wdiff rfc7378v5.txt rfc7378.txt

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

Trustworthy Location

Abstract

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

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

Status of This Memo

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

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7378.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

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

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, 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, 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
Device initiating the call. Where real-time assessment is not
possible, it is important to be able to determine the source of the
call in a post-incident investigation, so as to be able to enforce
accountability.

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

1.1. Terminology

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

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 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. 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 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 reused from "Requirements for a Location-by-Reference
Mechanism" [RFC5808].

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

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

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

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

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

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

"Location Theft" occurs when the attacker captures a Target's
location information (possibly including a signature) and presents it
as their own. Location theft can occur in a single instance or may
be continuous (e.g., where the attacker has gained control over the
victim's device). 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.

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.

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.

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.

As noted in [Swatting], many

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. 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.

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.

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

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

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

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

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

2.3. Location Spoofing

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

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

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

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

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 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 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.

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

3.1. Signed Location-by-Value

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

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

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

Figure 1: Location Signing

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

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 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 Device controlled by the
Target, for example, a mobile phone, but such devices 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 Device or user into a SIP
INVITE and send this to the PSAP. This cut-and-paste attack could
succeed even when a PIDF-LO is signed or when [RFC4474] is
implemented.

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

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

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

Location signing is also difficult when the host obtains location via
mechanisms such as GPS, unless trusted computing approaches, with
tamper-proof GPS modules, can be applied. Otherwise, an end host can
pretend to have a GPS device, 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.

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

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

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

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

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

3.2. Location-by-Reference

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

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

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.

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.

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

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

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

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

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

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

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

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

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 Adding 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.

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

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

...

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

As noted in [RFC6442], Section 3.3:

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

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

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

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 Device and at every IAP. This might impose interoperability
challenges.

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

4. Location Trust Assessment

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

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

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

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

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

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

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

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

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 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.

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 Devices that will find it difficult to utilize any of the
solutions described in Section 3. It is also unlikely that users
will be willing to upload their location information for
"verification" to a nearby location server located in the access
network.

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

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

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

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

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

6. Privacy Considerations

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

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
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

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

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

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

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

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

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

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

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

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

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

[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002, <http://www.rfc-editor.org/info/rfc3261>.

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

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

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

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

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

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

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

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

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

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

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

[RFC5491] Winterbottom, J., Thomson, M., and H. Tschofenig, "GEOPRIV
Presence Information Data Format Location Object (PIDF-LO)
Usage Clarification, Considerations, and Recommendations",
RFC 5491, March 2009, <http://www.rfc-editor.org/
info/rfc5491>.

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

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

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

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

[RFC6280] Barnes, R., Lepinski, M., Cooper, A., Morris, J.,
Tschofenig, H., and H. Schulzrinne, "An Architecture for
Location and Location Privacy in Internet Applications",
BCP 160, RFC 6280, July 2011, <http://www.rfc-editor.org/
info/rfc6280>.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

[TASMANIA] "Emergency services seek SIM-less calls block", ABC News
Online, August 18, 2006, <http://www.abc.net.au/elections/
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[UK] "Rapper makes thousands of prank 999 emergency calls to UK
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Acknowledgments

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

Authors' Addresses

Hannes Tschofenig
Austria

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

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

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

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

EMail: bernard_aboba@hotmail.com