Windows Vista: WAN Optimization Panacea or Pretender?

27.03.07

Microsoft’s new Windows Vista operating system is generating a lot of excitement in the business world. That’s because, in addition to the usual readability, stack and kernel updates, the new software, which replaces XP, features two changes that are of particular interest to business users: a new TCP/IP stack and a new Common Internet File System (CIFS) implementation.

The reason these two modifications are capturing the lion’s share of attention is that they both play a critical role in the performance of file sharing over networks, particularly wide-area networks (WANs). According to Nemertes Research, the number of branch offices is increasing 10 percent year-over-year. As enterprises continue to grow the number of remote employees, WAN performance is becoming a real hot-button issue for businesses whose productivity is compromised by the latency and sluggishness that are inherent in the wide-area environment.

For these users, Vista is expected to be a godsend: a new Windows operating system promising to address these long-distance performance issues and restore LAN-like performance for remote users accessing centralized Microsoft applications over the WAN. In fact, some even believe that Vista will do away with the need for application acceleration and WAN optimization solutions, which are widely deployed to overcome the limitations posed by the distributed enterprise.

The truth, however, is quite different. While Vista is a tremendous evolutionary step, and the focus on wide-area performance issues is appreciated, the fact remains that despite Vista — or even because of it — application acceleration and WAN optimization solutions are more important than ever.

To understand why, let’s explore the TCP/IP and CIFS changes that Vista will introduce.

Bigger Sweet Spot
For the new TCP/IP stack, Vista changes the congestion control algorithms to improve performance and allow more data to be sent at higher speeds.

The TCP/IP protocol slow-start process begins by sending a small fixed amount of data, slowly increasing the data window size and accelerating the delivery speed until congestion occurs and packets begin to drop. At that point, the protocol drops back to the original window speed and the process begins again. While this method is very efficient and reliable in a LAN, it presents problems in WAN environments, where the data must traverse long, latency-prone fixed bandwidth links.

Vista introduces a new algorithm called Compound TCP, which moderates the TCP flow control mechanism and increases the “sweet spot” in which TCP/IP can operate. The net result is that TCP performance is improved over higher-latency and lower-bandwidth links like those one would encounter in a WAN environment. Not only does Compound TCP improve data flow over long-distance links, it also reduces jitter, which is critical for delay-sensitive applications such as Voice over IP (VoIP).

CIFS Enhancements
The CIFS enhancements included with Windows Vista offer a much cleaner implementation of the protocol, which supports file and print sharing between devices.

CIFS, based on the Microsoft Server Message Block (SMB) protocol, works by breaking files down into small blocks of data, each of which is transmitted serially from the sender to the requester. The sender requires acknowledgement that the previous block has been received before it will send the next chunk of data, resulting in several hundred of even thousands of time-consuming round trips to transmit a single file. While this delivery technique works well in a LAN, it — even more so than TCP/IP — is highly inefficient in a high-latency, low-bandwidth WAN environment where each send and acknowledgment must traverse a long, fixed-capacity link.

The Windows Vista CIFS implementation, based on SMB version 2 (SMB2), adds functionality that streamlines the delivery of large files over the WAN by allowing two or more data blocks to be sent simultaneously over the WAN.

Desktop WAN Optimization
It’s easy to see how, based on the TCP/IP and CIFS improvements, some people might be led to believe that Windows Vista could potentially replace application acceleration and WAN optimization solutions. After all, it does address the obstacles to effective application performance over the WAN — namely latency and inefficient protocols.

However, a deeper understanding of the Vista operating system, and the TCP/IP and CIFS implementations in particular, lead to an entirely different conclusion.

First, the new TCP/IP stack, as mentioned earlier, is an incremental improvement that addresses a slightly larger portion of the overall market. However, it’s important to understand that Windows Vista is optimized for high-bandwidth, low-latency environments — in other words, LANs and high-speed metropolitan area networks. Vista has a negligible impact on high-latency environments, such as transcontinental and international links, satellite links, or low-bandwidth WANs, which make up the vast majority of global network deployments. These environments still need WAN optimization.

Second, as noted, the Windows Vista CIFS implementation is based on SMB2. In order to derive the greatest benefit from this new, ground-up implementation, Microsoft decided it could not support backward-compatibility between SMB2 and SMB1. Therefore, to ensure compatibility between all operating systems, Microsoft decided to equip all Windows Vista deployments with both SMB1 and SMB2. Windows Vista machines can only speak SMB2 to other Windows Vista machines. Any communication with a pre-Vista operating system reverts to the original CIFS (SMB1) implementation, which means no improvement at all.

The adoption rate of Windows Vista will ultimately dictate how rapidly SMB2 functionality will be absorbed into the workplace. Since this adoption will take some time, the original SMB1-based CIFS implementation will still be widely used — which means, despite the presence of Windows Vista, performance will largely be unchanged from what we see today.

Finally — and perhaps most important — today’s distributed enterprise supports a number of different types of applications, not just Microsoft file services. Oracle, PeopleSoft, SAP, even Outlook and Exchange — these and many other business-critical tools run over the WAN, and the new Windows Vista TCP/IP and CIFS implementations will have little, if any, impact on the performance of these applications over the WAN. They still need to be accelerated and optimized, and that requires solutions designed to provide such a service.

A Step In the Right Direction
Despite publicity to the contrary, Windows Vista is not the WAN optimization panacea some say it is.

That’s not to say it doesn’t offer significant and appreciable improvements. It does. For users in low-latency, high-bandwidth environments, Windows Vista will improve the poor file transfer performance that has ailed them before.

But for those still toiling away over high-latency, low-bandwidth WAN links, while the improvements offered by Windows Vista are certainly appreciated, they are far from the ultimate solution users need. Working in tandem with existing application acceleration and WAN optimization solutions, Windows Vista will dramatically improve the work environment. But the collaborative approach is a requirement, since the operating system alone can’t possibly deliver a complete solution.