3.1 File Transfer Tools and FTP Basics

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Questions and Answers

Which FTP command is used to change the working directory on the remote server?

  • DELE
  • PWD
  • RETR
  • CWD (correct)

In passive mode, the server uses which port for data transfer?

  • Any random port >1024 (correct)
  • Port 22
  • Port 21
  • Port 20

What is the purpose of the FTP command STOR?

  • Retrieve a file from the remote server
  • List the files in a remote directory
  • Store a file on the remote host (correct)
  • Change the working directory

Which FTP command would you use to abort a file transfer?

<p>ABOR (B)</p> Signup and view all the answers

Which of the following commands is NOT used for file transfer operations?

<p>LIST (A)</p> Signup and view all the answers

What does the FTP command RETR accomplish?

<p>Retrieve a remote file (B)</p> Signup and view all the answers

Which command is used to terminate the FTP connection?

<p>QUIT (A)</p> Signup and view all the answers

What does the command SIZE return?

<p>Size of a file (A)</p> Signup and view all the answers

What occurs when a duplicate datagram is received by either side in TFTP?

<p>A duplicate acknowledgment is generated. (A), Only the first acknowledgment is considered. (C)</p> Signup and view all the answers

What is the main issue known as Sorcerer's Apprentice Syndrome (SAS) in TFTP?

<p>Both sides retransmitting packets on receiving duplicates (A)</p> Signup and view all the answers

In what scenario would the modification to the TFTP specification become necessary?

<p>When duplicates of acknowledgments cause retransmission loops (B)</p> Signup and view all the answers

What modification did the TFTP specification introduce to handle SAS?

<p>Ignore all duplicated acknowledgments after the first instance (B)</p> Signup and view all the answers

What is the primary trigger for resending a block in the modified TFTP protocol?

<p>A timeout occurring (B)</p> Signup and view all the answers

Which layer of networking can cause loss of Ethernet frames leading to a timeout in TFTP?

<p>Data link layer (D)</p> Signup and view all the answers

What is indicated by a timeout event during a TFTP transfer?

<p>A failure to receive an acknowledgment in time (B)</p> Signup and view all the answers

What is the maximum size of each data block sent during a file transfer?

<p>512 bytes (A)</p> Signup and view all the answers

What must happen before the next data packet can be sent?

<p>Acknowledge the receipt of the last packet. (D)</p> Signup and view all the answers

What does a data packet of less than 512 bytes signal during a transfer?

<p>The transfer has been terminated. (C)</p> Signup and view all the answers

What strategy ensures that the sender stays synchronized with the receiver?

<p>Stop-Wait Protocol (A)</p> Signup and view all the answers

If a packet is lost in the network, what action does the intended recipient take?

<p>Retransmit its last packet or acknowledgment. (D)</p> Signup and view all the answers

How many packets does the sender keep for retransmission under the given protocol?

<p>One packet (B)</p> Signup and view all the answers

In the protocol discussed, which port is designated for TFTP transfer?

<p>Port 69 (B)</p> Signup and view all the answers

What initiates the transfer process in the Trivial File Transfer Protocol?

<p>A read request from the client. (B)</p> Signup and view all the answers

What does the Ack Block signify during the TFTP transfer process?

<p>The server acknowledges receipt of a data block. (A)</p> Signup and view all the answers

What packet type must the server respond with after sending a data block?

<p>Acknowledgment packet (A)</p> Signup and view all the answers

What is the purpose of the first packet sent in a TFTP file transfer?

<p>It requests a file transfer and specifies file details. (C)</p> Signup and view all the answers

How is the end of a file indicated in a TFTP transfer?

<p>By sending a block of less than 512 bytes. (B)</p> Signup and view all the answers

Which of the following correctly describes a TFTP DATA packet structure?

<p>2 byte OP Code, 2 bytes, Up to 512 bytes. (C)</p> Signup and view all the answers

What type of file modes can be specified in a TFTP request?

<p>Text or Binary (D)</p> Signup and view all the answers

What triggers the transfer in a TFTP write operation?

<p>WRQ (Write Request) (B)</p> Signup and view all the answers

What action is taken if an error message is sent during a TFTP transfer?

<p>The transfer is terminated. (B)</p> Signup and view all the answers

Which error code indicates that the requested file was not found?

<p>ERROR CODE 1 (B)</p> Signup and view all the answers

In TFTP, which port is typically used by the server to receive write requests?

<p>Port 69 (A)</p> Signup and view all the answers

What is the maximum size of a data block in a TFTP transfer?

<p>512 bytes (D)</p> Signup and view all the answers

What happens when a data packet is lost during a TFTP file transfer?

<p>The intended recipient times out and may resend the last packet. (D)</p> Signup and view all the answers

What occurs after sending Data Block 1 in a TFTP transfer?

<p>Data Block 2 is sent after receiving Ack Block 1. (C)</p> Signup and view all the answers

What type of acknowledgement does TFTP use to confirm receipt of a block?

