Supply-side platforms integrate with the publishers and send that inventory to the ad exchanges. These ad exchanges are integrated with multiple demand-side platforms and send them those requests. They conduct an auction by receiving the bids from the various demand-side platforms.
Some well known supply-side platforms are Magnite (previously Rubicon Project + Telaria), PubMatic, Index Exchange, Xandr, Smaato, OpenX, MoPub.
Do all bid requests and impressions go out to all DSPs or do SSPs discriminate? Is there a cost to an SSP to send requests out DSPs?
Yes, the DSP’s need to have an integration with the SSP’s. The SSP’s don’t universally send out requests to all the existing DSP’s.
Also, SSP allows the DSP to set the targeting parameters on their platform, using which the SSP only sends such traffic to the DSP’s filtering out unwanted traffic.