Hello Turtledove Authors,

I would like to understand Turtledove Interest group selection in the context of Header Bidding.

Context:
In Header bidding, Publishers request bids for an ad slot from multiple ad platforms via header (ad) calls. Bids from these ad platforms are passed on to the publisher ad server ad call (as an input). Header bidders bids and publisher ad servers bids compete for the ad slot auction and a winner is chosen.

The key aspect is that:
To participate in the auction; header bidders do NOT partner with the publisher ad network.

TURTLEDOVE Interest Group Selection Spec
I am using following part of the proposal as a spec for Interest Group selection :

(The TURTLEDOVE mechanism is triggered) when the browser is in an interest group and loads a web page that gets its ads from a publisher ad network that is one of the group's reader domains or one of their ad-partners.

Interpretation

Following diagram shows state of a browser and how I am interpreting the spec in direct or RTB-style auction.

1 third-ad-network.com (partners with) first-ad-network.com. 
2 IG bid win collapses iframe into restrictive mode and nothing can be communicated with the main frame.  
3 (Assumption) Turtledove will use domain name (first-ad-network.com) to determine eligible Interest Groups

Here is the interpretation for Header Bidding style auction.

4 fourth-ad-network.com interest group is not selected as it does NOT partner with publisher ad network domain.
5 Cannot perform IG based auction in the header call (as not the ad renderer). Even if such thing is enabled; nothing can come out with interaction of IG winner (?).

Questions:

1. Can you please confirm the interpretation of the Spec is correct? Particularly wrt header bidding where 'fourth-ad-network.com's interest group (puzzles) will not be part of the auction.
2. If yes, it seems the independence of header bidders would be broken for Interest based ads (even being within the privacy boundary). Do we need additional trust / partnership establishing mechanisms (& other related changes like in the case of /fetch_ad) to solve the issue?

Thanks,
Viraj