FTP, TFTP, and NFS
LESSON 25

Outline:
FTP, TFTP, and NFS

File Sharing:

• Client / Server File Interaction:
• File sharing is used to reduce storage costs.
• Remote file access extends the reach of system resources, further leveraging these costs.
• Distributed file storage and remote access allows multiple users to work on and modify the same file from anywhere on the network.
• There are two distinct forms of file sharing.

File Sharing:
On-line Access

• On-line access:
  • Allows multiple programs to access a single file concurrently.
  • This should be integrated with local files and access should be transparent.
• Advantage - no visible changes in the environment.
• Disadvantage - congested networks make file access slower.

File Sharing:
Whole-file Copying

• Whole-file copying:
  • This transfer scheme requires a two-step process in which the user first obtains a local copy of a file and then operates on it.
  • This is not integrated with the local file-system and requires the use of a client program.
  • Advantage - efficient manipulation operations.
  • Disadvantage - applying updates to stored file on server.

File Sharing:
Requirements

• Both methods of file sharing are difficult with heterogeneous machines.
• They must agree on:
  • Authorization.
  • Notions of file ownership and access protections.
  • Data formats.

File Sharing:
Version Control

• Another major concern when sharing files is version control:
  • Each user may make changes either locally or shared remotely.
  • The operating system must know which user’s changes to write first, and then notify all other users that the file has now changed.
  • This same issue plagues many different computer science areas.

File Transfer Protocol (FTP):

• File Transfer Protocol (FTP) - is a reliable file transfer protocol used within TCP/IP.
• It offers:
  • Interactive access with a human friendly interface.
  • File format specifications (ASCII, Binary)
  • Authentication Control (including login names and passwords)

Control and Data Transfer Connections:

• FTP allows concurrent access by multiple clients, using the transmission control protocol.
• Slave processes are used in FTP to accept and handle the control connection from the client.
• However, they use an additional process (or processes) to handle the separate data transfer connection(s) needed to send and receive files.

• Control processes and the control connection remain alive as long as the FTP "session" is going.
• A separate data transfer connection is setup and taken down for each file transfer.
• In addition to passing user commands to the server, FTP uses the control connection to allow the client and server control processes to coordinate their use of:
  • Dynamically assigned TCP protocol ports.
  • The creation of data transfer processes that use those ports.
• FTP uses Telnet's Network Virtual Terminal specification when passing data across the control connection.
• The basic definition of service is only used, no option negotiation is allowed.
• Commonly, FTP is shown to conceptually reside above TELNET in a layered protocols stack, strictly for this reason.

Interactive Transfer System:

• Interactive Transfer System (client performs operations repeatedly):
  • Read a line of input.
  • Parse the line to extract the command and arguments.
  • Execute the command with it’s arguments.

Anonymous FTP:

• Anonymous FTP access means the user does not need an account or password, but can freely login to a remote machine.
  • Login name: Anonymous
  • Password: Guest or full e-mail address.

Anonymous FTP:
Terminal-Based

• When anonymously logging into a remote site via an text-based connection, the user must explicitly interact with the machine using a set of commands (? from prompt).
• To download a file, the user must:
  • Login and send guest password
  • Select the directory the file exists in.
  • Change the transfer option based on file type.
  • Explicitly get the file and then exit.

Anonymous FTP:
WWW-based

• When accessing a FTP server through a web client (Netscape), no interactive commands are needed.
• Differences between terminal and WWW access:
  • No login required or file-type transfer option setting.
  • Graphical traversal of subdirectories.
  • No file uploading (PUT) allowed, only GET.
  • Applications can be spawned locally based on file type transferred.

Control and Error Information Messages:

• Control and error information messages between the FTP client and server begin with a 3-digit number followed by text.
• The software interprets the number.
• The text is solely meant for humans.
• The PORT command, used by a client, reports that a new TCP port number has been obtained for use in a data connection.
• The port number used for this connection increases each time a connection is made (by any client).
• To see more detailed interaction information, execute ftp with verbose mode on:
  • ftp -d hertz.njit.edu

Trivial File Transfer Protocol (TFTP):

• Trivial File Transfer Protocol (TFTP) - This simple transfer protocol:
  • Does not need a reliable stream transport service (uses UDP).
  • Is intended for applications that do not need complex interactions between the client and server.
• TFTP:
  • Restricts operations to small file transfers.
  • Does not provide authentication.
  • And is much smaller (software) than FTP.

TFTP:
Encoding and Transmission

• TFTP is commonly encoded in ROM (Read-Only Memory) on workstations and is used to obtain the initial bootstrap memory image (BOOTP).
• A TFTP transmission is symmetric (implements a time-out and retransmission mechanism) which helps ensure transfer will not fail after a single packet loss.
• Sorcerer’s Apprentice Bug:
  • This anomaly arises when an ACK for data packet K is delayed, but not lost.
  • It creates a cycle which continues indefinitely, with each packet transmitted at least twice.
• TFTP supports multiple file types and can be integrated with electronic mail.

Network File System (NFS):

• Network File System (Sun NFS):
  • Provides on-line shared file access that is transparent and integrated within operating systems.
  • NFS is usually embedded within the UNIX operating system and is commonly referred to NFS Mount.
  • NFS refers to the complete family of products (Sun calls it Open Network Computing).

• NFS has been extended from UNIX to allow PCs to access other computers using PC-NFS.
• NFS provides a resource-sharing network system with similar features to Novell Netware or Microsoft LAN Manager.
• The major difference is that NFS bases the major part of its functionality on the native O.S. available on the server hosts.
• NFS is not proprietary, yet is just as if not more powerful than commercial resource-sharing systems.
• NFS is usually implemented over TCP/IP, but is not exclusive to it.

NFS Architecture:

• NFS is made up of three layers:
  • RPC - which defines the format of messages used by remote procedure calls.
  • XDR - the eXternal Data Representation is a consistent representation of data between different machine architectures.
  • NFS - the Network File System is an application interface for file transfer, access, and management.

Network File System (NFS):

• The functionality of the layers maps very closely to the OSI functionality available in its equivalent layers (application, presentation, session).
• The operation of NFS is based on a stateless relationship between the client and server.
• NFS uses UDP and fixed retransmission time-outs to provide a low consistent delay over local area networks.

Remote Procedure Call (RPC):

• Remote Procedure Calls (RPC):
  • These are general purpose remote execution mechanisms which hide all details of lower-level protocol use.
  • They are used (in part) to implement NFS and other client / server programs.
  • RPC is used in realizing distributed computing systems.
• RPC and distributed computing has the ability to reduce the amount of data transferred across a network, especially where databases are concerned.
• The RPC protocol sends messages over the network to handle transactions, verification, and authorization.

RPC Message Specification:

External Data Representation (XDR):

• External Data Representation (XDR):
  • Provides a way to pass data among heterogeneous machines without the need for conversion routines.
• A program using XDR must know (in advance) what type of data to expect and provision for it accordingly.
• In an OSI system, each value is labeled with its type and size.
• XDR defines data in multiples of four bytes (32-bits), which is a definition of how the data should formed before network transmission.
• An NFS client formats data based on XDR so an NFS server will translate data into the form understood by the destination computer.
• UNIX and PC operating systems use the combination of NFS, RPC, and XDR to provide distributed system features.

Last Modification: (Sunday, August 25, 1996)

All work was written, produced, and is copyrighted by Daniel Z. Tabor Jr.
Page created by Daniel Z. Tabor Jr. Copyright ©1996 Illusion Industries Inc.