Operating Systems Lecture 12 Communicating over a Network Read Ch 4.6

1. Operating SystemsLecture 12 Communicating over a Network Read Ch 4.6 Operating System Concepts

2. Recall IPC • Interprocess communication (IPC) accomplished by message passing. • Direct Communication: Processes identify each other by name. • Indirect communication: Processes use a mailbox or a port. Operating System Concepts

3. Communication over a network • Communication between processes over a network is accomplished with sockets. • A socket is defined as an endpoint for communication. • Communication consists between a pair of sockets. • A socket is the concatenation of IP address and port number. • The socket 161.25.19.8:1625 refers to port 1625 on host 161.25.19.8 Operating System Concepts

4. Client-Server Architecture • The server "listens" to a specified port for incoming client requests. • When a request is received, the server accepts the connection from the client socket. • "Well known" port numbers: • telnet: 23 • ftp: 21 • http: 80 • All ports numbers below 1024 are considered well known. They are used for standard services. Operating System Concepts

5. Example Suppose there is a client process on host machine, X, with IP address, 146.86.5.20 The client contacts a web server at IP address, 161.25.19.8. Question: What is the port number for the server? Host, X, assigns the client a port, e.g. 1625. Result: Connection between 2 sockets: Client on host X Web Server: 146.86.5.20:1625 Q: What is this socket? Packets of information travel between the hosts and are delivered to the appropriate process by way of the port. Operating System Concepts

6. Socket Communication Operating System Concepts

7. Implementation in Java • Java provides a Socket class for establishing communication. (This makes is easier to implement than in C/C++) • Example: Implementation of a time-of-day server • Clients request the time of day from the server • The server listens to port 5155 (could choose any port number above 1024). Operating System Concepts

8. Server Side Application public class Server { public static void main( String [ ] args) { Socket client = null; //will hold socket for client PrintWriter pout = null; //allows easy communication ServerSocket sock = null; //Holds server socket . . . sock = new ServerSocket(5155); //Port for server socket while (true) { //Listen for requests client = sock.accept( ); //Waits for client to contact (blocked) //accept( ) returns socket from client //PrintWriter allows easy communication through socket pout = new PrintWriter(client.getOutputStream( ), true); pout.println(new java.util.Date().toString()); //write time of day to //client pout.close(); client.close(); ... } } Operating System Concepts

9. Client Side Application public class Client { public static void main(String[ ] args) { InputStream in = null; //for input stream from socket BufferedReader bin = null; //for reading in from socket Socket sock = null; //socket for contact with server ... sock = new Socket("127.0.0.1", 5155); //server socket in = sock.getInputStream( ); //Set up for reading bin = new BufferedReader(new inputStreamReader(in)); String line; // Holds input line while ((line = bin.readln( )) != null) //read to end System.out.println(line); //print out line ... } } //(Note: IP address 127.0.0.1 indicates the local host) Operating System Concepts

10. Remote Procedure Calls • Remote procedure call (RPC) abstracts procedure calls between processes on networked systems. • Messages are well structured (not just data packets). • Addressed to RPC daemon listening to a port on the remote system. • Contain identifier of function to be executed. • Contain parameters to pass to that function. • The port is identified by a number at the start of the message packet. • A system has one network address, but may have many ports. • To request a specific service, must address message to the proper port. Operating System Concepts

11. Invoking an RPC • Invoking a procedure on a remote host is the same as invoking one locally (The details are hidden). • Stubs – client-side proxy for the actual procedure on the server. • Procedure for executing a remote procedure call: • The client invokes the remote procedure • The RPC system calls the stub, passing it the parameters provided in the procedure call. • The client-side stub locates the port on the server. • The stub marshalls the parameters. (Packages them into a form that can be sent over the network). • The stub transmits a message to the server using message passing. • The server-side stub receives this message, unpacks the marshalled parameters, and performs the procedure on the server. • If necessary, return values are passed back to the client. Operating System Concepts

12. Which Port to Use? • With ordinary procedure calls there is a binding between the name of the procedure and the address of the procedure. • With an RPC, there is binding between the client and server ports. • How does the client know which port to use? • Predetermined binding: RPC has fixed port address. • Dynamic binding: O.S. provides a matchmaker daemon (rendezvous daemon) on a fixed RPC port. • The client sends a message to the matchmaker and requests the port address of the RPC. • The matchmaker returns the port number. • The RPC call is sent to that port. Operating System Concepts

13. Execution of RPC Operating System Concepts

14. Remote Method Invocation • Remote Method Invocation (RMI) is a Java mechanism similar to RPCs. • RMI allows a Java program on one machine to invoke a method on a remote object. • RMI works between Java Virtual Machines. Could be two JVM's on the same machine or on different machines. Operating System Concepts

15. RPC vs. RMI Difference between an RPC and an RMI: RPC Procedural: Only allows procedures or functions to be called. Parameters: Ordinary data structures RMI Object Based: Can invoke a method on a remote object. Parameters: Can pass objects as parameters. Allows for distributed Java applications Operating System Concepts

16. Process for RMI A stub on the client machine is invoked. The stub sends a parcel with the name of the method and the marshalled parameters. A skeleton on the server unmarshall's the parameters and invokes the method. The skeleton marshall's the return value into a parcel and returns it to the client. The stub on the client unmarshall's the return value and passes it to the client. Operating System Concepts

17. Marshalling Parameters Example: Client invokes someMethod(A, B) on a Server object. The method returns a boolean object. Operating System Concepts