Chapter 2. Speaking NetBIOS

Genuine Imitation

Well known oxymoron

The hardware part of IBM's PC Network is no longer in use and the protocol that actually ran on the wire is all but forgotten, yet the NetBIOS API remains. Vast sweeping hoards of programs including DOS itself were written to use NetBIOS. Like COBOL, it may never die.

Many vendors , eager for a piece of the Microsoft desktop pie, figured out how to implement the NetBIOS API on top of other protocols. There is NetBIOS over DECnet, NetBIOS over NetWare, NetBIOS over mashed potatoes and gravy with creamed corn, NetBIOS over SNA, NetBIOS over TCP/IP, and more. Of these, the most popular, tasty, and important is NetBIOS over TCP/IP, and that's what this chapter is really all about.

NetBIOS over TCP/IP is sometimes called NetBT or NBT . Folks from IBM for reasons unfathomable sometimes call it TCPBEUI . NBT is the simplest and most common name , so we'll stick with that.

On the 7-layer OSI reference model, NetBIOS is a session-layer (layer 5) API. Under DOS and its offspring, applications talk to NetBIOS by filling in a record structure known as a Network Control Block (NCB) and signaling an interrupt. The NCBs are used to pass commands and messages between applications and the underlying protocol stack.

Figure 2.1. The NetBIOS layer

The NetBIOS layer is sandwiched between the Server Message Block (SMB) filesharing protocol and the underlying network transport layer.

Fortunately, the NetBIOS API is specific to DOS and its kin. Unix and other systems do not need to implement the NetBIOS API, as there is no legacy of programs that use it. Instead, these systems participate in NBT networks by directly handling the TCP and UDP packets described in two IETF (Internet Engineering Task Force) Request for Comments documents: RFC 1001 and RFC 1002 (known collectively as Internet Standard #19). These RFCs describe a set of services that work together to create virtual NetBIOS LANs over IP.