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Versions: (draft-tschofenig-ecrit-trustworthy-location) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 RFC 7378

ECRIT Working Group                                        H. Tschofenig
INTERNET-DRAFT                                                  ARM Ltd.
Category: Informational                                   H. Schulzrinne
Expires: September 24, 2014                          Columbia University
                                                          B. Aboba (ed.)
                                                   Microsoft Corporation
                                                           17 March 2014

                          Trustworthy Location
              draft-ietf-ecrit-trustworthy-location-09.txt

Abstract

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

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

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

   This Internet-Draft will expire on September 24, 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
   (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  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Threats  . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Location Spoofing  . . . . . . . . . . . . . . . . . . . .  6
     2.2.  Identity Spoofing  . . . . . . . . . . . . . . . . . . . .  7
   3.  Solutions  . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.1.  Signed Location by Value . . . . . . . . . . . . . . . . .  8
     3.2.  Location by Reference  . . . . . . . . . . . . . . . . . . 11
     3.3.  Proxy Adding Location  . . . . . . . . . . . . . . . . . . 14
   4.  Location Trust Assessment  . . . . . . . . . . . . . . . . . . 16
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     7.1. Informative references  . . . . . . . . . . . . . . . . . . 20
   Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . . . 22
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23


















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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.

   Services that depend on location commonly experience security issues
   today.  While prank calls have been a problem for emergency services
   dating back to the time of street corner call boxes, with the move to
   IP-based emergency services, the ability to launch automated attacks
   has increased.  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, which in
   certain European countries can constitute as much as 70% of all
   emergency calls.  Reducing the number of prank 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.

   Since prank emergency calls can endanger bystanders or emergency
   services personnel, or divert resources away from legitimate
   emergencies, they can be life threatening.  A particularly dangerous
   form of prank call is "swatting" - a prank emergency call that draws
   a response from law enforcement (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.

   Many documented cases of "swatting" involve not only the faking of an
   emergency, but also the absence of accurate caller identification and
   the delivery of misleading location data.  Today these attacks are
   often carried out by providing false caller identification, since for
   circuit-switched calls from landlines, location provided to the
   Public Safety Answering Point (PSAP) is determined from a lookup
   using the calling telephone number.  With IP-based emergency
   services, in addition to the potential for false caller
   identification, it is also possible to attach misleading location
   information to the emergency call.

   Ideally, a call taker at a PSAP should be put in the position to
   assess, in real-time, the level of trust that can be placed on the



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   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 about the caller.  Where
   real-time assessment is not possible, it is important to be able to
   determine the source of the call in a post-mortem, so as to be able
   to enforce accountability.

   This document defines terminology (including the meaning of
   "trustworthy location") in Section 1.1, investigates security threats
   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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   The definition for "Target" is taken from "Geopriv Requirements"
   [RFC3693].

   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
   information.

   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"
   [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.




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   The following additional terms apply to location spoofing:

   "Place Shifting" is where the attacker constructs a 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
   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.  Where the
   Target and attacker collude, the term "location swapping" is used.

2.  Threats

   While previous IETF documents have analyzed aspects of the security
   of emergency services or threats to geographic location privacy,
   those documents do not cover the threats arising from unreliable
   location information.

   A threat analysis of the emergency services system is provided in
   "Security Threats and Requirements for Emergency Call Marking and
   Mapping" [RFC5069]. 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.  [RFC5069] also
   describes attacks against individuals, including attempts to prevent
   an individual from receiving aid, or to gain information about an
   emergency.  "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.

   This document focuses on threats from attackers providing false
   location information within emergency calls.  Since we do not focus
   on attackers gaining control of infrastructure elements (e.g.,
   location servers, call route servers) or the emergency services IP
   network, the threats arise from end hosts.  In addition to threats
   arising from the intentional forging of caller identification or
   location information, end hosts may be induced to provide
   untrustworthy location information.  For example, end hosts may
   obtain location from civilian GPS, which is vulnerable to spoofing



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   [GPSCounter] or from third party Location Service Providers (LSPs)
   which may be vulnerable to attack or may not provide location
   accuracy suitable for emergency purposes.

   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 intrastructure 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.

   In this document, we focus only on the malicious end host adversary
   model.

2.1.  Location Spoofing

   An adversary can provide false location information in an emergency
   call in order to misdirect emergency resources.  For calls
   originating within the Public Switched Telephone Network (PSTN) or
   via a fixed Voice over Internet Protocol (VoIP) service, this attack
   can be carried out via caller-id spoofing.  For example, where a
   Voice Service Provider enables setting of the outbound caller
   identification without checking it against the authenticated
   identity, forging caller identification is trivial.  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 location is attached to the emergency call by an end host,
   several avenues are available to provide false location information:

      1.  The end host could fabricate a Presence Information Data
      Format Location Object (PIDF-LO) and convey it within an emergency
      call;

      2.  The Voice Service Provider (VSP) (and indirectly a LIS) could



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      be fooled into using the wrong identity (such as an IP address)
      for location lookup, thereby providing the end host with
      misleading location information;

      3.  Inaccurate or out-of-date information (such as spoofed Global
      Positioning System (GPS) signals, a stale wiremap or an inaccurate
      access point location database) could be utilized by the LIS or
      the end host in its location determination, thereby leading to an
      inaccurate determination of location.

