ecrit B. Rosen
Internet-Draft NeuStar
Expires: August 31, 2006 J. Polk
Cisco Systems
February 27, 2006
Best Current Practice for Communications Services in support of
Emergency Calling
draft-rosen-sos-phonebcp-00.txt
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
Requesting help in an emergency using a communications device such as
a telephone or mobile is an accepted practice in most of the world.
As communications devices increasingly utilize the Internet to
interconnect and communicate, users will continue to expect to use
such devices to request help, regardless of whether or not they
communicate using IP. The emergency response community will have to
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upgrade their facilities to support the wider range of communications
services, but cannot be expected to handle wide variation in device
and service capability. The IETF has several efforts targeted at
standardizing various aspects of placing emergency calls. This memo
describes best current practice on how devices and services should
use such standards to reliably make emergency calls
Table of Contents
1. Requirements notation . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Which devices and services should support emergency calls . . 4
4. Determining Location . . . . . . . . . . . . . . . . . . . . . 4
5. Determining an emergency call . . . . . . . . . . . . . . . . 6
6. Session Signaling . . . . . . . . . . . . . . . . . . . . . . 7
6.1. SIP signaling requirements for User Agents . . . . . . . . 8
6.2. Mapping from Location to a PSAP URI . . . . . . . . . . . 9
6.3. Routing the call . . . . . . . . . . . . . . . . . . . . . 9
6.4. Responding to PSAP signaling . . . . . . . . . . . . . . . 10
6.5. Disabling of features . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. Normative References . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 14
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1. Requirements notation
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].
2. Introduction
In this memo, an emergency call refers to a communications session
established by a user to a "Public Safety Answering Point" (PSAP)
which is a call center established by response agencies to accept
emergency calls. We differentiate such calls from other sessions
which are created by responders using public communications
infrastructure often involving some kind of priority access as
defined in Emergency Telecommunications Service (ETS) in IP Telephony
(RFC4190). While current PSAPs are limited to voice sessions, often
with the additional capability to serve hearing impaired users with
text based "TTY" devices, envisioned upgrades to PSAPs will allow
sessions with audio, video, and several kinds of text including
interactive text (RFC4103) and Instant Messages.
Making an emergency call involves the use of location information,
referring to the physical location of the caller. Location is used
within the emergency calling system to route a call to the correct
PSAP, as well as by the PSAP to choose the correct responder, and
direct them to the person in need of assistance.
The steps involved in an emergency call are (with a rough ordering of
operation)
1. User dials visited location's emergency number
2. User device identifies call as emergency call
3. User device includes location indication in the call set-up
messaging
4. emergency call set-up is routed to appropriate PSAP based on
location of the caller
5. call is established with PSAP
6. caller's location is presented to PSAP operator for dispatch
As a quick overview for a typical Ethernet connected telephone using
SIP signaling:
o the phone would get location from the DHCP server [or an L7
server] when it boots.
o It would use "urn:service:sos" as the URI of an emergency call.
o It would put its location in the SIP INVITE as a PIDF-LO in the
body of the INVITE (or a reference to location in a Location
header) and forward the call to its first hop proxy.
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o The proxy recognize the call as an emergency call.
o The proxy would determine the PSAP's URI by using the [ecrit]
mapping server from the location provided in the signaling
o The proxy would use a SIP SRV record in the domain of the
resulting PSAP URI to determine where to send the call.
The (upgraded) PSAP would answer the call as SIP, with location
included.
3. Which devices and services should support emergency calls
Although present PSAPs have only support for voice calls placed
through PSTN facilities or systems connected to the PSTN, future
PSAPs will support Internet connectivity and a wider range of media
types. In general, if a user could reasonably expect to be able to
call for help with the device, then the device or service should
support emergency calling. Certainly, any device or service that
looks like and works like a telephone (wired or mobile) should
support emergency calling, but increasingly, users have expectations
that other devices and services should work.
Using current (evolving) standards, devices that create media
sessions and exchange audio, video and/or text, and have the
capability to establish sessions to a wide variety of addresses, and
communicate over private IP networks or the Internet, should support
emergency calls.
4. Determining Location
With Internet based communications services, determining where the
caller is located is more problematic than in PSTN and mobile
systems. Existing wired phones are tethered with a wire that is
connected directly to a call control device, a circuit switch.
Cellular phones are tethered via a radio channel to a cell tower,
which connects that cell phone to a circuit switch. The primary
difficulty with IP based phones is that the connectivity, whether
wired or radio channel, is decoupled from the call control device.
The communications service may not have any relationship with the
access network carrier, and, with NAT and VPN tunnels, may have no
way to even find out who the access carrier is.
