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.
Route optimization is generally handled by the Border Gateway Protocol, which does a slightly better job than random chance: if there are two links, the chance of getting the most efficient link randomly is 50 percent, and BGP will usually get the correct choice about 60 percent of the time. If you'd like to do better than 60 percent, then PathControl from RouteScience is the way to go -- but only for the largest Web sites.
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.