Download java.net.Socket

Programming application protocols
Goal: learn how to build client/server application that
communicate using sockets without understanding underlying
functions of TCP/IP
 Many possible programming interfaces
• Socket APIs (most common)
– BSD C Socket API (most common)
– Java socket API
– Python socket API
• Other APIs
– Client-side Java URLconnections
– Server-side Java servlets
– Python urllib
– Python HTTPServer
– RPC, CORBA, Java RMI (not covered)
2: Application Layer
1
Socket programming
Socket API
r introduced in BSD4.1
UNIX, 1981
r client/server paradigm
r two types of transport
service via socket API:
 unreliable datagram
 reliable, byte
stream-oriented
socket
a host-local,
application-created,
OS-controlled interface (a
“door”) into which
application process can
both send and
receive messages to/from
another application
process
2: Application Layer
2
Socket-programming using TCP
Socket: a door between application process and endend-transport protocol (UCP or TCP)
TCP service: reliable transfer of bytes from one
process to another
controlled by
application
developer
controlled by
operating
system
process
process
socket
TCP with
buffers,
variables
host or
server
internet
socket
TCP with
buffers,
variables
controlled by
application
developer
controlled by
operating
system
host or
server
2: Application Layer
3
Socket programming using TCP
Server setup
r server process must first
be running
r server must have created
a “listening” socket (door)
that welcomes client’s
contact
Client contacts server by:
r creating client-local TCP
socket
r specifying IP address, port
number of server process
r When client connects a
socket: client TCP
establishes connection to
server TCP
r


When client connects, server
TCP creates new socket for it to
communicate with client
different than listening socket
(allows server to talk with
multiple clients)
source IP and port numbers used
to distinguish b/w clients (more
in Chap 3)
application viewpoint
TCP provides reliable, in-order
transfer of bytes (“pipe”)
between client and server
2: Application Layer
4
Addressing processes w/ port numbers
r
For a process to receive
messages, it must have a
unique identifier
r
Each host has a unique32bit IP address
r
Q: Does the IP address of
the host on which the
process runs suffice for
identifying the process?

A: No, many processes can
be running on same host
r Identifier includes
both the IP address
and port numbers
associated with the
process on the host.
r Example port numbers:


HTTP server: 80
Mail server: 25
2: Application Layer
5
TCP sockets in action
*,SIP:80
*,SIP:80
CIP:1099,SIP:80
*,SIP:80
CIP:1099,SIP:80
CIP:1099,SIP:80
2: Application Layer
6
TCP sockets in action
CIP:1099,SIP:80
*,SIP:80
*,SIP:80
CIP:1099,SIP:80
CIP:1100,SIP:80
CIP:1099,SIP:80
CIP:1100,SIP:80
CIP:1099,SIP:80
CIP:1100,SIP:80
2: Application Layer
7
BSD sockets in C/Unix
r Socket API (socket.h)
• socket(): create unnamed socket (data structure)
– UDP (SOCK_DGRAM), TCP (SOCK_STREAM)
– IP (SOCK_RAW)
• bind(): name socket (bind local source port to socket)
• listen(): enable socket to accept connections
• accept(): get connect() request from listen queue, allocate
file descriptor for new socket
• connect(): initiate connection on a socket (TCP handshake)
• send(), sendto(), sendmsg(), writev(), write(): send data
• recv(), recvfrom(), recvmsg(), readv(), read(): receive data
• setsockopt(), getsockopt(): set socket options (such as
buffer sizes, flag fields)
• close(), shutdown(): teardown connection
2: Application Layer
8
BSD TCP sockets in action
client
server
socket()
socket()
bind()
connect()
listen()
accept()
write()
read()
write()
read()
2: Application Layer
9
Simple C socket example
r http://www.thefengs.com/wuchang/cou
rses/cs594/socket_example
r TCP socket code

Client
• tcpcli.c

Server
• tcpserv.c
2: Application Layer 10
Java network programming
Java network applications
java.net package
System-dependent
implementations
2: Application Layer 11
Java installation on linuxlab
r J2SE

javac
• java compiler

java
• java interpreter

http://docs.oracle.com/javase/8/
2: Application Layer 12
java.net classes
r Low-level networking classes

