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Communications Programming Concepts

Remote Procedure Call

Remote Procedure Call (RPC) is a protocol that provides the high-level communications paradigm used in the operating system. RPC presumes the existence of a low-level transport protocol, such as Transmission Control Protocol/Internet Protocol (TCP/IP) or User Datagram Protocol (UDP), for carrying the message data between communicating programs. RPC implements a logical client-to-server communications system designed specifically for the support of network applications.

This chapter provides the following information about programming RPC:

The RPC protocol is built on top of the eXternal Data Representation (XDR) protocol, which standardizes the representation of data in remote communications. XDR converts the parameters and results of each RPC service provided.

The RPC protocol enables users to work with remote procedures as if the procedures were local. The remote procedure calls are defined through routines contained in the RPC protocol. Each call message is matched with a reply message. The RPC protocol is a message-passing protocol that implements other non-RPC protocols such as batching and broadcasting remote calls. The RPC protocol also supports callback procedures and the select subroutine on the server side.

A client is a computer or process that accesses the services or resources of another process or computer on the network. A server is a computer that provides services and resources, and that implements network services. Each network service is a collection of remote programs. A remote program implements remote procedures. The procedures, their parameters, and the results are all documented in the specific program's protocol.

RPC provides an authentication process that identifies the server and client to each other. RPC includes a slot for the authentication parameters on every remote procedure call so that the caller can identify itself to the server. The client package generates and returns authentication parameters. RPC supports various types of authentication such as the UNIX and Data Encryption Standard (DES) systems.

In RPC, each server supplies a program that is a set of remote service procedures. The combination of a host address, program number, and procedure number specifies one remote service procedure. In the RPC model, the client makes a procedure call to send a data packet to the server. When the packet arrives, the server calls a dispatch routine, performs whatever service is requested, and sends a reply back to the client. The procedure call then returns to the client.

The RPC interface is generally used to communicate between processes on different workstations in a network. However, RPC works just as well for communication between different processes on the same workstation.

The Port Mapper program maps RPC program and version numbers to a transport-specific port number. The Port Mapper program makes dynamic binding of remote programs possible.

To write network applications using RPC, programmers need a working knowledge of network theory. For most applications, an understanding of the RPC mechanisms usually hidden by the rpcgen command's protocol compiler is also helpful. However, use of the rpcgen command circumvents the need for understanding the details of RPC.

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