Step 1 moves the HTTP origin from browser to server — the key shift vs 3. Bidders see a data-center IP, not the user, so cookies, UA, and IP must be forwarded or rebuilt via cookie sync. That hop costs 10-30% in match rates 10. Most teams run hybrid: identity-sensitive bidders stay client-side, latency-tolerant ones move to PBS 39.
FIELD NOTES
- Set s2sConfig.timeout to 50-75% of bidderTimeout for headroom
- PBS silently drops slow bidders; no error surfaces client-side
- 4KB uids cap gone server-side; user.ext.eids has no limit