<p>ACK with block number specified. (C)</p> Signup and view all the answers

In a TFTP process, what does 'Lost Ack (Timer operation)' refer to?

<p>The timeout when an acknowledgment of a data block is not received. (B)</p> Signup and view all the answers

How many bytes make up the OP Code in a TFTP ACK packet?

<p>2 bytes (C)</p> Signup and view all the answers

During a TFTP transfer, what is indicated by Ack Block n?

<p>The completion of the last data block transfer. (C)</p> Signup and view all the answers

Which operation begins any file transfer in TFTP?

<p>Request to read or write a file. (A)</p> Signup and view all the answers

What is the maximum size of each data block sent in TFTP?

<p>512 Bytes (D)</p> Signup and view all the answers

What type of request is sent by the client to initiate a read operation in TFTP?

<p>RRQ (Read Request) (A)</p> Signup and view all the answers

What action does the server take upon receiving an RRQ?

<p>It starts sending data blocks starting with Data Block 1. (C)</p> Signup and view all the answers

What is the role of the timeout in TFTP communication?

<p>To ensure lost packets are resent in case of no acknowledgment. (C)</p> Signup and view all the answers

Flashcards

Active Mode

A mode where the server connects back to the client on port 20 for data transfer.

Passive Mode

A mode where the client connects to a random port on the server for data transfer.

USER Command

FTP command to specify the user name during login.

PASS Command

FTP command to specify the user's password during login.

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LIST Command

FTP command to list files in the current directory.

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RETR Command

FTP command to retrieve or download a specified file.

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STOR Command

FTP command to upload a specified file to the server.

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QUIT Command

FTP command to close the current FTP connection.

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TFTP

A protocol for transferring files over a network with simple commands.

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Read Request Packet

A packet used to request a file transfer from the server to the client.

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Write Request Packet

A packet used to request a file transfer from the client to the server.

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Data Packet

Packet containing file data, includes block number and data bytes.

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Acknowledgement Packet

Packet sent by the receiver to confirm receipt of a data packet.

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Error Packet

Packet sent when there is an error during file transfer.

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Block Number

Each block of the file is numbered consecutively starting at 1.

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End of File Signal

A block of less than 512 bytes indicates the end of a file transfer.

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Error Codes

Standard codes defining specific errors during TFTP operations.

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TFTP Modes

Modes specify data transfer format; either Ascii or Binary.

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Timer operation in TFTP

How timer functions to ensure reliable data transfer in TFTP.

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Sorcerer's Apprentice Syndrome (SAS)

A bug in TFTP causing excessive duplicate transfers due to delayed packets.

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Why does SAS occur?

SAS happens when both sides retransmit due to receiving duplicates.

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How to fix SAS?

Modify TFTP to ignore duplicate acknowledgments after the first one.

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Data Block Size in TFTP

Data blocks in TFTP transfers are typically 512 bytes each.

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Timeout in TFTP

A mechanism to resend data if acknowledgment isn't received in time.

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Data Block

Chunk of data transferred in TFTP, typically 512 bytes.

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Ack Block

Acknowledgment block that confirms receipt of a data block in TFTP.

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Write Request (WRQ)

Initiates transfer from client to server in TFTP.

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Read Request (RRQ)

Initiates transfer from server to client in TFTP.

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Lost Packet

When a data packet fails to reach its destination in TFTP.

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Timeout

Period where the recipient waits for an acknowledgment before retrying in TFTP.

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Resend

Action taken when a packet is not acknowledged within a time frame.

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RFC 1350

The document that defines the TFTP protocol.

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Port 69

The port used for TFTP operations like data transfer requests.

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Fixed Length Blocks

Data is sent in consistent sizes, here, 512 bytes each.

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Retransmit

To send a packet again if the previous one was lost.

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Error Recovery

The process of restoring communication after a disruption during data transfer.

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Stop-and-Wait Protocol

A method where the sender waits for an acknowledgment before sending the next packet.

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Trivial File Transfer Protocol (TFTP)

A simple file transfer protocol designed for transferring files easily.

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Study Notes

File Transfer Tools

  • File Transfer Protocol (FTP) is a standard communication protocol used for transferring files over a network.
  • FTP is based on a client-server architecture.
  • FTP uses separate control and data connections.
  • FTP users authenticate using a clear-text sign-in protocol (username and password).
  • FTP allows anonymous connections if configured.
  • Clients include command-line interfaces, graphical user interfaces (GUIs), and web browsers (ftp://).

Motivation

  • Secure file transfer over the internet is necessary.
  • Some legacy industrial devices only support FTP or TFTP.
  • Secure firewall configuration is crucial.
  • Secure management of remote devices is essential.

