ecrit B. Rosen
Intended status: Standards Track H. Schulzrinne
January 9, March 22, 2008 Columbia U.
September 19, 2007
Framework for Emergency Calling using Internet Multimedia
Status of this Memo
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Copyright (C) The IETF Trust (2007).
Summoning emergency help by the public is a core feature
The IETF has several efforts targeted at standardizing various
aspects of telephone
networks. placing emergency calls. This document describes how various IETF protocols and
of those component parts are combined used to place support emergency calls. This includes how
these calls are routed to the correct Public Safety Answering Point
(PSAP) based on the physical location of the caller, while providing
the call taker the necessary information to dispatch a first
responder to that location from
citizens and visitors to call back the caller if necessary.
It describes at a high level how the pieces (recognizing a call as an
emergency call, marking it as such, determining the location of the
caller, routing the call based on location) go together, and
references the Internet standards that define the details of these
Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 4
3. Overview of How Emergency Calls how emergency calls are Placed placed . . . . . . . . . . 7
4. Which devices and services should support emergency calls . . 11
5. Identifying an Emergency Call emergency call . . . . . . . . . . . . . . . . 12
6. Location and Its Role its role in an Emergency Call emergency call . . . . . . . . . . 12
5.1. Introduction 13
6.1. Types of location information . . . . . . . . . . . . . . 14
6.2. Location Determination . . . . . . . . . 13
5.2. Types of Location Information . . . . . . . . . 16
6.2.1. User-entered location information . . . . . 13
5.3. Location Determination . . . . . 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 . . . . . 14
5.3.1. User-Entered Location Information . . . 18
6.3. Who adds location, endpoint or proxy . . . . . . . 15
5.3.2. Access Network "Wire Database" Location Information . 15
5.3.3. End-System Measured Location Information . . . 18
6.4. Location and references to location . . . . 16
5.3.4. Third-party Measured Location Information . . . . . . 16
5.4. Location and References to Location . 19
6.5. End system location configuration . . . . . . . . . . 17
5.5. End System Location Configuration . . 19
6.6. When location should be configured . . . . . . . . . . 17
5.6. Conveyance of Location . . 21
6.7. Conveying location in SIP . . . . . . . . . . . . . . . . 19
6.8. Location Updates updates . . . . . . . . . . . . . . . . . . . . . 19
5.8. Location Validation 22
6.9. Multiple locations . . . . . . . . . . . . . . . . . . . 20
5.9. Default . 23
6.10. Location validation . . . . . . . . . . . . . . . . . . . . . 21
5.10. Uninitialized Devices and Location . . . . . . . . . . . . 21
6. Routing the Call to the PSAP . . . . . . . . . . . . . . . . . 21
7. Signaling of Emergency Calls . . . . . . . . . . . . . . . . . 23
8. Caller Preferences . . 23
6.11. Default location . . . . . . . . . . . . . . . . . . . . 23
9. Including a Valid Call-Back Identifier . . . . . . . . . . . . 23
10. Mid-Call Services and Behavior . . 24
6.12. Other location considerations . . . . . . . . . . . . . . 24
11. Call Termination . . .
7. Uninitialized devices . . . . . . . . . . . . . . . . . . . . 24
12. Media . . . . .
8. Routing the call to the PSAP . . . . . . . . . . . . . . . . . 25
9. Signaling of emergency calls . . . . . . 25
13. Testing . . . . . . . . . . . 26
9.1. Use of TLS . . . . . . . . . . . . . . . . 25
14. Example Call Flows . . . . . . . . 26
9.2. SIP signaling requirements for User Agents . . . . . . . 27
9.3. SIP signaling requirements for proxy servers . . . . . . . 25
15. Alternatives Considered 27
10. Call backs . . . . . . . . . . . . . . . . . . . 25
15.1. tel URIs . . . . . . . 27
11. Mid-call behavior . . . . . . . . . . . . . . . . . . 26
16. Security Considerations . . . . 28
12. Call termination . . . . . . . . . . . . . . . 26
16.1. Caller Authentication . . . . . . . . 28
13. Disabling of features . . . . . . . . . . 27
16.2. Location Privacy . . . . . . . . . . 28
14. Media . . . . . . . . . . . 27
16.3. PSAP Impersonation . . . . . . . . . . . . . . . . . 28
15. Testing . . . 28
16.4. Preventing Call Misdirection . . . . . . . . . . . . . . . 28
16.5. Call Signaling Integrity . . . . . . . . . 29
16. Security Considerations . . . . . . . . 28
16.6. Media Integrity and Confidentiality . . . . . . . . . . . 28 29
17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 29 30
18. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29 30
18.1. Normative References . . . . . . . . . . . . . . . . . . . 29 30
18.2. Informative References . . . . . . . . . . . . . . . . . . 32 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 32 34
Intellectual Property and Copyright Statements . . . . . . . . . . 34 36
As a framework document, we do not define any new protocols or
articulate new behaviors. Thus we do not use RFC2119 [RFC2119]
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
(Emergency) call taker: see [I-D.ietf-ecrit-requirements]
ESRP (emergency service routing proxy): see
Access Network: 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.
Location Configuration: network
(Emergency) Call taker: The process by which person who answers an endpoint learns its
Location Conveyance: emergency call at
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%
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
Location conveyance: The process of sending location to another
Location Determination: 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. location or location may be determined by
administration using a wiremap database or similar
Location Information Server: Server (LIS): An element that stores location
information for retrieval by an authorized entity
Location Validation: see [I-D.ietf-ecrit-requirements]
Mapping: see [I-D.ietf-ecrit-requirements]
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.
PSAP (public safety answering point): see
SIP B2BUA: see [RFC3261]
SIP proxy: see [RFC3261]
SIP Server: see [RFC3261]
SIP UA (user agent): see [RFC3261]
Stationary device (user): 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 payphone. 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.
Mobile device (user): User agent that changes geographic another
Routing Location: The location
and possibly its network attachment point during of an endpoint that is used for
routing an emergency call.
Summoning police, May not be as precise as the fire department Dispatch
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 an ambulance a pay phone
Uncertainty The mathematically derived statistical estimate,
expressed in emergencies
is one of meters, indicating the fundamental and most-valued functions size of the telephone.
As area used in the
calculation of Confidence.
Requesting help in an emergency using a communications device such as
a telephone functionality moves from circuit-switched telephony to or mobile is an accepted practice in most of the world.
As communications devices increasingly utilize the Internet telephony, its to
interconnect and communicate, users rightfully expect that this core
functionality will continue to work at least as well 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 it has defined in Emergency
Telecommunications Service (ETS) in IP Telephony [RFC4190]. They
also can be distinguished from emergency warning systems that are
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 older technology. 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 which that
could be used to make a request for help which 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.
It is beyond the scope of this document to enumerate and discuss all
the differences between traditional (PSTN) (Public Switched Telephone
Network) and IP based telephony, but calling on the Internet is
o the interleaving of signaling and media data packets;
o the interleaving over the same infrastructure of a wider variety
o the separation of the access provider from the application
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; 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.
We distinguish an individual request for help, usually accomplished
by dialing a short digit sequence like 9-1-1 or 1-1-2, from a call
placed by specially designated persons who There are many legacy telephone networks that
will persist long after most systems have authority been upgraded to claim
priority on available Internet communications facilities. This
document only discusses a request for help by an ordinary user
answered at an IP
origination and termination of emergency call center (i.e. a PSAP). 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
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
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, through its related specifications 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. 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 interdomain
inter-domain VoIP calls would be placed. Other signaling protocols
may also use protocol translation to communicate with a SIP-enabled
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]. To ensure that,
[I-D.ietf-ecrit-phonebcp] recommends certain minimal capabilities in
that call taker user agents and PSAP-operated proxies should possess.
