Trustworthy Location

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Document Type Active Internet-Draft (ecrit WG)
Authors Hannes Tschofenig  , Henning Schulzrinne  , Bernard Aboba 
Last updated 2014-05-31
Replaces draft-tschofenig-ecrit-trustworthy-location
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ECRIT Working Group                                        H. Tschofenig
INTERNET-DRAFT                                                  ARM Ltd.
Category: Informational                                   H. Schulzrinne
Expires: December 1, 2014                            Columbia University
                                                          B. Aboba (ed.)
                                                   Microsoft Corporation
                                                             31 May 2014

                          Trustworthy Location


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

   This document focuses on the security issues arising from conveyance
   of location within an emergency call, and describes mechanisms
   availble to convey location in a manner that is inherently secure and
   reliable.  It also provides guidelines for assessing the
   trustworthiness of location information.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 1, 2014.

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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
   ( in effect on the date of
   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2   Literature review  . . . . . . . . . . . . . . . . . . . .  5
   2.  Threat Model   . . . . . . . . . . . . . . . . . . . . . . . .  7
     2.1.  Location Spoofing  . . . . . . . . . . . . . . . . . . . .  8
     2.2.  Identity Spoofing  . . . . . . . . . . . . . . . . . . . .  9
   3.  Solutions  . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.1.  Signed Location by Value . . . . . . . . . . . . . . . . . 10
     3.2.  Location by Reference  . . . . . . . . . . . . . . . . . . 14
     3.3.  Proxy Adding Location  . . . . . . . . . . . . . . . . . . 17
   4.  Location Trust Assessment  . . . . . . . . . . . . . . . . . . 18
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     7.1. Informative references  . . . . . . . . . . . . . . . . . . 22
   Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . . . 25
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25

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1.  Introduction

   Several public and commercial services depend upon location
   information in their operations.  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 only supporting 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" [I-D.ietf-
   stir-problem-statement].  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 after the fact, 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 the threat model in Section 2, outlines
   potential solutions 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",
   document are to be interpreted as described in [RFC2119].

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   The definitions of "Internet Access Provider (IAP)", "Internet
   Service Provider (ISP)" and "Voice Service Provider (VSP)" are taken
   from "Requirements for Emergency Context Resolution with Internet
   Technologies" [RFC5012].

   The definition of a "hoax call" is taken from "False Emergency Calls"

   The definition of "Target" and "Device" is taken from "An
   Architecture for Location and Location Privacy in Internet
   Applications" [RFC6280].

   The term "location determination method" refers to the mechanism used
   to determine the location of a Target.  This may be something
   employed by a location information server (LIS), or by the Target
   itself.  It specifically does not refer to the location configuration
   protocol (LCP) used to deliver location information either to the
   Target or the Recipient.  This term is re-used from "GEOPRIV 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

   Additionally, the terms Location-by-Value (LbyV), Location-by-
   Reference (LbyR), Location Configuration Protocol, Location
   Dereference Protocol, and Location Uniform Resource Identifier (URI)
   are re-used from "Requirements for a Location-by-Reference Mechanism"

   "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.

   The following additional terms apply to location spoofing:

   "Place Shifting" is where the attacker constructs 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" is where the attacker uses or re-uses location
   information that was valid in the past, but is no longer valid

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   because the attacker has moved.

   "Location Theft" is where the attacker captures a Target's location
   information 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.

   "Identity Spoofing" is where 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.

1.2.  Literature Review

   There is existing work on the problem of hoax calls, as well as
   analyses of aspects of the security of emergency services, threats to
   geographic location privacy, threats relating to spoofing of caller
   identification and modification of location information in transit.
   This section reviews the literature.

1.2.1.  Hoax Calls

   Hoax calls have been a problem for emergency services dating back to
   the time of street corner call boxes.  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."  As a result,
   considerable attention has been focused on the problem.

   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.

   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 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.

