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Protocol Layers

• Communications betweens processes takes place using agreed conventions - protocols
• Network communications requires protocols to cover high-level application communication all the way down to wire communication
• Complexity handled by encapsulation in protocol layers

ISO OSI Protocol

OSI layers

• Network layer provides switching and routing technologies
• Transport layer provides transparent transfer of data between end systems and is responsible for end-to-end error recovery and flow control
• Session layer establishes, manages and terminates connections between applications.
• Presentation layer provides independance from differences in data representation (e.g. encryption)
• Application layer supports application and end-user processes

TCP/IP Protocol

Some Alternative Protocols

• Firewire
• USB
• Bi-directional pipes
• Pseudo-terminals

Networking

• A network is a communications system for connecting end systems called hosts
• A local area network (LAN) connects computers that are close together, e.g. by ethernet
• A Wide Area Network (WAN) connects computers in different cities,
• A Metropolitan Area Network (MAN) is intermediate in size
• The Queen St Mall in Brisbane has a wireless network along its kilometre length. There doesn't seem to be a suitable term yet apart from "wireless hotspot"
An internet is a connection of two or more distinct LANs or WANs
• An intranet is an internet with all networks belonging to a single organisation

Gateways

• A gateway is a generic term for an entity used to connect two or more networks
• A repeater operates at the physical level copies the information from one subnet to another
• A bridge operates at the data link layer level and copies frames between networks
• A router operates at the network level and not only moves information between networks but also decides on the route

Packet encapsulation

Connection Models

• Connection oriented
• Connectionless

Connection oriented

• A single connection is established for the session
• Two-way communications flow along the connection
• When the session is over, the connection is broken.
• The analogy is to a phone conversation
• An example is TCP

Connectionless

• In a connectionless system, messages are sent independant of each other
• Ordinary mail is the analogy
• Connectionless messages may arrive out of order
• An example is the IP protocol
• Connection oriented transports may be established on top of connectionless ones - TCP over IP
• Connectionless transports my be established on top of connection oriented ones - HTTP over TCP

Communications Model

• Message passing
  

Communications Model (2)

• Event driven
• Procedure call
  • ordinary procedure call
  • dynamic link library procedure call
  • remote procedure call
    
  • lightweight remote procedure call

Distributed Computing Models

• Peer to peer
• Filter
• Client Server

Client/Server System

Client/Server Application

Server Distribution

• Single client, single server
  
• multiple clients, single server
  

Server Distribution (2)

• single client, multiple servers
  
• multiple clients, multiple servers

Component Distribution

• Presentation component
• Application logic
• Data access

Example: Distributed Database

• Gartner classification: 1

Example: Network File Service

• This allows remote clients acess to a shared file system
• Gartner classification: 2

Example: Web

• This is a distributed hypertext system, with many additional mechanisms
• Gartner classification: 3

Example: Terminal Emulation

• This allows a remote system to act as a normal terminal on a local system
• Gartner classification: 4

Example: Expect

• This acts as a wrapper around a classical system, either to provide an X Window interface or an alternative character-based one
• Gartner classification: 5

X Window System

• This decouples application view of windowing and display view of windowing
• Gartner classification: 5

Gartner Classification

Three Tier Models

Fat vs thin

• Clients and servers may be "fat" or "thin" (no normal size?)
• Fat clients use up space on each client they run on, may have complex installation requirements, etc (e.g. Netscape or IE - which "has" to be part of the O/S)
• Thin clients have reduced functionality, but are easier to manage
• The network computer model favours thin clients.

Middleware model

The middleware model is

Middleware

• The network services include things like TCP/IP
• The middleware layer is application-independent s/w using the network services
• Examples of middleware are: DCE, RPC, Corba
• Middleware may only perform one function (such as RPC) or many (such as DCE)

Middleware examples

• Primitive services such as terminal emulators, file transfer, email
• Basic services such as RPC
• Integrated services such as DCE, Network O/S
• Distributed object services such as CORBA, OLE/ActiveX
• Mobile object services such as RMI, Jini
• World Wide Web

Middleware functions

• Initiation of processes at different computers
• Session management
• Directory services to allow clients to locate servers
• remote data access
• Concurrency control to allow servers to handle multiple clients
• Security and integrity
• Monitoring
• Termination of processes both local and remote

Continuum of Processing

Points of Failure

• The client side of the application could crash
• The client system may have h/w problems
• The client's network card could fail
• Network contention could cause timeouts
• There may be network address conflicts
• Network elements such as routers could fail
• Transmission errors may lose messages
• The client and server versions may be incompatable
• The server's network card could fail
• The server system may have h/w problems
• The server s/w may crash
• The server's database may become corrupted

Acceptance Factors

• Reliability
• Performance
• Responsiveness
• Scalability
• Capacity
• Security

Transparency

• access transparency
• location transparency
• migration transparency
• replication transparency
• concurrency transparency
• scalability transparency
• performance transparency
• failure transparency

References

• P. E. Renaud Introduction to Client/Server Systems Wiley, 1993, ISBN 0-471-57774-X
• Don Libes, Expect O'Reilly Press
• A. Umar Distributed Computing and Client Server Systems Prentice-Hall, 1993
• A. Umar Object-Oriented Client/Server Internet Environments Prentice-Hall, 1997