Overview
When Apple introduced TCP with MacTCP and Open Transport in System 7 in the 1990s, they needed their file sharing protocol (AFP) to run on both TCP and AppleTalk. They introduced AppleTalk Session Protocol (ASP) and DSI for TCP coincidentally with AFP 2.x.
DSI is implemented directly into AFP clients such as in Mac OS and afpfs-ng.
Read more about this topic: Data Stream Interface