Defining NetBIOS

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Defining NetBIOS

Microsoft has been using NetBIOS for the upper layers of their networking architecture for years. This chapter looks at the NetBIOS standard and how it communicates. Mapping of NetBIOS functions to those found in TCP/IP also is discussed. This mapping is required for TCP/IP and any other network protocol installed in Windows NT so that the internal NetBIOS commands can traverse the network.

Although there are three main functions that need to be supported for NetBIOS to function—Name Management, Session Management and Data Transfer—there will be much emphasis given to the Name Management in this chapter. This is the key issue in using NetBIOS over TCP/IP because TCP/IP uses IP addressing, whereas NetBIOS uses computer names. The other functions present in NetBIOS are already present in TCP/IP.

NetBIOS is a networking standard based on the OSI (Open System Interconnect) model—also known as the seven-layer model. When referencing the OSI model, NetBIOS as implemented in Windows NT provides the services required for the top three layers: application, presentation, and session.

The application layer interacts with user programs (for example, Windows NT Explorer or Microsoft Word) and handles network access for those programs. When the application layer receives a request for network access, it turns the request into an SMB (Server Message Block). An SMB is a unit of work that tells the system at the other end what the user on this system wants to do (for example, read a file from the network). SMBs are considered Protocol Data Units (PDUs) and as such perform the work of moving requests and data between systems. All the other layers in the protocol stack simply serve to move the SMB from one system to another system.

After the SMB has been generated, the presentation layer prepares to deliver the information to the correct computer. This requires the services of the session layer, which creates or uses a session with the remote computer to deliver the information. In some cases (broadcasts), a session is not required. The presentation layer checks to see whether a session is required for the transmission—and, if a session is required, whether one already exists. If a session does not exist, the presentation layer uses the services of the session layer to create a session with the remote host. The presentation layer can then generate an NCB (Network Control Block) that tells the underlying layer what to do with the SMB (which is now the data to be transferred).

The session layer receives the NCB and acts on it normally by sending the data to the remote host. As already mentioned, the session layer is responsible for creating and terminating sessions with other hosts, as well as for controlling the flow of data. By using sessions, Windows NT adds a layer of security because the user’s credentials (access token) are checked and verified when the session is created. In addition, sessions enables extra checking of the information flowing across the network to verify that it has arrived in good order.

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