Tabular Data Stream (TDS) is an application layer protocol, used to transfer data between a database server and a client. It was initially designed and developed by Sybase Inc. for their Sybase SQL Server relational database engine in 1984, and later by Microsoft in Microsoft SQL Server.
During the early development of Sybase SQL Server, the developers at Sybase realized that there was no commonly accepted application-level protocol to transfer data between a database server and its client. To encourage the use of their products, Sybase came up with a solution through the use of a flexible pair of libraries, called netlib and db-lib, to implement standard SQL. A further library was included to implement "Bulk Copy" called blk. While netlib's job is to ferry data between the two computers through the underlying network protocol, db-lib provides an API to the client program, and communicates with the server via netlib. db-lib sends to the server a structured stream of bytes meant for tables of data, hence a Tabular Data Stream. blk provides, like db-lib, an API to the client programs and communicates with the server via netlib. Unlike SQL, it provides a proprietary but much faster protocol for loading data into a database table.
In 1990, Sybase entered into a technology sharing agreement with Microsoft which resulted in Microsoft marketing its own SQL Server — Microsoft SQL Server — based on Sybase's code. Microsoft kept the db-lib API and added ODBC. (Microsoft has since added additional APIs.) At about the same time, Sybase introduced a more powerful successor to db-lib, called ct-lib, and called the pair Open Client. db-lib is officially deprecated but still in widespread use.
The TDS protocol comes in several varieties, most of which had not been openly documented because they were considered to be proprietary technology. The exception was TDS 5.0, used exclusively by Sybase, for which documentation is available from Sybase.[1] This state changed when Microsoft published the TDS specification,[2] probably due to the Open Specification Promise.
A free native library implementation of the TDS protocol has been developed by the FreeTDS team,[3] licensed under the LGPL license, and the protocol has been decoded with WireShark.[4]