ECRIT H. Tschofenig
Internet-Draft Nokia Siemens Networks
Intended status: Informational H. Schulzrinne
Expires: March 25, 2011 Columbia University
B. Aboba
Microsoft Corporation
September 21, 2010
Trustworthy Location Information
draft-ietf-ecrit-trustworthy-location-00.txt
Abstract
For some location-based applications, such as emergency calling or
roadside assistance, it appears that the identity of the requestor is
less important than accurate and trustworthy location information.
To ensure adequate help location has to be left untouched by the end
point or by entities in transit.
This document lists different threats, an adversary model, outlines
three frequentlly discussed solutions and discusses operational
considerations. Finally, the document concludes with a suggestion on
how to move forward.
Status of this Memo
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This Internet-Draft will expire on March 25, 2011.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Emergency Services . . . . . . . . . . . . . . . . . . . . . . 5
4. Threats . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Location Spoofing . . . . . . . . . . . . . . . . . . . . 8
4.2. Call Identity Spoofing . . . . . . . . . . . . . . . . . . 8
5. Solution Proposals . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Location Signing . . . . . . . . . . . . . . . . . . . . . 10
5.2. Location by Reference . . . . . . . . . . . . . . . . . . 11
5.3. Proxy Adding Location . . . . . . . . . . . . . . . . . . 13
6. Operations Considerations . . . . . . . . . . . . . . . . . . 14
6.1. Attribution to a Specific Trusted Source . . . . . . . . . 14
6.2. Application to a Specific Point in Time . . . . . . . . . 18
6.3. Linkage to a Specific Endpoint . . . . . . . . . . . . . . 18
7. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 20
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
10.1. Normative References . . . . . . . . . . . . . . . . . . . 24
10.2. Informative references . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27
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1. Introduction
Much of the focus in trustable networks has been on ensuring the
reliability of personal identity information or verifying privileges.
However, in some cases, access to trustworthy location information is
more important than identity since some services are meant to be
widely available, regardless of the identity of the requestor.
Emergency services, such as fire department, ambulance and police,
but also commercial services such as food delivery and roadside
assistance are among those. Customers, competitors or emergency
callers lie about their location to harm the service provider or to
deny services to others, by tying up the service capacity. In
addition, if third parties can modify the information, they can deny
services to the requestor.
Physical security is often based on location. As a trivial example,
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 the HTTP client's IP address (that is then
translated to location). In all these cases, trustworthy location
information can be used to augment identity information or, in some
cases, avoid the need for role-based authorization.
A number of standardization organizations have developed mechanisms
to make civic and geodetic location available to the end host.
Examples for these protocols are LLDP-MED [LLDP-MED], DHCP extensions
(see [RFC4776], [RFC3825]), HELD
[I-D.ietf-geopriv-http-location-delivery], or the protocols developed
within the IEEE as part of their link-layer specifications. The
server offering this information is usually called a Location
Information Server (LIS). More common with high-quality cellular
devices is the ability for the end host itself to determine its own
location using GPS. 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 the security threats in Section 4, and
outlines three solutions that are frequently mentioned in Section 5.
We use emergency services an example to illustrate the security
problems, as the problems have been typically discussed in that
context since the stakes are high, but the issues apply also to other
examples as cited earlier. We also take a look at the operational
considerations in Section 6 since there is a cost associated with the
estbalishment of the necessary infrastructure. With the pros of the
available technology being described and the cons of the operational
complexity highlighted we offer a conclusion in Section 7.
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2. Terminology
This document re-uses a lot of the terminology defined in Section 3
of [RFC5012].
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3. Emergency Services
Users of the legacy 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.
Regulators have already started to demand emergency service support
for voice over IP. However, enabling such critical public services
using the Internet is challenging, as many of the assumptions of the
public switched telephone network (PSTN) / public land mobile network
(PLMN) no longer hold. In particular, while the local telephone
company provides both the physical access and the phone service, VoIP
allows and encourages to split these two roles between the Access
Infrastructure Provider (AIP) and Application (Voice) Service
Provider (VSP). The VSP may be located far away from the AIP and may
either have no business relationship with that AIP or may be a
competitor. It is also likely that the VSP will have no relationship
with the PSAP and will therefore be unknown.
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4. Threats
IP-based emergency calling faces many security threats, most of which
are well-known from other realms, such as protecting the privacy of
communications or against denial-of-service attacks using packet
flooding. Here, we focus specifically on a higher-layer threat that
is unique to services where semi-anonymous users can request
expensive services.
