By Kelly Jackson Higgins
Switching The Stack
Make room for another layer of switching. Layer 4 is here and the lines between routing and switching are more blurry than ever. The good news is that Layer 4 switching relies on the well-entrenched and familiar TCP and UDP (User Datagram Protocol), not proprietary schemes like those Ipsilon and other vendors have come up with for Layer 3 switching. Layer 4 provides a way for gigabit switches and routing switches to prioritize applications and help servers share the load. Gigabit start-up YAGO (Yet Another Gigabit Organization), Alteon Networks and Torrent Networking are among the Layer 4 generation pushing this feature of making the application traffic visible to the gigabit switch or routing switch. Then you can individually assign class of service and security policy.

Alteon, which does not offer Layer 3 switching and routing, is adding Layer 4 software to its gigabit switches. Once it does, its switches can determine the availability of different applications and different servers running in a farm. An Alteon switch with Layer 4 software can select the server with the most bandwidth during busy times, for instance, says Selina Lo, vice president of product marketing at Alteon. YAGO, meanwhile, will roll out backbone routing switches that do much the same, but in ASICs rather than software, to help prioritize traffic.

Layer 4 switching really isn't new. The Cisco 7500 router has always done Layer 4 "processing," but not at wire speed as these novel devices can do, says P.G. Nenon, director of marketing at YAGO, which plans to ship its new routing switch in the first quarter of 1998. The big difference: The packet lookup that Cisco and other routers did in software slowed the processing; Layer 4 devices work in hardware with high-speed ASIC chips, Nenon says.

You really need all three layers of switching, says Alteon's Lo. Layer 2 eases LAN congestion, Layer 3 smoothes router congestion, and Layer 4 addresses server congestion, she says. Look for Layer 4 switching to work with RSVP (Resource Reservation Protocol), with RSVP as the signaling mechanism and Layer 4 as a way to provide different qualities of service, says Dave Passmore, a principal at Decisys.

And switching can only keep going up, up, up the stack: Lo says these devices eventually will do some sort of upper-layer switching, where the HTTP protocol could look into a URL address, for instance, and go right to the Web server with that page. And the CEO's e-mail messages would get priority over the Webmaster's. But don't be baffled by the layers of terms. Layer 4 switching is mostly a marketing label for Gigabit Ethernet vendors to differentiate themselves in the already crowded market, Decisys' Passmore says.