Data Stream Interface
computer network protocol to run Apple Filing Protocol over TCP
The Data Stream Interface (DSI) is a session layer used to carry Apple Filing Protocol traffic over Transmission Control Protocol.
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.
Protocol
DSI is spoken between a client and an AFP server. All DSI communication contains the following DSI header:
Packet structure
The fields are:
Flags: whether the packet is a request (0x00) or a reply (0x01)
Command: one of 7 possible commands (see below)
Request ID: a sequential identifier set on the request and copied in the reply
Error code/ enclosed data offset:
For requests, this is left as 0, except when using the DSIWrite command.
For replies, this is an error code.
Total data length: the entire length of data after the DSI header
Reserved: for future expansion
Payload: this is where limited DSI data or more commonly AFP header is placed
Commands
There are seven possible commands:
Requests and replies
Upon receiving most DSI requests, the client or server sends a reply message. This reply contains:
the flags field set to 0x01 (reply)
the command field set to the same value as the request's command field
the same request ID sent in the request (used for the client to find the request being acknowledged)
totalDataLength set to the payload length (if applicable).
where applicable, the data payload itself following the DSI header.
The public source identifies “Data Stream Interface” as computer network protocol to run Apple Filing Protocol over TCP. This brief keeps that definition visible, then builds a research path around Data, Stream and Interface.
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This entry incorporates text from “Data Stream Interface” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.