Windows Vista WAN Optimization: Panacea or Pretender?
A close-up look at improvements to file sharing in Windows Vista
by Bobby Guhasarkar
Microsoft’s new Windows Vista operating system is generating a lot of excitement in the business world. In addition to the usual usability and stack and kernel updates, the XP replacement 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.
These two modifications are capturing the lion’s share of attention because they both play a critical role in the performance of file sharing, particularly over wide-area networks (WANs). According to Nemertes Research, the number of branch offices is increasing 10 percent annually. As enterprises continue to add remote employees, WAN performance has become a hot-button issue for businesses whose productivity is compromised by the latency and sluggishness inherent in the WAN environment.
Organizations held high hopes for a new Windows operating system that addressed these long-distance performance issues and could restore LAN-like performance for remote users accessing centralized Microsoft applications over the WAN. In fact, some even believed that Vista would 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. To understand why, let’s explore the TCP/IP and CIFS changes that Vista introduced.
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 introduced 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 such as those one would encounter in a WAN environment. Compound TCP improves data flow over long-distance links and reduces jitter, which is critical for delay-sensitive applications such as voice over IP (VoIP).
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 or even thousands of time-consuming round trips to transmit a single file. While this delivery technique works well in a LAN, it 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
Based on the TCP/IP and CIFS improvements, some might be led to believe that Windows Vista can 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 is an incremental improvement that addresses a slightly larger portion of the overall market. However, Windows Vista is optimized for high-bandwidth, low-latency environments—in other words, LANs and high-speed metropolitan area networks (MANs). 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, the Windows Vista CIFS implementation is based on SMB2. 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 most importantly—today’s distributed enterprise supports a number of different types of applications, not just Microsoft file services. Oracle, PeopleSoft, SAP, even Exchange and Notes—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
Windows Vista is not a WAN optimization panacea. It does offer significant and appreciable improvements, and for users in low-latency, high-bandwidth environments, Windows Vista does improve file transfer performance.
However, for those with high-latency, low-bandwidth WAN links, the improvements offered by Windows Vista—while welcome—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.
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Bobby Guhasarkar is a senior manager at Juniper Networks. You can reach the author at email@example.com