CIS 456 Open Systems Networking

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CIS 456 Open Systems Networking

• Theodore L. Nicholson
• Fall Semester 2001

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Protocol Layering

• Lecture 10

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Protocol Layering

• There is an analogy between the layering of
  network protocols and the languages used to write
  computer programs
• Protocols are to networking what languages are to
  computation
• Abstraction is used in both situations to divide
  a large problem into subproblems

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Protocol Layering

• For example The IP layer provides an abstraction
  for the lower level network details in the same
  way a high level programming language provides an
  abstraction for the low level details of the
  machine architecture and instruction set

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Protocol Layering

• Taken as a whole, networking is a complicated
  task which must address a number of problems
• Hardware failure
• Network Congestion
• Packet delay or loss
• Data corruption
• Data duplication or inverted arrivals

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Protocol Layering

• Conceptual Layers

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Protocol Layering

• Keep in mind the distinction between the
  conceptual organization of a protocol and the
  implementation details
• For example, the simple protocol diagram shown
  does not reveal the multiple protocols or
  interfaces at a particular level

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Protocol Layering
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Protocol Layering

• A message traversing the network will make use of
  only those layers necessary for the processing of
  the message at that node
• Only the sender and receiver will make use of the
  entire protocol stack intermediate nodes
  (routers) will use only the lower levels

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Protocol Layering
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Protocol Layering

• What functionality should reside in each layer?
• There are two well known approaches
• ISO 7-layer Reference Model
• TCP/IP 5-layer Reference Model

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Protocol Layering

• The ISO 7-layer Model is the older of the two and
  was developed by the International Organization
  for Standardization
• Implemented as X.25 in public data networks and
  used extensively in Europe

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ISO Model
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ISO Model

• Note Each host attaches to a packet switch using
  a serial connection
• The packet switches are connected to form the
  network

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ISO Model

• Application application programs
• Presentation shared i.e., data compression
• Session shared i.e., remote terminal access
• Transport end-to-end reliability
• Network destination addressing and routing
• Data Link frames
• Physical i.e., voltage and current

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ISO Model

• Note the ISO model was designed to describe
  protocols for a single network and thus, does not
  contain a specific layer for internetwork routing
  in the same way TCP/IP protocols do

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TCP/IP Model

• TCP/IP is used for most networking
• The Internet
• Also used for extensively for local networks in
  the U.S.

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TCP/IP Model
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TCP/IP Model

• Application Applications
• Transport End-to-end communication
• Internet Datagram routing
• Network e.g., device driver
• Hardware e.g. NIC card

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Protocol Layering

• Differences between ISO and Internet
• Link Level v. End-to-End Reliability
• Where are errors detected?
• ISO All lower levels (below Transport) have
  various error checking schemes
• TCP/IP Only the Transport level ensures reliable
  communication

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Protocol Layering

• Differences between ISO and Internet
• ISO Network is a service which host computers
  use
• Naïve hosts, smart network
• TCP/IP Requires hosts to participate in almost
  all network protocols error detection, routing,
  network control (i.e., ICMP)
• Simple packet delivery system, smart hosts

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Protocol Layering Principle

• Layered protocols are designed so that layer n at
  the destination receives exactly the same object
  sent by layer n at the source

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Protocol Layering Principle
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TCP/IP Layering

• TCP/IP layering does not adhere strictly to the
  protocol layering principle
• The protocol layering principle does apply to the
  Transport and Application layers
• i.e., packets received are identical to those
  sent
• However, the protocol layering principle does not
  apply, technically, to the Internet and Network
  layers

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TCP/IP Layering
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Two Important Boundaries

• The conceptual protocol layering creates two
  important boundaries
• A protocol address boundary that separates the
  use of high level (IP) addresses and low level
  (physical) addresses
• An operating system boundary that separates
  applications from the rest of the TCP/IP stack

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Two Important Boundaries
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Multiplexing/Demultiplexing

• Multiplexing and Demultiplexing occur at almost
  every protocol layer
• Each layer typically contains many modules the
  data for which must be standardized for the next
  lower level

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Multiplexing/Demultiplexing

• For outgoing data, the message must be packed up
  into a form that is standard for the next lower
  level
• This means adding a new wrapper around the data
  as it proceeds down each layer
• Header
• For incoming data, the reverse is true

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Multiplexing/Demultiplexing Frames
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Multiplexing/Demultiplexing Datagrams
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Disadvantages of Layering

• Inherently Inefficient
• However, abstraction benefits are great
• Adherence to strict layering scheme does not
  allow for optimization
• i.e., if connected directly to the destination
  network, packet size may be optimized for that
  network
• A relaxed layering scheme allows information like
  route selection and MTU to propagate upward

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Midterm Exam

• This Monday!!