Supporting emergency calling does not require any new specialized SIP
header fields, request methods, status codes, message bodies, or
packages. 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
impared. 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, response 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:
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
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 how emergency calls are Placed
We distinguish (Section 4) an placed
An emergency call can be distinguished (Section 5) from any other
call by a unique Service URN[I-D.ietf-ecrit-service-urn], which URN [I-D.ietf-ecrit-service-urn], that is
placed in the initial call set-up signaling when a home or visited emergency
dial string is detected. We route Because emergency calls based on the services are local to
specific geographic regions, a caller must obtain his location (
(Section 5)) of the caller. Section 6) prior to making emergency calls. To get this location we
location, either include a form of measuring (e.g. (e.g., GPS) ( (Section 5.3.3)) 6.2.3)
device location in the endpoint, endpoint is deployed, or the endpoint is
configured ( (Section 5.5)) 6.5) with its location from the access network's
Location Information Server (LIS) (LIS). The location is conveyed (
(Section 5.6)) 6.7) in the SIP signaling with the call. We route( (Section 6)) the The call is routed
(Section 8) based on location using the LoST protocol ( [I-D.ietf-ecrit-lost])
[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 which
(ESRP) that serves 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 [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
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
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 in
the emergency services network. That proxy would route to the
o The call is established with the PSAP and common media streams are
o The location of the caller is displayed to the call taker.
. . . . . . . . . . . . . . . . .
. +--------+ +----------+ .
. +--------+ | +----------+ | .
. | LIS | | | SIP | | .
. | |-+ | Registrar|-+ .
. +--------+ +----------+ .
. ^ ^ .
. . | . . . . . . . | . . . . . .
| | +--------+
|+--------------+ +--------+ |
|| | LoST | |
|||  [M3][M5] +--------+ +-------+
||| | PSAP2 |
|||  +-------+  +------+  [M6] +-------+  [M7]+------+ [M8]+-------+
Alice ------>| Proxy |---->| ESRP |---->| PSAP1 |-----> Call-Taker
+-------+ +------+ +-------+
| PSAP3 |
Figure 1: Generic ECRIT Emergency Call Component Topology
Figure 2 shows a generic emergency call establishment. This includes
o Alice - who will make the emergency call.
Alice Servers - Servers providing Alice's UA its IP
address and other configuration information, perhaps including
Location by-value or by-reference. In this flow, we use DHCP as
an example location configuration protocol. ESRP Server PSAP
[M1] LCP Request(s) (ask for location)
LCP Reply(s) (replies with location)
[M2] SIP REGISTER
SIP 200 OK (REGISTER)
[M3] Initial LoST Protocol Query (contains location)
Initial LoST Protocol Response (contains PSAP-URI and dial
*** Some time later, Alice dials/initiates emergency call ***
[M4] LCP Request (update location)
LCP Reply (replies with location)
[M5] Update LoST Protocol Query (contains location)
LoST Protocol Response (contains PSAP-URI)
[M6/7] INVITE (service URN, Location & PSAP URI)
[M8] INVITE (urm:service:sos, Location & PSAP-URI)
Emergency Session Established
Figure 2: General Flow of an Emergency Call Establishment
Figure 1 shows emergency call component topology and Figure 2 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 server, registrar for Alice's UA to
register Alice's UA to register with. UA. Most SIP
UAs will register
with a call server, 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 messages than 4 3 messages to complete.
o ESRP - The Emergency Services Routing Proxy Server 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 caller) to choose the actual PSAP which that handles the
call. In some jurisdictions, that this may involve another LoST query query.
o LoST Server 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 PSAP - Call center where emergency calls are destined for in times
of emergencies. for.
Generally, Alice's UA either has location configured manually, has an
integral location measurement mechanism, or it runs a location
configuration protocol LCP [M1] to
obtain location from the access (broadband) network. For most
devices, a location configuration protocol 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 Location-to-PSAP
SIP(S)-URI query [M3] to learn a URI, URI for use if the Lost Query LoST query
fails during an emergency call call, or to use it to test the emergency call
mechanism. The LoST query response may contain the dial string for
emergency calls appropriate for the location provided.
At some time after her device has hopefully passed since Alice's UA booted. In this
example, she dials or booted, Alice initiates an
emergency call. This She may have
been through her keypad with her locally known do this by dialing an emergency dial string.
It is important that this dial string be recognized by
valid for her UA
wherever Alice is because she may be in enough distress she forgets
what the traveled-to emergency dial string is; as there are more than
60 around the world. current ("local") location, or for her "home" location.
The UA recognizes the dial string, which means this is an emergency
call. string. The UA attempts to refresh its location,
location [M4], and with that location, to refresh the LoST mapping, 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 fails,
or takes too long) long, the UA uses it's cached values. values it has cached.
The UA creates an a SIP INVITE which [M6] request that includes the location.
[I-D.ietf-sip-location-conveyance] defines a SIP Location Geolocation header
that contains either contain the a location-by-reference URI, URI or a [RFC2396] "cid:" "cid"
URL indicating where in the message body the location-by-value is.
The INVITE message routes is routed to the ESRP, which ESRP [M7], that is the first
inbound proxy for the emergency services domain. This message, message is
then routed by the ESRP towards the most current appropriate PSAP for Alice's
location, which uses
location [M8], as determined by PSAP state, location and other state information
to choose this PSAP.
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
her Alice's emergency.
Alice Servers ESRP Server PSAP
[M1] DHCP Request(s) (may ask for Location)
DHCP Reply(s) (replies with location if asked)
[M2] SIP REGISTER
SIP 200 OK (REGISTER)
[M3] Initial LoST Protocol Query (contains Location)
Initial LoST Protocol Response (contains PSAP-URI)
***Some time later, Alice dials/initiates
4. Which devices and services should support emergency call***
[M4] DHCP Request(s) (update Location)
DHCP Reply(s) (replies with location)
[M5] Update LoST Protocol Query (contains Location)
LoST Protocol Response (contains PSAP-URI)
[M6/7] INVITE (sos URN, Location & early PSAP URI)
[M8] INVITE (sos, Location & PSAP-URI)
Emergency Session Established
Figure 2: General Flow of an Emergency Call Establishment
Support for voice calls and real-time text calls placed through PSTN
facilities or systems connected to the PSTN is found in present
PSAPs. Future PSAPs will however support Internet connectivity and a very rough example of the operation
wider range of an emergency call
establishment. There are no layer 3 routers in 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 message flow, device, then the device or service should support
emergency calling. Certainly, any device or service that looks like
whatever security messages exist in 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
Using current (evolving) standards, devices that create media
sessions and exchange audio, video and/or text, and have the call are not shown either.
Each of those aspects will be addressed individually,
capability to keep each
discussion in context establish sessions to a wide variety of that subject, for clarity.
4. addresses, and
communicate over private IP networks or the Internet, should support
5. Identifying an Emergency Call 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
infrastructure the telephone is connected to. However, this number
differs between localities, even though it is often the same for a
country or region, such 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 responder, type of responder needed, e.g., one for
police, another for fire. It is deemed impractical to change the dialed
digits to summon help. 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, we define common emergency call URIs which 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, the current appropriate emergency dial string should be strings
is translated to the appropriate corresponding service URN. URNs, carried in the Request-
URI of the INVITE. Such translation could ideally be
performed in 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
the emergency call, it can include location. Dial string recognition
could be performed in a signaling intermediary (proxy server). 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 preferences of the
emergency response professionals are such that
regulations may require the visited dialing
sequences must sequence(s) always work.
Having the home dial string work is optional.
The best answer seems to be for both to work.
The mechanism for obtaining the dialing sequences for a given
location is provided by LoST [I-D.ietf-ecrit-lost]. Where If the endpoint
does not support the translation of dial strings to telephone
numbers, the dialing sequence would be represented as a dial string [I-D.rosen-iptel-dialstring]
[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.
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.
6. Location and Its Role its role in an Emergency Call
Caller location plays a emergency call
Location is central role to the operation of emergency services. It is
frequently the case that the user in routing an emergency calls. 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 (overload area, with overload arrangements between PSAPs to
handle each others calls notwithstanding). 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, not "closest" the closest PSAP, or "best fit" algorithms.
5.2. Types of Location Information
There are four primary types the PSAP that serves
the nominal city name provided in the location may not be the correct
Accuracy of routing location information: civic, postal,
geospatial, and cellular cell tower is a complex subject. Calls must be
routed quickly, but accurately, and sector.
Civic: Civic location information describes the location of determination is often a person
time/accuracy tradeoff, especially with mobile devices or object by self
measuring mechanisms. It is considered acceptable to base a street address that corresponds routing
decision on an accuracy equal to the area of one sector of a building or
other structure. (This mobile
cell site if no more accurate routing location is sometimes also called "civil" available.
Routing to the most appropriate PSAP is always calculated on the
information.) Civic 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 more finely grained factors other than location information such (such as floor, room
caller media and cubicle. Civic
information comes in language preference, PSAP state, etc.). However,
location of the caller is the primary input to the routing decision.
