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ecrit                                                           B. Rosen
Internet-Draft                                                   NeuStar
Intended status: BCP                                             J. Polk
Expires: April 28, 2011                                    Cisco Systems
                                                        October 25, 2010

    Best Current Practice for Communications Services in support of
                           Emergency Calling


   The IETF and other standards organization have efforts targeted at
   standardizing various aspects of placing emergency calls on IP
   networks.  This memo describes best current practice on how devices,
   networks and services should use such standards to make emergency

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 28, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Overview of how emergency calls are placed . . . . . . . . . .  3
   4.  Which devices and services should support emergency calls  . .  4
   5.  Identifying an emergency call  . . . . . . . . . . . . . . . .  4
   6.  Location and its role in an emergency call . . . . . . . . . .  5
     6.1.  Types of location information  . . . . . . . . . . . . . .  5
     6.2.  Location Determination . . . . . . . . . . . . . . . . . .  6
       6.2.1.  User-entered location information  . . . . . . . . . .  6
       6.2.2.  Access network "wire database" location information  .  6
       6.2.3.  End-system measured location information . . . . . . .  6
       6.2.4.  Network-measured location information  . . . . . . . .  7
     6.3.  Who adds location, endpoint or proxy . . . . . . . . . . .  7
     6.4.  Location and references to location  . . . . . . . . . . .  8
     6.5.  End system location configuration  . . . . . . . . . . . .  8
     6.6.  When location should be configured . . . . . . . . . . . .  9
     6.7.  Conveying location in SIP  . . . . . . . . . . . . . . . . 10
     6.8.  Location updates . . . . . . . . . . . . . . . . . . . . . 10
     6.9.  Multiple locations . . . . . . . . . . . . . . . . . . . . 11
     6.10. Location validation  . . . . . . . . . . . . . . . . . . . 11
     6.11. Default location . . . . . . . . . . . . . . . . . . . . . 12
     6.12. Other location considerations  . . . . . . . . . . . . . . 12
   7.  LIS and LoST Discovery . . . . . . . . . . . . . . . . . . . . 12
   8.  Routing the call to the PSAP . . . . . . . . . . . . . . . . . 13
   9.  Signaling of emergency calls . . . . . . . . . . . . . . . . . 14
     9.1.  Use of TLS . . . . . . . . . . . . . . . . . . . . . . . . 14
     9.2.  SIP signaling requirements for User Agents . . . . . . . . 14
     9.3.  SIP signaling requirements for proxy servers . . . . . . . 15
   10. Call backs . . . . . . . . . . . . . . . . . . . . . . . . . . 16
   11. Mid-call behavior  . . . . . . . . . . . . . . . . . . . . . . 17
   12. Call termination . . . . . . . . . . . . . . . . . . . . . . . 17
   13. Disabling of features  . . . . . . . . . . . . . . . . . . . . 17
   14. Media  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
   15. Testing  . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   16. Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   17. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
   18. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
   19. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     19.1. Normative References . . . . . . . . . . . . . . . . . . . 20
     19.2. Informative References . . . . . . . . . . . . . . . . . . 23
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24

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1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   This document uses terms from [RFC3261], [RFC5012] and

2.  Introduction

   This document describes how access networks, Session Initiation
   Protocol [RFC3261] user agents, proxy servers and PSAPs support
   emergency calling, as outlined in [I-D.ietf-ecrit-framework], which
   is designed to complement the present document in section headings,
   numbering and content.  This BCP succinctly describes the
   requirements of end devices and applications (requirements prefaced
   by "ED-"), access networks (including enterprise access networks)
   (requirements prefaced by "AN-"), service providers (requirements
   prefaced by "SP-") and PSAPs to achieve globally interoperable
   emergency calling on the Internet.

   This document also defines requirements for "Intermediate" devices
   which exist between end devices or applications and the access
   network.  For example, a home router is an "Intermediate" device.
   Reporting location on an emergency call (see Section 6) may depend on
   the ability of such intermediate devices to meet the requirements
   prefaced by "INT-".

3.  Overview of how emergency calls are placed

   An emergency call can be distinguished (Section 5) from any other
   call by a unique Service URN [RFC5031], which is placed in the call
   set-up signaling when a home or visited emergency dial string is
   detected.  Because emergency services are local to specific
   geographic regions, a caller must obtain his location (Section 6)
   prior to making emergency calls.  To get this location, either a form
   of measuring (e.g., GPS) (Section 6.2.3) device location in the
   endpoint is deployed, or the endpoint is configured (Section 6.5)
   with its location from the access network's Location Information
   Server (LIS).  The location is conveyed (Section 6.7) in the SIP
   signaling with the call.  The call is routed (Section 8) based on
   location using the LoST protocol [RFC5222], which maps a location to
   a set of PSAP URIs.  Each URI resolves to a PSAP or an Emergency
   Services Routing Proxy (ESRP), which serves a group of PSAPs.  The
   call arrives at the PSAP with the location included in the SIP INVITE

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4.  Which devices and services should support emergency calls

   ED-1 A device or application SHOULD support emergency calling if a
   user could reasonably expect to be able to place a call for help with
   the device.  Some jurisdictions have regulations governing this.

   SP-1 If a device or application expects to be able to place a call
   for help, the service provider that supports it MUST facilitate
   emergency calling.  Some jurisdictions have regulations governing

   ED-2 Devices that create media sessions and exchange real-time audio,
   video and/or text, 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.  Some jurisdictions
   have regulations governing this.

