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 TEST CENTER Internet pathfinder By Logan G. Harbaugh April 26, 2002 FOR MANY CORPORATIONS, the public Web site has become critical to doing business. That's why many large companies are multihomed, using two or more Internet connections, so if one ISP goes down the other will hopefully continue to provide service. Multihoming brings its own issues, however, including how to ensure that Web site traffic is sent through the ISP that will be the most efficient way to reach the recipient.
PathControl is a specialized route control device that monitors actual delays from Web server to client and sends traffic over the appropriate link. By optimizing the use of multiple ISPs, PathControl can lead to significant savings in bandwidth and related costs, but it will be cost-effective only for Web sites with very heavy traffic. The typical usage scenario involves two or more DS-3 (45 Mbps) or higher speed connections and routers that support BGP (Border Gateway Protocol). The process PathControl uses to measure latency and network performance is relatively simple: a reference URL is placed on each Web page on the site, and when a user accesses that page, the URL causes a one pixel by one pixel GIF to be transferred from PathControl to the client. PathControl uses that transfer to measure the round-trip time to the client. Since the reference can easily be placed on each page, and uses very little bandwidth, network performance can be continuously monitored for every client attached to a site, and each client can be switched from one connection to another as network dynamics change. PathControl has a data-collection mode, which simply monitors usage and identifies the ideal route for each client connection and whether that route is the one that BGP has selected. In the active mode, PathControl will adjust the BGP routing tables for the routers to the ISPs to achieve optimum results. The data collected is fascinating in its own right, and might be enough to justify the purchase for some companies that are trying to find out if their service-level agreements are being met. During our testing, we were able to observe the statistics collected by a PathControl unit managing five separate ISP links in the San Francisco Bay Area. Because PathControl shows the average latency for each client connection over time, for each ISP, we were able to discover that each of the five ISPs was at times the best way to reach the client, and at other times, the worst. Also interesting was that the average latency often varied by 300ms (milliseconds) or so, and sometimes by a good deal more than that. Monitoring PathControl in use at the ISP showed variations between the best and worst connection of as much as 4 seconds. Considering that this was the round-trip latency for a one-bit file, and that each Web page actually consists of perhaps dozens of much larger files, cutting the latency for each transfer by 300ms would obviously represent a significant performance increase. And in fact, PathControl's reporting feature showed that 38 percent of the clients were better off being switched to another ISP connection, resulting in a 70 percent improvement in performance when using the PathControl with five ISPs. When PathControl is installed, the normal procedure is for a support engineer to perform a site survey, develop a network plan, and perform some on-site training before installing the device. On-site installation support is included in the price. Each installation is optimized for the purchaser, including specialized support to work with load balancers, firewalls, streaming video, VoIP (voice over IP), or other requirements as necessary. According to support engineer, Sean Finn, the standard one-bit GIF that the PathControl uses to measure latency works well for predicting the optimum route for all types of traffic, including video, audio, and other types of streaming media. This is due to the fact that problems with the Internet and the available routes do not generally affect only one type of traffic such as video, but all types, including the relatively simple packet round-trip that PathControl uses. Thus an increase in average latency or packet loss that would cause problems with video will also be detected by the GIF. When a Web site is used for streaming or file transfers, RouteScience recommends that the Web designer create a minimized window that refreshes regularly and displays the GIF. This is because the single measurement that is taken as each page is opened is fine under normal circumstances, but when opening that page causes a transfer that lasts for minutes or even hours, it is better to ensure that the PathControl continues to monitor the client connection and switch between ISPs if necessary. Spending $100,000 or more to provide better traffic flow might seem excessive, but PathControl has other benefits. It can weight ISPs depending on the cost for each connection, and with the optimization it provides, it might enable a company to switch from two tier-one ISPs to three or four less expensive ones, achieving lower costs and better response for customers at the same time. IT consultant Logan Harbaugh (logan@lharba.com) is the author of two books on networking.
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