File Transfer Protocol (FTP)

JMU

File Transfer Protocol (FTP)
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Introduction

Purpose:
- Transfer files
Specification:
- RFC 959

Channels/Connections in FTP

Control Channel:
- Uses port 21
- For things like: user ID, passwords, commands
Data Channel:
- Uses port 20
- For the actual file

Modes

Active Mode:
- The local host initiates communication on the control channel
- The remote host then initiates communication on the data channel
Passive Mode:
- The local host initiates communication on the control channel
- The remote host responds with host information to use to create the data channel
- The local host then uses that information to initiate communication on the data channel

Commands

Properties:
- Sent from client to server
- Use the control channel
- Sent in ASCII
Examples:
- USER SP username CRLF
- PASS SP password CRLF
- CWD SP directory CRLF
- LIST [SP pathname] CRLF
- RETR SP pathname CRLF
- STOR SP pathname CRLF

Reply Codes

First Digit:
- 1 - Positive preliminary reply
- 2 - Positive completion reply
- 3 - Positive intermediate reply
- 4 - Transient negative completion reply
- 5 - Permanent negative completion reply
Second Digit:
- 0 - Syntax error
- 1 - Request for information
- 2 - Related to the connection
- 3 - Related to authentication/accounting
- 5 - File system status
Third Digit:
- Details

State Information

The Concept:
- A sequence of commands/responses (usually starting with USER and ending with QUIT) defines a session
- The protocol allows for an infinite number of possible sequences of commands
The Implication:
- The server must maintain information about the user (e.g., whether the password has been transmitted, the current working directory)