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: 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

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.

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

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

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

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.

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

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

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

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

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

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

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

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

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

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

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

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.

New Data link level protocol added in 1990 RFC 1149: A Standard for the Transmission of IP Datagrams on Avian Carriers.A Standard for the Transmission of IP Datagrams on Avian Carriers Later followed up with RFC 2549: IP over Avian Carriers with Quality of ServiceIP over Avian Carriers with Quality of Service

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

Comparing The Internet & OSI

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

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