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\title{
The Right Place at the Right Time:\\
The Use of Network Location\\
in Authentication and Abuse Prevention}

\author{
\authorblockN{Geoffrey Goodell}
\authorblockA{goodell@eecs.harvard.edu\\
Harvard University}
\and
\authorblockN{Paul Syverson}
\authorblockA{syverson@itd.nrl.navy.mil\\
US Naval Research Laboratory}
}

\maketitle

\thispagestyle{empty}

The use of network location to draw conclusions about users has become quite
commonplace on today's Internet.  Numerous institutional subscription services
use IP address as the exclusive means of identifying users.  IP address is
sometimes also used as a criterion for fraud detection and abuse prevention,
for example, flagging discrepancies between geographic location associated with
an IP address and that associated with mailing address for credit card billing.
For abuse prevention, many websites that allow public contributions (e.g.
wikis, blogs, chat rooms, etc.) simply block the IP addresses from which
chronic abuse emerges.  Using IP address to categorize or identify users seems
like a reasonable approach in general, but there are some important caveats as
well.  We briefly examine the long-term architectural dangers as well as
short-term policy risks as we strive to put the costs and benefits into
perspective.

\section*{Practical Justification}

There is little doubt that some ISPs are more vigilant than others in
curtailing spam and abuse.  Many network administrators have accepted the idea
that it is easier or more effective to fight spam by restricting the flow of
mail instead of addressing the security vulnerabilities that make spam feasible
(compromised hosts send a large proportion of unsolicited mail) or the market
forces that make spam desirable (financial incentives foster the development of
spam distribution software and methods).  At least for the present, knowing the
ISP from which a connection originates certainly provides some statistically
meaningful information about whether the user responsible for the connection is
likely to engage in antisocial behavior.  Similarly, teaching Bayesian spam
filters about network location may enhance their effectiveness in reducing
spam.

Providers of online subscription services can be reasonably assured that most
connections from the IP address of institutions that provide limited access to
their systems are from authorized users of those systems. Also, the overhead of
setting up a mechanism that uses this information is far simpler than most
alternatives.

In recent years, a market has emerged for so-called ``geolocation'' services,
which provide a mapping from network location to physical, geographic
locations.  Service providers collect data from Internet service providers and
resell it to geolocation customers in the form of datasets or permission to
execute queries on their databases.  Geolocation products have been developed
for several uses, including fraud resolution, spam mitigation, targeted
advertising, and digital rights management.
% Is "showing the Olympics only in Europe because a certain company wanted
% to squeeze all they could out of the US market" included in one of the
% above, or is it an instance of an interesting other category, perhaps
% called "price discrimination"? -RD

An important premise of geolocation is that there is much to learn about
individual Internet users from how they are connected to the network, and such
information can be used as a basis for implicitly categorizing them by risk
level or market segment.  Geolocation affords advertisers the possibility of
offering products and services specific to particular localities.  Content
providers wishing to disallow people from certain countries or regions from
accessing certain content may, to a significant extent though not completely,
use network location information to achieve this goal.

Similarly, it is reasonable that credit charges from locations that are far
from the home of a credit card holder and that are known to be hotbeds of
credit fraud merit close scrutiny. But this can have problems too: a large
percentage of credit card use is associated with travel. Also, mail-order
catalogs and, still easier to distribute, web pages mean that even small local
shops with only telephonic credit transaction clearing can have many remote
customers.  The credit industry typically errs on the side of giving users easy
access to its services rather than denying undesired access when these goals
conflict. The content industry typically adopts the opposite approach.  For
this reason, IP address is only one of many factors in authorization and
identification for credit approval and fraud detection rather than an absolute
or sole discriminator.

So far it seems that the primary incentive for those who use network-layer
information for application-layer decisions is to provide an expedient means of
authorization, striking an acceptable balance between easy access for desired
usage and adequate deterrence against undesired usage.  In short, it works.  To
date, IP addresses have been a resource difficult enough to obtain or spoof
that they have fulfilled this role in authorization and fraud detection. More
proper authentication, however, would require user certificates, a PKI, or
other mechanisms that have proved difficult to set up for large, relatively
open systems and have not seen widespread user adoption where they do exist.
% I suggest a paragraph break here, and lead in with "We also note that"
% rather than "And," -RD
And, as long as end user systems not under institutional control remain as
vulnerable as they now are to root level intrusion, end-to-end authentication
could also be an illusory approach to security. However, that is a
much broader and separate problem than the one we are considering.

