Windows® Internals, Sixth Edition, Part 1

The republication cache contains both metadata (but no secrets) and actual data (chunked in segments and blocks) for the BranchCache content retrieved by the local BranchCache client. It is stored with the purpose of making those chunks of content available to other BranchCache clients. Republished content is stored for up to 28 days, but it can be flushed out earlier if the republication cache has reached its limit and space is needed for newer content to be republished. By default, the republication cache is constrained to consume no more than five percent of the volume on which it is located, which is by specified by %SystemRoot%\ServiceProfiles\NetworkService\AppData\Local\PeerDistRepub. The location and the size of the republication cache can be changed using NetSh:

netsh branchcache set localcache directory=C:\BranchCache\Localcache

netsh branchcache set localcache size=20 percent=TRUE

BranchCache attempts to persist the republication cache across system reboots through the use of an index file that contains the database of segment descriptors. When the system reboots, BranchCache validates the general integrity of the republication cache by checking that it was properly closed. If the republication cache fails this consistency check, it is discarded. The publication cache is not persisted across reboots. The private SMB-BranchCache publication cache needs no explicit persistence; persistence comes for free, as a result of the SMB-BranchCache integration (which was covered previously) and the fact that with the SMB all published content is actual files. The hashes are validated before access to the files in the cache is allowed.

Configuration

BranchCache can be configured using the Local Security Group Policy editor as shown in Figure 7-28, using the network shell (NetSh) as shown in Figure 7-29, or as part of Group Policy pushed by an administrator (within a domain).

Figure 7-28. Configuring BranchCache using the Group Policy editor

Figure 7-29. Configuring BranchCache using the network shell

BranchCache Implementationservice in %SystemRoot%\PeerDistSvc.dll. This service starts when the BranchCache is enabled on both clients and servers, and it interacts with the kernel-mode components (drivers).

HTTP extension driver in %SystemRoot%\System32\Drivers\PeerDistKM.sys. This driver registers with the Network Module Registrar (NMR) as a client of the http.sys driver and examines all HTTP packets going into and out of the system. It adds files to the cache and retrieves cached content information for published content from the BranchCache service, rather than sending the request to the web server.

BranchCache APIs (PeerDistXxx) are exported by %SystemRoot%\System32\PeerDist.dll, which uses LRPC/ALPC to communicate with the BranchCache service.

The BranchCache HTTP transport in %SystemRoot%\System32\PeerDistHttpTrans.dll implements the transport on top of which the Peer Content Caching and Retrieval: Retrieval Protocol [MS-PCCRR] exchanges data between BranchCache clients and/or hosted cache servers. Each MS-PCCRR message is encapsulated in a simple transport message, which in turn, is sent over an HTTP request.

The Web Services Discovery Provider in %SystemRoot%\System32\PeerDistWSDDiscoProv.dll implements the WS-D protocol to discover which clients on the LAN are caching a particular file (or part of a file).

The BranchCache Network Shell Helper in %SystemRoot%\System32\PeerDistSh.dll is an extension to the Network Shell (%SystemRoot%\System32\Netsh.exe) application that provides users with a means of monitoring and configuring the BranchCache service. Network Shell helper DLLs are installed by adding a string value to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\NetSh, which provides the Network Shell with the path to the helper DLL.

A standalone variant of all the BranchCache APIs are implemented in %SystemRoot%\System32\PeerDistHashPeerDistHash.dll (only present on Windows Server systems), which contains all of the BranchCache APIs and functionality and does not require the use of the BranchCache service. This component is designed for use by other Windows features that are tightly integrated with BranchCache, such as the SMB Groveler, which generates the hashes on the server.

Hash groveler service in %SystemRoot%\System32\smbhash.exe (only on Windows Server systems). The groveler runs on the file or web server and generates hashes when clients request a hash list. The groveler monitors a given namespace or share and ensures that the BranchCache hashes are updated for all files within that namespace. All groveler I/O runs at low I/O priority so as not to interfere with the normal operation of the system.

BranchCache uses the following protocols, which are documented at www.microsoft.com: