Virtual Server via Direct Routing This page contains information about working principle of the Direct Routing request dispatching technique and how to use it to contruct server clusters. Direct Routing request dispatching technique This request dispatching approach is similar to the one implemented in IBM's NetDispatcher. The virtual IP address is shared by real servers and the load balancer. The load balancer has an interface configured with the virtual IP address too, which is used to accept request packets, and it directly route the packets to the chosen servers. All the real servers have their non-arp alias interface configured with the virtual IP address or redirect packets destined for the virtual IP address to a local socket, so that the real servers can process the packets locally. The load balancer and the real servers must have one of their interfaces physically linked by a HUB/Switch. The architecture of virtual server via direct routing When a user accesses a virtual service provided by the server cluster, the packet destined for virtual IP address (the IP address for the virtual server) arrives. The load balancer(LinuxDirector) examines the packet's destination address and port. If they are matched for a virtual service, a real server is chosen from the cluster by a scheduling algorithm, and the connection is added into the hash table which records connections. Then, the load balancer directly forwards it to the chosen server. When the incoming packet belongs to this connection and the chosen server can be found in the hash table, the packet will be again directly routed to the server. When the server receives the forwarded packet, the server finds that the packet is for the address on its alias interface or for a local socket, so it processes the request and return the result directly to the user finally. After a connection terminates or timeouts, the connection record will be removed from the hash table. The load balancer simply changes the MAC address of the data frame to that of the chosen server and restransmits it on the LAN. This is the reason that the load balancer and each server must be directly connected to one another by a single uninterrupted segment of a LAN. If you meet some arp problem of the cluster, see the arp problem page for more information.