TCP/IP Application Interfaces

For applications to access the services offered by the core TCP/IP protocols in a standard way, network operating systems like Windows 2000 make industry standard application programming interfaces (APIs) available. Application interfaces are sets of functions and commands that are programmatically called by application code to perform network functions. For example, a Web browser application connecting to a Web site needs access to TCP's connection establishment service.

Figure 1.2 shows two common TCP/IP network interfaces, Windows Sockets and NetBIOS, and their relation to the core protocols.

Figure 1.2 APIs for TCP/IP

Windows Sockets Interface

The Windows Sockets API is a standard API under Windows 2000 for applications that use TCP and UDP. Applications written to the Windows Sockets API run on many versions of TCP/IP. TCP/IP utilities and the SNMP service are examples of applications written to the Windows Sockets interface.

Windows Sockets provides services that allow applications to bind to a particular port and IP address on a host, initiate and accept a connection, send and receive data, and close a connection. There are two types of sockets:

1. A stream socket provides a two-way, reliable, sequenced, and unduplicated flow of data using TCP.

2. A datagram socket provides the bi-directional flow of data using UDP.

A socket is defined by a protocol and an address on the host. The format of the address is specific to each protocol. In TCP/IP, the address is the combination of the IP address and port. Two sockets, one for each end of the connection, form a bi-directional communications path.

To communicate, an application specifies the protocol, the IP address of the destination host, and the port of the destination application. After the application is connected, information can be sent and received.

NetBIOS Interface

NetBIOS was developed for IBM in 1983 by Sytek Corporation to allow applications to communicate over a network. NetBIOS defines two entities, a session level interface and a session management and data transport protocol.

The NetBIOS interface is a standard API for user applications to submit network input/output (I/O) and control directives to underlying network protocol software. An application program that uses the NetBIOS interface API for network communication can be run on any protocol software that supports the NetBIOS interface.

NetBIOS also defines a protocol that functions at the session/transport level. This is implemented by the underlying protocol software (such as the NetBIOS Frames Protocol (NBFP), a component of NetBEUI, or NetBIOS over TCP/IP (NetBT)) to perform the network I/O required to accommodate the NetBIOS interface command set. NetBIOS over TCP/IP is defined in RFCs 1001 and 1002.

NetBIOS provides commands and support for NetBIOS Name Management, NetBIOS Datagrams, and NetBIOS Sessions.

NetBIOS Name Management

NetBIOS name management services provide the following functions:

• Name Registration and Release
  When a TCP/IP host initializes, it registers its NetBIOS names by broadcasting or directing a NetBIOS name registration request to a NetBIOS Name Server such as a Windows Internet Name Service (WINS) server. If another host has registered the same NetBIOS name, either the host or a NetBIOS Name Server responds with a negative name registration response. The initiating host receives an initialization error as a result.
  When the workstation service on a host is stopped, the host discontinues broadcasting a negative name registration response when someone else tries to use the name and sends a name release to a NetBIOS Name Server. The NetBIOS name is said to be released and available for use by another host.

• Name Resolution
  When a NetBIOS application wants to communicate with another NetBIOS application, the IP address of the NetBIOS application must be resolved. NetBIOS over TCP/IP performs this function by either broadcasting a NetBIOS name query on the local network or sending a NetBIOS name query to a NetBIOS Name Server.
  For more information about NetBIOS name resolution, see "NetBIOS Name Resolution" later in this chapter.

The NetBIOS name service uses UDP port 137.

NetBIOS Datagrams

The NetBIOS datagram service provides delivery of datagrams that are connectionless, nonsequenced, and unreliable. Datagrams can be directed to a specific NetBIOS name or broadcast to a group of names. Delivery is unreliable in that only the users who are logged on to the network receive the message. The datagram service can initiate and receive both broadcast and directed messages. The NetBIOS datagram service uses UDP port 138.

NetBIOS Sessions

The NetBIOS session service provides delivery of NetBIOS messages that are connection-oriented, sequenced, and reliable. NetBIOS sessions use TCP connections and provide session establishment, keepalive, and termination. The NetBIOS session service allows concurrent data transfers in both directions using TCP port 139.