VAST (Video Ad Serving Template)

VAST — Video Ad Serving Template — is the IAB Tech Lab XML specification that tells a video player how to fetch, render, and track a video ad. A VAST response is an XML document with a media-file URL, a click-through URL, a list of impression tracking pixels, and a list of event tracking pixels keyed to playback milestones (start, first-quartile, midpoint, third-quartile, complete).

VAST 2.0 launched in 2008; the current version is VAST 4.3 (2023), which added support for verification scripts (the OMID stitching point), universal ad IDs (ad de-duplication across creatives), and SIMID interactive overlays. VAST 4.x also formally separated media-file delivery from verification, which earlier versions had conflated through VPAID's combined-bundle model.

Every CTV ad pod, every DOOH ad slot served programmatically, and every web video pre-roll uses VAST. The protocol is the lingua franca: a DSP returns a VAST URL; the player fetches and parses the XML; the player fires pixels at each milestone. Trillboards screens render VAST natively rather than running a heavyweight ad-pod player — see the edge SDK for the parser implementation.

VAST also defines an error-handling protocol: a Wrapper that points to another Wrapper that points to an InLine creative. Each hop fires its own tracking pixels; each level can return an error code that maps to a defined enum (901 = ad blocked, 303 = no ad served, etc.).