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Internet Engineering Task Force (IETF) H. Tschofenig Request for Comments: 7378 Independent Category: Informational H. Schulzrinne ISSN: 2070-1721 Columbia University

                                                         B. Aboba, Ed.
                                                 Microsoft Corporation
                                                         December 2014
                        Trustworthy Location


 The trustworthiness of location information is critically important
 for some location-based applications, such as emergency calling or
 roadside assistance.
 This document describes threats to conveying location, particularly
 for emergency calls, and describes techniques that improve the
 reliability and security of location information.  It also provides
 guidelines for assessing the trustworthiness of location information.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Not all documents
 approved by the IESG are a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at

Tschofenig, et al. Informational [Page 1] RFC 7378 Trustworthy Location December 2014

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
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

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

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1. Introduction

 Several public and commercial services need location information to
 operate.  This includes emergency services (such as fire, ambulance,
 and police) as well as commercial services such as food delivery and
 roadside assistance.
 For circuit-switched calls from landlines, as well as for Voice over
 IP (VoIP) services that only support emergency service calls from
 stationary Devices, location provided to the Public Safety Answering
 Point (PSAP) is determined from a lookup using the calling telephone
 number.  As a result, for landlines or stationary VoIP, spoofing of
 caller identification can result in the PSAP incorrectly determining
 the caller's location.  Problems relating to calling party number and
 Caller ID assurance have been analyzed by the Secure Telephone
 Identity Revisited [STIR] working group as described in "Secure
 Telephone Identity Problem Statement and Requirements" [RFC7340].  In
 addition to the work underway in STIR, other mechanisms exist for
 validating caller identification.  For example, as noted in [EENA],
 one mechanism for validating caller identification information (as
 well as the existence of an emergency) is for the PSAP to call the
 user back, as described in [RFC7090].
 Given the existing work on caller identification, this document
 focuses on the additional threats that are introduced by the support
 of IP-based emergency services in nomadic and mobile Devices, in
 which location may be conveyed to the PSAP within the emergency call.
 Ideally, a call taker at a PSAP should be able to assess, in real
 time, the level of trust that can be placed on the information
 provided within a call.  This includes automated location conveyed
 along with the call and location information communicated by the
 caller, as well as identity information relating to the caller or the
 Device initiating the call.  Where real-time assessment is not
 possible, it is important to be able to determine the source of the
 call in a post-incident investigation, so as to be able to enforce
 This document defines terminology (including the meaning of
 "trustworthy location") in Section 1.1, reviews existing work in
 Section 1.2, describes threat models in Section 2, outlines potential
 mitigation techniques in Section 3, covers trust assessment in
 Section 4, and discusses security considerations in Section 5.

1.1. Terminology

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

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 We use the definitions of "Internet Access Provider (IAP)", "Internet
 Service Provider (ISP)", and "Voice Service Provider (VSP)" found in
 "Requirements for Emergency Context Resolution with Internet
 Technologies" [RFC5012].
 [EENA] defines a "hoax call" as follows: "A false or malicious call
 is when a person deliberately telephones the emergency services and
 tells them there is an emergency when there is not."
 The definitions of "Device", "Target", and "Location Information
 Server" (LIS) are taken from "An Architecture for Location and
 Location Privacy in Internet Applications" [RFC6280], Section 7.
 The term "Device" denotes the physical device, such as a mobile
 phone, PC, or embedded microcontroller, whose location is tracked as
 a proxy for the location of a Target.
 The term "Target" denotes an individual or other entity whose
 location is sought in the Geopriv architecture [RFC6280].  In many
 cases, the Target will be the human user of a Device, or it may be an
 object such as a vehicle or shipping container to which a Device is
 attached.  In some instances, the Target will be the Device itself.
 The Target is the entity whose privacy the architecture described in
 [RFC6280] seeks to protect.
 The term "Location Information Server" denotes an entity responsible
 for providing Devices within an access network with information about
 their own locations.  A Location Information Server uses knowledge of
 the access network and its physical topology to generate and
 distribute location information to Devices.
 The term "location determination method" refers to the mechanism used
 to determine the location of a Target.  This may be something
 employed by a LIS or by the Target itself.  It specifically does not
 refer to the location configuration protocol (LCP) used to deliver
 location information to either the Target or the Recipient.  This
 term is reused from "GEOPRIV Presence Information Data Format
 Location Object (PIDF-LO) Usage Clarification, Considerations, and
 Recommendations" [RFC5491].
 The term "source" is used to refer to the LIS, node, or Device from
 which a Recipient (Target or third party) obtains location

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 Additionally, the terms "location-by-value" (LbyV), "location-by-
 reference" (LbyR), "Location Configuration Protocol", "Location
 Dereference Protocol", and "Location Uniform Resource Identifier"
 (URI) are reused from "Requirements for a Location-by-Reference
 Mechanism" [RFC5808].
 "Trustworthy Location" is defined as location information that can be
 attributed to a trusted source, has been protected against
 modification in transmit, and has been assessed as trustworthy.
 "Location Trust Assessment" refers to the process by which the
 reliability of location information can be assessed.  This topic is
 discussed in Section 4.
 "Identity Spoofing" occurs when the attacker forges or obscures their
 identity so as to prevent themselves from being identified as the
 source of the attack.  One class of identity spoofing attack involves
 the forging of call origin identification.
 The following additional terms apply to location spoofing
 (Section 2.3):
 With "Place Shifting", attackers construct a Presence Information
 Data Format Location Object (PIDF-LO) for a location other than where
 they are currently located.  In some cases, place shifting can be
 limited in range (e.g., within the coverage area of a particular cell
 "Time Shifting" occurs when the attacker uses or reuses location
 information that was valid in the past but is no longer valid because
 the attacker has moved.
 "Location Theft" occurs when the attacker captures a Target's
 location information (possibly including a signature) and presents it
 as their own.  Location theft can occur in a single instance or may
 be continuous (e.g., where the attacker has gained control over the
 victim's Device).  Location theft may also be combined with time
 shifting to present someone else's location information after the
 original Target has moved.