5.  Identifying an emergency call

   ED-3 Endpoints SHOULD recognize dial strings of emergency calls.  If
   the service provider always knows the location of the device, then
   the service provider could recognize them.

   SP-2 Proxy servers SHOULD recognize emergency dial strings if for
   some reason the endpoint does not recognize them.

   ED-4/SP-3 Emergency calls MUST be marked with a Service URN in the
   Request-URI of the INVITE.

   ED-5/SP-4 Local dial strings MUST be recognized.

   ED-6/SP-5 Devices MUST be able to be configured with the home country
   from which the home dial string(s) can be determined.

   ED-7/SP-6 Emergency dial strings SHOULD be determined from LoST
   [RFC5222].  Dial Strings MAY be configured directly in the device.

   AN-1 LoST servers MUST return dial strings for emergency services

   ED-8 Endpoints which do not recognize emergency dial strings SHOULD
   send dial strings as per [RFC4967].

   SP-7 If a proxy server recognizes dial strings on behalf of its
   clients it MUST recognize emergency dial strings represented by

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   [RFC4967] and SHOULD recognize emergency dial strings represented by
   a tel URI [RFC3966].

   ED-9 Endpoints SHOULD be able to have home dial strings provisioned.

   SP-8 Service providers MAY provision home dial strings in devices.

   ED-10 Devices SHOULD NOT have one button emergency calling

   ED-11/SP-9 All sub-services for the 'sos' service specified in
   [RFC5031] MUST be recognized.

6.  Location and its role in an emergency call

   Handling location for emergency calling usually involves several
   steps to process and multiple elements are involved.  In Internet
   emergency calling, where the endpoint is located is "determined"
   using a variety of measurement or wiretracing methods.  Endpoints may
   be "configured" with their own location by the access network.  In
   some circumstances, a proxy server may insert location into the
   signaling on behalf of the endpoint.  The location is "mapped" to the
   URI to send the call to, and the location is "conveyed" to the PSAP
   (and other elements) in the signaling.  Likewise, we employ Location
   Configuration Protocols, the Location-to-Service Mapping Protocol,
   and Location Conveyance Protocols for these functions.  The Location-
   to-Service Translation protocol [RFC5222] is the Location Mapping
   Protocol defined by the IETF.

6.1.  Types of location information

   There are several forms of location.  In IETF location configuration
   and location conveyance protocols, civic and geospatial (geo) forms
   are both supported.  The civic forms include both postal and
   jurisdictional fields.  A cell tower/sector can be represented as a
   point (geo or civic) or polygon.  Other forms of location
   representation must be mapped into either a geo or civic for use in
   emergency calls.

   ED-12/INT-1/SP-10 Endpoints, Intermediate Devices and Service
   Providers MUST be prepared to handle location represented in either
   civic or geo form.

   ED-13/INT-2/SP-11/AN-2 Elements MUST NOT convert (civic to geo or geo
   to civic) from the form of location the determination mechanism

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6.2.  Location Determination

   ED-14/INT-3/AN-3 Any location determination mechanism MAY be used,
   provided the accuracy of the location meets local requirements.

6.2.1.  User-entered location information

   ED-15/INT-4/AN-4 Devices, intermediate Devices and/or access networks
   SHOULD support a manual method to override the location the access
   network determines.  Where a civic form of location is provided, all
   fields in the PIDF-LO [RFC4119] and [RFC5139] MUST be able to be

6.2.2.  Access network "wire database" location information

   AN-5 Access networks supporting copper, fiber or other hard wired IP
   packet service SHOULD support location configuration.  If the network
   does not support location configuration, it MUST require every device
   that connects to the network to support end system measured location.

   AN-6/INT-5 Access networks and intermediate devices providing wire
   database location information SHOULD provide interior location data
   (building, floor, room, cubicle) where possible.  It is RECOMMENDED
   that interior location be provided when spaces exceed approximately
   650 square meters.  See [I-D.ietf-ecrit-framework] Section 6.2.2 for
   a discussion of how this value was determined.

   AN-7/INT-6 Access networks and intermediate devices (including
   enterprise networks) which support intermediate range wireless
   connections (typically 100m or less of range) and which do not
   support a more accurate location determination mechanism such as
   triangulation, MUST support location configuration where the location
   of the access point is reflected as the location of the clients of
   that access point.

   AN-8/INT-7 Where the access network provides location configuration,
   intermediate devices MUST either be transparent to it, or provide an
   interconnected client for the supported configuration mechanism and a
   server for a configuration protocol supported by end devices
   downstream of the intermediate device such that the location provided
   by the access network is available to clients as if the intermediate
   device was not in the path.

6.2.3.  End-system measured location information

   ED-16/INT-8 Devices MAY support end-system measured location.  See
   [I-D.ietf-ecrit-framework] Section 6 for a discussion of accuracy of

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   ED-17/INT-9/AN-9 Devices that support endpoint measuring of location
   MUST have at least a coarse location capability (typically <1km
   accuracy when not location hiding) for routing of calls.  The
   location mechanism MAY be a service provided by the access network.

