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2: Application Layer 1
Socket Programming
2: Application Layer 2
Socket-programming using TCP
Socket: a door between application process and end-
end-transport protocol (UCP or TCP)
TCP service: reliable transfer of bytes from one
process to another
process TCP with buffers, variables socket controlled by application developer controlled by operating system
host or
server
process TCP with buffers, variables socket controlled by application developer controlled by operating system
host or
server
internet
2: Application Layer 3
Socket programming with TCP
Client must contact server ❒ server process must first be running ❒ server must have created socket (door) that welcomes client’s contact Client contacts server by: ❒ creating client-local TCP socket ❒ specifying IP address, port number of server process ❒ When client creates socket: client TCP establishes connection to server TCP ❒ When contacted by client, server TCP creates new socket for server process to communicate with client ❍ allows server to talk with multiple clients ❍ source port numbers used to distinguish clients (more in Chap 3)
TCP provides reliable, in-order
transfer of bytes (“pipe”)
between client and server
application viewpoint
2: Application Layer 4
Stream jargon
❒ A stream is a sequence
of characters that flow
into or out of a process.
❒ An input stream is
attached to some input
source for the process,
eg, keyboard or socket.
❒ An output stream is
attached to an output
source, eg, monitor or
socket.
outToServer to network from network inFromServer inFromUser keyboard monitor
Process
clientSocket stream^ input stream^ input output stream socket^ TCP
Client
process
client TCP socket 2: Application Layer 5
Socket programming with TCP
Example client-server app:
1) client reads line from
standard input ( inFromUser
stream) , sends to server via
socket ( outToServer
stream)
2) server reads line from socket
3) server converts line to
uppercase, sends back to
client
4) client reads, prints modified
line from socket
( inFromServer stream)
outToServer to network from network inFromServer inFromUser keyboard monitor
Process
clientSocket stream^ input stream^ input output stream socket^ TCP
Client
process
client TCP socket 2: Application Layer 6
Client/server socket interaction: TCP
wait for incoming connection request connectionSocket welcomeSocket.accept() = create socket, port= x , for incoming request: welcomeSocket = ServerSocket() create socket, connect to hostid , port= x clientSocket Socket() = close connectionSocket read reply from clientSocket close clientSocket
Server (running on hostid ) Client
read request from^ send request using clientSocket connectionSocket write reply to connectionSocket
TCP
connection setup
2: Application Layer 7 Example: Java client (TCP) import java.io.; import java.net.; class TCPClient { public static void main(String argv[]) throws Exception { String sentence; String modifiedSentence; BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in)); Socket clientSocket = new Socket("hostname", 6789); DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream()); Create input stream Create client socket, connect to server Create output stream attached to socket 2: Application Layer 8 Example: Java client (TCP), cont. BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream())); sentence = inFromUser.readLine(); outToServer.writeBytes(sentence + '\n'); modifiedSentence = inFromServer.readLine(); System.out.println("FROM SERVER: " + modifiedSentence); clientSocket.close(); } } Create input stream attached to socket Send line to server Read line from server 2: Application Layer 9 Example: Java server (TCP) import java.io.; import java.net.; class TCPServer { public static void main(String argv[]) throws Exception { String clientSentence; String capitalizedSentence; ServerSocket welcomeSocket = new ServerSocket(6789); while(true) { Socket connectionSocket = welcomeSocket.accept(); BufferedReader inFromClient = new BufferedReader(new InputStreamReader(connectionSocket.getInputStream())); Create welcoming socket at port 6789 Wait, on welcoming socket for contact by client Create input stream, attached to socket 2: Application Layer 10 Example: Java server (TCP), cont DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream()); clientSentence = inFromClient.readLine(); capitalizedSentence = clientSentence.toUpperCase() + '\n'; outToClient.writeBytes(capitalizedSentence); } } } Read in line from socket Create output stream, attached to socket Write out line to socket End of while loop, loop back and wait for another client connection 2: Application Layer 11 Socket programming with UDP
UDP: no “connection” between
client and server
❒ no handshaking
❒ sender explicitly attaches
IP address and port of
destination to each packet
❒ server must extract IP
address, port of sender
from received packet
UDP: transmitted data may
be received out of order,
or lost
application viewpoint
UDP provides unreliable transfer
of groups of bytes (“datagrams”)
between client and server
2: Application Layer 12 Client/server socket interaction: UDP close clientSocket Server (running on hostid ) read reply from clientSocket create socket, clientSocket = DatagramSocket() Client Create, address ( send datagram request hostid, port=x, using clientSocket create socket, port= x , for incoming request: serverSocket = DatagramSocket() read request from serverSocket write reply to serverSocket specifying client host address, port number
