App-V 5: On Using BranchCache vs. Multi-Range with HTTP Streaming
Like with 4.6, App-V 5 uses HTTP Streaming – however – the functionality varies depending on client configuration. Two important items to be made aware of:
App-V hooks into WinHTTP and shares the Internet Settings (i.e. what Internet Explorer uses.) This makes the App-V client adhere to the underlying settings with some minor exceptions.
App-V defaults to using multi-range requests instead of single range.
Multi-range HTTP was defined in the HTTP 1.1 specification (http://tools.ietf.org/html/draft-ietf-httpbis-p5-range-11#section-5.4) in order to optimize performance and to reduce delays on downloading large amounts of data. Otherwise, the normal operation of single-range HTTP is used. In the case of the App-V client using multi-range, the expectations are for ranges to come back in the same order they were requested in but if an out-of-order range does not come back in the proper order, the App-V Client will revert back to single range. This will most likely happen in Internet-facing scenarios or situations involving streaming from the Cloud (i.e. Azure.) This will also happen if a multi-range streaming request fails with an “invalid response range” return.
Why Single Range? Well BranchCache of Course!
If BranchCache is configured in your network, the App-V client can take advantage of BranchCache be it Distributed Cache Mode (distributed amongst the various client computers running Windows 7 or later) or in Hosted Cache Mode (where the content is hosted on a server running Windows Server 2008 R2 or later.) If you are not familiar with BranchCache, you can read up more on it here: http://technet.microsoft.com/en-us/library/dd637832(v=WS.10).aspx and here: http://technet.microsoft.com/en-us/library/hh831696.aspx. Remember that in order for BranchCache to work, the server hosting the content must be running IIS and have the BranchCache feature installed.
But . . . BranchCache does NOT support multi-range. Because of this fact, when App-V 5 was released initially, the client defaults to single-range if the client machine was set up for BranchCache. There was a configuration item exposed to override this by setting the value “MultirangeEvenIfBranchCacheEnabled” to 1 either through modifying the registry, PowerShell, group policy, or as an installation flag. Remember that turning this on will not allow the App-V client to take advantage of BranchCache.
Enter App-V 5 Service Pack 2
Amongst the many feedback items that were addressed in Service Pack 2 was some of the confusion amongst the configuration of the App-V client with BranchCache. The concern was brought forth that the default to single-range HTTP when BranchCache was detected was too final and led to some confusion revolving performance. Most customers were not leveraging BranchCache for the App-V content in internal networks or using Internet-facing scenarios to stream virtual applications.
So with App-V 5 SP2, the value was changed to SupportBranchCache and it is turned off by default. This means only those customers who wanted to explicitly stream content using BranchCache for HTTP can turn it on, but otherwise, standard installations of the App-V client will take advantage of multi-range for improved performance. This should improve performance for customers who do not use BranchCache. Performance for customers who do use BranchCache will remain the same as long as SupportBranchCache is set to 1.