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. A more recent concern is that massive prank calls
can be used to disrupt emergency services, e.g., during a mass-
casualty event and thus be used as a means to amplify the effect of a
terror attack, for example.
Emergency services have three finite resources subject to denial of
service attacks: the network and server infrastructure, call takers
and dispatchers, and the first responders, such as fire fighters and
police officers. Protecting the network infrastructure is similar to
protecting other high-value service providers, except that
trustworthy 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.
Currently, emergency services rely on the fact that location spoofing
is difficult for normal users. Additionally, the identity of most
callers can be ascertained, so that the threat of severe punishments
reduces prank calls. Mechanically placing a large number of
emergency calls that appear to come from different locations is also
difficult. Calls from payphones 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
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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 payphones 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
[I-D.ietf-ecrit-mapping-arch] or against the emergency services IP
network (including PSAPs), a larger region and a large number of
potential emergency callers are affected. The call takers themselves
are a particularly scarce resource and if human interaction by these
call takers is required then this can very quickly have severe
consequences.
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.
We will focus only on the malicious end host adversary model since it
follows today's most common adversary model on the Internet that
includes bot nets.
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4.1. Location Spoofing
An adversary can provide false location information in order to fool
the emergency personnel. Such an attack is particularly easy if
location information is attached to the emergency call by the end
host and is either not verified or cannot be verified by anyone.
Only entities that are close to the caller can verify the correctness
of location information. Another form of this attack is to fool a
VSP (and indirectly a LIS) in using a wrong identity (such as an IP
address) for the location lookup. This type of attack can be
accomplished in the PSTN today with the help of caller-id spoofing.
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.
4.2. Call Identity Spoofing
If an adversary can place emergency calls without disclosing its
identity, then prank calls are more difficult to be traced. There
are at least two different forms of authentication in this context:
(a) network access authentication (e.g., using the Extensible
Authentication Protocol (EAP) [RFC3748] and (b) authentication of the
emergency caller at the VoIP application layer. This differentiation
is created by the split between the AIP and the VSP. Note that
different identities are involved and that the are also managed by
different parties and thus making the linkage between the two quite
difficult.
Trying to find an adversary that did not authenticate itself to the
VSP is difficult even though there is still a chance if network
access authentication was executed. If there is no authentication
(neither to the PSAP, to the VSP nor to the AIP) then it is very
challenging to trace the call back in order to a make a particular
entity accountable. This might, for example, be the case with an
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open IEEE 802.11 WLAN access point even if the owner of the access
point can be determined.
However, unlike for the existing telephone system, it is possible to
imagine that VoIP emergency calls could require strong identity, as
providing such identity information is not necessarily coupled to
having a business relationship with the AIP, ISP or VSP. However,
due to the time-critical nature of emergency calls, it is unlikely
that multi-layers authentication can be used, so that in most cases,
only the device placing the call will be able to be identified,
making the system vulnerable to botnet attacks. Furthermore,
deploying additional credentials for emergency service purposes, such
as dedicated certificates, increases costs, introduces a significant
administrative overhead and is only useful if widely used.
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5. Solution Proposals
This section presents three solution approaches that have been
discussed in order to mitigate the threats discussed.
5.1. 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 [I-D.ietf-sip-location-conveyance] 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
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
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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.
5.2. 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
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.
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+-----------+ 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)
[I-D.ietf-geopriv-http-location-delivery] 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
information to limit the inaccuracy of user-provided location
information.
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5.3. 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 the procedures described in
[I-D.schulzrinne-ecrit-unauthenticated-access] are applicable and
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.
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6. Operations Considerations
6.1. Attribution to a Specific Trusted Source
[NENA-i2] Section 3.7 describes some of the aspects of attribution as
follows:
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.
6.1.1. 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
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of whether the LIS implementation utilizes an LCP operating above the
link layer (such as an application layer protocol such as HELD
[I-D.ietf-geopriv-http-location-delivery]), 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:
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
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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 autoconfiguration [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,
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.
6.1.2. 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
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report location:
1. The location object should be digitally signed.
2. The certificate for the signer (LIS operator) should be rooted
in VESA. For this purpose, VPC and ERDB operators should issue
certs to LIS operators.
3. The signature should include a timestamp.
4. Where possible, the Location Object should be refreshed
periodically, with the signature (and thus the timestamp) being
refreshed as a consequence.