Routing is but one of two forms:
Jurisdictional - This refers 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 civic
location using actual
political subdivisions, especially accurate enough for the community name.
Postal - dispatch may take as much as 30 seconds.
This refers is too long to a civic location used wait for emergencies. Accordingly, it is common,
especially in mobile systems to mail use a letter
to. The name of coarse location, for example,
the post office sometimes does not correspond
to cell site and sector serving the actual community name call, for routing purposes, and a postal address may contain
post office boxes or street addresses that do not correspond
an actual building. Postal addresses are generally unsuitable
for emergency call routing, but may be the only address
Geospatial: Geospatial addresses contain longitude, latitude and
altitude information based on an understood datum (starting point)
and earth shape model. While there have been many datum developed
over time, most modern systems are using or moving towards update the
Cell tower/sector: Cell tower location when a more precise value is known prior
to dispatch. In this document we use "routing location" and sectors identify
"dispatch location" when the cell tower distinction matters.
Accuracy of dispatch location is sometimes determined by local
regulation, and the antenna sector is constrained by available technology. The actual
requirement exceeds available technology. It is required that the mobile a
device is currently using.
Traditionally, making an emergency call close to the tower location is expressed as "demising" or separation
wall between two apartments in a point, and
routing decisions are made high rise apartment building report
location with sufficient accuracy to determine on that point. Cell/sector information
could also be transmitted as an irregularly shaped polygon what side of
geospatial coordinates reflecting the likely geospatial location
wall it is on. This implies perhaps a 3 cm accuracy requirement. As
of the mobile device.
In IETF protocols, civic date of this memo, typical assisted GPS uncertainty with 95%
confidence is 100 m.
Location usually involves several steps to process and geo forms multiple
elements are both supported. The civic
forms include both involved. In Internet emergency calling, where the postal and jurisdictional fields. The cell
tower/sector can be represented as
endpoint is located is "Determined" using a point.
5.3. Location Determination
Location information can be entered by the user or installer variety of a
device ("manual configuration"), can measurement or
wire-tracing methods. Endpoints may be measured "Configured" with their own
location by the end system,
can be delivered to the end system by access network. In some protocol or can be
measured by circumstances, a third party and inserted proxy
server may insert location into the call signaling. We
discuss these in detail below.
In some cases, an entity may have multiple sources signaling on behalf of location
information, possibly partially contradictory. This is particularly
likely if the
endpoint. The location information is determined both by "Mapped" to the end
system URI to send the call to,
and a third party. Handling multiple locations is discussed
in [I-D.ietf-geopriv-pdif-lo-profile]. Conflicting the location
information is particularly harmful if it points "Conveyed" to multiple distinct
PSAPs. Guidelines for dealing with multiple locations is also given the PSAP (and other elements) in [I-D.ietf-ecrit-lost].
All location objects MUST be delivered to
the PSAP. signaling. Likewise, we employ Location
information should contain information about 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 source of data, such
as GPS, manually entered or based on access network topology. In
addition, Location Mapping Protocol defined by the source
6.1. Types of the location information should
There are several ways location can be included
(PIDF "provided-by"). The ability of the UA to understand how it
learned its location, and include this specified:
Civic Civic location information element in describes the location of a person
or object by a street address that is sent corresponds to the PSAP, provides the call-taker
with many pieces of a building or
other structure. Civic location may include more finely grained
location information to make decisions upon, such as floor, room and guidance
for what cubicle. Civic
information comes in two forms:
Jurisdictional This refers to ask a civic location using actual
political subdivisions, especially for the caller and what community name.
Postal This refers to tell the responders.
The call should indicate which a civic location information has been used for
routing, so that mail delivery. The
name of the same location information is used 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 all
routing decisions. Otherwise, two proxies might pick different
location information from dispatch because the call request, resulting in different
routing decisions post office conventions
(for community name, for different transactions. example) do not match those known by
the responders. The location
conveyance mechanism [I-D.ietf-sip-location-conveyance] contains a
parameter which fact that they are unique can sometimes be used for this purpose
End systems and network elements can derive location information from
a variety of sources. It is not the goal of this document
exhaustively enumerate them, but we provide a few common examples in
the sections below.
5.3.1. 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 mapping between a postal address and a database.
Location information added by end users is almost always inferior to
measured or wire database information, as users may mistype
location information, may not know the meaning of geospatial
coordinates or may use address information that does not correspond suitable to dispatch a recognized civic address. A user-entered responder to. In IETF
location protocols, there is a element (Postal Community Name)
that can fail to be changed when the location of a device changes during or after
movement. For example, included in a user could move their residence to another
dwelling, not update their device/equipment with this new location,
and place an emergency call with old location information.
All that said, there are always a small number of cases where the
mechanisms used by the access network to determine location fail to
accurately reflect the actual location of the endpoint. For example, provide the user may deploy his own WAN behind an access network, effectively
remoting an endpoint some distance from post office
name as well as the access network's notion
of its location. actual jurisdictional community name.
There must be some mechanism provided to provision
a location is no other accommodation for postal addresses in these
Geospatial (geo): Geospatial addresses contain longitude, latitude
and altitude information based on an endpoint by the user understood datum and earth
shape model. While there have been many datums developed over
time, most modern systems are using or by moving towards the access network on
Cell tower/sector: Cell tower/sector is often used for identifying
the location of a user. The use mobile handset, especially for routing of
emergency calls. Cell tower and sectors identify the mechanism introduces cell tower
possibility of users falsely declaring themselves to be somewhere
they are not. As an aside, normally, if an emergency caller insists
he is at antenna sector that a location different from what any automatic location
determination system reports he is, responders will always be sent to mobile device is currently using.
Traditionally, the user's self-declared location. However this tower location is represented as a matter of local
policy point chosen
to be within a certain PSAP service boundary who agrees to take
calls originating from that tower/sector, and is outside the scope of this document.
5.3.2. Access Network "Wire Database" Location Information
Location routing decisions
are made on that point. Cell/sector information can could also 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 layouts at known
locations. In DSL installations, the local telephone carrier
maintains a mapping
represented as an irregularly shaped polygon of wire-pairs to subscriber addresses.
Even for IEEE 802.11 wireless access points, wire databases may
provide sufficient location resolution; geospatial
coordinates reflecting the likely geospatial location of the access
point may be sufficient location information for each of
mobile device. Whatever representation is used must route
correctly in the clients
served LoST database, where "correct" is determined by that access point. However, this may not be true for
larger scale systems such as IEEE 802.16
local PSAP management.
In IETF protocols, civic and IEEE 802.22 which
typically have larger cells than those of IEEE 802.11. A Wire
database may be the source of location information for geospatial forms are both
residential users of DSL supported.
The civic forms include both postal and Cable Modem installations, as well as
the only infrastructure at a WiFi hotspot, such jurisdictional fields. A
cell tower/sector can be represented as a coffee shop.
Each point (geo or civic) or
polygon. Other forms of these cases will have location representation must be mapped into
either a known geo or civic address for use in emergency calls.
For emergency call purposes, conversion of the dwelling/
business, likely providing sufficient location resolution. However,
the information from
civic location of an IEEE 802.16 base station may be of little
use to emergency personnel
Wire databases to the home are likely geo or vice versa prior to conveyance is not desirable. The
location should be sent in the most promising
solution for residential users where a service provider knows the
customer's service address. The service provider can then perform
address verification, similar 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 current system in some
5.3.3. End-System Measured 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 Information
Global Positioning System (GPS) sensors may Determination
Location information can be embedded directly in entered by the user or installer of a
device ("manual configuration"), measured by the end device. GPS produces relatively high precision location
fixes in open-sky conditions, but system, can be
delivered to the technology still faces several
challenges in terms end system by some protocol or measured by a third
party and inserted into the call signaling. Choice of performance (time-to-fix location
determination mechanisms and time-to-first-
fix), as well as obtaining successful their properties are out of scope for
In some cases, an entity may have multiple sources of location fixes within shielded
structures, or underneath the ground (tunnels, basements, etc.). It
also requires all devices to be equipped with
information, possibly partially contradictory. This is particularly
likely if the appropriate GPS
capability. GPS technology location information is improving (e.g. Galileo), determined both by the end
system and a third party. Although self measured location (e.g.
increasingly successful in attractive, access network provided location could be much
more difficult conditions such as dense
urban canyons accurate, and inside commercial structures. It is currently
accurate to tens of meters using more reliable in some kind of "assist", which may environments (indoor high
rise in dense urban areas for example).