6.2.4.  Network-measured location information

   AN-10 Access networks MAY provide network-measured location
   determination.  Wireless access networks that do not supply network
   measured location MUST require that all devices connected to the
   network have end-system measured location.  Uncertainty and
   confidence may be specified by local regulation.  Where not
   specified, uncertainty of less than 100 m with 95% confidence is
   recommended for dispatch location.

   AN-11 Access networks that provide network measured location MUST
   have at least a coarse location (typically <1km when not location
   hiding) capability at all times for routing of calls.

   AN-12 Access networks with range of <10 meters (e.g. personal area
   networks such as Bluetooth MUST provide a location to mobile devices
   connected to them.  The location provided SHOULD be that of the
   access point location unless a more accurate mechanism is provided.

6.3.  Who adds location, endpoint or proxy

   ED-18/INT-10 Endpoints SHOULD attempt to configure their own location
   using the LCPs listed in ED-21.

   SP-12 Proxies MAY provide location on behalf of devices if:
   o  The proxy has a relationship with all access networks the device
      could connect to, and the relationship allows it to obtain
   o  The proxy has an identifier, such as an IP address, that can be
      used by the access network to determine the location of the
      endpoint, even in the presence of NAT and VPN tunnels that may
      obscure the identifier between the access network and the service

   ED-19/INT-11/SP-13 Where proxies provide location on behalf of
   endpoints, the service provider MUST ensure that either the end
   device is provided with the local dial strings for its current
   location (where the end device recognizes dial strings), or the
   service provider proxy MUST detect the appropriate local dial strings
   at the time of the call.

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6.4.  Location and references to location

   ED-20/INT-12 Devices SHOULD be able to accept and forward location by
   value or by reference.  An end device that receives location by
   reference (and does not also get the corresponding value) MUST be
   able to perform a dereference operation to obtain a value.

6.5.  End system location configuration

   ED-21/INT-13 Devices MUST support both the DHCP location options
   [RFC4776], [RFC3825] and HELD [RFC5985].  When devices deploy a
   specific access network interface for which location configuration
   mechanisms such as LLDP-MED [LLDP-MED] or 802.11v are specified, the
   device SHOULD support the additional respective access network
   specific location configuration mechanism.

   AN-13/INT-14 The access network MUST support either DHCP location
   options or HELD.  The access network SHOULD support other location
   configuration technologies that are specific to the type of access
   network.  If the access network supports more than one location
   configuration protocol, all such protocols MUST return the same
   location, within the constraints of the protocols deployed.

   AN-14/INT-15 Where a router is employed between a LAN and WAN in a
   small (less than approximately 650 square meters) area, the router
   MUST be transparent to the location provided by the WAN to the LAN.
   This may mean the router must obtain location as a client from the
   WAN, and supply an LCP server to the LAN with the location it
   obtains.  Where the area is larger, the LAN MUST have a location
   configuration mechanism satisfying the requirements of this document.

   ED-22/INT-16 Endpoints SHOULD try all LCPs supported by the device in
   any order or in parallel.  The first one that succeeds in supplying
   location MUST be used.

   AN-15/INT-17 Access networks that support more than one LCP MUST
   reply with the same location information (within the limits of the
   data format for the specific LCP) for all LCPs it supports.

   ED-23/INT-18/SP-14 When HELD is the LCP, the request MUST specify a
   value of "emergencyRouting" for the "responseTime" parameter and use
   the resulting location for routing.  If a value for dispatch location
   will be sent, another request with the "responseTime" parameter set
   to "emergencyDispatch" must be completed, with the result sent for
   dispatch purposes.

   ED-24 Where the operating system supporting application programs
   which need location for emergency calls does not allow access to

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   Layer 2 and Layer 3 functions necessary for a client application to
   use DHCP location options and/or other location technologies that are
   specific to the type of access network, the operating system MUST
   provide a published API conforming to ED-12 through ED-23 and ED-25
   through ED-32.  It is RECOMMENDED that all operating systems provide
   such an API.

6.6.  When location should be configured

   ED-25/INT-19 Endpoints SHOULD obtain location immediately after
   obtaining local network configuration information.

   ED-26/INT-20 If the device is configured to use DHCP for
   bootstrapping, it MUST include both options for location acquisition
   (civic and geodetic), the option for LIS discovery, and the option
   for LoST discovery as defined in [RFC4776], [RFC3825], [RFC5986] and

   ED-27/INT-21 If the device sends a DHCPINFORM message, it MUST
   include both options for location acquisition (civic and geodetic),
   the option for LIS discovery, and the option for LoST discovery as
   defined in [RFC4776], [RFC3825], [RFC5986] and [RFC5223].

   ED-28/INT-22 To minimize the effects of VPNs that do not allow
   packets to be sent via the native hardware interface rather than via
   the VPN tunnel, location configuration SHOULD be attempted before
   such tunnels are established.

   ED-29/INT-23 Software which uses LCPs SHOULD locate and use the
   actual hardware network interface rather than a VPN tunnel interface
   to direct LCP requests to the LIS in the actual access network.

   AN-16 Network administrators MUST take care in assigning IP addresses
   such that VPN address assignments can be distinguished from local
   devices (by subnet choice, for example), and LISs SHOULD NOT attempt
   to provide location to addresses that arrive via VPN connections
   unless it can accurately determine the location for such addresses.

