ECRIT Working Group H. Tschofenig
INTERNET-DRAFT Nokia Siemens Networks
Category: Informational H. Schulzrinne
Expires: October 4, 2012 Columbia University
B. Aboba (ed.)
Microsoft Corporation
4 April 2012
Trustworthy Location Information
draft-ietf-ecrit-trustworthy-location-03.txt
Abstract
For some location-based applications, such as emergency calling or
roadside assistance, it is important to be able to determine whether
location information is trustworthy.
This document outlines potential threats to trustworthy location and
analyzes the operational issues with potential solutions.
Status of This Memo
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provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 4, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Threats . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Location Spoofing . . . . . . . . . . . . . . . . . . . . 6
3.2. Identity Spoofing . . . . . . . . . . . . . . . . . . . . 7
4. Solution Proposals . . . . . . . . . . . . . . . . . . . . . . 8
4.1. Location Signing . . . . . . . . . . . . . . . . . . . . . 8
4.2. Location by Reference . . . . . . . . . . . . . . . . . . 9
4.3. Proxy Adding Location . . . . . . . . . . . . . . . . . . 11
5. Operational Considerations . . . . . . . . . . . . . . . . . . 11
5.1. Attribution to a Specific Trusted Source . . . . . . . . . 11
5.2. Application to a Specific Point in Time . . . . . . . . . 16
5.3. Linkage to a Specific Endpoint . . . . . . . . . . . . . . 16
6. Security Considerations . . . . . . . . . . . . . . . . . . . 17
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.1. Informative references . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
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1. Introduction
The operation of a number of public and commercial services depend
upon location information in their operations. Emergency services,
such as fire department, ambulance and police, are among these, as
are commercial services such as food delivery and roadside
assistance.
Users of the telephone network can summon emergency services such as
ambulance, fire and police using a well-known emergency service
number (e.g., 9-1-1 in North America, 1-1-2 in Europe). Location
information is used to route emergency calls to the appropriate
regional Public Safety Answering Point (PSAP) that serves the caller
to dispatch first-level responders to the emergency site.
Physical security is often based on location. Light switches in
buildings are not typically protected by keycards or passwords, but
are only accessible to those within the perimeter of the building.
Merchants processing credit card payments already use location
information to estimate the risk that a transaction is fraudulent,
based on translation of the HTTP client's IP address to an estimated
location. In these cases, location information can be used to
augment identity or, in some cases, avoid the need for role-based
authorization.
For services that depend on the accuracy of location information in
their operation, the ability to determine the trustworthiness of
location information may be important. Prank calls have been a
problem for emergency services, dating back to the time of street
corner call boxes. Individual prank calls waste emergency services
and possibly endanger bystanders or emergency service personnel as
they rush to the reported scene of a fire or accident. However, a
recent increase in the frequency and sophistication of the attacks
has lead to the FBI issuing a warning [Swatting].
In situations where it is possible to place emergency calls without
accountability, experience has shown that the frequency of nuisance
calls can rise dramatically. For example, where emergency calls have
been allowed from handsets lacking a SIM card, or where ownership of
the SIM card cannot be determined, the frequency of nuisance calls
has often been unacceptably high [TASMANIA][UK][SA].
In emergency services deployments utilizing voice over IP, many of
the assumptions of the public switched telephone network (PSTN) and
public land mobile network (PLMN) no longer hold. While the local
telephone company provides both physical access and the phone
service, with VoIP there is a split between the role of the Access
Infrastructure Provider (AIP), and the Application (Voice) Service
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Provider (VSP). The VSP may be located far away from the AIP and may
either have no business relationship with the AIP or may be a
competitor. It is also likely that the VSP will have no relationship
with the PSAP.
In some situations it is possible for the end host to determine its
own location using technology such as the Global Positioning System
(GPS). Where the end host cannot determine location on its own,
mechanisms have been standardized to make civic and geodetic location
available to the end host, including LLDP-MED [LLDP-MED], DHCP
extensions [RFC4776][RFC6225], HELD [RFC5985], or link-layer
specifications such as [IEEE-802.11y]. The server offering this
information is known as a Location Information Server (LIS). The LIS
may be deployed by an AIP, or it may be run by a Location Service
Provider (LSP) which may have no relationship with the AIP, the VSP
or the PSAP. The location information is then provided, by reference
or value, to the service-providing entities, i.e. location
recipients, via application protocols, such as HTTP, SIP or XMPP.
