Transcript cap2-2006

Programming TCP Clients
InetAddress Class
• An IP address identifies uniquely a host in the internet,
which consists of 4 numbers (1 byte each one) in the IPV4.
The InetAddress class is an object which store such type of
data and has some functions to get the host name from the
IP number or vice-versa
• It has no constructors, but an object factory. A new object
of this class is created with
– InetAddress n = InetAddress.getLocalHost()
– InetAddress n = InetAddress.getByName(nombre)
• Following methods can be applied:
– String nombre = n.getHostName()
– String direccion = n.getHostAddress()
• See InetExample.java & Names.java
Host Addressing in TCP/IP Networks
Network Address
Host Address
The URL class
• URL = Uniform Resource Locator
• It is used to connect to a web server and
donload the resources it offers
PURL
The URL
• A URL is a UNIFORM RESOURCE LOCATOR. It is a unique
address of a resource a web server has released in the Internet.
• A URL consists of 3 principal components:
– the protocol descriptor . The most used may be http which stands for
HyperText Tranfer Protocol but there is also a File Transfer protocol (ftp),
Gopher, File o News.
– The hostname of the maschine which “serves” the resource
–The name of the resource in that maschine
• With Java, it is possible to open an URL and read its content as
it were stored in a file. For that, it is necessary to create an URL
object with at least the 3 components named above.
The URL
Constructors for a URL object:
–URL aRUL = new URL(“http://www.arminco.com/index.html”);
–URL aURL = new URL(“http”;”www.arminco.com”,”index.html”);
–URL aURL = new URL(“http”,”www.arminco.com”,80,”index.html”);
• While creating a URL object an exception of the
MalformedURLException class may be generated. Because of
this, some measurements must be taken (use a try-catch)
try {
}
URL miURL = new URL(....);
catch(MalFormedURLException e) {
// code for reacting to the exception
}
• Methods available for objects of the :
getProtocol(), getHost(), getPort(), getFile(),
openConnection()
The URL
If we know beforehand that the content of a URL is text, we can
retrieve it with the following piece of program:
import java.net.*;
import java,io.*;
public class LeerURL {
public static void main(String args[]) {
try {
URL miURL = new URL(“http://www.dcc.uchile.cl”);
URLConnection c = miURL.openConnection();
BufferedReader in = new BufferedReader (
new InputStreamReader(c.getInputStream()));
String line;
while ((line = in.readLine() != null)
System.out.prinln(line);
c.close();
catch(MalFormedURLException e) { System.out.pritnln(e);}
}
}
The TCP Client’s Socket
• A socket is an abstraction representing one end in a communication link
between two programs.
• When an application wants to send/receive data to/from the net it has to open
one
• A socket is always bound to a port (although sometimes this will be not
evident for the programmer, especially if the program is a client one). The port
is the way an application identifies a certain socket with the TCP/IP layer
• A server which runs on a server is associated also opens a socket bound to a
certain port and start listening to requests from the clients whishing to
establish a communication.
• In order to establish a communication a client has to previously know 1- the
port number, 2- the host address. With this information the client tries a
rendezvous where the server is already running and listening.
TCP Client in Java (1)
• For trying a rendezvous in Java (with a TCP server, which could have
been written in any language) we must create an object of the Socket class.
Socket(String host, int port)
Socket csocket = new Socket(“hostname”,7);
• A host address can be given as an IP number or name:
dichato.dcc.uchile.cl or 192.24.80.40. In the first case Java will do the
DNS lookup first.
• The creation of a socket is a blocking statement. This means that the
execution of the program will block until the creation returns a socket
connected to the server or a null if it fails.
• A failure to create a socket may arise if there is no such host in the
internet, if the host is not currently reachable, of if there is no server
listening to that port on that host. There is a timeout (default) for this
instruction.
• A failure will throw a checked Exception. It is therefore necessary to
program the socket creation within a try-and-catch block
TCP Client in Java (2)
• If the socket is created then we can open an inputStream and an
ouputStream from that socket in order to read data from an write data
into the server.
PrintWriter out = new PrintWriter(csocket.getOutputStream(), true);
BufferedReader In = new BufferedReader(new
InputStreamReader(calling.getInputStream()));
• Both getInputStream & getOutputStream open byte oriented data
streams. Printwriter & BufferedReader are “filters” which convert
bytes into text (string with end-of-line marks) and vice-versa.
out.print(“hello”); out.println(“how are you ?”);
String linea = in.readLine();
• readLine is a blocking sentence. For using it we must be sure the server
will send an eol mark.
A Client for the date server
Protocol: The date server just waits until someone tries a
rendezvous and answers with the current date of the server.
This means the client must try rendezvous (1) and then read the
response of the server (2). Then the server breaks the
communication (3).
