Protocol Hierarchy

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Lesson 5
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Protocol Hierarchy

• Question How can communication be provided to
  the top layer of the five-layer network of the
  following figure?

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Providing communication to Top Layer of 5-Layer
Network
Destination Machine
Source Machine
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Important thing about the Figure

• The peer process on layer 4, for example,
  conceptually think of their communication as
  being horizontal, using the layer 4 protocol
• Each layer is likely to have a procedure called
  something like SendToOtherSide and
  GetFromOtherSide, even though these procedures
  actually communicate with lower layers across the
  3/4 interface, not with the other side.

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Providing communication to Top Layer of a 5-Layer
Network

• A message M is produced by an application running
  in layer 5 and given to layer 4 for transmission
• Layer 4 puts a header in front of the message to
  identify the message and passes the result to
  layer 3

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The Header

• Headers contain control information, such as
  sequence numbers, to allow layer 4 on the
  destination machine to deliver messages in the
  right order if the lower layer do not maintain
  sequence.
• In some layers, headers can also contain sizes,
  times, and other control information.

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Providing communication to Top Layer of a 5-Layer
Network

• In many networks, there is no limit to the size
  of messages transmitted in the layer 4 protocol,
  but there is always a limit impose by the layer 3
  protocol. Consequently, layer 3 must break up the
  incoming messages into smaller units, packets,
  attaching a layer 3 header to each packet. In
  this example, M is split into two parts M1 and
  M2.

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Providing communication to Top Layer of a 5-Layer
Network

• Layer 3 decides which of the outgoing lines to
  use and passes the message to layer 2.
• Layer 2 adds not only a header to each piece, but
  also a trailer, and gives the resulting unit to
  layer 1 for physical transmission.

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Providing communication to Top Layer of a 5-Layer
Network

• At the receiving machine the message moves
  upward, from layer to layer, with headers being
  stripped off as it progresses.
• None of the headers below n are passed up to
  layer n.

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Important

• The peer process abstraction is crucial to all
  network design. Using it, the unmanageable task
  of designing the complete network can be broken
  down into several smaller, manageable design
  problems, namely, the design of individual
  layers.
• The lower layers of a protocol hierarchy are
  frequently implemented in hardware or firmware.

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Connection-Oriented Service and Connectionless
service

• Connection-oriented service means that a
  connection must be set up before any
  communication takes place over which all packets
  travel
• Example Telephone system
• Connectionless service packets are routed
  individually
• Example Snail mail

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The OSI Model

• The Open Systems Interconnection (OSI) model was
  developed by the International Standards
  Organization (IS0).
• Called the ISO OSI model because it deals with
  connecting open systemsthat is, systems that are
  open for communication with other systems. Will
  use OSI for short.
• Has seven layers

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Principles on which the Seven Layers are Based

• A layer should be created where a different
  abstraction is needed
• Each layer should perform a well-defined function
• The function of each layer should be chosen with
  an eye toward defining internationally
  standardized protocols.
• The layer boundaries should be chosen to minimize
  the information flow across the interfaces
• The number of layers should be large enough that
  distinct functions need not be thrown together in
  the same layer out of necessity and small enough
  that the architecture does not become unwieldly.

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The Seven Layers

• Layer 1 Physical Layer
• Layer 2 Data Link Layer
• Layer 3 Network Layer
• Layer 4 Transport Layer
• Layer 5 Session Layer
• Layer 6 Presentation Layer
• Layer 7 Application Layer

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Layer 1 Physical Layer

• Deals with basic network hardware.
• Design issues have to do with making sure that
  when one side sends a 1 bit, it is received by
  the other side as a 1 bit, and not a 0 bit

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Layer 2 Data Link Layer

• Specify how to organize data into frames
• Specify how to transmit frames over a network

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Layer 3 Network

• Specify how addresses are assigned
• Specify how packets are forwarded from one end of
  a network to the other

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Layer 4 Transport Layer

• Specify how to handle details of reliable transfer

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Session Layer

• Specify how to establish a communication session
  with a remote system (e.g. how to login to a
  remote time-sharing computer).
• Gives specification for security details such
  authentication using passwords.

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Layer 6 Presentation Layer

• Specify how to present data.
• Such protocols needed because different brands of
  computers use different internal representations
  for integers and characters
• In other words
• Layer 6 protocols needed to translate from the
  representation on one computer to the
  representation on another

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Layer 7 Application Layer

• Each layer 7 protocol specify how a particular
  application uses a network
• Example specification for an application that
  transfers files from one computer to the other

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TCP/IP Reference Model
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Internet Layer

• Connectionless
• Permits hosts to insert packets into any network
  and have them travel independently to the
  destination (potentially on different networks)
• Packets may arrive out of orderhigher layers
  deal with ordering
• Official packet format protocol Internet
  Protocol (IP)
• Internet layers job
• deliver IP packets to their destination (packet
  routing and congestion are major issues here)
• Similar to OSI network layer

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The Transport Layer

• Allow peer entities on the source and destination
  machines to carry on a conversation
• Two end-to-end protocols defined here
• TCP (transmission control protocol)
• UDP (User datagram protocol)

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Transmission Control Protocol (TCP)

• Reliable connectionless-oriented protocol
• Allows byte streams originating on one machine to
  be delivered without error to another machine in
  the internet
• Fragments incoming byte stream into discrete
  messages and passes each one to the internet
  layer
• At destination receiving TCP process reassembles
  the receive messages into the output stream.
• Handles flow control fast sender wont swamp
  slow receiver with more messages than it can
  handle

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UDP (User Datagram Protocol)

• Unreliable, connectionless protocol no
  sequencing or flow control

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The Application Layer

• Protocols
• telnet virtual terminal
• ftp File transfer
• smtp electronic mail
• Domain Name System (DNS) maps hosts names onto
  their network addresses
• NNTP the protocol for moving USENET news
  articles around
• HTTP protocol for fetching pages on the www
• Etc.

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Host-to-Network Layer

• Not well defined
• Varies from host to host and network to network
• Host must connect to network using some protocol
  so it can send IP packets to it