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RFC 7378

Informational
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Trustworthy Location

 


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Internet Engineering Task Force (IETF)                     H. Tschofenig
Request for Comments: 7378                                   Independent
Category: Informational                                   H. Schulzrinne
ISSN: 2070-1721                                      Columbia University
                                                           B. Aboba, Ed.
                                                   Microsoft Corporation
                                                           December 2014


                          Trustworthy Location

Abstract

   The trustworthiness of location information is critically important
   for some location-based applications, such as emergency calling or
   roadside assistance.

   This document describes threats to conveying location, particularly
   for emergency calls, and describes techniques that improve the
   reliability and security of location information.  It also provides
   guidelines for assessing the trustworthiness of location information.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see 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/rfc7378.

Page 2 
Copyright Notice

   Copyright (c) 2014 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.

Table of Contents

   1. Introduction ....................................................3
      1.1. Terminology ................................................3
      1.2. Emergency Services Architecture ............................5
   2. Threat Models ...................................................8
      2.1. Existing Work ..............................................8
      2.2. Adversary Model ............................................9
      2.3. Location Spoofing .........................................10
      2.4. Identity Spoofing .........................................11
   3. Mitigation Techniques ..........................................11
      3.1. Signed Location-by-Value ..................................12
      3.2. Location-by-Reference .....................................15
      3.3. Proxy-Added Location ......................................18
   4. Location Trust Assessment ......................................20
   5. Security Considerations ........................................23
   6. Privacy Considerations .........................................24
   7. Informative References .........................................26
   Acknowledgments ...................................................30
   Authors' Addresses ................................................30

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1.  Introduction

   Several public and commercial services need location information to
   operate.  This includes emergency services (such as fire, ambulance,
   and police) as well as commercial services such as food delivery and
   roadside assistance.

   For circuit-switched calls from landlines, as well as for Voice over
   IP (VoIP) services that only support emergency service calls from
   stationary Devices, location provided to the Public Safety Answering
   Point (PSAP) is determined from a lookup using the calling telephone
   number.  As a result, for landlines or stationary VoIP, spoofing of
   caller identification can result in the PSAP incorrectly determining
   the caller's location.  Problems relating to calling party number and
   Caller ID assurance have been analyzed by the Secure Telephone
   Identity Revisited [STIR] working group as described in "Secure
   Telephone Identity Problem Statement and Requirements" [RFC7340].  In
   addition to the work underway in STIR, other mechanisms exist for
   validating caller identification.  For example, as noted in [EENA],
   one mechanism for validating caller identification information (as
   well as the existence of an emergency) is for the PSAP to call the
   user back, as described in [RFC7090].

   Given the existing work on caller identification, this document
   focuses on the additional threats that are introduced by the support
   of IP-based emergency services in nomadic and mobile Devices, in
   which location may be conveyed to the PSAP within the emergency call.
   Ideally, a call taker at a PSAP should be able to assess, in real
   time, the level of trust that can be placed on the information
   provided within a call.  This includes automated location conveyed
   along with the call and location information communicated by the
   caller, as well as identity information relating to the caller or the
   Device initiating the call.  Where real-time assessment is not
   possible, it is important to be able to determine the source of the
   call in a post-incident investigation, so as to be able to enforce
   accountability.

   This document defines terminology (including the meaning of
   "trustworthy location") in Section 1.1, reviews existing work in
   Section 1.2, describes threat models in Section 2, outlines potential
   mitigation techniques in Section 3, covers trust assessment in
   Section 4, and discusses security considerations in Section 5.

1.1.  Terminology

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

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   We use the definitions of "Internet Access Provider (IAP)", "Internet
   Service Provider (ISP)", and "Voice Service Provider (VSP)" found in
   "Requirements for Emergency Context Resolution with Internet
   Technologies" [RFC5012].

   [EENA] defines a "hoax call" as follows: "A false or malicious call
   is when a person deliberately telephones the emergency services and
   tells them there is an emergency when there is not."

   The definitions of "Device", "Target", and "Location Information
   Server" (LIS) are taken from "An Architecture for Location and
   Location Privacy in Internet Applications" [RFC6280], Section 7.

   The term "Device" denotes the physical device, such as a mobile
   phone, PC, or embedded microcontroller, whose location is tracked as
   a proxy for the location of a Target.

   The term "Target" denotes an individual or other entity whose
   location is sought in the Geopriv architecture [RFC6280].  In many
   cases, the Target will be the human user of a Device, or it may be an
   object such as a vehicle or shipping container to which a Device is
   attached.  In some instances, the Target will be the Device itself.
   The Target is the entity whose privacy the architecture described in
   [RFC6280] seeks to protect.

   The term "Location Information Server" denotes an entity responsible
   for providing Devices within an access network with information about
   their own locations.  A Location Information Server uses knowledge of
   the access network and its physical topology to generate and
   distribute location information to Devices.

   The term "location determination method" refers to the mechanism used
   to determine the location of a Target.  This may be something
   employed by a LIS or by the Target itself.  It specifically does not
   refer to the location configuration protocol (LCP) used to deliver
   location information to either the Target or the Recipient.  This
   term is reused from "GEOPRIV Presence Information Data Format
   Location Object (PIDF-LO) Usage Clarification, Considerations, and
   Recommendations" [RFC5491].

   The term "source" is used to refer to the LIS, node, or Device from
   which a Recipient (Target or third party) obtains location
   information.

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   Additionally, the terms "location-by-value" (LbyV), "location-by-
   reference" (LbyR), "Location Configuration Protocol", "Location
   Dereference Protocol", and "Location Uniform Resource Identifier"
   (URI) are reused from "Requirements for a Location-by-Reference
   Mechanism" [RFC5808].

   "Trustworthy Location" is defined as location information that can be
   attributed to a trusted source, has been protected against
   modification in transmit, and has been assessed as trustworthy.

   "Location Trust Assessment" refers to the process by which the
   reliability of location information can be assessed.  This topic is
   discussed in Section 4.

   "Identity Spoofing" occurs when the attacker forges or obscures their
   identity so as to prevent themselves from being identified as the
   source of the attack.  One class of identity spoofing attack involves
   the forging of call origin identification.

   The following additional terms apply to location spoofing
   (Section 2.3):

   With "Place Shifting", attackers construct a Presence Information
   Data Format Location Object (PIDF-LO) for a location other than where
   they are currently located.  In some cases, place shifting can be
   limited in range (e.g., within the coverage area of a particular cell
   tower).

   "Time Shifting" occurs when the attacker uses or reuses location
   information that was valid in the past but is no longer valid because
   the attacker has moved.

   "Location Theft" occurs when the attacker captures a Target's
   location information (possibly including a signature) and presents it
   as their own.  Location theft can occur in a single instance or may
   be continuous (e.g., where the attacker has gained control over the
   victim's Device).  Location theft may also be combined with time
   shifting to present someone else's location information after the
   original Target has moved.