   AN-17 Placement of NAT devices where an LCP uses IP address for an
   identifier SHOULD consider the effect of the NAT on the LCP.  The
   address used to query the LIS MUST be able to correctly identify the
   record in the LIS representing the location of the querying device

   ED-30/INT-24 For devices which are not expected to change location,
   refreshing location on the order of once per day is RECOMMENDED.

   ED-31/INT-25 For devices which roam, refresh of location information
   SHOULD be more frequent, with the frequency related to the mobility

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   of the device and the ability of the access network to support the
   refresh operation.  If the device detects a state change that might
   indicate having moved, for example when it changes access points, the
   device SHOULD refresh its location.

   ED-32/INT-26/AN-18 It is RECOMMENDED that location determination not
   take longer than 250 ms to obtain routing location and systems SHOULD
   be designed such that the typical response is under 100 ms.  However,
   as much as 3 seconds to obtain routing location MAY be tolerated if
   location accuracy can be substantially improved over what can be
   obtained in 250 ms.

6.7.  Conveying location in SIP

   ED-33/SP-15 Location sent between SIP elements MUST be conveyed using

6.8.  Location updates

   ED-34/AN-19 Where the absolute location or the accuracy of location
   of the endpoint may change between the time the call is received at
   the PSAP and the time dispatch is completed, location update
   mechanisms MUST be implemented and used.

   ED-35/AN-20 Mobile devices MUST be provided with a mechanism to get
   repeated location updates to track the motion of the device during
   the complete processing of the call.

   ED-36/AN-21 The LIS SHOULD provide a location reference which permits
   a subscription with appropriate filtering.

   ED-37/AN-22 For calls sent with location-by-reference, with a SIP or
   SIPS scheme, the server resolving the reference MUST support a
   SUBSCRIBE [RFC3265] to the presence event [RFC3856].  For other
   location-by-reference schemes that do not support subscription, the
   PSAP will have to repeatedly dereference the URI to determine if the
   device moved.

   ED-38 If location was sent by value, and the endpoint gets updated
   location, it MUST send the updated location to the PSAP via a SIP re-
   INVITE or UPDATE request.  Such updates SHOULD be limited to no more
   than one update every 10 seconds, a value selected to keep the load
   on a large PSAP manageable, and yet provide sufficient indication to
   the PSAP of motion.

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6.9.  Multiple locations

   ED-39/SP-16 If the LIS has more than one location for an endpoint it
   MUST conform to the rules in Section 3 of [RFC5491]

   ED-40 If a UA has more than one location available to it, it MUST
   choose one location to route the call towards the PSAP.  If multiple
   locations are in a single PIDF, the procedures in [RFC5491] MUST be
   followed.  If the UA has multiple PIDFs, and has no reasonable basis
   to choose from among them, a random choice is acceptable.

   SP-17 If a proxy inserts location on behalf of an endpoint, and it
   has multiple locations available for the endpoint it MUST choose one
   location to use to route the call towards the PSAP.  If multiple
   locations are in a single PIDF, the procedures in [RFC5491] MUST be
   followed.  If the proxy has multiple PIDFs, and has no reasonable
   basis to choose from among them, a random choice is acceptable.

   SP-18 If a proxy is attempting to insert location but the UA conveyed
   a location to it, the proxy MUST use the UA's location for routing in
   the initial INVITE and MUST convey that location towards the PSAP.
   It MAY also include what it believes the location to be in a separate
   Geolocation header.

   SP-19 All location objects received by a proxy MUST be delivered to
   the PSAP.

   ED-41/SP-20 Location objects MUST be created with information about
   the method by which the location was determined, such as GPS,
   manually entered, or based on access network topology included in a
   PIDF- LO "method" element.  In addition, the source of the location
   information MUST be included in a PIDF-LO "provided-by" element.

   ED-42/SP-21 A location with a method of "derived" MUST NOT be used
   unless no other location is available.

6.10.  Location validation

   AN-23 A LIS should perform location validation of civic locations via
   LoST before entering a location in its database.

   ED-44 Endpoints SHOULD validate civic locations when they receive
   them from their LCP.  Validation SHOULD be performed in conjunction
   with the LoST route query to minimize load on the LoST server.

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6.11.  Default location

   AN-24 When the access network cannot determine the actual location of
   the caller, it MUST supply a default location.  The default SHOULD be
   chosen to be as close to the probable location of the device as the
   network can determine.  See [I-D.ietf-ecrit-framework]

   SP-22 Proxies handling emergency calls MUST insert a default location
   in the INVITE if the call does not contain a location and the proxy
   does not have a method for obtaining a better location.

   AN-25/SP-23 Default locations MUST be marked with method=Default and
   the proxy MUST be identified in provided-by element of the PIDF-LO.

6.12.  Other location considerations

   ED-45 If the LCP does not return location in the form of a PIDF-LO
   [RFC4119], the endpoint MUST map the location information it receives
   from the configuration protocol to a PIDF-LO.

   ED-46/AN-26 To prevent against spoofing of the DHCP server, elements
   implementing DHCP for location configuration SHOULD use [RFC3118]
   although the difficulty in providing appropriate credentials is

   ED-47 If S/MIME is used, the INVITE message MUST provide enough
   information unencrypted for intermediate proxies to route the call
   based on the location information included.  This would include the
   Geolocation header, and any bodies containing location information.
   Use of S/MIME with emergency calls is NOT RECOMMENDED.

   ED-48/SP-24 Either TLS or IPSEC [RFC3986] MUST be used to protect
   location (but see Section 9.1).

