Internet Relativism and the Hunt for Elusive “Ground Truth”

Networking and security research often rely on a notion of ground truth to evaluate the effectiveness of a solution.  “Ground truth” refers to a true underlying phenomenon that we would like to characterize, detect, or measure.  We often evaluate the effectiveness of a classifier, detector, or measurement technique by how well it reflects ground truth.

For example, an Internet link might have a certain upstream or downstream throughput; the effectiveness of a tool that measures throughput could be thus be quantified in terms of how close its estimates of upstream and downstream throughput are in comparison to the true throughput of the underlying link.  Since there is a physical link with actual upstream or downstream throughput characteristics—and the properties of that link are either explicitly known or can be independently measured—measuring error with respect to ground truth makes sense.  In the case of analyzing routing configuration to predict routing behavior (or detect errors), static configuration analysis can characterize where traffic in the network will flow and whether the configuration will give rise to erroneous behavior; either the predictions correctly characterize the behavior of the real network, or they don’t.  A spam filter might classify an email sender as a legitimate sender or a spammer; again, either the sender is a spammer or it is a legitimate mail server.  In this case, comparing against ground truth is more difficult, since if we had a perfect characterization of spammers and legitimate senders, we would already have the perfect spam filter.  The solution in these kinds of cases is to compare against an independent label (e.g., a blacklist) and somehow argue that the proposed detection mechanism is better than the existing approach to labeling or classification (e.g., faster, earlier, more lightweight, etc.).

Problem: Lack of ground truth.  For some Internet measurement problems, the underlying phenomenon simply cannot be known—even via an independent labeling mechanism—either because the perpetrator of an action won’t reveal his or her true intention, or sometimes because there actually is no “one true answer”. Sometimes we want to characterize scenarios or phenomena where the ground truth proves elusive.  

Consider the following two problems:

  • Network neutrality.The network neutrality debate centers around the question of whether Internet service providers should carry all traffic according to the same class of service, regardless of various properties such as what type of traffic it is (e.g., voice, video) or who is sending or receiving that traffic.
  • Filter bubbles.  Eli Pariser introduced the notion of a filter bubble in his book The Filter Bubble.  A filter bubble is the phenomenon whereby each Internet user sees different Internet content based on factors ranging from our demographic to our past search history to our stated preferences.  Briefly, each of us sees a different version of the Internet, based on a wide range of factors.

These two detection problems do not have a notion of ground truth that can be easily measured.  In the latter case, there is effectively no ground truth at all.

In the case of network neutrality, detection boils down to determining whether an ISP is providing preferential treatment to a certain class of applications or customers.  While ground truth certainly exists (i.e., either the ISP is discriminating against a certain class of traffic or it isn’t), discovering ground truth is incredibly challenging: ISPs may not reveal their policies concerning preferential treatment of different traffic flows, for example.

Similarly, in the case of filter bubbles, we want to determine whether a content provider or intermediary (e.g., search engine, news aggregator, social network feed) is manipulating content for particular groups of users (e.g., showing only certain news articles to Americans).  Again, there is a notion of ground truth—either the content is being manipulated or it isn’t—but the interesting aspect here is not so much whether content is being manipulated (we all know that it is), but rather what the extent of that manipulation is.  Characterizing the extent of manipulation is difficult, however, because personalization is so pervasive on the Internet: everyone effectively sees content that is tailored to their circumstances, and there is no notion of a baseline that reflects what a set of search results or a page of recommended products might look like before the contents were tailored for a particular user.  In many cases, personalization has been so ingrained in data mining and search that even the algorithm designers are unable to characterize what “ground truth” content (i.e., without manipulation) might look like.

Relativism: measuring how different perspectives give rise to inconsistencies.  In cases where ground truth is difficult to measure or impossible to know, we can still ask questions about consistency.  For example, in the case of network neutrality, we can ask whether different groups of users experience comparable performance.  In the case of filter bubbles, we can ask whether different groups of users see similar content.  When inconsistencies arise, we can then attempt to attribute a cause to these inconsistencies by controlling for all factors except for the factor we believe might be the underlying cause for the inconsistency.  One might call this Internet relativism, in a way: We concede that either there is no absolute truth, or that the absolute truth is so difficult to obtain that we might as well not try to know it.  Instead, we can explore how differences in perspective  or “input signals” (e.g., demographic, geography) give rise to different outcomes and try to determine which input differences triggered the inconsistency.  We have applied this technique to the design of two real-world systems that address these two respective problem areas.  In both of these problems, we would love to know the underlying intention of the ISP or information intermediary (i.e., “Is the performance problem I’m seeing a result of preferential treatment?”, “(How) is Google, Netflix, or Amazon manipulating my results based on my demographic?”).

