OpenRTB Native and Carousels

[Chris Naegelin (Magnite)]

Hi everyone -

I'm currently a Sr Director of Product at Magnite and we are trying to better understand how a "Carousel" style format would be handled within the specifications of OpenRTB Native 1.2

For visual clarity - here is a similar style carousel to what I am referencing: https://www.facebook.com/business/ads-guide/carousel/facebook-feed/traffic 

 One can think of a carousel as a single ad with a series of grouped assets. For example, in the facebook unit above there is a Title, Image, and Call to action multiple times within the same unit.   What I am struggling with is understanding how would an openrtb native 1.2 request be constructed for this use case? 

1)  If using a placement count >1 , what guarantees are there that title 1 and image 1 are correlated such that Title 2 and Image 1 are not accidentally placed together at render time. The SSP has no inherent knowledge of this relationship from the buy side. 

2) If using sequence its my understanding that this implies / signals that we are holding multiple auctions instead of one auction with multiple asset groups?

3) Is the proper way to do this simply to send a single group of assets in a single IMP object and request multiple IMP objects using roadblocking in the same bid request? Our understanding here is that many DSPs are not supporting multi-bid so this may also prove problematic.

Any specific request/response examples folks could share or general guidance would be very much appreciated.

Regards,

Chris Naegelin