1.2.  Emergency Services Architecture

   This section describes how location is utilized in the Internet
   Emergency Services Architecture, as well as the existing work on the
   problem of hoax calls.

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1.2.1.  Location

   The Internet architecture for emergency calling is described in
   "Framework for Emergency Calling Using Internet Multimedia"
   [RFC6443].  Best practices for utilizing the architecture to make
   emergency calls are described in "Best Current Practice for
   Communications Services in Support of Emergency Calling" [RFC6881].

   As noted in "An Architecture for Location and Location Privacy in
   Internet Applications" [RFC6280], Section 6.3:

      there are three critical steps in the placement of an emergency
      call, each involving location information:

      1. Determine the location of the caller.

      2. Determine the proper Public Safety Answering Point (PSAP) for
         the caller's location.

      3. Send a SIP INVITE message, including the caller's location, to
         the PSAP.

   The conveyance of location information within the Session Initiation
   Protocol (SIP) is described in "Location Conveyance for the Session
   Initiation Protocol" [RFC6442].  Conveyance of location-by-value
   (LbyV) as well as conveyance of location-by-reference (LbyR) are
   supported.  Section 7 of [RFC6442] ("Security Considerations")
   discusses privacy, authentication, and integrity concerns relating to
   conveyed location.  This includes discussion of transmission-layer
   security for confidentiality and integrity protection of SIP, as well
   as (undeployed) end-to-end security mechanisms for protection of
   location information (e.g., S/MIME).  Regardless of whether
   transmission-layer security is utilized, location information may be
   available for inspection by an intermediary that -- if it decides
   that the location value is unacceptable or insufficiently accurate --
   may send an error indication or replace the location, as described in
   [RFC6442], Section 3.4.

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   Although the infrastructure for location-based routing described in
   [RFC6443] was developed for use in emergency services, [RFC6442]
   supports conveyance of location within non-emergency calls as well as
   emergency calls.  Section 1 of "Implications of 'retransmission-
   allowed' for SIP Location Conveyance" [RFC5606] describes the overall
   architecture, as well as non-emergency usage scenarios (note: the
   [LOC-CONVEY] citation in the quote below refers to the document later
   published as [RFC6442]):

      The Presence Information Data Format for Location Objects (PIDF-LO
      [RFC4119]) carries both location information (LI) and policy
      information set by the Rule Maker, as is stipulated in [RFC3693].
      The policy carried along with LI allows the Rule Maker to
      restrict, among other things, the duration for which LI will be
      retained by recipients and the redistribution of LI by recipients.

      The Session Initiation Protocol [RFC3261] is one proposed Using
      Protocol for PIDF-LO.  The conveyance of PIDF-LO within SIP is
      specified in [LOC-CONVEY].  The common motivation for providing LI
      in SIP is to allow location to be considered in routing the SIP
      message.  One example use case would be emergency services, in
      which the location will be used by dispatchers to direct the
      response.  Another use case might be providing location to be used
      by services associated with the SIP session; a location associated
      with a call to a taxi service, for example, might be used to route
      to a local franchisee of a national service and also to route the
      taxi to pick up the caller.

1.2.2.  Hoax Calls

   Hoax calls have been a problem for emergency services dating back to
   the time of street corner call boxes.  As the European Emergency
   Number Association (EENA) has noted [EENA]:

      False emergency calls divert emergency services away from people
      who may be in life-threatening situations and who need urgent
      help.  This can mean the difference between life and death for
      someone in trouble.

   EENA [EENA] has attempted to define terminology and describe best
   current practices for dealing with false emergency calls.  Reducing
   the number of hoax calls represents a challenge, since emergency
   services authorities in most countries are required to answer every
   call (whenever possible).  Where the caller cannot be identified, the
   ability to prosecute is limited.

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   A particularly dangerous form of hoax call is "swatting" -- a hoax
   emergency call that draws a response from law enforcement prepared
   for a violent confrontation (e.g., a fake hostage situation that
   results in the dispatching of a "Special Weapons And Tactics" (SWAT)
   team).  In 2008, the Federal Bureau of Investigation (FBI) issued a
   warning [Swatting] about an increase in the frequency and
   sophistication of these attacks.

   Many documented cases of "swatting" (also sometimes referred to as
   "SWATing") involve not only the faking of an emergency but also
   falsification or obfuscation of identity [Swatting] [SWATing].  There
   are a number of techniques by which hoax callers attempt to avoid
   identification, and in general, the ability to identify the caller
   appears to influence the incidence of hoax calls.

   Where a Voice Service Provider allows the caller to configure its
   outbound caller identification without checking it against the
   authenticated identity, forging caller identification is trivial.
   Similarly, where an attacker can gain entry to a Private Branch
   Exchange (PBX), they can then subsequently use that access to launch
   a denial-of-service attack against the PSAP or make fraudulent
   emergency calls.  Where emergency calls have been allowed from
   handsets lacking a subscriber identification module (SIM) card,
   so-called non-service initialized (NSI) handsets, or where ownership
   of the SIM card cannot be determined, the frequency of hoax calls has
   often been unacceptably high [TASMANIA] [UK] [SA].

   However, there are few documented cases of hoax calls that have
   arisen from conveyance of untrustworthy location information within
   an emergency call, which is the focus of this document.

2.  Threat Models

   This section reviews existing analyses of the security of emergency
   services, threats to geographic location privacy, threats relating to
   spoofing of caller identification, and threats related to
   modification of location information in transit.  In addition, the
   threat model applying to this work is described.

2.1.  Existing Work

   "An Architecture for Location and Location Privacy in Internet
   Applications" [RFC6280] describes an architecture for privacy-
   preserving location-based services in the Internet, focusing on
   authorization, security, and privacy requirements for the data
   formats and protocols used by these services.

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   In Section 5 of [RFC6280] ("An Architecture for Location and Location
   Privacy in Internet Applications"), mechanisms for ensuring the
   security of the location distribution chain are discussed; these
   include mechanisms for hop-by-hop confidentiality and integrity
   protection as well as end-to-end assurance.

   "Geopriv Requirements" [RFC3693] focuses on the authorization,
   security, and privacy requirements of location-dependent services,
   including emergency services.  Section 8 of [RFC3693] includes
   discussion of emergency services authentication (Section 8.3), and
   issues relating to identity and anonymity (Section 8.4).

   "Threat Analysis of the Geopriv Protocol" [RFC3694] describes threats
   against geographic location privacy, including protocol threats,
   threats resulting from the storage of geographic location data, and
   threats posed by the abuse of information.

   "Security Threats and Requirements for Emergency Call Marking and
   Mapping" [RFC5069] reviews security threats associated with the
   marking of signaling messages and the process of mapping locations to
   Universal Resource Identifiers (URIs) that point to PSAPs.  RFC 5069
   describes attacks on the emergency services system, such as
   attempting to deny system services to all users in a given area, to
   gain fraudulent use of services and to divert emergency calls to
   non-emergency sites.  In addition, it describes attacks against
   individuals, including attempts to prevent an individual from
   receiving aid, or to gain information about an emergency, as well as
   attacks on emergency services infrastructure elements, such as
   mapping discovery and mapping servers.