\section*{Immediate Negative Side Effects}

The ability to block abuser IP addresses is a powerful but ill-suited tool for
some of the problems to which it has been applied: in a few cases, individual
sites have blocked access from IP addresses in a broad geographic area.  Two
well-known examples are the blocking by a major-party presidential campaign of
its web site from IP addresses outside the US just prior to the 2004 US
presidential election and the blocking of 2004 Olympics coverage from IP
addresses inside the US.  In 2002, Pennsylvania ISPs blocked access to 1.6
million innocuous Web sites in an attempt to satisfy a state mandate intended
to curtail child pornography~\cite{bradner04simple}.  More recently, the major
US ISP Verizon was the subject of lawsuits when it began blocking all email
from Europe and other continents by default as a spam
deterrent~\cite{leyden05verizon}; since that time, Verizon has agreed to a
settlement.

In general, binding the identity of users to how they are connected to the
Internet is not only impossible, but also undesirable.  The vast majority of
Internet users have dynamically assigned addresses, and while they may use the
same address from lease to lease, they also may not.  Proxies, workarounds,
dynamic addresses, mobility, system vulnerabilities, and other complications
make network location useful as a heuristic at best.  To the extent that it is
useful, the ability to use network location as an indicator of identity is a
technical shortcoming of the current Internet that can be overcome.

One of the main drawbacks to using network location for authorization is that
legitimate users cannot access a service when away from their home
institutions. It is possible for a user to set up tunneling such that her
access appears to be from an IP address within the permitted range, but this
may be onerous, technically difficult, or simply not possible as a matter of
policy.  A major expense associated with deploying and maintaining virtual
private network (VPN) infrastructure derives from the need for individuals and
businesses to access Internet resources that rely upon network location
information to differentiate between valid and invalid users.

Furthermore, abusers can deploy and use proxies in unblocked locations fairly
easily.  Individual proxies themselves can be blocked, but with multitudes of
newly compromised hosts emerging daily, the effectiveness of that approach is
limited.  Networks designed to protect honest users from traffic analysis such
as Tor~\cite{tor} can be blocked because they explicitly provide a means of
doing so, but abusers can take advantage of million-node botnets with no easily
discernible pattern of IP-address source.  The result would seem to block the
honest user from protecting herself while leaving the abusers unblocked and
harder to find.

\section*{Long-Term Security Risks}

We have noted some immediate practical problems, but solutions that avoid these
problems raise additional security concerns of their own. IP tunneling simply
to allow use of institutional subscriptions when it is not otherwise needed is
an extreme solution that may open the possibility of other intrusions to the
institution.  University librarians have long recognized the problem that
authorization by IP address poses for remote users trying to access
institutional services.  For example, consider what happens when an outsider
uses University-based research projects, such as research on proxies or
filtering, to access resources that are supposed to be limited to University
access.  This has been one of the prime motivators for development and
increasing adoption of systems such as Shibboleth\footnote{Shibboleth,
\url{http://shibboleth.Internet2.edu/}}, which provides single sign-on and
user-controlled credential management independent of IP address.  This can help
permit remote access without resorting to IP tunneling.

If legitimate users of credit systems have incentive via easier authorization
of their transactions to route from an IP address associated with their home
location, they reveal via routing information their interactions with merchants
and financial institutions, not just to those principals but to observers as
well. This may leave them more vulnerable to identity theft, spear phishing,
and the like.  And, as actual large-scale systematic fraudsters become aware of
the use of authentication by IP address, they are provided with specific
incentive to spoof authorized or trusted (low fraud) locations, or worse, to
break into systems in or near those locations. This approach thus has the
potential of greater vulnerability and risk for the legitimate user with a
false sense of protection against the actual adversaries.

\section*{The Role of Network Access Providers}

The fact that network location provides an effective way to assign blame for
malicious activity raises questions about the extent to which network access
providers ought to be responsible for the comportment of the systems for which
their networks serve as attachment points to the Internet.

Regulators have substantial interest in supporting the principle of enforcement
within the network, since the network could potentially provide convenient
points of control for execution of policy.  Furthermore, incentives for major
telecommunications carriers generally encourage a movement away from uniform,
open access and toward vertically-integrated ``silos'' in which carriers
determine the set of resources that customers may
access~\cite{bradner05misunderstanding,vanschewick05}.  So, there exist strong
industry and regulatory forces to empower network operators at the expense of
network neutrality and end-to-end connectivity.  A recent ITU proposal
advocates expanding the role of ISPs to require that they ensure that traffic
traversing their wires adheres to certain normative
requirements~\cite{palfrey06}.