7.  LIS and LoST Discovery

   ED-49 Endpoints MUST support one or more mechanisms that allow them
   to determine their public IP address, for example, STUN [RFC5389].

   ED-50 Endpoints MUST support LIS discovery as described in [RFC5986],
   and the LoST discovery as described in [RFC5223].

   ED-51 The device MUST have a configurable default LoST server
   parameter.  If the device is provided by or managed by a service
   provider, it is expected that the service provider will configure
   this option.

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   ED-52 DHCP LoST discovery MUST be used, if available, in preference
   to configured LoST servers.  That is, the endpoint MUST send queries
   to this LoST server first, using other LoST servers only if these
   queries fail.

   AN-27 Access networks which support DHCP MUST implement the LIS and
   LoST discovery options in their DHCP servers and return suitable
   server addresses as appropriate.

8.  Routing the call to the PSAP

   ED-53 Endpoints who obtain their own location SHOULD perform LoST
   mapping to the PSAP URI.

   ED-54 Mapping SHOULD be performed at boot time and whenever location
   changes beyond the service boundary obtained from a prior LoST
   mapping operation or the time-to-live value of that response has
   expired.  The value MUST be cached for possible later use.

   ED-55 The endpoint MUST attempt to update its location at the time of
   an emergency call.  If it cannot obtain a new location quickly (see
   Section 6), it MUST use the cached value.

   ED-56 The endpoint SHOULD attempt to update the LoST mapping at the
   time of an emergency call.  If it cannot obtain a new mapping
   quickly, it MUST use the cached value.  If the device cannot update
   the LoST mapping and does not have a cached value, it MUST signal an
   emergency call without a Route header containing a PSAP URI.

   SP-25 Networks MUST be designed so that at least one proxy in the
   outbound path will recognize emergency calls with a Request URI of
   the service URN in the "sos" tree.  An endpoint places a service URN
   in the Request URI to indicate that the endpoint understood the call
   was an emergency call.  A proxy that processes such a call looks for
   the presence of a SIP Route header field with a URI of a PSAP.
   Absence of such a Route header indicates the UAC was unable to invoke
   LoST and the proxy MUST perform the LoST mapping and insert a Route
   header field with the URI obtained.

   SP-26 To deal with old user agents that predate this specification
   and with UAs that do not have access to their own location data, a
   proxy that recognizes a call as an emergency call that is not marked
   as such (see Section 5) MUST also perform this mapping, with the best
   location it has available for the endpoint.  The resulting PSAP URI
   would be placed in a Route header with the service URN in the Request

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   SP-27 Proxy servers performing mapping SHOULD use location obtained
   from the access network for the mapping.  If no location is
   available, a default location (see Section 6.11) MUST be supplied.

   SP-28 A proxy server which attempts mapping and fails to get a
   mapping MUST provide a default mapping.  A suitable default mapping
   would be the mapping obtained previously for the default location
   appropriate for the caller.

   ED-57/SP-29 [RFC3261] and [RFC3263] procedures MUST be used to route
   an emergency call towards the PSAP's URI.

9.  Signaling of emergency calls

9.1.  Use of TLS

   ED-58/SP-30 Either TLS or IPsec MUST be used when attempting to
   signal an emergency call.

   ED-59/SP-31 If TLS session establishment is not available, or fails,
   the call MUST be retried without TLS.

   ED-60/SP-32 [RFC5626] is RECOMMENDED to maintain persistent TLS
   connections between elements when one of the element is an endpoint.
   Persistent TLS connection between proxies is RECOMMENDED using any
   suitable mechanism.

   ED-61/AN-28 TLS MUST be used when attempting to retrieve location
   (configuration or dereferencing) with HELD.  The use of [RFC5077] is
   RECOMMENDED to minimize the time to establish TLS sessions without
   keeping server-side state.

   ED-62/AN-29 When TLS session establishment fails, the location
   retrieval MUST be retried without TLS.

9.2.  SIP signaling requirements for User Agents

   ED-63 The initial SIP signaling method is an INVITE request:
   1.   The Request URI SHOULD be the service URN in the "sos" tree, If
        the device cannot interpret local dial strings, the Request-URI
        SHOULD be a dial string URI [RFC4967] with the dialed digits.
   2.   The To header field SHOULD be a service URN in the "sos" tree.
        If the device cannot interpret local dial strings, the To:
        SHOULD be a dial string URI with the dialed digits.
   3.   The From header field SHOULD contain the AoR of the caller.

