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CSCE 515:
Computer Network
Programming
------ Java Network Programming
reference: Dave Hollinger Wenyuan Xu
http://www.cse.sc.edu/~wyxu/csce515f07.html Department of Computer Science and Engineering University of South Carolina
Java Network Programming
Introduction
Java Socket Programming
InetAddress
Socket
ServerSocket
DatagramSocket
Multicast Socket
2007 CSCE515 – Computer Network Programming
Crash Course in Java
Why Java?
Network Programming in Java is very
different than in C/C++
much more language support
error handling
no pointers! (garbage collection)
Threads are part of the language.
some support for common application level protocols (HTTP).
Netprog: Java Intro^4
Java notes for C++ programmers
Everything is an object.
No code outside of class definition!
Single inheritance
an additional kind of inheritance: interfaces
All classes are defined in .java files
one top level public class per file
To print to stdout:
System.out.println();
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First Program: Simp.java
public class Simp {
public static void main(String args[]) {
System.out.println("Hello, Netprog");
public class Simp {
public static void main(String args[]) {
System.out.println("Hello, Netprog");
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6
Compiling and Running
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Simp.java
javac Simp.java
java Simp Simp.class
compile run
bytecode
source code
Java bytecode and interpreter
bytecode is an intermediate representation of the program (class).
The Java interpreter starts up a new “Virtual Machine”.
The VM starts executing the users class by running it’s main() method.
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Java Data Types
Primitive Data Types:
boolean true or false
char unicode! (16 bits)
byte signed 8 bit integer
short signed 16 bit integer
int signed 32 bit integer
long signed 64 bit integer
float , double IEEE 754 floating point
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not an int!
Other Data Types
Reference types (composite)
classes
arrays
strings are supported by a built-in class named String
string literals are supported by the language (as a special case).
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Classes and Objects
“All Java statements appear within
methods, and all methods are defined
within classes”.
Java classes are very similar to C++
classes (same concepts).
Instead of a “standard library”, Java
provides a lot of Class implementations.
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Defining a Class
One top level public class per .java file.
typically end up with many .java files for a single program.
One (at least) has a static public main() method.
Class name must match the file name!
compiler/interpreter use class names to figure out what file name is.
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Packages
You can organize a bunch of classes into
a package.
defines a namespace that contains all the classes.
You need to use some java packages in
your programs
java.lang java.io, java.util
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Importing classes and packages
Instead of #include , you use import
You don’t have to import anything, but
then you need to know the complete name
(not just the class, the package).
if you import java.io.File you can use File objects.
If not – you need to use java.io.File objects.
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Exceptions
Terminology:
throw an exception : signal that some condition (possibly an error) has occurred.
catch an exception : deal with the error (or whatever).
In Java, exception handling is necessary
(forced by the compiler)!
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Try/Catch/Finally
try {
// some code that can throw
// an exception
} catch (ExceptionType1 e1) { // code to handle the exception
} catch (ExceptionType2 e2) {
// code to handle the exception
} finally {
// code to run after the stuff in try
// can handle other exception types } Netprog: Java Intro^22
Exceptions and Network Programming
Exceptions take care of handling errors
instead of returning an error, some method calls will throw an exception.
A little hard to get used to, but forces the
programmer to be aware of what errors
can occur and to deal with them.
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Socket Programming
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Java Sockets Programming
The package java.net provides support for
sockets programming (and more).
Typically you import everything defined in
this package with:
import java.net.;*
Classes
InetAddress
Socket
ServerSocket
DatagramSocket
DatagramPacket
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java.net.InetAddress class
static methods you can use to create new
InetAddress objects. public static InetAdress getByName(String host) public static InetAdress getLocalHost() public static InetAdress[] getAllByName(String hostName)
InetAddress x = InetAddress.getByName(
“cse.sc.edu”);
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Sample Code: Lookup.java
Uses InetAddress class to lookup
hostnames found on command line.
¾ java Lookup www.yahoo.com
www.yahoo.com:209.191.93.
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try {
InetAddress a = InetAddress.getByName(hostname);
System.out.println(hostname + ":" + a.getHostAddress());
} catch (UnknownHostException e) {
System.out.println("No address found for " + hostname);
}
try {
InetAddress a = InetAddress.getByName(hostname);
System.out.println(hostname + ":" + a.getHostAddress());
} catch (UnknownHostException e) {
System.out.println("No address found for " + hostname);
}
Sample Code: getLocalhost.java
Uses InetAddress class to lookup
localhost
¾ java getLocalhost
broad/129.252.130.
