Transport Driver Interface
The Transport Dispatch Interface is the protocol understood by the upper edge of the Transport Provider layer of the Windows NT/2k/XP kernel network stack.
Transport Providers are implimentations of network protocols such as TCP/IP, NetBIOS and AppleTalk.
When user-mode binaries are created by compiling and linking, an entity called a TDI client is linked into the binary. TDI clients are provided with the compiler. The user-mode binary uses the user-mode API of whatever network protocol is being used, which in turns causes the TDI client to emit TDI commands into the Transport Provider.
Typical TDI commands are TDI_SEND, TDI_CONNECT, TDI_RECEIVE.
The purpose of the Transport Dispatch Interface is to provide an abstraction layer, permitting simplification of the TDI clients.
Source: Windows XP Driver Development Kit documentation.