This document investigates security threats in Section 3, and
outlines potential solutions in Section 4. Operational
considerations are provided in Section 5 and security considerations
are discussed in Section 6.
2. 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].
This document uses terms from [RFC5012] Section 3.
3. Threats
This document focuses on threats deriving from the introduction of
untrustworthy location information by end hosts, regardless of
whether this occurs intentionally or unintentionally.
In addition to threats arising from the intentional forging of
location information, end hosts may be induced to provide
untrustworthy location information. For example, end hosts may
obtain location from civilian GPS, which is vulnerable to spoofing
[GPSCounter] or from third party Location Service Providers (LSPs)
which may be vulnerable to attack or may not warrant the use of their
services for emergency purposes.
Emergency services have three finite resources subject to denial of
service attacks: the network and server infrastructure, call takers
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and dispatchers, and the first responders, such as fire fighters and
police officers. Protecting the network infrastructure is similar to
protecting other high-value service providers, except that location
information may be used to filter call setup requests, to weed out
requests that are out of area. PSAPs even for large cities may only
have a handful of PSAP call takers on duty, so even if they can, by
questioning the caller, eliminate a lot of prank calls, they are
quickly overwhelmed by even a small-scale attack. Finally, first
responder resources are scarce, particularly during mass-casualty
events.
Legacy emergency services rely on the ability to identify callers, as
well as on the difficulty of location spoofing for normal users to
limit prank calls. The ability to ascertain identity is important,
since the threat of severe punishments reduces prank calls.
Mechanically placing a large number of emergency calls that appear to
come from different locations is difficult. Calls from pay phones
are subject to greater scrutiny by the call taker. In the current
system, it would be very difficult for an attacker from country 'Foo'
to attack the emergency services infrastructure located in country
'Bar'.
One of the main motivations of an adversary in the emergency services
context is to prevent callers from utilizing emergency service
support. This can be done by a variety of means, such as
impersonating a PSAP or directory servers, attacking SIP signaling
elements and location servers.
Attackers may want to modify, prevent or delay emergency calls. In
some cases, this will lead the PSAP to dispatch emergency personnel
to an emergency that does not exist and, hence, the personnel might
not be available to other callers. It might also be possible for an
attacker to impede the users from reaching an appropriate PSAP by
modifying the location of an end host or the information returned
from the mapping protocol. In some countries, regulators may not
require the authenticated identity of the emergency caller, as is
true for PSTN-based emergency calls placed from pay phones or SIM-
less cell phones today. Furthermore, if identities can easily be
crafted (as it is the case with many VoIP offerings today), then the
value of emergency caller authentication itself might be limited. As
a consequence, an attacker can forge emergency call information
without the chance of being held accountable for its own actions.
The above-mentioned attacks are mostly targeting individual emergency
callers or a very small fraction of them. If attacks are, however,
launched against the mapping architecture (see [RFC5582] or against
the emergency services IP network (including PSAPs), a larger region
and a large number of potential emergency callers are affected. The
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call takers themselves are a particularly scarce resource and if
human interaction by these call takers is required then this can very
quickly have severe consequences.
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 LIS, the LoST infrastructure, used for
mapping locations to PSAP address, or call routing elements, may
act maliciously.
Malicious end host adversary model: The end host itself acts
maliciously, whether the owner is aware of this or whether it is
acting as a bot.
In this document, we focus only on the malicious end host adversary
model.
3.1. Location Spoofing
An adversary can provide false location information in an emergency
call in order to misdirect emergency resources. For calls
originating within the PSTN, this attack can be carried out via
caller-id spoofing. Where location is attached to the emergency call
by an end host, several avenues are available to provide false
location information:
1. The end host could fabricate a PIDF-LO and convey it within an
emergency call;
2. The VSP (and indirectly a LIS) could be fooled into using the
wrong identity (such as an IP address) for location lookup,
thereby providing the end host with misleading location
information;
3. Inaccurate or out-of-date information (such spoofed GPS
signals, a stale wiremap or an inaccurate access point location
database) could be utilized by the LIS or the endhost in its
location determination, thereby leading to an inaccurate
determination of location.
