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FTP - File Transfer Protocol

Notes

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Why do we need a FTP

Service?

Purpose: To Transfer files between two computers

Goals of FTP Service

Promote sharing of files (programs and/or data)

Encourage indirect/implicit use of remote

computers

Shield users from variations in file storage amonghosts

Transfer data reliably and efficiently

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Problems of File Transfer

At first, file transfer may seem simple

Heterogeneous systems use different:

Operating Systems

Character Sets

Naming Conventions

Directory Structures

File Structures and Formats

FTP need to address and resolve these problems

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FTP

RFC 959

uses two TCP Ports

one for control one for data transfers

command-response protocol

control port uses telnet protocol to negotiate

session US-ASCII

is end-of-line character

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Active Mode FTP

Client connect from a random unprivileged port (n > 1023) to the servers

command port (21) and sends port command to tell server to connect to n+1

then listens on the next higher unprivileged port (n+1) for server responses.

The server connects from its data port (20) to the client data port (n+1)

ClientServer

20 21 1026 1027

1

2

3

4

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Passive Mode FTP

Client opens two random unprivileged ports ( n > 1023 and n+1; ex 1026 and

1027) and connects the first port (n) to server command port 21 and issues a

pasv command (server sends port to use for data); client connects to servers

specified data port, server completes connection.

ClientServer

20 21 1026 1027

1

2

2024

3

4

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Transfer Files in a Heterogeneous Host Environment

Due to multiple hardware types and operating systems file

are converted to four environmentally neutral data type for

transport and the converted to local types at the destination

ASCII A NVT-ASCII

EBCDIC E EBCDIC Text

IMAGE I Raw binary, series of octets

LOCAL L Raw binary using a variable byte size

Client responsibility to tell server data type to use

Default data type, unless otherwise specified is ASCII

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File Structures

Operating System store files in different structures

FTP defined file structures for transporting files

File F Unstructured, sequence of bytes Record R Series of records

Page P Series of data blocks (pages)

Default file structure is File (F)

File Structure specified using STRU command

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Transmission Modes

Mode is used to specify additional coding or sequencing

performed on data

independent of data type and file structure Stream S stream of bytes, if record structure

EOF sent as record indication; if file

eof indicated by closing stream

Block B file sent as sequence of blocks

preceded by header info allows restartof an interruped transfer

Compressed C data compressed using run length

encoding

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FTP Commands

USER R User name, userid for access control

PASS O Password for access control

ACCT O Account info CWD O Change working directory

CDUP O Change to parent directory

SMNT O Structure mount, mount a different file system

QUIT R informs server that client wants out

REIN O restarts session at authentication phase

PORT R Host addr and data port to use

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FTP Commands (more)

PASV O Passive; informs server that client will contact

to set up data connections, ask server to sent

port info

TYPE R Data type, type of subsequent transfers

STRU R File structure

MODE R Transfer mode

RETR R Retrieve, download the file from server STOR R Store, upload the specified to server

STOU O Store unique, same as store but server picks

unique file name

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FTP Commands (more)

APPE O Append, upload file to server, if file name

exists, append the upload

ALLO O Allocate, sometimes used to preallocate space REST O Restart, restart an interrupted transfer

RNFR O Rename file from filename

RNTO O Rename file to

ABOR O Abort, ask server to abort last command

DELE O Delete specified file

RMD O Remove directory

MKD O Make directory

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FTP Commands (more)

PWD O Print working directory

LIST O Request directory listing

NLST O Request just a file name list SITE O Site parameters, allow client to specify site

specific options and parameters

SYST O request server operating system

STAT O Request server to send status of current xfr

HELP O general and command specific

NOOP R ask server to send a positive reply

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FTP Responses

Each command generates a server response

3 digit code, text,

use 3 digit code as driver for GUI Clients or programaticimplementations

use text for Command line clients

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Responses

1yz - Positive preliminary reply - command is being acted upon;

expect a final reply code before sending another command

2yz - Positive completion reply - command was successfully

executed; new command may be sent 3yz - Positive intermediate reply - command was accepted, but the

final result is being delayed because other information

needs to be supplied from the client; reply is used for

sequencing command groups

4yz - Transient negative completion reply - command failed, but thecondition is temporary

5yz - Permanent negative completion reply - command failed and

will always fail if given again; the command should not be

attempted again

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Response

x0z - Refers to command syntax

x1z - Indicates information returned by commands

requesting information such as status or help x2z - Refers to the state of the control or data connections

x3z - The reply is associated with the login process and

accounting procedures

x4z - Reserved for future use x5z - Refers to the state of the requested file transfer or

other file system command