For this reason, standards have been created for endpoints (devices)
to obtain location information. The endpoint is a subscriber to both
the access network and the communications service, and thus is in a
position to obtain its location from the access network, and supply
it to the communications service.
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DHCP (RFC2131) has been enhanced to provide the location of a device.
RFC3825 describes how a geo-location (lat/lon/alt) may be obtained
and draft-ietf-geopriv-dhcp-civil-09 describes how a civic (street
address) location can be obtained via DHCP.
[Placeholder for HELD, LCP or other L7 location determination
methods]
For devices that operate on a network where the network operator
controls the specification of every device connected to that network
that could be used for emergency calls, the method by which location
is determined need not be an IETF standard, but can be any method
that achieves the desired result. Such a method MUST be specified,
and every device MUST support it.
For devices that operate in a network where the network operator
controls the specification of every device connected to that network,
but the network attachment supports upstream networks to which
communications devices are connected (such as any network that
supports Ethernet connected telephones and terminal adapters), the
method by which location is determined need not be an IETF standard,
but can be any method which achieves the desired result. However,
the network attachment must support [both] DHCP [AND L7] for upstream
communications devices to obtain location. For smaller interior
(e.g, LAN) networks, the DHCP [or L7] server should simply repeat the
location obtained from the access network. For larger networks,
other mechanisms, such as a DHCP Relay Agent (RFC3046) must be used
to provide more accurate location of endpoints.
For devices that operate on a network where the network operator does
not control the specification of every device connected to the
network, DHCP [or L7] MUST be supported on the network.
Note: Self Reported location is generally unacceptable in emergency
calls, although it is being used prior to automatic location
determination schemes being fielded. Local laws may govern what is
acceptable in any country or area.
Devices should get location immediately after obtaining local network
configuration information. It is essential for the location to be
determined BEFORE any VPN tunnels are established. It is equally
essential that this location information is *not* overwritten by any
process engaged from establishing a VPN connection. In other words,
the established VPN to Chicago from the device in Dallas should not
overwrite the location of "Dallas".
It is desirable that location information be periodically refreshed.
For devices which are not expected to roam, refreshing on the order
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of once per day is recommended. For devices which roam, refresh of
location should be more frequent, with the frequency related to the
mobility of the device. There can be instances in which a device is
aware of when it moves, for example when it changes access points.
When this type of event occurs, the device should refresh its
location.
It is desirable for location information to be requested immediately
before placing an emergency call. However, if there is any delay in
getting more recent location, the call should be placed with the most
recent location information the device has. It is recommended that
the device not wait longer than 500 ms to obtain updated location,
and systems should be designed such that the typical response is
under 100ms.
5. Determining an emergency call
An emergency call is distinguished by the device (or a downstream
element) by an "address", which in most cases for Internet connected
devices is still a dialstring, although other user interfaces may be
used.
Note: It is undesirable to have a single "button" emergency call user
interface element. These mechanisms have a very high false call
rate. PSAPs prefer devices to use the local emergency call
dialstring.
While in some countries there is a single 3 digit dialstring that is
used for all emergency calls (i.e. 911 in North America), in some
countries there are several 3 digit numbers used for different types
of calls. For example, in Switzerland, 117 is used to call police,
118 is used to call the fire brigade, and 144 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.
http://www.ietf.org/internet-drafts/
draft-schulzrinne-sipping-service-01.txt introduces a universal
emergency service urn scheme. On the wire, emergency calls should
include this type of URI (in for example, the To: field of a SIP
call). The scheme includes a single emergency URN (urn:service:sos)
and responder specific ones (urn:service:sos.police). Using the
service sos urn scheme, emergency calls can be recognized as such
throughout the Internet.
Devices MUST use the service:sos urn scheme to mark emergency calls.
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To determine which calls are emergency calls, some entity needs to
map a user entered dialstring into this URN scheme. A user may
"dial" 1-1-2, but the call would be sent to urn:service:sos. This
mapping is ideally performed at the endpoint device, but may be
performed at an intermediate entity (such as a SIP proxy server).
Note: It is strongly suggested that devices recognize the emergency
dialstring(s) and map to the universal emergency urn. If devices
cannot do "dial plan interpretation", then the first signaling aware
element (first hop proxy in SIP signaled devices) should do the
mapping. It is important to not require a large number of active
elements handle a call before it is recognized as an emergency call
In systems that support roaming, there may be a concept of "visited"
and "home" networks. Even when there is not a "visited network", the
user may be roaming (or nomadic) in a different country from their
home. This gives rise to the problem of which dialstring(s) to
recognize, the "home" or "visited"? While it is desirable that the
"home" dialstrings be recognized, it is required (by law in some
countries) that the "visited" dialstrings be recognized. Dial plan
interpretation may need to take "visited" emergency dialstrings into
account.
