Editing NetBIOS (section)

==Services==
NetBIOS provides three distinct services:
*Name service (NetBIOS-NS) for name registration and [[Name resolution (computer systems)|resolution]].
*[[Datagram]] distribution service (NetBIOS-DGM) for connectionless communication.
*[[Session (computer science)|Session]] service (NetBIOS-SSN) for [[connection-oriented communication]].

(Note: [[Server Message Block|SMB]], an upper layer, is a service that runs on top of the Session Service and the Datagram service, and is not to be confused as a necessary and integral part of NetBIOS itself. It can now run atop TCP with a small adaptation layer that adds a length field to each SMB message; this is necessary because TCP only provides a byte-stream service with no notion of message boundaries.)

===Name service===
In order to start sessions or distribute datagrams, an application must register its NetBIOS name using the name service. NetBIOS names are 16 octets in length and vary based on the particular implementation. Frequently, the 16th octet, called the NetBIOS Suffix, designates the type of resource, and can be used to tell other applications what type of services the system offers.{{cn|date=June 2023|reason=Unclear from text what is meant by "octets". Possibly confuses octets with bytes.}} In [[NetBIOS over TCP/IP|NBT]], the name service operates on UDP port 137 (TCP port 137 can also be used, but rarely is).

The name service primitives offered by NetBIOS are:
*Add name – registers a NetBIOS name.
*Add group name – registers a NetBIOS "group" name.
*Delete name – un-registers a NetBIOS name or group name.
*Find name – looks up a NetBIOS name on the network.

[[IPv6|Internet Protocol Version 6 (IPv6)]] are not supported by the NetBIOS name resolution protocol.<ref>{{cite web |title=[MS-WPO]: WINS Management Protocol |url=https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-wpo/26b6d27b-7470-4a0d-bcfc-88670c6c50d4 |website=learn.microsoft.com |language=en-us |date=14 February 2019 |quote=Because the NetBIOS protocol, defined in [RFC1002], does not support the mapping between NetBIOS names and IPv6 addresses, the Remote Administrative Interface: WINS protocol applies only to IPv4 addresses. It does not apply to IPv6 addresses. |access-date=17 June 2023 |archive-date=17 June 2023 |archive-url=https://web.archive.org/web/20230617123311/https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-wpo/26b6d27b-7470-4a0d-bcfc-88670c6c50d4 |url-status=live }}</ref>

===Datagram distribution service===
Datagram mode is [[connectionless]]; the application is responsible for error detection and recovery. In [[NetBIOS over TCP/IP|NBT]], the datagram service runs on UDP port 138.

The datagram service primitives offered by NetBIOS are:
*Send Datagram – send a datagram to a remote NetBIOS name.
*Send Broadcast Datagram – send a datagram to all NetBIOS names on the network.
*Receive Datagram – wait for a packet to arrive from a Send Datagram operation.
*Receive Broadcast Datagram – wait for a packet to arrive from a Send Broadcast Datagram operation.

===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.