File Access

• Taxonomy of issues related to file access
• Characteristics of file systems
  • Structure
    • One unit with no subdivisions
    • Subdivision into blocks, records, fields
    • Byte by byte access with no other structure
  • Attributes
    • Vary in different file systems
    • Minimum --> name, size, date created
    • Some others
  • Operations
    • Operations on the file as an object
    • Operations on the contents of a file
• A model for generalized file access
• A virtual filestore
  • Structure
    • A general hierarchical structure
    • Representing an unstructured file
    • Representing a flat file
  • Attributes
  • Operations
• Some available tools
  • Kermit and similar file transfer tools
    • Limited operations (connect to remote system; send or receive a whole file)
    • Extends the user's reach beyond the local file system
  • FTP
    • Also extends reach beyond the local file system
    • Invokes Telnet, the Internet Virtual Terminal Protocol
      • User authentication capability
      • Optional anonymous access
    • Two modes:
      • Two system file movement
      • Three system (control connections separate from file transfer connections)
  • FTAM

OSI Virtual Filestore Attributes

• File name
• Allowed Operations
• Access Control
• Account number
• Date and time of file creation
• Date and time of last file modification
• Date and time of last file read
• Date and time of last attribute modification
• Owner
• Identity of last modifier
• Identity of last reader
• Identity of last attribute modifier
• File availability
• Contents type
• Encryption key
• Size
• Maximum future size
• Legal Qualifications
• Private use

OSI Virtual Filestore Operations

• Create file
• Delete file
• Select file
• Deselect file
• Open file
• Close file
• Read attribute
• Change attribute
• Locate a record
• Read data from file
• Insert new data
• Replace data
• Extend some record
• Erase a record