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NetBIOS over TCP Sessions

NetBIOS sessions are established between two names. For example, when a Windows 2000 Professional-based workstation makes a file-sharing connection to a server using NetBIOS over TCP/IP, the following sequence of events takes place: The NetBIOS name for the server is resolved to an IP address. The IP address is resolved to a media access control address. A TCP connection is established from the workstation to the server, using port 139. The workstation sends a NetBIOS Session Request to the server name over the TCP connection. If the server is listening on that name, it responds affirmatively, and a session is established. When the NetBIOS session has been established, the workstation and server negotiate which level of the SMB protocol to use. Microsoft networking uses only one NetBIOS session between two names at any time. Any additional file or print sharing connections are multiplexed over the same NetBIOS session using identifiers within the SMB header. NetBIOS keep-alives are used on each connection to verify that both the server and workstation are still able to maintain their session. Therefore, if a workstation is shut down ungracefully, the server eventually cleans up the connection and associated resources, and vice versa. NetBIOS keep-alives are controlled by the SessionKeepAlive registry parameter and default to once per hour. If LMhosts files are used and an entry is misspelled, it is possible to attempt to connect to a server using the correct IP address but an incorrect name. In this case, a TCP connection is still established to the server. However, the NetBIOS session request (using the wrong name) is rejected by the server, because there is no listen posted on that name. An Error 51, "Remote computer not listening," is returned.

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