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By analysis of the SIP headers, it may be possible to flag situations
where the conveyed location is suspect (e.g. potentially wrong city,
state, country or continent). However, in other situations only
entities close to the caller may be able to verify the correctness of
location information.
The following list presents threats specific to location information
handling:
Place shifting: Trudy, the adversary, pretends to be at an arbitrary
location. In some cases, place shifting can be limited in range,
e.g., to the coverage area of a particular cell tower.
Time shifting: Trudy pretends to be at a location she was a while
ago.
Location theft: Trudy observes Alice's location and replays it as
her own.
Location swapping: Trudy and Malory, located in different locations,
can collude and swap location information and pretend to be in
each other's location.
3.2. Identity Spoofing
With calls originating on an IP network, at least two forms of
identity are relevant, with the distinction created by the split
between the AIP and the VSP:
(a) network access identity such as might be determined via
authentication (e.g., using the Extensible Authentication Protocol
(EAP) [RFC3748]);
(b) caller identity, such as might be determined from authentication
of the emergency caller at the VoIP application layer.
If the adversary did not authenticate itself to the VSP, then
accountability may depend on verification of the network access
identity. However, this also may not have been authenticated, such
as in the case where an open IEEE 802.11 Access Point is used to
initiate a nuisance emergency call. Although endpoint information
such as the IP or MAC address may have been logged, tying this back
to the device owner may be challenging.
Unlike the existing telephone system, VoIP emergency calls could
require strong identity, which need not necessarily be coupled to a
business relationship with the AIP, ISP or VSP. However, due to the
time-critical nature of emergency calls, multi-layer authentication
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is undesirable, so that in most cases, only the device placing the
call will be able to be identified, making the system vulnerable to
bot-net attacks. Furthermore, deploying additional credentials for
emergency service purposes (such as certificates) increases costs,
introduces a significant administrative overhead and is only useful
if widely deployed.
4. Solution Proposals
This section presents three potential solutions to the described
threats: location signing (Section 4.1), location by reference
(Section 4.2) and proxy added location (Section 4.3).
4.1. Location Signing
One way to avoid location spoofing is to let a trusted location
server sign the location information before it is sent to the end
host, i.e., the entity subject to the location determination process.
The signed location information is then verified by the location
recipient and not by the target. Figure 1 shows the communication
model with the target requesting signed location in step (a), the
location server returns it in step (b) and it is then conveyed to the
location recipient in step (c) who verifies it. For SIP, the
procedures described in [RFC6442] are applicable for location
conveyance.
+-----------+ +-----------+
| | | Location |
| LIS | | Recipient |
| | | |
+-+-------+-+ +----+------+
^ | --^
| | --
Geopriv |Req. | --
Location |Signed |Signed -- Geopriv
Configuration |Loc. |Loc. -- Using Protocol
Protocol |(a) |(b) -- (e.g., SIP)
| v -- (c)
+-+-------+-+ --
| Target / | --
| End Host +
| |
+-----------+
Figure 1: Location Signing
Additional information, such as timestamps or expiration times, has
to be included together with the signed location to limit replay
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attacks. If the location is retrieved from a location server, even a
stationary end host has to periodically obtain a fresh signed
location, or incur the additional delay of querying during the
emergency call. Bot nets are also unlikely to be deterred by
location signing. However, accurate location information would limit
the usable subset of the bot net, as only hosts within the PSAP
serving area would be useful in placing calls.
To prevent location-swapping attacks it is necessary to include some
some target specific identity information. The included information
depends on the purpose, namely either real-time verification by the
location recipient or for the purpose of a post-mortem analysis when
the location recipient wants to determine the legal entity behind the
target for prosecution (if this is possible). As argued in Section 6
the operational considerations make a real-time verification
difficult. A strawman proposal for location signing is provided by
[I-D.thomson-geopriv-location-dependability].
Still, for large-scale attacks launched by bot nets, this is unlikely
to be helpful. Location signing is also difficult when the host
provides its own location via GPS, which is likely to be a common
occurrence for mobile devices. Trusted computing approaches, with
tamper-proof GPS modules, may be needed in that case. After all, a
device can always pretend to have a GPS device and the recipient has
no way of verifying this or forcing disclosure of non-GPS-derived
location information.
