Layer 4

From LB Wiki

Layer 4 refers to the 4th layer of the OSI model, representing the transport layer. In modern TCP/IP and load balancing, this essentially means the combination of IP address and TCP/UDP port. This is the opposite style to Layer 7.

One way to look at Layer 4 is that it is "protocol unaware", when you load balance on Layer 4, the load balancer doesn't care if you're running HTTP, SSH, or the something really absurd, like CMAP (Coffee Maker Application Protocol).

The load balancer takes a connection on an IP and a port, and forwards the packets with a little NAT to a server. Nothing more. In many ways, Layer 4 load balancing doesn't do much more than your basic home wireless router. It's just re-writing source and/or destination IP addresses and ports.

Performance Considerations

Generally, load balancers that can handle greater levels of Layer 4, since there's less work involved with Layer 4.

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