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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

    Internetworking With TCP/IP vol 1 -- Part 25 2 2005

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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

    Internetworking With TCP/IP vol 1 -- Part 25 3 2005

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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)

    Internetworking With TCP/IP vol 1 -- Part 25 4 2005

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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

    Internetworking With TCP/IP vol 1 -- Part 25 5 2005

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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.

    Internetworking With TCP/IP vol 1 -- Part 25 6 2005

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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

    Internetworking With TCP/IP vol 1 -- Part 25 7 2005

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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?

    Internetworking With TCP/IP vol 1 -- Part 25 8 2005

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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

    Internetworking With TCP/IP vol 1 -- Part 25 9 2005

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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

    Internetworking With TCP/IP vol 1 -- Part 25 10 2005

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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

    Internetworking With TCP/IP vol 1 -- Part 25 11 2005

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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

    Internetworking With TCP/IP vol 1 -- Part 25 12 2005

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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

    Internetworking With TCP/IP vol 1 -- Part 25 13 2005

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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

    Internetworking With TCP/IP vol 1 -- Part 25 14 2005

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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

    Internetworking With TCP/IP vol 1 -- Part 25 15 2005

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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 (appl