   "Secure Telephone Identity Threat Model" [RFC7375] analyzes threats
   relating to impersonation and obscuring of calling party numbers,
   reviewing the capabilities available to attackers, and the scenarios
   in which attacks are launched.

2.2.  Adversary Model

   To provide a structured analysis, we distinguish between three
   adversary models:

   External adversary model:  The end host, e.g., an emergency caller
      whose location is going to be communicated, is honest, and the
      adversary may be located between the end host and the location
      server or between the end host and the PSAP.  None of the
      emergency service infrastructure elements act maliciously.

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   Malicious infrastructure adversary model:  The emergency call routing
      elements, such as the Location Information Server (LIS), the
      Location-to-Service Translation (LoST) infrastructure (which is
      used for mapping locations to PSAP addresses), or call routing
      elements, may act maliciously.

   Malicious end host adversary model:  The end host itself acts
      maliciously, whether the owner is aware of this or the end host is
      acting under the control of a third party.

   Since previous work describes attacks against infrastructure elements
   (e.g., location servers, call route servers, mapping servers) or the
   emergency services IP network, as well as threats from attackers
   attempting to snoop location in transit, this document focuses on the
   threats arising from end hosts providing false location information
   within emergency calls (the malicious end host adversary model).

   Since the focus is on malicious hosts, we do not cover threats that
   may arise from attacks on infrastructure that hosts depend on to
   obtain location.  For example, end hosts may obtain location from
   civilian GPS, which is vulnerable to spoofing [GPSCounter], or from
   third-party Location Service Providers (LSPs) that may be vulnerable
   to attack or may not provide location accuracy suitable for emergency
   purposes.

   Also, we do not cover threats arising from inadequate location
   infrastructure.  For example, the LIS or end host could base its
   location determination on a stale wiremap or an inaccurate access
   point location database, leading to an inaccurate location estimate.
   Similarly, a Voice Service Provider (VSP) (and, indirectly, a LIS)
   could utilize the wrong identity (such as an IP address) for location
   lookup, thereby providing the end host with misleading location
   information.

2.3.  Location Spoofing

   Where location is attached to the emergency call by an end host, the
   end host can fabricate a PIDF-LO and convey it within an emergency
   call.  The following represent examples of location spoofing:

   Place shifting:  Mallory, the adversary, pretends to be at an
                    arbitrary location.

   Time shifting:   Mallory pretends to be at a location where she was
                    a while ago.

   Location theft:  Mallory observes or obtains Alice's location and
                    replays it as her own.

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2.4.  Identity Spoofing

   While this document does not focus on the problems created by
   determination of location based on spoofed caller identification, the
   ability to ascertain identity is important, since the threat of
   punishment reduces hoax calls.  As an example, calls from pay phones
   are subject to greater scrutiny by the call taker.

   With calls originating on an IP network, at least two forms of
   identity are relevant, with the distinction created by the split
   between the IAP and the VSP:

   (a) network access identity such as might be determined via
       authentication (e.g., using the Extensible Authentication
       Protocol (EAP) [RFC3748]);

   (b) caller identity, such as might be determined from authentication
       of the emergency caller at the VoIP application layer.

   If the adversary did not authenticate itself to the VSP, then
   accountability may depend on verification of the network access
   identity.  However, the network access identity may also not have
   been authenticated, such as in the case where an open IEEE 802.11
   Access Point is used to initiate a hoax emergency call.  Although
   endpoint information such as the IP address or Media Access Control
   (MAC) address may have been logged, tying this back to the Device
   owner may be challenging.

   Unlike the existing telephone system, VoIP emergency calls can
   provide an identity that need not necessarily be coupled to a
   business relationship with the IAP, ISP, or VSP.  However, due to the
   time-critical nature of emergency calls, multi-layer authentication
   is undesirable.  Thus, in most cases, only the Device placing the
   call will be able to be identified.  Furthermore, deploying
   additional credentials for emergency service purposes (such as
   certificates) increases costs, introduces a significant
   administrative overhead, and is only useful if widely deployed.

3.  Mitigation Techniques

   The sections that follow present three mechanisms for mitigating the
   threats presented in Section 2:

   1. Signed location-by-value (Section 3.1), which provides for
      authentication and integrity protection of the PIDF-LO.  There is
      only an expired straw-man proposal for this mechanism
      [Loc-Dependability]; thus, as of the time of this writing this
      mechanism is not suitable for deployment.

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   2. Location-by-reference (Section 3.2), which enables location to be
      obtained by the PSAP directly from the location server, over a
      confidential and integrity-protected channel, avoiding
      modification by the end host or an intermediary.  This mechanism
      is specified in [RFC6753].

   3. Proxy-added location (Section 3.3), which protects against
      location forgery by the end host.  This mechanism is specified in
      [RFC6442].

3.1.  Signed Location-by-Value

   With location signing, a location server signs the location
   information before it is sent to the Target.  The signed location
   information is then sent to the Location Recipient, who verifies it.

   Figure 1 shows the communication model with the Target requesting
   signed location in step (a); the location server returns it in
   step (b), and it is then conveyed to the Location Recipient, who
   verifies it (step (c)).  For SIP, the procedures described in
   "Location Conveyance for the Session Initiation Protocol" [RFC6442]
   are applicable for location conveyance.

                   +-----------+               +-----------+
                   |           |               | Location  |
                   |    LIS    |               | Recipient |
                   |           |               |           |
                   +-+-------+-+               +----+------+
                     ^       |                    --^
                     |       |                  --
       Geopriv       |Req.   |                --
       Location      |Signed |Signed        -- Protocol Conveying
       Configuration |Loc.   |Loc.        --   Location (e.g., SIP)
       Protocol      |(a)    |(b)       --     (c)
                     |       v        --
                   +-+-------+-+    --
                   | Target /  |  --
                   | End Host  +
                   |           |
                   +-----------+

                        Figure 1: Location Signing

   A straw-man proposal for location signing is provided in "Digital
   Signature Methods for Location Dependability" [Loc-Dependability].
   Note that since [Loc-Dependability] is no longer under development,
   location signing cannot be considered deployable at the time of this
   writing.

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   In order to limit replay attacks, that proposal calls for the
   addition of a "validity" element to the PIDF-LO, including a "from"
   sub-element containing the time that location information was
   validated by the signer, as well as an "until" sub-element containing
   the last time that the signature can be considered valid.

   One of the consequences of including an "until" element is that even
   a stationary Target would need to periodically obtain a fresh
   PIDF-LO, or incur the additional delay of querying during an
   emergency call.

