[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits] [IPR]
Versions: (draft-rosen-ecrit-framework) 00 01
02 03 04 05 06 07 08 09 10 11 12 13
RFC 6443
ecrit B. Rosen
Internet-Draft NeuStar
Intended status: Standards Track H. Schulzrinne
Expires: January 11, 2009 Columbia U.
J. Polk
Cisco Systems
A. Newton
TranTech/MediaSolv
July 10, 2008
Framework for Emergency Calling using Internet Multimedia
draft-ietf-ecrit-framework-06
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on January 11, 2009.
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
citizens and visitors to authorities.
Rosen, et al. Expires January 11, 2009 [Page 1]
Internet-Draft Emergency Call Framework July 2008
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 . . . . . . . . . . 12
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 . 16
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 . . . . . . . . . . . . . . . . 21
6.8. Location updates . . . . . . . . . . . . . . . . . . . . . 22
6.9. Multiple locations . . . . . . . . . . . . . . . . . . . . 23
6.10. Location validation . . . . . . . . . . . . . . . . . . . 23
6.11. Default location . . . . . . . . . . . . . . . . . . . . . 24
6.12. Location format conversion . . . . . . . . . . . . . . . . 24
7. LIS and LoST Discovery . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . 28
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. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
18. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 31
19. Informative References . . . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 35
Intellectual Property and Copyright Statements . . . . . . . . . . 37
Rosen, et al. Expires January 11, 2009 [Page 2]
Internet-Draft Emergency Call Framework July 2008
1. Terminology
This document uses terms from [RFC3261] and [RFC5012]. In addition
the following terms are used:
Access network: The access network supplies IP packet service to an
endpoint. Examples of access networks include digital subscriber
lines (DSL), cable modems, IEEE 802.11, WiMaX, enterprise local
area networks, or cellular data networks.
(Emergency) Call taker: An emergency call taker answers an emergency
call at the PSAP.
Confidence: Confidence is an estimate indicating how sure the
measuring system is that the actual location of the person is
within the bounds defined by the uncertainty value, expressed as a
percentage. For example, a value of 90% indicates that the actual
location is within the uncertainty nine times out of ten.
Dispatch Location: The dispatch location is the location used for
dispatching responders to the person in need of assistance. The
dispatch location must be sufficiently precise to easily locate
the callee; it typically needs to be more accurate than the
routing location.
Emergency services routing proxy (ESRP): An emergency services
routing proxy provides routing services for a group of PSAPs.
Location configuration: During location configuration, an endpoint
learns its physical location.
Location conveyance: Location conveyance delivers location
information to another element.
Location determination: Location determination finds where an
endpoint is physically located. For example, the endpoint may
contain a GPS receiver used to measure its own location or the
location may be determined by a network administrator using a
wiremap database.
Location Information Server (LIS): A Location Information Server
stores location information for retrieval by an authorized entity.
Mobile device: A mobile device is a user agent that may change its
physical location and possibly its network attachment point during
an emergency call.
NENA (National Emergency Number Association): The National Emergency
Number Association is an organization of professionals to "foster
the technological advancement, availability and implementation of
a universal emergency telephone number system." It develops
emergency calling specifications and procedures.
Nomadic device (user): A nomadic user agent 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.
Rosen, et al. Expires January 11, 2009 [Page 3]
Internet-Draft Emergency Call Framework July 2008
Physical Location: A physical location describes where a person or
device is located in physical space, described by a coordinate
system. It is distinguished from the network location, described
by a network address.
Routing Location: The routing location of a device is used for
routing an emergency call and may not be as precise as the
Dispatch Location.
Stationary device: An stationary device is not mobile and is
connected to the network at a fixed, long-term-stable physical
location. Examples include home PCs or pay phones.
Uncertainty: Uncertainty is an estimate, expressed in a unit of
length, indicating the diameter of a circle that contains the
device with the probability indicated by the confidence value.
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. This document
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 a 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 to use them to place
emergency calls. However, many of the technical advantages of
Internet multimedia require re-thinking of the traditional emergency
Rosen, et al. Expires January 11, 2009 [Page 4]
Internet-Draft Emergency Call Framework July 2008
calling architecture. This challenge also offers an opportunity to
improve the operation of emergency calling technology, while
potentially lowering its cost and complexity.
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 the potential mobility of all end systems, including endpoints
nominally thought of as fixed systems and not just those using
radio access technology. For example, consider 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. 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.
Rosen, et al. Expires January 11, 2009 [Page 5]
Internet-Draft Emergency Call Framework July 2008
The IETF has historically refused to create national variants of its
standards. Thus, this document attempts to take into account best
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 Media Gateway Controller
protocols such as MGCP or H.248/Megaco. The latter protocols can 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].
Since this document is a framework document it does not include
normative behavior. 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 (UAs) 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
Rosen, et al. Expires January 11, 2009 [Page 6]
Internet-Draft Emergency Call Framework July 2008
characterize an emergency call:
o The call is routed to the most appropriate PSAP, based primarily
on the location of the caller.
o The PSAP must be able to automatically obtain the location of the
caller with sufficient accuracy 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 disrupted.
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 [RFC5031] 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 6)
prior to making emergency calls. To get this location, either a form
of measuring, for example, GPS (Section 6.2.3) 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], which maps a location to a set of PSAP URIs.
Each URI resolves to a PSAP or an Emergency Services Routing Proxy
(ESRP) that serves as an incoming proxy for a 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 in civic [RFC4776] or
geo [RFC3825] forms, 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 Some time later, the user places an emergency call. The phone
recognizes an emergency call from the dial strings and uses
"urn:service:sos" [RFC5031] 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, which commonly involves an
Rosen, et al. Expires January 11, 2009 [Page 7]
Internet-Draft Emergency Call Framework July 2008
outgoing proxy.
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
the call to the PSAP.
o The call is established with the PSAP and mutually agreed upon
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
Rosen, et al. Expires January 11, 2009 [Page 8]
Internet-Draft Emergency Call Framework July 2008
The typical message flow for this example using Alice as the caller:
[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 or otherwise initiates an 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
Figure 1 shows emergency call component topology and the text above
shows call establishment. These include the following components:
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 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.
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.
Rosen, et al. Expires January 11, 2009 [Page 9]
Internet-Draft Emergency Call Framework July 2008
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 [RFC3986] "cid"
URL indicating where in the message body the location-by-value is.
The INVITE message is routed to the ESRP [M7], which 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 the 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 request 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
Current PSAPs support voice calls and real-time text calls placed
through PSTN facilities or systems connected to the PSTN. Future
Rosen, et al. Expires January 11, 2009 [Page 10]
Internet-Draft Emergency Call Framework July 2008
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.
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 [RFC5031]. For a single number environment the urn is
"urn:service:sos". Users are not expected to "dial" an emergency
URN. Rather, appropriate emergency dial strings are translated to
corresponding service URNs, carried in the Request-URI of the INVITE
request. Such translation is best done by the endpoint, among other
reasons, because emergency calls convey location in the signaling,
but non emergency calls do not normally do that. If the device
recognizes 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 dial strings should be used. Many users would
prefer that their home dialing sequences work no matter where they
Rosen, et al. Expires January 11, 2009 [Page 11]
Internet-Draft Emergency Call Framework July 2008
are. However, local laws and regulations may require the visited
dialing sequence(s) work. Therefore, the visited dial string must
work. Devices must allow at least the configuration of the home
country, from which a home dial string can be determined.
The mechanism for obtaining the dialing sequences for a given
location is provided by LoST [I-D.ietf-ecrit-lost]. Lost servers
must return dial strings for emergency services. If the endpoint
does not support the translation of dial strings to telephone
numbers, the dialing sequence is represented as a dial string
[RFC4967] and the outgoing proxy has to recognize the dial string and
translate to the service URN. To determine the local emergency 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. If a service provider is unable to guarantee that it can
correctly determine local emergency dialstrings, wherever its
subscribers may be, 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. Speed dial mechanisms
may effectively create single button emergency call invocation and
should not be permitted.
