1. Networking COMP

2. Basic Networking Network comunication Admin
Vast basket of things that different users want
Design metaphore to give:
Different interfacces for different users
And an understandable code structure
Admin

3. Networking
Computers are (mostly) networked i.e. connected to bits of hardware some being computers:
desktops,
laptops
embedded controllers
servers
phones
sensors
Users expect to be able communicate with the world
Computers increasingly require networking to operate
How does networking function?
How can we write programs that use the network?

4. What’s required to communicate?
Users do not need to understand what goes on inside this bubble
Java application programmers only need a shallow understanding of the bubble

5. Shopping basket of networking wants
Transmit a character over a specific wire or fibre
Send data to machine where no direct connection exists
Split a message into “packets” and rebuild
Deal with multiple packets from multiple computers on the same wire at the same time
Correct transmission errors
Real time data transmission
Manage a “conversation” with a distant server.
Software reconfiguration around hardware failure
Design metaphor to structure networking software

6. Levels of communication
Different users appear to communicate at different levels
Transmit a character over a specific wire or fibre
Send
Simultaneous multiple packets from multiple computers
Send Movie
Manage a “conversation” with a distant server.

7. Protocol Stack – design metaphor
Layers of programs that build on each other.
Each layer calls methods provided by the layer below
Applications only appear to have direct communication
Actual communication occurs at the lowest layer
It only appears that a mail program sends an .
Application Layer Application
Transport Layer
Internet Layer
Network Access Layer Network

8. TCP/IP Stack Network Access Layer (eg, Ethernet protocol)
Part of the operating system
Encoding a packet into signals to go over a wire/fibre/wireless to the computer at the other end of the wire, and decoding it.
Dealing with collisions – two computers sending at the same time

9. TCP/IP Stack Internet Layer (eg, IP protocol) Network Access Layer
Part of operating system
Encoding information into a packet with address information
Dealing with addresses and routing a packet to the target computer at the other side of the network.
Network Access Layer

10. TCP/IP Stack Transport Layer (eg TCP protocol) Internet Layer
There are other protocols that work differently
Transport Layer (eg TCP protocol)
Part of operating system
Ensuring sequence of packets in a message all arrive in order
Protocol to acknowledge, recover, resend, reorder.
Internet Layer
Network Access Layer

11. TCP/IP Stack Application Layer Transport Layer Internet Layer
Programs that need to communicate - Clients and Servers
Only need to understand part of the Transport layer
Transport Layer
Internet Layer
Network Access Layer

12. Application Layer view
Applications that want to communication with other applications on another computer can use TCP scokets.
TCP (Transport Layer) provides “Sockets”:
A socket is one end of a TCP connection
A scotet has two streams associated with it:
Input stream (from the other end of the connection) Application can read from it
Output stream (to the other end of the connection) Application can write/print to it
Scoket

13. Getting a Socket
Connection requires each computer to set up a socket connected to the other
How do you specify where you want the connection to go to?
Which program running on which computer is going to server you a web page?
Which computer sets up a socket first?
Do you need a socket at the other end before you set up yours?

14. Addressing a connection
Two parts to the address:
host computer: IP address, or Host name
“port” (an integer)
Each computer on the network must have an IP address or “debretts.ecs.vuw.ac.nz”
Operating system provides a set of “ports” that connections can be associated with.
Type echo $HOSTNAME in a terminal to find your name and use this to connect to a service on your own computer. 1 2 : 80
6667

15. Who goes first? Client – Server model:
Server applications can run on a remote host
Server application listens on a known port
Client application requests a connection to host : port
Server applications accept the request
Operating systems will set up a new socket on each end with a connection between them.
A Java ServerSocket only needs a port number;
A Java Socket needs a port number and the IP or host name of the computer the server is running on.
Setup the Server before the Client

16. Using sockets in Java Sockets provide streames that are easy to use!
Streams are sequences of characters
Like files, or UI text pane, or System.in/System.out
try {
Socket socket = get a socket somehow ;
Scanner inputFromTarget = new Scanner(socket.getInputStream());
PrintStream outputToTarget = new Scanner(socket.getOutputStream());
while (inputFromTarget.hasNext()){
String line = inputFromTarget.nextLine();
String response = work out a response ;
outputToTarget.println(response);
flush(response); }
} catch (IOException e){System.out.println(“connection failure ”+e); }

17. Addressing a connection
Host1 1 2 : 80
6667
Host2 1 2 : 80
6667 xx xx
Server Listens on port 6667
Client request connection to port 6667 on Host2
port xx is not specified
Server accepts connection
OS creates socket connected to port xx on Host1
OS creates socket connected to port 6667 on Host2
Server is still listening!

18. Setting up the sockets Server
try { ServerSocket serverSocket = new ServerSocket(PORT); while (true) { Socket socket = serverSocket.accept(); … set up Scanner and PrintStream and communicate via socket } } catch (IOException e){System.out.println("Failed connection "+ e); } Socket socket = new Socket(("irc.ecs.vuw.ac.nz", PORT); …set up Scanner and PrintStream and communicate via socket
Server port
Server port
Client