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   4.   A Route header field SHOULD be present with a PSAP URI obtained
        from LoST (see Section 8).  If the device does not interpret
        dial plans, or was unable to obtain a route from a LoST server,
        no such Route header field will be present.
   5.   A Contact header field MUST be globally routable, for example a
        GRUU [RFC5627], and be valid for several minutes following the
        termination of the call, provided that the UAC remains
        registered with the same registrar, to permit an immediate call-
        back to the specific device which placed the emergency call.  It
        is acceptable if the UAC inserts a locally routable URI and a
        subsequent B2BUA maps that to a globally routable URI.
   6.   Other header fields MAY be included as per normal SIP behavior.
   7.   A Supported header field MUST be included with the 'geolocation'
        option tag [I-D.ietf-sip-location-conveyance], unless the device
        does not understand the concept of SIP location.
   8.   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,
        by-reference or both.
   9.   If a device understands the SIP Location Conveyance extension
        and has its location unavailable or unknown to that device, it
        MUST include a Supported header field with a "geolocation"
        option tag, and MUST NOT include a Geolocation header field, and
        not include a PIDF-LO message body.
   10.  A SDP offer SHOULD be included in the INVITE.  If voice is
        supported the offer MUST include the G.711 codec, see
        Section 14.  As PSAPs may support a wide range of media types
        and codecs, sending an offerless INVITE may result in a lengthy
        return offer, but is permitted.  Cautions in [RFC3261] on
        offerless INVITEs should be considered before such use.
   11.  If the device includes location-by-value, the UA MUST support
        multipart message bodies, since SDP will likely be also in the
   12.  A UAC SHOULD include a "inserted-by" header parameter with its
        own hostname on all Geolocation header fields.  This informs
        downstream elements which device entered the location at this
        URI (either cid-URL or location-by-reference URI).

9.3.  SIP signaling requirements for proxy servers

   SP-33 SIP Proxy servers processing emergency calls:
   1.  If the proxy interprets dial plans on behalf of user agents, the
       proxy MUST look for the local emergency dial string at the
       location of the end device and MAY look for the home dial string.
       If it finds it, the proxy MUST:
       *  Insert a Geolocation header field.  Location-by-reference MUST
          be used because proxies must not insert bodies.

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       *  Include the Geolocation "inserted-by" and "used-for-routing"
          parameters with its own hostname (which should match the Via
          it inserts) on the inserted-by.
       *  Map the location to a PSAP URI using LoST.
       *  Add a Route header with the PSAP URI.
       *  Replace the Request-URI (which was the dial string) with the
          service URN appropriate for the emergency dial string.
       *  Route the call using normal SIP routing mechanisms.
   2.  If the proxy recognizes the service URN in the Request URI, and
       does not find a a Route header, it MUST query a LoST server.  If
       a location was provided (which should be the case), the proxy
       uses that location to query LoST.  The proxy may have to
       dereference a location by reference to get a value.  If a
       location is not present, and the proxy can query a LIS which has
       the location of the UA it MUST do so.  If no location is present,
       and the proxy does not have access to a LIS which could provide
       location, the proxy MUST supply a default location (See
       Section 6.11).  The location (in the signaling, obtained from a
       LIS, or default) MUST be used in a query to LoST with the service
       URN received with the call.  The resulting URI MUST be placed in
       a Route header added to the call.
   3.  The proxy SHOULD NOT modify any parameters in Geolocation header
       fields received in the call.  It MAY add a Geolocation header
       field.  Such an additional location SHOULD NOT be used for
       routing; the location provided by the UA should be used.
   4.  Either a P-Asserted-Identity [RFC3325] or an Identity header
       field [RFC4474], or both, SHOULD be included to identify the
       sender.  For services which must support emergency calls from
       unauthenticated devices, valid identity may not be available.
       Proxies encountering a P-Asserted-Identity will need to pass the
       header to the PSAP, which is in a different domain.  [RFC3325]
       requires a "spec(T)" to determine what happens if the "id"
       privacy service, or a Privacy header is present and requests
       privacy.  In the absence of another spec(T), such proxies should
       pass the header unmodified if and only if the connection between
       the proxy and the PSAP is, as far as the proxy can determine,
       protected by TLS with mutual authentication using keys reliably
       known by the parties, encrypted with no less strength than AES
       and the local regulations governing the PSAP do not otherwise

10.  Call backs

   ED-64/SP-34 Devices device SHOULD have a globally routable URI in a
   Contact: header field which remains valid for several minutes past
   the time the original call containing the URI completes unless the
   device registration expires and is not renewed.

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   SP-35 Call backs to the Contact: header URI received within 30
   minutes of an emergency call must reach the device regardless of call
   features or services that would normally cause the call to be routed
   to some other entity.

   SP-36 Devices MUST have a persistent AOR URI either in a P-Asserted-
   Identity header field or From protected by an Identity header field
   suitable for returning a call some time after the original call.
   Such a call back would not necessarily reach the device that
   originally placed the call.

11.  Mid-call behavior

   ED-65/SP-37 During the course of an emergency call, devices and
   proxies MUST initiate a call transfer upon receipt of REFER request
   within the dialog with method=INVITE and the Referred-by: header
   field [RFC3515] in that request.

12.  Call termination

   ED-66 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,
   and the session timer (if implemented) expires, the call MAY be
   declared lost.  If in the interval, an incoming call is received from
   the domain of the PSAP, the device MUST drop the old call and alert
   for the (new) incoming call.  Dropping of the old call MUST only
   occur if the user is attempting to hang up; the domain of an incoming
   call can only be determined from the From header, which is not
   reliable, and could be spoofed.  Dropping an active call by a new
   call with a spoofed From header field would be a DoS attack.

13.  Disabling of features

   ED-67/SP-38 User Agents and proxies MUST disable features that will
   interrupt an ongoing emergency call, such as:
   o  Call Waiting
   o  Call Transfer
   o  Three Way Call
   o  Hold
   o  Outbound Call Blocking
   when an emergency call is established.  Also see ED-74 in Section 14.

   ED-68/SP-39 The emergency dial strings SHOULD NOT be permitted in
   Call Forward numbers or speed dial lists.

