Transcript Programming TCP Clients

Programming TCP Clients
Version 2008
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
methods to get the host name and IP number
• 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
• 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 machine which “serves” the resource
–The name of the resource in that machine
• 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 class
• URL = Uniform Resource Locator
• It is used to connect to a web server and
donload the resources it offers
The URL
• Constructors for a URL object:
URL aRUL = new URL(“http://www.arminco.com/index.html”);
URL aURL = new URL(“ftp”;”www.arminco.com”,”mails.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 class :
getProtocol(), getHost(), getPort(), getFile(), openConnection()
• See PURL.java for an example
PURL
The TCP Client’s Socket
• TCP socket: an abstraction representing one end in a
communication link between two programs.
• When an application wants to send/receive data to/from a TCP/IP
network it has to open one
• Socket is always bound to a port (sometimes not evident for the
programmer, of client application.
• Port identifies a socket inside a computer
• In order to establish a TCP/IP communication with a server, a
client has to previously know 1- the port number, 2- the host
address. With this information client creates socket to try a
rendezvous with the server program already running.
TCP Client in Java (1)
• For trying a rendezvous in Java client we must create an
object of the Socket class.
Socket(String host, int port)
Socket csocket = new Socket(“hostname”,7);
• Host address 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, or 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-andcatch block
TCP Client in Java (2)
• After the socket is created we can open an InputStream and an
OuputStream from that socket in order to read data from an write data
into the server.
• We will start with text-oriented information exchange (protocol).
• We will use a PrintWriter to send (print) information and a
BufferedReader to recieve (read) information
PrintWriter out = new PrintWriter(
csocket.getOutputStream(), true);
BufferedReader In = new BufferedReader(new
InputStreamReader(csocket.getInputStream()));
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. No data can be read anymore (the result would be
null).
2- read data
Date server
13
1- connect
Client
3 Examples
DateClientBasic : This examples shows a basic date client with
the basic instructions. Normally this only will work inside a local
area network and will crash when something goes wrong.
DateClinetProxy : has the necessary instructions to connect to
the outside world using a proxy socks server.
Client
application
Proxy socks
server
Server
applicaiton
LAN
DateClient : an application as it would be programmed for real
use: very flexible and checking everything
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- write data
3- read answer
Echo server
7
1- connect
EchoClient
Client
A pop client for mail retrieving
The client first connects to the server on port 110. Then the client
can “dialogue” with the server following the protocol described in
the rfc 1939 (internet protocol descriptions)
See http://www.ietf.org/rfc.html
Go
2- follow protocol
mail pop
server
110
1- Connect
PopClientW.java
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- follow protocol
mail SMTP
server
1- Connect
25
ClienteSMTP
Client
The Sockets constructors
• Socket(String host, int port) The port must be in
the range 1-65,535
• Socket(InetAddress host, int port)The same but
with an InetAddress object as parameter
• Socket(String host, int port, InetAddress
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). If null is given, default address is used.
Sometimes it is necessary to specify the local port (firewalls). If 0
is given the system will assign a random available port number.
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
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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.
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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 a checked
exception, mostly from a type extended from IOException. 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)