File Transfer Protocol (FTP)

  • Uses TCP at the transport layer.
  • The server listens on port 21.
  • Uses separate control and data channels.
  • The control channel remains open during the session for authentication and directory listing (port 21).
  • The data channel supports file transfers and closes after each transfer (port 20 for Active mode, random port >1024 for Passive mode).
  • Two operational modes: Active and Passive mode.

Examples of FTP Commands (Authentication)

  • USER - specifies the username for login.
  • PASS - specifies the user's password.

Examples of FTP Commands (Exploring files)

  • LIST - lists files for a given specification.
  • CWD - changes the current directory.

Examples of FTP Commands (Downloading/Uplaoding Files)

  • TYPE - sets the file type (ASCII or binary).
  • RETR - retrieves a file.
  • STOR - uploads a file.

Examples of FTP Commands (Closing the Connection)

  • QUIT - closes the FTP connection.

Examples of FTP Commands (Additional)

  • ABOR - aborts a file transfer.
  • CWD - changes the working directory.
  • DELE - deletes a remote file.
  • LIST - lists remote files.
  • MDTM - returns the modification time of a file.
  • MKD - makes a remote directory.
  • NLST - names a list of remote directories.
  • PASV - enters passive mode.
  • PORT - opens a data port.
  • PWD - prints the working directory.
  • QUIT - terminates the connection.
  • RETR - retrieves a remote file.
  • RMD - removes a remote directory.
  • RNFR - renames from
  • RNTO - renames to
  • SITE - site-specific commands
  • SIZE - returns the size of a file
  • STOR - stores a file on the remote host
  • TYPE - sets transfer type

Active Mode (Simplified)

  • Client chooses its port number for the data channel.
  • The server initiates the data channel connection to the client's port.

Problem with Active Mode

  • Firewalls block incoming connections to the client's port.
  • The incoming port can be in range 1024 - 65,536.
  • Active mode requires firewall setup to allow connections from the server to the client for data transfer.

Passive Mode

  • Server chooses a port for the data channel and notifies the client.
  • Client initiates the data channel connection to the server's port.

Trivial File Transfer Protocol (TFTP)

  • RFC 1350.
  • Simple protocol for file transfers within a Local Area Network (LAN).
  • Used for transferring configuration and firmware images to networking devices.
  • Only allows reading or writing a file for a client, without authentication.
  • Uses UDP at the transport layer.
  • Only 5 commands: RRQ (Read Request), WRQ (Write Request), DATA (Data Block), ACK (Acknowledgement), and ERROR.
  • Error control with block numbers and acknowledged data transfer.

TFTP commands (RFC defines as packets)

  • Read Request (OP Code 1) includes filename and mode (ASCII or binary)
  • Write Request (OP Code 2) includes filename and mode (ASCII or binary)
  • Data packets are numbered consecutively starting at 1, contain up to 512 bytes of data.
  • Each data packet must be acknowledged by the receiver.
  • Error messages (OP Code 5) can be sent.

Trivial File Transfer Protocol (TFTP) Overview

  • Transfer starts with a request to read or write a file.
  • The server opens a logical connection.
  • Files sent in 512-byte data blocks.
  • Each block acknowledged by receiver before next sent.
  • Transfers terminate with data less than 512 bytes.
  • Receiver may resend if data block lost, due to Time Out.
  • Stop-and-wait protocol used.

TFTP Commands (Simplified)

  • Requests (RRQ, WRQ)
  • Data Blocks (DATA)
  • Acknowledgements (ACK)
  • Errors (ERROR)

Trivial File Transfer Protocol (TFTP) Issues

  • Connectionless and unreliable UDP used (need for error control).
  • Time outs & retransmission required.
  • Packet loss in network may cause time-out & retransmission
  • Block numbering (data packet integrity)

FTP Secure (FTPS)

  • Secure FTP protocol that encrypts files exchanged using SSL (Secure Sockets Layer).
  • Two types:
    • Explicit FTPS (port 21) encryption on data channel after authentication.
    • Implicit FTPS (port 990) encryption on command channels.
  • Encrypts file transfer with strong algorithms (AES and triple DES).
  • Authentication uses usernames/passwords &/or certificates.

Secure File Transfer Protocol (SFTP)

  • Secure FTP protocol using SSH.
  • Offers high protection for file transfers using TCP port 22.
  • Uses strong encryption algorithms including AES, triple DES and others.
  • Authentication methods include user names/passwords, SSH keys, and combinations.

Briefly contrasting SFTP and FTPS

  • FTPS can be difficult connecting to secured firewalls since it uses multiple port numbers (control & data channels).
  • SFTP is simpler, easier to configure firewalls since it uses a single port (port 22).
  • SFTP uses SSH keys allowing shared authentication between trading partners

The choice - SFTP or FTPS

  • IT department and/or partners may dictate which protocol is used.
  • Authentication with certificates may be required by customers and partners
  • Firewall configuration can be a deciding factor.
  • Customers may only use one port which makes SFTP preferable.

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