Internet Protocol Version 6

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					Protocol Layering


   Myungchul Kim
  mckim@icu.ac.kr
  Tel: 042-866-6127

                      1
• Protocols and protocol layering
   – peer entities
   – a protocol is a set of rules and formats that govern the
     communication between communicating peers.
   – layering: the lower layer provides a service used by the higher
     layer in its execution.
   – service access point (SAP): the interface between the lower and the
     upper layer
   – protocol data units (PDU) between peer entities in the same layer.
   – service data unit (SDU): handed to a layer by an upper layer.
   – protocol stack -> information hiding -> reduce system performance


• The importance of layering
   – a complex problem into smaller, more manageable pieces
   – the implementation details of a layer are hidden (abstracted) from
     other layers.
   – many upper layers can share the services provided by a lower layer.
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• Problems with layering
   – layering is a form of information hiding
   – lead to poor performance
   – example of layers between transport and link: TCP over a lossy
     wireless link


• ISO OSI reference model
   – open system: a system that implements open protocol standards
   – reference model: formally defines what is meant by a layer, a
     service, a service access point, name, etc.
   – service architecture: describes the services provided by each layer.
   – protocol architecture: the set of protocols that implement the
     service architecture.




                                                                            3
• The seven layers




   – physical layer
      • coding scheme, connector shape and size, bit-level
        synchronization


                                                             4
5
6
– datalink layer
   • framing: insert markets in the bit stream
   • frame packets
   • medium access control (MAC): datalink-layer address and
      access to the medium
   • logical link control: link error control and flow control
   • Ethernet card: physical and datalink layers
– network layer
   • concatenate logically a set of links to form the abstraction of an
      end-to-end link.
   • compute a route
   • segmentation and reassembly
   • unique network-wide addresses
   • in datagram network, routing and data forwarding
   • in connection-oriented network, the data plane and the control
      plane.
                                                                      7
    • the beauty of IP is that we can layer it over practically any
      datalink layer technology, because it makes very few
      assumptions about the datalink layer.
    • type-of-service in the IP
– transport layer
    • create the abstraction of an error-controlled, and flow-
      controlled, end-to-end link
    • flow control
    • multiplex multiple applications to the same end-to-end
      connection
    • port number
    • User Datagram Protocol (UDP): multiplexing, but not error
      recovery or flow control for audio, video, network file system.
    • Transmission Control Protocol (TCP): WWW, file transfer.


                                                                        8
– session layer
    • provides the abstraction of full-duplex service, expedited data
      delivery and session synchronization.
    • The Internet does not have a standard session layer protocol.
– presentation layer
    • deals with data
    • encrypt data
    • The Internet does not support a standard presentation layer.
– application layer
    • WWW: browser, server, navigation
    • file transfer using TCP
    • real-time audio and video retrieval using UDP




                                                                        9
• why seven layers ?
   – the case for the session and presentation layers appears to be weak.
   – error control can be placed at the application, transport, network,
     or datalink layers.
   – flow control can be at the network or transport layers.


• why did the ISO OSI protocol stack fail?
   – standardization process was slow, cumbersome,…
   – it was specified before there was much experience. (the Internet
     philosophy is to standardize only after implementation)
   – a game of user numbers.




                                                                        10
      Limitations of the Layered Model
• Some layers are not always cleanly separated.
   – Inter-layer dependencies in implementations for performance
     reasons
   – Some dependencies in the standards (header checksums)
• Higher layers not always well defined.
   – Session, presentation, application layers
• Lower layers have “sublayers”.
   – Sublayers well defined in the standards
• Interfaces are not really standardized.
   – It would be hard to mix and match layers from independent
     implementations
   – Many cross-layer assumptions, e.g. buffer management


                                                                   11
The Internet Protocol Suite

 Application
                     Applications
Presentation         Presentation
  Session              Session
                      UDP TCP
 Transport
  Network
  Data link           Data Link
  Physical             Physical


                 The Hourglass Model
                                       12
    Internetworking Options
7               7   7                    7
6               6   6                    6
5               5   5                    5
4               4   4                    4
3               3   3      data link     3
2    physical   2   2          2         2
1       1       1   1       1     1      1

    repeater                bridge
                        (e.g. 802 MAC)

7               7   7                    7
6               6   6                    6
5               5   5                    5
4    network    4   4
                             ...         4
3       3       3   3       3    3       3
2     2   2     2   2       2    2       2
1     1   1     1   1       1    1       1

     router               gateway
                                             13
   Relevant Standardization
           Bodies
• Trend toward a priori standards.
   – a mixed blessing
• ITU-TS (formerly CCITT) - Telecommunications Sector
  of the International Telecommunications Union.
   – government representatives (PTTs/State Department)
   – responsible for international “recommendations”
• T1 - telecom committee reporting to American National
  Standards Institute.
   – T1/ANSI formulate US positions
   – interpret/adapt ITU standards for US use
   – represents US in ISO


                                                          14
                   More Bodies
• IEEE - Institute of Electrical and Electronics
  Engineers.
   – responsible for many LAN physical layer and datalink layer
     standards
• IETF - Internet Engineering Task Force.
   – standards for network layer and “higher”
• ATM Forum.
   – voting membership mostly manufacturers
   – comparatively rapid evolution of recommendations
• ISO - International Standards Organization.
   – covers a broad area



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