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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

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   The list of current Internet-Drafts can be accessed at

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   This Internet-Draft will expire on December 27, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).


   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",
   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-

   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

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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
   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

   [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

   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

   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

   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

   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

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
   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
   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
   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
   4.  Support Session Timer (or equivalent) to guard against session

   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
   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

              Hardie, T., "LoST: A Location-to-Service Translation
              Protocol", draft-ietf-ecrit-lost-00 (work in progress),
              June 2006.

              Tschofenig, H., "GEOPRIV PIDF-LO Usage Clarification,
              Considerations and Recommendations",
              draft-ietf-geopriv-pdif-lo-profile-04 (work in progress),
              May 2006.

              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.

              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.

              Polk, J. and B. Rosen, "Session Initiation Protocol
              Location Conveyance",
              draft-ietf-sip-location-conveyance-02 (work in progress),
              March 2006.

              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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              Schulzrinne, H., "DHCP Option for Civil Location",
              draft-schulzrinne-geopriv-dhcp-civil-01 (work in
              progress), February 2003.

              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
   470 Conrad Dr.
   Mars, PA  16046

   Phone: +1 724 382 1051
   Email: br@brianrosen.net

   James M. Polk
   Cisco Systems
   3913 Treemont Circle
   Colleyville, TX  76034

   Phone: +1-817-271-3552
   Email: jmpolk@cisco.com

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Intellectual Property Statement

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Copyright Statement

   Copyright (C) The Internet Society (2006).  This document is subject
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   Funding for the RFC Editor function is currently provided by the
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