2
3
Date server
13
1
DateClient
Client
A Client for the echo server
Protocol: The echo server waits until someone makes a rendezvous. Then, it
reads “line-by-line” what the client sends and answers with the same. The
connection must be broken by the client. So the client must 1) try a rendezvous,
2) send a line, 3) read the answer. Then repeat 2 & 3 until the client user brakes
the dialogue
2
3
Echo server
7
1
EchoClient
Client
A finger client
The server waits for a request, gets a command line from the client, analyses it
and then sends a (variable) set of lines, according to the query and the status.
After that, the server breakes the communication. The client has to 1) try a
rendezvous, 2) send the query string, 3) read lines until the communication is
broken.
Finger [-l] [<username>]@host
3
Finger server
7
1
Finger
Client
A pop client for mail retrieving
This protocol is described in the rfc 1939 (internet protocol
descriptions)
See http://www.ietf.org/rfc.html
cliente_pop <host> <username> <password>
3
mail pop server
110
1
Cliente_pop
Client
A SMTP client for mail sending
We will provide a graphic interface for sending mails from a
local computer to an open STMP server (most of them are
open for computers inside a LAN. The protocol consists in a
dialogue for identifying sender, receiver, subjet and body of
the mail.
2
mail SMTP
server
25
1
ClienteSMTP
Client
A generic client in Java ?
import java.io.*;
import java.net.*;
public class Cliente {
public static void main(String[] args) throws IOException {
Socket echoSocket = null; PrintWriter out = null; BufferedReader in = null;
if (args.length != 2)
System.out.println(“Use: java GClient <host> <port> ”);
int nport = Integer.parseInt(args[1]);
try {
echoSocket = new Socket(args[0], nport);
out = new PrintWriter(echoSocket.getOutputStream(), true);
in = new BufferedReader(new
InputStreamReader(echoSocket.getInputStream()));
} catch (UnknownHostException e) {
System.err.println("Don't know about host: "+ args[0]);
System.exit(1);
} catch (IOException e) {
System.err.println("Couldn't get I/O for the connection to: "+args[0]);
System.exit(1);
}
<<Process protocol according to port number>>
}
}
The Sockets constructors
• Socket(String host, int port) The port must be in
the range 1-65,535
• Socket(Inet Address host, int port)The same but
with an InetAddress object as parameter
• Socket(String host, int port, String
localHost, int localport) Every TCP communication
consists of a local and remote host and a local and remote port.
This constructor allows us to specify all them. Specifying local
address makes only sense when local computer is multihomed
(more than one address). Null = default address. Sometimes it is
necessary to specify the local port (firewalls). 0 = the system will
assign a random port number from the still available. Numbers
from 1 to 1025 should not be used as they are reserved for “well
known services” like echo, telnet finger, ftp.
More Socket methods in Java
• InetAddress getInetAddress() returns the IP address of
•
•
•
•
•
the remote host to which the socket is connected
int getPort() returns the port number to which the socket at
the other extreme is bound
InetAddress getLocalAddress() returns the IP address of
the local host
int getLocalPort() returns the port number to to which the
socket is bound.
void setSoTimeout(int timeout)sets timeout in
milliseconds for a read operation on this socket. 0 = no timeout,
this can block the operation indefinitely. If the reading operation
is not completed in that time an InterruptedIOException is
thrown
int getSoTimeout() returns the timeout of the socket
More Socket methods in Java
• void setTcpNoDelay(boolean on) Disables/Enables using the
Nagel’s algorithm which makes TCP more efficient by delaying the
writing (sending) of small amounts of data until there is enough data to
send. This may introduce some unacceptable delays for some applications.
• boolean getTcpNoDelay() returns whether the Nagel’s algorithm is
working or not
• void setSoLinger(boolean on, int val) allows to set a linger
time-out (in milliseconds). Linger is the time the socket communication
remains “open” by the system after the program closes it. This will allow
to receive packages for confirmation which are still delayed and avoid the
using of the same port on the same machine for some 4 min.
• int getSoLinger () returns the current linger setting or –1 if not set.
•
•
•
•
void
int
void
int
setSendBufferSize(int size)
getSendBufferSize()
setReceiveBufferSize(int size)
getReceiveBufferSize()
Sockets.java
Socket originated Exceptions
• Many of the Socket constructors and methods throw an
exception. These instructions should be programmed inside a
try-and-catch block
• Most of the thrown exceptions are objects from a subclass of
the IOException class
–BindException: the requested local port or address could not be
used. Typically when the port is already used or it is a system port
or the local address is not a valid one.
–ConnectException: connection refused because there was no
server listening to that port on the remote host.
–NoRouteToHostException: remote host could not be reached
typically because of network problems or a firewall
• UnknownHostException: the given host address is not valid
(DNS Lookup filed)