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 caller reporting 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
Rosen, et al. Expires January 11, 2009 [Page 12]
Internet-Draft Emergency Call Framework July 2008
on actual 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, such as time of day, caller media requests and
language preference, call load, 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, which must be very precise.
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 call 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 "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.
Rosen, et al. Expires January 11, 2009 [Page 13]
Internet-Draft Emergency Call Framework July 2008
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.
This document provides guidance for generic network configurations
with respect to location. It is recognized that unique issues may
exist in some network deployments. The IETF will continue to
investigate these unique situations and provide further guidance, if
warranted, in future documents.
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 fine grained
location information such as floor, room and cubicle. Civic
information comes in two forms:
Jurisdictional refers to a civic location using actual political
subdivisions, especially for the community name.
Postal 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 (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 an 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
also an element for a postal code. 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]
datum.
Rosen, et al. Expires January 11, 2009 [Page 14]
Internet-Draft Emergency Call Framework July 2008
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, both civic and geospatial forms are 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. Where PSAPs 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's, and the database used is not exactly the same
one the PSAP uses, the double conversion has a high probability of
introducing an error.
6.2. Location determination
As noted above, location information can be entered by the user or
installer of a device ("manual configuration"), measured by the end
system, can be delivered to the end system by some protocol or
measured by a third party and inserted into the call signaling.
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 such as indoor
high rises in dense urban areas. 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 to use when multiple conflicting
Rosen, et al. Expires January 11, 2009 [Page 15]
Internet-Draft Emergency Call Framework July 2008
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 forget 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.
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 [RFC5139] 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. This value is derived from fire brigade recommendations of
Rosen, et al. Expires January 11, 2009 [Page 16]
Internet-Draft Emergency Call Framework July 2008
spacing of alarm pull stations. However, 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 provided.
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 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
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 enabled and thus location will always be
available accurately immediately (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. In such
circumstances, 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 have end-system measured location mechanisms but
need a cold start period lasting more than a couple seconds need
another way to get a routing location. Typically this would be a
location associated with a radio link (cell site/sector).
Rosen, et al. Expires January 11, 2009 [Page 17]
Internet-Draft Emergency Call Framework July 2008
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:
* Time Difference Of Arrival - TDOA
* Uplink Time Difference Of Arrival - U-TDOA
* Angle of Arrival - AOA
* RF fingerprinting
* Advanced Forward Link Trilateration - AFLT
* 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. Where devices do not support
location, proxy servers may have to add location to emergency calls.
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 with a LIS to determine the
location of the device. Systems designers are discouraged from
relying on proxies to add location. The technique may be useful in
some limited circumstances as devices are upgraded 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 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.
Rosen, et al. Expires January 11, 2009 [Page 18]
Internet-Draft Emergency Call Framework July 2008
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 (i.e. as 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 including DHCP, HELD and LLDP:
DHCP can deliver civic [RFC4776] 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 [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 carried in HTTP.
Link-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 mandatory-to-implement 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 would be
Rosen, et al. Expires January 11, 2009 [Page 19]
Internet-Draft Emergency Call Framework July 2008
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 expires. 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 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 or 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 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.
Rosen, et al. Expires January 11, 2009 [Page 20]
Internet-Draft Emergency Call Framework July 2008
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 for HELD.
In some cases, such as residential NAT devices, the NAT is placed
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
so that 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 1,000,000 / (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
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. Three
seconds, the current nominal time for a quick fix, is a very long
time to wait for help. Systems designers should attempt to provide
accurate routing location in much less time then that.
NENA recommends [NENAi3TRD]IP based systems complete calls in two
seconds (i.e., 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 call signaling. This is referred to as "conveyance" to
Rosen, et al. Expires January 11, 2009 [Page 21]
Internet-Draft Emergency Call Framework July 2008
distinguish it from "configuration". In SIP, the location
information is conveyed following the procedures in
[I-D.ietf-sip-location-conveyance]. Since 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 from the LCP form to PIDF may be required.