6.2.1. User-entered location information
Location information can be
operated maintained by the access network (A-GPS) end user or by a government (WAAS).
Newer multi-frequency systems will improve accuracy without assist.
GPS equipped devices vary depending on which element initiates
requests, which element actually determines final location, assist
mechanisms, etc. Some common implementations include:
1. GPS S/A (standalone), device initiated
2. GPS S/A, network initiated
3. AGPS-device initiated, network determined
4. AGPS-device initiated, network augmented
5. AGPS-network initiated, network determined
6. AGPS-network initiated, network augmented
5.3.4. Third-party Measured Location Information
Wireless triangulation: Elements in the network infrastructure
triangulate end systems based on signal strength, angle
installer of arrival an endpoint in the endpoint itself, 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
5. Advanced Forward Link Trilateration - AFLT
6. Enhanced Forward Link Trilateration - EFLT
Sometimes triangulation and measured mechanisms are combined, for
example A-GPS with AFLT in a database.
Location beacons: A short range wireless beacon, e.g., using
Bluetooth information provided by end users is almost always less
reliable than measured or infrared, announces its wire database information, as users may
mistype location to mobile devices in
5.4. Location and References to Location
Location information or may be expressed as the actual enter civic or geo
value but address information
that does not correspond to a recognized (i.e. valid, see Section
Section 6.10) address. Users can be transmitted as by-value (wholly contained within the
signaling message) or by-reference (a URI pointing neglect to change the value
residing on data when the
location of a remote node waiting to be dereferenced). There are
pros and cons to each form:
pro- Value available to each device along the path immediately
for further processing.
con- Size, especially if constrained to changes during or after movement.
All that said, there are always a UDP transport. Value
fixed at the time the value is acquired from small number of cases where the access
network. Value can be changed
automated mechanisms used by the endpoint, which may be
considered untrustworthy for this critical usage.
pro- Small size. Value can be fixed at time of dereference.
Value cannot be changed by endpoint
con- URI resolution requires location source be available and
accessible by dereferencer. Dereferencing takes time.
Dereferencing may fail.
5.5. End System Location Configuration
Unless a user agent has access network to provisioned or locally measured determine location information, it must obtain it
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.
network's notion of its location. There are several Location Configuration Protocols (LCPs) that can must be
used for this purpose.
DHCP can deliver civic [RFC4676] or geospatial [RFC3825]
information. User agents would need some mechanism
provided to support both formats.
Note that provision a location for an endpoint by the user agent can or by
the access network on behalf of a user. The use DHCP, via of the DHCP REQUEST or
INFORM messages, even if it uses other means to acquire its IP
Insert reference mechanism
introduces the possibility of users falsely declaring themselves to L7 acquisition protocol document> is another
Link-Layer Discovery Protocol [LLDP]), with proposed extensions
[LLDP-MED], may also
be used to deliver location information.
SUPL OMA <insert reference> somewhere they are not. As an aside, normally, if an emergency
caller insists that he is yet another choice.
Other LCPs may at a location different from what any
automatic location determination system reports he is, responders
will always be devised by other standards bodies. Each LCP has
limitations in sent to the kinds of networks that can reasonably support it.
For user's self-declared location. However
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), unless every matter of local policy and is outside the scope of this
6.2.2. Access network
supported every protocol, "wire database" location information
Location information can be maintained by the access network,
relating some form of identifier for the end subscriber or alternatively, every device supported
every protocol, serious incompatibilities would ensue.
[I-D.ietf-ecrit-phonebcp] contains 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 (short) list mapping of protocols such
devices must support.
Where an access network can control the specification
wire-pairs to subscriber addresses.
Accuracy of EVERY
endpoint that could make an emergency call that is directly connected location historically has been to the network, or indirectly connected (for example, a device on street address level.
However, this is not sufficient for larger structures. The PIDF-LO
[RFC4119] with a
LAN behind recent extension [I-D.ietf-geopriv-revised-civic-lo]
permits interior building/floor/room and even finer specification of
location within a network attachment unit), it may specify any protocol it
wishes street address. When possible, interior location
should be supported.
The threshold for each endpoint. This when interior location is a very unusual case; nearly every
access network can 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 used considered. The
ultimate goal is to support an Ethernet based LAN behind it
For example, existing mobile networks are being used be able to support
routers and LANs behind a find the person in need quickly if
responders arrive at the location given.
Even for IEEE 802.11 wireless data network WAN connection, with
Ethernet connected phones connected to that. It is possible that access points, wire databases may
provide sufficient location resolution. The location of the access network supports a protocol not on
point as determined by the phonebcp list, and
every handset supported in that network could use that protocol wiremap may be supplied as the location
emergency calls. However, unless another element which each of the access
network provider controls clients of the specification 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 can acquire IEEE 802.11.
The civic location
using that protocol and then that element can support one of the
phonebcp's list an IEEE 802.16 base station may be of protocols, little
use to emergency personnel, since the Ethernet connected phone won't endpoint could be
kilometers away from the base station.
Wire databases to acquire location. In this case, if the access network
provider supplies a router which includes home are likely to be the most promising
solution for residential users where a DHCP server, it can
acquire location using service provider knows the access network specific protocol, and
customer's service address. The service provider can then
address validation (see Section 6.10), similar to the current system
in some jurisdictions.
6.2.3. End-system measured location information to supply it to its clients
Global Positioning System (GPS) and similar satellite based (e.g. the
Ethernet connected phone) via DHCP.
For most networks, it will not
Galileo) receivers may be practical to control embedded directly in the
specification end device. GPS
produces relatively high precision location fixes in open-sky
conditions, but the technology still faces several challenges in
terms of every device, performance (time-to-fix and time-to-first-fix), as well as
obtaining successful location fixes within shielded structures, or arrange interworking with network
specific LCPs. For this reason, most
underground. It also requires all devices will need to support
ALL of be equipped with the LCPs in [I-D.ietf-ecrit-lost], and access networks will
appropriate GPS capability. GPS-derived locations are currently
accurate to support at least one tens of these LCPs.
Location for non-mobile meters. Many mobile devices is normally expected to require using some
kind of "assist", that may be acquired
at operated by the access network attachment time and retained (A-GPS)
or by a government (WAAS).
GPS systems may be always on; where location will always be available
accurately (assuming the device. It should device can "see" enough satellites). Mobile
devices may not be
refreshed when able to sustain the cached value becomes invalid (for example, if DHCP
is power levels required to keep
the acquisition protocol, refresh of location may occur measuring system active. This means that when the
IP address lease location is renewed). At
needed, the time 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, the call. For this reason, devices that don't have
end-system measured location should mechanisms always on need another way to
get a routing location. Typically this would be refreshed, a location
associated with the retained a radio link (cell site/sector).
6.2.4. Network measured location used if 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
5. Advanced Forward Link Trilateration - AFLT
6. Enhanced Forward Link Trilateration - EFLT
Sometimes multiple mechanisms are combined, for example A-GPS with
Location beacons: A short range wireless beacon, e.g., using
Bluetooth or infrared, announces its location acquisition does not immediately return a value. Mobile to mobile devices may determine in
the vicinity. This allows devices to get location from the beacon
6.3. Who adds location, endpoint or proxy
The IETF emergency call architecture prefers endpoints to learn their
location at network attachment time and
periodically thereafter as a backup in case supply it on the call. Outbound proxies that support
devices that do not support location may have to add location determination to
emergency calls at a proxy server. Some calling networks have
relationships with all access networks the time of call does not work. Mobile device location may be
refreshed when a TTL expires, connected
to, and that may allow the device moves beyond some boundaries
(as provided by [I-D.ietf-ecrit-lost]), etc. Normally, mobile
devices will acquire its proxy to accurately determine location at call time for use in an
emergency call routing, but see Section 5.7
5.6. Conveyance of Location
When an emergency call is placed,
the endpoint (normally) puts
location information in endpoint. However NATs and other middleboxes often make it
impossible to determine a reference identifier the signaling with access network
could use to determine the call. We refer location. Systems designers are
discouraged from relying on proxies to
that add location. The technique
may be useful in some limited circumstances as "conveyance" devices are upgraded
to distinguish it from "configuration".
