Notes On Chapter Thirty-Four -- File Transfer and Remote File Access

• 34.1 Introduction
  
  
  • File Transfer Protocol (FTP) is well-established software that uses the client-server paradigm
  
  
• 34.2 Generalized File Transfer
  
  
  • In the early days of networking it quickly became apparent that there was a need for general file transfer software - to transfer various types of files between various types of computers.
    
    
  • Complications:
    • conventions for naming files vary.
    • file formats, protection methods, and data representation vary.
  
  
• 34.3 The File Transfer Protocol (FTP)
  
  
  • FTP:
    • deals well with differences between computing systems,
    • permits transfer of arbitrary data, and
    • deals well with file permissions and ownership.
    
    
  • FTP has been around longer than IP or TCP.
  
  
• 34.4 FTP General Model and User Interface
  
  
  • There are many different programs that use FTP. (You have to make a distinction between a program named "ftp" and the protocol itself "FTP.")
    
    
  • FTP accommodates both batch and interactive styles of file transfer, but it is most commonly used interactively.
    
    
  • A program can run FTP code without user intervention.
  
  
• 34.5 FTP Commands
  
  
  • Many FTP programs are based on the old BSD unix file transfer program (ftp)
    
    
  • Figure 34.1 on page 515 of the fourth edition lists the commands provided by BSD ftp.
  
  
• 34.6 Connections, Authorization, and File Permission
  
  
  • A user of FTP has to login as a user on the remote machine.
    
    
  • After this authentication FTP establishes a control connection between the client and server.
    
    
  • The client uses the control connection to send commands to the server.
  
  
• 34.7 Anonymous File Access
  
  
  • There is a special mode of FTP use where the user logs in as "anonymous" [usually the alias "ftp" is accepted - for those of us who don't want to (or can't :-) spell out "anonymous".]
    
    
  • Access is granted with no password, or just by requiring the user to enter an e-mail address.
    
    
  • Anonymous FTP is useful for allowing downloads and uploads of material that is "posted" in a shared public area of a host's file system - electronic bulletin boards.
  
  
• 34.8 File Transfer In Either Direction
  
  
  • One can transfer files from local to remote with send, put, or mput.
    
    
  • One can transfer files from remote to local with get or mget.
    
    
  • However these operations are constrained by whatever access restrictions have been placed on directories and files.
  
  
• 34.9 Wildcard Expansion in File Names
  
  
  • One may use wildcard characters with mput or mget.
    
    
• 34.10 File Name Translation
  
  
  • Sometimes when we transfer files from one computer to another, we need to change the names of files because of incompatible naming rules. BSD ftp has commands for specifying how names should be changed. For example you can use mget with a wildcard and all the filenames will be mapped from lower-case to upper-case and/or extensions of .txt will be changed to .ASC.
    
    
• 34.11 Changing Directories and Listing Contents
  
  
  • To allow users to deal with the directory system on the remote computer, FTP supports pwd, cd, cdup, dir, and ls commands.
    
    
• 34.12 File Types and Transfer Modes
  
  
  • FTP can transfer in text mode or binary mode.
    
    
  • Text mode is for files that consist solely of sequences of characters separated into lines.
    
    
  • Binary mode is for everything else.
    
    
  • Text mode will handle translation between text representation differences between machines. For example it will substitute the linefeed only convention of unix for the carriage-return, linefeed convention of DOS and Windows.
    
    
  • Binary mode does no translation -- just transfers bit-for-bit.
    
    
• 34.13 Example FTP Session
  
  
  • The example is on pages 520-522 of the fourth edition. You can successfully connect to ftp.cs.purdue.edu with ftp and perform the interaction illustrated. Try it!
    
    
• 34.14 Verbose Output
  
  
  • ftp has a verbose command that toggles verbose and quiet modes of operation.
    
    
• 34.15 Client-Server Interaction in FTP
  
  
  • The client does not just pass all the characters typed by the user to the server.
    
    
  • The client interprets what the user types and sends commands to the server (when necessary) that achieve the result desired by the user.
    
    
  • The client and server communicate using the language of FTP -- the protocol.
  
  
• 34.16 Control and Data Connections
  
  
  • For each file transfer the client and the server form a separate connection -- distinct from the original control connection.
    
    
  • The control connection persists for the duration of a session (what happens between an open and a close) but each data connection is terminated after transferring one file.
    
    
  • The FTP server on the remote host acts as a client when it opens a data connection back to the local host.
    
    
• 34.17 Data Connections And End Of File
  
  
  • Advantages of having separate control and data connections:
    
    
    1. Implementation is simple - no problem distinguishing data from control commands.
       
       
    2. Client can control the file transfer interactively -- for example s/he can send an abort command in the middle of the transfer.
       
       
    3. The sender does not have to state the size of the file before starting the transfer.
       
       The sender may write to the end of the file while the (beginning of) the file is being transferred.
       
       The sender closes the data connection to signal that the transfer has completed. The receiver gets the end-of-file condition from the underlying protocol.
       
       
• 34.18 Trivial File Transfer Protocol
  
  
  • TFTP is Trivial File Transfer Protocol.
    
    
  • TFTP uses UDP and FTP uses TCP.
    
    
  • TFTP is not interactive.
    
    
  • TFTP can only get public files that it already "knows about."
    
    
  • TFTP software can be small enough so that TFTP, UDP, and IP will all fit in ROM.
    
    
  • Diskless workstations commonly use TFTP when they boot.
    
    
  • A ROM program in the client broadcasts a request, establishes a connection with a boot server, uses TFTP to download an image of an OS, and executes the OS.
    
    
  • It can be cost-effective to use diskless clients that mount file systems from file servers (or clients with only low-capacity local disks).
    
    
  • It is easy to "upgrade" the OS of a large number of diskless workstations. Just change the software they download from the boot server.
    
    
• 34.19 Network File System
  
  
  • NFS uses UDP to transfer file blocks across a network between a client host and a server.
    
    
  • NFS makes it possible for a client to mount a remote file system onto an arbitrary location in its directory "tree."
    
    
  • NFS is an alternative to FTP. NFS is for block-at-a-time transfers while FTP is oriented towards transferring whole files.
    
    
• 34.20 Summary