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Socket Methods
void close();
InetAddress getInetAddress(); getpeername
InetAddress getLocalAddress(); getsockname
InputStream getInputStream();
OutputStream getOutputStream();
Lots more (setting/getting socket options,
partial close, etc.)
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Socket I/O
Socket I/O is based on the Java I/O
support (in the package java.io ).
InputStream and OutputStream are
abstract classes
common operations defined for all kinds of InputStreams, OutputStreams…
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InputStream Basics
// reads some number of bytes and
// puts in buffer array b
int read(byte[] b);
// reads up to len bytes
int read(byte[] b, int off, int len);
Both methods can throw IOException.
Both return –1 on EOF.
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OutputStream Basics
// writes b.length bytes
void write(byte[] b);
// writes len bytes starting
// at offset off void write(byte[] b, int off, int len);
Both methods can throw IOException.
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Output stream example
2007 CSCE515 – Computer Network Programming
InetAddress myServerAddr; Socket MyClient;
DataOutputStream output;
try { … output = new DataOutputStream(MyClient.getOutputStream()); output.writeBytes(“hello”); output.writeBytes("DATA\n"); } catch (IOException e) { System.out.println(e); }
InetAddress myServerAddr; Socket MyClient;
DataOutputStream output;
try { … output = new DataOutputStream(MyClient.getOutputStream()); output.writeBytes(“hello”); output.writeBytes("DATA\n"); } catch (IOException e) { System.out.println(e); }
Sample Client
smtpClient.java
Simple client code to send an email via an smtp server
Client:
Open a socket.
Open an input and output stream to the socket.
Read from and write to the socket according to the server's protocol.
Clean up.
2007 CSCE515 – Computer Network Programming
ServerSocket Class –Servers
(TCP Passive Socket)
Constructors:
ServerSocket(int port);
ServerSocket(int port, int backlog);
ServerSocket(int port, int backlog,
InetAddress bindAddr);
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ServerSocket Constructors
ServerSocket(int port, int backlog);
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ServerSocket MyService;
try { MyService = new ServerSocket(PortNumber, 90);
} catch (IOException e) { System.out.println(e); }
ServerSocket MyService;
try { MyService = new ServerSocket(PortNumber, 90);
} catch (IOException e) { System.out.println(e); }
ServerSocket Methods
Socket accept();
void close();
InetAddress getInetAddress();
int getLocalPort();
throw IOException, SecurityException
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Sample Echo Server
TCPEchoServer.java
Server:
1. Create a ServerSocket
2. accept incoming request, get a Socket
3. Open an input and output stream to the Socket.
4. Read from and write to the Socket according to the
server's protocol.
5. Close the Socket
6. Return to step 2
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UDP Sockets
DatagramSocket class
DatagramPacket class needed to specify
the payload (incoming or outgoing).
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DatagramSocket Constructors
DatagramSocket();
DatagramSocket(int port);
DatagramSocket(int port, InetAddress a);
All can throw SocketException or SecurityException.
When should each constructors be used?
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Sample UDP code
UDPEchoServer.java
Simple UDP Echo server.
Test using nc as the client (netcat):
> nc –u hostname port
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Multicast
2007
CSCE515 – Computer Network Programming
Multicasting
Unicast
A flow from one source to one destination
IP packets contain destination IP address
Broadcast
A flow from one source to all destinations
IP packets contain broadcast address 255.255.255.
Multicast
A flow from one source transmits to a Group of destinations
IP packets contain a class D address for destination
Ranges from 224.0.0.0 to 239.255.255.255 (256K addresses)
2007 CSCE515 – Computer Network Programming
Multicast vs. Multiple Unicast
2007 CSCE515 – Computer Network Programming
Advantages of Multicasting
Advantages:
Lower overhead at the source Source sends only one packet
Bandwidth is conserved on shared links Only one copy of each packet is sent on each link
Requirements:
Group address management – Network/router participation
Packet duplication at routing nodes
Disadvantages
Security
Business models
No Incentives for deployment
2007 CSCE515 – Computer Network Programming
Multicast Addressing
The set of hosts listening to a particular multicast address is called a host group
Multicast packets, at least for now, are sent only as UDP packets (WHY?)