   Although privacy-preserving procedures may be disabled for emergency
   calls, by design, PIDF-LO objects limit the information available for
   real-time attribution.  As noted in [RFC5985], Section 6.6:

      The LIS MUST NOT include any means of identifying the Device in
      the PIDF-LO unless it is able to verify that the identifier is
      correct and inclusion of identity is expressly permitted by a Rule
      Maker.  Therefore, PIDF parameters that contain identity are
      either omitted or contain unlinked pseudonyms [RFC3693].  A
      unique, unlinked presentity URI SHOULD be generated by the LIS for
      the mandatory presence "entity" attribute of the PIDF document.

      Optional parameters such as the "contact" and "deviceID" elements
      [RFC4479] are not used.

   Also, the Device referred to in the PIDF-LO may not necessarily be
   the same entity conveying the PIDF-LO to the PSAP.  As noted in
   [RFC6442], Section 1:

      In no way does this document assume that the SIP user agent client
      that sends a request containing a location object is necessarily
      the Target.  The location of a Target conveyed within SIP
      typically corresponds to that of a Device controlled by the
      Target, for example, a mobile phone, but such Devices can be
      separated from their owners, and moreover, in some cases, the user
      agent may not know its own location.

   Without the ability to tie the Target identity to the identity
   asserted in the SIP message, it is possible for an attacker to cut
   and paste a PIDF-LO obtained by a different Device or user into a SIP
   INVITE and send this to the PSAP.  This cut-and-paste attack could
   succeed even when a PIDF-LO is signed or when [RFC4474] is
   implemented.

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   To address location-spoofing attacks, [Loc-Dependability] proposes
   the addition of an "identity" element that could include a SIP URI
   (enabling comparison against the identity asserted in the SIP
   headers) or an X.509v3 certificate.  If the Target was authenticated
   by the LIS, an "authenticated" attribute is added.  However, because
   the inclusion of an "identity" element could enable location
   tracking, a "hash" element is also proposed that could instead
   contain a hash of the content of the "identity" element.  In
   practice, such a hash would not be much better for real-time
   validation than a pseudonym.

   Location signing cannot deter attacks in which valid location
   information is provided.  For example, an attacker in control of
   compromised hosts could launch a denial-of-service attack on the PSAP
   by initiating a large number of emergency calls, each containing
   valid signed location information.  Since the work required to verify
   the location signature is considerable, this could overwhelm the PSAP
   infrastructure.

   However, while DDoS attacks are unlikely to be deterred by location
   signing, accurate location information would limit the subset of
   compromised hosts that could be used for an attack, as only hosts
   within the PSAP serving area would be useful in placing emergency
   calls.

   Location signing is also difficult when the host obtains location via
   mechanisms such as GPS, unless trusted computing approaches, with
   tamper-proof GPS modules, can be applied.  Otherwise, an end host can
   pretend to have GPS, and the Recipient will need to rely on its
   ability to assess the level of trust that should be placed in the end
   host location claim.

   Even though location-signing mechanisms have not been standardized,
   [NENA-i2], Section 4.7 includes operational recommendations relating
   to location signing:

      Location configuration and conveyance requirements are described
      in NENA 08-752[27], but guidance is offered here on what should be
      considered when designing mechanisms to report location:

      1. The location object should be digitally signed.

      2. The certificate for the signer (LIS operator) should be rooted
         in VESA.  For this purpose, VPC and ERDB operators should issue
         certificates to LIS operators.

      3. The signature should include a timestamp.

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      4. Where possible, the Location Object should be refreshed
         periodically, with the signature (and thus the timestamp) being
         refreshed as a consequence.

      5. Antispoofing mechanisms should be applied to the Location
         Reporting method.

   (Note: The term "Valid Emergency Services Authority" (VESA) refers to
   the root certificate authority.  "VPC" stands for VoIP Positioning
   Center, and "ERDB" stands for the Emergency Service Zone Routing
   Database.)

   As noted above, signing of location objects implies the development
   of a trust hierarchy that would enable a certificate chain provided
   by the LIS operator to be verified by the PSAP.  Rooting the trust
   hierarchy in the VESA can be accomplished either by having the VESA
   directly sign the LIS certificates or by the creation of intermediate
   Certificate Authorities (CAs) certified by the VESA, which will then
   issue certificates to the LIS.  In terms of the workload imposed on
   the VESA, the latter approach is highly preferable.  However, this
   raises the question of who would operate the intermediate CAs and
   what the expectations would be.

   In particular, the question arises as to the requirements for LIS
   certificate issuance, and how they would compare to requirements for
   issuance of other certificates such as a Secure Socket
   Layer/Transport Layer Security (SSL/TLS) web certificate.

3.2.  Location-by-Reference

   Location-by-reference was developed so that end hosts can avoid
   having to periodically query the location server for up-to-date
   location information in a mobile environment.  Additionally, if
   operators do not want to disclose location information to the end
   host without charging them, location-by-reference provides a
   reasonable alternative.  Also, since location-by-reference enables
   the PSAP to directly contact the location server, it avoids potential
   attacks by intermediaries.

   As noted in "A Location Dereference Protocol Using HTTP-Enabled
   Location Delivery (HELD)" [RFC6753], a location reference can be
   obtained via HELD [RFC5985].  In addition, "Location Configuration
   Extensions for Policy Management" [RFC7199] extends location
   configuration protocols such as HELD to provide hosts with a
   reference to the rules that apply to a location-by-reference so that
   the host can view or set these rules.

Top      ToC       Page 16 
   Figure 2 shows the communication model with the Target requesting a
   location reference in step (a); the location server returns the
   reference and, potentially, the policy in step (b), and it is then
   conveyed to the Location Recipient in step (c).  The Location
   Recipient needs to resolve the reference with a request in step (d).
   Finally, location information is returned to the Location Recipient
   afterwards.  For location conveyance in SIP, the procedures described
   in [RFC6442] are applicable.

                   +-----------+  Geopriv      +-----------+
                   |           |  Location     | Location  |
                   |    LIS    +<------------->+ Recipient |
                   |           | Dereferencing |           |
                   +-+-------+-+ Protocol (d)  +----+------+
                     ^       |                    --^
                     |       |                  --
       Geopriv       |Req.   |LbyR +          --
       Location      |LbyR   |Policy        -- Protocol Conveying
       Configuration |(a)    |(b)         --   Location (e.g., SIP)
       Protocol      |       |          --     (c)
                     |       V        --
                   +-+-------+-+    --
                   | Target /  |  --
                   | End Host  +
                   |           |
                   +-----------+

                      Figure 2: Location-by-Reference

   Where location-by-reference is provided, the Recipient needs to
   dereference the LbyR in order to obtain location.  The details for
   the dereferencing operations vary with the type of reference, such as
   an HTTP, HTTPS, SIP, secure SIP (SIPS), or SIP Presence URI.

   For location-by-reference, the location server needs to maintain one
   or several URIs for each Target, timing out these URIs after a
   certain amount of time.  References need to expire to prevent the
   Recipient of such a Uniform Resource Locator (URL) from being able to
   permanently track a host and to offer garbage collection
   functionality for the location server.