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   ED-69/SP-40 The User Agent and Proxies MUST disable call features
   which would interfere with the ability of call backs from the PSAP to
   be completed such as:
   o  Do Not Disturb
   o  Call Forward (all kinds)

   ED-70 Call backs SHOULD be determined by retaining the domain of the
   PSAP which answers an outgoing emergency call and instantiating a
   timer which starts when the call is terminated.  If a call is
   received from the same domain and within the timer period, sent to
   the Contact: or AoR used in the emergency call, it should be assumed
   to be a call back.  The suggested timer period is 5 minutes.
   [RFC4916] may be used by the PSAP to inform the UA of the domain of
   the PSAP.  Recognizing a call back from the domain of the PSAP will
   not always work, and further standardization will be required to give
   the UA the ability to recognize a call back.

14.  Media

   ED-71 Endpoints MUST send and receive media streams on RTP [RFC3550].

   ED-72 Normal SIP offer/answer [RFC3264] negotiations MUST be used to
   agree on the media streams to be used.

   ED-73 Endpoints supporting voice MUST support G.711 A law (and mu Law
   if they are intended be used in North America) encoded voice as
   described in [RFC3551].  It is desirable to include wideband codecs
   such as AMR-WB in the offer.

   ED-74 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.

   ED-75 Endpoints supporting Instant Messaging (IM) MUST support both
   [RFC3428] and [RFC4975].

   ED-76 Endpoints supporting real-time text MUST use [RFC4103].  The
   expectations for emergency service support for the real-time text
   medium are described in [RFC5194], Section 7.1.

   ED-77 Endpoints supporting video MUST support H.264 per

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15.  Testing

   ED-78 INVITE requests to a service URN ending in ".test" indicates a
   request for an automated test.  For example,
   "urn:service.sos.fire.test".  As in standard SIP, a 200 (OK) response
   indicates that the address was recognized and a 404 (Not found) that
   it was not.  A 486 (Busy Here) MUST be returned if the test service
   is busy, and a 404 (Not found) MUST be returned if the PSAP does not
   support the test mechanism.

   ED-79 In its response to the test, the PSAP MAY include a text body
   (text/plain) indicating the identity of the PSAP, the requested
   service, and the location reported with the call.  For the latter,
   the PSAP SHOULD return location-by-value even if the original
   location delivered with the test was by-reference.  If the location-
   by-reference was supplied, and the dereference requires credentials,
   the PSAP SHOULD use credentials supplied by the LIS for test
   purposes.  This alerts the LIS that the dereference is not for an
   actual emergency call and location hiding techniques, if they are
   being used, may be employed for this dereference.  Use of SIPS for
   the request would assure the response containing the location is kept

   ED-80 A PSAP accepting a test call SHOULD accept a media loopback
   test [I-D.ietf-mmusic-media-loopback] and SHOULD support the "rtp-
   pkt-loopback" and "rtp-start-loopback" options.  The user agent would
   specify a loopback attribute of "loopback-source", the PSAP being the
   mirror.  User Agents should expect the PSAP to loop back no more than
   3 packets of each media type accepted (which limits the duration of
   the test), after which the PSAP would normally send BYE.

   ED-81 User agents SHOULD perform a full call test, including media
   loopback, after a disconnect and subsequent change in IP address not
   due to a reboot.  After an initial test, a full test SHOULD be
   repeated approximately every 30 days with a random interval.

   ED-82 User agents MUST NOT place a test call immediately after
   booting.  If the IP address changes after booting, the UA should wait
   a random amount of time (in perhaps a 30 minute period, sufficient
   for any avalanche restart to complete) and then test.

   ED-83 PSAPs MAY refuse repeated requests for test from the same
   device in a short period of time.  Any refusal is signaled with a 486
   or 488 response.

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16.  Security Considerations

   Security considerations for emergency calling have been documented in
   [RFC5069], and [I-D.ietf-geopriv-arch].

17.  IANA Considerations

   This document has no actions for IANA.

18.  Acknowledgements

   Work group members participating in the creation and review of this
   document include include Hannes Tschofenig, Ted Hardie, Marc Linsner,
   Roger Marshall, Stu Goldman, Shida Schubert, James Winterbottom,
   Barbara Stark, Richard Barnes and Peter Blatherwick.

19.  References

19.1.  Normative References

              Wang, Y., Even, R., Kristensen, T., and R. Jesup, "RTP
              Payload Format for H.264 Video",
              draft-ietf-avt-rtp-rfc3984bis-12 (work in progress),
              October 2010.

              Sivachelvan, C., Venna, N., Jones, P., Stratton, N.,
              Roychowdhury, A., and K. Hedayat, "An Extension to the
              Session Description Protocol (SDP) for Media Loopback",
              draft-ietf-mmusic-media-loopback-14 (work in progress),
              July 2010.

              Polk, J. and B. Rosen, "Location Conveyance for the
              Session Initiation Protocol",
              draft-ietf-sip-location-conveyance-13 (work in progress),
              March 2009.

              TIA, "ANSI/TIA-1057 Link Layer Discovery Protocol - Media
              Endpoint Discovery".

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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   [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,
              June 2002.

   [RFC3263]  Rosenberg, J. and H. Schulzrinne, "Session Initiation
              Protocol (SIP): Locating SIP Servers", RFC 3263,
              June 2002.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              June 2002.

