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Lecture 10 FTP & TFTP

Lecture 10 FTP & TFTP. CPE 401 / 601 Computer Network Systems. slides are modified from Dave Hollinger and Michael mgunes. Overview. File Transfer Protocol (RFC 959) Why FTP? FTP’s connections FTP in action FTP commands/responses Trivial File Transfer Protocol (RFC 1350)

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Lecture 10 FTP & TFTP

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  1. Lecture 10FTP & TFTP CPE 401 / 601Computer Network Systems slides are modified from Dave Hollinger and Michael mgunes

  2. Overview • File Transfer Protocol (RFC 959) • Why FTP? • FTP’s connections • FTP in action • FTP commands/responses • Trivial File Transfer Protocol (RFC 1350) • TFTP and TFTP’s message formats • FTP and TFTP compared FTP

  3. 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 among hosts • Transfer data reliably and efficiently FTP

  4. 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 FTP

  5. The FTP Model PI: Protocol Interpreter DTP: Data Transfer Protocol User Interface User Control Server PI User PI Data File System File System Server DTP User DTP FTP

  6. Control and Data Connections • Control functions (commands) and reply codes are transferred over the control connection. • All data transfer takes place over the data connection. • The control connection must be “up” while data transfer takes place. FTP

  7. Control Connection • The control connection is the “well known” service. • The control connection uses the TELNET protocol. • Commands and replies are all line oriented text (default is ASCII). FTP

  8. * Persistent command and reply connection Non-persistent data connection Server is listening on port 21 for connection requests * Insulates users from “raw” FTP commands *Server uses port 20 for data connections * Routes “raw” FTP commands * Receives server’s replies FTP’s Connections client User Interface server User Protocol Interpreter Server Protocol Interpreter Control Connection 21 User Data Transfer Function Data Connection Server Data Transfer Function 20

  9. FTP’s Connections – Establishment ftp> open cse.unr.edu Connected to cse.unr.edu 220 cse FTP server ready. 530 Please login with USER and PASS client 331 Password req for mgunes. Password: User Interface 230 User mgunes logged in. ftp> USER mgunes PASS mypass server User Protocol Interpreter Server Protocol Interpreter Control Connection User Data Transfer Function Server Data Transfer Function Data Connection

  10. FTP’s Connections Data Transfer client ls client.txt -rw-r--r-- mgunes client.txt User Interface 226 Closing Data Connection PORT 192,168,100,173,19,137 LIST client.txt 200 Port Command Sucessful 150 Data Connection will be open shortly server User Protocol Interpreter User Protocol Interpreter Server Protocol Interpreter Server Protocol Interpreter Control Connection Passive open on Port 5001 User Data Transfer Function Server Data Transfer Function Data Connection Establish Data Connection

  11. FTP’s Connections – Connection Closing bye client User Interface 221 Service Closing QUIT server User Protocol Interpreter Server Protocol Interpreter Control Connection User Data Transfer Function Server Data Transfer Function Data Connection

  12. FTP Connections FTP

  13. FTP Client Commands (issued by user interface) * Sent to server as multiple command by User Protocol Interpreter

  14. Access Control Commands USER specify user PASS specify password CWD change directory CDUP change directory to parent QUIT logout FTP

  15. Transfer Parameter Commands PORT publish local data port PASV server should listen TYPE establish data representation MODE establish transfer mode STRU establish file structure FTP

  16. Data Transfer Modes • STREAM: • file is transmitted as a stream of bytes. • BLOCK: • file is transmitted as a series of blocks preceded by headers containing count and descriptor code (EOF, EOR, restart marker). • COMPRESSED: • uses a simple compression scheme - compressed blocks are transmitted. FTP

  17. Service Commands RETR retrieve file STOR send file STOU send file and save as unique APPE send file and append ABOR abort prev. service command PWD print working directory LIST transfer list of files over data link FTP

  18. FTP Replies • All replies are sent over control connection. • Replies are a single line containing • 3 digit status code (sent as 3 numeric chars). • text message. • The FTP spec. includes support for multiline text replies. FTP

  19. FTP Reply Status Code First digit of status code indicates type of reply: ‘1’: Positive Preliminary Reply (got it, but wait). ‘2’: Positive Completion Reply (success). ‘3’: Positive Intermediate Reply (waiting for more information). ‘4’: Transient Negative Completion (error - try again). ‘5’: Permanent Negative Reply (error - can’t do). FTP

  20. FTP Reply Status Code • 2nd digit indicates function groupings. ‘0’: Syntax (problem with command syntax). ‘1’: Information (reply to help or status cmds). ‘2’: Connections (problem with a connection). ‘3’: Authentication (problem with login). ‘4’: Unspecified. ‘5’: File system (related to file system). • 3rd digit indicates specific problem within function group. FTP

  21. Example FTP Responses • 120 Service will be ready shortly • 200 Command OK • 230 User login OK • 331 User name OK; password is needed • 421 Service not available • 530 User not logged in • 552 Requested action aborted; exceeded storage allocation FTP

  22. RFC 959 • The RFC includes lots more information and many details including: • parameters for commands • lists of reply status codes • protocol state diagrams • support for a variety of file structures • sample sessions FTP

