Editing NetBIOS over TCP/IP (section)

===Session service===
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Session mode lets two computers establish a connection for a "conversation", allows larger messages to be handled, and provides error detection and recovery.

Sessions are established by exchanging packets. The computer establishing the session attempts to make a [[Transmission Control Protocol|TCP]] connection to port 139 on the computer with which the session is to be established. If the connection is made, the computer establishing the session then sends over the connection a "Session Request" packet with the NetBIOS names of the application establishing the session and the NetBIOS name to which the session is to be established. The computer with which the session is to be established will respond with a "Positive Session Response" indicating that a session can be established or a "Negative Session Response" indicating that no session can be established (either because that computer isn't listening for sessions being established to that name or because no resources are available to establish a session to that name).

Data is transmitted during an established session by Session Message packets.

TCP handles flow control and retransmission of all session service packets, and the dividing of the data stream over which the packets are transmitted into [[Internet Protocol|IP]] datagrams small enough to fit in link-layer packets.

Sessions are closed by closing the TCP connection.