an efficient protocol for communication. One of the first business uses of RPC was by Xerox under the name "Courier" in 1981. The first popular implementation of RPC on Unix was Sun's RPC (now called ONC RPC), used as the basis for Network File System (NFS). In the 1990s, with the popularity of object-oriented programming, an alternative model of remote method invocation (RMI) was widely implemented, such as in Common Object Request Broker Architecture (CORBA, 1991) and Java remote method invocation. RMIs, in turn, fell in popularity with the rise of the internet, particularly in the 2000s. Message passing RPC is a request–response protocol. An RPC is initiated by the client, which sends a request message to a known remote server to execute a specified procedure with supplied parameters. The remote server sends a response to the client, and the application continues its process. While the server is processing the call, the client is blocked (it waits until the server has finished processing before resuming execution), unless the client sends an asynchronous request to the server, such as an XMLHttpRequest. There are many variations and subtleties in various implementations, resulting in a variety of different (incompatible) RPC protocols. An important difference between remote procedure calls and local calls is that remote calls can fail because of unpredictable network problems. Also, callers generally must deal with such failures without knowing whether the remote procedure was actually invoked. Idempotent procedures (those that have no additional effects if called more than once) are easily handled, but enough difficulties remain that code to call remote procedures is often confined to carefully written low-level subsystems. Sequence of events The client calls the client stub. The call is a local procedure call, with parameters pushed on to the stack in the normal way. The client stub packs the parameters into a message and makes a system call to send the message. Packing the parameters is called marshalling. The client's local operating system sends the message from the client machine to the server machine. The local operating system on the server machine passes the incoming packets to the server stub. The server stub unpacks the parameters from the message. Unpacking the parameters is called unmarshalling. Finally, the server stub calls the server procedure. The reply traces the same steps in the reverse direction. Standard contact mechanisms To let different clients access servers, a number of standardized RPC