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BEST CURRENT PRACTICE

Internet Engineering Task Force (IETF)                          B. Rosen
Request for Comments: 6881                                       NeuStar
BCP: 181                                                         J. Polk
Category: Best Current Practice                            Cisco Systems
ISSN: 2070-1721                                               March 2013


          Best Current Practice for Communications Services in
                      Support of Emergency Calling

Abstract

   The IETF and other standards organizations 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 using IETF protocols should use such standards
   to make emergency calls.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   BCPs is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6881.

Copyright Notice

   Copyright (c) 2013 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
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.





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Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................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 ..............................6
      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 ............7
           6.2.4. Network Measured Location Information ...............7
      6.3. Who Adds Location?  The Endpoint, or the Proxy? ............8
      6.4. Location and References to Location ........................8
      6.5. End System Location Configuration ..........................8
      6.6. When Location Should Be Configured ........................10
      6.7. Conveying Location ........................................11
      6.8. Location Updates ..........................................11
      6.9. Multiple Locations ........................................12
      6.10. Location Validation ......................................12
      6.11. Default Location .........................................13
      6.12. Other Location Considerations ............................13
   7. LIS and LoST Discovery .........................................13
   8. Routing the Call to the PSAP ...................................14
   9. Signaling of Emergency Calls ...................................15
      9.1. Use of TLS ................................................15
      9.2. SIP Signaling Requirements for User Agents ................16
      9.3. SIP Signaling Requirements for Proxy Servers ..............17
   10. Callbacks .....................................................18
   11. Mid-Call Behavior .............................................19
   12. Call Termination ..............................................19
   13. Disabling of Features .........................................19
   14. Media .........................................................20
   15. Testing .......................................................21
   16. Security Considerations .......................................22
   17. IANA Considerations ...........................................22
      17.1. Test Service URN .........................................22
      17.2. 'test' Subregistry .......................................23
   18. Acknowledgements ..............................................23
   19. References ....................................................23
      19.1. Normative References .....................................23
      19.2. Informative References ...................................27






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1.  Introduction

   This document describes how access networks, Session Initiation
   Protocol [RFC3261] user agents, proxy servers, and Public Safety
   Answering Points (PSAPs) support emergency calling, as outlined in
   [RFC6443], which is designed to complement the present document in
   section headings, numbering, and content.  Understanding [RFC6443] is
   necessary to understand this document.  This Best Current Practice
   (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
   that 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-".

   The access network requirements apply to those networks that may be
   used to place emergency calls using IETF protocols.  Local
   regulations may impact the need to support this document's access
   network requirements.

   Other organizations, such as the National Emergency Number
   Association (NENA), define the PSAP interface.  NENA's documents
   reference this document.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

   This document uses terms from [RFC3261], [RFC5012], and [RFC6443].

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] that is placed in the call
   setup 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) ([RFC6443] Section 6.2.3) device location in



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   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 Location-to-Service Translation (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) that
   serves a group of PSAPs.  The call arrives at the PSAP with the
   location included in the SIP INVITE request.

4.  Which Devices and Services Should Support Emergency Calls?

   ED-1:  A device or application that implements SIP calling SHOULD
      support emergency calling.  Some jurisdictions have regulations
      governing which devices need to support emergency calling, and
      developers are encouraged to ensure that devices they develop meet
      relevant regulatory requirements.  Unfortunately, the natural
      variation in those regulations also makes it impossible to
      accurately describe the cases when developers do or do not have to
      support emergency calling.

   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
      this.

   ED-2:  Devices that create media sessions and exchange real-time
      audio, video, and/or text and that 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
      (the correct dial string depends on which country a caller is in),
      the service provider may recognize them; see SP-2.

   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:  Geographically local dial strings MUST be recognized.





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   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 into the
      device.

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

   ED-8:  Endpoints that 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
      [RFC4967] and SHOULD recognize the 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
      initiation.

