Sockets For Clients

Instructors: Fu-Chiung Cheng

(鄭福炯 )Associate Professor

Computer Science & EngineeringTatung University

Contents

• The Socket class

• Telnet and Telnet protocols

• Socket information

• Socket Options

• InetAddress class

Socket

• A socket is a connection between two hosts• Socket: client and server socket

– Socket class (client) and ServerSocket (server)

• Client socket operations:– connect to a remote machine (a.k.a. server)– send data– receive data– close a connection

Server Socket

• Server Socket operations:– connect to a remote machine (a.k.a. server)– send data– receive data– close a connection– bind to a port (register the port to host) – listen for incoming data– accept connections from remote machines on the bound part

Telnet and Telnet Protocols

• Telnet command

– %telnet localhost 25 or telnet 140.129.20.87 25

• Port 25 provides SMTP (Simple Mail Transfer Protocol) service.

• Protocol: Set of rules and conventions used by communicating participants

• check page 303 for the SMTP protocol – Type: HELO command

– Reponse: 250 gamma. Hello alpha.cse.ttu.edu.tw [140.129.20.247], pleased to meet you

Telnet and Telnet Protocols

– Type HELP …

214-Commands:214- HELO MAIL RCPT DATA RSET214- NOOP QUIT HELP VRFY EXPN214-For more info use "HELP <topic>".214-smtp214-To report bugs in the implementation contact Sun

Microsystems214-Technical Support.214-For local information contact postmaster at this site.214 End of HELP info

Socket Class

• Client-side TCP operations• URL, URLConnection, Applet and JEditorPane use Socket for client

communication.• Constructors: (page 307-310)

– Socket public Socket(InetAddress address,int port)– public Socket(String host, int port, InetAddress localAddr, int localPort)– public Socket(InetAddress address, int port, InetAddress localAddr, int localPort)

Socket Class

• Socket theSocket = new Socket("140.129.20.87", 13);– Connect to host "140.129.20.87“ and – use port 13 service

• InetAddress fddi = new InetAddress(“fddisunsie.oit.unc.edu”); Socket theSocket = new Socket(www.oreilly.com, 80, fddi, 0)

– Use fddi network interface– Local port 1024~65535

Socket Information

• public InetAddress getInetAddress()– Returns the address to which the socket is

connected. – Returns: the remote IP address to which this

socket is connected.

• public InetAddress getLocalAddress() – Gets the local address to which the socket is

bound.– Returns:the local address to which the socket is

bound.

Socket Information

• public int getPort() – Returns the remote port to which this socket is

connected.– Returns:the remote port number to which this

socket is connected

• public int getLocalPort() – Returns the local port to which this socket is

bound.– Returns:the local port number to which this socket

is connected.

Socket Information

• Example 10-3 (SocketInfo) on page 313• Check www.ttu.edu.tw and 140.129.20.83• Outputs:

– Connected to 140.129.20.83/140.129.20.83 on port 80 from port 1118 of JAVA.ttu.edu.tw/140.129.20.89

– Connected to www.ttu.edu.tw/140.129.21.6 on port 80 from port 1122 of JAVA.ttu.edu.tw/140.129.20.89

Socket Information

• public InputStream getInputStream()– Get the input stream for this socket.– Returns:an input stream for reading bytes

from this socket.

• public OutputStream getOutputStream()– Get the output stream for this socket.– Returns:an output stream for writing bytes

to this socket.

Socket Information

• Example 10-4 (Daytime Protocol Client) on page 314

• Daytime protocol: Connect to 140.129.20.87 and port 13

• Use getInputStream to read the daytime • Outputs:

– It is Tue Mar 27 09:45:43 2001 at 140.129.20.87

Socket Information

• Example 10-4 (Daytime Protocol Client) on page 314

• Daytime protocol: Connect to 140.129.20.87 and port 13

• Use getInputStream to read the daytime • Outputs:

– It is Tue Mar 27 09:45:43 2001 at 140.129.20.87

• More complicate sample: Time protocol on page 317

Socket Information

• Example 10-6 (Echo Protocol Client) on page 321

• Echo protocol: Connect to 140.129.20.87 and port 7

• You may try telnet 140.129.20.87 7• Use getInputStream to read the echo string

from server • Use getOutputStream to write string to server

Closing the Socket

• It is very good practice to close sockets.• System may hit max number of sockets before GC

kicks in. (Especially for Browser app.)• public void close()

– Closes this socket.

• public void shutdownInput() – Places the input stream for this socket at "end of stream". – Any data sent to the input stream side of the socket is

acknowledged and then silently discarded. – If you read from a socket input stream after invoking

shutdownInput() on the socket, the stream will return EOF.

