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The OSI Reference ModelWILLIAM W. MCMILLAN

State of Affairs before the OSI Model

u By the 1970s many protocols and standards were being defined for

u Low-level data frame definition

u Routing

u Packet formation

u Error handling

u Contention management

u Address ing, etc., etc.

u Some protocols like X.25 apply to several levels of abstraction

u Some might apply only to one level

u We would like different sorts of networks and devices to interconnect

u … and we don’t want s tandards to be compatible at one level, but clash at other levels

u E.g., we have various des igns of railroads and trains . A s tandard or practice that defines gauge dimens ions for tracks should not also define safety s tandards for s team boilers in train engines, or loading dock heights , or water s tation placement

u … but this is what was happening in network development

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Early Attempts at Standardization

u Protocols were cropping up without systematic attempts to s tandardize through profess ional organizations

u Initial forms of packet switching, flow control, arbitration

u Various bodies s tarted to develop technical s tandards

u Recognition that a framework was needed within which to define s tandards

u … especially to open up systems

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CCITT: Comité Consultatif International Téléphonique et Télégraphique

u Formed in 1956 to define internatinal telephone and telegraph standards, merging exis ting committees working on s tandards

u Now known as ITU-T: International Telecommunications Union, Telecommunication Sector (ITU has its origins in the 1860s)

u The interests of telephone companies were top priority

u Virtual circuits were des ired

u Like a phone conversation, you establish a “connection” that pers is ts through a “conversation”

u Developed the X.25 protocol

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Computer and Telecom Companies

u Telephone companies – as well as IBM, Digital Equipment, Datapoint, and others – had their s tandards, some applying to all levels of abstraction of network des ign

u Many not interested in defining a layered framework for s tandards and protocols

u They wanted people to use their proprietary s tuff, preferably as turnkey systems

u Inter-networking and open standards for networking were not especially des ired

u … maybe even feared

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CYCLADES Computer Network

u France,1972-198 1, for research, funded by government

u Led by Louis Pouzin, who had worked at MIT on early time-sharing (CTSS)

u (Pouzin coined the term shell for an OS command language processor)

u Worked with, and influenced, ARPANET developers

u Host – or node – respons ible for getting data delivered

u Based on datagrams or packets that included “To” and “From” info

u Pouzin cons idered to be the inventor of the datagram

u Transmiss ion not acknowledged for each datagram

u CYCLADES Included time-sharing support

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(Networking) Power to the People!6

Russell, A. L. (2014). Open Standards and the Digital Age: His tory, Ideology, and Networks . Cambridge. (p. 198)

OSI Is Born, 1984

u INWG failed, but individuals involved in those efforts went to the International Standards Organization ( ISO) to get backing in encouraging open standards

u In 1977 ISO formed a committee on Open Systems Interconnection (OSI)

u The thrust was to a great extent to break down barriers created by proprietary s tandards

u … allowing users to “mix and match” products and technologies at different levels of detail

u IBM especially was cons idered by many to be a kind of Lord Voldemort, from whom they wished to escape

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Nature of the OSI Model

u The OSI model is not a set of networking s tandards or protocols

u Think of OSI as a partition of networking is sues into layers of abstraction

u Once networks are seen as layers of technology, it becomes poss ible to mix technologies

u Des ign at any one level just concerns itself with that level

u … same with implementation, configuration, trouble shooting, …

u Don’t mess with the layer above or below

u … except that at each level you have to cons ider how to interact with the level below and the level above

u E.g., when you fix my car’s broken heater core, don’t change how the heater controls on the dash work – though you have to connect the core to the electrical system and controls

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OSI Model Analogy

We are designing a (physical) mail system for a company (ignoring intra-company mail). At the lowest level we specify how to connect with the U.S. Postal Service (offloading incoming mail and loading outgoing mail). At the next level, we say how mail will be delivered to and picked up from departments. The next higher level handles how in the departments mail is collected from, and delivered to, individuals. There are three levels of abstraction in this example (unless we include the levels within the U.S. Postal Service). To design protocols for UPS deliveries and pickups, or intra-company mail, we might only have to define new practices at the lowest level of abstraction.

