Common Internet File System

The Common Internet File System (CIFS) is the standard way that computer users share files across corporate intranets and the Internet. An enhanced version of the Microsoft open, cross-platform Server Message Block (SMB) protocol, CIFS is a native file-sharing protocol in Windows 2000.

CIFS defines a series of commands used to pass information between networked computers. The redirector packages requests meant for remote computers in a CIFS structure. CIFS can be sent over a network to remote devices. The redirector also uses CIFS to make requests to the protocol stack of the local computer. The CIFS messages can be broadly classified as follows:

• Connection establishment messages consist of commands that start and end a redirector connection to a shared resource at the server.
• Namespace and File Manipulation messages are used by the redirector to gain access to files at the server and to read and write them.
• Printer messages are used by the redirector to send data to a print queue at a server and to get status information about the print queue.
• Miscellaneous messages are used by the redirector to write to mailslots and named pipes.

Some of the platforms that CIFS supports are:

• Microsoft Windows 2000, Microsoft® Windows NT®, Microsoft® Windows® 98, Microsoft® Windows® 95
• Microsoft® OS/2 LAN Manager
• Microsoft® Windows® for Workgroups
• UNIX
• VMS
• Macintosh
• IBM LAN Server
• DEC PATHWORKS
• Microsoft® LAN Manager for UNIX
• 3Com 3+Open
• MS-Net

CIFS complements Hypertext Transfer Protocol (HTTP) while providing more sophisticated file sharing and file transfer than older protocols, such as FTP. CIFS is shown servicing a user request for data from a networked server in Figure B.17.

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Figure B.17 CIFS Architecture

When there is a request to open a shared file, the I/O calls the redirector, which in turn requests the redirector to choose the appropriate transport protocol. For NetBIOS requests, NetBIOS is encapsulated in the IP protocol and transported over the network to appropriate server. The request is passed up to the server, which sends data back to satisfy the request.

Components in the redirector provide support for CIFS, such as:

• Rdbss.sys

  All kernel-level interactions are encapsulated in this driver. This includes all cache managers, memory managers, and requests for remote file systems so the specified protocol can use the requested server.

• Mrxsmb.sys

  This mini-redirector for CIFS has commands specific to CIFS.

• Mrxnfs.sys

  This mini-redirector for the Network File System (NFS) provides support for NFS. Mrxnfs.sys is included in Services for Unix.

In Windows NT 4.0, Windows Internet Name Service (WINS), and Domain Name System (DNS) name resolution was accomplished by using TCP port 134. Extensions to CIFS and NetBT now allow connections directly over TCP/IP with the use of TCP port 445. Both means of resolution are still available in Windows 2000. It is possible to disable either or both of these services in the registry.

Features that CIFS offers are:

Integrity and Concurrency   CIFS allows multiple clients to access and update the same file while preventing conflicts by providing file sharing and file locking. File sharing and file locking is the process of allowing one user to access a file at a time and blocking access to all other users. These sharing and locking mechanisms can be used over the Internet and intranets. They also permit aggressive caching and read-ahead and write-behind without loss of integrity. File caches of buffers must be cleared before the file is usable by other clients. These capabilities ensure that only one copy of a file can be active at a time, preventing data corruption.

Optimization for Slow Links   The CIFS protocol has been tuned to run well over slow-speed dial-up lines. The effect is improved performance for users who access the Internet using a modem.

Security   CIFS servers support both anonymous transfers and secure, authenticated access to named files. File and directory security policies are easy to administer.

Performance and Scalability   CIFS servers are highly integrated with the operating system, and are tuned for maximum system performance.

Unicode File Names   File names can be in any character set, not just character sets designed for English or Western European languages.

Global File Names   Users do not have to mount remote file systems, but can refer to them directly with globally significant names (names that can be located anywhere on the Internet), instead of ones that have only local significance (on a local computer or LAN). Distributed File Systems (DFS) allows users to construct an enterprise-wide namespace. Uniform Naming Convention (UNC) file names are supported so a drive letter does not need to be created before remote files can be accessed.

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