5. Antispoofing 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.
6.1.3. 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.winterbottom-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 [I-D.ietf-ecrit-location-hiding-req]). As
such, the "Authorization via Access Control Lists" model is likely to
be the preferred model for many AIPs and subject for discussion in
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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
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.
6.2. 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
LISs. As a result, recipients need to validate the timestamp in
order to determine whether it is credible.
6.3. Linkage to a Specific Endpoint
As noted in the "HTTP Enabled Location Delivery (HELD)"
[I-D.ietf-geopriv-http-location-delivery] 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
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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
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.
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7. Conclusion
Emergency services raise a number of architectural questions, see
[I-D.ietf-ecrit-framework],
[I-D.schulzrinne-ecrit-unauthenticated-access],
[I-D.ietf-ecrit-location-hiding-req], and
[I-D.tschofenig-ecrit-architecture-overview]. With the generalized
emergency architecture considered within the ECRIT working group
various security challenges need to be addressed, including the
ability to report faked location and other attacks against the
emergency services infrastructure. These types of attacks also show
that the attack characteristics play an important role when dealing
with the problems and lower-layer solutions, as they have been
proposed as solutions for Denial of Service prevention (for example
using cryptographic puzzles), have limited applicability.
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 make it difficult to utilize any of the security
mechanisms described in Section 5. It will be very 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
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
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emergencies might be valuable for the emergency services community.
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8. IANA Considerations
This document does not require actions by IANA.
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9. 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.
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10. References
10.1. Normative References
[RFC5012] Schulzrinne, H. and R. Marshall, "Requirements for
Emergency Context Resolution with Internet Technologies",
RFC 5012, January 2008.
10.2. Informative references
[I-D.ietf-ecrit-framework]
Rosen, B., Schulzrinne, H., Polk, J., and A. Newton,
"Framework for Emergency Calling using Internet
Multimedia", draft-ietf-ecrit-framework-11 (work in
progress), July 2010.
[I-D.ietf-ecrit-location-hiding-req]
Schulzrinne, H., Liess, L., Tschofenig, H., Stark, B., and
A. Kuett, "Location Hiding: Problem Statement and
Requirements", draft-ietf-ecrit-location-hiding-req-04
(work in progress), February 2010.
[I-D.ietf-ecrit-mapping-arch]
Schulzrinne, H., "Location-to-URL Mapping Architecture and
Framework", draft-ietf-ecrit-mapping-arch-04 (work in
progress), March 2009.
[I-D.ietf-geopriv-http-location-delivery]
Barnes, M., Winterbottom, J., Thomson, M., and B. Stark,
"HTTP Enabled Location Delivery (HELD)",
draft-ietf-geopriv-http-location-delivery-16 (work in
progress), August 2009.
[I-D.ietf-sip-location-conveyance]
Polk, J. and B. Rosen, "Location Conveyance for the
Session Initiation Protocol",
draft-ietf-sip-location-conveyance-13 (work in progress),
March 2009.
[I-D.schulzrinne-ecrit-unauthenticated-access]
Schulzrinne, H., McCann, S., Bajko, G., Tschofenig, H.,
and D. Kroeselberg, "Extensions to the Emergency Services
Architecture for dealing with Unauthenticated and
Unauthorized Devices",
draft-schulzrinne-ecrit-unauthenticated-access-08 (work in
progress), July 2010.
[I-D.thomson-geopriv-location-dependability]
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Thomson, M. and J. Winterbottom, "Digital Signature
Methods for Location Dependability",
draft-thomson-geopriv-location-dependability-06 (work in
progress), August 2010.
[I-D.tschofenig-ecrit-architecture-overview]
Tschofenig, H. and H. Schulzrinne, "Emergency Services
Architecture Overview: Sharing Responsibilities",
draft-tschofenig-ecrit-architecture-overview-00 (work in
progress), July 2007.
[I-D.winterbottom-geopriv-deref-protocol]
Winterbottom, J., Tschofenig, H., Schulzrinne, H.,
Thomson, M., and M. Dawson, "A Location Dereferencing
Protocol Using HELD",
draft-winterbottom-geopriv-deref-protocol-05 (work in
progress), January 2010.
[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.
[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.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)",
RFC 3748, June 2004.
[RFC3825] Polk, J., Schnizlein, J., and M. Linsner, "Dynamic Host
Configuration Protocol Option for Coordinate-based
Location Configuration Information", RFC 3825, July 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.
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[RFC4293] Routhier, S., "Management Information Base for the
Internet Protocol (IP)", RFC 4293, April 2006.
[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.
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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: bernarda@microsoft.com
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