Configuration gets location from meet the requirements of this document, or where relationships
between access network networks and calling networks are feasible and can be
relied upon to endpoint,
conveyance sends get accurate location.
Proxy insertion of location from endpoint to elements that route the
call based complicates dial string recognition. As
noted in Section Section 6, local dial strings depend on that the location object and
of the PSAP. Using SIP, caller. If the
location information is conveyed following device does not know its own location, it
cannot use the procedures in
[I-D.ietf-sip-location-conveyance]. LoST service to learn the local emergency dial
strings. The form of calling network must provide another way for the device
to learn the local dial string (and update it when the user moves to
information obtained by where the acquisition protocol dial string(s) change) or do the dial string
6.4. Location and references to location
Location information may not be expressed as the same actual civic or
geospatial value but can be transmitted as by value (wholly contained
within the conveyance protocol uses (PIDF-LO [RFC4119]). Mapping signaling message) or by reference (a URI pointing to the
value residing on a remote node waiting to be required. Calling networks which support devices
which do not support location may have dereferenced). Each
form is better suited to add some applications than others.
When location to emergency
calls. Some calling networks have relationships with is transmitted by value, the access
network that may allow it location information is
available to accurately determine each device; on the other hand, location objects can be
large, and only represent a single snapshot of the
endpoint, although NATs device's location.
Location references are small and other middleboxes usually make it
impossible can be used to determine represent a reference identifier the access network
could use to determine time-
varying location, but the location.
For emergency call purposes, conversion added complexity of the dereference step
introduces a risk that location information from
civic will not be available to geo or vice versa prior parties that
6.5. End system location configuration
Unless a user agent has access to conveyance is not desirable. The provisioned or locally measured
location should be sent in the form information, it was determined. The PSAP may
convert, if must obtain it needs to, and if conversion resulted from an earlier
conversion, unacceptable errors may be introduced.
5.7. Location Updates
Location information may not the access network.
There are several location configuration protocols (LCPs) that can be available at call setup time
mobile devices. For example, if a GPS-enabled cell phone is turned
on and then immediately places an emergency call, it 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 take
significant additional time before use DHCP, via the cell phone acquires a GPS fix
and its location. Thus, while it is desirous to base emergency
routing on precise caller location information, DHCP REQUEST or INFORM
messages, even if it is not possible in
all circumstances uses other means to do so. In some cases, the initial call setup
will proceed based on, for example, cell and sector information and
then add location information during 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 call, rather than delaying IP address of the initial call setup by requestor as an unacceptable amount of time.
identifier and returns the location of a mobile caller, e.g., in a vehicle value or
aircraft, reference associated
with that identifier. HELD is typically transported on HTTP.
Link-Layer Discovery Protocol Layer Discovery Protocol [LLDP] with
Media Endpoint Device extensions [LLDP-MED] can change significantly during the emergency call. The
PSAP must be able used to get updated deliver
location information while it is
processing directly from the call.
Location updates where Layer 2 network
infrastructure, and also supports both civic and geospatial
formats identical in format to DHCP methods.
Each LCP has limitations in the location kinds of networks that can reasonably
support it. For this reason, it is conveyed by value may be
conveyed either in not possible to choose a single
mandatory-to-deploy LCP. For endpoints with common network
connections (such as an Ethernet jack or a re-INVITE WiFi connection) serious
incompatibilities would ensue unless every network supported every
protocol, or UPDATE [RFC3311] request message
(where UPDATE alternatively, every device supported every protocol.
For this reason, a list of LCPs is preferred) or the PSAP may subscribe established in
[I-D.ietf-ecrit-phonebcp]. Every endpoint that could be used to the location
place emergency calls must implement all of the caller, using SIP presence mechanisms RFC 3856
[RFC3856]). Authorization for subscriptions protocols on the
list. Every access network must deploy at least one of them. It is for future study.
recognized that this is conveyed by reference, additional dereference
operations yield updated location.
5.8. Location Validation
In some jurisdictions, location must an onerous requirement, that it would be validated prior
desirable to a device
placing an actual emergency call, and eliminate. However, since it is always a recommended
practice. Validation in this context means both the variability of the
networks that there is prevent a
mapping single protocol from being acceptable, it
must be the address endpoints that implement all of them, and to accommodate
a PSAP wide range of devices, networks must deploy at least one of them.
Often, network operators and device designers believe that the PSAP understands how
to direct responders they have
a simpler environment and some other network specific mechanism can
be used to the provide location. This is not as easy as Unfortunately, it
sounds. There are, for example, many cases of two names for the same
street, or two streets with is very rare to
actually be able to limit the same name in range of devices that may be connected
to a city. In some
countries, the current system provides validation. network.
For example, in 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 United States,
access network could support a protocol not on the Master Street Address Guide (MSAG) records all
valid street addresses list, and is used require
every handset in that network to ensure use that protocol for emergency
calls. However, the service
addresses in Ethernet connected phone billing records correspond won't be able to valid emergency
service street addresses. Validation
acquire location, and the user of the phone is normally a concern for civic
addresses, although there could unlikely to be a concern
dissuaded from placing an emergency call on that a given geo is
within at least one PSAP service boundary; phone. The
widespread availability of gateways, routers and other network-
broadening devices means that is, a "valid" geo is
one indirectly connected endpoints are
possible on nearly every network. Network operators and vendors are
cautioned that shortcuts to meeting this requirement are seldom
Location for which there non-mobile devices is a mapping.
The LoST resolver[I-D.ietf-ecrit-lost] includes a validation
function. Validation normally expected to be acquired
at network attachment time and retained by the device. It should ideally be performed
refreshed when a the cached value becomes invalid. For example, if
DHCP is the acquisition protocol, refresh of location may occur when
the IP address lease is
entered into 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 Location Information Server (which is normally value. Mobile
devices may determine location at network attachment time and
periodically thereafter as a
provisioning mechanism backup in case location determination at
the access carrier's operation and support
system). It 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.
6.6. When location should be confirmed periodically, because the mapping
database undergoes slow change; configured
Devices should get routing location immediately after obtaining local
network configuration information. The presence of NAT and VPN
tunnels (that assign new streets are added or removed,
community names change, postal codes change, etc. Endpoints may wish IP addresses to validate locations they receive from 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, network demarcation point and will
need to validate manually entered locations. Proxies which insert
location may wish to validate locations they receive from a LIS.
Test functions (Section 13) should also re-validate.
5.9. Default Location
Occasionally, a failure may occur where thus the IP address
seen by the access network cannot
determine is the actual right identifier for location of
the caller. residence. In these 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, it must
supply a default 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. The default
It is desirable that routing location should information be as
accurate as the network can determine. For example, in a cable
refreshed. A LIS supporting a default location for million subscribers each Cable Modem Termination System
(CMTS), with refreshing
once per day would need to support a representative location query rate of 1,000,00 / (24 *
60 * 60) = 12 queries per second.
It is desirable for all cable modems served by
that CMTS could routing location information to be provided requested
immediately before placing an emergency call. However, if the network there is unable to resolve the
any unit less than significant delay in getting more recent location, the CMTS. Default locations must call
marked as such (how?) so that the PSAP knows that placed with the most recent location information the device
has. In mobile handsets, routing is not
5.10. Uninitialized Devices often accomplished with the cell
site and Location
Support sector of devices that are not registered, the tower serving the call, because it can take
many seconds to start up the location determination mechanism and don't have valid
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
back identifiers quickly is complex. In some jurisdictions, required. However,
if location can be substantially improved by waiting a short time
(e.g. for some
services, support sort of emergency calls from so called "uninitialized"
devices, "quick fix"), it's preferable to wait. 3
seconds, that is the current nominal time for example, a mobile phone which does not have 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 active
service contract emergency call is placed, the endpoint should put location in
the United States signaling with the call. That is required referred to support calls as "conveyance" to
distinguish it from "configuration". In SIP, the location
information is attractive for such devices to 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 able the same
as the conveyance protocol uses (PIDF-LO [RFC4119]). Mapping by the
endpoint to PIDF may be used in an
emergency. However, the requirement required.
