Protocol Suites and Layering Models
      Defn:  Protocols are agreements about how communication should take place. 
      They specify:
            • Format of messages
            • Meaning of messages
            • Rules for exchanging messages
            • Procedures for handling problems
      Network hardware functions at a very low level. 
      Hardware related problems can occur that need to be addressed by protocols:
            • Bits can be corrupted or destroyed
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            • Entire packets can be lost
            • Packets can be duplicated
            • Packets can be delivered out of order
      Protocols can also be used to distinguish among:
            • Multiple computers on a network
            • Multiple applications on a computer
            • Multiple copies of a single application on a computer
     Sets of Protocols
     • Sets of protocols are designed to work together.
     • Each protocol solves a small part of the communications problem
     • Sets of protocols are known as:
             -- Protocol Suites
             -- Protocol families
     • They are designed in layers.
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        Protocol Design
        • Protocols are divided into layers
        • Each layer is devoted to one sub-problem
                                           Example: The ISO (International Standards 
                                       Organization) OSI (Open System Interconnection)  7-
                                                   layer Reference Model
       The OSI 7-Layer Model
       The 7-Layer Model:
         • Was defined fairly early in the development of networks
         • Is now somewhat dated
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         • Does not include the internet layer
      The OSI 7-Layers
      Layer 1: Physical                        Layer 5: Session
          • The underlying hardware                • Login and passwords
      Layer 2: Data Link (media access)        Layer 6: Presentation
          • Hardware frame definitions             • Data representation
      Layer 3: Network                         Layer 7: Application
          • Packet forwarding                      • Individual application programs
      Layer 4: Transport
          • Reliability
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    The Layering Model
    Protocol software follows the 
    layering model, with:
      • One software module per layer
      • Modules that work together
      • Incoming or outgoing data 
       passing from one module to 
       another
        The entire set of protocol 
         The entire set of protocol 
       layers (or modules) is known 
       layers (or modules) is known 
             as a stack.
              as a stack.
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