1.2. Emergency Services Architecture

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

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1.2.1. Location

 The Internet architecture for emergency calling is described in
 "Framework for Emergency Calling Using Internet Multimedia"
 [RFC6443].  Best practices for utilizing the architecture to make
 emergency calls are described in "Best Current Practice for
 Communications Services in Support of Emergency Calling" [RFC6881].
 As noted in "An Architecture for Location and Location Privacy in
 Internet Applications" [RFC6280], Section 6.3:
    there are three critical steps in the placement of an emergency
    call, each involving location information:
    1. Determine the location of the caller.
    2. Determine the proper Public Safety Answering Point (PSAP) for
       the caller's location.
    3. Send a SIP INVITE message, including the caller's location, to
       the PSAP.
 The conveyance of location information within the Session Initiation
 Protocol (SIP) is described in "Location Conveyance for the Session
 Initiation Protocol" [RFC6442].  Conveyance of location-by-value
 (LbyV) as well as conveyance of location-by-reference (LbyR) are
 supported.  Section 7 of [RFC6442] ("Security Considerations")
 discusses privacy, authentication, and integrity concerns relating to
 conveyed location.  This includes discussion of transmission-layer
 security for confidentiality and integrity protection of SIP, as well
 as (undeployed) end-to-end security mechanisms for protection of
 location information (e.g., S/MIME).  Regardless of whether
 transmission-layer security is utilized, location information may be
 available for inspection by an intermediary that -- if it decides
 that the location value is unacceptable or insufficiently accurate --
 may send an error indication or replace the location, as described in
 [RFC6442], Section 3.4.

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 Although the infrastructure for location-based routing described in
 [RFC6443] was developed for use in emergency services, [RFC6442]
 supports conveyance of location within non-emergency calls as well as
 emergency calls.  Section 1 of "Implications of 'retransmission-
 allowed' for SIP Location Conveyance" [RFC5606] describes the overall
 architecture, as well as non-emergency usage scenarios (note: the
 [LOC-CONVEY] citation in the quote below refers to the document later
 published as [RFC6442]):
    The Presence Information Data Format for Location Objects (PIDF-LO
    [RFC4119]) carries both location information (LI) and policy
    information set by the Rule Maker, as is stipulated in [RFC3693].
    The policy carried along with LI allows the Rule Maker to
    restrict, among other things, the duration for which LI will be
    retained by recipients and the redistribution of LI by recipients.
    The Session Initiation Protocol [RFC3261] is one proposed Using
    Protocol for PIDF-LO.  The conveyance of PIDF-LO within SIP is
    specified in [LOC-CONVEY].  The common motivation for providing LI
    in SIP is to allow location to be considered in routing the SIP
    message.  One example use case would be emergency services, in
    which the location will be used by dispatchers to direct the
    response.  Another use case might be providing location to be used
    by services associated with the SIP session; a location associated
    with a call to a taxi service, for example, might be used to route
    to a local franchisee of a national service and also to route the
    taxi to pick up the caller.

1.2.2. Hoax Calls

 Hoax calls have been a problem for emergency services dating back to
 the time of street corner call boxes.  As the European Emergency
 Number Association (EENA) has noted [EENA]:
    False emergency calls divert emergency services away from people
    who may be in life-threatening situations and who need urgent
    help.  This can mean the difference between life and death for
    someone in trouble.
 EENA [EENA] has attempted to define terminology and describe best
 current practices for dealing with false emergency calls.  Reducing
 the number of hoax calls represents a challenge, since emergency
 services authorities in most countries are required to answer every
 call (whenever possible).  Where the caller cannot be identified, the
 ability to prosecute is limited.

Tschofenig, et al. Informational [Page 7] RFC 7378 Trustworthy Location December 2014

 A particularly dangerous form of hoax call is "swatting" -- a hoax
 emergency call that draws a response from law enforcement prepared
 for a violent confrontation (e.g., a fake hostage situation that
 results in the dispatching of a "Special Weapons And Tactics" (SWAT)
 team).  In 2008, the Federal Bureau of Investigation (FBI) issued a
 warning [Swatting] about an increase in the frequency and
 sophistication of these attacks.
 Many documented cases of "swatting" (also sometimes referred to as
 "SWATing") involve not only the faking of an emergency but also
 falsification or obfuscation of identity [Swatting] [SWATing].  There
 are a number of techniques by which hoax callers attempt to avoid
 identification, and in general, the ability to identify the caller
 appears to influence the incidence of hoax calls.
 Where a Voice Service Provider allows the caller to configure its
 outbound caller identification without checking it against the
 authenticated identity, forging caller identification is trivial.
 Similarly, where an attacker can gain entry to a Private Branch
 Exchange (PBX), they can then subsequently use that access to launch
 a denial-of-service attack against the PSAP or make fraudulent
 emergency calls.  Where emergency calls have been allowed from
 handsets lacking a subscriber identification module (SIM) card,
 so-called non-service initialized (NSI) handsets, or where ownership
 of the SIM card cannot be determined, the frequency of hoax calls has
 often been unacceptably high [TASMANIA] [UK] [SA].
 However, there are few documented cases of hoax calls that have
 arisen from conveyance of untrustworthy location information within
 an emergency call, which is the focus of this document.

2. Threat Models

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

2.1. Existing Work

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

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 In Section 5 of [RFC6280] ("An Architecture for Location and Location
 Privacy in Internet Applications"), mechanisms for ensuring the
 security of the location distribution chain are discussed; these
 include mechanisms for hop-by-hop confidentiality and integrity
 protection as well as end-to-end assurance.
 "Geopriv Requirements" [RFC3693] focuses on the authorization,
 security, and privacy requirements of location-dependent services,
 including emergency services.  Section 8 of [RFC3693] includes
 discussion of emergency services authentication (Section 8.3), and
 issues relating to identity and anonymity (Section 8.4).
 "Threat Analysis of the Geopriv Protocol" [RFC3694] describes threats
 against geographic location privacy, including protocol threats,
 threats resulting from the storage of geographic location data, and
 threats posed by the abuse of information.
 "Security Threats and Requirements for Emergency Call Marking and
 Mapping" [RFC5069] reviews security threats associated with the
 marking of signaling messages and the process of mapping locations to
 Universal Resource Identifiers (URIs) that point to PSAPs.  RFC 5069
 describes attacks on the emergency services system, such as
 attempting to deny system services to all users in a given area, to
 gain fraudulent use of services and to divert emergency calls to
 non-emergency sites.  In addition, it describes attacks against
 individuals, including attempts to prevent an individual from
 receiving aid, or to gain information about an emergency, as well as
 attacks on emergency services infrastructure elements, such as
 mapping discovery and mapping servers.
 "Secure Telephone Identity Threat Model" [RFC7375] analyzes threats
 relating to impersonation and obscuring of calling party numbers,
 reviewing the capabilities available to attackers, and the scenarios
 in which attacks are launched.

