By changing the destination MAC address of a forwarded packet, the balancer can leave all the information in upper layers intact, so the backend receives the original source and destination IP addresses. The backend can then send a reply directly to the original sender — a technique known as Direct Server Response (DSR). If user requests are small and replies are large (e.g., most HTTP requests), DSR provides tremendous savings, because only a small fraction of traffic need traverse the load balancer.