  • NANO: Network Access Neutrality Observatory.We developed NANO several years ago to characterize ISP discrimination for different classes of traffic flows.  In contrast to existing work in this area (e.g., Glasnost), which requires a hypothesis about the type of discrimination that is taking place, NANO operates without any a priori hypothesis about discrimination rules and simply looks for systematic deviation from “normal” behavior for a certain class of traffic (e.g., all traffic from a certain ISP, for a certain application, etc.).  The tricky aspect involved in this type of detection is that there is no notion of normal.  For example, ISP Y might also be performing similar type of discrimination, so there is no firm ground truth against which to compare.  Ideally, what we’d like to ask is “What would be the performance that this user see using ISP X vs. the performance they would see if they were not using ISP X?”  Unfortunately, there is no reasonable way to test the performance that a user would experience as a result of not using an ISP.  (This is in contrast to randomized treatment in clinical trials, where it makes sense to have a group of users who, say, are subject to a particular treatment or not.)  To address this problem, the best we could do to establish a baseline was to average the performance seen by all users from other ISPs and compare those statistics against the performance seen by a group of users for the ISP under test.
  • Bobble: Exposing inconsistent search results.  We recently developed Bobble to characterize the inconsistencies that exist in Web search results that users see, as a result of both personalization and geography.  Ideally, we would like to measure the extent of manipulation against some kind of baseline.  Unfortunately, however, the notion of a baseline is almost meaningless, since no Internet user is subject to such a baseline—even a user who has no search history may still see personalized results based on geography, time of day, device type, and other features, for example.  In this scenario, we established a baseline by comparing the search results of a signed-in user against a user with no search history, making our best attempt to hold all other factors constant.  We also performed the same experiment with users who were not signed in and had no search history, varying only geography.  Unlike NANO, in the case of Bobble, there is not even a notion of an “average” user; the best we can hope for are meaningful characterizations of inconsistencies.

Takeaways and general principles.  These two problems both involve an attempt to characterize an underlying phenomenon without any hope of observing “ground truth”.  In these cases, it seems that our best hope is to approximate a baseline and compare against that (as we did in NANO); failing that, we can at least characterize inconsistencies.  In any case, when looking for these inconsistencies, it is important to (1) enumerate all factors that could possibly introduce inconsistencies; and (2) hold those factors fixed, to the extent possible.  For example, in NANO, one can only compare a user against average performance for a group of users that have identical (or at least similar) characteristics for anything that could affect the outcome.  If, for example, browser type (or other features) might affect performance, then the performance of a user for an ISP “under test” must be compared against users with the same browser (or other features), with the ISP being the only differing feature that could possibly affect performance.  Similarly, in the case of Bobble, we must hold other factors like browser type and device type fixed when attempting to isolate the effects of geography or search history.  Enumerating all of these features that could introduce  inconsistencies is extremely challenging, and I am not aware of any good way to determine whether a list of such features is exhaustive.

I believe networking and security researchers will continue to encounter phenomena that they would like to measure, but where the nature of underlying phenomenon cannot be known with certainty.  I am curious as to whether others have encountered problems that call for Internet relativism, and whether it may time to develop sound experimental methods to characterize Internet relativism, rather than simply blindly clamoring for “ground truth” when none may even exist.

About Nick Feamster
Nick Feamster is a professor in the Department of Computer Science at Princeton University. Before joining the faculty at Princeton, he was a professor in the School of Computer Science at Georgia Tech. He received his Ph.D. in Computer science from MIT in 2005, and his S.B. and M.Eng. degrees in Electrical Engineering and Computer Science from MIT in 2000 and 2001, respectively. His research focuses on many aspects of computer networking and networked systems, including the design, measurement, and analysis of network routing protocols, network operations and security, and anonymous communication systems. In December 2008, he received the Presidential Early Career Award for Scientists and Engineers (PECASE) for his contributions to cybersecurity, notably spam filtering. His honors include the Technology Review 35 "Top Young Innovators Under 35" award, a Sloan Research Fellowship, the NSF CAREER award, the IBM Faculty Fellowship, and award papers at SIGCOMM 2006 (network-level behavior of spammers), the NSDI 2005 conference (fault detection in router configuration), Usenix Security 2002 (circumventing web censorship using Infranet), and Usenix Security 2001 (web cookie analysis).

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: