The tunnel mode looks like the Direct Server Return mode, except that traffic between the load balancer and the server can be routed. The load balancer encapsulates the request in an IP tunnel to the server. The server recovers the client request from the load balancer, process it, and forwards the response directly to the client.
TCP Connection Overview
The load balancer takes client requests and then encapsulates them into an IP tunnel to forward them to the server.
The client traffic between the server and the load balancer is tunneled and can be routed between both of them.
The server will answer directly to the client.
Pros & Cons
backends from multiple data centers can be used
load-balancer network bandwidth is not a bottleneck anymore
total output bandwidth is the sum of each backend bandwidth
requires patched backend to be able to tunnel IP traffic
no layer 7 advanced features are available
When to Use This Architecture?
when the only way to reach backends is routing.
where no intelligence is required
when the output capacity of the load-balancer could be the bottleneck
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