   Off-path adversaries must be prevented from obtaining the Target's
   location.  The reference contains a randomized component that
   prevents third parties from guessing it.  When the Location Recipient
   fetches up-to-date location information from the location server, it
   can also be assured that the location information is fresh and not
   replayed.  However, this does not address location theft.

Top      ToC       Page 17 
   With respect to the security of the dereference operation, [RFC6753],
   Section 6 states:

      TLS MUST be used for dereferencing location URIs unless
      confidentiality and integrity are provided by some other
      mechanism, as discussed in Section 3.  Location Recipients MUST
      authenticate the host identity using the domain name included in
      the location URI, using the procedure described in Section 3.1 of
      [RFC2818].  Local policy determines what a Location Recipient does
      if authentication fails or cannot be attempted.

      The authorization by possession model (Section 4.1) further relies
      on TLS when transmitting the location URI to protect the secrecy
      of the URI.  Possession of such a URI implies the same privacy
      considerations as possession of the PIDF-LO document that the URI
      references.

      Location URIs MUST only be disclosed to authorized Location
      Recipients.  The GEOPRIV architecture [RFC6280] designates the
      Rule Maker to authorize disclosure of the URI.

      Protection of the location URI is necessary, since the policy
      attached to such a location URI permits anyone who has the URI to
      view the associated location information.  This aspect of security
      is covered in more detail in the specification of location
      conveyance protocols, such as [RFC6442].

   For authorizing access to location-by-reference, two authorization
   models were developed: "Authorization by Possession" and
   "Authorization via Access Control Lists".  With respect to
   "Authorization by Possession", [RFC6753], Section 4.1 notes:

      In this model, possession -- or knowledge -- of the location URI
      is used to control access to location information.  A location URI
      might be constructed such that it is hard to guess (see C8 of
      [RFC5808]), and the set of entities that it is disclosed to can be
      limited.  The only authentication this would require by the LS is
      evidence of possession of the URI.  The LS could immediately
      authorize any request that indicates this URI.

      Authorization by possession does not require direct interaction
      with a Rule Maker; it is assumed that the Rule Maker is able to
      exert control over the distribution of the location URI.
      Therefore, the LIS can operate with limited policy input from a
      Rule Maker.

Top      ToC       Page 18 
      Limited disclosure is an important aspect of this authorization
      model.  The location URI is a secret; therefore, ensuring that
      adversaries are not able to acquire this information is paramount.
      Encryption, such as might be offered by TLS [RFC5246] or S/MIME
      [RFC5751], protects the information from eavesdroppers.

      ...

      Using possession as a basis for authorization means that, once
      granted, authorization cannot be easily revoked.  Cancellation of
      a location URI ensures that legitimate users are also affected;
      application of additional policy is theoretically possible but
      could be technically infeasible.  Expiration of location URIs
      limits the usable time for a location URI, requiring that an
      attacker continue to learn new location URIs to retain access to
      current location information.

   In situations where "Authorization by Possession" is not suitable
   (such as where location hiding [RFC6444] is required), the
   "Authorization via Access Control Lists" model may be preferred.

   Without the introduction of a hierarchy, it would be necessary for
   the PSAP to obtain credentials, such as certificates or shared
   symmetric keys, for all the LISs in its coverage area, to enable it
   to successfully dereference LbyRs.  In situations with more than a
   few LISs per PSAP, this would present operational challenges.

   A certificate hierarchy providing PSAPs with client certificates
   chaining to the VESA could be used to enable the LIS to authenticate
   and authorize PSAPs for dereferencing.  Note that unlike PIDF-LO
   signing (which mitigates modification of PIDF-LOs), this merely
   provides the PSAP with access to a (potentially unsigned) PIDF-LO,
   albeit over a protected TLS channel.

   Another approach would be for the local LIS to upload location
   information to a location aggregation point who would in turn manage
   the relationships with the PSAP.  This would shift the management
   burden from the PSAPs to the location aggregation points.

3.3.  Proxy-Added Location

   Instead of relying upon the end host to provide location, is possible
   for a proxy that has the ability to determine the location of the end
   point (e.g., based on the end host IP or MAC address) to retrieve and
   add or override location information.  This requires deployment of
   application-layer entities by ISPs, unlike the two other techniques.
   The proxies could be used for emergency or non-emergency
   communications, or both.

Top      ToC       Page 19 
   The use of proxy-added location is primarily applicable in scenarios
   where the end host does not provide location.  As noted in [RFC6442],
   Section 4.1:

      A SIP intermediary SHOULD NOT add location to a SIP request that
      already contains location.  This will quite often lead to
      confusion within LRs.  However, if a SIP intermediary adds
      location, even if location was not previously present in a SIP
      request, that SIP intermediary is fully responsible for addressing
      the concerns of any 424 (Bad Location Information) SIP response it
      receives about this location addition and MUST NOT pass on
      (upstream) the 424 response.  A SIP intermediary that adds a
      locationValue MUST position the new locationValue as the last
      locationValue within the Geolocation header field of the SIP
      request.

      ...

      A SIP intermediary MAY add a Geolocation header field if one is
      not present -- for example, when a user agent does not support the
      Geolocation mechanism but their outbound proxy does and knows the
      Target's location, or any of a number of other use cases (see
      Section 3).

   As noted in [RFC6442], Section 3.3:

      This document takes a "you break it, you bought it" approach to
      dealing with second locations placed into a SIP request by an
      intermediary entity.  That entity becomes completely responsible
      for all location within that SIP request (more on this in
      Section 4).

   While it is possible for the proxy to override location included by
   the end host, [RFC6442], Section 3.4 notes the operational
   limitations:

      Overriding location information provided by the user requires a
      deployment where an intermediary necessarily knows better than an
      end user -- after all, it could be that Alice has an on-board GPS,
      and the SIP intermediary only knows her nearest cell tower.  Which
      is more accurate location information?  Currently, there is no way
      to tell which entity is more accurate or which is wrong, for that
      matter.  This document will not specify how to indicate which
      location is more accurate than another.

Top      ToC       Page 20 
   The disadvantage of this approach is the need to deploy application-
   layer entities, such as SIP proxies, at IAPs or associated with IAPs.
   This requires that a standardized VoIP profile be deployed at every
   end Device and at every IAP.  This might impose interoperability
   challenges.

   Additionally, the IAP needs to take responsibility for emergency
   calls, even for customers with whom they have no direct or indirect
   relationship.  To provide identity information about the emergency
   caller from the VSP, it would be necessary to let the IAP and the VSP
   interact for authentication (see, for example, "Diameter Session
   Initiation Protocol (SIP) Application" [RFC4740]).  This interaction
   along the Authentication, Authorization, and Accounting
   infrastructure is often based on business relationships between the
   involved entities.  An arbitrary IAP and VSP are unlikely to have a
   business relationship.  If the interaction between the IAP and the
   VSP fails due to the lack of a business relationship, then typically
   a fall-back would be provided where no emergency caller identity
   information is made available to the PSAP and the emergency call
   still has to be completed.

4.  Location Trust Assessment

   The ability to assess the level of trustworthiness of conveyed
   location information is important, since this makes it possible to
   understand how much value should be placed on location information as
   part of the decision-making process.  As an example, if automated
   location information is understood to be highly suspect or is absent,
   a call taker can put more effort into verifying the authenticity of
   the call and obtaining location information from the caller.

   Location trust assessment has value, regardless of whether the
   location itself is authenticated (e.g., signed location) or is
   obtained directly from the location server (e.g., location-by-
   reference) over security transport, since these mechanisms do not
   provide assurance of the validity or provenance of location data.

   To prevent location-theft attacks, the "entity" element of the
   PIDF-LO is of limited value if an unlinked pseudonym is provided in
   this field.  However, if the LIS authenticates the Target, then the
   linkage between the pseudonym and the Target identity can be
   recovered in a post-incident investigation.

Top      ToC       Page 21 
   As noted in [Loc-Dependability], if the location object was signed,
   the Location Recipient has additional information on which to base
   their trust assessment, such as the validity of the signature, the
   identity of the Target, the identity of the LIS, whether the LIS
   authenticated the Target, and the identifier included in the "entity"
   field.

   Caller accountability is also an important aspect of trust
   assessment.  Can the individual purchasing the Device or activating
   service be identified, or did the call originate from a non-service
   initialized (NSI) Device whose owner cannot be determined?  Prior to
   the call, was the caller authenticated at the network or application
   layer?  In the event of a hoax call, can audit logs be made available
   to an investigator, or can information relating to the owner of an
   unlinked pseudonym be provided, enabling investigators to unravel the
   chain of events that led to the attack?

   In practice, the source of the location data is important for
   location trust assessment.  For example, location provided by a
   Location Information Server (LIS) whose administrator has an
   established history of meeting emergency location accuracy
   requirements (e.g., United States Phase II E-911 location accuracy)
   may be considered more reliable than location information provided by
   a third-party Location Service Provider (LSP) that disclaims use of
   location information for emergency purposes.

   However, even where an LSP does not attempt to meet the accuracy
   requirements for emergency location, it still may be able to provide
   information useful in assessing how reliable location information is
   likely to be.  For example, was location determined based on the
   nearest cell tower or 802.11 Access Point (AP), or was a
   triangulation method used?  If based on cell tower or AP location
   data, was the information obtained from an authoritative source
   (e.g., the tower or AP owner), and when was the last time that the
   location of the tower or access point was verified?

   For real-time validation, information in the signaling and media
   packets can be cross-checked against location information.  For
   example, it may be possible to determine the city, state, country, or
   continent associated with the IP address included within SIP Via or
   Contact header fields, or the media source address, and compare this
   against the location information reported by the caller or conveyed
   in the PIDF-LO.  However, in some situations, only entities close to
   the caller may be able to verify the correctness of location
   information.

Top      ToC       Page 22 
   Real-time validation of the timestamp contained within PIDF-LO
   objects (reflecting the time at which the location was determined) is
   also challenging.  To address time-shifting attacks, the "timestamp"
   element of the PIDF-LO, defined in [RFC3863], can be examined and
   compared against timestamps included within the enclosing SIP
   message, to determine whether the location data is sufficiently
   fresh.  However, the timestamp only represents an assertion by the
   LIS, which may or may not be trustworthy.  For example, the Recipient
   of the signed PIDF-LO may not know whether the LIS supports time
   synchronization, or whether it is possible to reset the LIS clock
   manually without detection.  Even if the timestamp was valid at the
   time location was determined, a time period may elapse between when
   the PIDF-LO was provided and when it is conveyed to the Recipient.
   Periodically refreshing location information to renew the timestamp
   even though the location information itself is unchanged puts
   additional load on LISs.  As a result, Recipients need to validate
   the timestamp in order to determine whether it is credible.

   While this document focuses on the discussion of real-time
   determination of suspicious emergency calls, the use of audit logs
   may help in enforcing accountability among emergency callers.  For
   example, in the event of a hoax call, information relating to the
   owner of the unlinked pseudonym could be provided to investigators,
   enabling them to unravel the chain of events that led to the attack.
   However, while auditability is an important deterrent, it is likely
   to be of most benefit in situations where attacks on the emergency
   services system are likely to be relatively infrequent, since the
   resources required to pursue an investigation are likely to be
   considerable.  However, although real-time validation based on
   PIDF-LO elements is challenging, where LIS audit logs are available
   (such as where a law enforcement agency can present a subpoena),
   linking of a pseudonym to the Device obtaining location can be
   accomplished during an investigation.

   Where attacks are frequent and continuous, automated mechanisms are
   required.  For example, it might be valuable to develop mechanisms to
   exchange audit trail information in a standardized format between
   ISPs and PSAPs / VSPs and PSAPs or heuristics to distinguish
   potentially fraudulent emergency calls from real emergencies.  While
   a Completely Automated Public Turing test to tell Computers and
   Humans Apart (CAPTCHA) may be applied to suspicious calls to lower
   the risk from bot-nets, this is quite controversial for emergency
   services, due to the risk of delaying or rejecting valid calls.

Top      ToC       Page 23 
5.  Security Considerations

   Although it is important to ensure that location information cannot
   be faked, the mitigation techniques presented in this document are
   not universally applicable.  For example, there will be many GPS-
   enabled Devices that will find it difficult to utilize any of the
   solutions described in Section 3.  It is also unlikely that users
   will be willing to upload their location information for
   "verification" to a nearby location server located in the access
   network.

   This document focuses on threats that arise from conveyance of
   misleading location information, rather than caller identification or
   authentication and integrity protection of the messages in which
   location is conveyed.  Nevertheless, these aspects are important.  In
   some countries, regulators may not require the authenticated identity
   of the emergency caller (e.g., emergency calls placed from Public
   Switched Telephone Network (PSTN) pay phones or SIM-less cell
   phones).  Furthermore, if identities can easily be crafted (as is the
   case with many VoIP offerings today), then the value of emergency
   caller authentication itself might be limited.  As a result,
   attackers can forge emergency calls with a lower risk of being held
   accountable, which may encourage hoax calls.

   In order to provide authentication and integrity protection for the
   Session Initiation Protocol (SIP) messages conveying location,
   several security approaches are available.  It is possible to ensure
   that modification of the identity and location in transit can be
   detected by the Location Recipient (e.g., the PSAP), using
   cryptographic mechanisms, as described in "Enhancements for
   Authenticated Identity Management in the Session Initiation Protocol
   (SIP)" [RFC4474].  However, compatibility with Session Border
   Controllers (SBCs) that modify integrity-protected headers has proven
   to be an issue in practice, and as a result, a revision of [RFC4474]
   is in progress [SIP-Identity].  In the absence of an end-to-end
   solution, SIP over Transport Layer Security (TLS) can be used to
   provide message authentication and integrity protection hop by hop.

