CS212 Lecture 6
Networking
7/7/03


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0) Announcements
  - phase 2 of project should be up soon

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1)Networking Basics

 + COMMUNICATION PROTOCOLS FOR NETWORKING
  - 2 common protocols: UDP and TCP
  - messages/data are "chopped up" and sent in packets, which are
    are collected and reassembled at the receiving end
  - UDP and TCP have their own strengths/weaknesses... different uses

 + TCP (Transmission Control Protocol)
  - reliable (no lost packets, no lost data)
  - connection-oriented (the 2 sides must establish a TCP connection
    before exchanging data)
  - has flow-control and congestion-control, which serve to limit the
    rate at which the sender sends.  This helps both the receiver (who
    may be slow or running out of buffer space) and the network in \
    general (to prevent congestion)
  - TCP sends "packets" of data.  These may need to be reassembled into 
    at the receiver if a message was placed in several packets
  - useful for applications that require no lost data (ex: instant
    messaging, web browsers/HTTP)

 + UDP (User Datagram Protocol)
  - not reliable: packets can be lost.  Useful for applications that
    require speed but can tolerate lost info (streaming audio, games)
  - no flow- or congestion-control: UDP sends as fast as it can
  - sends independent packets of data, called "datagrams"
  - not connection oriented:  no connection setup and teardown.  UDP
    just starts sending.

 + PORTS 
  - computer usually has 1 connection to network.  How does it know
    which application to send received data to?
  - use ports!  These are just numbers.  Your computer has 2^16 (65536)
    ports.  Some (0-1023) are reserved for well-known services
    such as HTTP, FTP, telnet, etc...
  - each packets sent over network has an IP address, which specifies
    which computer it should go to, and a port number, indicating 
    which application on that computer it should go to.

 + SOCKETs (see 3 below)

+ IN JAVA
  - use URL, URLConnection, Socket, and ServerSocket classes for TCP
    communication
  - use DatagramPacket, DatagramSocket, and MulticastSocket for UDP
    communication

java.sun.com/docs/books/tutorial/networking/overview/networking.html

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2) URLs

 + URL (Uniform Resorce Locator)
  - an address to a resource on the internet
  - commonly identifies webpages, but can identify many other types of
    resources
  - has 2 parts: Protocol Identifier, Resource Name.  Seperated by colon
  - example: PI = "http", RN = "//java.sun.com"
  - can also specify the port (80 here): http://www.cnn.com/:80

 + CREATING URLs
  - Create a URL object, pass in the String representation of the URL
    into the constructor of the URL object
	URL url  = new URL("http://www.cnn.com/");

 + not really necessary for our project, but good to know about

java.sun.com/docs/books/tutorial/networking/urls/definition.html

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3) Sockets

 + SOCKETS
  - one endpoint of a 2-way connection between two programs over network
  - URL and URLConnection objects provide high-level mechanism for
    accessing internet resources.  We can use TCP and UDP for 
    lower-level access.  Useful for writing client-server apps.
  - TCP uses sockets for communication between computers (TCP "binds
    to" a socket) on a specific port.
  - A socket can both read and write
  - in Java, use Socket on client side and ServerSocket on server side

 + STEPS OF COMMUNICATION
  - server binds to a socket on a specific port and listens for 
    connection requests from clients
  - to connect to server, client opens a connection to the server
    at the specified port number (it should know IP address of server
    and port number of app on server beforehand)
  - server can accept of reject the connection
  - upon successful acceptance of connection, the connection on the
    server side gets a different socket (with a different port number)
    so the original port can still be used to listen for connections
  - client and server can now communicate by reading from and writing
    to their sockets

java.sun.com/docs/books/tutorial/networking/sockets/definition.html

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4) Sockets: reading and writing

 + CREATING A SOCKET
  - many different constructors for Socket object.  One useful one is:
	Socket(String host, int port)
  - ex) Socket socket = new Socket("localhost", 7000);
	- creates a socket to the same machine (Java can do this) over
	  TCP to port 7000
  - when developing, I recommend connecting to "localhost" so you
    can develop on the same computer

 + READING FROM A SOCKET
  - use socket.getInputStream() to get the Socket's InputStream
  - can connect it to any other type of input stream, or an 
    InputStreamReader, for example.
  - use the stream's read methods.  For example, BufferedReader has
    the readLine() method.  ObjectInputStream has readObject()
  - the read method should return null if the end of the stream has
    been reached.  Check API to be sure.

 + WRITING TO A SOCKET
  - use socket.getOutputStream() method to get the OutputStream
  - can connect that output stream to other output streams, such
    as a PrintWriter, ObjectOutputStream, etc.
  - use the outer stream's write method (such as println() in 
    PrintWriter, or writeObject() in ObjectOutputStream)

 + OVERVIEW
  1) Open a socket.
  2) Open an input stream and output stream on the socket.
  3) Read from and write to the stream according to application's
     protocol.
  4) Close the streams.
  5) Close the socket.

 + Lets look at the EchoClient example from the tutorial

java.sun.com/docs/books/tutorial/networking/sockets/readingWriting.html

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5) Sockets on the server side

 + SERVER SOCKETS
  - use the ServerSocket class on the server side to wait for client
    connection requests
  - the ServerSocket is ONLY used for accepting connection requests,
    not for any other communication
  - use the accept() method of the ServerSocket object to wait for a
    a request.  When one comes in, accept() returns with a new Socket
    object, which you can use to communicate with the client
  - From then on just read and write to the socket as explained above
    - the protocol (including order of messages) must be reflected
      in your code (see below for more info)

 + SUPPORTING MULTIPLE CLIENTS
  - in this case you must use threads
  - have a loop that continually accepts new client connections, thus
    getting a new Socket for each new client
  - spawn a new thread to handle communication with that client.  The 
    thread must have access to the Socket used to communicate with the
    client
  - by spawning a new thread, that thread can handle communication on
    its own, while the main thread can go back to accepting more 
    client connections

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6) Overall

 + PROTOCOL
  - the language and rules governing how two entities communicate.
    May take into account, syntax, ordering of messages, etc.
  - can be very simple or very complex
  - example:  i say "hello", you respond with "hello"
  - your protocol for communication should be well-thought-out
  - for example: client asks for elections, server responds by sending
    election data.  Also: client votes, server responds by sending
    a success or failure message.
  - your code must implement your protocol.  For example, if the 
    client must respond to every "hello" message with "hello", you
    must program that into your server