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Computer Networks &
PROTOCOLS
(CSI 4118)
FALL 2005
Professor Robert L. Probert
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Part I
Introduction
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Topic and Scope
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Computer networks and internets: an
overview of concepts, terminology, and
technologies that form the basis for digital
communication in private corporate networks
and the global Internet.
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You Will Learn
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Internets and Internetworking
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Motivation and concept
Internet Protocol (IP) datagram format and addressing
Internet routers and routing
Address binding (ARP)
Internet control messages (ICMP)
User Datagram Protocol (UDP)
Transmission Control Protocol (TCP)
Protocol ports and demultiplexing
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You Will Learn (continued)
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Some network applications!
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Client-server paradigm
Domain name system (DNS)
File transfer (FTP)
Remote login (TELNET)
Email transfer (SMTP)
Web technologies and protocols
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HTTP, PHP, CGI, Java
Security
Voice over IP, …
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Background Required
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Ability to program in Java, ability to read C code
Knowledge of low-level programming constructs
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Pointers
Bit fields in structures
Familiarity with basic tools
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Text editor
Compiler / linker / loader
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Background Required
(continued)
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Basic knowledge of operating systems
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Terminology
Functionality
Processes and concurrent processing
CSI 3103/3503
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Motivation for Networking
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Information access
Interaction among cooperative application
programs
Resource sharing
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The Internet concept
the illusion of a single network that TCP/IP software
provides to users and applications
User’s
computers
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The Internet concept
the underlying physical structure in which a computer attaches to
one physical network and routers interconnect the networks
Net 2
Net 1
Net 4
Net 3
router
Net 5
Physical net
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Figure 2.1
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Figure 2.2
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Figure 2.3
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Figure 2.4
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Figure 2.5
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Figure 2.6
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Practical Examples
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Email
File transfer / access
Web browsing
Remote login / execution
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What is a Network?
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Transmission hardware (media) [Ch. 4]
Special-purpose hardware devices [Ch. 9, 10]
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Interconnect transmission media
Control transmission
Run protocol software
Protocol software [Ch. 16 and following]
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Encodes and formats data
Detects and corrects problems
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What does a Network Do?
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Provides communication that is
Reliable
 Fair
 Efficient [15.14]
 From one application to another
(allows arbitrary applications to communicate via
Network (Distributed) Programming)
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What Does a Network Do?
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Reliable
Fair
Efficient
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Delay
Throughput
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Delay and Throughput
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Delay
 Propagation Delay
 Switching Delay
 Access Delay
 Queuing Delay
Throughput
 Rate of Data Transmission
 How many bits can enter (or leave) network in fixed unit of time
 Effective Throughput
 Capacity
 Congestion
 Degree of Utilization
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What Does a Network Do?
(continued)
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Automatically detects and corrects
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Data corruption e,g., CRC [7.9, 7.10]
Data loss
Duplication
Out-of-order delivery e.g., sequencing [16.10]
Automatically finds optimal path from source
to destination, routing [Ch. 13]
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Network (Distributed)
Programming [Ch. 3]
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Network allows arbitrary applications to
communicate
Programmer does not need to understand
network technologies
Network facilities are accessed through an
Application Program Interface
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Basic Paradigm for Pairwise
Internet Communication
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Establish contact
Exchange data (bi-directional)
Terminate contact
Note: NO data processing by the network
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Establishing Contact
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Performed by pair of applications
One application waits for contact (called
server)
Other application initiates contact (called
client)
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Identifying a Waiting
Application
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Conceptually two items specified
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Computer
Application on that computer
Terminology
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Computer identified by domain name
Application identified by program name
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Representations and
Translations
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Humans use names such as
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computer: www.netbook.cs.purdue.edu
application: ftp
Network protocols require binary values
Library routines exist to translate from names
to numbers
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Example API
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Simplified API
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Simplified API (Cont’d)
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Simplified API (Cont’d)
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The Await_Contact Function
connection await_contact(appnum a)
The argument specifies a number that identifies the server
application
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Simplified API (Cont’d)
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The Make_Contact Function
connection make_contact(computer c, appnum a)
The client uses the return value, which is of type
connection, to transfer data
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Simplified API (Cont’d)
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The appname_to_appnum Function