6.8. Location updates
As discussed above, it make take some time for some measurement
mechanisms to do so has caused get a huge
number of prank calls location accurate enough for dispatch, and a
routing location with less accuracy may be provided to get 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
established early. The PSAP needs the phone works. An unitialized device that can place
an emergency call must supply dispatch location before it
sends the same as a fully enabled
6. Routing the Call call to the PSAP
Emergency calls are routed based on one or more responder. This requires an update of the following
criteria expressed in
In addition, the call setup request (INVITE):
Location: Since each PSAP serves location of a limited geographic region and
transferring existing calls delays mobile caller, e.g., in a vehicle or
aircraft, can change significantly during the emergency response, calls
need to be routed to the call. While
most appropriate PSAP. In often this
architecture, emergency call setup requests contain change is not significant, the PSAP must be able to
get updated location
information, expressed in civic or geospatial coordinates, information while it is processing the call.
Subscription is preferred so that
allows such routing. If there 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 or imprecise (e.g., cell
tower and sector) information at call setup time, an on-going
emergency call may also be transferred way to another PSAP based on
location information that becomes available in mid-call.
Type request an update of emergency service: In some jurisdictions, emergency calls
for fire, police, ambulance a location-by-value. If
the UAC gets new location, it must reINVITE or mountain rescue are directed UPDATE to
just those emergency-specific PSAPs. We support this mechanism by
optionally labeling calls with a service identifier
Media capabilities of caller: In some cases, emergency call centers supply the
Generally, the PSAP can wait for specific caller media preferences, such as typed text or
video, are separate from voice systems. Also, even if media
capability does not affect an accurate location for dispatch.
However, there is no fixed limit known in advance; it depends on the selection
nature of the PSAP, there may be
call takers within emergency. At some point the PSAP that are specifically trained, e.g., must dispatch. In a
subscription environment, the PSAP could update the parameters in interactive text or sign language communications. Again, we
use the caller capabilities [RFC3840] mechanism
filter (immediate response required). In a HELD dereference, there
is no way to label cancel and route
Routing the PSAP will have to choose a ResponseTime
that it will wait for calls by 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 and by service information
is the primary function
LoST [I-D.ietf-ecrit-lost] provides. particularly harmful if different routes (PSAPs) result from LoST accepts a query
queries for the multiple locations. Guidelines for dealing with
multiple locations are also given in either civic or geo form, plus a service
identifier, and returns an xml data structure containing [I-D.ietf-ecrit-lost].
Generally, if a URI (or
set of URIs) to route the call to. Normal SIP [RFC3261] routing
functions are used to resolve UA gets multiple locations, it must choose the URI one to
use. If a next hop destination.
The endpoint can complete the LoST mapping from its proxy is inserting location at boot
time, and periodically thereafter. It should attempt has multiple locations, it
must choose the one to obtain a
"fresh" location, use.
The ability of the UA or proxy to understand how and from that a current mapping when whom it places an
emergency call, and if accessing either
learned its location acquisition
function or mapping function fails, it should use location, and include this cached value.
The call would follow its normal outbound call processing. Networks
that support devices that do not implement LoST mapping themselves
would have information element in the outbound proxy do
location object that is sent to the mapping. The proxy must have PSAP, provides the location call-taker
with many pieces of the endpoint, which is often difficult information to make decisions upon, and guidance
for what to ask the
calling network caller and what to accurately determine. The endpoint may have its
location, but would not normally include it on tell the responders.
The call signaling.
There should indicate the location information that has been used
for routing, so that the same location information is no used for all
call routing decisions. The location conveyance mechanism provided in
to allow contains a proxy parameter for this
6.10. Location validation
It is recommended that location must be validated prior to a device
placing an actual emergency call; some jurisdictions require the endpoint supply location, because that would open
this be done. Validation in this context means both that there is a
mapping from the endpoint address to an attack by any proxy on a PSAP and that the path PSAP understands how
get it direct responders to reveal the location. The Proxy CAN redirect a call to Determining the
service URN which, if addresses that
are valid can be difficult. There are, for example, many cases of
two names for the device recognized same street, or two streets with the significance, would
include location same name in a
city. In some countries, 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, current system provides validation.
For example, in the proxy routes United States, the call. Normal SIP[RFC3261] mechanisms are Master Street Address Guide
(MSAG) records all valid street addresses and is used to route calls to the URI obtained from the LoST query.
Often, ensure that
the SIP routing of an emergency call will first route to an
incoming call proxy service addresses in the domain operated by the phone billing records correspond to valid
That proxy service street addresses. Validation is called an "Emergency Services Routing Proxy" (ESRP).
The ESRP, which 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 normal SIP proxy server, 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 use a variety of
PSAP state information, the location of wish to validate locations they receive
from the caller, access network, and other
criteria will need to onward route the call validate manually entered
locations. Proxies that insert location may wish to the PSAP.
7. Signaling of Emergency Calls
As discussed above, validate
locations they receive from a LIS. Test functions (Section 15)
should also re-validate.
6.11. Default location is carried in all emergency calls in
call signaling. Since emergency calls carry privacy-sensitive
information, they are subject to access network cannot determine the requirements for geospatial
protocols [RFC3693]. actual location
of the caller. In particular, signaling information these cases, it must supply a default location.
The default location should be
carried in TLS, i.e., in 'sips' mode. While requiring TLS is
actually the way as accurate as the standards are written, it is unacceptable to
have an emergency call fail to complete because network can
determine. For example, in a TLS connection was
not created, cable network, a default location for any reason. In many cases, persistent TLS
each Cable Modem Termination System (CMTS), with a representative
location for all cable modems served by that CMTS could be maintained between elements to minimize provided
if the time
needed network is unable to establish them.
The use of SIP Identity [RFC4474] resolve the subscriber to protect any unit less
than the headers of CMTS. Default locations must be marked as such so that the
message could improve end-to-end integrity
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 information.
Details LCP did not directly
return a PIDF.
To prevent against spoofing of how the DHCP server, devices implementing
DHCP for location is carried in call signaling can be found in
8. Caller Preferences
SIP Caller Preferences [RFC3841] configuration should use DHCP security mechanisms
Location may be used to signal how the PSAP
should handle for routing by multiple proxy servers on the call. For example, a language preference expressed
path. Mechanism such as S/MIME in an Accept-Language header may SIP signaling [RFC3261] cannot be
used because they obscure location. Only hop-by-hop mechanisms such
as a hint to cause the PSAP to
route the call to a call taker who speaks the requested language.
9. Including a Valid Call-Back Identifier
The call-taker TLS should be used. Location information is sensitive and must be able to reach the emergency caller if the
protected [RFC3693]. Although support of TLS is mandatory in
[RFC3261], many devices do not support it. Implementing location
conveyance in SIP mandates inclusion of TLS support.
7. Uninitialized devices
Support of devices that are not registered, or that don't have valid
call back identifiers is disconnected. complex. In traditional emergency calls,
wireline and wireless some jurisdictions, for some
services, support of emergency calls include a callback identifier
for this purpose. In SIP systems, from so-called "uninitialized"
devices is required. For example, cellular providers in the caller should include a
Contact header field indicating its device URI, if available, or
possibly a GRUU[I-D.ietf-sip-gruu] if United
States must support calls need to be routed via 9-1-1 from a
proxy. This identifier would mobile phone that does not
have an active service contract. It is attractive for such devices
to be used able to initiate call-backs
immediately by the call-taker if, for example, the call is
In addition, a call-back identifier should be included either as the
URI used in an emergency. However, the From header field [RFC3261] preferably verified by SIP
Identity[RFC4474]. This identifier would be used requirement to initiate a call-
do so has caused a later time and may reach the caller, not necessarily on huge number of prank calls to the
service. In some countries, it is common to attempt to place an
emergency call from an unitialized device (and at the same location) as in the original emergency
call. Both local bazaars to
prove to a would-be purchaser that the Contact and From specific requirements are detailed
Emergency phone works. For this reason,
PSAP authorities generally discourage support of for unititialized devices.
devices (see Section 5.10. If an uninitialized device does that can place an emergency call, some kind of call back URI must be provided.
Finally, there may be two other call identifiers included in an
emergency call. An identifier may be included which can be used to
location the caller, same as opposed to the device or the subscriber of a
specific calling service. This identifier may fully enabled device, must carry a call back
URI that can be used to retrieve
information about the caller that is independent of calling service.
For example, Alice may have home, office and mobile telephony
services, but she is call the same Alice in all of them. Information
about Alice may be kept by an entity independent of any telephony
service provider. The caller identity is a URI device back, and is placed should have
identifiers in a
SIP Call-Info header [RFC3261] using the token "?" following the
recommendations in [I-D.ietf-ecrit-phonebcp].
