Notes

Outline

Overview of Distributed Programming Introduction Internetworking Internet Addresses (and resolving them in Java) Introduction to Distributed Programming Distributed Programming Techniques Distributed Programming Support in Java Overview of Java Security

Introduction A distributed systems consists of a collection of computers linked together by a network Distributed system software enables computers to coordinate activities and share resources (software, hardware, data)

Internetworking Network: a set of computers and peripherals (printers, modems, etc.) connected together by a medium. (direct or indirect connection). Devices communicate with one another through a protocol A Protocol is a language or set of rules Network architecture: Devices in same room or building (LAN) vs. WAN

Protocols A protocol: a set of rules that two or more computers must follow to exchange messages The key communication protocol is Internet Protocol (IP) Messages travel in packets (or datagrams) Later on we will discuss TCP (Transmission Control protocol) UDP (User Datagram Protocol)

The OSI Reference Model A protocol stack with seven layer

Naming and Routing To make datagram delivery possible, each computer on the net is assigned a unique address This address could be symbolic (e.g. www.yahoo.com) or numeric IP (216.32.74.50)  IP addresses are 32-bit numeric identifiers containing network and host identifiers The identifiers uniquely identify the network and a host on that network

Naming and Routing Translation from symbolic names to IP addresses is done through a naming service (DNS) IP addresses are used when a host is sending packets to other hosts on same (or other) network The process of locating networks and hosts and delivering packets to them is called routing While data is being routed, it may get lost between networks (the transport protocol is responsible for  verification

Internet Addresses Five classes of Internet addresses: A, B, C, D, E A B C D E

Internet Addresses Different classes meet the requirements of different organizations Class A for large networks (>65,536 hosts) Class B for med-size networks (256<size<65,536) Class C for networks with up to 256 hosts Class D is used for multicasts Class E for future use

Resolving Internet Addresses The java.net.InetAddress class Examples Reference: (page 28 – 30) from the book

Distributed Programming Client/Server Model One program communicates with another program to exchange information Both programs speak the same language (protocol) Object-based Model A collection of objects that isolates the requesters of services (clients) from providers (servers) by interface

Distributed Programming Techniques Sockets Message destinations are specified as socket addresses Messages queued at sending/receiving sockets Remote Procedure Calls This figure tries to compare RPC to Agents

Distributed Programming is Good Multiuser applications Resource sharing Scalability Efficiency Fault tolerance and availability Transparency

Distributed Programming is Hard Multiple failure modes Security issues Use of multiple technologies Testing and debugging

Facilities in Java Sockets RMI JavaIDL JavaSpaces Jini

Overview of Java Security The Java security model is composed of three layers: The Java language itself The Java compiler and run-time system The SecurityManager class The three layers provide a restricted environment known as the “sandbox”

Security…. The compiler and runtime system provide a simple secure execution environment that consists of three sublayers: The bytecode interpreter and class format verifier A mechanism for loading and checking libraries at runtime Automated Garbage Collection

Security…. The built-in Java safety features ensures that the Java system is not subverted by invalid code. It is not able to protect against malicious code File f = new File(“path to a sensitive file”); if (f.delete() == true) {     System.out.println(“file has been deleted”); } else {     System.out.println(“operation is not allowed”); }

The SecurityManager The SecurityManager class (in java.lang) provides the necessary mechanism for creating a security policy that defines tasks that an application can and cannot do: class MySecurityManager extends SecurityManager {     private  boolean checkDelete = true;     public void checkDelete(String file) {         if (checkDelete) {           throw new SecurityException(“not allowed”);         }    } }

How does it work? How does the Java interpreter’s SecurityManager actually work? public boolean XX(Type arg1) {    SecurityManager sm = System.getSecurityManager();    if(security != null) {      security.checkXX(arg1);    } }

The sandbox (JDK1.0) A restricted environment in which to run untrusted code (applets). JDK1.0 Security Model

The Evolving Sandbox (JDK1.1) JDK1.1 has introduced the concept of signed applets. A correctly signed applet is treated as trusted local code.