   [RFC3265]  Roach, A., "Session Initiation Protocol (SIP)-Specific
              Event Notification", RFC 3265, June 2002.

   [RFC3428]  Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C.,
              and D. Gurle, "Session Initiation Protocol (SIP) Extension
              for Instant Messaging", RFC 3428, December 2002.

   [RFC3515]  Sparks, R., "The Session Initiation Protocol (SIP) Refer
              Method", RFC 3515, April 2003.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC3551]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
              Video Conferences with Minimal Control", STD 65, RFC 3551,
              July 2003.

   [RFC3825]  Polk, J., Schnizlein, J., and M. Linsner, "Dynamic Host
              Configuration Protocol Option for Coordinate-based
              Location Configuration Information", RFC 3825, July 2004.

   [RFC3841]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
              Preferences for the Session Initiation Protocol (SIP)",
              RFC 3841, August 2004.

   [RFC3856]  Rosenberg, J., "A Presence Event Package for the Session
              Initiation Protocol (SIP)", RFC 3856, August 2004.

   [RFC3966]  Schulzrinne, H., "The tel URI for Telephone Numbers",
              RFC 3966, December 2004.

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   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC4103]  Hellstrom, G. and P. Jones, "RTP Payload for Text
              Conversation", RFC 4103, June 2005.

   [RFC4119]  Peterson, J., "A Presence-based GEOPRIV Location Object
              Format", RFC 4119, December 2005.

   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474, August 2006.

   [RFC4776]  Schulzrinne, H., "Dynamic Host Configuration Protocol
              (DHCPv4 and DHCPv6) Option for Civic Addresses
              Configuration Information", RFC 4776, November 2006.

   [RFC4916]  Elwell, J., "Connected Identity in the Session Initiation
              Protocol (SIP)", RFC 4916, June 2007.

   [RFC4967]  Rosen, B., "Dial String Parameter for the Session
              Initiation Protocol Uniform Resource Identifier",
              RFC 4967, July 2007.

   [RFC4975]  Campbell, B., Mahy, R., and C. Jennings, "The Message
              Session Relay Protocol (MSRP)", RFC 4975, September 2007.

   [RFC5031]  Schulzrinne, H., "A Uniform Resource Name (URN) for
              Emergency and Other Well-Known Services", RFC 5031,
              January 2008.

   [RFC5139]  Thomson, M. and J. Winterbottom, "Revised Civic Location
              Format for Presence Information Data Format Location
              Object (PIDF-LO)", RFC 5139, February 2008.

   [RFC5222]  Hardie, T., Newton, A., Schulzrinne, H., and H.
              Tschofenig, "LoST: A Location-to-Service Translation
              Protocol", RFC 5222, August 2008.

   [RFC5223]  Schulzrinne, H., Polk, J., and H. Tschofenig, "Discovering
              Location-to-Service Translation (LoST) Servers Using the
              Dynamic Host Configuration Protocol (DHCP)", RFC 5223,
              August 2008.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008.

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   [RFC5491]  Winterbottom, J., Thomson, M., and H. Tschofenig, "GEOPRIV
              Presence Information Data Format Location Object (PIDF-LO)
              Usage Clarification, Considerations, and Recommendations",
              RFC 5491, March 2009.

   [RFC5626]  Jennings, C., Mahy, R., and F. Audet, "Managing Client-
              Initiated Connections in the Session Initiation Protocol
              (SIP)", RFC 5626, October 2009.

   [RFC5627]  Rosenberg, J., "Obtaining and Using Globally Routable User
              Agent URIs (GRUUs) in the Session Initiation Protocol
              (SIP)", RFC 5627, October 2009.

   [RFC5985]  Barnes, M., "HTTP-Enabled Location Delivery (HELD)",
              RFC 5985, September 2010.

   [RFC5986]  Thomson, M. and J. Winterbottom, "Discovering the Local
              Location Information Server (LIS)", RFC 5986,
              September 2010.

19.2.  Informative References

              Rosen, B., Schulzrinne, H., Polk, J., and A. Newton,
              "Framework for Emergency Calling using Internet
              Multimedia", draft-ietf-ecrit-framework-11 (work in
              progress), July 2010.

              Barnes, R., Lepinski, M., Cooper, A., Morris, J.,
              Tschofenig, H., and H. Schulzrinne, "An Architecture for
              Location and Location Privacy in Internet Applications",
              draft-ietf-geopriv-arch-03 (work in progress),
              October 2010.

   [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.

   [RFC5012]  Schulzrinne, H. and R. Marshall, "Requirements for
              Emergency Context Resolution with Internet Technologies",
              RFC 5012, January 2008.

   [RFC5069]  Taylor, T., Tschofenig, H., Schulzrinne, H., and M.
              Shanmugam, "Security Threats and Requirements for
              Emergency Call Marking and Mapping", RFC 5069,
              January 2008.

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   [RFC5077]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
              "Transport Layer Security (TLS) Session Resumption without
              Server-Side State", RFC 5077, January 2008.

   [RFC5194]  van Wijk, A. and G. Gybels, "Framework for Real-Time Text
              over IP Using the Session Initiation Protocol (SIP)",
              RFC 5194, June 2008.

Authors' Addresses

   Brian Rosen
   470 Conrad Dr.
   Mars, PA  16046

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

   James Polk
   Cisco Systems
   3913 Treemont Circle
   Colleyville, TX  76034

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

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