2.2. Adversary Model

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

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 Malicious infrastructure adversary model:  The emergency call routing
    elements, such as the Location Information Server (LIS), the
    Location-to-Service Translation (LoST) infrastructure (which is
    used for mapping locations to PSAP addresses), or call routing
    elements, may act maliciously.
 Malicious end host adversary model:  The end host itself acts
    maliciously, whether the owner is aware of this or the end host is
    acting under the control of a third party.
 Since previous work describes attacks against infrastructure elements
 (e.g., location servers, call route servers, mapping servers) or the
 emergency services IP network, as well as threats from attackers
 attempting to snoop location in transit, this document focuses on the
 threats arising from end hosts providing false location information
 within emergency calls (the malicious end host adversary model).
 Since the focus is on malicious hosts, we do not cover threats that
 may arise from attacks on infrastructure that hosts depend on to
 obtain location.  For example, end hosts may obtain location from
 civilian GPS, which is vulnerable to spoofing [GPSCounter], or from
 third-party Location Service Providers (LSPs) that may be vulnerable
 to attack or may not provide location accuracy suitable for emergency
 Also, we do not cover threats arising from inadequate location
 infrastructure.  For example, the LIS or end host could base its
 location determination on a stale wiremap or an inaccurate access
 point location database, leading to an inaccurate location estimate.
 Similarly, a Voice Service Provider (VSP) (and, indirectly, a LIS)
 could utilize the wrong identity (such as an IP address) for location
 lookup, thereby providing the end host with misleading location

2.3. Location Spoofing

 Where location is attached to the emergency call by an end host, the
 end host can fabricate a PIDF-LO and convey it within an emergency
 call.  The following represent examples of location spoofing:
 Place shifting:  Mallory, the adversary, pretends to be at an
                  arbitrary location.
 Time shifting:   Mallory pretends to be at a location where she was
                  a while ago.
 Location theft:  Mallory observes or obtains Alice's location and
                  replays it as her own.

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2.4. Identity Spoofing

 While this document does not focus on the problems created by
 determination of location based on spoofed caller identification, the
 ability to ascertain identity is important, since the threat of
 punishment reduces hoax calls.  As an example, calls from pay phones
 are subject to greater scrutiny by the call taker.
 With calls originating on an IP network, at least two forms of
 identity are relevant, with the distinction created by the split
 between the IAP and the VSP:
 (a) network access identity such as might be determined via
     authentication (e.g., using the Extensible Authentication
     Protocol (EAP) [RFC3748]);
 (b) caller identity, such as might be determined from authentication
     of the emergency caller at the VoIP application layer.
 If the adversary did not authenticate itself to the VSP, then
 accountability may depend on verification of the network access
 identity.  However, the network access identity may also not have
 been authenticated, such as in the case where an open IEEE 802.11
 Access Point is used to initiate a hoax emergency call.  Although
 endpoint information such as the IP address or Media Access Control
 (MAC) address may have been logged, tying this back to the Device
 owner may be challenging.
 Unlike the existing telephone system, VoIP emergency calls can
 provide an identity that need not necessarily be coupled to a
 business relationship with the IAP, ISP, or VSP.  However, due to the
 time-critical nature of emergency calls, multi-layer authentication
 is undesirable.  Thus, in most cases, only the Device placing the
 call will be able to be identified.  Furthermore, deploying
 additional credentials for emergency service purposes (such as
 certificates) increases costs, introduces a significant
 administrative overhead, and is only useful if widely deployed.

3. Mitigation Techniques

 The sections that follow present three mechanisms for mitigating the
 threats presented in Section 2:
 1. Signed location-by-value (Section 3.1), which provides for
    authentication and integrity protection of the PIDF-LO.  There is
    only an expired straw-man proposal for this mechanism
    [Loc-Dependability]; thus, as of the time of this writing this
    mechanism is not suitable for deployment.

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 2. Location-by-reference (Section 3.2), which enables location to be
    obtained by the PSAP directly from the location server, over a
    confidential and integrity-protected channel, avoiding
    modification by the end host or an intermediary.  This mechanism
    is specified in [RFC6753].
 3. Proxy-added location (Section 3.3), which protects against
    location forgery by the end host.  This mechanism is specified in

3.1. Signed Location-by-Value

 With location signing, a location server signs the location
 information before it is sent to the Target.  The signed location
 information is then sent to the Location Recipient, who verifies it.
 Figure 1 shows the communication model with the Target requesting
 signed location in step (a); the location server returns it in
 step (b), and it is then conveyed to the Location Recipient, who
 verifies it (step (c)).  For SIP, the procedures described in
 "Location Conveyance for the Session Initiation Protocol" [RFC6442]
 are applicable for location conveyance.
                 +-----------+               +-----------+
                 |           |               | Location  |
                 |    LIS    |               | Recipient |
                 |           |               |           |
                 +-+-------+-+               +----+------+
                   ^       |                    --^
                   |       |                  --
     Geopriv       |Req.   |                --
     Location      |Signed |Signed        -- Protocol Conveying
     Configuration |Loc.   |Loc.        --   Location (e.g., SIP)
     Protocol      |(a)    |(b)       --     (c)
                   |       v        --
                 +-+-------+-+    --
                 | Target /  |  --
                 | End Host  +
                 |           |
                      Figure 1: Location Signing
 A straw-man proposal for location signing is provided in "Digital
 Signature Methods for Location Dependability" [Loc-Dependability].
 Note that since [Loc-Dependability] is no longer under development,
 location signing cannot be considered deployable at the time of this

