How VAST works
The player, or a server-side stitching service, requests a VAST tag: a URL that points to the ad server. The server replies with XML describing one or more ads. The player picks a suitable media file, plays it and calls the tracking URLs at the specified moments.
A response can also be a wrapper that points to another ad server's tag, which is how an ad passes through several intermediaries before reaching the player.
What a VAST response contains
- Media files: the ad video in several formats and bitrates
- Duration, and whether and when the ad can be skipped
- Impression URL: called when the ad starts
- Tracking events: start, first quartile, midpoint, third quartile, complete, mute, pause
- Click-through URL
- Companion ads and verification scripts
VAST, VMAP and VPAID
- VAST: describes an ad
- VMAP: describes a schedule of ad breaks within a piece of content, with a VAST tag for each
- VPAID: an older standard for interactive ads that ran code in the player, now deprecated in favor of newer VAST features and separate standards for interactivity and measurement
Versions
VAST 2 and 3 remain widely used; VAST 4 added features for server-side insertion, higher-quality media files and standardized verification. A player and an ad source must support a common version, which is worth confirming before signing with an ad partner.
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