Internet DRAFT - draft-rosen-sos-phonebcp
draft-rosen-sos-phonebcp
ecrit B. Rosen
Internet-Draft NeuStar
Expires: December 27, 2006 J. Polk
Cisco Systems
June 25, 2006
Best Current Practice for Communications Services in support of
Emergency Calling
draft-rosen-sos-phonebcp-01.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 . . . . . . . . . . . . . . . . . . . . . . 8
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. and [I-D.ietf-
sipping-toip]
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 from an IP based device are
(with a rough ordering of operation)
1. Device connects to access network, and obtains initial location
2. User dials visited location's emergency number
3. User device identifies call as emergency call
4. User device includes location indication (by value or by
reference) in the call set-up messaging
5. emergency call set-up is routed to appropriate PSAP based on
location of the caller
6. call is established with PSAP
7. 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 "boots" and connects to its access network
o the phone would get location from the DHCP server [or an L7
server].
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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.
o The proxy recognize the call as an emergency call.
o The proxy would determine the PSAP's URI by using the [I-D.ietf-
ecrit-lost] 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.
[RFC4504] details Best Current Practice for SIP user agents. This
memo can be considered an addition to it for endpoints.
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
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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.
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 [I-D.schulzrinne-geopriv-dhcp-civil] 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
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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
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 and the ability of the access network to
support the refresh operation. 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. These numbers are empiracilly derived, but are intended
to keep total call signaling time below 2 seconds. There are
conflicts between the time it takes to generate location when
measuring techniques are used and the desire to route the call
quickly. If an accurate location cannot be determined quickly, a
rough location SHOULD be returned within 500ms which can be used to
route the call.
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 their 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,
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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.
[I-D.schulzrinne-sipping-service] 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.
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 RECOMMENDED 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 Paris, 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
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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.
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. <vspace blankLines="1"/>NOTE: unintialized devices may
not have an AoR available
3. A Via: header MUST be present and SHOULD include the URI of the
device
4. A Route header MAY be present if the device has performed a
fallback mapping function (see Section 4)
5. Either a P-Asserted-Identity [RFC3325] or an Identity header
[I-D.ietf-sip-identity], or both, SHOULD be included to identify
the sender.
6. A Contact header SHOULD be present (which might contain a GRUU
[I-D.ietf-sip-gruu]) to permit an immediate call-back to the
specific device which placed the emergency call.
7. Other headers MAY be included as per normal sip behavior
8. A Supported: header MUST be included with the 'location' option
tag, unless the device does not understand the concept of SIP
Location ;
9. 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;
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10. if a device understands the SIP Location Conveyance [I-D.ietf-
sip-location-conveyance] extension and has its location
available, it MUST include location either by-value or by-
reference. If it is by-value, the INVITE contains a Supported
header with a "location" option tag, and a "cidURL" indicating
which message body part contains the PIDF-LO. If the INVITE
contains a location by-reference, it includes the same Supported
header with the "location" option tag, and includes the URI of
the PIDF-LO on a remote node in a Location header. [I-D.ietf-
geopriv-pdif-lo-profile] MUST be used
11. If a device understand the SIP Location Conveyance extension and
has its location unavailable or unknown to that device, it MUST
include a Supported header with a "location" option tag, and not
include a Location header, and not include a PIDF-LO message
body.;
12. 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.
6.2. Mapping from Location to a PSAP URI
To route an emergency call, we make use of the [I-D.ietf-ecrit-lost]
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.
User agents that can obtain location information MUST perform the
mapping from location information to PSAP URI using [I-D.ietf-ecrit-
lost]. The mapping is performed whenever the UA acquires new
location information that is outside the bounds of the current PSAP
coverage region specified in the LoST response or the time-to-live
value of that response has expired.
To deal with old user agents that predate this specification and with
UAs that do not have access to their own location data, proxies that
recognize a call as an emergency call that is not marked as such (see
Section 5) or where the Request-URI is a service:sos URN MUST also
perform this mapping.
6.3. Routing the call
Normal routing mechanisms for the specified URI should be used. For
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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.
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:
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o Call Waiting (all kinds)
o Do Not Disturb
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. Implementers are
admonished to ,,,
8. Normative References
[I-D.ietf-ecrit-lost]
Hardie, T., "LoST: A Location-to-Service Translation
Protocol", draft-ietf-ecrit-lost-00 (work in progress),
June 2006.
[I-D.ietf-geopriv-pdif-lo-profile]
Tschofenig, H., "GEOPRIV PIDF-LO Usage Clarification,
Considerations and Recommendations",
draft-ietf-geopriv-pdif-lo-profile-04 (work in progress),
May 2006.
[I-D.ietf-sip-gruu]
Rosenberg, J., "Obtaining and Using Globally Routable User
Agent (UA) URIs (GRUU) in the Session Initiation Protocol
(SIP)", draft-ietf-sip-gruu-09 (work in progress),
June 2006.
[I-D.ietf-sip-identity]
Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", draft-ietf-sip-identity-06
(work in progress), October 2005.
[I-D.ietf-sip-location-conveyance]
Polk, J. and B. Rosen, "Session Initiation Protocol
Location Conveyance",
draft-ietf-sip-location-conveyance-02 (work in progress),
March 2006.
[I-D.ietf-sipping-toip]
Wijk, A., "Framework for real-time text over IP using
SIP", draft-ietf-sipping-toip-04 (work in progress),
March 2006.
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[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.
[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.
[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.
[RFC3325] Jennings, C., Peterson, J., and M. Watson, "Private
Extensions to the Session Initiation Protocol (SIP) for
Asserted Identity within Trusted Networks", RFC 3325,
November 2002.
[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.
[RFC4504] Sinnreich, H., Lass, S., and C. Stredicke, "SIP Telephony
Device Requirements and Configuration", RFC 4504,
May 2006.
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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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