Location verification may be most useful if it is used in conjunction
with other mechanisms. For example, a call taker can verify that the
region that corresponds to the IP address of the media stream roughly
corresponds to the location information reported by the caller. To
make the use of bot nets more difficult, a CAPTCHA-style test may be
applied to suspicious calls, although this idea is quite
controversial for emergency services, at the danger of delaying or
even rejecting valid calls.
4.2. Location by Reference
The location-by-reference concept was developed so that end hosts
could 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.
Figure 2 shows the communication model with the target requesting a
location reference in step (a), the location server returns the
reference in step (b), and it is then conveyed to the location
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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 [I-D.ietf-sip-
location-conveyance] are applicable.
+-----------+ Geopriv +-----------+
| | Location | Location |
| LIS +<------------->+ Recipient |
| | Dereferencing | |
+-+-------+-+ Protocol (d) +----+------+
^ | --^
| | --
Geopriv |Req. | --
Location |LbyR |LbyR -- Geopriv
Configuration |(a) |(b) -- Using Protocol
Protocol | | -- (e.g., SIP)
| V -- (c)
+-+-------+-+ --
| Target / | --
| End Host +
| |
+-----------+
Figure 2: Location by Reference
The details for the dereferencing operations vary with the type of
reference, such as a HTTP, HTTPS, SIP, SIPS URI or a SIP presence
URI. HTTP-Enabled Location Delivery (HELD) [RFC5985] is an example
of a protocol that is able to return such references.
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 URL from being able to permanently track a host
and to offer garbage collection functionality for the location
server.
Off-path adversaries must be prevented from obtaining the target's
location. The reference contains a randomized component that
prevents third parties from guessing it. When the location recipient
fetches up-to-date location information from the location server, it
can also be assured that the location information is fresh and not
replayed. However, this does not address location swapping.
However, location-by-reference does not offer significant security
benefits if the end host uses GPS to determine its location. At
best, a network provider can use cell tower or triangulation
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information to limit the inaccuracy of user-provided location
information.
4.3. Proxy Adding Location
Instead of making location information available to the end host, it
is possible to allow an entity in the AIP, or associated with the
AIP, to retrieve the location information on behalf of the end point.
This solution is possible when the application layer messages are
routed through an entity with the ability to determine the location
information of the end point, for example based on the end host's IP
or MAC address.
When the untrustworthy end host does not have the ability to access
location information, it cannot modify it either. Proxies can use
various authentication security techniques, including SIP Identity
[RFC4474], to ensure that modifications to the location in transit
can be detected by the location recipient (e.g., the PSAP). As noted
above, this is unlikely to work for GPS-based location determination
techniques.
The obvious disadvantage of this approach is that there is a need to
deploy application layer entities, such as SIP proxies, at AIPs or
associated with AIPs. This requires a standardized VoIP profile to
be deployed at every end device and at every AIP, for example, based
on SIP. This might impose a certain interoperability challenge.
Additionally, the AIP more or less takes the responsibility for
emergency calls, even for customers they have no direct or indirect
relationship with. To provide identity information about the
emergency caller from the VSP it would be necessary to let the AIP
and the VSP to interact for authentication (see, for example,
[RFC4740]). This interaction along the Authentication, Authorization
and Accounting infrastructure (see ) is often based on business
relationships between the involved entities. The AIP and the VSP are
very likely to have no such business relationship, particularly when
talking about an arbitrary VSP somewhere on the Internet. In case
that the interaction between the AIP 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.
5. Operational Considerations
5.1. Attribution to a Specific Trusted Source
[NENA-i2] Section 3.7 describes some of the aspects of attribution as
follows:
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The i2 solution proposes a Location Information Server (LIS) be
the source for distributing location information within an access
network. Furthermore the validity, integrity and authenticity of
this information are directly attributed to the LIS operator.
Section 6.1.1 describes the issues that arise in ensuring the
validity of location information provided by the LIS operator.
Section 6.1.2 and Section 6.1.3 describe operational issues that
arise in ensuring the integrity and authenticity of location
information provided by the LIS operator.
5.1.1. Validity
In existing networks where location information is both determined by
the access/voice service provider as well as communicated by the AIP/
VSP, responsibility for location validity can be attributed entirely
to a single party, namely the AIP/VSP.
