Windows® Internals, Sixth Edition, Part 1

MailSlot (part of MRxSMB.SYS). Mailslots are handled very differently from named pipes. The surrogates are not called for I/Os sent to a mailslot, and prefix caching is not used. (All paths having “mailslot” as the share name are targeted directly at the mailslot mini-redirector.) There can be, at most, one mailslot mini-redirector, and it is currently reserved for the SMB redirector.

Network File System (NFS) is an optional component that was formerly installed with Services For Unix (SFU) and is now an optional Windows component (available on all Server editions, but only Enterprise and Ultimate editions of Windows client) that can be installed using the Programs and Features control panel. (Click Turn Windows Features On Or Off, and then select Services For NFS.) NFS protocol versions 2 and 3 are supported.

Offline Files, covered in a following section, optionally enables disk caching and offline access to files accessed through the SMB protocol. Offline Files also registers as a MUP surrogate provider.

Server Message Block and Sub-Redirectors

The Server Message Block (SMB) protocol is the primary remote file-access protocol used by Windows clients and servers, and dates back to the 1980s. SMB version 1.0 (generally referred to as just SMB) was designed to operate in a friendly LAN environment, where speeds were typically 10 Mb/s and no one was trying to steal your data. To accomplish many common tasks required a series of synchronous messages between the client and the server. Little thought was given to WANs, because WANs were scarce at the time. In 1996, SMB was submitted to the IETF as the Common Internet File System (CIFS). Microsoft documents the CIFS/SMB protocol in the MS-CIFS and MS-SMB protocol documents.

The SMB 2.0 protocol was released in Windows Vista and Windows Server 2008, and it was a complete redesign of the main remote file protocol for Windows. SMB 2.0 provides a number of improvements over SMB, such as the following:

Greatly reduced complexity. The number of opcodes was reduced from over 100 to just 19.

Reduced the chattiness of the protocol to make it more suitable for running across WANs, which generally have much longer latencies and lower bandwidth than LANs.

Compound requests allow multiple requests to be sent in a single network packet.

Pipelining requests allow multiple requests and data to be sent before the answer to a previous request is received (also known as credit-based flow control).

Larger reads and writes.

Caching of folder and file properties.

Improved message-signing algorithm (HMAC SHA-256 replaced MD5).

Improved scalability of file sharing.

Works well with Network Address Translation (NAT).

Support for symbolic links.

Version 2.1 of the SMB protocol (released with Windows 7 and Windows Server 2008/R2) is a minor release (documented in the MS-SMB2 protocol specification). It adds the following improvements:

A new opportunistic lock (oplock) leasing model, which allows greater file and handle caching opportunities—without requiring changes to existing applications

Support for even larger transmission units (large MTU), from a previous maximum of 64 KB to 1 MB (by default, but configurable up to 8 MB via the registry).

To maintain backward compatibility with SMB servers, an SMB2 client uses the existing SMB connection setup mechanisms, and then advertises that it supports a higher version of the protocol. The SMB mini-redirector contains all the functionality that is common between the different versions of the protocol, with a separate sub-redirector implementing each variant of the SMB protocol. An SMB2 client establishes a connection and sends an SMB negotiate request that contains both the supported SMB and SMB2 dialects. If the server supports SMB2, it responds with an SMB2 negotiate response, and the client hands the connection to the SMB2 sub-redirector. At that point, all messages on the connection are SMB2. If the server does not support SMB2, it responds with an SMB negotiate response, and the client hands the connection to the SMB1 sub-redirector:

The common portions are implemented by %SystemRoot%\System32\Drivers\MRxSMB.sys.

The SMB 1 protocol is implemented by %SystemRoot%\System32\Drivers\MRxSMB10.sys.

The SMB 2 protocol is implemented by %SystemRoot%\System32\Drivers\MRxSMB20.sys.

Distributed File System Namespace