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  37.     <div id="doc" class="markdown-body container-fluid comment-enabled comment-inner" data-hard-breaks="true"><h1 id="Week-1---Stop-and-Wait-ARQ" data-id="Week-1---Stop-and-Wait-ARQ"><a class="anchor hidden-xs" href="#Week-1---Stop-and-Wait-ARQ" title="Week-1---Stop-and-Wait-ARQ"><span class="octicon octicon-link"></span></a><span>Week 1 - </span><a href="https://en.wikipedia.org/wiki/Stop-and-wait_ARQ" target="_blank" rel="noopener"><span>Stop-and-Wait ARQ</span></a></h1><blockquote>

  38. <p><span>Distributed Systems and Network Programming - Spring 2023</span></p>

  39. </blockquote><h2 id="Task" data-id="Task"><a class="anchor hidden-xs" href="#Task" title="Task"><span class="octicon octicon-link"></span></a><span>Task</span></h2><ul>

  40. <li><span>Your task for this lab is to write a UDP server using Python </span><a href="https://docs.python.org/3/library/socket.html" target="_blank" rel="noopener"><span>socket</span></a><span> module that communicates with a given client. The client reads a file from the local file system and uploads it to the server.</span></li>

  41. </ul><h3 id="Client-Implementation" data-id="Client-Implementation"><a class="anchor hidden-xs" href="#Client-Implementation" title="Client-Implementation"><span class="octicon octicon-link"></span></a><span>Client Implementation</span></h3><ul>

  42. <li><span>The client starts by sending the following string message </span><code>s|0|filename.ext|filesize</code><span> where:</span>

  43. <ul>

  44. <li><code>s</code><span> indicates that the message type is </span><code>start</code></li>

  45. <li><code>|</code><span> is the field delimiter.</span></li>

  46. <li><code>0</code><span> is the initial sequence number.</span></li>

  47. <li><code>filename.ext</code><span> is the name of the file to be sent (with extension).</span></li>

  48. <li><code>filesize</code><span> is the total size of the file to be sent </span><u><span>in bytes</span></u><span>.</span></li>

  49. </ul>

  50. </li>

  51. <li><span>The client expects a reply from the server in the format </span><code>a|seqno</code><span> where:</span>

  52. <ul>

  53. <li><code>a</code><span> indicates that the message type is </span><code>acknowledgement</code><span>.</span></li>

  54. <li><code>seqno</code><span> equals </span><code>(x+1)%2</code><span> where </span><code>x</code><span> is the sequence number of the message to be acknowledged.</span></li>

  55. </ul>

  56. </li>

  57. <li><span>If the expected server acknowledgement was received successfully, the client does the following:</span>

  58. <ol>

  59. <li><span>Divide the file into chunks so that the size of a single data packet (including headers) does not exceed the buffer size of the server.</span></li>

  60. <li><span>Start sending file chunks, one by one. Each message has the format </span><code>d|seqno|data</code><span> where:</span>

  61. <ul>

  62. <li><code>d</code><span> indicates that the message type is </span><code>data</code></li>

  63. <li><code>seqno</code><span> is the sequence number of the data message, it alternates between </span><code>1</code><span> and </span><code>0</code><span>, starting from </span><code>1</code><span>.</span></li>

  64. <li><code>data</code><span> is the raw bytes of the file.</span></li>

  65. </ul>

  66. </li>

  67. <li><span>The client waits for an acknowledgement message after sending each chunk.</span></li>

  68. </ol>

  69. </li>

  70. <li><span>If an expected acknowledgement message does not arrive within 1 second, the client retransmits the message.</span></li>

  71. <li><span>If an acknowledgement message arrives with an unexpected sequence number, the client ignores that duplicate ACK and keeps waiting for the expected ACK.</span></li>

  72. </ul><h3 id="Server-Implementation-your-task" data-id="Server-Implementation-your-task"><a class="anchor hidden-xs" href="#Server-Implementation-your-task" title="Server-Implementation-your-task"><span class="octicon octicon-link"></span></a><span>Server Implementation (your task)</span></h3><ol>

  73. <li><span>Parse one integer argument, the port number to listen on.</span></li>

  74. <li><span>Create a UDP socket and start listening for incoming messages on </span><code>0.0.0.0:&lt;port&gt;</code><span>.</span>

  75. <ul>

  76. <li><span>The server should use a fixed receiver buffer size of </span><code>20480</code><span> bytes (20 Kibibytes).</span></li>

  77. </ul>

  78. </li>

  79. <li><span>Upon receiving a message from a client, inspect the message type (first character).</span>

  80. <ul>

  81. <li><span>If the message type is </span><code>s</code><span>, prepare to receive a file from the client with the given name and size.</span></li>

  82. <li><span>If the message type is </span><code>d</code><span>, write the delivered chunk to the file system.</span></li>

  83. <li><span>Otherwise, terminate gracefully with an error.</span></li>

  84. </ul>

  85. </li>

  86. <li><span>In both cases, reply with an acknowledge message in the format </span><code>a|seqno</code><span> where</span>

  87. <ul>

  88. <li><code>a</code><span> indicates that the message type is </span><code>acknowledgement</code><span>.</span></li>

  89. <li><code>seqno</code><span> equals </span><code>(x+1)%2</code><span> where </span><code>x</code><span> is the sequence number of the message to be acknowledged.</span></li>

  90. </ul>

  91. </li>

  92. <li><span>Once the file is received completely, the server should print an indicating message, write the content to the file system, and close the file.</span></li>

  93. <li><span>If an existing file with the same name is present in the server directory, the server should print an indicating message and overwrite that file with the new one.</span></li>

