Security Threats and Requirements for
Emergency Call Marking and Mapping",
draft-ietf-ecrit-security-threats-05 (work in progress),
August 2007.
[I-D.ietf-ecrit-service-urn]
Schulzrinne, H., "A Uniform Resource Name (URN) for
Emergency and Other Well-Known Services",
draft-ietf-ecrit-service-urn-07 (work in progress),
August 2007.
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[I-D.ietf-geopriv-http-location-delivery]
Barnes, M., Winterbottom, J., Thomson, M., and B. Stark,
"HTTP Enabled Location Delivery (HELD)",
draft-ietf-geopriv-http-location-delivery-03 (work in
progress), November 2007.
[I-D.ietf-geopriv-pdif-lo-profile]
Winterbottom, J., Thomson, M., and H. Tschofenig, "GEOPRIV
PIDF-LO Usage Clarification, Considerations and
Recommendations", draft-ietf-geopriv-pdif-lo-profile-10
(work in progress), October 2007.
[I-D.ietf-geopriv-revised-civic-lo]
Thomson, M. and J. Winterbottom, "Revised Civic Location
Format for PIDF-LO",
draft-ietf-geopriv-revised-civic-lo-06 (work in progress),
October 2007.
[I-D.ietf-sip-gruu]
Rosenberg, J., "Obtaining and Using Globally Routable User
Agent (UA) URIs (GRUU) in the Session Initiation Protocol
(SIP)", draft-ietf-sip-gruu-15 (work in progress),
October 2007.
[I-D.ietf-sip-location-conveyance]
Polk, J. and B. Rosen, "Location Conveyance for the
Session Initiation Protocol",
draft-ietf-sip-location-conveyance-08 (work in progress),
July 2007.
[I-D.ietf-sip-outbound]
Jennings, C. and R. Mahy, "Managing Client Initiated
Connections in the Session Initiation Protocol (SIP)",
draft-ietf-sip-outbound-10 (work in progress), July 2007.
[I-D.ietf-sip-sips]
Audet, F., "The use of the SIPS URI Scheme in the Session
Initiation Protocol (SIP)", draft-ietf-sip-sips-07 (work
in progress), November 2007.
[I-D.ietf-sipping-config-framework]
Channabasappa, S., "A Framework for Session Initiation
Protocol User Agent Profile Delivery",
draft-ietf-sipping-config-framework-14 (work in progress),
November 2007.
[I-D.thomson-geopriv-lis-discovery]
Thomson, M. and J. Winterbottom, "Discovering the Local
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Location Information Server (LIS)",
draft-thomson-geopriv-lis-discovery-03 (work in progress),
September 2007.
[LLDP] IEEE, "IEEE802.1ab Station and Media Access Control",
Dec 2004.
[LLDP-MED]
TIA, "ANSI/TIA-1057 Link Layer Discovery Protocol - Media
Endpoint Discovery".
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2396] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396,
August 1998.
[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[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.
[RFC3263] Rosenberg, J. and H. Schulzrinne, "Session Initiation
Protocol (SIP): Locating SIP Servers", RFC 3263,
June 2002.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific
Event Notification", RFC 3265, June 2002.
[RFC3311] Rosenberg, J., "The Session Initiation Protocol (SIP)
UPDATE Method", RFC 3311, October 2002.
[RFC3325] Jennings, C., Peterson, J., and M. Watson, "Private
Extensions to the Session Initiation Protocol (SIP) for
Asserted Identity within Trusted Networks", RFC 3325,
November 2002.
[RFC3428] Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C.,
and D. Gurle, "Session Initiation Protocol (SIP) Extension
for Instant Messaging", RFC 3428, December 2002.
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[RFC3515] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65, RFC 3551,
July 2003.
[RFC3693] Cuellar, J., Morris, J., Mulligan, D., Peterson, J., and
J. Polk, "Geopriv Requirements", RFC 3693, February 2004.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC3825] Polk, J., Schnizlein, J., and M. Linsner, "Dynamic Host
Configuration Protocol Option for Coordinate-based
Location Configuration Information", RFC 3825, July 2004.
[RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
"Indicating User Agent Capabilities in the Session
Initiation Protocol (SIP)", RFC 3840, August 2004.
[RFC3841] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
Preferences for the Session Initiation Protocol (SIP)",
RFC 3841, August 2004.
[RFC3856] Rosenberg, J., "A Presence Event Package for the Session
Initiation Protocol (SIP)", RFC 3856, August 2004.
[RFC3920] Saint-Andre, P., Ed., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 3920, October 2004.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[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.
[RFC4190] Carlberg, K., Brown, I., and C. Beard, "Framework for
Supporting Emergency Telecommunications Service (ETS) in
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IP Telephony", RFC 4190, November 2005.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006.
[RFC4507] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 4507, May 2006.
[RFC4676] Schulzrinne, H., "Dynamic Host Configuration Protocol
(DHCPv4 and DHCPv6) Option for Civic Addresses
Configuration Information", RFC 4676, October 2006.
[RFC4967] Rosen, B., "Dial String Parameter for the Session
Initiation Protocol Uniform Resource Identifier",
RFC 4967, July 2007.
18.2. Informative References
[I-D.ietf-sipping-service-examples]
Johnston, A., "Session Initiation Protocol Service
Examples", draft-ietf-sipping-service-examples-13 (work in
progress), July 2007.
[RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers",
RFC 3966, December 2004.
[WGS84] NIMA, "NIMA Technical Report TR8350.2, Department of
Defense World Geodetic System 1984, Its Definition and
Relationships With Local Geodetic Systems, Third Edition",
July 1997.
Authors' Addresses
Brian Rosen
NeuStar, Inc.
470 Conrad Dr
Mars, PA 16046
US
Email: br@brianrosen.net
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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
James Polk
Cisco Systems
3913 Treemont Circle
Colleyville, Texas 76034
US
Phone: +1-817-271-3552
Email: jmpolk@cisco.com
Andrew Newton
TranTech/MediaSolv
4900 Seminary Road
Alexandria, VA 22311
US
Phone: +1 703 845 0656
Email: andy@hxr.us
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