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Application Layer

u Can usually be thought of as the user level of communication

u E.g., a Skype conversation between two people separated by a great dis tance

u Abstractly, communication is person-to-person, person-to-webs ite, appliction-to-application,…

u Shopping, regis tering for classes , an FTP file transfer, doing an Internet search,…

u Ideally, end-users think and work only at this level

u Units of interaction may be cons idered to be user transactions

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Presentation Layer

u Your computer (HW and SW) presents information to you that came over the network

u E.g., systems software delivers data from a socket (connection) to a browser

u Data might have to be reformatted

u Software tools like plugins might have to be invoked

u This layer is local to one station

u Other s tations might use other encoding and formatting

u Here things differ quite a bit between Windows, Mac OS, Linux, etc.

u The job is about the same as getting the electric ity from the pole into the house and presenting it to the electrical outlets – details will look very different in a house built in 1880 and one built in 2015

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Session Layer

u E.g., a login and shopping sess ion with www.toysrus .com

u Are you talking to me? (0:50 – 1:10; warning: foul language outside thi s range)

u Defines initiation and termination of a connection, a temporarily pers is tent dialog

u … and maybe reestablishes a broken connection

u Each party is “stateful,” meaning it has a s tate and remembers it’s in a conversation, with whom, when the conversation has been terminated, etc.

u Sending login credentials , queries , names and addresses , credit card numbers , shopping selections

u … but the sess ion layer doesn’t really “understand” these

u E.g., a telephone operator establishes a connection between parties . When someone hangs up, the operator disconnects the path

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Transport Layer

u E.g., a file (“segment”) containing a photo of a cat is sent

u Point-to-point, virtual “s tream” of data

u Overall transmiss ion can vary in length

u … as opposed to, say, packets, that are fixed in length

u Goal is to send a data unit that is semantically useful (typically a file)

u Independent of how sent at higher levels , e.g., in email, from clicking on a URL,…

u I.e., we don’t have to know about the sess ion defined at the next higher level

u Error detection and correction – Do we have the whole photo? What parts are miss ing? Can we get another copy of the miss ing part?

u Typically, packets are the units managed at this level

u E.g. dividing up my loose-leaf notebook into envelopes and reassembliing it at the receiving end in the dean’s office

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Network Layer

u Getting the pieces to where they are intended to go, often over great dis tances and across various networks

u This is the main job of the U.S. Postal Service within the U.S.

u Dealing with arbitrary parts or units that don’t by themselves have semantic integrity

u (Maybe this is unlike the Postal Service, except perhaps a multi-package shipment)

u In a circuit-switched connection, the job is to connect all the intermediate switches, but then becomes trivial once the connection is established

u In a packet-switched connection, many busy bees must help all the pieces of data get from point A to point B

u Internet routers are good at this

u Parties involved have no knowledge of applications , sess ions , miss ing packets ,…

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Data Link Layer

u Mainly a local network is sue

u How do we get data from a workstation to the server?

u Data broken up into frames

u Protocols like Ethernet and token-ring arbitration are relevant

u Low-level hardware addresses like MAC addresses and s imilar are at the heart of data link communication

u Aspects of multiplexing are defined here

u Like a univers ity internal campus mail system or an office phone connecting via a compay switchboard to another office

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Physical Layer

u How are data transmitted over the wires , wireless , optical fiber, etc. from one station to another?

u What are the speeds and capacities of the media?

u How do we share a connection (“wire”)?

u Is sues like data-to-electrical-pulse encoding are critical

u The nature of hardware and network interfaces is critical

u Bits are the units of concern, not packets or other organized units

u In a phone system, the electrical encoding of voice on wires is a phys ical-layer concern

u In the U.S Mail system, the phys ical layer includes the mail carrier’s hand, mail pouch, trucks , roadways, airplanes,…

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