Transcript COS 420 day 25

COS 420
DAY 25
Agenda
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Assignment 5 posted
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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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Meeting Timelines
Deliverables
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Program requirements
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Protocol Definition
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Due March 30
15%
Due April 13
15%
Due May 4
Due May 1
25%
25%
Due May 4
10%
Better but I hope to see improvement by May1
Working Network Application
Final Paper
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late
10%
User Manual
Protocol
Program requirements
Technical Specifications
Presentation
On-Line File Sharing
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Always a popular application
Two basic paradigms
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Whole-file copying
Piecewise file access
Piecewise access mechanism
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Opaque: application uses special facilities to
access remote file
Transparent: application uses same facilities to
access local and remote files
File Transfer
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Whole file copying
Client
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Contacts server
Specifies file
Specifies transfer direction
Server
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Maintains set of files on local disk
Waits for contact
Honors request from client
File Transfer Protocol (FTP)
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Major TCP/IP protocol for whole-file
copying
Uses TCP for transport
Features
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Interactive access
Format specification (ASCII or EBCDIC)
Authentication control (login and
password)
FTP Process Model
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Separate processes handle
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Interaction with user
Individual transfer requests
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.
Control Connection Vs. Data
Connection
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For data transfer, client side becomes server
and server side becomes client
Client
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Creates process to handle data transfer
Allocates port and sends number to server over
control connection
Process waits for contact
Server
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Receives request
Creates process to handle data transfer
Process contacts client-side
Question For Discussion
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What special relationship is required
between FTP and NAT?
Interactive Use Of FTP
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Initially a command-line interface
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User
User
User
User
invokes client and specifies remote server
logs in and enters password
issues series of requests
closes connection
Currently
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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
Anonymous FTP
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Login anonymous
Password guest
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Some servers require you to enter an email
address
Used for ‘‘open’’ FTP site (where all files
are publicly available
Typically used by browsers
Secure File Transfer Protocols
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Secure Sockets Layer FTP (SSL-FTP)
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Secure File Transfer Program (sftp)
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Uses secure sockets layer technology
All transfers are confidential
Almost nothing in common with FTP
Uses ssh tunnel
Secure Copy (scp)
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Derivative of Unix remote copy (rcp)
Uses ssh tunnel
Trivial File Transfer Protocol
(TFTP)
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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
TFTP Packet Types
TFTP Retransmission
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Symmetric (both sides implement
timeout and retransmission)
Data block is request for ACK
ACK is request for next data block
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
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Network File System (NFS)
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Protocol for file access, not copying
Developed by Sun Microsystems, now
part of TCP/IP standards
Transparent (application cannot tell that
file is remote)
NFS Implementation
Remote Procedure Call (RPC)
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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
Summary
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Two paradigms for remote file sharing
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File Transfer Protocol (FTP)
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Whole file copying
Piecewise file access
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
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SSL-FTP, sftp, and scp
Summary (continued)
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Trivial File Transfer Protocol (TFTP)
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Alternative to FTP that uses UDP
Symmetric retransmission scheme
Packet duplication can result in Sorcerer’s
Apprentice problem
Network File System (NFS)
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Standard protocol for piecewise file access
Uses RPC and XDR