   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 entities are involved.  In Internet
   emergency calling, where the endpoint is located is determined using
   a variety of measurement or wire-tracing methods.  Endpoints can be
   configured with their own location by the access network.  In some
   circumstances, a proxy server can 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
   entities) in the signaling.  Likewise, we employ Location
   Configuration Protocols (LCPs), 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.








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6.1.  Types of Location Information

   There are several forms of location.  All IETF location configuration
   and location conveyance protocols support both civic and geospatial
   (geo) forms.  The civic forms include both postal and jurisdictional
   fields.  A cell tower/sector can be represented as a point (geo or
   civic) or polygon.  Endpoints, intermediate devices, and service
   providers receiving other forms of location representation MUST map
   them 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:  Entities MUST NOT convert (civic to geo or
      geo to civic) from the form of location that the determination
      mechanism (see Section 6.2) supplied prior to receipt by the PSAP.

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.  When the override location is
      supplied in civic form, it MUST be possible for the resultant
      Presence Information Data Format Location Object (PIDF-LO)
      received at the PSAP to contain any of the elements specified in
      [RFC4119] and [RFC5139].

6.2.2.  Access Network "Wire Database" Location Information

   AN-5:  Access networks supporting copper, fiber, or other hard-wired
      IP packet services SHOULD support location configuration.  If the
      network does not support location configuration, it MUST require
      every device or intermediate 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 [RFC6443] Section 6.2.2 for
      a discussion of how this value was determined.




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   AN-7/INT-6:  Access networks and intermediate devices (including
      enterprise networks) that support intermediate range wireless
      connections (typically 100 m or less of range) and that 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
      [RFC6443] Section 6 for a discussion of accuracy of location.

   ED-17/INT-9/AN-9:  Devices that support endpoint measuring of
      location MUST have at least a coarse location capability
      (typically <1 km accuracy) for the 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 every device or
      intermediate device connected to the network to support end system
      measured location.  Uncertainty and confidence may be specified by
      local regulation.  Where not specified, uncertainty of less than
      100 meters 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 <1 km when not location
      hiding) capability at all times for the routing of calls.

   AN-12:  Access networks with a 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
      reported by the upstream access network unless a more accurate
      mechanism is available.





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6.3.  Who Adds Location?  The Endpoint, or the Proxy?

   ED-18/INT-10:  Endpoints SHOULD attempt to configure their own
      location using the Location Configuration Protocols (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
      location.

   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
      provider.

   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.

6.4.  Location and References to Location

   ED-20/INT-12:  Devices SHOULD be able to accept and forward location-
      by-value or location-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

   Obtaining location from the access network may be preferable even if
   the device can measure its own location, especially indoors where
   most measurement mechanisms are not accurate enough.  The
   requirements listed in this section do not apply to devices that can
   accurately measure their own location.











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   ED-21/INT-13:  Devices MUST support both the Dynamic Host
      Configuration Protocol (DHCP) location options [RFC4776] [RFC6225]
      and HTTP-Enabled Location Delivery (HELD) [RFC5985].  When devices
      deploy a specific access network interface for which location
      configuration mechanisms such as Link Layer Discovery Protocol -
      Media Endpoint Discovery (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.

   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
      that need location for emergency calls does not allow access to
      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.





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6.6.  When Location Should Be Configured

   If an endpoint is manually configured, the requirements in this
   section are not applicable.

   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 and does not use its own measurement to determine
      location, 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], [RFC6225], [RFC5986],
      and [RFC5223], respectively.

   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], [RFC6225], [RFC5986], and
      [RFC5223], respectively.

   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 that 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 they can accurately determine the
      location for such addresses.

   AN-17:  Placement of NAT devices where an LCP uses an 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 that are not expected to change location,
      refreshing location on the order of once per day is RECOMMENDED.





