What is SSP (supply-side platform)?

An SSP works for the publisher. For each impression it packs a bid request with details such as the page address, ad size, country, device and, where consent allows, a user ID. It sends that request to many DSPs at once, runs the auction, picks the top bid, takes its fee and returns the winning ad.

SSPs and DSPs speak a shared standard, OpenRTB. In header bidding, each SSP connects through its own bid adapter, and its best bid is passed to your ad server, which makes the final choice. An SSP and an exchange are often the same company.

Most lines in a publisher’s ads.txt file name SSPs, because they are the companies authorised to sell the inventory. Adding SSPs brings competition, but many SSPs reach the same DSPs, so each extra one tends to add less than the one before, while every bidder adds requests and waiting time.