What is ad serving in CTV advertising?

Ad serving in CTV is the process of selecting and delivering a video ad to a viewer during a content break. When the app detects an ad break, it sends a request to the publisher's ad server. The ad server runs an auction, selects the winning ad, and returns a VAST tag — an XML file that tells the player where to fetch the video creative and what tracking events to fire. The player fetches the video from the CDN, plays the ad, and fires tracking beacons (impression, quartile marks, complete) back to the ad server, DSP, and any third-party verification vendors. This full sequence — from break detection to beacon completion — typically takes 1–3 seconds in India, depending on network conditions and CDN proximity.

CSAI (client-side ad insertion) means the CTV app player makes a live ad request during the break, fetches the VAST tag, and retrieves the creative from a CDN in real time. SSAI (server-side ad insertion) means the ad is stitched into the content stream on the server before it reaches the device — the viewer receives a single continuous stream with ads already embedded. SSAI eliminates buffering at ad break points (since there is no live CDN fetch) and is highly resistant to ad blocking. JioHotstar uses SSAI for its premium inventory. Most other India CTV publishers use CSAI via Google Ad Manager. The trade-off: SSAI provides better viewer experience but limited third-party measurement visibility; CSAI supports full VAST wrapper tracking but introduces a buffering risk at breaks.

The most common India CTV ad serving failures: (1) VAST 303 — no eligible ad: the ad server found no matching creative for the break (targeting too narrow, floor underbid, or no active line items). (2) VAST 400 — video file not found: the CDN URL in the VAST tag returned a 404; re-upload the creative and verify the CDN link is live. (3) VAST 403 — geo-blocked creative: the CDN serving the creative blocks requests from India IP addresses; ensure the CDN has India delivery enabled. (4) VAST 101 — wrong VAST version: the publisher's player only supports VAST 2.0 but received a VAST 4.0 tag. Check VAST error logs in the ad server or DSP reports — error codes identify exactly where in the delivery chain the failure occurred.