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					Enterprise Network Systems

             TCP
             Mark Clements
    Last Week – Client/ Server

       Cost effective way of providing more
        computing power
       High specs for server hardware
       Thin client is cheap and often desirable
       Client is proactive, server reactive
       Client software is user friendly
       Must have a network to operate

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    This week – TCP/IP & Protocols

     How  data crosses networks
     Communications model architecture
     TCP/ IP model
     Transport layer and what it contains
     Internet layer and what it contains


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    Protocol Architecture

       When data is transferred across networks, it
        must do so in an orderly fashion
       A physical path is not sufficient on its own
       End (and intermediate) devices must co-
        operate
       Communication is required between all
        devices other than just the data that is being
        transferred

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    Tasks for data Transfer

       Source system application has TWO choices
       Use reliable TCP
        –   Sets up communication path
        –   Determines whether remote system is ready to
            accept data
        –   Sends data
        –   Tears down communication path
       Use unreliable UDP
        –   Uses UDP and just sends data „as is‟
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    More tasks

       When data transfer is complete, end systems
        must communicate their readiness to break
        the connection (TCP)
       UDP is connectionless – hopes message
        arrives at destination safely




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    Other Data Transfer Tasks

       If data incompatibility occurs, a translation
        must be performed
       If data are lost there must be recovery
        mechanisms in place
       These are just a few of the tasks that have to
        be performed
       Logic required to implement all this is too
        complex for a single software module

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    Solution

       Break down the tasks into a set of simpler
        sub-tasks
        –   implemented separately as logic modules
            (software)
        –   arranged in a vertical stack
        –   each layer then performs a sub-set of the entire
            logic
        –   any layer may be changed without affecting any
            other layer

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    ISO OSI Model – a blueprint




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     Seven Layer Model

        The seven layers set out the tasks that must
         be performed for data transfer to take place
        Top four layers reside in PC
        Bottom three implemented in NIC and the
         network
        Other communication models exist too e.g.
         SNA, ATM, WAP TCP/ IP etc.

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     TCP/ IP Protocol Suite

        Today the TCP/ IP protocol suite is used by
         most computers worldwide
        Developed in 1970s before OSI by USA
        TCP/ IP protocol ready to „go‟ when OSI was
         still teething
        Does in four layers what OSI takes seven
         layers to accomplish

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     OSI versus TCP/ IP

     Some similarities exist:
     Both have Transport layer
     and Application layer
     TCP/ IP is a practical
     implementation of the OSI
     „blueprint‟



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     Application Addressing

        Transport Layer Duties
        When data arrives at a computer it must be delivered
         to the correct application
         –   We don‟t want email going to FTP program etc.
        Transmission Control Protocol (TCP) deals with this
         –   Applications are given a port number
         –   “Well known” port numbers – 21, 23, 25, 53, 80, 110
        Port numbers are sent along with the data we wish to
         transfer to identify the process on the end system that
         the data being carried is intended for

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     Example - Web Page Request

        Segment formed at transport layer
        Destination port 80 (on web server)
        Source port taken from stack of free ports e.g. 5645
        This relates to the browser window (or tab) you made
         the request from
        Network delivers request to web server
        Reply segment has source port 80 and destination port
         5645
        Your PC now knows it has web content and which
         window (or tab) to display results in
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     Computer Addressing

        To have a world-wide delivery system we
         need a global addressing system
        Each entity needs a unique address
        Internet Protocol (IPv4) addressing
        e.g. 193.60.61.124
        This is dealt with at the Internet layer


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     TCP/ IP Concepts




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     Peer & Protocol Layers

        Application layer communicates with
         application layer on each of the two end
         entities
        Transport layer communicates with transport
         layer on end entities
        Network, Data-link & Physical layers are
         protocol layers


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     What is a PDU?

        PDU is a protocol data unit
        This is the data that is passed from layer to
         layer in host systems
        Data are added as the original data from the
         application is processed by the layers
        Our communication works its way down the
         stack to the Physical layer
        Crosses the network then rises back up the
         opposite stack in the far end-system
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     Application layer to Cabling




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     User-data Progression

        Application makes data e.g. email client
         –   Only email program understands an email
        Reliable – uses TCP
        Data are passed to TCP layer and TCP
         header added
         –   email port identity
         –   sequence number & checksum


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     TCP PDU progress

        Passed to IP for network header
         containing
         –  destination addressing using IP number
          – source addressing using IP number
          – facilities requests e.g. priority
        Now the PDU is passed to Network Access
         Layer

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     Network Access Layer

        Here a header and trailer is added with:
         –   subnetwork destination (MAC) address
                 e.g. router, communications server, host etc.
         –   subnet facilities requests
         –   error checking data appended
        This header is used for the next hop only
        Now the complete PDU can be transmitted onto the
         Physical medium
         –   off to subnet destination address


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     Arrival at destination

        When the data arrive at final destination, the
         PDU is passed back up the protocol stack
        At each layer the header contents are read
         and acted on then the header is removed
        Data arrive at correct application process
         port - email program, web browser etc.



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     TCP/ IP Applications

        HTTP, Hypertext Transfer Protocol which is
         used for request and delivery of web pages.
        SMTP, Simple Mail Transfer Protocol for
         emails
        FTP, File Transfer Protocol for transfer of
         data
        TELNET, for remote logging into computers

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     Conclusion

        Protocol architecture needed to contain a set
         of communication rules
        TCP/ IP dominates world communications
        SMTP, FTP & TELNET all rely on TCP/ IP
        Port numbers are associated with application
         processes in Transport Layer
        IP addressing is dealt with in the Internet layer

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