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Network Architectures

Network Architectures. Week 3 – OSI and The Internet. What do we want from a communications network?. We want to transfer messages from a process in one computer to a process in another computer reliably quickly and in an understandable form. We also know that:.

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Network Architectures

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  1. Network Architectures Week 3 – OSI and The Internet

  2. What do we want from a communications network? • We want to • transfer messages from a process in one computer • to a process in another computer • reliably • quickly • and in an understandable form.

  3. We also know that: • Messages have to pass through a number of switching nodes from one host to another, and thus the network has to route messages correctly • The various links may be of different media at different bandwidths • The signals carrying data are subject to interference and degradation, and thus messages may be corrupted or lost • The byte coding structure in one computer may be different from the other

  4. We need a series of protocols to address these issues • Remember the definition of a protocol: • “A protocol defines the format and order of messages exchanged between two communicating entities, and the actions taken on receipt or transmission of a message.” • Those “communicating entities” may be as limited as the two devices on either end of a physical link.

  5. Network Architectures • Data communication developed in the 60’s • By early 70s suppliers were developing own architectures – IBM/SNA DEC/DECnet • But these did not help inter-organisation data communications • Two approaches: • OSI model • Internet’s TCP/IP

  6. OSI – Open Systems Interconnect • Developed by the International Standards Organisation in 1974 • It is a reference model • Describes a network and a framework for developing network protocols • Incorporates work done by a number of organisations, particularly DEC • Not ever fully implemented

  7. OSI Continued • Its real benefit is that is defines a layered architecture and thus the model is still used • The objectives of the model were: • Must provide a high degree of connectivity • Must be reliable • Must be easy to implement, to use and to modify

  8. OSI Layers • 7 – Application • 6 – Presentation • 5 – Session • 4 – Transport • 3 – Network • 2 – Data link • 1 - Physical As a layered architecture, each layer is isolated from the others, And thus its internals can be modified or replaced.

  9. Key points about a layered approach • The source process only wants to know that the message it sent gets to the destination process – it does not need to know how • The Network stack is implemented in the operating system of both hosts and the various switches • As such the software in each operating system will be different code – but implementing the same protocol

  10. Application – Layer 7 • Provides an application or a service to the user application • Examples are • FTAM – File transfer Access Mode • X.400 – message handling • X.500 – network directory services

  11. Presentation – Layer 6 • Structures data in agreed format • Carries out code conversion (ASCII to EBCDIC • Carries out data compression • Carries out data encryption

  12. Session – Layer 5 • Co-ordinates connection & disconnection of dialogs between processes • Synchronises the flow of data - checkpoints • Re-establishes the connection if it fails

  13. Transport – Layer 4 • Provides end-to-end, error free delivery of messages, based on level of service required • These include: • Error control • Flow control • Partitioning and reassembling messages

  14. Network – Layer 3 • Responsible for end-to-end routing of data packets across the network • Logical addressing • Routing • Performs network management • Formats packages

  15. Data Link – layer 2 • Responsible for reliable transfer of data across a link in frames • Provides for error detection and control • Organises data into frames • Provides flow control – if receiver slower than transmitter • Negotiates access control between the two devices

  16. Physical – Layer 1 • Transmits bits across the physical medium • Accepts data in frames and translates into signals on the medium • Concerned about the medium being used , the signalling scheme and the connectors

  17. Why is OSI not the prevailing standard? • A case of De facto prevailing over De jure • While OSI was defined, manufacturers were slow to implement it • Europeans were keen, but the US had TCP/IP • US government proclaimed GOSIP (Government OSI Profile) in 1992 • But TCP/IP took off with the Internet in the early 90s • It worked, was common, and most organisations just accepted it

  18. TCP/IP, Internet & The WWW • TCP/IP are two protocols at the Transport and Network levels • The Internet is a “Network of Networks” that use TCP/IP as key layers in its protocol stack • The World Wide Web is an application that runs on the Internet

  19. TCP/IP • TCP (Transmission Control Protocol • IP (Internet Protocol) • Developed at the same time as OSI, but as a product not an international model • Developed for the ARPANET – Dept of Defence, defence contractors, Universities and the Military • To enable communication across analogue lines, packet radios and Ethernet networks • To be a Network of networks

  20. TCP/IP (cont.) • Developed by Vint Cerf & Robert Kahn • Uni of California included it is BSD UNIX • National Science Foundation mandated it and ran the backbone 1985 • While the requirements were much the same as for OSI, it was not built as a layered product. • Many of its attributes reflect the environment it was developed in • Commercial services started in the 1980’s and NSF stopped providing the backbone in 1995.

  21. New Data link level protocol added in 1990 • RFC 1149: A Standard for the Transmission of IP Datagrams on Avian Carriers. • www.faqs.org/rfcs/rfc1149.html • Later followed up with • RFC 2549: IP over Avian Carriers with Quality of Service

  22. The Principles set out for TCP/IP • Autonomy – a network should be able to work on its own without change • Best effort service – Lost messages would be retransmitted • Stateless servers – Routers should not need to maintain the state of a connection • Decentralised control – No global control over the Internet

  23. Comparing The Internet & OSI

  24. Internet “layers” PDU • Application – layer 5 Message • Transport – layer 4 Segment • Network – layer 3 Datagram • Data Link – layer 2 Frame • Physical – layer 1 Bit PDU (Protocol Data Unit) Layers as per Kurose & Rose

  25. Generic functions that may be at each layer • Error control • Flow or congestion control • Segmentation & re-assembly • Multiplexing – higher level sessions sharing a single lower level connection • Connection set-up

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