However, on the Internet, not only may the AIP and VSP represent
different parties, but location determination may depend on
information contributed by parties trusted by neither the AIP nor
VSP, or even the operator of the Location Information Server (LIS).
In such circumstances, mechanisms for enhancing the integrity or
authenticity of location data contribute little toward ensuring the
validity of that data.
It should be understood that the means by which location is
determined may not necessarily relate to the means by which the
endpoint communicates with the LIS. Just because a Location
Configuration Protocol (LCP) operates at a particular layer does not
imply that the location data communicated by that protocol is derived
solely based on information obtained at that layer. In some
circumstances, LCP implementations may base their location
determination on information gathered from a variety of sources which
may merit varying levels of trust, such as information obtained from
the calling endpoint, or wiremap information that is time consuming
to verify or may rapidly go out of date.
For example, consider the case of a Location Information Server (LIS)
that utilizes LLDP-MED [LLDP-MED] endpoint move detection
notifications in determining calling endpoint location. Regardless
of whether the LIS implementation utilizes an LCP operating above the
link layer (such as an application layer protocol such as HELD
[RFC5985]), the validity of the location information conveyed would
be dependent on the security properties of LLDP-MED.
[LLDP-MED] Section 13.3 defines the endpoint move detection
notification as follows:
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lldpXMedTopologyChangeDetected NOTIFICATION-TYPE
OBJECTS { lldpRemChassisIdSubtype,
lldpRemChassisId,
lldpXMedRemDeviceClass
}
STATUS current
DESCRIPTION
"A notification generated by the local device
sensing a change in the topology that
indicates a new remote device attached to a
local port, or a remote device disconnected
or moved from one port to another."
::= { lldpXMedNotifications 1 }
Figure 3: Interworking Architecture
As noted in Section 7.4 of [LLDP-MED], the lldpRemChassisIdSubtype,
lldpRemChassisId and lldpXMedRemDeviceClass variables are determined
from the Chassis ID (1) and LLDP-MED Device Type Type-Length-Value
(TLV) tuples provided within the LLDP advertisement of the calling
device. As noted in [LLDP-MED] Section 9.2.3, all Endpoint Devices
use the Network address ID subtype (5) by default. In order to
provide topology change notifications in a timely way, it cannot
necessarily be assumed that a Network Connectivity devices will
validate the network address prior to transmission of the move
detection notification. As a result, there is no guarantee that the
network address reported by the endpoint will correspond to that
utilized by the device.
The discrepancy need not be due to nefarious reasons. For example,
an IPv6-capable endpoint may utilize multiple IPv6 addresses.
Similarly, an IPv4-capable endpoint may initially utilize a Link-
Local IPv4 address [RFC3927] and then may subsequently acquire a
DHCP-assigned routable address. All addresses utilized by the
endpoint device may not be advertised in LLDP, or even if they are,
endpoint move detection notification may not be triggered, either
because no LinkUp/LinkDown notifications occur (e.g. the host adds or
changes an address without rebooting) or because these notifications
were not detectable by the Network Connectivity device (the endpoint
device was connected to a hub rather than directly to a switch).
Similar issues may arise in situations where the LIS utilizes DHCP
lease data to obtain location information. Where the endpoint
address was not obtained via DHCP (such as via manual assignment,
stateless auto-configuration [RFC4862] or Link-Local IPv4 self-
assignment), no lease information will be available to enable
determination of device location. This situation should be expected
to become increasingly common as IPv6-capable endpoints are deployed,
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and Location Configuration Protocol (LCP) interactions occur over
IPv6.
Even in scenarios in which the LIS relies on location data obtained
from the IP MIB [RFC4293] and the Bridge MIB [RFC4188], availability
of location determination information is not assured. In an
enterprise scale network, maintenance of current location information
depends on the ability of the management station to retrieve data via
polling of network devices. As the number of devices increases,
constraints of network latency and packet loss may make it
increasingly difficult to ensure that all devices are polled on a
sufficiently frequent interval. In addition, in large networks, it
is likely that tables will be large so that when UDP transport is
used, query responses will fragment, resulting in increasing packet
loss or even difficulties in firewall or NAT traversal.
