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embOS/IP FTP Client

The embOS/IP FTP client is an optional extension which adds the client part of FTP protocol to the stack. FTP stands for File Transfer Protocol. It is the basic mechanism for moving files between machines over TCP/IP based networks such as the Internet. FTP is a client/server protocol, meaning that one machine, the client, initiates a file transfer by contacting another machine, the server and making requests.

The FTP client implements the relevant parts of the following RFCs. 

RFC# Description
[RFC 959] FTP - File Transfer Protocol
Direct download: ftp://ftp.rfc-editor.org/in-notes/rfc959.txt

 


Features

  • Low memory footprint.
  • Multiple connections supported.
  • Independent of the file system: Any file system can be used.
  • Independent of the TCP/IP stack: Any stack with sockets can be used.
  • Demo application included.
  • Project for executable on PC for Microsoft Visual Studio included.

Requirements

TCP/IP stack

The embOS/IP FTP client requires a TCP/IP stack. It is optimized for embOS/IP, but any RFC-compliant TCP/IP stack can be used. The shipment includes a Win32 simulation, which uses the standard Winsock API and an implementation which uses the socket API of embOS/IP.

 


FTP backgrounds

The File Transfer Protocol (FTP) is an application layer protocol. FTP is an unusual service in that it utilizes two ports, a 'Data' port and a 'CMD' (command) port. Traditionally these are port 21 for the command port and port 20 for the data port. FTP can be used in two modes, active and passive. Depending on the mode, the data port is not always on port 20.

 

When an FTP client contacts a server, a TCP connection is established between the two machines. The server does a passive open (a socket is listen) when it begins operation; thereafter clients can connect with the server via active opens. This TCP connection persists for as long as the client maintains a session with the server, (usually determined by a human user) and is used to convey commands from the client to the server, and the server replies back to the client. This connection is referred to as the FTP command connection.

The FTP commands from the client to the server consist of short sets of ASCII characters, followed by optional command parameters. For example, the FTP command to display the current working directory is PWD (Print Working Directory). All commands are terminated by a carriage return-linefeed sequence (CRLF) (ASCII 10,13; or Ctrl-J, Ctrl-M). The servers replies consist of a 3 digit code (in ASCII) followed by some explanatory text. Generally codes in the 200s are success and 500s are failures. See the RFC for a complete guide to reply codes. Most FTP clients support a verbose mode which will allow the user to see these codes as commands progress.

If the FTP command requires the server to move a large piece of data (like a file), a second TCP connection is required to do this. This is referred to as the FTP data connection (as opposed to the aforementioned command connection). In active mode the data connection is opened by the server back to a listening client. In passive mode the client opens also the data connection. The data connection persists only for
transporting the required data. It is closed as soon as all the data has been sent.

Active mode FTP

In active mode FTP the client connects from a random unprivileged port P (P > 1023) to the FTP server's command port, port 21. Then, the client starts listening to port P+1 and sends the FTP command PORT P+1 to the FTP server. The server will then connect back to the client's specified data port from its local data port, which is port 20.

Passive mode FTP

In passive mode FTP the client connects from a random unprivileged port P (P > 1023) to the FTP server's command port, port 21. In opposite to an active mode FTP connection where the client opens a passive port for data transmission and waits for the connection from server-side, the client sends in passive mode the "PASV" command to the server and expects an answer with the information on which port the server is listening for the data connection.

After receiving this information, the client connects to the specified data port of the server from its local data port.

Supported client FTP commands

embOS/IP FTP client supports a subset of the defined FTP commands. Refer to [RFC 959] for a complete detailed description of the FTP commands. The following FTP commands are implemented:

FTP command Description
CDUP Change to parent directory
CWD Change working directory
LIST List directory
MKD Make directory
PASS Password
PWD Print the current working directory
RETR Retrieve
RMD Remove directory
STOR Store
TYPE Transfer type
USER User name

 


Resource usage

The ROM usage depends on the compiler options, the compiler version and the used CPU. The memory requirements of the FTP client presented in the tables below have been measured on an ARM7 and a Cortex-M3 system. Details about the further configuration can be found in the sections of the specific example.

Configuration used

#define FTPC_BUFFER_SIZE 512
#define FTPC_CTRL_BUFFER_SIZE 256
#define FTPC_SERVER_REPLY_BUFFER_SIZE 128  // Only required in debug builds with enabled logging.

ROM usage on an ARM7 system

The following resource usage has been measured on an ARM7 system using IAR Embedded Workbench V6.30.6, Thumb mode, no interwork, size optimization.

Add-on ROM
embOS/IP FTP client Approximately 2 KByte

ROM usage on a Cortex-M3 system

The following resource usage has been measured on a Cortex-M3 system using IAR Embedded Workbench V6.30.6, size optimization.

Add-on ROM
embOS/IP FTP client Approximately 1.7 KByte

RAM usage

Almost all of the RAM used by the web server is taken from task stacks. The amount of RAM required for every child task depends on the configuration of your client. The table below shows typical RAM requirements for your task stacks.

Build Description RAM
Release A task used for the FTP client without debugging features and disabled debug outputs. Approximately 1400 Byte

 

The approximately task stack size required for the FTP client can be calculated as fol-
lows:
TaskStackSize = 2 * FTPC_BUFFER_SIZE + FTPC_CTRL_BUFFER_SIZE

 

Build Description RAM
Debug A task used for the FTP client with debugging features and enabled debug outputs. Approximately 1550 Byte

 

The approximately task stack size required for the FTP client can be calculated as fol-
lows:
TaskStackSize = 2 * FTPC_BUFFER_SIZE + FTPC_CTRL_BUFFER_SIZE + FTPC_SERVER_REPLY_BUFFER_SIZE