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Design and Implementation I

Apr 23, 2024, 6 min read

Learning Outcomes

  • To define the basics of client-server communication.

Client-Server

  • Client-Server programs consist of software processes that interact via messages.
  • Each process is connected via program constructs (sockets) to an input/output interface in a computer called a “port”.
  • A port is a memory location shared with network software (actually more complex than this).
  • Software on the other side of the port makes sure that the message you send gets there (TCP/IP).
  • Java package java.net is the Java version of the socket API.

Sockets

There are four possible operations that a socket can perform:

  • Connect to a remote machine.
  • Send data.
  • Receive data.
  • Close the connection.

A socket may not be connected to more than a host at a time.

A socket may not reconnect after it is closed (you have to start again).

Connecting

  • You must at least specify the remote host and port to connect to.

  • The host may be specified as either a string, like xxx.brunel.ac.uk or as an InetAddress object.

  • The port should be an int between 1 and 65535.

  • You cannot just connect to any port on any host, the remote host must actually be listening for connections on that port.

  • You can use the constructors to determine which ports on a host are listening for connections.

  • Sending and receiving operations are implemented via the socket interface (connected to a port).

    • Sending is usually asynchronous.
    • Receiving is usually synchronous.

Protocols

  • Client and server.
  • Tend to send, receive, send, receive, etc.
  • Special instructions using Socket API.
  • Can however just send, send, send but the server needs to receive, receive, receive.

Servers

  • Sockets connect clients and servers.
  • Servers wait for a client to connect to it.
  • A server uses a Server Socket.
  • A server socket binds to a particular port on the local machine and waits (listens).
  • When a server detects a connection attempt, it accepts the connection.
    • This allows the client and server to communicate.

Multiple Clients

  • Several clients can connect to the same port on the server at the same time.
  • Incoming data is distinguished by the port to which it is addressed and the client host and port from which it came.
  • Incoming connections are stored in a queue until the server can accept them.
  • Once the queue fills up, further incoming connections are refused until space in the queue opens up.

Internet Services Approach

  • Each service has a port number that if you connect to (via a socket) you access the service.
  • Most are text based.

HTTP Request Message

HTTP Reply Message

Sockets and Ports

Other Network Services and Protocols

  • Two types:
    • Connectionless (UDP)
    • Connection-oriented (TCP)
  • echo 7/tcp
  • echo 7/udp
  • ftp 21/tcp
  • telnet 23/tcp
  • smtp 25/tcp
  • whois 43/tcp
  • finger 79/tcp
  • http 80/tcp

Processes

They communicate with other processes via sockets connected to ports onto the internetworking layers and then onto a network.

Berkley Socket API

  • socket():
    • creates a new socket of a certain socket type, identified by an integer number, and allocates system resources to it.
  • bind():
    • is typically used on the server side, and associates a socket with a socket address structure, i.e. a specified local port number and IP address.
  • listen():
    • is used on the server side, and causes a bound TCP socket to enter listening state.
  • connect():
    • is used on the client side, and assigns a free local port number to a socket. In case of a TCP socket, it causes an attempt to establish a new TCP connection.
  • accept():
    • is used on the server side. It accepts a received incoming attempt to create a new TCP connection from the remote client, and creates a new socket associated with the socket address pair of this connection.
  • send() and recv(), or write() and read(), or sendto() and recvfrom():
    • are used for sending and receiving data to/from a remote socket.
  • close():
    • causes the system to release resources allocated to a socket. In case of TCP, the connection is terminated.
  • gethostbyname() and gethostbyaddr():
    • are used to resolve host names and addresses. IPv4 only.
  • select():
    • is used to pend, waiting for one or more of a provided list of sockets to be ready to read, ready to write, or that have errors.
  • poll():
    • is used to check on the state of a socket in a set of sockets. The set can be tested to see if any socket can be written to, read from or if an error occurred.
  • getsockopt():
    • is used to retrieve the current value of a particular socket option for the specified socket.
  • setsockopt():
    • is used to set a particular socket option for the specified socket.

A “Client” Program

  • It is a template that you can use to connect to any server on any machine.
  • Need to be aware of the socket number and the machine address or name.

EchoServer Network Application

Simple client-server program that sends and receives (echos) from client to the server.

//echoserver
WHILE NOT TERMINATED
    RECEIVE messageX from echoClient
    SEND messageX to echoClient
ENDWHILE
//echoclient
WHILE NOT TERMINATED
    READ messageX FROM user
    SEND messageX TO echoServer
    RECEIVE messageX FROM echoServer
    PRINT messageX
ENDWHILE

Single Client Knock, Knock

// KKClient
WHILE NOT TERMINATED
    #C3 RECEIVE “Knock Knock” FROM KKSERVER
    #C1 SEND “Who’s there?” TO KKSERVER
    RECEIVE message FROM KKSERVER
    IF message = “You are supposed to say “Who’s there?”” THEN
        GOTO #C1
    ELSE
        [message is First_part_of_joke]
        #C2 SEND First_part_of_joke + “who?” TO KKSERVER
    ENDIF
    RECEIVE message FROM KKSERVER
    IF message = “You are supposed to say Joke+“Who?” THEN
        GOTO #C2
    ELSE
        [message is answer + “Would you like another go?”]
        IF YES THEN
            SEND “y” TO KKSERVER
            GOTO #C3
        ELSE
            SEND “n” to KKSERVER
        ENDIF
    ENDIF
    [Terminate]
ENDWHILE
// KKServer
[accept KKClient connection]
WHILE NOT TERMINATED
    #S1 SEND “Knock Knock” TO KKCLIENT
    RECEIVE message FROM KKCLIENT
    IF message = “Who’s there?” THEN
        [Select first part of joke]
        SEND First_part_of_joke TO KKCLIENT
    ELSE
        SEND “You are supposed to say “Who’s there?”” TO KKCLIENT
    ENDIF
    RECEIVE message FROM KKCLIENT
    IF message = First_part_of_joke + “who?” THEN
        [Select answer]
        SEND answer + “Would you like another go?” TO KKCLIENT
    ELSE
        SEND “You are supposed to say Joke+Who?” TO KKCLIENT
    ENDIF
    RECEIVE message FROM KKCLIENT
    IF message = “y” THEN
        GOTO #S1
    ENDIF
    [message is “n”]
    [Terminate]
ENDWHILE

Multi-Client Knock, Knock

kkmserver
create a serversocket object and connect it to port 4444
while not terminated
    wait for a client to connect
    make the connection and start a new thread of execution using kkmsthread
endwhile

kkmsthread
begin
    perform the same tasks as the original kkserver
end

Different Systems

Message Passing Protocols

We can represent in a table:

What really happens

Example

Text based protocol that asks someone for 50p:

Simultaneously:

Reading

This is very general and lots of support material can be found on the web – basically you are looking for socket programming in Java.

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