Over the past few weeks, regulators have rekindled their interest in an online Do Not Track proposal in hopes of better protecting consumer privacy. FTC Chairman Jon Leibowitz told a Senate Commerce subcommittee last month that Do Not Track is “one promising area” for regulatory action and that the Commission plans to issue a report in the fall about “whether this is one viable way to proceed.” Senator Mark Pryor (D-AR), who sits on the subcommittee, is also reportedly drafting a new privacy bill that includes some version of this idea, of empowering consumers with blanket opt-out powers over online tracking.

Details are sparse at this point about how a Do Not Track mechanism might actually be implemented. There are a variety of possible technical and regulatory approaches to the problem, each with its own difficulties and limitations, which I’ll discuss in this post.

An Adaptation of “Do Not Call”

Because of its name, Do Not Track draws immediate comparisons to arguably the most popular piece of consumer protection regulation ever instituted in the US—the National Do Not Call Registry. If the FTC were to take an analogous approach for online tracking, a consumer would register his device’s network identifier—its IP address—with the national registry. Online advertisers would then be prohibited from tracking devices that are identified by those IP addresses.

Of course, consumer devices rarely have persistent long-term IP addresses. Most ISPs assign IP addresses dynamically (using DHCP) and a single device might be assigned a new IP address every few minutes. Consumer devices often also share the same IP address at the same time (using NAT) so there’s no stable one-to-one mapping between IPs and devices. Things could be different with IPv6, where each device could have its own stable IP address, but the Do Not Call framework, directly applied, is not the best solution for today’s online world.

The comparison is still useful though, if only to caution against the assumption that Do Not Track will be as easy, or as successful, as Do Not Call. The differences between the problems at hand and the technologies involved are substantial.

A Registry of Tracking Domains

Back in 2007, a coalition of online consumer privacy groups lobbied for the creation of a national Do Not Track List. They proposed a reverse approach: online advertisers would be required to register with the FTC all domain names used to issue persistent identifiers to user devices. The FTC would then publish this list, and it would be up to the browser to protect users from being tracked by these domains. Notice that the onus here is fully on the browser—equipped with this list—to protect the user from being uniquely identified. Meanwhile, online advertisers would still have free rein to try any method they wish to track user behavior, so long as it happens from these tracking domains.

We’ve learned over the past couple of years that modern browsers, from a practical perspective, can be limited in their ability to protect the user from unique identification. The most stark example of this is the browser fingerprinting attack, which was popularized by the EFF earlier this year. In this attack, the tracking site runs a special script that gathers information about the browser’s configurations, which are unique enough to identify the browser instance in nearly every case. The attack takes advantage of the fact that much of the gathered information is used frequently for legitimate purposes—such as determining which plugins are available to the site—so a browser which blocks the release of this information would surely irritate the user. As these kinds of “side-channel” attacks grow in sophistication, major browser vendors might always be playing catch-up in the technical arms race, leaving most users vulnerable to some form of tracking by these domains.

The x-notrack Header

If we believe that browsers, on their own, will be unable to fully protect users, then any effective Do No Track proposal will need to place some restraints on server tracking behavior. Browsers could send a signal to the tracking server to indicate that the user does not want this particular interaction to be tracked. The signaling mechanism could be in the form of a standard pre-defined cookie field, or more likely, an HTTP header that marks the user’s tracking preference for each connection.

In the simplest case, the HTTP header—call it x-notrack—is a binary flag that can be turned on or off. The browser could enable x-notrack for every HTTP connection, or for connections to only third party sites, or for connections to some set of user-specified sites. Upon receiving the signal not to track, the site would be prevented, by FTC regulation, from setting any persistent identifiers on the user’s machine or using any other side-channel mechanism to uniquely identify the browser and track the interaction.

While this approach seems simple, it could raise a few complicated issues. One issue is bifurcation: nothing would prevent sites from offering limited content or features to users who choose to opt-out of tracking. One could imagine a divided Web, where a user who turns on the x-notrack header for all HTTP connections—i.e. a blanket opt-out—would essentially turn off many of the useful features on the Web.

By being more judicious in the use of x-notrack, a user could permit silos of first-party tracking in exchange for individual feature-rich sites, while limiting widespread tracking by third parties. But many third parties offer useful services, like embedding videos or integrating social media features, and they might require that users disable x-notrack in order to access their services. Users could theoretically make a privacy choice for each third party, but such a reality seems antithetical to the motivations behind Do Not Track: to give consumers an easy mechanism to opt-out of harmful online tracking in one fell swoop.

The FTC could potentially remedy this scenario by including some provision for “tracking neutrality,” which would prohibit sites from unnecessarily discriminating against a user’s choice not to be tracked. I won’t get into the details here, but suffice it to say that crafting a narrow yet effective neutrality provision would be highly contentious.

Privacy Isn’t a Binary Choice

The underlying difficulty in designing a simple Do Not Track mechanism is the subjective nature of privacy. What one user considers harmful tracking might be completely reasonable to another. Privacy isn’t a single binary choice but rather a series of individually-considered decisions that each depend on who the tracking party is, how much information can be combined and what the user gets in return for being tracked. This makes the general concept of online Do Not Track—or any blanket opt-out regime—a fairly awkward fit. Users need simplicity, but whether simple controls can adequately capture the nuances of individual privacy preferences is an open question.

Another open question is whether browser vendors can eventually “win” the technical arms race against tracking technologies. If so, regulations might not be necessary, as innovative browsers could fully insulate users from unwanted tracking. While tracking technologies are currently winning this race, I wouldn’t call it a foregone conclusion.

The one thing we do know is this: Do Not Track is not as simple as it sounds. If regulators are serious about putting forth a proposal, and it sounds like they are, we need to start having a more robust conversation about the merits and ramifications of these issues.