   PSAPs remain vulnerable to distributed denial-of-service attacks,
   even where the mitigation techniques described in this document are
   utilized.  Placing a large number of emergency calls that appear to
   come from different locations is an example of an attack that is
   difficult to carry out within the legacy system but is easier to
   imagine within IP-based emergency services.  Also, in the current
   system, it would be very difficult for an attacker from one country
   to attack the emergency services infrastructure located in another
   country, but this attack is possible within IP-based emergency
   services.

Top      ToC       Page 24 
   While manually mounting the attacks described in Section 2 is
   non-trivial, the attacks described in this document can be automated.
   While manually carrying out a location theft would require that the
   attacker be in proximity to the location being spoofed, or to collude
   with another end host, an attacker able to run code on an end host
   can obtain its location and cause an emergency call to be made.
   While manually carrying out a time-shifting attack would require that
   the attacker visit the location and submit it before the location
   information is considered stale, while traveling rapidly away from
   that location to avoid apprehension, these limitations would not
   apply to an attacker able to run code on the end host.  While
   obtaining a PIDF-LO from a spoofed IP address requires that the
   attacker be on the path between the HELD requester and the LIS, if
   the attacker is able to run code requesting the PIDF-LO, retrieve it
   from the LIS, and then make an emergency call using it, this attack
   becomes much easier.  To mitigate the risk of automated attacks,
   service providers can limit the ability of untrusted code (such as
   WebRTC applications written in JavaScript) to make emergency calls.

   Emergency services have three finite resources subject to denial-of-
   service attacks: the network and server infrastructure; call takers
   and dispatchers; and the first responders, such as firefighters and
   police officers.  Protecting the network infrastructure is similar to
   protecting other high-value service providers, except that location
   information may be used to filter call setup requests, to weed out
   requests that are out of area.  Even for large cities, PSAPs may only
   have a handful of call takers on duty.  So, even if automated
   techniques are utilized to evaluate the trustworthiness of conveyed
   location and call takers can, by questioning the caller, eliminate
   many hoax calls, PSAPs can be overwhelmed even by a small-scale
   attack.  Finally, first-responder resources are scarce, particularly
   during mass-casualty events.

6.  Privacy Considerations

   The emergency calling architecture described in [RFC6443] utilizes
   the PIDF-LO format defined in [RFC4119].  As described in the
   location privacy architecture [RFC6280], privacy rules that may
   include policy instructions are conveyed along with the location
   object.

Top      ToC       Page 25 
   The intent of the location privacy architecture was to provide strong
   privacy protections, as noted in [RFC6280], Section 1.1:

      A central feature of the Geopriv architecture is that location
      information is always bound to privacy rules to ensure that
      entities that receive location information are informed of how
      they may use it.  These rules can convey simple directives ("do
      not share my location with others"), or more robust preferences
      ("allow my spouse to know my exact location all of the time, but
      only allow my boss to know it during work hours")...  The binding
      of privacy rules to location information can convey users' desire
      for and expectations of privacy, which in turn helps to bolster
      social and legal systems' protection of those expectations.

   However, in practice this architecture has limitations that apply
   within emergency and non-emergency situations.  As noted in
   Section 1.2.2, concerns about hoax calls have led to restrictions on
   anonymous emergency calls.  Caller identification (potentially
   asserted in SIP via P-Asserted-Identity and SIP Identity) may be used
   during emergency calls.  As a result, in many cases location
   information transmitted within SIP messages can be linked to caller
   identity.  For example, in the case of a signed LbyV, there are
   privacy concerns arising from linking the location object to
   identifiers to prevent replay attacks, as described in Section 3.1.

   The ability to observe location information during emergency calls
   may also represent a privacy risk.  As a result, [RFC6443] requires
   transmission-layer security for SIP messages, as well as interactions
   with the location server.  However, even where transmission-layer
   security is used, privacy rules associated with location information
   may not apply.

   In many jurisdictions, an individual requesting emergency assistance
   is assumed to be granting permission to the PSAP, call taker, and
   first responders to obtain their location in order to accelerate
   dispatch.  As a result, privacy policies associated with location are
   implicitly waived when an emergency call is initiated.  In addition,
   when location information is included within SIP messages in either
   emergency or non-emergency uses, SIP entities receiving the SIP
   message are implicitly assumed to be authorized Location Recipients,
   as noted in [RFC5606], Section 3.2:

      Consensus has emerged that any SIP entity that receives a SIP
      message containing LI through the operation of SIP's normal
      routing procedures or as a result of location-based routing should
      be considered an authorized recipient of that LI.  Because of this
      presumption, one SIP element may pass the LI to another even if
      the LO it contains has <retransmission-allowed> set to "no"; this

Top      ToC       Page 26 
      sees the passing of the SIP message as part of the delivery to
      authorized recipients, rather than as retransmission.  SIP
      entities are still enjoined from passing these messages
      outside the normal routing to external entities if
      <retransmission-allowed> is set to "no", as it is the passing to
      third parties that <retransmission-allowed> is meant to control.

   Where LbyR is utilized rather than LbyV, it is possible to apply more
   restrictive authorization policies, limiting access to intermediaries
   and snoopers.  However, this is not possible if the "authorization by
   possession" model is used.

7.  Informative References

   [EENA]     EENA, "False Emergency Calls", EENA Operations Document,
              Version 1.1, May 2011, <http://www.eena.org/ressource/
              static/files/2012_05_04-3.1.2.fc_v1.1.pdf>.

   [GPSCounter]
              Warner, J. and R. Johnston, "GPS Spoofing
              Countermeasures", Los Alamos research paper LAUR-03-6163,
              December 2003.

   [Loc-Dependability]
              Thomson, M. and J. Winterbottom, "Digital Signature
              Methods for Location Dependability", Work in Progress,
              draft-thomson-geopriv-location-dependability-07,
              March 2011.

   [NENA-i2]  NENA 08-001, "NENA Interim VoIP Architecture for Enhanced
              9-1-1 Services (i2)", Version 2, August 2010.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000,
              <http://www.rfc-editor.org/info/rfc2818>.

   [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, <http://www.rfc-editor.org/info/rfc3261>.

   [RFC3693]  Cuellar, J., Morris, J., Mulligan, D., Peterson, J., and
              J. Polk, "Geopriv Requirements", RFC 3693, February 2004,
              <http://www.rfc-editor.org/info/rfc3693>.

Top      ToC       Page 27 
   [RFC3694]  Danley, M., Mulligan, D., Morris, J., and J. Peterson,
              "Threat Analysis of the Geopriv Protocol", RFC 3694,
              February 2004, <http://www.rfc-editor.org/info/rfc3694>.

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, Ed., "Extensible Authentication Protocol
              (EAP)", RFC 3748, June 2004,
              <http://www.rfc-editor.org/info/rfc3748>.

   [RFC3863]  Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr,
              W., and J. Peterson, "Presence Information Data Format
              (PIDF)", RFC 3863, August 2004,
              <http://www.rfc-editor.org/info/rfc3863>.