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 In order to limit replay attacks, that proposal calls for the
 addition of a "validity" element to the PIDF-LO, including a "from"
 sub-element containing the time that location information was
 validated by the signer, as well as an "until" sub-element containing
 the last time that the signature can be considered valid.
 One of the consequences of including an "until" element is that even
 a stationary Target would need to periodically obtain a fresh
 PIDF-LO, or incur the additional delay of querying during an
 emergency call.
 Although privacy-preserving procedures may be disabled for emergency
 calls, by design, PIDF-LO objects limit the information available for
 real-time attribution.  As noted in [RFC5985], Section 6.6:
    The LIS MUST NOT include any means of identifying the Device in
    the PIDF-LO unless it is able to verify that the identifier is
    correct and inclusion of identity is expressly permitted by a Rule
    Maker.  Therefore, PIDF parameters that contain identity are
    either omitted or contain unlinked pseudonyms [RFC3693].  A
    unique, unlinked presentity URI SHOULD be generated by the LIS for
    the mandatory presence "entity" attribute of the PIDF document.
    Optional parameters such as the "contact" and "deviceID" elements
    [RFC4479] are not used.
 Also, the Device referred to in the PIDF-LO may not necessarily be
 the same entity conveying the PIDF-LO to the PSAP.  As noted in
 [RFC6442], Section 1:
    In no way does this document assume that the SIP user agent client
    that sends a request containing a location object is necessarily
    the Target.  The location of a Target conveyed within SIP
    typically corresponds to that of a Device controlled by the
    Target, for example, a mobile phone, but such Devices can be
    separated from their owners, and moreover, in some cases, the user
    agent may not know its own location.
 Without the ability to tie the Target identity to the identity
 asserted in the SIP message, it is possible for an attacker to cut
 and paste a PIDF-LO obtained by a different Device or user into a SIP
 INVITE and send this to the PSAP.  This cut-and-paste attack could
 succeed even when a PIDF-LO is signed or when [RFC4474] is

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 To address location-spoofing attacks, [Loc-Dependability] proposes
 the addition of an "identity" element that could include a SIP URI
 (enabling comparison against the identity asserted in the SIP
 headers) or an X.509v3 certificate.  If the Target was authenticated
 by the LIS, an "authenticated" attribute is added.  However, because
 the inclusion of an "identity" element could enable location
 tracking, a "hash" element is also proposed that could instead
 contain a hash of the content of the "identity" element.  In
 practice, such a hash would not be much better for real-time
 validation than a pseudonym.
 Location signing cannot deter attacks in which valid location
 information is provided.  For example, an attacker in control of
 compromised hosts could launch a denial-of-service attack on the PSAP
 by initiating a large number of emergency calls, each containing
 valid signed location information.  Since the work required to verify
 the location signature is considerable, this could overwhelm the PSAP
 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
 tamper-proof GPS modules, can be applied.  Otherwise, an end host can
 pretend to have GPS, and the Recipient will need to rely on its
 ability to assess the level of trust that should be placed in the end
 host location claim.
 Even though location-signing mechanisms have not been standardized,
 [NENA-i2], Section 4.7 includes operational recommendations relating
 to location signing:
    Location configuration and conveyance requirements are described
    in NENA 08-752[27], but guidance is offered here on what should be
    considered when designing mechanisms to report location:
    1. The location object should be digitally signed.
    2. The certificate for the signer (LIS operator) should be rooted
       in VESA.  For this purpose, VPC and ERDB operators should issue
       certificates to LIS operators.
    3. The signature should include a timestamp.

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    4. Where possible, the Location Object should be refreshed
       periodically, with the signature (and thus the timestamp) being
       refreshed as a consequence.
    5. Antispoofing mechanisms should be applied to the Location
       Reporting method.
 (Note: The term "Valid Emergency Services Authority" (VESA) refers to
 the root certificate authority.  "VPC" stands for VoIP Positioning
 Center, and "ERDB" stands for the Emergency Service Zone Routing
 As noted above, signing of location objects implies the development
 of a trust hierarchy that would enable a certificate chain provided
 by the LIS operator to be verified by the PSAP.  Rooting the trust
 hierarchy in the VESA can be accomplished either by having the VESA
 directly sign the LIS certificates or by the creation of intermediate
 Certificate Authorities (CAs) certified by the VESA, which will then
 issue certificates to the LIS.  In terms of the workload imposed on
 the VESA, the latter approach is highly preferable.  However, this
 raises the question of who would operate the intermediate CAs and
 what the expectations would be.
 In particular, the question arises as to the requirements for LIS
 certificate issuance, and how they would compare to requirements for
 issuance of other certificates such as a Secure Socket
 Layer/Transport Layer Security (SSL/TLS) web certificate.

3.2. Location-by-Reference

 Location-by-reference was developed so that end hosts can avoid
 having to periodically query the location server for up-to-date
 location information in a mobile environment.  Additionally, if
 operators do not want to disclose location information to the end
 host without charging them, location-by-reference provides a
 reasonable alternative.  Also, since location-by-reference enables
 the PSAP to directly contact the location server, it avoids potential
 attacks by intermediaries.
 As noted in "A Location Dereference Protocol Using HTTP-Enabled
 Location Delivery (HELD)" [RFC6753], a location reference can be
 obtained via HELD [RFC5985].  In addition, "Location Configuration
 Extensions for Policy Management" [RFC7199] extends location
 configuration protocols such as HELD to provide hosts with a
 reference to the rules that apply to a location-by-reference so that
 the host can view or set these rules.