Protection Domains (JDK1.2) In JDK1.1, local and signed code enjoy full access to the system, but not remote. This has changed in JDK1.2. All code will get access to system resources based on a policy file. If a security manager is not loaded then there is no need for a security policy If you load a security manager, you must create a security policy

Security Policies Example grant {    Permission java.io.FilePermission “some path-”, “read”;    Permission java.net.SocketPermission “some port”, “connect, accept”; } ; More on security policies later in the course….

Sockets Programming Client-Server Computing What are Sockets Sockets Programming in Java Programming Examples

Client/Server Computing Simple idea: Some hosts (clients, typically desk top computers) are specialized to interact with users: Gather input from users Present information to users Other hosts (servers) are specialized to manage large data, process that data The Web is a good example: Client (Browser) & Server (HTTP server)

Client/Server Computing Other examples: E-mail

Client/Server Computing Other examples: Chatroom

Tiered Client/Server Architecture 1-tier: single program 2-tier: client/server (e.g. the Web) 3-tier: application logic and databases on different servers (e.g. the Web with CGI and databases)

Client/Server Communication Two related processes on a single machine may communicate through a pipe A pipe is a pseudo-file that can be used to connect two processes together

Client/Server Communication Two UNRELATED processes may communicate through files (process A write to a file and process B reads from it) But HOW two processes located on two different machines communicate? Solution: Berkeley sockets.

What are sockets A socket is an end point of a connection Or: the interface between user and network Two sockets must be connected before they can be used to transfer data (case of TCP) A number of connections to choose from: TCP, UDP, Multicast Types of Sockets SOCK_STREAM, SOCK_DGRAM, SOCK_RAW

Sockets Message destinations are specified as socket adresses Each socket address is a communication identifier: Internet address Port number The port number is an integer that is needed to distinguish between services running on the same machine Port numbers between 0 .. 1023 are reserved

Which transport protocol (TCP v. UDP) TCP -- Transmission Control Protocol UDP -- User Datagram Protocol What should I use? TCP is a reliable protocol, UDP is not TCP is connection-oriented, UDP is connectionless TCP incurs overheads, UDP incurs fewer overheads UDP has a size limit of 64k, in TCP no limit

Socket Communication Sequence of steps normally taken to set up socket communication and exchange data between C/S

Sockets Programming in Java Streams The basic of all I/O in Java is the data stream A pipeline of data put info into the pipeline (write) and get it (read) Programming with Sockets (TCP) Opening a Socket Creating a data input stream Creating a data output stream Closing the socket(s)

Opening a socket Client-side:       Socket myClient = null;  try {      MyClient = new Socket(“host”, PotNumber);  } catch (UnknownHostException uhe) {      uhe.printStackTrace();  } “host” can be symbolic name or IP address

Opening a socket Server-side        ServerSocket myService = null;         try {           myService = new ServerSocket(portNumber);         } catch (UnknownHostException uhe) {           uhe.printStackTrace();         }         Socket serviceSocket;         serviceSocket = myService.accept();

Creating an input stream Client-side:        BufferedReader is = null;         try {            is = new BufferedReader(new              InputStreamReader(myClient.getInputStream()));         } catch (IOException ioe) {             ioe.printStackTrace();         }

Creating an input stream Server-side:         BufferedReader is = null;         try {            is = new BufferedReader(new         InputStreamReader(serviceClient.getInputStream()));         } catch(IOException ioe) {            ioe.printStackTrace();         }

Creating an output stream Client-side:        DataOutputStream os = null;         try {            os = new               DataOutputStream(myClient.getOutputStream());         } catch (IOException e) {            e.printStrackTrace();         }

Creating an output stream Server-side:        DataOutputStream os = null;         try {            os = new         DataOutputStream(serviceClient.getOutputStream());         } catch(IOException e) {            e.printStackTrace();         }

Closing sockets Client-side:         try {            os.close();            is.close();            myClient.close();         } catch(IOException e) {             e.printStrackTrace();         }

Closing sockets Server-side:         try {            os.close();            is.close();            serviceSocket.close();            myService.close();         } catch(IOException e) {            e.printStackTrace();         }

Sockets Programming Examples Clients: SMTP client Echo client Time Client Clients/Servers: HiClient, HiServer MathClient, MathServer