ClientServer Interaction

1
Client-Server Interaction

• Chapters 21,22,23,24

2
Client - Server Interaction
3
The client-Server Paradigm

• A server application waits passively for contact,
  while a client application initiates a contact
  directly.

4
Characteristics of Clients Software

• is an arbitrary application program which becomes
  a client when remote access is needed.
• Is invoked directly by a user and executes for
  one session
• runs locally on a users computer.
• actively initiates contact with server
• can access multiple services as needed, but
  actively contacts one remote server at a time.
• does not require special hardware or a
  sophisticated operating system.

5
Characteristics of Server Software

• is a special purpose privileged program dedicated
  to providing one service, but can handle multiple
  clients at the same time.
• is generally invoked automatically when a system
  boots, and continues to execute through many
  sessions.
• runs on a shared computer.
• waits passively for contact from arbitrary
  clients.
• accepts contacts from arbitrary clients, but
  offers a single service.

6
Multiple Services on one computer
Two servers on a single computer accessed by two
other clients.
7
Identifying a service

• TCP uses 16 bit integers known as protocol port
  numbers to identify services.
• TCP assigns a unique port number to each
  service.

• A client specifies the port number of the desired
  service when sending a request.
• Conceptually TCP software on the servers computer
  uses the port number in an incoming message to
  determine which server should receive the request

8
Concurrent Servers

• Each copy of the service executes concurrently in
  its own thread.
• Servers may be created statically or dynamically

• Transport protocols assign an identifier to each
  client as well as to each service.
• Protocol software on the servers machine uses the
  combination of client and server identifiers to
  choose the correct server copy.

9
Connection-Oriented Conectionless Transport

• Connectionless transport used datagrams in
  TCP/IP this is the User Datagram Protocol (UDP).
• To use Connection oriented transport two
  applications must establish a connection and the
  send data across the connection. When complete
  the connection must be closed.
• The same service can be made available over two
  or more transport protocols.

10
The Socket interface

• Stream sockets are the end points of a
  communications link between two processes. The
  streams connecting the two sockets are
  bi-directional perfect error free communication
  streams. The lower level TCP protocol performs
  all error recovery.
• Sockets are appropriate for applications that
  require guaranteed delivery of information.

• Terminal sessions
• Remote login
• File Transfer applications
• Mail applications
• Hypertext applications.

11
Stream Sockets

• Stream sockets are slower than datagram sockets
  because each packet from the sender must be
  confirmed as arriving at the receiver as error
  free and in the correct sequence, otherwise the
  sender will re-transmit the packet. There is also
  an initiation phase where the connection is
  established so that communication cannot start
  immediately.
• If two processes wish to communicate they must
  each create a socket. Once created the socket
  must then be bound to a specific network address.
  One process must initiate the connection (This
  process is called the client). The other process
  must wait for a connection request (This process
  is the server). This delegation of
  responsibilities is highlighted in the diagrams
  below.

12
Socket Communication
13
Java Socket API

• The java socket API is at a slightly higher level
  that the c/C api illustrated in Comer chapter
  22.
• However the lower level functions such as listen
  etc are present in a Class library called
  java.net.Sockimpl

14
Domain Name System

• Computers are organised into groups of machines.
  These groups are called domains
• A computer can belong to more than one domain.
• Each domain name consists of a sequence of
  alpha-numeric segments separated by periods.
• doc.ntu.ac.uk
• domains are organised hierarchically.

15
Computing Domain
16
Domain names
uk - united kingdom ac - academic institution
17
Name servers are hierarchical
18
Resolving a name

• looking up the actual IP address of a computer
  name is called resolving a name.
• Socket APIs have functions which call the DNS
  servers. In Java you can use the InetAddress
  class
• InetAddress.getByName(String) - to look up the IP
  of a host given as text
• InetAddress.getLocalHost() - to get the IP
  address of the computer on which the process is
  running
• InetAddress.getAllByName(String) - to get the
  names of all hosts registered on a particular
  domain. This function returns all the machines in
  an array.