Passback

What is a Passback?

A passback occurs when an ad network or demand source cannot fill an ad request and redirects it to another demand source for a second chance at monetization. Instead of showing a blank space when the primary network has no suitable ad, the request is "passed back" to a backup network, which can either fill the impression or pass it further down the chain.

Passbacks are a key component of the waterfall auction model. When the top-priority demand source passes on an impression, the passback tag triggers the next source in line. This process can continue through multiple levels until either the impression is filled or all sources are exhausted. In header bidding setups, passbacks are less critical because all sources bid simultaneously, but they still serve as a safety net for impressions that fall through the primary auction.

Why It Matters for Publishers

Passbacks help publishers minimize unfilled impressions and recover revenue that would otherwise be lost. Every unfilled impression represents a missed monetization opportunity. A well-configured passback chain ensures that even low-demand traffic (from regions with few advertisers or during low-spend periods) has the best chance of being monetized.

However, passbacks add latency. Each level of passback adds 100-500ms of delay as the request travels to another network and waits for a response. Multiple passback levels can significantly slow ad loading, hurting user experience and potentially lowering viewability scores.

Tips for Optimization

• Limit passback depth: No more than 2-3 passback levels. Beyond that, the added latency outweighs the marginal revenue from filling low-value impressions.
• Use header bidding instead: If you're still relying heavily on passbacks, implement header bidding. Parallel auctions are more efficient than sequential passback chains.
• Set timeout limits: Configure passback tags with strict timeouts (500-1000ms) so slow-responding networks don't hold up the entire chain indefinitely.
• Use a house ad as final fallback: At the end of your passback chain, serve a house ad or self-promotion rather than a blank space. This recovers value from otherwise unfilled impressions.