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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 and, potentially, the policy in step (b), and it is then
 conveyed to the Location Recipient in step (c).  The Location
 Recipient needs to resolve the reference with a request in step (d).
 Finally, location information is returned to the Location Recipient
 afterwards.  For location conveyance in SIP, the procedures described
 in [RFC6442] are applicable.
                 +-----------+  Geopriv      +-----------+
                 |           |  Location     | Location  |
                 |    LIS    +<------------->+ Recipient |
                 |           | Dereferencing |           |
                 +-+-------+-+ Protocol (d)  +----+------+
                   ^       |                    --^
                   |       |                  --
     Geopriv       |Req.   |LbyR +          --
     Location      |LbyR   |Policy        -- Protocol Conveying
     Configuration |(a)    |(b)         --   Location (e.g., SIP)
     Protocol      |       |          --     (c)
                   |       V        --
                 +-+-------+-+    --
                 | Target /  |  --
                 | End Host  +
                 |           |
                    Figure 2: Location-by-Reference
 Where location-by-reference is provided, the Recipient needs to
 dereference the LbyR in order to obtain location.  The details for
 the dereferencing operations vary with the type of reference, such as
 an HTTP, HTTPS, SIP, secure SIP (SIPS), or SIP Presence URI.
 For location-by-reference, the location server needs to maintain one
 or several URIs for each Target, timing out these URIs after a
 certain amount of time.  References need to expire to prevent the
 Recipient of such a Uniform Resource Locator (URL) from being able to
 permanently track a host and to offer garbage collection
 functionality for the location server.
 Off-path adversaries must be prevented from obtaining the Target's
 location.  The reference contains a randomized component that
 prevents third parties from guessing it.  When the Location Recipient
 fetches up-to-date location information from the location server, it
 can also be assured that the location information is fresh and not
 replayed.  However, this does not address location theft.

Tschofenig, et al. Informational [Page 16] RFC 7378 Trustworthy Location December 2014

 With respect to the security of the dereference operation, [RFC6753],
 Section 6 states:
    TLS MUST be used for dereferencing location URIs unless
    confidentiality and integrity are provided by some other
    mechanism, as discussed in Section 3.  Location Recipients MUST
    authenticate the host identity using the domain name included in
    the location URI, using the procedure described in Section 3.1 of
    [RFC2818].  Local policy determines what a Location Recipient does
    if authentication fails or cannot be attempted.
    The authorization by possession model (Section 4.1) further relies
    on TLS when transmitting the location URI to protect the secrecy
    of the URI.  Possession of such a URI implies the same privacy
    considerations as possession of the PIDF-LO document that the URI
    Location URIs MUST only be disclosed to authorized Location
    Recipients.  The GEOPRIV architecture [RFC6280] designates the
    Rule Maker to authorize disclosure of the URI.
    Protection of the location URI is necessary, since the policy
    attached to such a location URI permits anyone who has the URI to
    view the associated location information.  This aspect of security
    is covered in more detail in the specification of location
    conveyance protocols, such as [RFC6442].
 For authorizing access to location-by-reference, two authorization
 models were developed: "Authorization by Possession" and
 "Authorization via Access Control Lists".  With respect to
 "Authorization by Possession", [RFC6753], Section 4.1 notes:
    In this model, possession -- or knowledge -- of the location URI
    is used to control access to location information.  A location URI
    might be constructed such that it is hard to guess (see C8 of
    [RFC5808]), and the set of entities that it is disclosed to can be
    limited.  The only authentication this would require by the LS is
    evidence of possession of the URI.  The LS could immediately
    authorize any request that indicates this URI.
    Authorization by possession does not require direct interaction
    with a Rule Maker; it is assumed that the Rule Maker is able to
    exert control over the distribution of the location URI.
    Therefore, the LIS can operate with limited policy input from a
    Rule Maker.

Tschofenig, et al. Informational [Page 17] RFC 7378 Trustworthy Location December 2014

    Limited disclosure is an important aspect of this authorization
    model.  The location URI is a secret; therefore, ensuring that
    adversaries are not able to acquire this information is paramount.
    Encryption, such as might be offered by TLS [RFC5246] or S/MIME
    [RFC5751], protects the information from eavesdroppers.
    Using possession as a basis for authorization means that, once
    granted, authorization cannot be easily revoked.  Cancellation of
    a location URI ensures that legitimate users are also affected;
    application of additional policy is theoretically possible but
    could be technically infeasible.  Expiration of location URIs
    limits the usable time for a location URI, requiring that an
    attacker continue to learn new location URIs to retain access to
    current location information.
 In situations where "Authorization by Possession" is not suitable
 (such as where location hiding [RFC6444] is required), the
 "Authorization via Access Control Lists" model may be preferred.
 Without the introduction of a hierarchy, it would be necessary for
 the PSAP to obtain credentials, such as certificates or shared
 symmetric keys, for all the LISs in its coverage area, to enable it
 to successfully dereference LbyRs.  In situations with more than a
 few LISs per PSAP, this would present operational challenges.
 A certificate hierarchy providing PSAPs with client certificates
 chaining to the VESA could be used to enable the LIS to authenticate
 and authorize PSAPs for dereferencing.  Note that unlike PIDF-LO
 signing (which mitigates modification of PIDF-LOs), this merely
 provides the PSAP with access to a (potentially unsigned) PIDF-LO,
 albeit over a protected TLS channel.
 Another approach would be for the local LIS to upload location
 information to a location aggregation point who would in turn manage
 the relationships with the PSAP.  This would shift the management
 burden from the PSAPs to the location aggregation points.

3.3. Proxy-Added Location

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

Tschofenig, et al. Informational [Page 18] RFC 7378 Trustworthy Location December 2014

 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 4).
 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
    location is more accurate than another.

Tschofenig, et al. Informational [Page 19] RFC 7378 Trustworthy Location December 2014

 The disadvantage of this approach is the need to deploy application-
 layer entities, such as SIP proxies, at IAPs or associated with IAPs.
 This requires that a standardized VoIP profile be deployed at every
 end Device and at every IAP.  This might impose interoperability
 Additionally, the IAP needs to take responsibility for emergency
 calls, even for customers with whom they have no direct or indirect
 relationship.  To provide identity information about the emergency
 caller from the VSP, it would be necessary to let the IAP and the VSP
 interact for authentication (see, for example, "Diameter Session
 Initiation Protocol (SIP) Application" [RFC4740]).  This interaction
 along the Authentication, Authorization, and Accounting
 infrastructure is often based on business relationships between the
 involved entities.  An arbitrary IAP and VSP are unlikely to have a
 business relationship.  If the interaction between the IAP and the
 VSP fails due to the lack of a business relationship, then typically
 a fall-back would be provided where no emergency caller identity
 information is made available to the PSAP and the emergency call
 still has to be completed.

