Layer 4 switching worth the hype?

Make no mistake; Layer 4 switching is real.

Just ask the folks at Yahoo, Digex, Concentric Network, WebTV or Disney Online. They’ve all deployed Layer 4 switching technology to optimize their Web data centre infrastructures, minimize application response times and improve overall traffic control.

Layer 4 switching is about managing and switching application sessions, not just individual packets. For years, the ability to prioritize bandwidth by application session or TCP flow has been dubbed a Layer 4 service, but it’s not Layer 4 switching. Layer 4 switches can identify and process TCP/IP sessions at wire speed, thus providing new services.

For example, using a single virtual IP address, Layer 4 switches can load balance user requests to a virtual server made up of multiple physical servers that support the same applications and content. A Layer 4 switch intercepts new TCP requests to the virtual server, determines the best “real” server to handle each request and forwards packets associated with each TCP connection to each chosen server.

In the process, the Layer 4 switch must substitute the virtual IP address in the destination address field of each packet for the actual IP address of the chosen servers while monitoring the state of the TCP connection. This move ensures that all packets within each TCP connection are forwarded to the same real server in the proper packet sequence.

Layer 4 switching also enables application redirection. Layer 4 switching technology can be used to transparently intercept application traffic, such as HTTP, File Transfer Protocol and Domain Name System (DNS), and redirect the traffic to specific servers regardless of the destination IP address. Layer 4 switching technology can be expanded to redirect or load balance user HTTP requests based on information within the session content, allowing different object types to be physically stored on servers optimized for them.

So can Layer 2 and 3 switches simply add Layer 4 switching intelligence without fundamental architecture changes? If history is a good indicator, the answer is a resounding “no.” Bridges added IP routing support, but no good multiprotocol router today ever resulted from architectures designed for bridges. Layer 2 and 3 switches that cram one or two Layer 4 services into their centralized management processors will ultimately collapse under the pressures of exploding Internet and intranet traffic.

There’s no doubt that Layer 4 switching technology has become a critical data centre component of Web hosts, content providers, ISPs and progressive Web-oriented enterprises. From now on, switches will never be the same.