To give an example of this difference in dialstrings: If the device
is from North America, the home and visited emergency dialstring is
"9-1-1". If that devices roams to the UK, the home emergency
dialstring is still "9-1-1", but the visited emergency dialstring
would become "9-9-9". If the device roams to London, the home
dialstring remains the same, "9-1-1", but the visited dialstring
changes from 999 to "1-1-2".
The home emergency dialstrings may be provisioned into the device (or
other element doing dialstring to universal emergency call urn
mapping). The visited dialstring may be discovered by a lower layer
protocol that is used by the access network, such as DHCP, or with a
higher layer protocol like SIP (using a REGISTER Request) or HTTP
(using a GET Request) once the device learns its location. It could
be that the device knows more than one way to learn the visited
emergency dialstring, and using the methods in some configured order
(until an answer is received).
6. Session Signaling
SIP signaling (RFC3261) is expected be supported by upgraded PSAPs.
Gateways may be used between Internet connected devices and older
PSAPs. Some countries may support other signaling protocols into
PSAPs.
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6.1. SIP signaling requirements for User Agents
Initial signaling Method is INVITE. The Request URI must be a
service:sos URN unless the device does not do emergency dialstring
interpretation. If the device does not do emergency dialstring
interpretation, the expectation is that the request URI will be a tel
URI with the dialed digits, or a sips uri with the dialed digits and
a USER=PHONE parameter (e.g. sips:911@example.com;user=phone). The
call would normally be sent to the first hop proxy of the
communications service.
1. The To: header must be present and should be the same as the
Request URI
2. The From: header must be present and should be the AoR of the
caller
3. NOTE: this may be the contact address of this device to ensure
callback is to this specific device
4. A Via: header must be present and should include the URI of the
device
5. A Route header MAY be present if the device has performed a
fallback mapping function (see section 4)
6. Other headers MAY be included as per normal sip behavior
7. A Supported: header MUST be included with the 'location' option
tag, unless the device does not understand the concept of SIP
Location ;
8. If the device's location is by-reference, a Location: header
MUST be present containing the URI of the PIDF-LO reference for
that device;
9. If the device does not know its location, but understands the
concept of SIP Location, the Location: header MUST be present
with an unknown option tag;
10. If the device knows its location and chooses to include this by-
value in the INVITE message, the request MUST contain a PIDF-LO
message body part [RFC4119], which implies Content-Type: header
value of MIME/Multipart
11. A normal SDP offer SHOULD be included in the INVITE. The offer
SHOULD NOT include compressed audio codecs, although a wideband
codec offer may be included.
Note: Silence suppression (Voice Activity Detection methods) MUST NOT
be used on emergency calls. PSAP call takers sometimes get
information on what is happening in the background to determine how
to process the call.
It is RECOMMENDED that location by-value is used by the device to
convey location during emergency calling.
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6.2. Mapping from Location to a PSAP URI
To route an emergency call, we make use of the [ecrit] mapping
service which takes a location expressed by a PIDF-LO and returns one
or more PSAP URIs. The request includes the service urn which is
used to determine which entity should receive the call. The URI
would replace the Request URI in a SIP INVITE.
It is desirable for the endpoint to do the mapping from location to
PSAP URI. This will allow for a fallback map to be know. It is
desirable for the first active signaling element (first hop proxy for
SIP) to do an [ecrit] mapping as well, as this will be the freshest
and most up-to-date URI of the appropriate PSAP. If the device
included a PSAP URI from fallback mapping in the SIP INVITE, the
first SIP element SHOULD do its mapping and use this result to
forward the message towards the PSAP. The fallback mapping's purpose
is in case the SIP element's mapping failed, the element would have a
route already in the message, just not one that is the freshest.
At the time of a call, it may be the case that the element doing the
mapping cannot reach the mapping server, or the server could be
congested. To guard against the inability to correctly route, or to
avoid undue delay, it is desirable that an older mapping be kept
(cached). The device, if it is doing the mapping, should request a
mapping whenever it gets location and retain that mapping for use in
unusual circumstances. If the first active element maps, it has the
problem that it will not get location for the device until an
emergency call is placed. Thus the first hop proxy cannot do a
mapping in advance.
In this case, the DHCP server [or L7 server] will return the mapping
for the supplied location, which can be retained by the device. When
an emergency call is placed, it sends this cached mapping in a
(loose) Route header.
