COP3252 Advanced Java Programming

1
COP3252Advanced Java Programming

• 19-Feb-08
• Lecture Set 11
• Networking

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Networking

• Fundamental networking capabilities come from
  package java.net.
  
• Offers
• Stream-based communications
• Allow applications to view networking as streams
  of data
  
• Packet-based communications
• Allows for transmission of individual packets of
  information
  
• Often used for streaming audio or video

3
Socket-based communications

• Allow an application to view networking as if it
  were just file I/O
  
• Programs can read from and write to a socket just
  like reading/writing a file
  
• Two types of sockets Stream and Datagram

4
Stream and Datagram sockets

• Stream sockets Connection-oriented
• Process establishes connection to another
  process
  
• While connection is in place, data flows in
  continuous stream
  
• Uses TCP (transmission control protocol)
• Datagram sockets Connectionless
• Individual packets of information are
  transmitted
  
• Does not guarantee delivery, or that packets
  arrive in correct order
  
• Uses UDP (user datagram protocol)
• Less overhead than TCP (thus, commonly used for
  steaming broadcast data like video or audio)
  

5
A simple server using Stream Sockets

• Requires 5 steps
• 1 Create a Server-Socket object
• ServerSocket s new ServerSocket(port,queuelngth
  )
  
• Specifies TCP port number to use and number of
  clients that can wait to connect to the server
  
• Port number is often called handshake point.
  Used by the client to locate the server at the ip
  address.
  
• The above constructor binds the server to the
  port. Only one application at a time can be
  bound to a specific port on the server.
  
• If queue is full, new client connections are
  denied.
  
• Port number can be between 0 and 65,535. Note
  that ports below 1024 are ephemeral ports
  (reserved by the operating system in most cases)
  and should not be used when creating your server
  application unless there is a good reason to do
  so.

6
A simple server using Stream Sockets

• 2 Tell server to listen for client connection
  attempts
  
• Socket connection s.accept()
• This causes the server to listen indefinitely
  (called blocking) for an attempt by a client to
  connect.
  
• After connection attempt on listening port, the
  connection is pushed off onto another port
  (allowing server to accept a new connection on
  the listening port)

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A simple server using Stream Sockets

• 3 Get the OutputStream and InputStream objects
  that enable the server to communication with the
  client
  
• At this point, we have two options byte
  oriented or character oriented network i/o
  streams.

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Byte-Oriented I/O Streaming

• ObjectInputStream input new ObjectInputStream
  (connection.getInputStream())
  
• ObjectOutputStream output new
  ObjectOutputStream
  
• (connection.getOutputStream())

9
Character Oriented I/O Streaming

• This looks and acts just like the file I/O that
  we talked about before
  
• BufferedReader is new BufferedReader(new
  InputStreamReader(connection.getInputStream()))
  
• PrintStream os new PrintStream(
  connection.getOutputStream())
  

10
A simple server using Stream Sockets

• 4 Processing phase
• Server and client communicate via the input and
  output streams
  
• Keep in mind that this is usually done according
  to a defined protocol
  
• 5 End
• When transmission is complete, server should
  close connection by invoking the close method on
  the streams and on the socket.
  
• This is extremely important! A multi-threaded
  network application server must close all streams
  and sockets in order to avoid locking up system
  resources.

11
Server example

• See ChatServer.java

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A simple client using Stream Sockets

• Clients are a 4 step process
• 1 Create socket to connect to the server
• Socket connection new Socket(serverAddress,port)
  
  
• Can throw an IOException
• Common exception is UnknownHostException (when
  client is unable to resolve server address to
  corresponding IP address).

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A simple client using Stream Sockets

• 2 Get the OutputStream and InputStream objects
  that enable the client to connunicate with the
  server
  
• This is done the same as when creating the server
  IO streams
  
• 3 Processing phase
• 4 End

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Client Example

• See ChatClient.java

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Class InetAddress

• Java Class that represents an Internet Protocol
  (IP) address
  
• Doesnt have any public constructors
• Pass the hostname or string format of the dotted
  quad address to the static InetAddress.getByName()
  method
  
• Contains many useful methods
• getLocalHost()
• Returns the local host name
• getHostName()
• Returns the hose name for this IP address
• getHostAddress()
• Determines the IP address of a host, given the
  hosts name
  
• See API for others
• http//java.sun.com/javase/6/docs/api/java/net/Ine
  tAddress.html

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InetAddress Example

• See InetAddressExample.java

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Connectionless Client/Server interaction with
Datagrams

• Uses classes DatagramSocket and DatagramPacket
• See DatagramSend.java and DatagramReceive.java

18
A simple socket port scanner (local host)

• This example shows you how to determine which
  ports are open and which are closed on the local
  machine
  
• See PortScanExample.java

19
Connecting to a REAL server

• Lets say we want to connect to an existing
  server.
  
• how do we communicate?
• All protocols are documented in RFCs (request
  for comments)
  

20
Various RFCs

• Daytime protocol (RFC-867) http//www.ietf.org/r
  fc/rfc0867.txt
  
• Time protocol (RFC-868) http//www.ietf.org/rfc/
  rfc0868.txt
  
• NTP protocol (RFC-1305) http//www.ietf.org/rfc/
  rfc1305.txt
  
• SMTP protocol (RFC-821) http//www.ietf.org/rfc/
  rfc0821.txt

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Communicating with a NIST Daytime Server

• NIST (National Institute of Standards and
  Technology) runs many time servers (daytime,
  time, NTP).
  
• The servers are publicly accessible.
• See http//www.time.gov/ and
  http//tf.nist.gov/tf-cgi/servers.cgi
  

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NIST Daytime

• The Daytime protocol is widely used by small
  computers running MS-DOS and similar operating
  systems.
  
• The server listens on port 13, and responds to
  requests in either tcp/ip or udp/ip formats.
  
• The standard does not specify an exact format for
  the Daytime Protocol, but requires that the time
  is sent using standard ASCII characters.
  
• NIST chose a time code format similar to the one
  used by its dial-up Automated Computer Time
  Service (ACTS)
  

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NIST Daytime Format

• JJJJJ YR-MO-DA HHMMSS TT L H msADV UTC(NIST)
  OTM
  
• JJJJ is Modified Julian Date (starting point of
  Nov. 17, 1858).
  
• YR-MO-DA is the date. Last two digits of year,
  month, and current day of month.
  
• HHMMSS is time in hours, minutes, seconds in
  UTC.
  
• Apply offset for local time (-5 for EST)
• TT is 00 to 99.
• 00 US is on Standard time.
• 50 US is on Daylight Savings Time.
• In month of DST change, number changes to days
  till change and will decrement every day until
  change occurs.
  
• L is code indicating leap second addition. (??)
• H is health of server.
• 0 healthy, 1 possible time error up to 5 sec,
  2 time error 5 sec, 3 server h/w or s/w
  failure, 4 maintenance mode
  
• msADV millisecond advance on time to compensate
  for network delays
  
• UTC(NIST) label to indicate where it came from
• OTM on time marker (asterisk). Means that time
  is current when asterisk is printed.
  

24
Simple DayTime example

• See DaytimeExample.java

25
Assignment 4

• Posted see assignments page