   As noted in [EENA], many documented cases of "swatting" involve not

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   only the faking of an emergency, but also falsification or
   obfuscation of identity.  In general, the ability to identify the
   caller also appears to influence the incidence of hoax calls.  Where
   a Voice Service Provider enables setting of the 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 to make fraudulent emergency calls.
   Where emergency calls have been allowed from handsets lacking a SIM
   card, or where ownership of the SIM card cannot be determined, the
   frequency of hoax calls has often been unacceptably high

   However, to date there have been 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

1.2.2.  Existing IETF Work

   The Internet architecture for emergency calling is described in
   "Framework for Emergency Calling Using Internet Multimedia" [RFC6443]
   and "Best Current Practice for Communications Services in Support of
   Emergency Calling" [RFC6881].  The conveyance of location information
   within the Session Initiation Protocol (SIP) is described in
   "Location Conveyance for the Session Initiation Protocol" [RFC6442],
   which in the Security Considerations (Section 7) includes discussion
   of privacy, authentication and integrity concerns relating to
   conveyed location.  Note that while [RFC6442] does not prohibit the
   conveyance of location within non-emergency calls, in practice,
   location conveyance requires additional infrastructure as described
   in [RFC6443].  As a result, privacy issues inherent in conveyance of
   location within non-emergency calls are not considered within

   "Secure Telephone Identity Threat Model" [I-D.ietf-stir-threats]
   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.

   "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.  Within the Security
   Considerations (Section 5), mechanisms for ensuring the security of
   the location distribution chain are discussed;  these include

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   mechanisms for hop-by-hop confidentiality and integrity protection as
   well as end-to-end assurance.  As noted in 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

   "Geopriv Requirements" [RFC3693] focuses on the authorization,
   security and privacy requirements of location-dependent services,
   including emergency services.  Within the Security Considerations
   (Section 8), this 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 signalling 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.

2.  Threat 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

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      emergency service infrastructure elements act maliciously.

   Malicious infrastructure adversary model:  The emergency call routing
      elements, such as the Location Information Server (LIS), the
      Location-to-Service Translation (LoST) infrastructure, used for
      mapping locations to PSAP address, 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 whether it 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) which may be vulnerable
   to attack or may not provide location accuracy suitable for emergency

   Also, we do not cover threats arising from inadequate location
   infrastructure.  For example, a stale wiremap or an inaccurate access
   point location database could be utilized by the Location Information
   Server (LIS) or the end host in its location determination, thereby
   leading to an inaccurate determination of location.  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.

2.1.  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:  Trudy, the adversary, pretends to be at an
      arbitrary location.

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

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

2.2.  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, this also may 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 or 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, so that 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.  Solutions

   This section presents two mechanisms which can be used to enable
   location to be authenticated: signed location by value (Section 3.1),
   which provides for authentication and integrity protection of the
   PIDF-LO, and location-by-reference (Section 3.2), which enables
   location to be obtained by the PSAP via direct contact with the
   location server.  In addition, a mechanism is presented which

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   protects against location forgery by the end host: proxy added
   location (Section 3.3).  Since at the time of this writing there is
   no completed specification for signed location by value, only an
   expired straw-man proposal, it should be understood that only the
   location-by-reference and proxy added location mechanisms are
   suitable for deployment.

   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"
   [RFC4474].  However, compatibility with Session Border Controllers
   (SBCs) that modify integrity-protected headers has proven to be an
   issue in practice.  As a result, SIP over Transport Layer Security
   (TLS) is currently a more deployable mechanism to provide per-message
   authentication and integrity protection hop-by-hop.

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 in step (c) who
   verifies it.  For SIP, the procedures described in "Location
   Conveyance for the Session Initiation Protocol" [RFC6442] are
   applicable for location conveyance.

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                +-----------+               +-----------+
                |           |               | 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" [I-D.thomson-geopriv-
   location-dependability].  Note that since this document is no longer
   under development, location signing cannot be considered deployable
   at the time of this writing.

   In order to limit replay attacks, this document proposes 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

   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.

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      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 [RFC4474] is implemented.

   To address location-spoofing attacks, [I-D.thomson-geopriv-location-
   dependability] proposes addition of an "identity" element which 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, inclusion of an "identity" attribute could enable location
   tracking, so that a "hash" element is also proposed which could
   contain a hash of the content of the "identity" element instead.  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

   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

   Location signing is also difficult when the host obtains location via
   mechanisms such as GPS, unless trusted computing approaches, with

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   tamper-proof GPS modules, can be applied.  Otherwise, an end host can
   pretend to have a GPS device, 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.

   [NENA-i2] Section 3.7 includes operational recommendations relating
   to location signing:

      Location determination is out of scope for NENA, but we can offer
      guidance 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 certs to LIS operators.