Furthermore, even in situations where the location data can be
presumed to exist and be valid, there may be issues with the
integrity of the retrieval process. For example, where the LIS
depends on location information obtained from a MIB notification or
query, unless SNMPv3 [RFC3411] is used, data integrity and
authenticity is not assured in transit between the network
connectivity device and the LIS.
From these examples, it should be clear that the availability or
validity of location data is a property of the LIS system design and
implementation rather than an inherent property of the LCP. As a
result, mechanisms utilized to protect the integrity and authenticity
of location data do not necessarily provide assurances relating to
the validity or provenance of that data.
5.1.2. Location Signing
[NENA-i2] Section 3.7 includes recommendations relating to location
signing:
Location determination is out of scope for NENA, but we can offer
guidance on what should be considered when designing mechanisms to
report location:
1. The location object should be digitally signed.
2. The certificate for the signer (LIS operator) should be
rooted in VESA. For this purpose, VPC and ERDB operators
should issue certs to LIS operators.
3. The signature should include a timestamp.
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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. Anti-spoofing mechanisms should be applied to the Location
Reporting method.
[Note: The term Valid Emergency Services Authority (VESA) refers
to the root certificate authority.]
Signing of location objects implies the development of a trust
hierarchy that would enable a certificate chain provided by the LIS
operator to be verified by the PSAP. Rooting the trust hierarchy in
VESA can be accomplished either by having the VESA directly sign the
LIS certificates, or by the creation of intermediate 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 whether they are significantly different
from say, requirements for issuance of an SSL/TLS web certificate.
5.1.3. Location by Reference
Where location by reference is provided, the recipient needs to
deference the LbyR in order to obtain location. With the
introduction of location by reference concept two authorization
models were developed, see [I-D.ietf-geopriv-deref-protocol], namely
the "Authorization by Possession" and "Authorization via Access
Control Lists" model. With the "Authorization by Possession" model
everyone in possession of the reference is able to obtain the
corresponding location information. This might, however, be
incompatible with other requirements typically imposed by AIPs, such
as location hiding (see [RFC6444]). As such, the "Authorization via
Access Control Lists" model is likely to be the preferred model for
many AIPs and subject for discussion in the subsequent paragraphs.
Just as with PIDF-LO signing, the operational considerations in
managing credentials for use in LbyR dereferencing can be
considerable without the introduction of some kind of hierarchy. It
does not seem reasonable for a PSAP to manage client certificates or
Digest credentials for all the LISes in its coverage area, so as to
enable it to successfully dereference LbyRs. In some respects, this
issue is even more formidable than the validation of signed PIDF-
LOs. While PIDF-LO signing credentials are provided to the LIS
operator, in the case of de-referencing, the PSAP needs to be obtain
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credentials compatible with the LIS configuration, a potentially more
complex operational problem.
As with PIDF-LO signing, the operational issues of LbyR can be
addressed to some extent by introduction of hierarchy. Rather than
requiring the PSAP to obtain credentials for accessing each LIS, the
local LIS could be required to upload location information to
location aggregation points who would in turn manage the
relationships with the PSAP. This would shift the management burden
from the PSAPs to the location aggregation points.
5.2. Application to a Specific Point in Time
PIDF-LO objects contain a timestamp, which reflects the time at which
the location was determined. Even if the PIDF-LO is signed, the
timestamp only represents an assertion by the LIS, which may or may
not be trustworthy. For example, the recipient of the signed PIDF-LO
may not know whether the LIS supports time synchronization, or
whether it is possible to reset the LIS clock manually without
detection. Even if the timestamp was valid at the time location was
determined, a time period may elapse between when the PIDF-LO was
provided and when it is conveyed to the recipient. Periodically
refreshing location information to renew the timestamp even though
the location information itself is unchanged puts additional load on
LISes. As a result, recipients need to validate the timestamp in
order to determine whether it is credible.
5.3. Linkage to a Specific Endpoint
As noted in the "HTTP Enabled Location Delivery (HELD)" [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" element and the
"deviceID" element [RFC4479] are not used.
Given the restrictions on inclusion of identification information
within the PIDF-LO, it may not be possible for a recipient to verify
that the entity on whose behalf location was determined represents
the same entity conveying location to the recipient.
