Download - 12 - File Transfer - Comer

8/2/2019 12 - File Transfer - Comer

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PART XXV

APPLICATIONS:FILE TRANSFER AND ACCESS

(FTP, TFTP, NFS)

Internetworking With TCP/IP vol 1 -- Part 25 1 2005


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On-Line File Sharing

d

Always a popular applicationd

Two basic paradigms

Whole-file copying

Piecewise file access

d Piecewise access mechanism

Opaque: application uses special facilities to access

remote file

Transparent: application uses same facilities to accesslocal and remote files



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File Transfer

d

Whole file copyingd

Client

Contacts server

Specifies file

Specifies transfer direction

Server

Maintains set of files on local disk

Waits for contact

Honors request from client



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File Transfer Protocol (FTP)

dMajor TCP/IP protocol for whole-file copying

dUses TCP for transport

dFeatures

Interactive access

Format specification (ASCII or EBCDIC)

Authentication control (login and password)



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FTP Process Model

TCP/IPinternet

operatingsystem

datatransfer

controlprocess

operatingsystem

controlprocess

datatransfer

server systemclient system

client dataconnection

client controlconnection

server controlconnection

server dataconnection

d Separate processes handle

Interaction with user

Individual transfer requests



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FTPs 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.



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Control Connection Vs. Data Connection

dFor data transfer, client side becomes server and server side

becomes client

d Client

Creates process to handle data transfer

Allocates port and sends number to server over control

connection

Process waits for contact

d

Server Receives request

Creates process to handle data transfer

Process contacts client-side



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32AA 2009-2010

performances Il client apre la connessione TCP di controllo con il server; apertaIl client apre la connessione TCP di controllo con il server; aperta

la connessione, invia la richiesta di trasferimento del file su questola connessione, invia la richiesta di trasferimento del file su questocanale.canale.

s Il server quindi apre una separata connessione TCP per trasferirviIl server quindi apre una separata connessione TCP per trasferirviil file.il file.

s Per aprire ogni connessione si richiede un round-trip time di 1Per aprire ogni connessione si richiede un round-trip time di 1

RTT.RTT.x 1/2 RTT perch la richiesta giunga al server, e un altro1/2 RTT perch la richiesta giunga al server, e un altro

1/2 RTT perch la risposta torni indietro, oltre al tempo di1/2 RTT perch la risposta torni indietro, oltre al tempo ditrasmissione del file.trasmissione del file.

s Tempo complessivo per una transazione FTP:Tempo complessivo per una transazione FTP:

1 RTT control-channel OPEN1 RTT control-channel OPEN

0.5 RTT send request on control-channel0.5 RTT send request on control-channel

1 RTT file-channel OPEN1 RTT file-channel OPEN

0.5 RTT file starts to arrive on file-channel0.5 RTT file starts to arrive on file-channel

Ftrans time to transmit the fileFtrans time to transmit the file

----------------

33RTT + Ftrans = time to get the first file in FTPRTT + Ftrans = time to get the first file in FTP


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35AA 2009-2010

modello grafico-temporale

s consente di evidenziare, in termini di pacchetti inviati, leconsente di evidenziare, in termini di pacchetti inviati, lecaratteristiche del:caratteristiche del:

1.1. protocollo applicativoprotocollo applicativo

2.2. protocollo di trasportoprotocollo di trasporto

1.1. connesso vs non connessoconnesso vs non connesso

3.3. meccanismi di trasmissionemeccanismi di trasmissione

1.1. slowstartslowstart vs trasmissionevs trasmissione voracevorace

calcolo del troughput in termini di RTTcalcolo del troughput in termini di RTT


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33AA 2009-2010

Schema trasferimento primo file

InIn rossorosso le interazionile interazionicon il control channelcon il control channel

ininblublu quelle sul canalequelle sul canale

per il trasferimento fileper il trasferimento file

slow start ...slow start ...


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34AA 2009-2010

Schema trasferimento file seguente

Il control channel (inIl control channel (in rossorosso)) gi aperto. Il server ricevuta gi aperto. Il server ricevuta

la richiesta apre il canale per illa richiesta apre il canale per il

trasferimento (intrasferimento (inblublu))

0.5 RTT send request on control-channel

1 RTT file-channel OPEN

0.5 RTT file starts to arrive on file-channel

Ftrans time to transmit the file

--------

2 RTT + Ftrans = time to get subsequent files

slow start ...slow start ...


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Question For Discussion

dWhat special relationship is required between FTP and

NAT?



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Interactive Use Of FTP

dInitially 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

dCurrently

Most FTP initiated through browser

User enters URL or clicks on link

Browser uses FTP to contact remote server and obtainlist of files

User selects file for download



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Anonymous FTP

dLogin anonymous

dPassword guest

dUsed for open FTP site (where all files are publicly

available

d Typically used by browsers



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Secure File Transfer Protocols

dSecure Sockets Layer FTP (SSL-FTP)

Uses secure sockets layer technology

All transfers are confidential

dSecure File Transfer Program (sftp)

Almost nothing in common with FTP

Uses ssh tunnel

d Secure Copy (scp)

Derivative of Unix remote copy (rcp)

Uses ssh tunnel



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Trivial File Transfer Protocol (TFTP)

dAlternative to FTP

dWhole-file copying

dNot as much functionality as FTP

dCode is much smaller

d Intended for use on Local Area Network

d Runs over UDP

d Diskless machine can use to obtain image at bootstrap



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TFTP Packet Types

READ REQ. (1) FILENAME 0 MODE 02-octet opcode n octets 1 octet n octets 1 octet

WRITE REQ. (2) FILENAME 0 MODE 0

2-octet opcode n octets 1 octet n octets 1 octet

DATA (3) BLOCK # DATA OCTETS...

2-octet opcode 2 octets up to 512 octets

ACK (4) BLOCK #

2-octet opcode 2 octets

ERROR (5) ERROR CODE ERROR MESSAGE 0

2-octet opcode 2 octets n octets 1 octet



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TFTP Retransmission

dSymmetric (both sides implement timeout and

retransmission)

d Data block is request for ACK

d ACK is request for next data block



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Sorcerers Apprentice Bug

dConsequence of symmetric retransmission

dDuplicate packet is perceived as second request, which

generates another transmission

d Duplicate response triggers duplicate packets from the other

end

dCycle continues



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37AA 2009-2010

ritrasmissione simmetrica?

sorcerers apprentice bugsorcerers apprentice bug


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Network File System (NFS)

dProtocol for file access, not copying

dDeveloped by Sun Microsystems, now part of TCP/IP

standards

d Transparent (application cannot tell that file is remote)



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NFS Implementation

application

localfile

system

local

disk

NFSclient

network connectionto NFS server

local / remote decision



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Remote Procedure Call (RPC)

dAlso developed by Sun Microsystems, now part of TCP/IP

standards

d Used in implementation of NFS

d Relies on eXternal Data Representation (XDR) standard for

conversion of data items between heterogeneous computers



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Summary

dTwo paradigms for remote file sharing

Whole file copying

Piecewise file access

dFile Transfer Protocol (FTP)

Standard protocol for file copying

Separate TCP connection for each data transfer

Client and server roles reversed for data connection

d Examples of secure alternatives to FTP

SSL-FTP, sftp, and scp



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Summary

(continued)

d Trivial File Transfer Protocol (TFTP)

Alternative to FTP that uses UDP

Symmetric retransmission scheme

Packet duplication can result in Sorcerers Apprentice

problem

d Network File System (NFS)

Standard protocol for piecewise file access

Uses RPC and XDR