      3.  The signature should include a timestamp.

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

      5.  Anti-spoofing 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 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 an SSL/TLS web certificate.

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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 HTTP-Enabled Location Delivery
   (HELD) [RFC5985].

   Figure 2 shows the communication model with the target requesting a
   location reference in step (a), the location server returns the
   reference 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

                +-----------+  Geopriv      +-----------+
                |           |  Location     | Location  |
                |    LIS    +<------------->+ Recipient |
                |           | Dereferencing |           |
                +-+-------+-+ Protocol (d)  +----+------+
                  ^       |                    --^
                  |       |                  --
    Geopriv       |Req.   |                --
    Location      |LbyR   |LbyR          -- 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
   deference the LbyR in order to obtain location.  The details for the
   dereferencing operations vary with the type of reference, such as a
   HTTP, HTTPS, SIP, SIPS URI or a SIP presence URI.

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   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.

   With respect to the security of the de-reference 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

      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

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      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 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.

      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 hierarchy, it would be necessary for the
   PSAP to obtain client certificates or Digest credentials for all the
   LISes in its coverage area, to enable it to successfully dereference
   LbyRs.  In situations with more than a few LISes 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 against 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.

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3.3.  Proxy Adding 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.

   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

      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

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

      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

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      location is more accurate than another.

   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 a standardized VoIP profile to be deployed at every end
   device and at every IAP.  This might impose interoperability

   Additionally, the IAP needs to take responsibility for emergency
   calls, even for customers they have no direct or indirect
   relationship with.  To provide identity information about the
   emergency caller from the VSP it would be necessary to let the IAP
   and the VSP to 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.  In case 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 to 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 after the fact.

   As noted in [I.D.thomson-geopriv-location-dependability], if the
   location object was signed, the location recipient has additional
   information on which to base their trust assessment, such as the

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   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 lead 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. Phase II) 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 about 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: headers, 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

   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

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   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 LISes.  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 lead 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 in a

   Where attacks are frequent and continuous, automated mechanisms are
   required.  For example, it might be valuable to develop mechanisms to
   exchange audit trails 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.

5.  Security Considerations

   IP-based emergency services face a number of security threats that do
   not exist within the legacy system.  Mechanically placing a large
   number of emergency calls that appear to come from different
   locations is difficult in a legacy environment.  Also, in the current
   system, it would be very difficult for an attacker from country 'Foo'
   to attack the emergency services infrastructure located in country

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   However, within an IP-based emergency services a number of these
   attacks become much easier to mount.  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 fire fighters 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 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.

   Attackers may want to modify, prevent or delay emergency calls.  In
   some cases, this will lead the PSAP to dispatch emergency personnel
   to an emergency that does not exist and, hence, the personnel might
   not be available to other callers.  It might also be possible for an
   attacker to impede the users from reaching an appropriate PSAP by
   modifying the location of an end host or the information returned
   from the mapping protocol.  In some countries, regulators may not
   require the authenticated identity of the emergency caller (e.g.
   emergency calls placed from PSTN pay phones or SIM-less cell phones).
   Furthermore, if identities can easily be crafted (as it 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 call information with a lower risk of being held

   The above-mentioned attacks are mostly targeting individual emergency
   callers or a very small fraction of them.  If attacks are, however,
   launched against the mapping architecture (see "Location-URL Mapping
   Architecture and Framework" [RFC5582] or against the emergency
   services IP network (including PSAPs), a larger region and a large
   number of potential emergency callers are affected.  The call takers
   themselves are a particularly scarce resource and if human
   interaction by these call takers is required then this can very
   quickly have severe consequences.

   Although it is important to ensure that location information cannot
   be faked 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.

   Nevertheless, it should be understood that mounting several of the
   attacks described in this document is non-trivial.  Location theft

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   requires the attacker to be in proximity to the location being
   spoofed, or to either collude with another endhost or gain control of
   an endhost so as to obtain its location.  Time shifting attacks
   require that the attacker visit the location and submit it before the
   location information is considered stale, while travelling rapidly
   away from that location to avoid apprehension.  Obtaining a PIDF-LO
   from a spoofed IP address requires that the attacker be on the path
   between the HELD requester and the LIS.

