Redirector

A redirector is a component exposed by a network provider to an operating system that accepts and processes remote I/O service requests. It does this by formulating service request messages and sending them to a remote computer's redirector server service. The remote computer's redirector server service receives the request and services it by making local I/O requests. Because a redirector provides I/O services to higher-level services such as MUP, a redirector hides the details of the network layer from applications so that applications don't have to supply protocol-specific parameters to a redirector. Thus, a network provider is protocol-independent: applications can operate in almost any network configuration.

The MSNP provides a redirector that works directly with the networking transport layer and NetBIOS to form communication between a client and a server. The NetBIOS API discussed in Chapter 1 provides a programming interface with these same transports. This redirector provided by MSNP is often referred to as the LAN manager redirector because it is designed around the old Microsoft LAN manager software that provided network operating system capability to MS-DOS applications in the past. (For more detailed information about the NetBIOS programming interface, see Chapter 1.) The NetBIOS interface is capable of communicating over numerous network protocols. This makes the MSNP redirector protocol-independent: your application does not have to concern itself with the specific details of a network protocol. When your application uses the MSNP redirector, it can communicate over TCP/IP, NetBEUI, or even IPX/SPX. This is a rather nice feature because it allows applications to communicate no matter what the physical network comprises. However, one important detail needs to be considered. For two applications to communicate with each other over the network, the two workstations must have at least one transport protocol in common. For example, if workstation A has only TCP/IP installed and workstation B has only IPX installed, the MSNP redirector will not be able to establish communication between the two workstations over a network.

The MSNP redirector communicates with other workstations by sending messages to a remote workstation's redirector server service. These messages are set up in a well-defined structure known as SMB. The actual protocol for how the redirector sends and receives messages to a remote workstation is known as the Server Message Block File Sharing Protocol, or simply the SMB protocol.