   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.

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

   Location swapping:  Trudy and Malory collude and swap location
      information, pretending to be in each other's location.

2.2.  Identity Spoofing

   With calls originating on an IP network, at least two forms of
   identity are relevant, with the distinction created by the split
   between the Internet Access Provider (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 prank 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 a strong identity that need not necessarily be coupled to a
   business relationship with the IAP, Internet Service Provider (ISP)



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   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,
   making the system vulnerable to bot-net attacks.  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 three mechanisms which can be used to convey
   location securely: signed location by value (Section 3.1), location
   by reference (Section 3.2) and proxy added location (Section 3.3).

   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 end host, (the entity subject to
   the location determination process).  The signed location information
   is then verified by the location recipient and not by the target.  A
   straw-man proposal for location signing is provided in "Digital
   Signature Methods for Location Dependability" [I-D.thomson-geopriv-
   location-dependability].

   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        -- Geopriv
    Configuration |Loc.   |Loc.        --   Using Protocol
    Protocol      |(a)    |(b)       --     (e.g., SIP)
                  |       v        --       (c)
                +-+-------+-+    --
                | Target /  |  --
                | End Host  +
                |           |
                +-----------+

                        Figure 1: Location Signing

   In order to limit replay attacks, [I.D.thomson-geopriv-location-
   dependability] 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
   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



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   [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-swapping 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 is unlikely to deter attacks launched by bot-nets,
   since the work required to verify the location signature is
   considerable.  However, while bot-nets are unlikely to be deterred by
   location signing, accurate location information would limit the
   subset of the bot-net 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 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:



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      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.

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.  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] or the Dynamic Host Configuration Protocol (DHCP) location
   URI option [DHCP-URI-OPT].




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   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
   applicable.

                +-----------+  Geopriv      +-----------+
                |           |  Location     | Location  |
                |    LIS    +<------------->+ Recipient |
                |           | Dereferencing |           |
                +-+-------+-+ Protocol (d)  +----+------+
                  ^       |                    --^
                  |       |                  --
    Geopriv       |Req.   |                --
    Location      |LbyR   |LbyR          -- Geopriv
    Configuration |(a)    |(b)         --   Using Protocol
    Protocol      |       |          --     (e.g., SIP)
                  |       V        --       (c)
                +-+-------+-+    --
                | 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.

   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 swapping.




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   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
      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 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



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      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.

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



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      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.

   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
   challenges.

   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,



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   "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, a call taker
   can put more effort into obtaining location information from the
   caller.

   Location trust assessment has value regardless of whether the
   location has been conveyed securely (via signed location, location-
   by-reference or proxy-added location) or not (via location-by-value
   without location signing), since secure conveyance does not provide
   assurance relating to the validity or provenance of location data.

   To prevent location-swapping 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 post-mortem.

   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
   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 prank 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,



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   the ability to identify a caller may decrease the likelihood of
   caller misbehavior.  For example, 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 nuisance calls has
   often been unacceptably high [TASMANIA][UK][SA].

   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
   information.

   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.



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   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 prank 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
   post-mortem.

   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 Touring 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.  In order to limit prank calls,
   legacy emergency services rely on the ability to identify callers, as
   well as on the difficulty of location spoofing for normal users.  The
   ability to ascertain identity is important, since the threat of
   punishment reduces prank calls; as an example, calls from pay phones
   are subject to greater scrutiny by the call taker.

   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 'Bar'.

   However, within an IP-based emergency services a number of these



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   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.  PSAPs even for large cities may only have a handful of PSAP
   call takers on duty, so even if they can, by questioning the caller,
   eliminate a lot of prank calls, they are quickly overwhelmed by even
   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 consequence, an
   attacker can forge emergency call information without the chance of
   being held accountable for its own actions.

   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
   requires the attacker to be in proximity to the location being



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   spoofed, and location swapping requires the attacker to collude with
   someone who was at the spoofed 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

[DHCP-URI-OPT]
          Polk, J., "Dynamic Host Configuration Protocol (DHCP) IPv4 and
          IPv6 Option for a Location Uniform Resource Identifier (URI)",
          Internet draft (work in progress), draft-ietf-geopriv-dhcp-
          lbyr-uri-option-19, February 2013.

[EENA]    EENA, "False Emergency Calls", EENA Operations Document,
          Version 1.0, March 2011,
          http://www.eena.org/ressource/static/files/
          2011_03_15_3.1.2.fc_v1.0.pdf

[GPSCounter]
          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.

[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



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          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
          2006.

[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.

[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.

[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.




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[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.

[SA]      "Saudi Arabia - Illegal sale of SIMs blamed for surge in prank
          calls", Arab News, May 4, 2010,
          http://www.menafn.com/qn_news_story_s.asp?StoryId=1093319384

[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

[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, Henning Schulzrinne 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 Bert Wijnen and
   Meral Shirazipour who provided review comments in IETF last call.














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Authors' Addresses

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

   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
   US

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

   Bernard Aboba
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   US

   Email:  bernard_aboba@hotmail.com























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