Remote Procedure Call Concept RPC

1
Lecture 12

• Remote Procedure Call Concept (RPC)

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Overview

• Introduction
• RPC Model
• Paradigm for Distributed Programs
• Conventional Procedure Call Model
• Client/Server and RPC
• Sun Microsystems RPC Definition
• Remote Programs and Procedures
• Management for RPCs
• Communication Semantics
• RPC Mapping
• RPC Data Marshaling
• Summary

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Introduction

• Remote Procedure Call (RPC) is a protocol that
  one program can use to request a service from a
  program located in another computer.
• RPC uses the client/server model.
• The requesting program is a client and the
  service-providing program is the server.
• Similar to a local procedure call, an RPC is a
  synchronous operation requiring the requesting
  program to be suspended until the results of the
  remote procedure are returned.
• Threads or lightweight processes that share the
  same address space allow multiple RPCs to be
  performed concurrently.
• The RPC concept simplifies the design of
  client/server software
• Makes the programs easier to understand
• Uses symmetric data conversion (XDR)

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Remote Procedure Call Model

• A conceptual framework for building distributed
  programs
• Remote Procedure Call model or RPC model
• Uses familiar concepts from conventional programs
  as the basis for the design of distributed
  applications

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Distributed Programs Paradigms

• Communication-Oriented Design
• Begin with the communication protocol
• Design a message format and syntax
• Design the client/server by specifying how each
  reacts to incoming/outgoing messages
• A communication-oriented design may lead to
  problems
• Protocol may not provide all the needed
  functionality
• Inexperience with protocol design could lead to
  inefficient protocols
• Difficult to understand or modify
• Application-Oriented Design
• Begin with the application
• Design a conventional application program
• Build and test a working version as a
  conventional program
• Divide the program into several pieces to execute
  on separate computers
• The remote procedure call paradigm focuses on the
  application
• Emphasizes the problem to be solved instead of
  the communication mechanism
• Does not require major code changes when
  splitting procedures between local/remote
  machines
• RPC separates the solution of a problem from the
  task of making the solution operate in a
  distributed environment

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Conceptual Model for Conventional Procedure Calls
main
proc1
proc2
proc3
proc4
proc5
proc6
proc7
proc8
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Extension of the Procedural Model
computer1
computer2
main
proc1
proc2
proc3
proc4
proc5
proc6
proc7
proc8
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Conventional Procedure Call Execution

• Single thread of control through all procedures
• Execution begins in main and continues until it
  encounters a procedure call
• Execution branches to the code in the procedure
• Execution continues in the procedure until it
  encounters a return statement

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Distributed Procedural Model

• Single thread of execution executes in a
  distributed environment
• Client/Server corresponds to a procedure call and
  return
• Client request corresponds to a procedure call
• Server response corresponds to the return function

Main Program machine 1 (client)
Procedure A machine 2 (server)
Procedure B machine 3 (server)
main
call remote proc A
call remote proc A
respond to caller
respond to caller
exit
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Distributed Computation

• Paradigm helps programmers design distributed
  programs easily
• Distributed program invokes a procedure to access
  a remote service
• Remote procedures are accessed the same as local
  procedures
• Distributed are as easy to construct as
  conventional programs

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Sun Microsystems' RPC Definition

• Sun Microsystems developed a specific form of RPC
  (Sun RPC, Open Network Computing (ONC) RPC or
  simply RPC)
• Received wide acceptance including NFS
• ONC RPC defines the format of messages
• Allows the calling program to use UDP or TCP
• Uses XDR to represent procedure arguments as well
  as data items in an RPC message header
• ONC RPC includes a compiler system to aid in
  building distributed applications

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Remote Programs and Procedures

• RPC defines a remote program as the basic unit of
  software that executes on a remote machine
• Each remote program corresponds to a server and
  contains a set of one or more procedures plus
  global data
• The procedures inside a remote program all share
  access to its global data

A single remote program
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Identifying Remote Programs and Procedures

• The ONC RPC standard specifies that each remote
  program be assigned a unique 32-bit integer to
  identify it
• ONC RPC assigns an integer to each remote
  procedure inside a given remote program
• The procedures are numbered 1, 2, N
• ONC RPC divided the set of program numbers into 8
  groups of 32-bit numbers
• Each remote program is assigned a unique number
• ONC RPC includes an integer version number for
  each remote program
• The RPC specification permits a computer to run
  multiple versions of a remote program

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Mutual Exclusion for Procedures in a Remote
Program

• The ONC RPC mechanism specifies that at most one
  remote procedure can execute in a remote program
  at one time
• RPC provides automatic mutual exclusion among
  procedures within a given remote program
• Important for remote programs that maintain a
  shared data area

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Communication Semantics

• ONC RPC does not enforce reliable semantics
• Can use TCP or UDP as a transport protocol
• When using UDP as the transport protocol the
  application must be able to tolerate zero-or-more
  execution semantics
• Each remote procedure call needs to be idem
  potent
• The standard does not specify additional
  protocols or mechanisms for reliable delivery
• RPC semantics are defined as a function of the
  underlying transport protocol

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RPC Retransmission

• ONC RPC includes a simple timeout and
  retransmission strategy
• Does not guarantee reliability
• Default timeout mechanism implements a fixed
  timeout with a fixed number of retries

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Map a Remote Program to a Protocol Port

• TCP/UDP use 16-bit protocol numbers
• RPC uses 32-bit numbers to identify remote
  programs
• RPC programs can outnumber protocol ports
• Unable to map RPC program to protocol ports
  directly
• Each RPC program can obtain a protocol port
  number if port assignments are temporary

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Dynamic Port Mapping

• ONC RPC mechanism includes a dynamic mapping
  service to solve the identification problem
• Each machine that supports RPC provides a
  mechanism that allows a mapping of RPC program
  numbers to protocol ports

RPC program register
RPC program (server)
RPC port mapper (server)
RPC program socket used by RPC
socket for well-known port
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RPC Port Mapper Algorithm

• Create a passive socket bound to the well-known
  port assigned to the ONC RPC port mapper service
  (111)
• Repeatedly accept request to register an RPC
  program number or to look up a protocol port
  given an RPC program number

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ONC RPC Message Format

• ONC RPC does not use a fixed format for messages
• The standard defines the general format of RPC
  messages
• The XDR language defines the data items
• The XDR language defines how to assemble a message

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Data Marshaling Arguments

• RPC must represent all arguments in an external
  form for transfer
• Complex data structures must be encoded into a
  compact representation
• marshal, linearize or serialize
• Client side marshals the arguments into the
  message and the server side un-marshals them

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Authentication

• RPC uses several forms of authentication
• Simple scheme that relies on UNIX
• Complex scheme that uses the Data Encryption
  Standard (DES)
• RPC leaves the format and interpretation up to
  the authentication subsystem

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Example RPC Message
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Summary

• The remote procedure model helps make distributed
  programs easy to design and understand
• The RPC model views each server as implementing
  one or more procedures
• A message from a client corresponds to a call and
  a response from the server corresponds to a
  return
• Remote procedures accept arguments and return one
  or more results
• Sun Microsystems developed the de-facto standard
  form of remote procedure call (ONC RPC)
• ONC RPC use a dynamic binding mechanism called
  the RPC Port Mapper