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For example, you could define network filters that mark all Lotus Notes packets as medium priority and tag them with the low-latency ToS flag. This would provide Notes users with preferential service over less-critical traffic, routing this traffic over specific network segments. Conversely, you could define another set of filters that mark all RealAudio traffic as lower priority and also set the high-bandwidth ToS flag, forcing that traffic to use an alternate--and more appropriate--route.

As long as you own the end-to-end connection between the source and destination systems, you can do whatever you want with these packets. Keep in mind, however, that most ISPs will not treat these packets any differently than unmarked packets. Indeed, if you need a certain ToS from an ISP, you will most likely end up paying for a dedicated PVC (Permanent Virtual Circuit) between your sites, since you won't be able to prioritize packets using these services. In this regard, a private WAN is still your best option.

However, you also could apply filters to incoming Internet data, and at least be able to manage it from that point through the rest of your network, giving you a minimum of bandwidth manageability.

OS and Application Support Issues Apart from the network, there are significant hurdles to getting the precedence and ToS bits into your IP packets. There are two ways to circumvent these problems: You could have the applications write this information into the packets while they are being sent, or you could have network devices write this information using application-specific traffic filters. In either case, you will be dependent on the vendors of your applications, operating systems and infrastructure equipment to support these attributes.

Surprisingly, there is very little support for this undertaking in the commercial market. Only a few operating systems, for example, have mechanisms in their IP stacks for writing the precedence and ToS bits into a packet. The WINSOCK.DLL that comes with Microsoft Corp.'s Windows95 and Windows NT does not allow this functionality at all, returning "invalid operation" errors when the "setsockopt(IP_TOS)" function is called. Other OSes, including Irix, HP-UX and Solaris, are only slightly better, providing some support within the OS.

In fact, of all the operating systems we've used, the only two that offer high levels of support for the Type of Service byte are Linux and Digital Unix. These systems excelled in our tests not only because they supported these functions directly but because they incorporated these services into their bundled applications.

Both systems, for example, offer a telnet client and server that set the low-latency ToS flag, whereas none of the others we tested provide this basic functionality. The FTP client and server bundled with Linux and Digital Unix use the low-latency flag for the FTP command channel while using the high-throughput flag for the data channel. This lets an FTP command such as "abort operation" be routed through a fast network, getting it to the server quickly (thus canceling the download faster).

Why don't more applications support ToS? Because, for the most part, the operating systems they run on don't offer the necessary support. Until Microsoft fixes the WINSOCK.

DLL provided with Windows NT, application vendors such as Lotus Development Corp., Netscape Communications Corp. and Oracle Corp. will be unable to implement application-specific prioritization services directly within their applications.

Implement in Infrastructure

There are ways around the vendor-specific shortcomings, however--the most common route is to implement IP prioritization services in the infrastructure rather than in the end-point applications and operating systems. For years, many of the largest and busiest networks have been building manual prioritization controls using per-application filters within their routers.

In this type of model, you can manually define a filter that queues and sends Notes traffic at a higher priority than FTP traffic, for example. While such tools are crude, they get the job done on a per-hop basis, if not on an enterprisewide level. Network managers may also find it's worth exploring some of the new bandwidth-management products, such as CLASS Data's Classifier (recently acquired by Cisco Systems), which uses end-point agents to adjust IP prioritization and ToS services within the network infrastructure.