  23. Trivial FTP (TFTP) • Used only to read and write files from/to a remote server • Cannot list directories • Useful for bootstrapping diskless systems • Workstations • X terminals • Simple and small: • 5 message formats • Runs on UDP • Designed to fit in ROM • Uses a “stop and wait” protocol • NO BUILT IN SECURITY FEATURES (login) TFTP

  24. Diskless Workstation Booting 1The call for help Help! I don't know who I am! My Ethernet address is: 4C:23:17:77:A6:03 Diskless Workstation RARP TFTP

  25. The answer from the all-knowing RARP Server I know all! You are to be know as: 128.113.45.211 Diskless Workstation RARP REPLY TFTP

  26. The request for instructions I need the file named boot-128.113.45.211 Diskless Workstation TFTP Request (Broadcast) TFTP

  27. The dialog TFTP Server here is part 1 I got part 1 here is part 2 Diskless Workstation boot file TFTP File Transfer TFTP

  28. TFTP Protocol 5 message types: • Read request • Write request • Data • ACK (acknowledgment) • Error • Each is an independent UDP Datagram • Each has a 2 byte opcode (1st 2 bytes) • The structure of the rest of the datagram depends on the opcode. TFTP

  29. FILENAME MODE DATA ERROR MESSAGE TFTP Message Formats OPCODE 0 0 OPCODE BLOCK# OPCODE BLOCK# OPCODE BLOCK# 0 2 bytes 2 bytes TFTP

  30. TFTP transfer modes • octet : for transferring binary files. • no translation done. • netascii : for transferring text files. • all lines end with \r\n (CR,LF). • provides standard format for transferring text files. • both ends responsible for converting to/from netascii format. TFTP

  31. NetAscii Transfer Mode Unix - end of line marker is just '\n' • receiving a file • you need to remove '\r' before storing data. • sending a file • you need to replace every '\n' with "\r\n" before sending TFTP

  32. Read Request 01 filename 0 mode 0 null terminated ascii string containing name of file null terminated ascii string containing transfer mode 2 byte opcode network byte order variable length fields! TFTP

  33. Write Request 02 filename 0 mode 0 null terminated ascii string containing name of file null terminated ascii string containing transfer mode 2 byte opcode network byte order variable length fields! TFTP

  34. TFTP Data Packet 03 block # data 0 to 512 bytes 2 byte block number network byte order 2 byte opcode network byte order all data packets have 512 bytes except the last one. TFTP

  35. TFTP Acknowledgment 04 block # 2 byte block number network byte order 2 byte opcode network byte order TFTP

  36. TFTP Error Packet 05 errcode errstring 0 null terminated ascii error string 2 byte opcode network byte order 2 byte error code network byte order TFTP

  37. TFTP Error Codes (16 bit int) 0 - not defined 1 - File not found 2 - Access violation 3 - Disk full 4 - Illegal TFTP operation 5 - Unknown port 6 - File already exists 7 - No such user TFTP

  38. TFTP Connection Establishment TFTP

  39. TFTP Session TFTP

  40. Lost Data Packets - Original Protocol Specification • Sender uses a timeout with retransmission. • sender could be client or server. • Duplicate data packets must be recognized and ACK retransmitted. • This original protocol suffers from the "sorcerer’s apprentice syndrome". TFTP

  41. Sorcerer’s Apprentice Syndrome send DATA[n] (time out) retransmit DATA[n] receive ACK[n] send DATA[n+1] receive ACK[n] (dup) send DATA[n+1](dup) ... receive DATA[n] send ACK[n] receive DATA[n] (dup) send ACK[n] (dup) receive DATA[n+1] send ACK[n+1] receive DATA[n+1] (dup) send ACK[n+1] (dup) TFTP

  42. The Fix • Sender should not resend a data packet in response to a duplicate ACK. • If sender receives ACK[n] • don’t send DATA[n+1] if the ACK was a duplicate. TFTP

  43. Concurrency • TFTP servers use a "well known address" (UDP port number). • How would you implement a concurrent server? • forking (alone) may lead to problems! • Can provide concurrency without forking, but it requires lots of bookkeeping. TFTP

  44. TFTP Concurrency • According to the protocol, the server may create a new udp port and send the initial response from this new port. • The client should recognize this, and send all subsequent messages to the new port. TFTP

  45. When is it over? • There is no length of file field sent! • All data messages except the last one contain 512 bytes of data. • message length is 2 + 2 + 512 = 516 • The last data message might contain 0 bytes of data! TFTP

  46. Issues What if more than 65535 chunks are sent? • 65536 blocks x 512 bytes/block = 33,554,432 bytes. • The RFC does not address this issue! • Remember that the network can duplicate packets! TFTP

  47. FTP vs. TFTP • FTP provides (minimal) security through login procedure • TFTP has NO login procedure • FTP Provides a reliable service through its use of TCP • TFTP must handle its own retransmissions since it uses UDP • FTP uses two connections • TFTP uses one connection (stop and wait) • FTP provides many commands • TFTP can only read and write files TFTP

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