   [RFC4119]  Peterson, J., "A Presence-based GEOPRIV Location Object
              Format", RFC 4119, December 2005,
              <http://www.rfc-editor.org/info/rfc4119>.

   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474, August 2006,
              <http://www.rfc-editor.org/info/rfc4474>.

   [RFC4479]  Rosenberg, J., "A Data Model for Presence", RFC 4479,
              July 2006, <http://www.rfc-editor.org/info/rfc4479>.

   [RFC4740]  Garcia-Martin, M., Ed., Belinchon, M., Pallares-Lopez, M.,
              Canales-Valenzuela, C., and K. Tammi, "Diameter Session
              Initiation Protocol (SIP) Application", RFC 4740,
              November 2006, <http://www.rfc-editor.org/info/rfc4740>.

   [RFC5012]  Schulzrinne, H. and R. Marshall, Ed., "Requirements for
              Emergency Context Resolution with Internet Technologies",
              RFC 5012, January 2008,
              <http://www.rfc-editor.org/info/rfc5012>.

   [RFC5069]  Taylor, T., Ed., Tschofenig, H., Schulzrinne, H., and M.
              Shanmugam, "Security Threats and Requirements for
              Emergency Call Marking and Mapping", RFC 5069,
              January 2008, <http://www.rfc-editor.org/info/rfc5069>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

Top      ToC       Page 28 
   [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,
              <http://www.rfc-editor.org/info/rfc5491>.

   [RFC5606]  Peterson, J., Hardie, T., and J. Morris, "Implications of
              'retransmission-allowed' for SIP Location Conveyance",
              RFC 5606, August 2009,
              <http://www.rfc-editor.org/info/rfc5606>.

   [RFC5751]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Message
              Specification", RFC 5751, January 2010,
              <http://www.rfc-editor.org/info/rfc5751>.

   [RFC5808]  Marshall, R., Ed., "Requirements for a Location-by-
              Reference Mechanism", RFC 5808, May 2010,
              <http://www.rfc-editor.org/info/rfc5808>.

   [RFC5985]  Barnes, M., Ed., "HTTP-Enabled Location Delivery (HELD)",
              RFC 5985, September 2010,
              <http://www.rfc-editor.org/info/rfc5985>.

   [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,
              <http://www.rfc-editor.org/info/rfc6280>.

   [RFC6442]  Polk, J., Rosen, B., and J. Peterson, "Location Conveyance
              for the Session Initiation Protocol", RFC 6442,
              December 2011, <http://www.rfc-editor.org/info/rfc6442>.

   [RFC6443]  Rosen, B., Schulzrinne, H., Polk, J., and A. Newton,
              "Framework for Emergency Calling Using Internet
              Multimedia", RFC 6443, December 2011,
              <http://www.rfc-editor.org/info/rfc6443>.

   [RFC6444]  Schulzrinne, H., Liess, L., Tschofenig, H., Stark, B., and
              A.  Kuett, "Location Hiding: Problem Statement and
              Requirements", RFC 6444, January 2012,
              <http://www.rfc-editor.org/info/rfc6444>.

   [RFC6753]  Winterbottom, J., Tschofenig, H., Schulzrinne, H., and M.
              Thomson, "A Location Dereference Protocol Using HTTP-
              Enabled Location Delivery (HELD)", RFC 6753, October 2012,
              <http://www.rfc-editor.org/info/rfc6753>.

Top      ToC       Page 29 
   [RFC6881]  Rosen, B. and J. Polk, "Best Current Practice for
              Communications Services in Support of Emergency Calling",
              BCP 181, RFC 6881, March 2013,
              <http://www.rfc-editor.org/info/rfc6881>.

   [RFC7090]  Schulzrinne, H., Tschofenig, H., Holmberg, C., and M.
              Patel, "Public Safety Answering Point (PSAP) Callback",
              RFC 7090, April 2014,
              <http://www.rfc-editor.org/info/rfc7090>.

   [RFC7199]  Barnes, R., Thomson, M., Winterbottom, J., and H.
              Tschofenig, "Location Configuration Extensions for Policy
              Management", RFC 7199, April 2014,
              <http://www.rfc-editor.org/info/rfc7199>.

   [RFC7340]  Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
              Telephone Identity Problem Statement and Requirements",
              RFC 7340, September 2014,
              <http://www.rfc-editor.org/info/rfc7340>.

   [RFC7375]  Peterson, J., "Secure Telephone Identity Threat Model",
              RFC 7375, October 2014,
              <http://www.rfc-editor.org/info/rfc7375>.

   [SA]       "Saudi Arabia - Illegal sale of SIMs blamed for surge in
              hoax calls", Arab News, April 5, 2010,
              <http://www.arabnews.com/node/341463>.

   [SIP-Identity]
              Peterson, J., Jennings, C. and E. Rescorla, "Authenticated
              Identity Management in the Session Initiation Protocol
              (SIP)", Work in Progress, draft-ietf-stir-rfc4474bis-02,
              October 2014.

   [STIR]     IETF, "Secure Telephone Identity Revisited (stir) Working
              Group", October 2013,
              <http://datatracker.ietf.org/wg/stir/charter/>.

   [SWATing]  "SWATing 911 Calls", Dispatch Magazine On-Line,
              April 6, 2013, <http://www.911dispatch.com/
              swating-911-calls/>.

   [Swatting] "Don't Make the Call: The New Phenomenon of 'Swatting'",
              Federal Bureau of Investigation, February 4, 2008,
              <http://www.fbi.gov/news/stories/2008/february/
              swatting020408>.

Top      ToC       Page 30 
   [TASMANIA] "Emergency services seek SIM-less calls block", ABC News
              Online, August 18, 2006, <http://www.abc.net.au/elections/
              tas/2006/news/stories/1717956.htm?elections/tas/2006/>.

   [UK]       "Rapper makes thousands of prank 999 emergency calls to UK
              police", Digital Journal, June 24, 2010,
              <http://www.digitaljournal.com/article/293796?tp=1>.

Acknowledgments

   We would like to thank the members of the IETF ECRIT working group,
   including Marc Linsner and Brian Rosen, for their input at IETF 85
   that helped get this document pointed in the right direction.  We
   would also like to thank members of the IETF GEOPRIV working group,
   including Richard Barnes, Matt Lepinski, Andrew Newton, Murugaraj
   Shanmugam, and Martin Thomson for their feedback on previous versions
   of this document.  Alissa Cooper, Adrian Farrel, Pete Resnick, Meral
   Shirazipour, and Bert Wijnen provided helpful review comments during
   the IETF last call.

Top      ToC       Page 31 
Authors' Addresses

   Hannes Tschofenig
   Austria

   EMail: Hannes.tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at


   Henning Schulzrinne
   Columbia University
   Department of Computer Science
   450 Computer Science Building
   New York, NY  10027
   United States

   Phone: +1 212 939 7004
   EMail: hgs@cs.columbia.edu
   URI:   http://www.cs.columbia.edu


   Bernard Aboba (editor)
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   United States

   EMail: bernard_aboba@hotmail.com