The communications service provider may also include an identifier signaling that may can be used to retrieve information specific to identify the device.
8. Routing the call held by to the service provider. This identifier, also a URI may be placed in PSAP
Emergency calls are routed based on one or more of the Call-Info header using following
criteria expressed in the token "?" per
10. Mid-Call Services and Behavior
A call setup request (INVITE):
Location: Since each PSAP may need to REFER[RFC3515] serves a call limited geographic region and
transferring existing calls delays the emergency response, calls
need to a bridge for
conferencing. The caller should also be prepared routed to have the most appropriate PSAP. In this
architecture, emergency call
transferred (usually attended, but possibly blind) as
While setup requests contain location
information, expressed in a call, a number of other call features, civic or geospatial coordinates, that
allows such as call
waiting, must be disabled. This 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 discussed be transferred to another PSAP based on
location information that becomes available in
11. Call Termination
It is undesirable mid-call.
Type of emergency service: In some jurisdictions, emergency calls
for the caller fire, police, ambulance or mountain rescue are directed to terminate an
just those emergency-specific PSAPs. This mechanism is supported
by marking emergency call.
Strategies calls with the proper service identifier
Media capabilities of caller: In some cases, emergency call centers
for devices to handle specific caller attempts to terminate 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 [I-D.ietf-ecrit-phonebcp]. interactive text or sign language communications, where routing
within the PSAP call termination is
accomplished with normal SIP call termination procedures.
PSAPs should accept media streams based on RTP [RFC3550]. Traditionally,
voice has been the only media stream accepted offer would be provided.
Routing for calls by PSAPs. In some
countries, text, in location and by service is the form of BAUDOT codes primary function
LoST [I-D.ietf-ecrit-lost] provides. LoST accepts a query with
location (by-value) in either civic or similar tone encoded
signaling within geospatial form, plus a voiceband is accepted ("TTY") for persons who have
hearing disabilities. With the Internet comes
service identifier, and returns a wider array URI (or set of
potential media which a PSAP should accept. Using SIP signaling
includes the capability URIs) to negotiate media. route the
call to. Normal SIP offer/answer
[RFC3264] negotiations should be [RFC3261] routing functions are used to agree on resolve
the media streams URI to be used. PSAPs should accept real-time text [RFC4103]. All PSAPs a next hop destination.
The endpoint can complete the LoST mapping from its location at boot
time, and periodically thereafter. It 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 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 accept IM with
at least [RFC3428] as well as [RFC3920].
Since use this cached value. The call
would follow its normal outbound call processing.
Determining when the emergency calling architecture consists of 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
pieces operated by independent entities, it is important to points) polygon for
geo reported location [I-D.ietf-geopriv-pdif-lo-profile]. This can
to test whether an emergency call enclosing subset of the area when the reported point is likely to succeed without
actually occupying not
near an edge or a smaller edge section when the human resources at reported location is
near an edge. Civic location is uncommon for mobile devices, but
reporting that the same mapping is good within a PSAP. Both signaling and
media paths need to community name, or
even a street, may be tested since NATs very helpful for WiFi connected devices that
roam and firewalls may allow obtain civic location from the
session setup request to reach AP they are connected to.
Networks that support devices that do not implement LoST mapping
themselves would have the PSAP, while preventing outbound proxy do the
exchange mapping. The proxy
must have the location of media.
[I-D.ietf-ecrit-phonebcp] includes 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 description of an automated test
procedure proxy to require the
endpoint supply location, because that validates routing, signaling and media path
continuity. This test would be used at boot time, and whenever 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 changes enough that a new PSAP mapping is returned
from LoST. A manual operation for in the test redirected
call. All networks should also be possible.
14. Example Call Flows
15. Alternatives Considered
This is a non-normative appendix. During discussions of emergency
calling, a number of suggestions are commonly made. Below, we
discuss some of the reasons why these alternatives do not satisfy the
requirements of detect emergency calling.
15.1. tel URIs
Instead of providing URIs to call calls and supply default
location and/or routing proxies or end systems, it
has been suggested that end systems be configured with a "tel" URI
[RFC3966]. Such a "tel" URI would have to be routed to a
geographically appropriate telephony gateway, as if it is unlikely that
every building, enterprise not already performed.
With the URI obtained from mapping, whether by the endpoint or residence will have its own gateway.
VoIP devices can be used in networks that are completely unaware of
VoIP services, with VoIP service providers that the
proxy, the proxy routes the call. Normal SIP [RFC3261] and [RFC3263]
mechanisms are physically far
removed used to route calls to the URI obtained from the caller's network location. Thus, LoST
Often, the use SIP routing of a tel
URI simply moves the problem an emergency call will first route to an
incoming call proxy in the outbound proxy, domain operated by the emergency service.
That proxy is called an "Emergency Services Routing Proxy" (ESRP).
The ESRP, which has to is a normal SIP proxy server, may use a variety of
PSAP state information, the caller's location of the caller, and other
criteria to determine onward route the appropriate telephony gateway.
In addition, emergency telephone numbers are far from universal, with
some such numbers used for non-emergency purposes elsewhere. Thus,
an outbound proxy would have call to ascertain the location of PSAP. In order for the caller ESRP
to guess whether route on media choice, the "tel" URI identifies initial INVITE has to supply an SDP
9. Signaling of emergency call or some
Thus, "tel" URIs are not likely to be appropriate or sufficient for
9.1. Use of TLS
As discussed above, location is carried in all emergency calls and do not, by themselves, solve in the
16. Security Considerations
Connecting ANY service signaling. Since emergency calls carry privacy-sensitive
information, they are subject to the Internet creates threads requirements for geospatial
protocols [RFC3693]. In particular, signaling information should be
carried in TLS, i.e., in 'sips' mode. However, it is unacceptable to the service
which did not exist before. The
have an emergency call service is especially
critical compared to other services lately connected fail to the Internet.
It must work reliably even in case of complete because a major disaster when thousands
of citizens call TLS connection was
not created, for help simultaneously. Not only does any reason. Thus the service
need to call should be protected attempted with
TLS, but also the liberties of if the citizens who might
need to use TLS session establishment fails, the service must call should be considered.
The emergency service is an obvious target for a deliberate attack,
and specifically a denial of service attack. Mechanisms must
automatically retried without TLS. In many cases, persistent TLS
connections can be
provided maintained between elements to help minimize the emergency networks survive such attacks while
continuing to provide service time
needed to genuine callers.
Failure establish them [I-D.ietf-sip-outbound]. In other
circumstances, use of any security mechanism should normally not prevent 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 to be established. Unlike most systems, suspicious
calls (that is, those where normal security mechanisms are not
attempted or they fail to produce expected valid credentials) are
normally not dropped, but are processed route will create SIP INVITE messages with the call taker made
aware that Service
URN in the information given (location, Request URI, the LoST-determined URI for example), may not be
accurate. As the discussion PSAP in Section 5 shows, providing accurate a
Route header, and the location in the presence of a very wide variety of circumstances is
challenging. Exceptions may result 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 some of the security
mechanisms not being path of an emergency call must be able to be deployed, and yet
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 may for the caller.
10. Call backs
The call-taker must be valid.
When able to reach the emergency service is under deliberate attack, caller if the policies
original call acceptance may be changed. More stringent compliance to
security recommendations may be enforced, 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 at least
possibly a GRUU [I-D.ietf-sip-gruu] if calls with full
security mechanisms in place may need to be processed before calls without
The decision whether other security mechanisms should 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:
In addition, a call-back identifier must be tried 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
call be dropped depends caller, not necessarily on the policy of the citizen, same device (and at the policy of same
location) as the original emergency call router and the policy of the PSAP and out as per normal SIP rules.
Emergency authorities generally discourage support of the scope unitialized
devices (see Section 7. If an uninitialized device does place an
emergency call, some kind of
16.1. Caller Authentication
Fraudulent calls to PSAPs is call back URI must be provided (e.g. a significant concern. Current systems
rely on inherent security mechanisms
GRUU) in the PSTN to make sure the
identity of the owner of the telephone Contact: header. It is known. As Internet
technologies are increasingly used useful to place calls, it is becoming
easier be able to hide call the identity
device back some time later as well by including some form of URI in
a caller. Use of the SIP Identity
mechanism [RFC4474] is recommended. If SIP Identity cannot be
provided, carriers network asserted identity.
