Intro

Reading : https://en.wikipedia.org/wiki/OSI_model
  Proposed 1983 
   - combined work of International Organization for Standardization (ISO)
   and International Telegraph and Telephone Consultative Committee (CCITT)


  Application Layer - defines overall protocols, user interface, and actions.
    HTTP (IE, Firefox), telnet & ssh (Putty), FTP (Filezilla), etc, 
    i.e display of windows that allow you to drag and drop a file name to 
      trigger the FTP transfer.

  Presentation Layer - guarantees common syntax of data transmitted.
    When submitting a program to MVS, convert ASCII text to EBCDIC. 

    Also, tasks such as data encryption or compression.

  Session Layer - provides a specific transaction between machines.
    Establishes a negotiated reserved connection and port assignment.
   
    Works to keep this open and assigned for duration of 'session'.

    Establishes task definition and control.
      Example :
        FTP - connect, login, transfer of multi-packet files without having
        to log in each time, close.

        Putty (ssh) - connect, login, continued interaction between screen 
          and keyboard, log-off and close.

      And guarantees that the file transmitted was correctly reassembled,
       i.e; if connection lost and re-established, 
         transfer continues at interruption.

    Part of the TCP protocol works at this layer.

  Transport Layer - parses data into packets of workable size for network
    layer.

    Connects software client to server.
      Establishes the 'virtual' connection between two system.

      Service access point between individual system/software and network
      technology. i.e Support of specific protocol, FTP, ssh, HTTP, etc.

    Responsible for sequencing created packets and rebuilding them on 
      delivery with minimal errors for a particular transaction.

      Transfer of a particular file with FTP.

    Level at which communicating machines interact. 

      TCP, UDP, etc.

    Responsible for success of most 'message' delivery 
      between source and destination.

layer details