6.8. Location updates
As discussed above, it may 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 some mobile callers, 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.
A PSAP has no way to request an update of a location-by-value. If
the UAC gets new location, it must signal the PSAP using a reINVITE
or UPDATE method with a new Geolocation header to supply the new
location.
With the wide variation in determination mechanisms, the PSAP doesn't
know when accurate location may be available to it. Therefore, the
preferred mechanism is that the LIS notifies the PSAP when accurate
location is updated rather than requiring a poll operation from the
PSAP to the LIS. The SIP Presence subscription [RFC3856] provides a
suitable mechanism.
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. If
the LIS is notifying the PSAP with a SUBSCRIBE/NOTIFY mechanism, the
PSAP could update the parameters in a filter on the subscription.
(immediate response required).
When using a HELD dereference, the PSAP must specify the value
"emergencyDispatch" for the ResponseTime parameter. The LIS should
be aware of the needs of the PSAP as they are local to one another.
Rosen, et al. Expires January 11, 2009 [Page 22]
Internet-Draft Emergency Call Framework July 2008
6.9. Multiple locations
Getting multiple locations all purported to describe the location of
the caller is confusing to all, and should be avoided. Handling
multiple locations at the point where a PIDF is created 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. When they occur
anyway, the general guidance is that the entity earliest in the chain
generally has more knowledge than later elements to make an
intelligent decision, especially about which location will be used
for routing. It is permissible to send multiple locations towards
the PSAP, but the element that chooses the route must select one (and
only one) location to use with LoST.
Guidelines for dealing with multiple locations are also given in
[I-D.ietf-ecrit-lost]. If a UA gets multiple locations, it must
choose the one to use for routing, but it may send all of the
locations it has in the signaling. If a proxy is inserting location
and has multiple locations, it must choose the one to use to route
and send any others it has.
The UA or proxy should have the ability to understand how and from
whom it learned its location, and should include this information in
the location objects that are sent to the PSAP. That labeling
provides the call-taker with many pieces of information to make
decisions upon, as well as guidance for what to ask the caller and
what to tell the responders.
The call must 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 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
Rosen, et al. Expires January 11, 2009 [Page 23]
Internet-Draft Emergency Call Framework July 2008
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"
geo is one where there is a mapping.
LoST [I-D.ietf-ecrit-lost] includes a location validation function.
Validation is normally 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. When the Test functions (Section 15) are
invoked, the location used should be re-validated.
When validation fails, the location given must not be used for an
emergency call. If validation is complete when location is first
loaded into a LIS, any problems can be found and fixed before devices
could get the bad location. Failure of validation arises because an
error is made in determining the location, although occasionally the
LoST database is not up to date or has faulty information. In either
case, the problem must be identified and corrected before using the
location.
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 anything more
precise than the CMTS. Default locations must be marked as such so
that the PSAP knows that the location is not accurate.
6.12. Location format conversion
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.
7. LIS and LoST Discovery
Endpoints must be able to discover a LIS (if the HELD protocol is
used), and a LoST server. DHCP options are defined for this purpose
Rosen, et al. Expires January 11, 2009 [Page 24]
Internet-Draft Emergency Call Framework July 2008
[I-D.ietf-geopriv-lis-discovery] and
[I-D.ietf-ecrit-dhc-lost-discovery]
Until such DHCP records are widely available, it may be necessary for
the service provider to provision a LoST server address in the
device. The endpoint can also do an SRV query within its SIP domain
to find a LoST server. In any environment, more than one of these
mechanisms may yield a LoST server, and they may be differernt. The
recommended priority is DHCP first, provisioned value second, and SRV
query in the SIP domain third.
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 specific emergency services such as 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 [RFC5031]. Even in single
number jurisdictions, not all services are dispatched by PSAPs and
may need alternate URNs to route calls to the appropriate call
center.
