1. File Transfer Protocol CS-328 Dick Steflik

2. 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

3. 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) ClientServer 202110261027 1 2 3 4

4. 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 202110261027 1 2 2024 3 4

5. 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 –ASCIIANVT-ASCII –EBCDICEEBCDIC Text –IMAGEIRaw binary, series of octets –LOCALLRaw binary using a variable byte size Client responsibility to tell server data type to use Default data type, unless otherwise specified is ASCII

6. File Structures Operating System store files in different structures FTP defined file structures for transporting files –FileFUnstructured, sequence of bytes –RecordRSeries of records –PagePSeries of data blocks (pages) Default file structure is File (F) File Structure specified using STRU command

7. Transmission Modes Mode is used to specify additional coding or sequencing performed on data independent of data type and file structure –StreamSstream of bytes, if record structure EOF sent as record indication; if file eof indicated by closing stream –BlockBfile sent as sequence of blocks preceded by header info allows restart of an interruped transfer –CompressedCdata compressed using run length encoding

8. FTP Commands USERRUser name, userid for access control PASSO Password for access control ACCTO Account info CWDOChange working directory CDUPOChange to parent directory SMNTOStructure mount, mount a different file system QUITRinforms server that client wants out REINOrestarts session at authentication phase PORTRHost addr and data port to use

9. FTP Commands (more) PASVOPassive; informs server that client will contact to set up data connections, ask server to sent port info TYPERData type, type of subsequent transfers STRURFile structure MODERTransfer mode RETRRRetrieve, download the file from server STORRStore, upload the specified to server STOUOStore unique, same as store but server picks unique file name

10. FTP Commands (more) APPEOAppend, upload file to server, if file name exists, append the upload ALLOOAllocate, sometimes used to preallocate space RESTORestart, restart an interrupted transfer RNFRORename file from filename RNTOORename file to ABOROAbort, ask server to abort last command DELEODelete specified file RMDORemove directory MKDOMake directory

11. FTP Commands (more) PWDOPrint working directory LISTORequest directory listing NLSTORequest just a file name list SITEOSite parameters, allow client to specify site specific options and parameters SYSTOrequest server operating system STATORequest server to send status of current xfr HELPOgeneral and command specific NOOPRask server to send a positive reply

12. FTP Responses Each command generates a server response –3 digit code, text, use 3 digit code as driver for GUI Clients or programatic implementations use text for Command line clients

13. 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 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

14. 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

15. Trivial File Transfer Protocol RFC 1782 UDP port 69 Simple protocol, usually used to transfer configuration files Usually used for transferring boot file for diskless hosts (X-Stations) or updating NVRAM

16. TFTP Typically used in short distance, low noise environments Server is usually implement in firmware for updating things like routers, bios...

17. TFTP Because of its compact size: –no error recovery like TCP based FTP –no command structure like FTP –cannot list directories –transfers to server are to a single configured directory

18. Transfer Modes netascii (8-bit ASCII) raw binary mail (obsolete, not used)

19. TFTP Transfers client send a RRQ or WRQ to server server sends ACK 0 client sends DAT1 repeat until done

20. TFTP Formats Type Op # Format without header 2 bytes string 1 byte string 1 byte ------------------------------------------------------ RRQ/ | 01/02 | Filename | 0 | Mode | 0 | WRQ ------------------------------------------------------- 2 bytes 2 bytes n bytes --------------------------------- DATA | 03 | Block # | Data | --------------------------------- 2 bytes 2 bytes ----------------------------- ACK | 04 | Block # | ----------------------------- 2 bytes 2 bytes string 1 byte ----------------------------------------------------- ERROR | 05 | ErrorCode | ErrMsg | 0 | -----------------------------------------------------