appnum appname_to appnum(char *a)
Clients and servers both use appname_to_appnum to
translate from a human-readable name for a service to
an internal binary value
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Simplified API (Cont’d)
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The Cname_to_comp Function
computer cname_to_comp(char *c)
Clients call cname_to_comp convert from a humanreadable computer name to the internal binary value
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Simplified API (Cont’d)
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The Send Function
int send (connection con, char *buffer, int length, int flags)
Both clients and servers use send to transfer data across
the network
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Simplified API (Cont’d)
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The Recv And Recvln Function
int recv (connection con, char *buffer, int length, int flags)
Both clients and servers use recv to access data that arrives across the
network
int recvln (connection con, char *buffer, int length)
Recvln repeatedly calls recv until an entire line of test has been
received
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Simplified API (Cont’d)
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The Send_eof Function
int send_eof (connection con)
Both the client and server must use send_eof after sending
data to inform the other side that no further transmission
will occur
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Simplified API (Cont’d)
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Example #1: Echo
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Useful for network testing
Server returns exact copy of data sent
User on computer X runs
echoserver 22000
User on another computer runs
echoclient X 22000
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Example Code Using API:
Echoserver
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Echoserver (2 of 2)
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Actually works on the Internet
API calls replace conventional I/O
No networking knowledge required
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Example #2: Chat
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Miniature version of Internet chat service
Allows two users to communicate
User on computer X runs
chatserver 25000
User on another computer runs
chatclient X 25000
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Example Application: Web
Server
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User on computer X runs
webserver 27000
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User on another computer runs browser and
enters URL:
http://X:27000/index.html
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Example Code Using API:
Webserver
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Webserver (2 of 6)
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Webserver (3 of 6)
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Webserver (4 of 6)
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Webserver (5 of 6)
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Webserver (6 of 6)
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Network Programming in Java
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The java.net package provides two classes-Socket and ServerSocket--that implement the
client side of the connection and the server
side of the connection, respectively.
java.io package also has to be used together
with socket API.
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Example API in Java (v1.3+)
Method
Class
Socket (String host, int port) java.net.Socket
ServerSocket (int port)
accept
println
readLine
close
close
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Meaning
Used by client to create a socket
to connect to the specified server
host and port
java.net.ServerSocket Used by server to create a socket
on a specified port
java.net.ServerSocket Used by server to accept
connection from client
java.io.PrintWriter
Used by either client or server to
java.io.PrintStream
send data
java.io.BufferedReader Used by either client or server to
receive data
java.net.Socket
Used by client to close this
socket
java.net.ServerSocket Used by server to close this
socket
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The steps for programming in Java:
1.
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4.
5.
Open a socket.
Open an input stream and output stream to the
socket.
Read from and write to the stream according to
the server's protocol.
Close the streams.
Close the socket.
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Example Code Using Java API:
(1) EchoServer.java
import java.io.*;
import java.net.*;
public class EchoServer {
public static void main(String args[]) {
// declare a server socket and a client socket for the server
ServerSocket echoServer = null;
String line;
BufferedReader br;
PrintStream os;
Socket clientSocket = null;
// open a server socket on a specfic port. args[0] is the port number
try {
echoServer = new ServerSocket(Integer.parseInt(args[0]));
} catch (IOException e) { }
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try {
// open a socket to listen and accept connections from client
clientSocket = echoServer.accept();
br = new BufferedReader(new
InputStreamReader(clientSocket.getInputStream()));
os = new PrintStream(clientSocket.getOutputStream());
// once receiving data from client, echo it back to the client.
while (true) {
line = br.readLine();
os.println(line);
} } catch (IOException e) { }
}//EchoServer.java
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(2) EchoClient.java
import java.io.*;
import java.net.*;
public class EchoClient {
public static void main(String[] args) throws IOException {
Socket echoSocket = null;
PrintWriter out = null;
BufferedReader in = null;
//open a socket to a specified host and port.
try { // args[0] is host address, args[1] is port number
echoSocket = new Socket(args[0], Integer.parseInt(args[1]));
out = new PrintWriter(echoSocket.getOutputStream(), true);
in = new BufferedReader(new InputStreamReader(
echoSocket.getInputStream()));
} catch (Exception e) {}
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BufferedReader stdIn = new BufferedReader(new
InputStreamReader(System.in));
String userInput;
//send user's input to server, and retrieve what the server sends back.
while ((userInput = stdIn.readLine()) != null) {
out.println(userInput);
System.out.println("echo: " + in.readLine());
}
//close everything gracefully
out.close();
in.close();
stdIn.close();
echoSocket.close();
}
}//EchoClient.java
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(3) Test Run
1.
Compile EchoServer.java and EchoClient.java.
2.
Start server program:
java EchoServer localhost 4118
3.
Start client program:
java EchoClient 4118
Then type in something at prompt to see what
will happen.
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Summary
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Studying networks is important because
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The world is interconnected
Applications now operate in a distributed
environment
This course
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Covers networking and internetworking
Explains the mystery of applications
Will be hard work
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Summary (continued)
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Computer Internetworks
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Deliver data from source to destination
Automatically find optimal paths
Handle problems that occur
We will review how
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