Windows® Internals, Sixth Edition, Part 1

Some redirectors require one or more of the following optional components:

A service process to assist the DLL and possibly store sensitive information or information that is global across client applications using a particular network or share. For example, the Workstation service (running in an SVCHOST process) keeps track of drive-letter to \\server\share mappings.

A network protocol driver that implements the legacy Transport Driver Interface (TDI) on its upper edge is required if the redirector uses a network protocol not supplied by Windows. (In essence, this means anything other than TCP/IP.) Such a protocol driver is responsible for implementing communications with the remote system.

A service process to assist the redirector. For example, the WebDav redirector forwards file-access operations to the WebClient user-mode service, which in turn issues the actual WebDav network protocol requests using HTTP APIs.

A redirector presents resources that are attached to remote systems as if they were attached to the local system. In Windows, there are no special file I/O APIs required to access resources on a remote system. When accessing a resource, an application generally does not know—nor does it care—whether the resource is located on the local system or on a remote system. The name “redirector” is used because it redirects file system operations to the remote system and returns to the application the responses from the remote system.

All redirectors that ship with Windows are implemented using the mini-redirector architecture, where protocol-specific code is implemented in a mini-redirector driver that imports the RDBSS library. RDBSS is implemented like a class driver, and the mini-redirectors are akin to port drivers. RDBSS registers with MUP by calling FsRtlRegisterUncProviderEx.

When a mini-redirector registers with RDBSS via RxRegisterMiniRdr, RDBSS in turn registers with MUP by calling FsRtlRegisterUncProviderEx. MUP routes requests (IRPs) to RDBSS, which performs processing that is common to all remote file systems, and then issues simplified requests via callback routines that mini-redirectors linked against it have registered. RDBSS provides common functionality such as a data structure and locking model, Cache Manager and Memory Manager integration, and handling of IRPs. This simplifies the implementation of the mini-redirectors, and it vastly reduces the amount of code that needs to be written and debugged.

Because RDBSS integrates with Cache Manager, RDBSS mini-redirectors might not directly see read and write requests on buffered handles (handles opened without specifying the FILE_FLAG_NO_BUFFERING flag to the CreateFile API); changes are cached by the cache manager on the local system until they need to be written back to the remote system. This improves response time, and it saves network bandwidth by aggregating writes and eliminating duplicate reads. RDBSS relies on the mini-redirector to tell it when it is safe to cache data for read and/or write. For example, the SMB mini-redirector uses opportunistic locks (more commonly known as oplocks, which are discussed in Chapter 12 in Part 2) to manage caching. An oplock is a cache coherency mechanism that allows file-system consumers to dynamically alter their caching state for a given file or stream (see Chapter 12 in Part 2 for more information about file system streams), while maintaining cache coherency between multiple concurrent users of a file. If the file (or stream) is not currently opened for read or write by another accessor (either locally or remotely), a client can locally cache reads, writes, and byte range locks. If the file is open by others but is not being written, writes and locks will not be locally cached, but reads can still be cached.

Mini-Redirectors

A mini-redirector implements a protocol necessary to contact a remote system and access its shared resources. The mini-redirector tries to make access to remote resources as transparent as possible to the local client application. For example, if there are network problems, a redirector might retry a request multiple times before it returns an error to the client application.

There are several mini-redirectors included with Windows:

RDPDR (Remote Desktop Protocol Device Redirection), which allows access from a Terminal Server system to the client system’s files and printers (%SystemRoot%\System32\Drivers\rdpdr.sys)

SMB (Server Message Block), which is the standard remote file system used by Windows (also known as CIFS, or Common Internet File System) (%SystemRoot%\System32\Drivers\MRxSMB.SYS). MRxSMB.SYS will load sub-redirectors, which are covered in the next section.

WebDAV (Web Differencing and Versioning), which enables access to files over the HTTP(S) protocol (%SystemRoot%\System32\Drivers\MRxDAV.SYS).