4. Location Trust Assessment

 The ability to assess the level of trustworthiness of conveyed
 location information is important, since this makes it possible to
 understand how much value should be placed on location information as
 part of the decision-making process.  As an example, if automated
 location information is understood to be highly suspect or is absent,
 a call taker can put more effort into verifying the authenticity of
 the call and obtaining location information from the caller.
 Location trust assessment has value, regardless of whether the
 location itself is authenticated (e.g., signed location) or is
 obtained directly from the location server (e.g., location-by-
 reference) over security transport, since these mechanisms do not
 provide assurance of the validity or provenance of location data.
 To prevent location-theft attacks, the "entity" element of the
 PIDF-LO is of limited value if an unlinked pseudonym is provided in
 this field.  However, if the LIS authenticates the Target, then the
 linkage between the pseudonym and the Target identity can be
 recovered in a post-incident investigation.

Tschofenig, et al. Informational [Page 20] RFC 7378 Trustworthy Location December 2014

 As noted in [Loc-Dependability], if the location object was signed,
 the Location Recipient has additional information on which to base
 their trust assessment, such as the validity of the signature, the
 identity of the Target, the identity of the LIS, whether the LIS
 authenticated the Target, and the identifier included in the "entity"
 Caller accountability is also an important aspect of trust
 assessment.  Can the individual purchasing the Device or activating
 service be identified, or did the call originate from a non-service
 initialized (NSI) Device whose owner cannot be determined?  Prior to
 the call, was the caller authenticated at the network or application
 layer?  In the event of a hoax call, can audit logs be made available
 to an investigator, or can information relating to the owner of an
 unlinked pseudonym be provided, enabling investigators to unravel the
 chain of events that led to the attack?
 In practice, the source of the location data is important for
 location trust assessment.  For example, location provided by a
 Location Information Server (LIS) whose administrator has an
 established history of meeting emergency location accuracy
 requirements (e.g., United States Phase II E-911 location accuracy)
 may be considered more reliable than location information provided by
 a third-party Location Service Provider (LSP) that disclaims use of
 location information for emergency purposes.
 However, even where an LSP does not attempt to meet the accuracy
 requirements for emergency location, it still may be able to provide
 information useful in assessing how reliable location information is
 likely to be.  For example, was location determined based on the
 nearest cell tower or 802.11 Access Point (AP), or was a
 triangulation method used?  If based on cell tower or AP location
 data, was the information obtained from an authoritative source
 (e.g., the tower or AP owner), and when was the last time that the
 location of the tower or access point was verified?
 For real-time validation, information in the signaling and media
 packets can be cross-checked against location information.  For
 example, it may be possible to determine the city, state, country, or
 continent associated with the IP address included within SIP Via or
 Contact header fields, or the media source address, and compare this
 against the location information reported by the caller or conveyed
 in the PIDF-LO.  However, in some situations, only entities close to
 the caller may be able to verify the correctness of location

Tschofenig, et al. Informational [Page 21] RFC 7378 Trustworthy Location December 2014

 Real-time validation of the timestamp contained within PIDF-LO
 objects (reflecting the time at which the location was determined) is
 also challenging.  To address time-shifting attacks, the "timestamp"
 element of the PIDF-LO, defined in [RFC3863], can be examined and
 compared against timestamps included within the enclosing SIP
 message, to determine whether the location data is sufficiently
 fresh.  However, the timestamp only represents an assertion by the
 LIS, which may or may not be trustworthy.  For example, the Recipient
 of the signed PIDF-LO may not know whether the LIS supports time
 synchronization, or whether it is possible to reset the LIS clock
 manually without detection.  Even if the timestamp was valid at the
 time location was determined, a time period may elapse between when
 the PIDF-LO was provided and when it is conveyed to the Recipient.
 Periodically refreshing location information to renew the timestamp
 even though the location information itself is unchanged puts
 additional load on LISs.  As a result, Recipients need to validate
 the timestamp in order to determine whether it is credible.
 While this document focuses on the discussion of real-time
 determination of suspicious emergency calls, the use of audit logs
 may help in enforcing accountability among emergency callers.  For
 example, in the event of a hoax call, information relating to the
 owner of the unlinked pseudonym could be provided to investigators,
 enabling them to unravel the chain of events that led to the attack.
 However, while auditability is an important deterrent, it is likely
 to be of most benefit in situations where attacks on the emergency
 services system are likely to be relatively infrequent, since the
 resources required to pursue an investigation are likely to be
 considerable.  However, although real-time validation based on
 PIDF-LO elements is challenging, where LIS audit logs are available
 (such as where a law enforcement agency can present a subpoena),
 linking of a pseudonym to the Device obtaining location can be
 accomplished during an investigation.
 Where attacks are frequent and continuous, automated mechanisms are
 required.  For example, it might be valuable to develop mechanisms to
 exchange audit trail information in a standardized format between
 ISPs and PSAPs / VSPs and PSAPs or heuristics to distinguish
 potentially fraudulent emergency calls from real emergencies.  While
 a Completely Automated Public Turing test to tell Computers and
 Humans Apart (CAPTCHA) may be applied to suspicious calls to lower
 the risk from bot-nets, this is quite controversial for emergency
 services, due to the risk of delaying or rejecting valid calls.

