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Addressing VDI challenges with a WAN optimizer

More and more companies are implementing virtual desktop infrastructure (VDI) environments: Forty-seven percent of 427 respondents to Storage magazine’s fall storage Purchasing Intentions

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survey, for example, indicated their organizations have virtual desktops, a 4% increase over the prior year’s survey. For IT administrators to provide acceptable performance for users in remote locations, VDI-specific WAN optimization techniques and technologies are usually needed.

Before you look to products to handle WAN optimization, there’s some work you’ll need to do on your own. The first step is to create a well-audited build of the virtual desktop guest operating system. The goal is to strip out some of the unnecessary options within the operating system -- such as “smooth scrolling” and background logos -- to reduce the amount of bandwidth required for screen updates. If your installation of Windows is unmodified after installation, that default build will have lots of graphical enhancements that cause bandwidth-hogging screen updates.

While these tweaks to the virtual desktop operating system can and do deliver benefits, when it comes to optimizing for VDI over WAN, much depends on the VDI protocol that’s in use, which in turn generally correlates with the VDI platform being used. There are three dominant VDI protocols available that are used across the WAN – VMware Inc.’s PC over IP (PCoIP), for use with VMware View; Citrix Systems Inc.’s High Definition User Experience (HDX), formerly branded as Independent Computing Architecture, or ICA, for use with Citrix XenApp; and Microsoft’s Remote Desktop Protocol (RDP), which can be used with VMware View, Citrix XenApp or Microsoft’s Virtual Desktop Infrastructure.

Anyone seriously considering a VDI project would likely regard Microsoft’s RDP as a “legacy” protocol that can’t effectively deliver a rich, graphically dense environment to the user over the wire. But because of Microsoft RDP’s largely unoptimized status, it has the most to gain from a third-party WAN optimizer. For example, many third-party WAN optimization vendors can deliver a performance improvement of up to 12 times on standard Microsoft RDP. That is to say, they can put 12 times the number of RDP sessions on the same link with the same round-trip latency. In the main, the other protocols -- PCoIP and HDX -- do not benefit as much as Microsoft RDP. To some degree, WAN accelerators do level the VDI latency playing field for Microsoft but they do so at an additional cost to the customer. Make no mistake about it: Microsoft RDP is a legacy protocol when compared with the new kids on the block.

In contrast, the new generation of VDI protocols from VMware and Citrix have already been heavily optimized. Although both VMware and Citrix would like customers to believe that there is some “battle of the protocols,” it is actually a phony war. There really isn’t much separating the two protocols from a performance perspective -- and the vendors risk coming across as two politicians from the same party trying to convince the voters that they are substantially different from each other. Putting that to one side, if these protocols are already so good across the WAN, is there any optimization left to do? The answer is a decided yes, but there’s a smaller return on investment than in the past, and you will struggle to find all the features being supported by single device. 

WAN optimization for a VDI environment is restricted to a narrow set of companies that specialize in the field, primarily F5 Networks Inc., Riverbed Technology, Citrix (with its NetScaler) and Silver Peak Systems Inc. This is because to carry out the WAN improvements, they need access to the encryption keys that protect the data stream encapsulated in the protocol (PCoIP, HDX) from the user to the virtual desktop. Without these encryption keys, it’s not possible for any old WAN optimizer vendor to create a VDI-specific product because without the encryption keys they can’t get to the protocol data they require. That restricts the number of WAN optimization vendors to a small group of companies that have developed partnerships with the new generation of VDI vendors and gained access to their proprietary protocols. Let’s talk about how these products deliver bandwidth savings.

No matter which WAN optimization vendor you choose, pay close attention to their feature support. Not all vendors support all the protocols equally well. For example, Citrix NetScaler is a popular WAN optimization tool used with the Citrix HDX protocol, but it doesn’t support multistreaming (in which multiple TCP sessions are used to carry HDX traffic between the client and server), whereas its competitors often do.

The tools that handle WAN optimization for VDI optimize the traffic in a number of ways. Let’s go over each of those techniques and how they work.  

For starters, a WAN optimizer can deliver a small gain if it’s able to compress and cache some of the contents of the protocol data. However, as this protocol data already is compressed by the VDI platform, this additional compression load wouldn’t add a great deal of benefit.

In addition to compression and caching, many third-party vendors’ WAN optimization products, such as F5’s Big-IP, have a feature referred to as per-session prioritization; with access to the encryption keys, the WAN optimization technology can work out which stream of desktop data belongs to the CEO and which belongs to Joey the Copy Guy. With that information, IT administrators could give a higher priority to the CEO’s data and a better user experience to the CEO. But not all third-party WAN optimization products support this functionality, and for those that do support it, it’s not available across every protocol since the WAN optimization vendor may be limited by the capabilities of the VDI protocol.

There’s also a feature – called multisession deduplication -- in some third-party vendors’ WAN optimization products, such as F5’s Big-IP, that can cut down on the amount of data being transmitted. Very often, the same blocks of graphics, such as the Microsoft Office ribbon bar, are transmitted for multiple users. This data could be transmitted just once and cached at the branch office WAN optimization device. However, many of the VDI clients have their own client-side cache for this very purpose -- in the main, it’s more efficient for the caching to happen at the WAN optimization layer rather than for each client to have to maintain its own caching data. Everyone benefits from a shared cache, whereas a client cache benefits only one user.

In addition to those traffic reduction techniques, WAN optimization tools also offer improvements in user experience via features such as session reliability and single-sign-on support. With these two features, if a user is disconnected from virtual desktop, he is automatically reconnected and logged in again once the network pipe becomes available.

Mike Laverick is a VMware forum moderator and member of the London VMware User Group. He is also the man behind the virtualization website and blog RTFM Education, where he publishes free guides and utilities for VMware customers.

This was first published in April 2012

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