Tabular Data Stream Protocol

Microsoft® SQL Server™ 2000 uses an application-level protocol called Tabular Data Stream (TDS) for communication between client applications and SQL Server. The TDS packets are encapsulated in the packets built for the protocol stack used by the Net-Libraries. For example, if you are using the TCP/IP Sockets Net-Library, then the TDS packets are encapsulated in the TCP/IP packets of the underlying protocol.

The contents of the packets that send result sets back to the application depends on whether FOR XML is specified in the Transact-SQL statement transmitted to the database engine:

• If FOR XML is not specified, the database engine sends a relational result set back to the application. The TDS packets contain the rows of the result set, with each row comprised of one or more columns, as specified in the select list of the SELECT statement.
  
  
• If FOR XML is specified, the database engine streams an XML document back to the application. The XML document is formatting in the TDS packets as if it were a single, long Unicode value, with each packet being approximately 4 KB in size.

You can configure the SQL Server packet size, which is the size of the TDS packets. The size of the TDS packets defaults to 4 KB on most clients (DB-Library applications default to 512 bytes), which testing has shown to be the optimal TDS packet size in almost all scenarios. The size of the TDS packets can be larger than the size of the packets in the underlying protocol. If this is the case, the protocol stack on the sending computer disassembles the TDS packets automatically into units that fit into the protocol packets, and the protocol stack on the client computer reassembles the TDS packets on the receiving computer.