Closing the Socket

• public void shutdownOutput()– Disables the output stream for this socket. – For a TCP socket, any previously written

data will be sent followed by TCP's normal connection termination sequence.

– If you write to a socket output stream after invoking shutdownOutput() on the socket, the stream will throw an IOException.

Closing the Socket

• Example: PortScan on page 323• Code sample: close a socket

finally {

try {

if (connection != null) connection.close();

}

catch (IOException e) {}

}

Socket Options

• public void setTcpNoDelay(boolean on)– Enable/disable TCP_NODELAY

(disable/enable Nagle's algorithm).– Parameters:on - true to enable

TCP_NODELAY, false to disable

• public boolean getTcpNoDelay()– Tests if TCP_NODELAY is enabled.– Returns:a boolean indicating whether or

not TCP_NODELAY is enabled.

Socket Options

• Datagrams not yet been sent but Socket is closed• public void setSoLinger(boolean on, int linger)

– Enable/disable SO_LINGER with the specified linger time in seconds.

– The maximum timeout value is platform specific. – The setting only affects socket close.

• public int getSoLinger()– Returns setting for SO_LINGER. – -1 returns implies that the option is disabled.

• Example:– If (s.getSoLinger() == -1) s.setSoLinger(true, 240);

Socket Options

• public void setSoTimeout(int timeout)– Enable/disable SO_TIMEOUT with the specified

timeout, in milliseconds. – With this option set to a non-zero timeout, a read() call

on the InputStream associated with this Socket will block for only this amount of time.

– If the timeout expires, a java.io.InterruptedIOException is raised, though the Socket is still valid.

– The option must be enabled prior to entering the blocking operation to have effect.

– The timeout must be > 0. – A timeout of zero is interpreted as an infinite timeout.

Socket Options

• public int getSoTimeout()– Returns setting for SO_TIMEOUT. – 0 returns implies that the option is disabled

(i.e., timeout of infinity).

• Example:– If (s.getSoTimeout() == 0) s.setSoTimeout(180000);

Socket Options

• public void setSendBufferSize(int size)– Sets the SO_SNDBUF option to the specified value for

this Socket. – The SO_SNDBUF option is used by the platform's

networking code as a hint for the size to set the underlying network I/O buffers.

– Increasing buffer size can increase the performance of network I/O for high-volume connection, while decreasing it can help reduce the backlog of incoming data.

– Because SO_SNDBUF is a hint, applications that want to verify what size the buffers were set to should call getSendBufferSize()

Socket Options

• public int getSendBufferSize()– Get value of the SO_SNDBUF option for this

Socket, that is the buffer size used by the platform for output on this Socket

Socket Options

• public void setReceiveBufferSize(int size)– Sets the SO_RCVBUF option to the specified value for

this Socket. – The SO_RCVBUF option is used by the platform's

networking code as a hint for the size to set the underlying network I/O buffers.

– Increasing buffer size can increase the performance of network I/O for high-volume connection, while decreasing it can help reduce the backlog of incoming data.

– Because SO_RCVBUF is a hint, applications that want to verify what size the buffers were set to should call getReceiveBufferSize()

Socket Options

• public int getReceiveBufferSize()– Gets the value of the SO_RCVBUF option for

this Socket, that is the buffer size used by the platform for input on this Socket

Socket Options

• If SO_KEEPALIVE is turned on, then the client will occasionally send a data packet over an idle connection (once every two hours) just to make sure the server hasn’t crashed.

• public void setKeepAlive(boolean on)– Enable/disable SO_KEEPALIVE.– Parameters:on - whether or not to have socket keep

alive turned on.

• public boolean getKeepAlive()– Tests if SO_KEEPALIVE is enabled.

InetAddress Class

• public static InetAddress getByName(String host)– Determines the IP address of a host, given the host's

name. – The host name can either be a machine name, such as

"java.sun.com", or a string representing its IP address, such as "206.26.48.100".

• public static InetAddress[] getAllByName(String host)– Determines all the IP addresses of a host, given the host's

name. • public static InetAddress getLocalHost()

– Returns the local host.

InetAddress Class

• public InetAddress(String host)– Create a InetAddress of a host, given the host's

name. • public public byte[] getAddress()

– Returns the raw IP address of this InetAddress object. – The result is in network byte order: the highest order byte

of the address is in getAddress()[0].

• public String getHostAddress()– Returns the IP address string "%d.%d.%d.%d".

InetAddress Class

• public String getHostName() – Gets the host name for this IP address.

• public boolean isMulticastAddress() – Utility routine to check if the InetAddress is an IP

multicast address. – IP multicast address is a Class D address i.e first

four bits of the address are 1110.