Upper bounds on referrals

  A first approach to detect hit shaving is to modify the click-through sequence to elicit a notification from the user's browser to the referring site A when the user clicks the link to pageB.html in pageA.html. In this way, A can monitor how many times users have clicked through pageA.html to pageB.html by monitoring how many such notifications it receives. In this section we show two approaches for achieving this, or more specifically for turning the exchange of Figure 1 into one that looks like Figure 2. As Figure 2 shows, once the user clicks on the link to pageB.html, a notification is sent back to A (message 2) and then pageB.html is retrieved (messages 3,4). Neither of the methods we propose requires cooperation from site B, and both are invisible to the user.

  

Figure 2: Bounding referrals from above: User U retrieves pageA.html from A (message 1). When the user clicks on the link to pageB.html, the user's browser sends a notification to A (message 2) and then retrieves pageB.html (messages 3,4).

Though effective, the methods of this section enable site A only to record an upper bound on the number of referrals for which A should be credited, not an exact count. The reason for this is that the notification sent to A (message 2 in Figure 2) is an indication only that the user's browser will request pageB.html, not that B has received this request. To see the importance of this distinction, the webmaster of site B could plausibly claim that site B was down or heavily overloaded for some significant period of time (causing requests to be dropped), and thus no referrals were completed (or thus credited to A's account) during that time. An approach that enables A to additionally record a lower bound on its number of referrals is the topic of Section 3.