Sockets and Packets

java.net.Socket

java.net.ServerSocket

java.net.DatagramSocket

java.net.DatagramPacket

java.net.InetAddress
r High-level URL networking classes

java.net.URL

java.net.URLConnection
• java.net.HttpURLConnection

java.net.URLencoder
2: Application Layer 13
java.net.Socket
r Constructors

Socket(InetAddress, int)

Socket(String, int)

Socket(InetAddress, int, InetAddress, int)
r Some methods

getInputStream()

getOutputStream()

getInetAddress()

getPort()

getLocalAddress()

getLocalPort()

get/set individual socket options
2: Application Layer 14
java.net.ServerSocket
r Constructors

ServerSocket(int)

ServerSocket(int, int) // port, backlog specified

// port specified
ServerSocket(int, int, InetAddress) // port, backlog, IP
address
r Some methods

accept()

getInetAddress()

getLocalPort()
2: Application Layer 15
Stream jargon for Java
network programming
r A stream is a sequence of characters that flow
into or out of a process.
r An input stream is attached to some input source
for the process, e.g., keyboard or socket.
r An output stream is attached to an output source,
e.g., monitor or socket.
2: Application Layer 16
Socket programming with TCP
Example client-server app:
1) client reads line from
standard input (inFromUser
stream) , sends to server via
socket (outToServer
stream)
Client
process
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)
client TCP
socket
2: Application Layer 17
Client/server socket interaction: TCP
Server (running on hostid)
Client
create socket,
port=x, for
incoming request:
welcomeSocket =
ServerSocket()
TCP
wait for incoming
connection request connection
connectionSocket =
welcomeSocket.accept()
read request from
connectionSocket
write reply to
connectionSocket
close
connectionSocket
setup
create socket,
connect to hostid, port=x
clientSocket =
Socket()
send request using
clientSocket
read reply from
clientSocket
close
clientSocket
2: Application Layer 18
Example: Java client (TCP)
import java.io.*;
import java.net.*;
class TCPClient {
public static void main(String argv[]) throws Exception
{
String sentence;
String modifiedSentence;
Create
input stream
Create
client socket,
connect to server
Create
output stream
attached to socket
BufferedReader inFromUser =
new BufferedReader(new InputStreamReader(System.in));
Socket clientSocket = new Socket("hostname", 6789);
DataOutputStream outToServer =
new DataOutputStream(clientSocket.getOutputStream());
2: Application Layer 19
Example: Java client (TCP), cont.
BufferedReader inFromServer =
new BufferedReader(new
InputStreamReader(clientSocket.getInputStream()));
Create
input stream
attached to socket
sentence = inFromUser.readLine();
Send line
to server
outToServer.writeBytes(sentence + '\n');
Read line
from server
modifiedSentence = inFromServer.readLine();
System.out.println("FROM SERVER: " + modifiedSentence);
clientSocket.close();
}
}
2: Application Layer 20
Example: Java server (TCP)
import java.io.*;
import java.net.*;
class TCPServer {
Create
welcoming socket
at port 6789
Wait, on welcoming
socket for contact
by client
Create input
stream, attached
to socket
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()));
2: Application Layer 21
Example: Java server (TCP), cont
Create output
stream, attached
to socket
DataOutputStream outToClient =
new DataOutputStream(connectionSocket.getOutputStream());
Read in line
from socket
clientSentence = inFromClient.readLine();
capitalizedSentence = clientSentence.toUpperCase() + '\n';
Write out line
to socket
outToClient.writeBytes(capitalizedSentence);
}
}
}
End of while loop,
loop back and wait for
another client connection
2: Application Layer 22
Socket programming using UDP
UDP: no “connection” between
client and server
r
no handshaking
r
sender explicitly attaches
IP address and port of
destination to each packet
r
server must extract IP
address, port of sender
from received packet
application viewpoint
UDP provides unreliable transfer
of groups of bytes (“datagrams”)
between client and server
UDP: transmitted data may be
received out of order, or
lost
2: Application Layer 23
BSD UDP sockets in action
client
server
socket()
socket()
bind()
bind()
sendto()
recvfrom()
sendto()
recvfrom()
2: Application Layer 24
BSD example
r http://www.thefengs.com/wuchang/cou
rses/cs594/socket_example
r UDP socket code