Some of the addresses 224.0.0.0 to 224.0.0.15 are reserved for well-known groups 239.0.0.0 to 239.255.255.255 are for local/administratively scoped applications
Like any IP address, a multicast address can have a hostname, e.g. 224.0.0.2 -- all-routers.mcast.net (All routers on the local subnet)
2007 CSCE515 – Computer Network Programming
Class D 1 1 1 0 multicast group ID
Mapping to Ethernet Addresses
Ethernet has a 48-bit address field
It has its own multicast address range
01.00.5e.00.00.00 through 01.00.5e.7f.ff.ff
Lower order 23 bits can be used for multicast addresses
IP multicast address has 28 bits for specifying a group address
Thus, only the lower order 23 bits of IP multicast address are copied into the Ethernet address
2007 CSCE515 – Computer Network Programming
Link-Layer Multicast Addresses
Ranges from 01:00:5E:00:00:00 to 01:00:5E:FF:FF:FF
Map low-order 23 bits of class D address to lower order 23 bits of ethernet multicast address space
Upper 5 bits of multicast group ID are ignored in the mapping, thus mapping is not unique
For point-to-point links: no mapping needed.
2007 CSCE515 – Computer Network Programming
Multicast service model
Uses the notion of host groups
Sender sends to a group address
Any receiver who has joined this group gets this packets
Implications?
No limits on number or location of receivers
Best effort delivery (same as in unicast)
Dynamic membership
Host can join/leave at will (no synchronization required among group members)
Scope control
Can limit the distribution using TTL
2007 CSCE515 – Computer Network Programming
Components of the IP Multicast Architecture
2007 CSCE515 – Computer Network Programming
Multicast Routers
Biggest restriction on multicasting:
Availability of special multicast routers
Check whether your routers support
multicasting: wyxu@broad % ping all-routers.mcast.net all-routers.mcast.net is alive
2007 CSCE515 – Computer Network Programming
IGMP
Internet Group Management Protocol (IGMP)
Allows a router to know which of its directly connected
hosts belongs to which multicast group
IGMP is required to support TRPB, RPM, CBT and PIM
protocols
2007 CSCE515 – Computer Network Programming
73 2/17/
Class MulticastSocket
Extend class DatagramSocket and add
support for IP multicast
Multiple MulticastSockets can listen to
same port on same machine
Constructors
MulticastSocket()
MulticastSocket(int port)
MulticastSocket(SocketAddress
bindAddress)
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Class MulticastSocket
try {
MulticastSocket ms = new MulticastSocket(); //send some datagrams
} catch (SocketException e) {
System.out.println(e);
}
try {
MulticastSocket ms = new MulticastSocket(); //send some datagrams
} catch (SocketException e) {
System.out.println(e);
}
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Class MulticastSocket
Methods void joinGroup(InetAddress group) throws IOException void leaveGroup(InetAddress group) throws IOException void setTimeToLive(int ttl) throws IOException void setTTL(byte ttl) throws IOException int getTimeToLive() throws IOException byte getTTL() throws IOException void send(DatagramPacket packet, byte ttl) throws IOException void setInterface(InetAddress address) throws SocketException InetAddress getInterface() throws SocketException
Exceptions IOException SecurityException
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Sending Multicast Packets
// byte[] data // InetAddress multicastGroup // int multicastPort MulticastSocket socket = new MulticastSocket();
DatagramPacket packet = new DatagramPacket (data, data.length, multicastGroup, multicastPort);
socket.send(packet, (byte) 64);
socket.close();
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Receiving Multicast Packets
MulticastSocket socket = new MulticastSocket(multicastPort); Socket.joinGroup(multicastGroup);
byte buffer[] = new byte[65508]; DatagramPacket packet = new DatagramPacket();
socket.receive(packet);
InetAddress fromAddress = packet.getAddress(); int fromPort = packet.getPort(); int length = packet.getLength(); byte[] data = packet.getData(); // …
socket.leaveGroup(multicastGroup); socket.close();
Sample MulticastSocket code
MulticastSender.java
MulticastSniffer.java
Receiver:
Broad % java MulticastSniffer all- systems.mcast.net 4000
Sender:
Broad % java MulticastSender all- systems.mcast.net 4000
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79 2/17/
A Peer-to-Peer Multicast Chat System
Each client multicasts its message to other
clients
No server is involved; all clients
communicate as peers
Open a chat frame and start a thread that
listens for incoming packets