Where "Enhancements for Authenticated Identity Management in the
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Session Initiation Protocol (SIP)" [RFC4474] is used, it is possible
for the recipient to verify the identity assertion in the From:
header. However, if PIDF parameters that contain identity are
omitted or contain an unlinked pseudonym, then it may not be possible
for the recipient to verify whether the conveyed location actually
relates to the entity identified in the From: header.
This lack of binding between the entity obtaining the PIDF-LO and the
entity conveying the PIDF-LO to the recipient enables cut and paste
attacks which would enable an attacker to assert a bogus location,
even where both the SIP message and PIDF-LO are signed. As a result,
even implementation of both [RFC4474] and location signing does not
guarantee that location can be tied to a specific endpoint.
6. Security Considerations
IP-based emergency services face many security threats. "Security
Threats and Requirements for Emergency Call Marking and Mapping"
[RFC5069] describes attacks on the emergency services system, such as
attempting to deny system services to all users in a given area, to
gain fraudulent use of services and to divert emergency calls to non-
emergency sites. [RFC5069] also describes attacks against
individuals, including attempts to prevent an individual from
receiving aid, or to gain information about an emergency.
"Threat Analysis of the Geopriv Protocol" [RFC3694] describes threats
against geographic location privacy, including protocol threats,
threats resulting from the storage of geographic location data, and
threats posed by the abuse of information.
This document focuses on threats deriving from the introduction of
untrustworthy location information by end hosts, regardless of
whether this occurs intentionally or unintentionally.
Although it is important to ensure that location information cannot
be faked there will be a larger number of GPS-enabled devices out
there that will find it difficult to utilize any of the security
mechanisms described in Section 5. It is unlikely that end users
will upload their location information for "verification" to a nearby
location server located in the access network.
Given the practical and operational limitations in the technology, it
may be worthwhile to consider whether the goals of trustworthy
location, as for example defined by NENA i2 [NENA-i2], are
attainable, or whether lesser goals (such as auditability) should be
substituted instead.
The goal of auditability is to enable an investigator to determine
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the source of a rogue emergency call after the fact. Since such an
investigation can rely on audit logs provided under court order, the
information available to the investigator could be considerably
greater than that present in messages conveyed in the emergency call.
As a consequence the emergency caller becomes accountable for his
actions. For example, in such a situation, information relating to
the owner of the unlinked pseudonym could be provided to
investigators, enabling them to unravel the chain of events that lead
to the attack. Auditability is likely to be of most benefits 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.
Where attacks are frequent and continuous, a reliance on non-
automated mechanisms is unlikely to be satisfactory. As such,
mechanisms to exchange audit trails information in a standardized
format between ISPs and PSAPs / VSPs and PSAPs or heuristics to
distinguish potentially fraudulent emergency calls from real
emergencies might be valuable for the emergency services community.
7. IANA Considerations
This document does not require actions by IANA.
8. Acknowledgments
We would like to thank the members of the IETF ECRIT and the IETF
GEOPRIV working group for their input to the discussions related to
this topic. We would also like to thank Andrew Newton, Murugaraj
Shanmugam, Richard Barnes and Matt Lepinski for their feedback to
previous versions of this document. Martin Thomson provided valuable
input to version -02 of this document.
9. References
9.1. Informative References
[GPSCounter]
Warner, J. S. and R. G. Johnston, "GPS Spoofing
Countermeasures", Los Alamos research paper LAUR-03-6163,
December 2003.
[I-D.thomson-geopriv-location-dependability]
Thomson, M. and J. Winterbottom, "Digital Signature Methods
for Location Dependability", draft-thomson-geopriv-location-
dependability-07 (work in progress), March 2011.
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[I-D.ietf-geopriv-deref-protocol]
Winterbottom, J., Tschofenig, H., Schulzrinne, H., Thomson,
M., and M. Dawson, "A Location Dereferencing Protocol Using
HELD", draft-ietf-geopriv-deref-protocol-04 (work in
progress), October 2011.
[IEEE-802.11y]
Information technology - Telecommunications and information
exchange between systems - Local and metropolitan area
networks - Specific requirements - Part 11: Wireless LAN
Medium Access Control (MAC) and Physical Layer (PHY)
specifications Amendment 3: 3650-3700 MHz Operation in USA,
November 2008.
