Major There have been problems with http clients connecting to the RPC servers, which causes the RPC to try and allocate huge buffers and get OutOfMemoryExceptions. I want to have a fixed prefix that is sent first that identifies it as an RPC client. To make the system compatible as much as possible, I'd make the servers accept both forms for a Hadoop release and then change the clients to send the prefix, and finally make the servers only accept the new form.