What Is Layer 7 Server Load Balancing?

We saw in Chapter 2, Understanding Layer 2, 3, and 4 Protocols , what information is available at each of the layers defined by the OSI Seven Layer Model, from basic Ethernet and IP addressing information used by traditional Layer 2 and 3 switches, to identification of protocols and applications at Layer 4. Layer 7 server load balancing, along with other Layer 7 applications such as Web cache redirection, raises the stakes once more by examining information being generated by the application itself. This could be anything from the URL the user is requesting, the name of the video the user wants to watch, or even the name being requested in a DNS request. Typical applications of Layer 7 server load balancing, which we will look at in more detail later, are:

• HTTP URL parsing: Using the location and page being requested via HTTP.

• HTTP header inspection: Using other information in the HTTP request, such as language or browser type.

• HTTP cookies: Reading, rewriting, and inserting HTTP cookies for persistence and preferential user services.

• FTP parsing: Translating information in the FTP Control Channel to ensure correct operation when implementing Server Load Balancing.

• DNS parsing: Using the requested domain or host name to decide on the DNS server to send to.

• RTSP stream parsing: Identifying the video or audio stream requested by the user and serving the content from the correct resource.

In general, Layer 7 server load balancing enhances the user experience by making best use of the available resources.