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Transmission Control Protocol (TCP)
TCP provides a connection-oriented, reliable, byte stream
service.
TCP vs UDP
- TCP provides automatic error control
- The IP layer gives no guarantee that datagrams will be
delivered properly; it is up to TCP to timeout and
retransmit as needed.
- TCP provides reliability of service; datagram packets that
were received out of sequence will be reassembled in the
right order to form the stream of bytes.
- TCP is used by applications such as Telnet, Rlogin, FTP,
and SMTP.
TCP Service Model
- TCP service is obtained by creating end points called
sockets. Each socket consists of a socket number and
port number.
- A single daemon (called the inetd in Unix) waits on
multiple ports for an incoming connection.
IP HEADER
(20 Bytes)
TCP HEADER
(20 Bytes)
TCP Data
IP Datagram
TCP Segment
- Some well-known ports are 21(FTP), 23(telnet),
25(SMTP), 79(finger), and 80(HTTP).
- TCP connections are full-duplex and point-to-point.
- TCP connection is byte stream not message stream.
- TCP segment size is restricted by the network’s Maximum
Transfer Unit(MTU), generally 1500 bytes.
- Uses the sliding window protocol.
The TCP Header
- a port + IP address forms a unique 48-bit end-point
- Sequence number is used to mark the first byte; the
acknowledgement number is the next byte that is
expected. Note that each byte is marked with sequence
number.
- Header Length tells how many 32-bit words are in the
header.
- Following the Header Length is an unused 6 bit field
- URG is the urgent pointer (set to 1 if used). Indicates a
byte offset from the current sequence number at which
urgent data are to be found. Typically not used.
Source Port Destination Port
Sequence Number
Acknowledgement Number
Header Length
U R G
A C K
P S H
P S T
S Y N
F I N Window Size
Checksum Urgent Pointer
Options (or or more 32 bit words)
Options (or or more 32 bit words)
Options (or or more 32 bit words
The java.net.Socket class
Constructor methods:
protected Socket( ) – creates an unconnected socket
Socket ( InetAddress address, int numPort ) throws
java.io.IOException, java.lang.SecurityException – creates a
socket connected to address in port numPort.
Socket ( InetAddress address, int numPort, InetAddress
localAddress, int localPort ) throws java.io.IOException,
java.lang.SecurityException – creates a socket connected to
address in port numPort and is bound to the specified local
address in the local port.
Socket ( String hostName, int numPort ) throws
java.net.UnknownHostException, java.io.IOException,
java.lang.SecurityException – creates a socket connected to a
specified hostname and port.
Socket ( String hostName, int numPort, InetAddress
localAddress, int localPort ) throws
java.net.UnknownHostException, java.io.IOException,
java.lang.SecurityException – creates a socket connected to a
specified hostName in port numPort and is bound to the specified
local address in the local port.
Note: When a socket is instantiated, a connection is automatically
established. Although, at times, the constructor method may block
waiting for the connection to get through.
Socket class methods:
void close( ) throws java.io.IOException – closes the socket
connection.
InetAddress getInetAddress( )
InetAddress getLocalAddress( )
- returns the remote (local) address that is connected to (used
by) the socket.
InputStream getInputStream( ) throws java.io.IOException
OutputStream getOutputStream( ) throws
java.io.IOException
- returns an input (output) stream, which reads from (writes to)
the application the socket is connected to.
int getLocalPort( )
int getPort( )
- returns the port number that the socket is bound (connected) to
in the local (remote) machine.
int getReceiveBufferSize( ) throws java.net.SocketException
void setReceiveBufferSize(int size ) throws
java.net.SocketException
- returns (sets) the receive buffer size used by the socket. It is
advisable to use the BufferedInputStream or the BufferedReader
to buffer incoming TCP data.
int getSendBufferSize( ) throws java.net.SocketException
void setSendBufferSize(int size ) throws
java.net.SocketException
- returns (sets) the send buffer size used by the socket.
A Simple TCP Client Example
package NetProg; import java.net.; import java.io.; public class TCPClient { public static void main(String [] args) { InputStreamReader in = new InputStreamReader(System.in); BufferedReader bufR = new BufferedReader(in); String message = ""; while (true) { try { //create a socket and connect Socket socket = new Socket(“205.174.40.142”,4400);
System.out.print("Enter a MESSAGE: "); message = bufR.readLine();
socket.setSendBufferSize(message.length());
//connect to a buffered reader BufferedOutputStream bos = new BufferedOutputStream(socket.getOutputStream(), socket.getSendBufferSize()); for (int i=0; i<message.length(); i++){ bos.write((int) message.charAt(i));}
bos.close();
if (message.equals("bye")){ System.out.println("Closing.... goodbye!"); break; }
socket.close(); }//end of try catch (Exception e) { System.out.println("Error " + e.getMessage()); } }//end of while }//end of main }//end of class definition
A Simple TCP Server Example
package NetProg; import java.net.; import java.io.; public class TCPServer { public static void main(String [ ] args) { String message; try { //create a socket and connect ServerSocket server=new ServerSocket(4400); while (true){ Socket socket = server.accept(); //connect to a buffered reader BufferedInputStream binst = new BufferedInputStream(socket.getInputStream(), socket.getReceiveBufferSize()); System.out.print("\n RECEIVED MESSAGE: "); message = ""; for (int i=0;i<socket.getReceiveBufferSize();i++) { int data = binst.read(); if (data == -1) break; else{ System.out.print ((char) data) ; message += (char) (data); } }//end of for message = message.trim(); socket.close(); if (message.equals("bye")){ System.out.println("\n Closing ... goodbye!"); break; }//end of if }//end of while }//end of try catch (Exception e) { System.out.println("Error " + e.getMessage()); }// end of catch }//end of main }//end of class definition