Mapping elements should start a timer when requesting a mapping (TLS
establishment if appropriate), with a time-out of 500ms. If the
timer expires before the mapping is complete, the cached mapping
should be used.
6.3. Routing the call
Normal routing mechanisms for the specified URI should be used. For
SIP signaled devices, the domain of the URI should be extracted, and
the DNS consulted for a sip (or sips) SRV. The resulting NAPTR, if
present, should be used for the FQDN of the server.
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6.4. Responding to PSAP signaling
The PSAP is expected to use normal signaling (e.g. SIP) as per IETF
standards. Devices and proxies should expect to:
1. Be REFERed to a conference bridge; PSAPs often include
dispatchers, responders or specialists on a call.
2. Be REFERed to a secondary PSAP. Some responder's dispatchers are
not located in the primary PSAP. The call may have to be
transferred to another PSAP. Most often this will be an attended
transfer, or a bridged transfer.
3. (For devices that are Mobile) SUBSCRIBE to the Presence of the
AoR (or equivalent for other signaling schemes) to get location
updates.
4. Support Session Timer (or equivalent) to guard against session
corruption
Devices MUST NOT send a BYE (or equivalent for other non-SIP
signaling). The PSAP must be the only entity that can terminate a
call. If the user "hangs up" an emergency call, the device should
ring, and when answered, reconnect the caller to the PSAP.
There can be a case where the session signaling path is lost, and the
user agent does not receive the BYE. If the call is hung up, the
session timer expires, and 5 minutes elapses from the last message
received by the device from the PSAP, the call may be declared lost.
If in the 5 minute interval an incoming call is received from the
domain of the PSAP, the device should drop the old call and alert for
the (new) incoming call.
6.5. Disabling of features
The device and/or service should disable outgoing call features such
as:
o Call Waiting
o Call Transfer
o Three Way Call
o Flash hold
o Outbound Call Blocking
The emergency dialstrings should not be permitted in Call Forward
numbers or speed dial lists.
The device and/or service should disable the following incoming call
features on calls from the PSAP:
o Call Waiting (all kinds)
o Do Not Disturb
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o Call Forward (all kinds) (if the PSAP calls back within some
(30min?) interval)
7. Security Considerations
There are no new security considerations beyond those in the
normative references. This memo does not introduce any new
protocols; it specifies use of several of them. Implementors are
admonished to ,,,
8. Normative References
[I-D.ietf-sip-location-conveyance]
Polk, J. and B. Rosen, "Session Initiation Protocol
Location Conveyance",
draft-ietf-sip-location-conveyance-01 (work in progress),
July 2005.
[I-D.ietf-sipping-toip]
Wijk, A., "Framework of requirements for real-time text
conversation using SIP", draft-ietf-sipping-toip-03 (work
in progress), September 2005.
[I-D.schulzrinne-geopriv-dhcp-civil]
Schulzrinne, H., "DHCP Option for Civil Location",
draft-schulzrinne-geopriv-dhcp-civil-01 (work in
progress), February 2003.
[I-D.schulzrinne-sipping-service]
Schulzrinne, H., "A Uniform Resource Name (URN) for
Services", draft-schulzrinne-sipping-service-01 (work in
progress), October 2005.
[I-D.sinnreich-sipdev-req]
Sinnreich, H., "SIP Telephony Device Requirements and
Configuration", draft-sinnreich-sipdev-req-08 (work in
progress), October 2005.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, March 1997.
[RFC3046] Patrick, M., "DHCP Relay Agent Information Option",
RFC 3046, January 2001.
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[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3825] Polk, J., Schnizlein, J., and M. Linsner, "Dynamic Host
Configuration Protocol Option for Coordinate-based
Location Configuration Information", RFC 3825, July 2004.
[RFC4103] Hellstrom, G. and P. Jones, "RTP Payload for Text
Conversation", RFC 4103, June 2005.
[RFC4119] Peterson, J., "A Presence-based GEOPRIV Location Object
Format", RFC 4119, December 2005.
[RFC4190] Carlberg, K., Brown, I., and C. Beard, "Framework for
Supporting Emergency Telecommunications Service (ETS) in
IP Telephony", RFC 4190, November 2005.
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Authors' Addresses
Brian Rosen
NeuStar
470 Conrad Dr.
Mars, PA 16046
US
Phone: +1 724 382 1051
Email: br@brianrosen.net
James M. Polk
Cisco Systems
3913 Treemont Circle
Colleyville, TX 76034
US
Phone: +1-817-271-3552
Email: jmpolk@cisco.com
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