Server Message Block

Server Message Block

The SMB protocol was originally developed by Microsoft and Intel in the late 1980s to allow remote file systems to be transparently accessed by MS-DOS applications. Today this protocol simply allows a Windows MSNP redirector to communicate with a remote workstation's MSNP server service using an SMB data structure. An SMB data structure contains three basic components: a command code, command-specific parameters, and user data.

The SMB protocol is centered on a simple client request and server response messaging model. An MSNP redirector client creates an SMB structure with a specific request indicated in the command code field. If the command requires sending data, such as an SMB Write instruction, data accompanies the request. The SMB structure is then sent over a transport protocol such as TCP/IP to a remote workstation's server service. This server service processes the client's request and transmits an SMB response data structure back to the client.

Now that we've covered the basics of the components used in forming communication through the MSNP redirector, let's follow how each component communicates when we try to open \\Myserver\Myshare\Sample.mp3 across a network. It does so by following these steps:

1. An application submits a request to the local operating system to open \\Myserver\Myshare\Sample.mp3 using the CreateFile API function.

2. The local operating system's file system determines that the I/O request is destined for a remote machine named \\Myserver based on the UNC path description, so it passes the request to MUP.

3. MUP determines that this I/O request is destined for the MSNP provider because the MSNP provider finds \\Myserver on the network using NetBIOS name resolution.

4. The I/O request is passed to the MSNP provider's redirector.

5. The redirector formats the I/O request as an SMB message to open the file SAMPLE.MP3 that is contained in the remote \Myshare directory.

6. The formatted SMB message is transmitted over a network transport protocol.

7. The server named \\Myserver receives the SMB request from the network and passes the request to the server's MSNP redirector server service.

8. The server's redirector server service submits a local I/O request to open the SAMPLE.MP3 file that is located on the \Myshare share point.

9. The server's redirector server service formats an SMB response message regarding the success or failure of the local file open I/O request.

10. The server's SMB response message is sent back to the client over a network transport protocol.

11. The MSNP redirector receives the server's SMB response and passes a return code back to the local operating system.

12. The local operating system returns the return code to the application CreateFile API request.

As you can see, the MSNP redirector must go through quite a few steps to grant applications access to remote resources. The MSNP redirector also provides access control to resources on a network as a form of network security.