On the structure and characteristics of user agent string.

User agent (UA) strings transmitted during HTTP transactions convey client system configuration details to ensure that content returned by a server is appropriate for the requesting host. As such, analysis of UA strings and their structure offers a unique perspective on active client systems in the Internet and when tracked longitudinally, offers a perspective on the nature of system and configuration dynamics. In this paper, we describe our study of UA string characteristics. Our work is based on analyzing a unique corpus of over 1B UA strings collected over a period of 2 years by comScore. We begin by analyzing the general characteristics of UA strings, focusing on the most prevalent strings and dynamic behaviors. We identify the top 10 most popular User Agents, which account for 26% of total daily volume. These strings describe the expected instances of popular platforms such as Microsoft, Apple and Google. We then report on the characteristics of low-volume UA strings, which has important implications for unique device identification. We show that this class of user agent generates the overwhelming majority of traffic, with between 2M and 10M instances observed each day. We show that the distribution of UA strings has temporal dependence and we show the distribution measured depends on the type of content served. Finally, we report on two large-scale UA anomalies characterized by web browsers sending false and misleading UAs in their web requests.