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   ED-31/INT-25:  For devices that roam, refresh of location information
      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.  If the device detects a link 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 time 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

   ED-33/SP-15:  Location sent between SIP entities MUST be conveyed
      using the extension described in [RFC6442].

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 that
      permits a subscription with appropriate filtering.

   ED-37/AN-22:  For calls sent with location-by-reference, with a SIP
      or Session Initiation Protocol Secure (SIPS) scheme, the server
      resolving the reference MUST support a SUBSCRIBE [RFC6665] 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 an
      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 an endpoint 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 Presence Information Data
      Format (PIDF), the procedures in [RFC5491] MUST be followed.  If
      the endpoint 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 endpoint
      conveyed a location to it, the proxy MUST use the endpoint'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-43:  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 [RFC6443].

   SP-22:  Proxies handling emergency calls MUST insert a default
      location in the INVITE if the incoming INVITE 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 a PIDF-LO "provided-by"
      element.

6.12.  Other Location Considerations

   ED-44:  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-45/AN-26:  To prevent against spoofing of the DHCP server,
      entities implementing DHCP for location configuration SHOULD use
      DHCPv4 message authentication [RFC3118] or DHCPv6 message
      authentication [RFC3315], although the difficulty in providing
      appropriate credentials is significant.

   ED-46:  If S/MIME [RFC5751] 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 for this reason.

   ED-47/SP-24:  Transport Layer Security (TLS) [RFC5746] MUST be used
      to protect location (but see Section 9.1).  All SIP
      implementations of this specification MUST support TLS.

7.  LIS and LoST Discovery

   ED-48:  Endpoints MUST support one or more mechanisms that allow them
      to determine their public IP address, for example, Session
      Traversal Utilities for NAT (STUN) [RFC5389].

   ED-49:  Endpoints MUST support LIS discovery as described in
      [RFC5986] and LoST discovery as described in [RFC5223].




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   ED-50:  The device MUST have a configurable default LoST server
      parameter.

   ED-51:  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 that 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-52:  Endpoints that obtain their own location SHOULD perform LoST
      mapping to the PSAP URI.

   ED-53:  Mapping SHOULD be performed at boot time and whenever a
      location changes beyond the service boundary obtained from a prior
      LoST mapping operation, or when the time-to-live value of that
      response has expired.  The value MUST be cached for possible later
      use.

   ED-54:  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-55:  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.  The absence of such a Route header indicates that the
      endpoint was unable to invoke LoST, and the proxy MUST perform the
      LoST mapping and insert a Route header field with the URI
      obtained.







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   SP-26:  To deal with old user agents that predate this specification
      and with endpoints 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 URI.

   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 that 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-56/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-57/SP-30:  TLS is the primary mechanism used to secure the
      signaling for emergency calls.  IPsec [RFC4301] MAY be used
      instead of TLS for any hop.  Either TLS or IPsec MUST be used when
      attempting to signal an emergency call.

   ED-58/SP-31:  If TLS session establishment is not available or fails,
      the call MUST be retried without TLS.

   ED-59/SP-32:  Following the procedures described in [RFC5626] is
      RECOMMENDED to maintain persistent TLS connections between
      entities when one of the entities is an endpoint.  Persistent TLS
      connection between proxies is RECOMMENDED using any suitable
      mechanism.

   ED-60/AN-28:  TLS SHOULD be used when attempting to retrieve location
      (configuration or dereferencing) with HELD.  The use of the
      mechanism described in [RFC5077] is RECOMMENDED to minimize the
      time to establish TLS sessions without keeping server-side state.
      IPsec MAY be used instead of TLS.

   ED-61/AN-29:  When TLS session establishment fails, the location
      retrieval MUST be retried without TLS.





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9.2.  SIP Signaling Requirements for User Agents

   ED-62: 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 does not interpret local dial strings, the
        Request-URI MUST 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 does not interpret local dial strings, the To:
        MUST be a dial string URI with the dialed digits.

   3.   The From header field SHOULD contain the address of record (AoR)
        of the caller.

   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
        Globally Routable User Agent URI (GRUU) [RFC5627], and be valid
        for several minutes following the termination of the call,
        provided that the User Agent Client (UAC) remains registered
        with the same registrar, to permit an immediate callback to the
        specific device that placed the emergency call.  It is
        acceptable if the UAC inserts a locally routable URI and a
        subsequent back-to-back user agent (B2BUA) maps that to a
        globally routable URI.

   6.   Other header fields MAY be included as per normal SIP behavior.

   7.   If a geolocation URI is included in the INVITE, a Supported
        header field MUST be included with a 'geolocation-sip' or
        'geolocation-http" option tag, as appropriate [RFC6442].

   8.   If a device understands the SIP location conveyance [RFC6442]
        extension and has its location available, it MUST include
        location as either location-by-value or location-by-reference,
        or both, according to the rules within RFC 6442.

   9.   An SDP offer SHOULD be included in the INVITE.  If voice is
        supported, the offer SHOULD 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.



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   10.  If the device includes location-by-value, the user agent (UA)
        MUST support multipart message bodies, since SDP will likely be
        also in the INVITE.

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 are not allowed to insert bodies.

       *  Insert the Geolocation-Routing header with appropriate
          parameters.

       *  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 Route header, it MUST query a LoST server
       immediately.  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 that 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 that 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 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.






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   4.  Either a P-Asserted-Identity [RFC3325] or an Identity header
       field [RFC4474], or both, SHOULD be included to identify the
       sender.  For services that 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 either 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 specify
       otherwise.

   5.  Proxies SHOULD NOT return a 424 error.  They should process the
       INVITE as best they can.

   6.  Proxies SHOULD NOT obey a Geolocation-Routing value of "no" or a
       missing value if they must query LoST to obtain a route.
       Emergency calls are always routed by location.

10.  Callbacks

   ED-63/SP-34:  Devices SHOULD have a globally routable URI in a
      Contact header field that remains valid for several minutes past
      the time the original call containing the URI completes, unless
      the device registration expires and is not renewed.

   SP-35:  Callbacks to the Contact header URI received within
      30 minutes of an emergency call must reach the device regardless
      of call features (e.g., do not disturb) or services (e.g., call
      forwarding) 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 sometime after the
      original call.  Such a callback would not necessarily reach the
      device that originally placed the call.










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11.  Mid-Call Behavior

   ED-64/SP-37:  During the course of an emergency call, PSAPs and
      responders may need to transfer the call to some other entity.
      The request for such a transfer is signaled by a REFER request
      within the dialog with method=INVITE and a Refer-To header field
      [RFC3515].  Devices MUST react to such a transfer request with the
      appropriate INVITE.

12.  Call Termination

   ED-65:  Normal [RFC3261] procedures for termination MUST be used for
      termination of the call.

13.  Disabling of Features

   ED-66/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, but see ED-65 with respect to
   call waiting.  Also see ED-73 in Section 14.

   ED-67/SP-39:  The emergency dial strings SHOULD NOT be permitted in
      call forward numbers or speed dial lists.

   ED-68/SP-40:  The user agent and proxies MUST disable call features
      that would interfere with the ability of callbacks from the PSAP
      to be completed, such as:

   o  Do not disturb

   o  Call forward (all kinds)

   These features SHOULD be disabled for approximately 30 minutes
   following termination of an emergency call.






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   ED-69:  Callbacks SHOULD be determined by retaining the domain of the
      PSAP that answers an outgoing emergency call and instantiating a
      timer that starts when the call is terminated.  If a call is
      received from the same domain and within the timer period, and it
      is sent to the URI in a Contact header or the AoR used in the
      emergency call, then it should be assumed to be a callback.  The
      suggested timer period is 5 minutes.  The mechanism described in
      [RFC4916] can be used by the PSAP to inform the endpoint of the
      PSAP's domain.  Recognizing a callback from the domain of the PSAP
      will not always work, and further standardization will be required
      to give the endpoint the ability to recognize a callback.