[LLDP-MED]
"Telecommunications: IP Telephony Infrastructure: Link Layer
Discovery Protocol for Media Endpoint Devices, ANSI/
TIA-1057-2006", April 2006.
[NENA-i2] "08-001 NENA Interim VoIP Architecture for Enhanced 9-1-1
Services (i2)", December 2005.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3411] Harrington, D., Presuhn, R., and B. Wijnen, "An Architecture
for Describing Simple Network Management Protocol (SNMP)
Management Frameworks", STD 62, RFC 3411, December 2002.
[RFC3693] Cuellar, J., Morris, J., Mulligan, D., Peterson, J., and J.
Polk, "Geopriv Requirements", RFC 3693, February 2004.
[RFC3694] Danley, M., Mulligan, D., Morris, J. and J. Peterson, "Threat
Analysis of the Geopriv Protocol", RFC 3694, February 2004.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)", RFC
3748, June 2004.
[RFC3927] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
Configuration of IPv4 Link-Local Addresses", RFC 3927, May
2005.
[RFC4188] Norseth, K. and E. Bell, "Definitions of Managed Objects for
Bridges", RFC 4188, September 2005.
[RFC4293] Routhier, S., "Management Information Base for the Internet
Protocol (IP)", RFC 4293, April 2006.
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[RFC4474] Peterson, J. and C. Jennings, "Enhancements for Authenticated
Identity Management in the Session Initiation Protocol (SIP)",
RFC 4474, August 2006.
[RFC4479] Rosenberg, J., "A Data Model for Presence", RFC 4479, July
2006.
[RFC4740] Garcia-Martin, M., Belinchon, M., Pallares-Lopez, M., Canales-
Valenzuela, C., and K. Tammi, "Diameter Session Initiation
Protocol (SIP) Application", RFC 4740, November 2006.
[RFC4776] Schulzrinne, H., "Dynamic Host Configuration Protocol (DHCPv4
and DHCPv6) Option for Civic Addresses Configuration
Information", RFC 4776, November 2006.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
[RFC5012] Schulzrinne, H. and R. Marshall, "Requirements for Emergency
Context Resolution with Internet Technologies", RFC 5012,
January 2008.
[RFC5069] Taylor, T., Tschofenig, H., Schulzrinne, H. and M. Shanmugam,
"Security Threats and Requirements for Emergency Call Marking
and Mapping", RFC 5069, January 2008.
[RFC5582] Schulzrinne, H., "Location-to-URL Mapping Architecture and
Framework", RFC 5582, September 2009.
[RFC5985] Barnes, M., "HTTP Enabled Location Delivery (HELD)", RFC 5985,
September 2010.
[RFC6225] Polk, J., Linsner, M., Thomson, M. and B. Aboba, "Dynamic Host
Configuration Protocol Options for Coordinate-based Location
Configuration Information", RFC 6225, July 2011.
[RFC6442] Polk, J., Rosen, B. and J. Peterson, "Location Conveyance for
the Session Initiation Protocol", RFC 6442, December 2011.
[RFC6444] Schulzrinne, H., Liess, L., Tschofenig, H., Stark, B., and A.
Kuett, "Location Hiding: Problem Statement and Requirements",
RFC 6444, January 2012.
[SA] "Saudi Arabia - Illegal sale of SIMs blamed for surge in prank
calls", Arab News, May 4, 2010,
http://www.menafn.com/qn_news_story_s.asp?StoryId=1093319384
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[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/news/newsitems/200608/s1717956.htm
[UK] "Rapper makes thousands of prank 999 emergency calls to UK
police", Digital Journal, June 24, 2010,
http://www.digitaljournal.com/article/293796?tp=1
Authors' Addresses
Hannes Tschofenig
Nokia Siemens Networks
Linnoitustie 6
Espoo 02600
Finland
Phone: +358 (50) 4871445
Email: Hannes.Tschofenig@gmx.net
URI: http://www.tschofenig.priv.at
Henning Schulzrinne
Columbia University
Department of Computer Science
450 Computer Science Building, New York, NY 10027
US
Phone: +1 212 939 7004
Email: hgs@cs.columbia.edu
URI: http://www.cs.columbia.edu
Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
Email: bernard_aboba@hotmail.com
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