  In recent years, Header Bidding (HB) has gained popularity among web
publishers, challenging the status quo in the ad ecosystem. Contrary to the
traditional waterfall standard, HB aims to give back to publishers control of
their ad inventory, increase transparency, fairness and competition among
advertisers, resulting in higher ad-slot prices. Although promising, little is
known about how this ad protocol works: What are HB's possible implementations,
who are the major players, and what is its network and UX overhead? To address
these questions, we design and implement HBDetector: a novel methodology to
detect HB auctions on a website at real time. By crawling 35,000 top Alexa
websites, we collect and analyze a dataset of 800k auctions. We find that: (i)
14.28% of top websites utilize HB. (ii) Publishers prefer to collaborate with a
few Demand Partners who also dominate the waterfall market. (iii) HB latency
can be significantly higher (up to 3x in median case) than waterfall.