14.  Media

   ED-70:  Endpoints MUST send and receive media streams on RTP
      [RFC3550].

   ED-71:  Normal SIP offer/answer [RFC3264] negotiations MUST be used
      to agree on the media streams to be used.

   ED-72/SP-41:  G.711 A-law (and mu-law if they are intended to be used
      in North America) encoded voice as described in [RFC3551] MUST be
      supported.  If the endpoint cannot support G.711, a transcoder
      MUST be used so that the offer received at the PSAP contains
      G.711.  It is desirable to include wideband codecs such as G.722
      and Adaptive Multi-Rate - WideBand (AMR-WB) in the offer.  PSAPs
      SHOULD support narrowband codecs common on endpoints in their area
      to avoid transcoding.

   ED-73:  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-74:  Endpoints supporting Instant Messaging (IM) MUST support
      either [RFC3428] or [RFC4975].

   ED-75:  Endpoints supporting real-time text MUST comply with
      [RFC4103].  The expectations for emergency service support for the
      real-time text medium are described in [RFC5194] Section 7.1.

   ED-76:  Endpoints supporting video MUST support H.264 per [RFC6184].









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15.  Testing

   ED-77:  INVITE requests to a service URN starting with "test."
      indicate a request for an automated test, for example,
      "urn:service:test.sos.fire".  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-78:  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 location-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 therefore
      location-hiding techniques, if they are being used, may be
      employed for this dereference.  Use of SIPS for the request would
      assure that the response containing the location is kept private.

   ED-79:  A PSAP accepting a test call SHOULD accept a media loopback
      [RFC6849] test and SHOULD support the "rtp-pkt-loopback" and
      "rtp-media-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-80:  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-81:  User agents MUST NOT place a test call immediately after
      booting.  If the IP address changes after booting, the endpoint
      should wait a random amount of time (in perhaps a 30-minute
      period, sufficient for any avalanche-restart event to complete)
      and then test.

   ED-82:  PSAPs MAY refuse repeated requests for test from the same
      device in a short period of time.  Any refusal is signaled with a
      486 (busy here) or 488 (not acceptable here) response.





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16.  Security Considerations

   Security considerations for emergency calling have been documented in
   [RFC5069] and [RFC6280].  This document suggests that security (TLS
   or IPsec) be used hop by hop on a SIP call to protect location
   information, identity, etc.  It also suggests that if the attempt to
   create a security association fails the call be retried without the
   security.  It's more important to get an emergency call through than
   to protect the data; indeed, in many jurisdictions privacy is
   explicitly waived when making emergency calls.  Placing a call
   without security may reveal user information, including location.
   The alternative -- failing the call if security cannot be established
   -- is considered unacceptable.

17.  IANA Considerations

   This document registers service URNs in the Service URN Labels
   registry per [RFC5031] for testing.

17.1.  Test Service URN

   A new entry in the URN Service Label registry has been added.  The
   new service is "test", the reference is this document, and the
   description is "self-test".



























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17.2.  'test' Subregistry

   A new subregistry has been created: 'test' Sub-Services.  The
   registration process is Expert Review per [RFC5226].  The expert
   review should consider that the entries in this registry nominally
   track the entries in the 'sos' subregistry, although it is not
   required that every entry in 'sos' have an entry in 'test', and it is
   possible that entries in the 'test' subregistry may not necessarily
   be in the 'sos' subregistry.  For example, testing of non-emergency
   URNs may be allowed.  The reference is this document.  The initial
   content of the subregistry is:

