Local and Remote Procedure Calls

The communications method in computing includes Local procedure calls and Remote procedure calls.

Remote Procedure Call

This is a protocol that one program can use to request the services from other located in other machine in a network without having to understand the network details. Usually when a program using RPC are compiled into an executable program, a stub is included which acts as the representative of remote procedure code. When the program is run, and the procedure issue the stub receives a request and forwards it to the client runtime in the local computer by the daemons. The client runtime program knows the address of the remote computer and server application. It then sends the request across the network .The server also have a runtime program and stub that interface with remote procedure. The result is returned the same way.

Local Procedure Call

A local procedure call (LPC) is an interprocess communication facility for high-speed message passing.

In Windows NT, client-subsystem communication happens in a fashion similar to that in the MACH operating system. Each subsystem contains a client-side DLL that links with the client executable. The DLL contains stub functions for the subsystem’s API. Whenever a client process–an application using the subsystem interface–makes an API call, the corresponding stub function in the DLL passes on the call to the subsystem process. The subsystem process, after the necessary processing, returns the results to the client DLL. The stub function in the DLL waits for the subsystem to return the results and, in turn, passes the results to the caller. The client process simply resembles calling a normal procedure in its own code. In the case of RPC, the client actually calls a procedure sitting in some remote server over the network–hence the name remote procedure call. In Windows NT, the server runs on the same machine; hence the mechanism is called as a local procedure call.

LPC is designed to allow three methods of exchanging messages:

Message that is shorter than 256 bytes can be sent by calling LPC with a buffer containing the message. That is a small message. This message is then copied from the address space of the sending process into system address space, and then to the address space of the receiving process.

If a client and a server want to exchange more than 256 bytes of data, they can choose to use a shared section to which both are mapped. The sender places message data in the shared section and then sends a small message to the receiver with pointers to where the data is to be found in the shared section.

When a server wants to read or write larger amount of data than will fit in a shared section, data can be directly read from or written to a client's address space. The LPC component supplies two functions that a server can use to accomplish this. A message sent by the first method is used to synchronize the message passing.
There are three types of LPC. The first type sends small messages up to 304 bytes. The second type sends larger messages. The third type of LPC is called as Quick LPC and used by the Win32 subsystem in Windows NT 3.51. The first two types of LPC use port objects for communication. Ports resemble the sockets or named pipes in Unix. A port is a bi-directional communication channel between two processes. However, unlike sockets, the data passed through ports is not streamed. The ports preserve the message boundaries. Simply put, you can send and receive messages using ports. The subsystems create ports with well-known names. The client processes that need to invoke services from the subsystems open the corresponding port using the well-known name. After opening the port, the client can communicate, with the server, over the port.