VAST Tag

A VAST tag is the URL a video player calls to fetch a VAST XML response. It's the entry point of the ad-serving handshake: the player goes "I have an ad slot opening" → the VAST tag URL is called → the server returns VAST XML → the player parses the XML and either renders the inline creative or follows the wrapper to the next VAST tag.

VAST tags are typically heavily parameterized — a single template URL with macros for cache-busting, session ID, ad-slot index, deal ID, user signals, and so on. The IAB publishes a recommended macro list (RFC-style) so the same VAST tag template works across players.

In OpenRTB, the DSP's bid response includes either an "adm" field with raw VAST XML or a "nurl" pointing to a VAST tag the SSP fetches before passing to the player. The "adm" path is faster (no extra hop); the "nurl" path lets the DSP delay creative selection until the moment of impression.

Wrapper chains — one VAST tag pointing to another — are bounded to about 5 hops industry-wide. Deeper chains either time out or fire VAST error 302 ("wrapper limit reached"). On DOOH, where slot latency budgets are looser than CTV, longer wrapper chains are tolerated; on CTV pre-roll, the budget is tight enough that 3 hops is the practical limit.

See also IMA SDK, the Google library most players use to call VAST tags.