Data Stream Interface
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: 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 DSITickle and DSICloseSession commands do not trigger a reply. Session creation, maintenance and teardown. A session is set up by the client sending a DSIOpenSession, which will include the size of the receive buffer the client has for packets (called the request quantum, typically 1024 bytes). The server acknowledges the request and returns the size of its data receive buffer (typically 256k on Mac OS X Leopard). Session  closure can be initiated by either side by sending DSICloseSession. The sender does not need to wait for a reply and should immediately close the session after sending the message. Maintaining the connection is done by "tickling". DSI provides a mechanism for ensuring that client and server know that the other is still active. Every 30 seconds of inactivity, the server sends a tickle request to the client. Similarly, the client also sends its own tickle. (This is NOT a response packet.) Either the client or server can terminate the DSI session if they fail to hear from the other  for 120 seconds. The client may also disconnect if a request is in flight and neither a response nor tickle is received within 60 seconds (in Mac OS X v.10.2 and later). Getting server information with GetStatus. This DSI command encapsulates an FPGetSrvrInfo packet. It is used by a client to get information from a server it isn't logged into. The data elements are organized in the packet with a catalog of indices pointing to structured data. The request to a DSIGetStatus request will cause the server to respond with the following information: DSIGetStatus reply format is identical to  AFP's FPGetSrvrInfo and is used for ASPGetStatus. Error codes. The error codes returned are AFP result codes. Further research. DSI is never documented separately, and is sufficiently simple and static that older references are suitable for modern implementations. The concepts of DSI are identical to AppleTalk Session Protocol (ASP), and the overview in "Inside AppleTalk, Second Edition" can be helpful. The most succinct guide is the "AFP over TCP" chapter of Apple Filing Protocol Programming Guide. A significant source of information in understanding DSI can be found by analyzing communication between AFP clients and servers using a packet sniffer