The Network APIs, Windows Sockets, and NetBIOS

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The Network APIs, Windows Sockets, and NetBIOS

Notice that in Figure 2.31, the Application layer does not have protocols, but APIs. Recall that the Application layer provides the interface between applications and the transport protocols. Microsoft supports two APIs for applications to use: Windows Sockets and NetBIOS. This functionality is included because Microsoft networks still use NetBIOS for a number of internal mechanisms within the Windows NT operating system. It is also used because it provides a standard interface to a number of other protocols as well. TCP/IP, NetBEUI, and NWLink all have a NetBIOS interface to which applications can be written to use networking protocols. Strict Unix flavors of TCP/IP may not support the NetBIOS interface and may only support Windows Sockets as their API; Microsoft’s implementation of TCP/IP therefore includes support for both.

The Windows Sockets interface defines an industry standard specification for how windows applications communicate with the TCP/IP protocol. This specification includes definitions for how to use the transport protocols and how to transfer data between two machines, including the establishment of connection-oriented sessions (TCP three-way handshake) and non-connection-oriented datagrams (broadcasts). The Windows Sockets API also defines how to uniquely address packets destined for a particular application on another machine. The concept of a socket (the combination of the TCP/IP address and the port number) is a common example of the relative ease of uniquely identifying a communications path. Because of the ease and standardization of the Windows Sockets specifications, this API is enjoying a tremendous amount of exposure and success, particularly in terms of its use in Internet applications.

Windows Sockets uniquely identifies machines through their IP address, so machine names in the TCP/IP environment are entirely optional. Given that it is tremendously more difficult for users to remember a hundred IP addresses over some form of an alias for these machines, a name space was created to help identify machines on a TCP/IP network. A name space is a hierarchical naming scheme that uniquely identifies machine aliases to IP addresses. This scheme allows two machines to have the same alias as long as they are not in the same domain. This is very useful for people, but entirely unnecessary for applications, since applications can use the IP address. This is why you can use any alias you want to establish a connection to a particular machine. As long as the name resolution method (DNS, hosts file) returns a valid IP address, a communication path can be created. The IP address is what’s most important. With the NetBIOS API, the IP address is only part of the information necessary to establish communication between two machines, and the name of the machine is required.

The NetBIOS API was developed on local area networks and has evolved into a standard interface for applications to use to access networking protocols in the Transport layer for both connection-oriented and non-connection-oriented communications. NetBIOS interfaces have been written for the NetBEUI, NWLink, and TCP/IP protocols so that applications need not worry about which of these protocols is providing the transport services. Because each of these protocols supports the NetBIOS API, all the functionality for establishing sessions and initiating broadcasts is provided. Unlike Windows Sockets, NetBIOS requires not only an IP address to uniquely identify a machine, but a NetBIOS name as well.

Every machine on a network must be uniquely identified with a NetBIOS name. This name is required for establishing a NetBIOS session or sending out a broadcast. When utilizing names through a NetBIOS session, the sending machine must be able to resolve the NetBIOS name to an IP address. Because both an IP address and name are needed, all name resolution methods have to supply the correct IP address before successful communication can occur.

The Microsoft TCP/IP stack supports connection-oriented and non-connection-oriented communications established through either of these popular APIs. Microsoft includes NetBT (NetBIOS over TCP/IP) for applications that would like to utilize the NetBIOS API over a TCP/IP network. This small, seemingly insignificant piece of software is what prevents your machine from having to run two protocols, one for Windows Sockets, and one for NetBIOS. By providing NetBT with Microsoft’s TCP/IP protocol stack, all NetBIOS calls an application may initiate are supported.

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