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. ESRPs are
expected to be able to provide routing based on media. 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
Rosen, et al. Expires January 11, 2009 [Page 25]
Internet-Draft Emergency Call Framework July 2008
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 its 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. 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 may need the outbound proxy do the mapping. If the
endpoint recognized the call was an emergency call, provided the
correct service URN and/or included location on the call in a
Geolocation header, a proxy server could easily accomplish the
mapping.
However, if the endpoint did not recognize the call was an emergency
call, and thus did not include location, the proxy's task is more
difficult. It is often difficult for the calling network to
accurately determine the endpoint's location by itself. The endpoint
may have its location, but would not normally include it on the call
signaling unless it knew it was an emergency call. 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 attempt to redirect a call to the
service URN which, if the device recognizes the significance, would
include location in the redirected call from the device. All
networks should detect emergency calls and supply default location
and/or routing if it is not already performed.
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
Rosen, et al. Expires January 11, 2009 [Page 26]
Internet-Draft Emergency Call Framework July 2008
to route on media choice, the initial INVITE request has to supply an
SDP offer.
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 with a ciphersuite which
includes strong encryption (e.g., AES). 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. Never the less, protection
from eavesdropping of location by encryption should be provided.
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 receives a request over a TLS connection should attempt
to create a TLS connection to the next hop.
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 [RFC5077] is recommended. IPSEC [RFC4301] is an
acceptable alternative to TLS when used with an equivalent crypto
suite.
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.
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
Rosen, et al. Expires January 11, 2009 [Page 27]
Internet-Draft Emergency Call Framework July 2008
request 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
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 the
Contact: header 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.
11. Mid-call behavior
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. Therefore a
PSAP may need to REFER [RFC3515] a call to a bridge for conferencing.
Relay services for communication with people with disabilities may be
included in the call in this way.
The UA should also be prepared to have the call transferred (usually
attended, but possibly blind) as per
[I-D.ietf-sipping-service-examples].
SIP Caller Preferences [RFC3841] can be used to signal how the PSAP
should handle the call. For example, a language preference expressed
Rosen, et al. Expires January 11, 2009 [Page 28]
Internet-Draft Emergency Call Framework July 2008
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.
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 terminates a call using the 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 such as call forwarding can interfere with calls
from a PSAP and should be disabled. A UA can determine a PSAP call
back by examining the domain of incoming calls after placing an
emergency call and comparing that to the domain of the answering PSAP
from the 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 "pager-mode" MESSAGE [RFC3428] as well
as Message Session Relay Protocol [RFC4975]. Video may be important
to support Video Relay Service (Sign language interpretation) as well
Rosen, et al. Expires January 11, 2009 [Page 29]
Internet-Draft Emergency Call Framework July 2008
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
to support SRTP can be made yet 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 within some random interval
after 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
[RFC5069], and [I-D.barnes-geopriv-lo-sec].
17. IANA Considerations
This document has no actions for IANA.
Rosen, et al. Expires January 11, 2009 [Page 30]
Internet-Draft Emergency Call Framework July 2008
18. 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
comments and input were provided by Richard Barnes, Barbara Stark and
James Winterbottom.
19. Informative References
[I-D.barnes-geopriv-lo-sec]
Barnes, R., Lepinski, M., Tschofenig, H., and H.
Schulzrinne, "Security Requirements for the Geopriv
Location System", draft-barnes-geopriv-lo-sec-02 (work in
progress), February 2008.
[I-D.ietf-ecrit-dhc-lost-discovery]
Schulzrinne, H., Polk, J., and H. Tschofenig, "A Dynamic
Host Configuration Protocol (DHCP) based Location-to-
Service Translation Protocol (LoST) Discovery Procedure",
draft-ietf-ecrit-dhc-lost-discovery-03 (work in progress),
May 2008.
[I-D.ietf-ecrit-lost]
Hardie, T., Newton, A., Schulzrinne, H., and H.
Tschofenig, "LoST: A Location-to-Service Translation
Protocol", draft-ietf-ecrit-lost-10 (work in progress),
May 2008.
