File transfer protocol
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File Transfer Protocol. TCP/IP suit 4th Edition by Behrouz A Forouzan. FTP. 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 <crlf> is end-of-line character. connections.

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File Transfer Protocol

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File Transfer Protocol

TCP/IP suit 4th Edition byBehrouz A Forouzan


Internet Computing (CS-413)


  • 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

    • <crlf> is end-of-line character

Internet Computing (CS-413)


  • Control connection

    • Server issues a passive open on port 21 and waits for a client

    • Client uses an ephemeral port and issues an active response

    • The connection remains open for the entire session

    • IP uses minimal delay service because it is an interactive connection between a client and a server

Internet Computing (CS-413)

Control connection

Internet Computing (CS-413)

Data connection

  • Uses port 20 on the server side

    • Client issues a passive open using an ephemeral port..

    • Client sends this port number to the server through PORT command

    • Server receives the port number and issues an active open using well known port number 20

Internet Computing (CS-413)

DATA Connection

Internet Computing (CS-413)

Communication over control connection

  • FTP uses NVT ASCII character set

  • Commands and responses

  • Each line is terminated with two character end of line token

Internet Computing (CS-413)

Communication over data connection

  • File type

    • ASCII

    • EBCDIC

    • Image

  • Non print

    • File cannot be printed w/o further processing

    • Default for text file


    • Contains NVT ASCII vertical characters like carriage return & line feed.

    • File is printable after transfer

Internet Computing (CS-413)

Communication over data connection

  • Data structure

    • File structure (default)

      • File has no structure

      • It is a continuous stream of bytes

    • Record structure

      • File is organized as records

      • Used only with text files

    • Page structure

      • File is divided into pages, each with page number and a header

Internet Computing (CS-413)

Communication over data connection

  • Transmission modes

    • Stream mode (default)

      • Data sent from FTP to TCP as stream of bytes

      • TCP segments the data

      • If file structure is used, no end-of-file marked; only closing of data connection

      • If record structure is used, 1-byte end-of-record and 1-byte end-of-file marked

    • Block mode

      • Data delivered from FTP to TCP in blocks, each with 3-byte header, including 1B block descriptor and 2B block size (in bytes)

    • Compressed mode

      • Data can be compressed using run length encoding

        • Consecutive occurrences of data replaced by one occurrence and repetition count

Internet Computing (CS-413)

Command processing

  • Command types

    • Access commands

    • File management commands

    • Data formatting commands

    • Port defining commands

    • File transfer commands

    • Miscellaneous commands

Internet Computing (CS-413)


  • Every FTP command generates at least one response

  • Response has two parts

    • Numeric code

      • 3 digit

        • 1st & 2nd digits describe status of command

        • 3rd digit provides additional information

    • Text

      • Needed parameters or explanations

Internet Computing (CS-413)

Responses – 1st digit of code

  • 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 the condition is temporary

  • 5yz- Permanent negative completion reply - command failed and will always fail if given again; the command should not be attempted again

Internet Computing (CS-413)

Responses- 2nd digit of code

  • 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

Internet Computing (CS-413)

File transfer

Internet Computing (CS-413)

File transfer- example of listing

Internet Computing (CS-413)

File transfer- example of storing

Internet Computing (CS-413)

Anonymous FTP - example

% ftp

Connected to

220 Server ready


331 Guest login OK, send “guest” as password


ftp >pwd

257 ’/’ is current directory

Internet Computing (CS-413)

Anonymous FTP - example

  • ftp >ls

  • 200 OK

  • 150 Opening ASCII mode

  • bin

  • ftp> close

  • 221 Goodbye

  • ftp> quit

Internet Computing (CS-413)

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 it’s data port (20) to the client data port (n+1)











Internet Computing (CS-413)

Active mode FTP

  • From the server-side firewall's standpoint, to support active mode FTP the following communication channels need to be opened:

    • FTP server's port 21 from anywhere (Client initiates connection)

    • FTP server's port 21 to ports > 1023 (Server responds to client's control port)

    • FTP server's port 20 to ports > 1023 (Server initiates data connection to client's data port)

    • FTP server's port 20 from ports > 1023 (Client sends ACKs to server's data port)

  • The main problem with active mode FTP actually falls on the client side. The FTP client doesn't make the actual connection to the data port of the server--it simply tells the server what port it is listening on and the server connects back to the specified port on the client. From the client side firewall this appears to be an outside system initiating a connection to an internal client--something that is usually blocked.

Internet Computing (CS-413)

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.












Internet Computing (CS-413)

Passive mode ftp

  • Passive mode FTP solves many of the problems from the client side, it opens up a whole range of problems on the server side. The biggest issue is the need to allow any remote connection to high numbered ports on the server. Fortunately, many FTP daemons, including the popular WU-FTPD allow the administrator to specify a range of ports which the FTP server will use

  • The second issue involves supporting and troubleshooting clients which do (or do not) support passive mode. Necessitating a third-party FTP client.

  • Most browsers, acting as FTP client, only support passive mode when accessing ftp:// URLs. This can either be good or bad depending on what the servers and firewalls are configured to support

Internet Computing (CS-413)

  • Login