Use DSR to Take a Load Off Your Load Balancer
Direct server return (DSR) is a load balancing scheme that allows service requests to come in via the load balancer virtual IP (VIP). The responses are communicated by the back-end servers directly to the client. The load is taken off the load balancer as the return traffic is sent directly to the client from the back-end server, bypassing it entirely. You may want to do this if you have larger files to be served or traffic that doesn’t need to be transformed at all on its way back to the client.
Here’s how it works: Incoming requests are assigned a VIP address on the load balancer itself. Then the load balancer passes the request to the appropriate server while only modifying the destination MAC address to one of the back-end servers.
You need to be aware of the following when using DSR:
- Address resolution protocol (ARP) requests for the VIP must be ignored by the back-end servers if the load balancer and back-end servers are on the same subnet. If not, the VIP traffic routing will be bypassed as the back-end server establishes a direct connection with the client.
- The servers handling the DSR requests must respond to heartbeat requests with their own IP and must respond to requests for content with the load balancer VIP.
- Application acceleration is not a possibility because the load balancer does not handle the responses from the backend servers.