Editing NetBIOS (section)

===Session service===
Session mode lets two computers establish a connection, allows messages to span multiple packets, and provides error detection and recovery. In [[NetBIOS over TCP/IP|NBT]], the session service runs on TCP port 139.

The session service primitives offered by NetBIOS are:
*Call – opens a session to a remote NetBIOS name.
*Listen – listen for attempts to open a session to a NetBIOS name.
*Hang Up – close a session.
*Send – sends a packet to the computer on the other end of a session.
*Send No Ack – like Send, but doesn't require an acknowledgment.
*Receive – wait for a packet to arrive from a Send on the other end of a session.

In the original protocol used to implement NetBIOS services on PC-Network, to establish a session, the initiating computer sends an Open request which is answered by an Open acknowledgment. The computer that started the session will then send a Session Request packet which will prompt either a Session Accept or Session Reject packet.

During an established session, each transmitted packet is answered by either a positive-acknowledgment (ACK) or negative-acknowledgment (NAK) response. A NAK will prompt retransmission of the data. Sessions are closed by the non-initiating computer by sending a close request. The computer that started the session will reply with a close response which prompts the final session closed packet.