Tschofenig, et al. Informational [Page 22] RFC 7378 Trustworthy Location December 2014

5. Security Considerations

 Although it is important to ensure that location information cannot
 be faked, the mitigation techniques presented in this document are
 not universally applicable.  For example, there will be many GPS-
 enabled Devices that will find it difficult to utilize any of the
 solutions described in Section 3.  It is also unlikely that users
 will be willing to upload their location information for
 "verification" to a nearby location server located in the access
 This document focuses on threats that arise from conveyance of
 misleading location information, rather than caller identification or
 authentication and integrity protection of the messages in which
 location is conveyed.  Nevertheless, these aspects are important.  In
 some countries, regulators may not require the authenticated identity
 of the emergency caller (e.g., emergency calls placed from Public
 Switched Telephone Network (PSTN) pay phones or SIM-less cell
 phones).  Furthermore, if identities can easily be crafted (as is the
 case with many VoIP offerings today), then the value of emergency
 caller authentication itself might be limited.  As a result,
 attackers can forge emergency calls with a lower risk of being held
 accountable, which may encourage hoax calls.
 In order to provide authentication and integrity protection for the
 Session Initiation Protocol (SIP) messages conveying location,
 several security approaches are available.  It is possible to ensure
 that modification of the identity and location in transit can be
 detected by the Location Recipient (e.g., the PSAP), using
 cryptographic mechanisms, as described in "Enhancements for
 Authenticated Identity Management in the Session Initiation Protocol
 (SIP)" [RFC4474].  However, compatibility with Session Border
 Controllers (SBCs) that modify integrity-protected headers has proven
 to be an issue in practice, and as a result, a revision of [RFC4474]
 is in progress [SIP-Identity].  In the absence of an end-to-end
 solution, SIP over Transport Layer Security (TLS) can be used to
 provide message authentication and integrity protection hop by hop.
 PSAPs remain vulnerable to distributed denial-of-service attacks,
 even where the mitigation techniques described in this document are
 utilized.  Placing a large number of emergency calls that appear to
 come from different locations is an example of an attack that is
 difficult to carry out within the legacy system but is easier to
 imagine within IP-based emergency services.  Also, in the current
 system, it would be very difficult for an attacker from one country
 to attack the emergency services infrastructure located in another
 country, but this attack is possible within IP-based emergency

Tschofenig, et al. Informational [Page 23] RFC 7378 Trustworthy Location December 2014

 While manually mounting the attacks described in Section 2 is
 non-trivial, the attacks described in this document can be automated.
 While manually carrying out a location theft would require that the
 attacker be in proximity to the location being spoofed, or to collude
 with another end host, an attacker able to run code on an end host
 can obtain its location and cause an emergency call to be made.
 While manually carrying out a time-shifting attack would require that
 the attacker visit the location and submit it before the location
 information is considered stale, while traveling rapidly away from
 that location to avoid apprehension, these limitations would not
 apply to an attacker able to run code on the end host.  While
 obtaining a PIDF-LO from a spoofed IP address requires that the
 attacker be on the path between the HELD requester and the LIS, if
 the attacker is able to run code requesting the PIDF-LO, retrieve it
 from the LIS, and then make an emergency call using it, this attack
 becomes much easier.  To mitigate the risk of automated attacks,
 service providers can limit the ability of untrusted code (such as
 WebRTC applications written in JavaScript) to make emergency calls.
 Emergency services have three finite resources subject to denial-of-
 service attacks: the network and server infrastructure; call takers
 and dispatchers; and the first responders, such as firefighters and
 police officers.  Protecting the network infrastructure is similar to
 protecting other high-value service providers, except that location
 information may be used to filter call setup requests, to weed out
 requests that are out of area.  Even for large cities, PSAPs may only
 have a handful of call takers on duty.  So, even if automated
 techniques are utilized to evaluate the trustworthiness of conveyed
 location and call takers can, by questioning the caller, eliminate
 many hoax calls, PSAPs can be overwhelmed even by a small-scale
 attack.  Finally, first-responder resources are scarce, particularly
 during mass-casualty events.

6. Privacy Considerations

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

Tschofenig, et al. Informational [Page 24] RFC 7378 Trustworthy Location December 2014

 The intent of the location privacy architecture was to provide strong
 privacy protections, as noted in [RFC6280], Section 1.1:
    A central feature of the Geopriv architecture is that location
    information is always bound to privacy rules to ensure that
    entities that receive location information are informed of how
    they may use it.  These rules can convey simple directives ("do
    not share my location with others"), or more robust preferences
    ("allow my spouse to know my exact location all of the time, but
    only allow my boss to know it during work hours")...  The binding
    of privacy rules to location information can convey users' desire
    for and expectations of privacy, which in turn helps to bolster
    social and legal systems' protection of those expectations.
 However, in practice this architecture has limitations that apply
 within emergency and non-emergency situations.  As noted in
 Section 1.2.2, concerns about hoax calls have led to restrictions on
 anonymous emergency calls.  Caller identification (potentially
 asserted in SIP via P-Asserted-Identity and SIP Identity) may be used
 during emergency calls.  As a result, in many cases location
 information transmitted within SIP messages can be linked to caller
 identity.  For example, in the case of a signed LbyV, there are
 privacy concerns arising from linking the location object to
 identifiers to prevent replay attacks, as described in Section 3.1.
 The ability to observe location information during emergency calls
 may also represent a privacy risk.  As a result, [RFC6443] requires
 transmission-layer security for SIP messages, as well as interactions
 with the location server.  However, even where transmission-layer
 security is used, privacy rules associated with location information
 may not apply.
 In many jurisdictions, an individual requesting emergency assistance
 is assumed to be granting permission to the PSAP, call taker, and
 first responders to obtain their location in order to accelerate
 dispatch.  As a result, privacy policies associated with location are
 implicitly waived when an emergency call is initiated.  In addition,
 when location information is included within SIP messages in either
 emergency or non-emergency uses, SIP entities receiving the SIP
 message are implicitly assumed to be authorized Location Recipients,
 as noted in [RFC5606], Section 3.2:
    Consensus has emerged that any SIP entity that receives a SIP
    message containing LI through the operation of SIP's normal
    routing procedures or as a result of location-based routing should
    be considered an authorized recipient of that LI.  Because of this
    presumption, one SIP element may pass the LI to another even if
    the LO it contains has <retransmission-allowed> set to "no"; this

Tschofenig, et al. Informational [Page 25] RFC 7378 Trustworthy Location December 2014

    sees the passing of the SIP message as part of the delivery to
    authorized recipients, rather than as retransmission.  SIP
    entities are still enjoined from passing these messages
    outside the normal routing to external entities if
    <retransmission-allowed> is set to "no", as it is the passing to
    third parties that <retransmission-allowed> is meant to control.
 Where LbyR is utilized rather than LbyV, it is possible to apply more
 restrictive authorization policies, limiting access to intermediaries
 and snoopers.  However, this is not possible if the "authorization by
 possession" model is used.