  94. <li><span>Your server will be tested under constant delay and packet loss. The following Linux command can be used to simulate 15% packet loss and 1100 milliseconds constant delay over the </span><code>lo</code><span> interface. File transfer should still succeed after applying the command. To undo the effect use </span><code>del</code><span> instead of </span><code>add</code><span>.</span><pre><code class="bash hljs">sudo tc qdisc add dev lo root netem loss 15% delay 1100ms

  95. </code></pre>

  96. </li>

  97. <li><span>The server stays running unless a fatal error occurs or a </span><code>KeyboardInterrupt</code><span> is received.</span></li>

  98. </ol><h2 id="Testing" data-id="Testing"><a class="anchor hidden-xs" href="#Testing" title="Testing"><span class="octicon octicon-link"></span></a><span>Testing</span></h2><ul>

  99. <li>

  100. <p><span>The project structure looks like this</span></p>

  101. <pre><code class="bash hljs">.

  102. ├── client

  103. │   ├── client.py

  104. │   ├── image.png

  105. │   └── note.txt

  106. └── NameSurname.py

  107. </code></pre>

  108. </li>

  109. <li>

  110. <p><span>Example run and output</span></p>

  111. <pre><code class="bash hljs">$ python3 NameSurname.py 8080

  112. (<span class="hljs-string">'0.0.0.0'</span>, 8080):    Listening...

  113. (<span class="hljs-string">'127.0.0.1'</span>, 48256): s|0|note.txt|446

  114. (<span class="hljs-string">'127.0.0.1'</span>, 48256): d|1|chunk1

  115. (<span class="hljs-string">'0.0.0.0'</span>, 8080):    Received note.txt.

  116. ^C(<span class="hljs-string">'0.0.0.0'</span>, 8080):  Shutting down...

  117. </code></pre>

  118. <pre><code class="bash hljs">$ <span class="hljs-built_in">cd</span> client

  119. $ python3 client.py 127.0.0.1:8080 note.txt

  120. Client: s|0|note.txt|446

  121. Server: a|1

  122. Client: d|1|chunk1

  123. Server: a|0

  124. </code></pre>

  125. </li>

  126. <li>

  127. <p><span>Example session visualization</span></p>

  128. </li>

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ry="3" rx="3" height="65" width="150" stroke="#666" fill="#eaeaea" y="261" x="0"></rect><text style="text-anchor: middle; font-size: 14px; font-weight: 400; font-family: &quot;Open Sans&quot;, sans-serif;" class="actor" alignment-baseline="central" dominant-baseline="central" y="293.5" x="75"><tspan dy="0" x="75">Client</tspan></text></g><g><rect class="actor" ry="3" rx="3" height="65" width="150" stroke="#666" fill="#eaeaea" y="261" x="222"></rect><text style="text-anchor: middle; font-size: 14px; font-weight: 400; font-family: &quot;Open Sans&quot;, sans-serif;" class="actor" alignment-baseline="central" dominant-baseline="central" y="293.5" x="297"><tspan dy="0" x="297">Server</tspan></text></g></svg></pre><ul>

  130. <li>

  131. <p><span>Resulting directory structure. The text file should arrive intact.</span></p>

  132. <pre><code class="bash hljs">.

  133. ├── client

  134. │   ├── client.py

  135. │   ├── image.png

  136. │   └── note.txt

  137. ├── NameSurname.py

  138. └── note.txt

  139. </code></pre>

  140. </li>

  141. </ul><h2 id="Checklist" data-id="Checklist"><a class="anchor hidden-xs" href="#Checklist" title="Checklist"><span class="octicon octicon-link"></span></a><span>Checklist</span></h2><p><span>Your submitted code should satisfy the following requirements. Failing to satisfy an item will result in partial grade deduction or an assignment failure (depending on the severity).</span></p><ul class="contains-task-list">

  142. <li class="task-list-item enabled"><input class="task-list-item-checkbox" type="checkbox" disabled="disabled"><span> One submitted source file named according to the format </span><code>NameSurname.py</code></li>

  143. <li class="task-list-item enabled"><input class="task-list-item-checkbox" type="checkbox" disabled="disabled"><span> The source code executes successfully under the </span><a href="https://www.python.org/downloads/" target="_blank" rel="noopener"><span>latest stable Python interpreter</span></a><span>.</span></li>

  144. <li class="task-list-item enabled"><input class="task-list-item-checkbox" type="checkbox" disabled="disabled"><span> The code only imports dependencies from the </span><a href="https://docs.python.org/3/library/index.html" target="_blank" rel="noopener"><span>standard library</span></a><span> (no external dependencies).</span></li>

  145. <li class="task-list-item enabled"><input class="task-list-item-checkbox" type="checkbox" disabled="disabled"><span> A sent text/binary file and the corresponding received one are identical (files do not get corrupted in transit).</span></li>

  146. <li class="task-list-item enabled"><input class="task-list-item-checkbox" type="checkbox" disabled="disabled"><span> The code works under constant delay and packet loss.</span></li>

  147. <li class="task-list-item enabled"><input class="task-list-item-checkbox" type="checkbox" disabled="disabled"><span> The code is readable and nicely formatted (e.g., according to </span><a href="https://peps.python.org/pep-0008/" target="_blank" rel="noopener"><span>PEP8</span></a><span>)</span></li>

  148. <li class="task-list-item enabled"><input class="task-list-item-checkbox" type="checkbox" disabled="disabled"><span> The source code is the author’s original work. Both parties will be penalized for detected plagiarism.</span></li>

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