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134 lines
3.8 KiB
HTML
134 lines
3.8 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
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"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<!-- TFTP.html (C) K. J. Turner 18/12/14 -->
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<title>Trivial File Transfer Protocol</title>
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<link rev="made" href="http://www.cs.stir.ac.uk/~kjt/"/>
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</head>
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<body background="simulator.jpeg">
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<div style="text-align: center">
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<h1>Trivial File Transfer Protocol Simulator</h1>
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<img src="simulator.gif" alt="Simulator Logo"/>
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</div>
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<h2>Protocol Description</h2>
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<p>
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TFTP (Trivial File Transfer Protocol) is an elementary file transfer
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protocol that is typically used by a discless workstation to download its
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bootstrap file. TFTP operates over <a href="UDP.html">UDP (User Datagram
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Protocol)</a> and often in conjunction with <a href="BOOTP.html">BOOTP
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(Boot Protocol)</a>.
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</p>
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<p>
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A client may initially issue:
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</p>
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<ul>
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<li>
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<em>RRQ</em>: read request, to retrieve a named file from the server
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</li>
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<li><em>WRQ</em>: write request, to send a named file to the server</li>
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</ul>
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<p>
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During data transfer, the client or server as appropriate sends:
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</p>
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<ul>
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<li>
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<em>DATA</em>: with data content numbered D<em>n</em>, the last data
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being labelled <em>Dlast</em>
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</li>
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<li>
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<em>ACK</em>: an acknowledgement with the same number as the
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<em>DATA</em> being acknowledged
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</li>
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<li><em>ERROR</em>: to send an error message due to a serious problem</li>
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</ul>
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<p>
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<em>DATA</em> blocks are numbered from 1 onwards with just a name
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<em>D</em>n for their data contents. Normally an <em>ACK</em> quotes this
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number, but in the case of <em>WRQ</em> an acknowledgement with number 0
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is sent. All <em>DATA</em> blocks except the last one are full. The final
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block is less than the normal size, shown with contents D<em>last</em>.
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</p>
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<p>
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The protocol is unusual in that both sender and receiver may time out. If
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one party does not receive a fresh <em>DATA</em> block after sending an
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<em>ACK</em>, it may retransmit the <em>ACK</em> in case it was lost. A
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complication is that the <em>ACK</em> for the last <em>DATA</em> may be
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lost. For this reason, the receiver must wait until it is satisfied that
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data transfer is complete. If decision "presume all transmissions
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over" is made prematurely, the sender will become stuck because it
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cannot obtain the last <em>ACK</em> it needs.
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</p>
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<p>
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After completing transfer of a file, the client may start a new transfer.
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<em>ERROR</em> messages may be sent in situations such as incorrect
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sequence numbers or local disc errors. The full protocol allows for
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different modes of transfer, but the simulation supports only octet mode
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(the norm).
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</p>
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<h2>Protocol Parameters</h2>
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<p>
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This simulation has no parameters.
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</p>
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<h2>Protocol Simulation</h2>
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<p>
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The protocol simulation shows a time-sequence diagram with client and
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server protocol entities, and a communications medium that carries
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messages. The client and server exchange the messages described above,
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starting with an <em>RRQ</em> or <em>WRQ</em> from the client. Several
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files may be transferred consecutively.
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</p>
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<center>
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<applet code="simulator.ProtocolSimulator.class"
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archive="ProtocolSimulator.jar" width="750" height="700"
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name="ProtocolSimulator">
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<param name="protocol" value="TFTP"/>
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</applet>
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</center>
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<hr/>
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<p>
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<a href="index.html"><img src="uparrow.gif" alt="Up Arrow"/></a>
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Up one level to <a href="index.html">Protocol Simulators</a>
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</p>
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</body>
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</html>
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