6.  IANA Considerations

   This document does not require actions by IANA.

7.  References

7.1.  Informative References

          Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
          Telephone Identity Problem Statement", Internet draft (work in
          progress), draft-ietf-stir-problem-statement-05.txt, May 2014.

          Peterson, J., "Secure Telephone Identity Threat Model",
          Internet draft (work in progress), draft-ietf-stir-
          threats-02.txt, February 2014.

[EENA]    EENA, "False Emergency Calls", EENA Operations Document,
          Version 1.1, May 2011,

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

[NENA-i2] "08-001 NENA Interim VoIP Architecture for Enhanced 9-1-1
          Services (i2)", December 2005.

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

[RFC2818] Rescorla, E., "HTTP over TLS", RFC 2818, May 2000.

[RFC3693] Cuellar, J., Morris, J., Mulligan, D., Peterson, J., and J.
          Polk, "Geopriv Requirements", RFC 3693, February 2004.

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[RFC3694] Danley, M., Mulligan, D., Morris, J. and J. Peterson, "Threat
          Analysis of the Geopriv Protocol", RFC 3694, February 2004.

[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
          Levkowetz, "Extensible Authentication Protocol (EAP)", RFC
          3748, June 2004.

[RFC3863] Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr, W. and
          J. Peterson, "Presence Information Data Format (PIDF)", RFC
          3863, August 2004.

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

[RFC4479] Rosenberg, J., "A Data Model for Presence", RFC 4479, July

[RFC4740] Garcia-Martin, M., Belinchon, M., Pallares-Lopez, M., Canales-
          Valenzuela, C., and K. Tammi, "Diameter Session Initiation
          Protocol (SIP) Application", RFC 4740, November 2006.

[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.

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Level Security
          (TLS) Protocol Version 1.2", RFC 5246, August 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.

[RFC5582] Schulzrinne, H., "Location-to-URL Mapping Architecture and
          Framework", RFC 5582, September 2009.

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

[RFC5808] Marshall, R., "Requirements for a Location-by-Reference
          Mechanism", RFC 5808, May 2010.

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

[RFC6280] Barnes, R., et. al, "An Architecture for Location and Location
          Privacy in Internet Applications", RFC 6280, July 2011.

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

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

[RFC6444] Schulzrinne, H., Liess, L., Tschofenig, H., Stark, B., and A.
          Kuett, "Location Hiding: Problem Statement and Requirements",
          RFC 6444, January 2012.

[RFC6753] Winterbottom, J., Tschofenig. H., Schulzrinne, H. and M.
          Thomson, "A Location Dereference Protocol Using HTTP-Enabled
          Location Delivery (HELD)", RFC 6753, October 2012.

[RFC6881] Rosen, B. and J. Polk, "Best Current Practice for
          Communications Services in Support of Emergency Calling", BCP
          181, RFC 6881, March 2013.

[RFC7090] Schulzrinne, H., Tschofenig, H., Holmberg, C. and M. Patel,
          "Public Safety Answering Point (PSAP) Callback", RFC 7090,
          April 2014.

[SA]      "Saudi Arabia - Illegal sale of SIMs blamed for surge in hoax
          calls", Arab News, May 4, 2010,

[STIR]    IETF, "Secure Telephone Identity Revisited (stir) Working
          Group",, October

          "Don't Make the Call: The New Phenomenon of 'Swatting',
          Federal Bureau of Investigation, February 4, 2008,

          "Emergency services seek SIM-less calls block", ABC News
          Online, August 18, 2006,


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[UK]      "Rapper makes thousands of prank 999 emergency calls to UK
          police", Digital Journal, June 24, 2010,


   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 documented pointed in the right direction.  We
   would also like to thank members of the IETF GEOPRIV WG, including
   Andrew Newton, Murugaraj Shanmugam, Martin Thomson, Richard Barnes
   and Matt Lepinski for their feedback to previous versions of this
   document.  Thanks also to Pete Resnick, Adrian Farrel, Alissa Cooper,
   Bert Wijnen and Meral Shirazipour who provided review comments in
   IETF last call.

Authors' Addresses

   Hannes Tschofenig
   ARM Ltd.
   110 Fulbourn Rd
   Cambridge  CB1 9NJ
   Great Britain


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

   Phone:  +1 212 939 7004

   Bernard Aboba
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052


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