What is it?
Remote Procedure Call (RPC) is a protocol used in operating systems to enable high-level communication between programs. RPC relies on lower-level transport protocols like TCP/IP or UDP to transfer data between communicating applications. It establishes a client-server communication model specifically designed for network applications.
Built on the eXternal Data Representation (XDR) protocol, RPC standardizes data representation for remote communication. XDR handles the conversion of parameters and results for each RPC service.
RPC allows remote procedures to be used as though they were local. Defined routines within RPC handle remote calls, with each request paired with a response message. In addition to supporting message-passing, RPC includes non-RPC features like batching, broadcasting, server-side callbacks, and the select subroutine.
In RPC terminology, a client is a process or device that accesses resources or services from another device or process on the network, which acts as a server. A server provides services and implements network resources as a collection of remote programs, which define remote procedures along with their parameters and results in the program’s protocol.
RPC includes an authentication process that lets clients and servers identify one another. Each RPC call has a slot for authentication parameters, allowing the client to identify itself to the server in a supported authentication type.
Common authentication types UNIX and Data Encryption Standard (DES) systems are supported in RPC.
UNIX system: This type of authentication uses information based on the UNIX operating system’s user accounts, like usernames or user IDs, to verify who the client is. The server can then use this information to check if the client has the right permissions.
Data Encryption Standard (DES): DES is a method for encrypting data so only someone with the right “key” can read it. With DES, RPC can send a special encrypted message between the client and server to verify their identities securely.
Each RPC server provides a program with a set of remote procedures. To specify a remote procedure, a combination of host address, program number, and procedure number is used. When a client calls a procedure, a data packet is sent to the server. The server then executes the requested service and returns a response to the client, completing the procedure.
While RPC is commonly used for communication across networked workstations, it also works effectively between processes on the same machine.
The Port Mapper program is used in RPC to map program and version numbers to a transport-specific port, allowing dynamic binding of remote programs.
RPC Model
The remote procedure call (RPC) model is much like the local procedure call model. In a local call, the caller puts the procedure’s arguments in a specific location, like a result register.
Afterward, the caller hands control over to the procedure. Once the procedure finishes, the caller takes back control, retrieves the procedure’s results, and continues running.
RPC follows a similar process but operates across two processes: the caller and the server. The caller sends a call message, which includes the procedure’s parameters, to the server and then waits (or “blocks”) for a response. Meanwhile, the server process, which stays inactive until a call message arrives, takes the parameters from the message, performs the required computation, and sends back a reply. The server then waits for the next call. When the caller receives the reply, it extracts the results and resumes execution.
Complete RPC Mechanism
In the RPC model, only one of the two processes—the client or server—is active at a time. However, this is just one example. The RPC protocol does not impose any specific concurrency model, so other approaches are possible. For instance, an implementation may use asynchronous Remote Procedure Calls, allowing the client to continue working while awaiting the server’s response. Similarly, the server can spawn a separate task to handle incoming requests, allowing it to stay available for other requests.