PC Gen: Novell NetWare and NCP Calls -- General Information (92369)

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The NetWare shell (Netx) is responsible for determining if an application's file request is for the operating system (MS-DOS) or for the network. If a request is for a network file service, the shell intercepts it and prepares it for transmission to the appropriate server. Netx works with IPX, which is NetWare's communication protocol. IPX is bound to the workstation network interface card (NIC) driver that allows the request to be sent out over the topology to the server.

The shell and NetWare communicate using a language called NetWare Core Protocol (NCP). NCP is used for all client requests for service and for the responses to those requests. NCP is the single most important application layer protocol that NetWare provides. Without NCP, none of the file services offered by NetWare would be possible.

The definition of NCP is not available to the public. It is proprietary information.

NCP manages communications for these services:

• Opening files under different modes
• Closing open files
• Reading data blocks from open files
• Writing data blocks to open files

In addition, NCP provides high-level connection services and synchronization operations.

The shell accomplishes its redirection tasks by hooking several MS-DOS interrupts. Once loaded, the shell monitors any calls that applications make to Interrupt 21h and redirects those that require access to a server or the network.

To determine if a request is for a local drive or a network drive, the shell maintains a set of tables that keep track of network drives and mappings. The shell compares the network drive table to the internal operating system drive table (A-E, or whatever the LASTDRIVE= setting is). If the request is for a local drive, the shell passes it to the operating system. If the request is for a network drive, the shell passes it to the network.

When Microsoft Mail makes a file open request for a file located on a NetWare server, Mail issues an Interrupt 21h. The shell intercepts the request and finds that the file resides on the server or on a network drive (for example, drive M). Then the shell creates an IPX packet with the NCP information. The shell hands this off to the LAN driver, and the LAN driver passes it to the network. The response from the server is the same, except in reverse order. The shell reads the NCP response, and returns the Interrupt 21h request.