CLUSTER parameter* == CLUSTER=[protoList]:ServerList ServerList == [/R,]Server[,ServerList] Server == Host[..Port]
If a list is prefixed with "/R", the servers in the list are tried in random order (the first server to be tried is selected randomly and other servers are tried in the round-robin order). It could be useful for load balancing among equivalent (proxy) servers.
The retrial by this parameter is commonly applied to servers of any protocols in the phase of establishment of a TCP connection to a server. The retrial covers the authentication phase for several protocols. In HTTP origin/gateway servers, the retrial may be caused depending on the response from the server, including the response code 503 (Service Unavailable) and 404 (Not Found) for example.