The remote procedure call model is similar to the local procedure call model. In the local case, the caller places arguments to a procedure in some well-specified location. It then transfers control to the procedure, and eventually gains back control. At that point, the results of the procedure are extracted from a well-specified location, and the caller continues execution.
The remote procedure call is similar, in that one thread of control logically winds through two processes. One is the caller's process, the other is a server's process. Conceptually, the caller process sends a call message to the server process and waits (blocks) for a reply message. The call message contains the procedure's parameters, among other things. The reply message contains the procedure's results, among other things. Once the reply message is received, the results of the procedure are extracted, and the caller's execution is resumed.
On the server side, a process is dormant awaiting the arrival of a call message. When one arrives, the server process extracts the procedure's parameters, computes the results, sends a reply message, and then awaits the next call message.
Note that in this description, only one of the two processes is active at any given time. However, this need not be the case. The RPC protocol makes no restrictions on the concurrency model implemented. For example, an implementation may choose to have RPC calls be asynchronous, so that the client may do useful work while waiting for the reply from the server. Another possibility is to have the server create a task to process an incoming request, so that the server can be free to receive other requests.
The RPC protocol is independent of transport protocols. That is, RPC does not care how a message is passed from one process to another. The protocol deals only with specification and interpretation of messages.
It is important to point out that RPC does not attempt to ensure transport reliability. In this regard, the application must be aware of the type of transport protocol underneath RPC. If the RPC service knows it is running on top of a reliable transport such as TCP/IP, then most of the work is already done for it. On the other hand, if RPC is running on top of an unreliable transport such as UDP/IP, the service must devise its own retransmission and time-out policy. RPC does not provide this service.
Because of transport independence, the RPC protocol does not attach specific semantics to the remote procedures or their execution. Semantics can be inferred from (but should be explicitly specified by) the underlying transport protocol. For example, consider RPC running on top of an unreliable transport such as UDP/IP. If an application retransmits RPC messages after short time-outs, the only thing it can infer if it receives no reply is that the procedure was executed zero or more times. If it does receive a reply, then it can infer that the procedure was executed at least once.
A server may want to remember previously granted requests from a client and not regrant them to insure some degree of execute-at-most-once semantics. A server can do this by taking advantage of the transaction ID that is packaged with every RPC request. The main use of this transaction ID is by the RPC client for matching replies to requests. However, a client application may choose to reuse its previous transaction ID when retransmitting a request. The server application, knowing this fact, may choose to remember this ID after granting a request and not regrant requests with the same ID. The server is not allowed to examine this ID in any other way except as a test for equality.
On the other hand, if using a reliable transport such as TCP/IP, the application can infer from a reply message that the procedure was executed exactly once, but if it receives no reply message, it cannot assume the remote procedure was not executed. Note that even if a connection-oriented protocol like TCP is used, an application still needs time-outs and reconnection to handle server crashes.
The act of binding a client to a service is not part of the remote procedure call specification. This important and necessary function is left up to some higher-level software. (The software may use RPC itself; see ``rpcbind protocol''.)
Implementors should think of the RPC protocol as the jump-subroutine instruction (``JSR'') of a network; the loader (binder) makes JSR useful, and the loader itself uses JSR to accomplish its task. Likewise, the network makes RPC useful, using RPC to accomplish this task.
The RPC protocol provides the fields necessary for a client to identify itself to a service and vice-versa. Security and access control mechanisms can be built on top of the message authentication. Several different authentication protocols can be supported. A field in the RPC header specifies the protocol being used. More information on authentication protocols can be found in ``Authentication protocols''.