Windows® Internals, Sixth Edition, Part 1

BranchCache is similar to Offline Files, but it differs in several important ways. The most important of which is that content in the BranchCache is not available if the WAN is down. This is because the content is identified by a hash list generated and stored on the server, which the client uses to locate the cached content within the BranchCache (distributed or hosted). Some BranchCache features the following behaviors:

Data transfer uses AES encryption.

For content that is not file-based, BranchCache caches only content that is larger than 64 KB. (This can be changed by editing the registry value HKLM\System\CurrentControlSet\Services\PeerDistKM\Parameters\MinContentLength on the server.)

Caching Modes

BranchCache maintains two different local caches on each BranchCache-enabled system (which can be BranchCache content servers on one side of the WAN link, and BranchCache clients and BranchCache hosted cache servers on other side):

The publication cache stores content information metadata for content published using the BranchCache Server APIs (PeerDistServerXxx). The content information structure contains hashes of the various segments and blocks in which BranchCache breaks up the content into chunks, along with the secret needed to generate public and private content identifiers and the encryption key.

Publishing is usually thought of as a server-side operation, though any BranchCache client can publish content. With regard to publishing, BranchCache offers two different approaches to its client applications/protocols for generating/managing/storing BranchCache content information metadata:

An application and/or protocol that uses BranchCache acceleration can ask BranchCache to store content information metadata on its behalf (in the BranchCache publication cache), allowing BranchCache to manage the lifetime of that metadata according to rules, timelines, and limits shared across multiple applications using BranchCache. This is achieved by publishing via the PeerDistServerXxx APIs, and it is what the HTTP-BranchCache and BITS-BranchCache integrations do.

Alternatively, an application/protocol that wants to use BranchCache acceleration can ask BranchCache to generate only content information metadata without storing it, and instead simply return the metadata to the application or protocols. In this case, the application or protocol has to implement its own way to store or manage that metadata. This is what the SMB-BranchCache integrations does.

In both cases, the protocol integrated with BranchCache or the application using BranchCache directly is responsible for transporting that content information metadata through the WAN link from the publishing content server to the clients in the remote branches. BranchCache does not have, or control, a data channel crossing the WAN link. The transport of content information metadata is intentionally left to the protocol or application using BranchCache acceleration, so that the metadata can be transported with the same level of security, authentication, and authorization that would have been used for retrieving the actual content when BranchCache is not used. This is consistent with the fact that, from a security standpoint, owning a copy of the BranchCache content information for a given content is equivalent to owning the entire content and therefore being authorized to retrieve a copy of it from other BranchCache entities (clients, hosted cache servers, or third-party implementations).

The publication cache does not store any actual data of the published content; it stores only content information metadata. Publications tend to last for long periods of time, though the actual length of time is determined by the application that publishes the content. By default, the publication cache is constrained to consume no more than one percent of the volume on which it is located, which is specified by %SystemRoot%\ServiceProfiles\NetworkService\AppData\Local\PeerDistPub. The size and location of the publication cache can be changed using NetSh:

netsh branchcache set publicationcache directory=C:\PublicationCacheFolder

netsh branchcache set publicationcachesize size=20 percent=TRUE