Which standard protocol was utilized for Progressive Downloading in early web servers?
Hypertext Transfer Protocol (HTTP).
Progressive downloading was an early step toward continuous media delivery that leveraged existing, well-established web technology. This method allowed web servers to begin sending the media file data, and the browser could start rendering or playing the media once a sufficient initial buffer was established. The primary protocol used to facilitate this transmission of data segments over the wide area network was the standard Hypertext Transfer Protocol (HTTP), the same protocol used for standard web page retrieval. Although it provided a smoother experience than waiting for a 100% download, it was still fundamentally constrained by HTTP’s design, which was not optimized for the interactive control demanded by true streaming.

#Videos
Yuriy Reznik - Streaming in 1970s. NVP & ST - YouTube