Most publishers work with multiple SSPs, so it would be least disruptive if Fledge/TD continued to support this model. It is unclear from the FLEDGE explainer how this would work. Can we please fill in the gaps at a high level?

What does it mean to work with multiple SSPs?

At a high level, it means that every SSP gets to look at all the demand it represents and place its best bid. Today, Prebid.js allows each SSP to request bids and then orchestrates the final selection between SSPs according to the business rules defined in the publisher's Prebid.js config and then GAM makes the final decision.

In a Fledge world there are several changes that need to be accommodated

1. An SSP needs to be able to select a winning IG bid from all the IG demand it has access to; and
2. An SSP needs to be able to decide between its winning contextual bid and its winning IG bid.

1. What IG Bids does an SSP have access to?

DSPs have the ability to choose which SSPs they bid through and how they bid through those SSPs. They may choose to bid with $1.50 for Campaign A through SSP and bid with $1.30 on Campaign B through another SSP and not bid at all through SSP C. The manner in which they bid may vary based on the relationship they have with an SSP and/or the type of information the SSP provides them to help value the request. They also can choose to do supply chain optimization. I believe this functionality needs to be preserved in the Interest Bid world.

Ultimately, the DSP should decide which SSPs it allows to represent its bids. This decision will be based in part on which SSPs it has a relationship with. If no relationship exists between the SSP and DSP it is impossible to represent the bid as there's no way for the publisher to get paid.

To do this, the DSP needs to know which SSP(s) are eligible to represent the publisher in the auction.

2. How does an SSP decide between its winning contextual bid and its winning IG bid?

In general, this happens by passing the contextual bid information into the runAdAuction call. The question is whether each SSP gets its own runAdAuction call with Fledge providing a means for resolving multiple calls (discussed here), or if there's a single uber-auction call that must resolve all the SSPs and DSPs connections (this is explored more here).

I think it would be cleaner to have multiple runAdAuction calls with a resolution mechanism, but either is doable.

cc @brodrigu @fbastello @sbelov @appascoe @dialtone who I've talked to about this.