   Service                    Reference   Description
   ------------------------------------------------------------------
   test.sos                   RFC 6881    test for sos
   test.sos.ambulance         RFC 6881    test for sos.ambulance
   test.sos.animal-control    RFC 6881    test for sos.animal-control
   test.sos.fire              RFC 6881    test for sos.fire
   test.sos.gas               RFC 6881    test for sos.gas
   test.sos.marine            RFC 6881    test for sos.marine
   test.sos.mountain          RFC 6881    test for sos.mountain
   test.sos.physician         RFC 6881    test for sos.physician
   test.sos.poison            RFC 6881    test for sos.poison
   test.sos.police            RFC 6881    test for sos.police

18.  Acknowledgements

   Working group members participating in the creation and review of
   this document 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

   [LLDP-MED]  ANSI/TIA, "Link Layer Discovery Protocol - Media Endpoint
               Discovery", TIA Standard, TIA-1057, April 2006.

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

   [RFC3118]   Droms, R. and W. Arbaugh, "Authentication for DHCP
               Messages", RFC 3118, June 2001.







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   [RFC3261]   Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
               A., Peterson, J., Sparks, R., Handley, M., and E.
               Schooler, "SIP: Session Initiation Protocol", RFC 3261,
               June 2002.

   [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.

   [RFC3315]   Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
               and M. Carney, "Dynamic Host Configuration Protocol for
               IPv6 (DHCPv6)", RFC 3315, July 2003.

   [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.

   [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.

   [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.

   [RFC4301]   Kent, S. and K. Seo, "Security Architecture for the
               Internet Protocol", RFC 4301, December 2005.




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   [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.

   [RFC5226]   Narten, T. and H. Alvestrand, "Guidelines for Writing an
               IANA Considerations Section in RFCs", BCP 26, RFC 5226,
               May 2008.

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

   [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.




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   [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.

   [RFC5746]   Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
               "Transport Layer Security (TLS) Renegotiation Indication
               Extension", RFC 5746, February 2010.

   [RFC5751]   Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
               Mail Extensions (S/MIME) Version 3.2 Message
               Specification", RFC 5751, January 2010.

   [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.

   [RFC6184]   Wang, Y., Even, R., Kristensen, T., and R. Jesup, "RTP
               Payload Format for H.264 Video", RFC 6184, May 2011.

   [RFC6225]   Polk, J., Linsner, M., Thomson, M., and B. Aboba,
               "Dynamic Host Configuration Protocol Options for
               Coordinate-Based Location Configuration Information",
               RFC 6225, July 2011.

   [RFC6442]   Polk, J., Rosen, B., and J. Peterson, "Location
               Conveyance for the Session Initiation Protocol",
               RFC 6442, December 2011.

   [RFC6665]   Roach, A., "SIP-Specific Event Notification", RFC 6665,
               July 2012.

   [RFC6849]   Kaplan, H., Ed., Hedayat, K., Venna, N., Jones, P., and
               N. Stratton, "An Extension to the Session Description
               Protocol (SDP) and Real-time Transport Protocol (RTP) for
               Media Loopback", RFC 6849, February 2013.









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19.2.  Informative References

   [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.

   [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.

   [RFC6280]   Barnes, R., Lepinski, M., Cooper, A., Morris, J.,
               Tschofenig, H., and H. Schulzrinne, "An Architecture for
               Location and Location Privacy in Internet Applications",
               BCP 160, RFC 6280, July 2011.

   [RFC6443]   Rosen, B., Schulzrinne, H., Polk, J., and A. Newton,
               "Framework for Emergency Calling Using Internet
               Multimedia", RFC 6443, December 2011.



















Rosen & Polk              Best Current Practice                [Page 27]

RFC 6881                Emergency Call Phone BCP              March 2013


Authors' Addresses

   Brian Rosen
   NeuStar
   470 Conrad Dr.
   Mars, PA  16046
   USA

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


   James Polk
   Cisco Systems
   3913 Treemont Circle
   Colleyville, TX  76034
   USA

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































Rosen & Polk              Best Current Practice                [Page 28]


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