NetBIOS Services

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NetBIOS Services

As previously established, Windows NT uses NetBIOS internally, and any protocol (not just TCP/IP) that works with NT must have some means of translating NetBIOS to a native format. You have already seen that three ports are used: 137, 138, and 139. This section looks at the NetBIOS services and maps them to TCP/IP services and the port numbers indicated.

Three main services need to be handled by the protocol to enable communications over the network. There is a requirement to find the remote computer, because NetBIOS uses computer names rather than IP addresses; this is probably the most critical service in enabling NetBIOS to function over TCP/IP. Name resolution is handled on port 137, which is the NetBIOS Name Service port.

Another function that needs to be handled by an underlying protocol (such as TCP/IP) is Session Management. NetBIOS creates a session with a remote computer when it wants to communicate. As previously noted, this provides better security because the user has to be identified, and also enables extra checking in the transfer of large files. TCP also has the capability to create a session (by use of the three-way handshake). Creating a NetBIOS session, therefore, first creates a TCP session that provides base-level communications. This requires TCP and therefore uses the NetBIOS Session port, which is TCP port 139, as listed in Table 7.1.

The last—and for users, most important—function is the capability to transfer data from one system to another (whether it is a print job going to the printer, or a file being saved on the network). Two types of Data Transfer are available to NetBIOS and therefore must be available in the underlying protocol: connection-oriented (for transferring files, and so on) and connectionless (for broadcasts, logon requests, and so forth). The first uses the services of a session, and therefore uses TCP and the NetBIOS session port (139). Connectionless transfer simply sends the information and doesn’t care if the information is received; this suits the UDP transport protocol. As you may have guessed, connectionless transfers use the NetBIOS Datagram Service on UDP port 138.

As you can see, most of the functions map out easily. Session Management is a natural for TCP. Data Transfer is a basic network function, and TCP/IP already has connection-oriented and connectionless transfer methods available. The only function of NetBIOS that does not translate well is Name Management. Unfortunately, neither of the other functions works without the capability to resolve a NetBIOS name to a TCP/IP address.

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