\section*{Function Creep and Expedience}

Making use of the routing infrastructure itself to protect participants from
each other is an arms race that sacrifices as collateral damage the neutrality
characteristics of the Internet that provide its principal advantages over
alternative interconnection paradigms.

The consistent response that proposers of such methods offer to proponents of
end-to-end services is that it may be too late to salvage such principles, and
that compromise is in order.  Indeed, network-layer techniques have shown
promise as expedient short-term remedies to exigent security threats, and as a
result, governments and regulators have called upon ISPs to implement technical
solutions within the network~\cite{zittrain06}.  Vint Cerf recently argued that
it does not make sense to use the network to compensate for operating systems
that protect themselves inadequately.  In particular, having the network
perform application-level security checks on traffic to attempt to authenticate
end users or to look for viruses and malware makes it less efficient at moving
data around.·

\begin{quote}

``It's really hard to have a
network-level thing do this stuff, which means you have to assemble the packets
into something bigger and thus violate all the protocols,'' Cerf says. ``That
takes a heck of a lot of resources.''~\cite{talbot05}

\end{quote}

In other words, if we start requiring the network to perform tasks other than
routing, then we undermine the ability of the network to do its most essential
job.  Another problem with such function creep is that it can become
entrenched. Once a technique such as authentication by IP address is widely
established, if legitimate technical reasons to substantially change how
addressing and routing is done should arise, they may be harder to implement
and establish because they are hamstrung with the use of IP address as
authenticator or anti-fraud mechanism, even if that was originally only
introduced as an expedient.

\section*{Separating Identification from Routing}

IP addresses were introduced to allow routing of IP packets.  As we have
already seen, if IP addresses are used for other purposes, and if
identification and authorization become tied up in routing, then the purpose
for which they were designed is undermined: both legitimate users and attackers
end up using IP addresses not because of routing, but to appear as authorized
users.  Onion Routing was introduced ten years ago as an infrastructure that
``separates identification from routing''~\cite{reed96}. ``Parties are free to
(and usually should) identify themselves within a message. But the use of a
public network should not automatically give away the identities and location
of the communicating parties''~\cite{onion-routing-ih96}.  Anonymity from one's
communication partner is not the primary motivation for Onion Routing; users
may simply need to protect their points of attachment from attackers, whether
personal (e.g., stalkers or identity thieves) or enterprise (e.g., corporate
competitors gathering intelligence). In each case, the security benefits of
separating routing from identification are substantial, even if the challenges
it poses to the security models of some services are similarly great.

How will increased user mobility, increased use of anonymity networks for
security by honest users, etc. interplay with use of IP-address information for
authentication and authorization? Intuitively, it seems that these two
technologies are headed for a clash.

\section*{Discussion}

The extent to which network location information is used in authentication and
fraud and abuse detection will have a substantial impact on the Internet
environment for the next several years.  Adversaries may or may not adapt to
these techniques before the techniques become entrenched in the architecture of
critical services.  Although, if history is a guide, they will adapt at some
point, and all the faster if IP-address location technology increases their
incentives to do so. If we are to avoid arriving, therefore, at an entrenched
burden with no ultimate benefit, we must understand the technology that we are
using to do the job. In this brief article we have set out some of the
unanticipated ways in which relevant technologies interact. One response to
understanding this is through governance and policy, but our focus herein is
the technology itself.  The complexity, brittleness, and overhead involved in
the deployment of solutions that use network-layer address for authorization
may stifle innovation in the future. But it's not all bad news.  Systems like
Shibboleth provide a technological path that can continue to lead institutions
away from authentication by IP address.  Similarly, since network location is
only one of many factors going into fraud detection scores used by the credit
industry, the technical framework already exists to allow an abandonment of
network location as a factor as it diminishes in significance. This need not
mean the end of geolocation services either; for example, people
will still want to know about nearby restaurants and services. Geolocation
services will just need to be based on information other than IP address if
they are to continue serving a useful purpose.  If the already existing
security and functionality problems arising from IP-address-based abuse
deterrence do not lead to its abandonment, then the incentives it provides
to network attackers ultimately should.


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