[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-04 (work in progress),
February 2008.
[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-08 (work in
progress), July 2008.
[I-D.ietf-geopriv-lis-discovery]
Thomson, M. and J. Winterbottom, "Discovering the Local
Rosen, et al. Expires January 11, 2009 [Page 31]
Internet-Draft Emergency Call Framework July 2008
Location Information Server (LIS)",
draft-ietf-geopriv-lis-discovery-01 (work in progress),
June 2008.
[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-11
(work in progress), February 2008.
[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-10 (work in progress),
February 2008.
[I-D.ietf-sip-outbound]
Jennings, C. and R. Mahy, "Managing Client Initiated
Connections in the Session Initiation Protocol (SIP)",
draft-ietf-sip-outbound-15 (work in progress), June 2008.
[I-D.ietf-sip-sips]
Audet, F., "The use of the SIPS URI Scheme in the Session
Initiation Protocol (SIP)", draft-ietf-sip-sips-08 (work
in progress), February 2008.
[I-D.ietf-sipping-service-examples]
Johnston, A., Sparks, R., Cunningham, C., Donovan, S., and
K. Summers, "Session Initiation Protocol Service
Examples", draft-ietf-sipping-service-examples-14 (work in
progress), February 2008.
[LLDP] IEEE, "IEEE802.1ab Station and Media Access Control",
Dec 2004.
[LLDP-MED]
TIA, "ANSI/TIA-1057 Link Layer Discovery Protocol - Media
Endpoint Discovery".
[NENAi3TRD]
NENA, "08-751 NENA i3 Technical Requirements for", 2006.
Rosen, et al. Expires January 11, 2009 [Page 32]
Internet-Draft Emergency Call Framework July 2008
[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.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 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.
[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.
[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.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Rosen, et al. Expires January 11, 2009 [Page 33]
Internet-Draft Emergency Call Framework July 2008
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 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
IP Telephony", RFC 4190, November 2005.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474, August 2006.
[RFC4776] Schulzrinne, H., "Dynamic Host Configuration Protocol
(DHCPv4 and DHCPv6) Option for Civic Addresses
Configuration Information", RFC 4776, November 2006.
[RFC4967] Rosen, B., "Dial String Parameter for the Session
Initiation Protocol Uniform Resource Identifier",
RFC 4967, July 2007.
[RFC4975] Campbell, B., Mahy, R., and C. Jennings, "The Message
Session Relay Protocol (MSRP)", RFC 4975, September 2007.
[RFC5012] Schulzrinne, H. and R. Marshall, "Requirements for
Emergency Context Resolution with Internet Technologies",
RFC 5012, January 2008.
[RFC5031] Schulzrinne, H., "A Uniform Resource Name (URN) for
Emergency and Other Well-Known Services", RFC 5031,
January 2008.
[RFC5069] Taylor, T., Tschofenig, H., Schulzrinne, H., and M.
Shanmugam, "Security Threats and Requirements for
Emergency Call Marking and Mapping", RFC 5069,
January 2008.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, January 2008.
Rosen, et al. Expires January 11, 2009 [Page 34]
Internet-Draft Emergency Call Framework July 2008
[RFC5139] Thomson, M. and J. Winterbottom, "Revised Civic Location
Format for Presence Information Data Format Location
Object (PIDF-LO)", RFC 5139, February 2008.
[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
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
Rosen, et al. Expires January 11, 2009 [Page 35]
Internet-Draft Emergency Call Framework July 2008
Andrew Newton
TranTech/MediaSolv
4900 Seminary Road
Alexandria, VA 22311
US
Phone: +1 703 845 0656
Email: andy@hxr.us
Rosen, et al. Expires January 11, 2009 [Page 36]
Internet-Draft Emergency Call Framework July 2008
Full Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Rosen, et al. Expires January 11, 2009 [Page 37]
Html markup produced by rfcmarkup 1.129b, available from
https://tools.ietf.org/tools/rfcmarkup/