Client
• udpcli.c

Server
• udpserv.c
2: Application Layer 25
java.net.DatagramSocket
r Constructors

DatagramSocket()

DatagramSocket(int) // bind to specific port

DatagramSocket(int, InetAddress) // specify local
address
r Some methods

getLocalAddress()

getLocalPort()

receive(DatagramPacket)

send(DatagramPacket)

get/set individual socket options
2: Application Layer 26
java.net.DatagramPacket
r Constructors
 DatagramPacket(byte[], int) // receiving packets
 DatagramPacket(byte[], int, InetAddress, int) // sending
packets
r Some methods
 getAddress() // remote address
 getPort() // remote port
 getLength() // get packet length
 getData() // return data received or to be sent
 setAddress(InetAddress) // set remote address
 setData(byte[]) // set packet data
 setLength(int) // set packet length
 setPort(int) // set remote port
2: Application Layer 27
Client/server socket interaction: UDP
Server (running on hostid)
create socket,
port=x, for
incoming request:
serverSocket =
DatagramSocket()
read request from
serverSocket
write reply to
serverSocket
specifying client
host address,
port number
Client
create
socket,
clientSocket
=
DatagramSocket()
Create, address (hostid, port=x,
send datagram request
using clientSocket
read reply from
clientSocket
close
clientSocket
2: Application Layer 28
Example: Java client (UDP)
Client
process
Input: receives
packet (recall that
TCP received “byte
stream”)
Output: sends
packet (recall
that TCP sent
“byte stream”)
client UDP
socket
2: Application Layer 29
Example: Java client (UDP)
import java.io.*;
import java.net.*;
Create
input stream
class UDPClient {
public static void main(String args[]) throws Exception
{
Create
client socket
Translate
hostname to IP
address using DNS
BufferedReader inFromUser =
new BufferedReader(new InputStreamReader(System.in));
DatagramSocket clientSocket = new DatagramSocket();
InetAddress IPAddress = InetAddress.getByName("hostname");
byte[] sendData = new byte[1024];
byte[] receiveData = new byte[1024];
String sentence = inFromUser.readLine();
sendData = sentence.getBytes();
2: Application Layer 30
Example: Java client (UDP), cont.
Create datagram with
DatagramPacket sendPacket =
data-to-send,
new DatagramPacket(sendData, sendData.length, IPAddress, 9876);
length, IP addr, port
Send datagram clientSocket.send(sendPacket);
to server
DatagramPacket receivePacket =
new DatagramPacket(receiveData, receiveData.length);
Read datagram
from server
clientSocket.receive(receivePacket);
String modifiedSentence =
new String(receivePacket.getData());
System.out.println("FROM SERVER:" + modifiedSentence);
clientSocket.close();
}
}
2: Application Layer 31
Example: Java server (UDP)
import java.io.*;
import java.net.*;
Create
datagram socket
at port 9876
class UDPServer {
public static void main(String args[]) throws Exception
{
DatagramSocket serverSocket = new DatagramSocket(9876);
byte[] receiveData = new byte[1024];
byte[] sendData = new byte[1024];
while(true)
{
Create space for
received datagram
Receive
datagram
DatagramPacket receivePacket =
new DatagramPacket(receiveData, receiveData.length);
serverSocket.receive(receivePacket);
2: Application Layer 32
Example: Java server (UDP), cont
String sentence = new String(receivePacket.getData());
Get IP addr
port #, of
sender
InetAddress IPAddress = receivePacket.getAddress();
int port = receivePacket.getPort();
String capitalizedSentence = sentence.toUpperCase();
sendData = capitalizedSentence.getBytes();
Create datagram
to send to client
DatagramPacket sendPacket =
new DatagramPacket(sendData, sendData.length, IPAddress,
port);
Write out
datagram
to socket
serverSocket.send(sendPacket);
}
}
}
End of while loop,
loop back and wait for
another datagram
2: Application Layer 33
High-level Java networking
classes
r Socket/Packet
Low level building blocks
 Must implement all application-level logic

r Many protocols based on URLs and/or
tunneled in HTTP
Program at a higher-level to hide underlying
protocol details
 Do not re-implement HTTP, URL parsing, MIME
handling for each application

2: Application Layer 34
High-level client-side Java
networking classes
r java.net.URL

Represent a URL object
r java.net.URLConnection

Represent a connection to a URL which can be
read and written from
r java.net.HttpURLConnection

Subclass of URLConnection for http:// URLs
2: Application Layer 35
Java high-level client-side
networking classes example
r http://www.thefengs.com/wuchang/courses
/cs594/java_example
import java.net.*;
import java.io.*;
public class Test {
public static void main(String argv[]) {
try {
URL u=new URL(http://www.google.com/);
URLConnection uc=u.openConnection();
Object o = (Object) uc.getContent();
} catch (Exception e) {
}
}
2: Application Layer 36
High-level server-side Java
networking classes
r Servlets
 Dynamically generate content
 Implement common protocol header logic
• Example http servlets
– Cookies
– Content-type
– Content-length
r Servlet classes
 javax.servlet.Servlet javax.servlet.HttpServlet
• init()
• service()
• destroy()


javax.servlet.ServletRequest
javax.servlet.ServletResponse
javax.servlet.HttpServletRequest
2: Application Layer 37
Python network programming
Python network applications
Python network packages
(socket, URLlib, HTTPServer)
System-dependent
implementations
2: Application Layer 38
Python network programming
r Python
Scripting language
 No compilation required
 Language reference: http://www.python.org
 Provides APIs similar to Java

• socket
– Low-level socket interface
• urllib
– HTTP client
• SimpleHTTPServer
– HTTP server
2: Application Layer 39
Python sockets
r Similar to C and Java

Client
import socket
host = “localhost”
port = 7
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((host,port))
s.send(“some data to echo”)
print s.recv(20)
s.close
2: Application Layer 40
Python sockets
r Similar to C and Java

Server
import socket
host = “”
port = 7
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind((host,port))
s.listen(1)
while (1):
conn, addr = s.accept()
data = conn.recv(20)
conn.send(data)
conn.close()
2: Application Layer 41
Python urllib/urllib2
r Client-side HTTP code similar to Java’s
java.net.URLConnection

Hides socket creation, HTTP request
formatting, HTTP response parsing
Python urllib
Python urllib2
url = sys.argv[1]
sock = urllib.urlopen(url)
htmlSource = sock.read()
sock.close()
print htmlSource
url = sys.argv[1]
txdata = None
txheaders = {
‘Accept-Language’: ‘en-us’
}
req = urllib2.Request(url, txdata, txheaders)
u = urlib2.urlopen(req)
headers = u.info()
print u.read()
2: Application Layer 42
Python BasicHTTPServer
r Server-side HTTP processing libraries
similar to javax.servlet.HTTPServlet
Hides socket creation, HTTP request parsing,
HTTP response formatting
 SocketServer.TCPServer.BaseHTTPServer

2: Application Layer 43
Python network programming
URLs
r http://docs.python.org/lib/internet.html
r http://docs.python.org/lib/module-socket.html
r http://docs.python.org/lib/module-urllib.html
r http://docs.python.org/lib/module-urllib2.html
r http://docs.python.org/lib/module-
SimpleHTTPServer.html
r http://docs.python.org/lib/module-
BaseHTTPServer.html
r http://www.w3journal.com/6/s3.vanrossum.html
2: Application Layer 44
Programming your projects
r Multiple clients per server
r Two options
Single threaded server that manages all client
connections simultaneously
 Multi-threaded server with one thread per
client connection

2: Application Layer 45
Source code examples
r http://www.thefengs.com/wuchang/courses
/cs594/select_example

Manage I/O across multiple file descriptors
simultaneously
• Can be blocking (wait until any FD has event)
• Can be non-blocking (return immediately with status
of which FDs need service, if any)

May use poll() instead of select() as well
• Provides the same functionality with a different
interface (fd_set bit mask versus pollfd structures)
• Some systems map them to one another
– System V => select() is a wrapper to poll()
– BSD => poll() is a wrapper to select()
2: Application Layer 46