11. Mid-call behavior
A PSAP may need to REFER [RFC3515] a call to a bridge for
conferencing. The caller should make use of P-Asserted-Identity, [RFC3325]
In keeping with established customs in circuit-switched emergency
calling, authentication cannot also be made a prerequisite prepared to have the call
transferred (usually attended, but possibly blind) as per
12. Call termination
It is undesirable for routing or
accepting the caller to terminate an emergency call. However, a
PSAP call taker may be more
suspicious termination is accomplished with normal SIP call
13. Disabling of features
Certain features that can be invoked while a caller and request additional information if normal call is active
are not permitted when the call
authenticity cannot be verified.
16.2. Location Privacy
Location is sensitive information, it must an emergency call. Services such
as Call Waiting, Call Transfer, Three Way Call and Flash Hold should
Certain features can interfere with calls from a PSAP and should be protected against
disclosure to unauthorized persons. In most jurisdictions placing
disabled. The domain of a PSAP can be determined from the domain
answering an emergency call. A time limit after an emergency call implies disclosure of location to all
should be established during which any call from the entities
needing location to properly route same domain and respond
directed to the call.
Nevertheless, even supplied Contact: or AoR should be accepted as a
call-back from the PSAP.
PSAPs should always accept RTP media streams [RFC3550].
Traditionally, voice has been the only media stream accepted by
PSAPs. In some countries, text, in an emergency, callers the form of BAUDOT codes or
similar tone encoded signaling within a voiceband is accepted ("TTY")
for persons who have an expectation that
their location will not be divulged outside hearing disabilities. With the Internet comes a
wider array of potential media that implied release.
During acquisition of a PSAP should accept. Using SIP
signaling includes the location information, an eavesdropper or
impersonator may obtain location. When DHCP is used, authentication
[RFC3118] capability to negotiate media. Normal SIP
offer/answer [RFC3264] negotiations should be used to protect agree on the location option. Use of TLS
media streams to be used. PSAPs should accept real-time text
[RFC4103]. All PSAPs should accept G.711 A law (and mu Law in other LCPs 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 used. Similarly, TLS important to support Video Relay Service (Sign language
interpretation) as well as modern video phones.
Media should be used with SIP
signaling when location kept secure, preferably by use of Secure RTP
Since the emergency calling architecture consists of a number of
pieces operated by independent entities, it is conveyed. However, failure important to establish a
security association should never be used able
to drop test whether an emergency call.
Rather, the operation should be attempted without the security
16.3. PSAP Impersonation
See Section 16.4.
With LoST-based call routing (Section 6), an attacker could modify
the mapping entries for one or more locations, re-routing calls
destined for them. The security mechanisms for provisioning the data
in the LoST database must be robust.
LoST is likely to succeed without
actually occupying the human resources at a distributed database, with many replicas of authoritative
data. An attacker may impersonate a valid LoST server PSAP. Both signaling and supply
fraudulent data. An attacker
media paths need to be tested since NATs and firewalls may also perpetrate a denial of service
attack on LoST servers. These issues are addressed in
Finally, allow the URI LoST returns would normally contain a domain name.
The domain can be hijacked by several known attacks. TLS should be
session setup request to place calls, with reach the domain name verified. Using DNSSEC
[RFC4033] on PSAP, while preventing the DNS entries is recommended. As above, failure
the security mechanism must not impede the processing media.
[I-D.ietf-ecrit-phonebcp] includes a description of an emergency
call; 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 should proceed without security rather than
abandoning for the call.
16.4. Preventing Call Misdirection
We need to prevent an emergency call reaching a destination other
than a PSAP. For example, a rogue UA able test should also be possible.
The PSAP needs to intercept SIP requests
might be able to impersonate a PSAP.
In the absence control frequency and duration of a globally recognized certificate that ensures that the owner
test, and since the process could be overused, it may temporarily or
permanently suspend its operation.
There is a legitimate PSAP, we rely on concern associated with testing during a so-called
"avalanche-restart" event where, for example a large power outage
affects a chain large number of trust enforced
by the 'sips' URI schema. The 'sips' URI schema forces each SIP hop
to route the call only endpoints, that, when power is restored,
all attempt to destinations supporting TLS transport.
Each ESRP verifies that the next-hop destination chosen as described
in Section 6 corresponds reboot and, possibly, test. Devices need to the server certificate offered by that
16.5. Call Signaling Integrity
their initiation of a malicious outsider from manipulating call information in
SIP requests can be assured by using "sips" (that is, TLS, hop-by-hop
from caller boot time test to avoid the problem.
16. Security Considerations
Security considerations for emergency call taker.
16.6. Media Integrity calling have been documented in
[I-D.ietf-ecrit-security-threats], and Confidentiality
Media integrity [I-D.barnes-geopriv-lo-sec].
Ed. Note: go through that doc and confidentiality can be assured by make sure any actions needed are
captured in the use of
SRTP[RFC3711]. BCP text.
This draft was created from a
draft-schulzrinne-sipping-emergency-arch-02 together with sections
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 was provided by Richard Barnes, Barbara Stark and
18.1. Normative References
Barnes, R., "Threats to GEOPRIV Location Objects",
draft-barnes-geopriv-lo-sec-00 (work in progress),
Hardie, T., "LoST: A Location-to-Service Translation
Protocol", draft-ietf-ecrit-lost-05 draft-ietf-ecrit-lost-06 (work in progress),
Rosen, B. and J. Polk, "Best Current Practice for
Communications Services in support of Emergency Calling",
draft-ietf-ecrit-phonebcp-01 (work in progress),
Schulzrinne, H. and R. Marshall, "Requirements for
Emergency Context Resolution with Internet Technologies",
draft-ietf-ecrit-requirements-13 (work in progress),
Taylor, T., "Security Threats and Requirements for
Emergency Call Marking and Mapping",
draft-ietf-ecrit-security-threats-05 (work in progress),
Schulzrinne, H., "A Uniform Resource Name (URN) for
Emergency and Other Well-Known Services", draft-ietf-ecrit-service-urn-06
draft-ietf-ecrit-service-urn-07 (work in progress), March
Barnes, M., "HTTP Enabled Location Delivery (HELD)",
draft-ietf-geopriv-http-location-delivery-01 (work in
progress), July 2007.
Tschofenig, H., "GEOPRIV PIDF-LO Usage Clarification,
Considerations and Recommendations",
draft-ietf-geopriv-pdif-lo-profile-08 (work in progress),
Thomson, M. and J. Winterbottom, "Revised Civic Location
Format for PIDF-LO",
draft-ietf-geopriv-revised-civic-lo-05 (work in progress),
Rosenberg, J., "Obtaining and Using Globally Routable User
Agent (UA) URIs (GRUU) in the Session Initiation Protocol
(SIP)", draft-ietf-sip-gruu-14 (work in progress),
Polk, J. and B. Rosen, "Session "Location Conveyance for the
Session Initiation Protocol",
draft-ietf-sip-location-conveyance-08 (work in progress),
Jennings, C. and R. Mahy, "Managing Client Initiated
Connections in the Session Initiation Protocol
draft-ietf-sip-outbound-10 (work in progress),
February July 2007.
Petrie, D. and S. Channabasappa, "A Framework for Session
Initiation Protocol User Agent Profile Delivery",
draft-ietf-sipping-config-framework-12 (work in progress),
Rosen, B., "Dialstring parameter for the Session
Initiation Protocol Uniform Resource Identifier",
draft-rosen-iptel-dialstring-05 (work in progress),
[LLDP] IEEE, "IEEE802.1ab Station and Media Access Control",
TIA, "ANSI/TIA-1057 Link Layer Discovery Protocol - Media
[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,
[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP
Messages", RFC 3118, June 2001.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
[RFC3263] Rosenberg, J. and H. Schulzrinne, "Session Initiation
Protocol (SIP): Locating SIP Servers", RFC 3263,
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
[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,
[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,
[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
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
Johnston, A., "Session Initiation Protocol Service
Examples", draft-ietf-sipping-service-examples-12 draft-ietf-sipping-service-examples-13 (work in
progress), January 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",
470 Conrad Dr
Mars, PA 16046
Department of Computer Science
450 Computer Science Building
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Phone: +1 212 939 7042
3913 Treemont Circle
Colleyville, Texas 76034
8045 Leesburg Pike, Suite 300
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Alexandria, VA 22182 22311
Phone: +1 703 636 8052 845 0656
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