At the intermediary transport layer, Layer 4 balancing deals with message delivery without respect to message content.
Transmission Control Protocol is the Layer 4 protocol for HTTP communication (TCP).
These load balancers do nothing except forward packet headers to or from the upstream server, with no regard for what is in the packets. By looking at the first few TCP packets, they can make some rudimentary routing judgments.
Layer 7 flexibility and scalability deals with the real substance of each communication at the high-level application layer. When it comes to website traffic, HTTP is the protocol of choice at Layer 7.
For TCP-based traffic, Layer 7 loading balancers distribute net flow significantly more intelligently than Layer 4 network switches.
The network traffic is terminated and the message is read by a Layer 7 scheduler. It's capable of making a load-balancing choice built on the message's stuff (the URL and cookie, for example).
Next, a new TCP link is made with the downstream server (or recycles an existent one using HTTP keep lives) and the request is sent to the server via HTTP POST...
