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COS 420

COS 420. DAY 25. Agenda. Assignment 5 posted Chap 22-26 Due May 4 Final exam will be take home and handed out May 4 and Due May 10 Latest version of Protocol Definition is better Today we will discuss File Transfer And Access (FTP,TFTP, NFS). Project 2 Grading.

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COS 420

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  1. COS 420 DAY 25

  2. Agenda • Assignment 5 posted • Chap 22-26 • Due May 4 • Final exam will be take home and handed out May 4 and Due May 10 • Latest version of Protocol Definition is better • Today we will discuss File Transfer And Access (FTP,TFTP, NFS)

  3. Project 2 Grading • Meeting Timelines 10% • Deliverables • Program requirements Due March 30 15% • late • Protocol Definition Due April 13 15% • Better but I hope to see improvement by May1 • Working Network Application Due May 4 25% • Final Paper Due May 1 25% • User Manual • Protocol • Program requirements • Technical Specifications • Presentation Due May 4 10%

  4. On-Line File Sharing • Always a popular application • Two basic paradigms • Whole-file copying • Piecewise file access • Piecewise access mechanism • Opaque: application uses special facilities to access remote file • Transparent: application uses same facilities to access local and remote files

  5. File Transfer • Whole file copying • Client • Contacts server • Specifies file • Specifies transfer direction • Server • Maintains set of files on local disk • Waits for contact • Honors request from client

  6. File Transfer Protocol (FTP) • Major TCP/IP protocol for whole-file copying • Uses TCP for transport • Features • Interactive access • Format specification (ASCII or EBCDIC) • Authentication control (login and password)

  7. FTP Process Model • Separate processes handle • Interaction with user • Individual transfer requests

  8. FTP’s Use of TCP Connections Data transfer connections and the data transfer processes that use them can be created dynamically when needed, but the control connection persists throughout a session. Once the control connection disappears, the session is terminated and the software at both ends terminates all data transfer processes.

  9. Control Connection Vs. Data Connection • For data transfer, client side becomes server and server side becomes client • Client • Creates process to handle data transfer • Allocates port and sends number to server over control connection • Process waits for contact • Server • Receives request • Creates process to handle data transfer • Process contacts client-side

  10. Question For Discussion • What special relationship is required between FTP and NAT?

  11. Interactive Use Of FTP • Initially a command-line interface • User invokes client and specifies remote server • User logs in and enters password • User issues series of requests • User closes connection • Currently • Most FTP initiated through browser • User enters URL or clicks on link • Browser uses FTP to contact remote server and obtain list of files • User selects file for download

  12. Anonymous FTP • Login anonymous • Password guest • Some servers require you to enter an email address • Used for ‘‘open’’ FTP site (where all files are publicly available • Typically used by browsers

  13. Secure File Transfer Protocols • Secure Sockets Layer FTP (SSL-FTP) • Uses secure sockets layer technology • All transfers are confidential • Secure File Transfer Program (sftp) • Almost nothing in common with FTP • Uses ssh tunnel • Secure Copy (scp) • Derivative of Unix remote copy (rcp) • Uses ssh tunnel

  14. Trivial File Transfer Protocol (TFTP) • Alternative to FTP • Whole-file copying • Not as much functionality as FTP • Code is much smaller • Intended for use on Local Area Network • Runs over UDP • Diskless machine can use to obtain image at bootstrap

  15. TFTP Packet Types

  16. TFTP Retransmission • Symmetric (both sides implement timeout and retransmission) • Data block is request for ACK • ACK is request for next data block

  17. Sorcerer’s Apprentice Bug • Consequence of symmetric retransmission • Duplicate packet is perceived as second request, which generates another transmission • Duplicate response triggers duplicate packets from the other end • Cycle continues

  18. Network File System (NFS) • Protocol for file access, not copying • Developed by Sun Microsystems, now part of TCP/IP standards • Transparent (application cannot tell that file is remote)

  19. NFS Implementation

  20. Remote Procedure Call (RPC) • Also developed by Sun Microsystems, now part of TCP/IP standards • Used in implementation of NFS • Relies on eXternal Data Representation (XDR) standard for conversion of data items between heterogeneous computers

  21. Summary • Two paradigms for remote file sharing • Whole file copying • Piecewise file access • File Transfer Protocol (FTP) • Standard protocol for file copying • Separate TCP connection for each data transfer • Client and server roles reversed for data connection • Examples of secure alternatives to FTP • SSL-FTP, sftp, and scp

  22. Summary (continued) • Trivial File Transfer Protocol (TFTP) • Alternative to FTP that uses UDP • Symmetric retransmission scheme • Packet duplication can result in Sorcerer’s Apprentice problem • Network File System (NFS) • Standard protocol for piecewise file access • Uses RPC and XDR

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