ICSA 411: Week 4 Protocol Architectures

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ICSA 411 Week 4Protocol Architectures

• Elizabeth Lane Lawley, Instructor

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Simple Protocol Architecture

• Not an actual architecture, but a model for how
  they work
• Similar to pseudocode, used for teaching
  programming
• Once we understand the building blocks, we can
  look at specific examples
• Open Systems Interconnection (OSI) 7-Layer Model
• TCP/IP
• Systems Network Architecture (SNA)

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Why Use Protocol Architecture?

• Data communications requires complex procedures
• Sender identifies data path/receiver
• Systems negotiate preparedness
• Applications negotiate preparedness
• Translation of file formats
• For all tasks to occur, high level of cooperation
  is required

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Modular Approach

• Break tasks into subtasks
• Each module handles specific subset of tasks
• Communication occurs
• between different modules on the same system
• between similar modules on different systems

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Simple Modular Example

• File transfer facility
• Three modules
• File transfer module could handle translation and
  inter-application communication
• Communication service module could handle
  negotiation of preparedness, data flow
• Network access module could handle data path

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Advantages of Modularity

• Easier application development
• Network can change without all programs being
  modified

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Three-Layer Model

• Distributed data communications involves three
  primary components
• Applications
• Computers
• Networks
• Three corresponding layers
• Network access layer
• Transport layer
• Application layer

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Network Access Layer

• Concerned with exchange of data between computer
  and network
• Includes addressing, routing, prioritizing, etc
• Different networks require different software at
  this layer
• Example X.25 standard for network access
  procedures on packet-switching networks

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Transport Layer

• Concerned with reliable transfer of information
  between applications
• Independent of the nature of the application
• Includes aspects like flow control and error
  checking

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Application Layer

• Logic needed to support various applications
• Each type of application (file transfer, remote
  access) requires different software on this layer

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Addressing

• Each computer on a network requires a unique
  address on that network
• Each application on the computer must have a
  unique address within the computer to allow the
  transport layer to support multiple applications
• Data units must include network and application
  addresses

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Standardized Protocol Architectures

• Vendors like standards because they make their
  products more marketable
• Customers like standards because they enable
  products from different vendors to interoperate
• Two protocol standards are well-known
• TCP/IP widely implemented
• OSI well-known, less used, still useful for
  modeling/conceptualizing

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OSI

• Application
• Presentation
• Session
• Transport
• Network
• Data Link
• Physical

• Open Systems Interconnection
• Developed by ISO
• Contains seven layers(see page 358)

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OSI Lower Layers

• Physical
• Data Link
• Network

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OSI Physical Layer

• Responsible for transmission of bits
• Always implemented through hardware
• Encompasses mechanical, electrical, and
  functional interfaces
• e.g. RS-232

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OSI Data Link Layer

• Responsible for error-free, reliable transmission
  of data
• Flow control, error correction
• e.g. HDLC

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OSI Network Layer

• Responsible for routing of messages through
  network
• Concerned with type of switching used
• Handles routing between networks, as well as
  through packet-switching networks

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OSI Upper Layers

• Transport
• Session
• Presentation
• Application

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OSI Transport Layer

• Isolates messages from lower and upper layers
• Breaks down message size
• Monitors quality of communications channel
• Selects most efficient communication service
  necessary for a given transmission

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OSI Session Layer

• Establishes connections between systems
• Manages log-ons, password exchange, log-offs
• Tracks physical location of files on both sides
  of a transfer

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OSI Presentation Layer

• Provides format and code conversion services
• Examples
• File conversion from ASCII to EBDIC
• Invoking character sequences to generate bold,
  italics, etc on a printer

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OSI Application Layer

• Provides access to network for end-user
• Users capabilities are determined by what items
  are available on this layer

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OSI in Action Outgoing File Transfer

• FTP program issues command to Application Layer
• Application passes it to Presentation, which may
  reformat, passes to Session
• Session requests a connection, passes to
  Transport
• Transport breaks file into chunks, passes to
  Network

• Network selects the datas route, passes to Data
  Link
• Data Link adds error-checking info, passes to
  Physical
• Physical transmits data, which includes
  information added by each layer

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OSI in Action Incoming File Transfer

• Physical receives bits, passes to Data Link
• Data Link checks for errors, passes to Network
• Network verifies routing, passes to Transport
• Transport reassembles data, passes to Session
• Session determines if transfer is complete, may
  end session, passes to Presentation

• Presentation may reformat, perform conversions,
  pass to Application layer
• Application presents results to user (e.g.
  updates FTP program display)

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TCP/IP v. OSI

• See diagram page 359 for relationships
• Most production software uses TCP/IP rather
  than OSI
• Why has OSI lost the war? Two primary reasons
• Not as mature as TCP/IP
• Unnecessarily complex (seven layers rather than
  five)

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Why Study OSI?

• Still the best model for conceptualizing and
  understanding protocol architectures
• Later ICSA networking classes expect you to know
  and understand this model
• Key points
• Modular
• Hierarchical
• Boundaries between layersinterfaces

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TCP/IP

• Transmission Control Protocol/Internet Protocol
• Developed by DARPA
• No official protocol standard
• Can identify five layers

• Application
• Host-to-Host (transport)
• Internet
• Network Access
• Physical

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TCP/IP Physical Layer

• Physical interface between a DTE (e.g. computer
  or terminal) and a transmission medium
• Specifies
• Characteristics of medium
• Nature of signals
• Data rate
• Similar to mechanical aspects of RS-232

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TCP/IP Network Access

• Exchange of data between end system and network
• Address of host and destination
• Prioritization of transmission
• Software at this layer depends on network (e.g.
  packet-switching vs. Ethernet)
• Segregation means that no other software needs to
  be concerned about net specifics

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TCP/IP Internet Layer

• An Internet is an interconnection of two or more
  networks
• Internet layer handles tasks similar to network
  access layer, but between networks rather than
  between nodes on a network
• Uses IP for addressing
• Implemented in workstations and routers

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TCP/IP Transport Layer

• Also called host-to-host layer
• Reliable exchange of data between applications
• Uses TCP protocols for transmission

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TCP/IP Application Layer

• Logic needed to support variety of applications
• Separate module supports each type of application
  (e.g. file transfer)