7. Informative References

 [EENA]     EENA, "False Emergency Calls", EENA Operations Document,
            Version 1.1, May 2011, <
            Warner, J. and R. Johnston, "GPS Spoofing
            Countermeasures", Los Alamos research paper LAUR-03-6163,
            December 2003.
            Thomson, M. and J. Winterbottom, "Digital Signature
            Methods for Location Dependability", Work in Progress,
            March 2011.
 [NENA-i2]  NENA 08-001, "NENA Interim VoIP Architecture for Enhanced
            9-1-1 Services (i2)", Version 2, August 2010.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997,
 [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000,
 [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
            A., Peterson, J., Sparks, R., Handley, M., and E.
            Schooler, "SIP: Session Initiation Protocol", RFC 3261,
            June 2002, <>.
 [RFC3693]  Cuellar, J., Morris, J., Mulligan, D., Peterson, J., and
            J. Polk, "Geopriv Requirements", RFC 3693, February 2004,

Tschofenig, et al. Informational [Page 26] RFC 7378 Trustworthy Location December 2014

 [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, Ed., "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,
 [RFC4119]  Peterson, J., "A Presence-based GEOPRIV Location Object
            Format", RFC 4119, December 2005,
 [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
            Authenticated Identity Management in the Session
            Initiation Protocol (SIP)", RFC 4474, August 2006,
 [RFC4479]  Rosenberg, J., "A Data Model for Presence", RFC 4479,
            July 2006, <>.
 [RFC4740]  Garcia-Martin, M., Ed., Belinchon, M., Pallares-Lopez, M.,
            Canales-Valenzuela, C., and K. Tammi, "Diameter Session
            Initiation Protocol (SIP) Application", RFC 4740,
            November 2006, <>.
 [RFC5012]  Schulzrinne, H. and R. Marshall, Ed., "Requirements for
            Emergency Context Resolution with Internet Technologies",
            RFC 5012, January 2008,
 [RFC5069]  Taylor, T., Ed., Tschofenig, H., Schulzrinne, H., and M.
            Shanmugam, "Security Threats and Requirements for
            Emergency Call Marking and Mapping", RFC 5069,
            January 2008, <>.
 [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.2", RFC 5246, August 2008,

Tschofenig, et al. Informational [Page 27] RFC 7378 Trustworthy Location December 2014

 [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,
 [RFC5606]  Peterson, J., Hardie, T., and J. Morris, "Implications of
            'retransmission-allowed' for SIP Location Conveyance",
            RFC 5606, August 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., Ed., "Requirements for a Location-by-
            Reference Mechanism", RFC 5808, May 2010,
 [RFC5985]  Barnes, M., Ed., "HTTP-Enabled Location Delivery (HELD)",
            RFC 5985, September 2010,
 [RFC6280]  Barnes, R., Lepinski, M., Cooper, A., Morris, J.,
            Tschofenig, H., and H. Schulzrinne, "An Architecture for
            Location and Location Privacy in Internet Applications",
            BCP 160, RFC 6280, July 2011,
 [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,

Tschofenig, et al. Informational [Page 28] RFC 7378 Trustworthy Location December 2014

 [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,
 [RFC7199]  Barnes, R., Thomson, M., Winterbottom, J., and H.
            Tschofenig, "Location Configuration Extensions for Policy
            Management", RFC 7199, April 2014,
 [RFC7340]  Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
            Telephone Identity Problem Statement and Requirements",
            RFC 7340, September 2014,
 [RFC7375]  Peterson, J., "Secure Telephone Identity Threat Model",
            RFC 7375, October 2014,
 [SA]       "Saudi Arabia - Illegal sale of SIMs blamed for surge in
            hoax calls", Arab News, April 5, 2010,
            Peterson, J., Jennings, C. and E. Rescorla, "Authenticated
            Identity Management in the Session Initiation Protocol
            (SIP)", Work in Progress, draft-ietf-stir-rfc4474bis-02,
            October 2014.
 [STIR]     IETF, "Secure Telephone Identity Revisited (stir) Working
            Group", October 2013,
 [SWATing]  "SWATing 911 Calls", Dispatch Magazine On-Line,
            April 6, 2013, <
 [Swatting] "Don't Make the Call: The New Phenomenon of 'Swatting'",
            Federal Bureau of Investigation, February 4, 2008,

Tschofenig, et al. Informational [Page 29] RFC 7378 Trustworthy Location December 2014

 [TASMANIA] "Emergency services seek SIM-less calls block", ABC News
            Online, August 18, 2006, <
 [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 document pointed in the right direction.  We
 would also like to thank members of the IETF GEOPRIV working group,
 including Richard Barnes, Matt Lepinski, Andrew Newton, Murugaraj
 Shanmugam, and Martin Thomson for their feedback on previous versions
 of this document.  Alissa Cooper, Adrian Farrel, Pete Resnick, Meral
 Shirazipour, and Bert Wijnen provided helpful review comments during
 the IETF last call.

Tschofenig, et al. Informational [Page 30] RFC 7378 Trustworthy Location December 2014

Authors' Addresses

 Hannes Tschofenig
 Henning Schulzrinne
 Columbia University
 Department of Computer Science
 450 Computer Science Building
 New York, NY  10027
 United States
 Phone: +1 212 939 7004
 Bernard Aboba (editor)
 Microsoft Corporation
 One Microsoft Way
 Redmond, WA  98052
 United States

Tschofenig, et al. Informational [Page 31]

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