331 lines
11 KiB
HTML
331 lines
11 KiB
HTML
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<!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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<!-- TCPss.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>Transmission Control Protocol (Slow Start)</title>
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<link rev="made" href="http://www.cs.stir.ac.uk/~kjt/"/>
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<script type="text/javascript" language="JavaScript">
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<!--
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var simulator;
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var userAMessageSize;
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var userAMessageSizeOld = 100;
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var userBMessageSize;
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var userBMessageSizeOld = 300;
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var windowSizeA;
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var windowSizeAOld = 500;
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var windowSizeB;
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var windowSizeBOld = 400;
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var maxSendPacket;
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var maxSendPacketOld = 200;
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function validInteger(name, value, min, max) {
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if (!value.match("\\d+") || value < min || value > max) {
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alert("Value for " + name + " must be in range " + min + " to " + max);
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return false;
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}
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else
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return true;
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}
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function validFloat(name, value, min, max) {
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if (!value.match("\\d+\.\\d+") || value < min || value > max) {
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alert("Value for " + name + " must be in range " + min + " to " + max);
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return false;
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}
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else
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return true;
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}
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function set() {
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userAMessageSize = document.settings.userAMessageSize.value;
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userBMessageSize = document.settings.userBMessageSize.value;
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windowSizeA = document.settings.windowSizeA.value;
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windowSizeB = document.settings.windowSizeB.value;
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maxSendPacket = document.settings.maxSendPacket.value;
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lossRate = document.settings.lossRate.value;
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if (validInteger("User A Message Size", userAMessageSize, 10, 1000) &&
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validInteger("User B Message Size", userBMessageSize, 10, 1000) &&
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validInteger("Protocol A Receive Window", windowSizeA, 10, 1000) &&
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validInteger("Protocol B Receive Window", windowSizeB, 10, 1000) &&
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validInteger("Medium Maximum Packet Size", maxSendPacket, 10, 1000) &&
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validFloat("Loss Rate", lossRate, 0.0, 1.0)) {
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if (userAMessageSize != userAMessageSizeOld ||
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userBMessageSize != userBMessageSizeOld ||
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windowSizeA != windowSizeAOld ||
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windowSizeB != windowSizeBOld ||
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maxSendPacket != maxSendPacketOld)
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simulator.restart();
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userAMessageSizeOld = userAMessageSize;
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userBMessageSizeOld = userBMessageSize;
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windowSizeAOld = windowSizeA;
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windowSizeBOld = windowSizeB;
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maxSendPacketOld = maxSendPacket;
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simulator.setParameter("userAMessageSize", userAMessageSize);
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simulator.setParameter("userBMessageSize", userBMessageSize);
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simulator.setParameter("windowSizeA", windowSizeA);
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simulator.setParameter("windowSizeB", windowSizeB);
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simulator.setParameter("maxSendPacket", maxSendPacket);
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simulator.setParameter("lossRate", lossRate);
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simulator.updateActionList();
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}
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}
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//-->
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</script>
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</head>
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<body background="simulator.jpeg"
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onload="simulator=document.ProtocolSimulator; set();">
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<div style="text-align: center">
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<h1>
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Transmission Control Protocol Simulator
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<br/>
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(Slow Start)
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</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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TCP (Transmission Control Protocol) is a connection-oriented protocol for
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transferring data reliably in either direction between a pair of users. TCP
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is a rather complex protocol, so it is easy to lose track of the simulation.
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Try not to do anything too complicated! There are <a
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href="TCPcs.html">client-server</a> and <a href="TCPpp.html">peer-peer</a>
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simulations as alternatives that illustrate opening and closing connections.
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</p>
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<p>
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This simulation focuses on dynamic window management - specifically what is
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called "slow start". This is designed to avoid the protocol
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flooding the network with packets when it starts, and to ensure that the
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protocol recovers slowly following message loss and timeout due to
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congestion.
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</p>
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<p>
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Users simply send messages of a fixed size; the content of messages is not
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identified. The medium maximum packet size is the protocol segment size.
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Depending on this, messages may be sent as a number of fragments. Data
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transfer is also subject to the current window size of the receiver, and may
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be held up if the receiver's window becomes full.
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</p>
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<p>
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When data arrives, it is not immediately delivered to the receiving user.
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Instead, data is accumulated and can be delivered later ("Deliver
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octets to user"). If the receiving window becomes full, new requests to
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send data will be buffered. When the receiving window opens again, this
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buffered data can be sent ("Send octets to peer").
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</p>
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<p>
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Messages may contain a send sequence number (the offset of where the message
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starts in the user's octet stream), an acknowledgement sequence number
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(the offset of the next octet expected), and the current window (how many
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octets can be received).
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</p>
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<p>
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Slow start is governed by two protocol variables: <var>cwind</var>
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(congestion window) and <var>ssthresh</var> (slow start threshold). These
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are measured in octets for this simulation, but are sometimes counted as
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numbers of segments. (Multiply the number of segments by the segment size to
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get the number of octets.) The slow start procedure operates as follows:
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</p>
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<dl>
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<dt><b>Initial Phase</b></dt>
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<dd>
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The minimum of the congestion window size and the receiver window size
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determines how much data can be sent. The protocol starts conservatively
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with <var>cwind</var> set to the segment size. As each acknowledgement
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arrives, <var>cwind</var> is increased by the segment size. The amount of
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data that can be sent thus typically doubles in each round-trip time (i.e.
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the time to send a message and get a response). The amount is ultimately
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limited by the receiver's window. The available window thus increases
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exponentially (shown as "exp." in the simulation when
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<var>cwind</var> is increased).
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</dd>
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<dt><b>Congestion Action</b></dt>
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<dd>
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When a timeout occurs and the protocol has to retransmit, the congestion
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window is set back to one segment size. At this point, the threshold
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<var>ssthresh</var> is set to half the previous value of
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<var>cwind</var>. Suppose that the segment size is 200, so that
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<var>cwind</var> starts at 200. On subsequent acknowledgements,
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<var>cwind</var> could rise to 1600. If congestion causes loss at this
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point, <var>cwind</var> will be set back to 200 and <var>ssthresh</var>
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will be set to 800 (half of 1600).
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</dd>
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<dt><b>Congestion Recovery</b></dt>
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<dd>
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Now the window is built up exponentially again. Once it reaches
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<var>ssthresh</var>, it is no longer increased so aggressively. Instead,
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<var>cwind</var> increases by one segment size for each round-trip time.
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This is irrespective of how many acknowledgements arrive during this
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period, unlike the exponential phase where every acknowledgement causes
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the window to double per round trip time. During the recovery phase, the
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available window increases linearly (shown as "lin." in the
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simulation). The window will then become 900, 1000, etc. up to the
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receiver maximum. If congestion again causes loss, the procedure is
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repeated.
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</dd>
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</dl>
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<p>
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Because the simulation does not run in real time, it does not reflect
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round-trip times. As a result, its behaviour during the exponential and
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linear phases appears similar. However, the phase is indicated depending on
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whether <var>cwind</var> is below or above the threshold.
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</p>
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<p>
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TCP is rather complex, so the simulation does not attempt to faithfully
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reflect all its details. Although the main paths should work as expected, it
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may be possible to get the simulation into unusual states in which it does
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not behave correctly.
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</p>
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<p>
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Things the simulation does not cover include the following. See advanced
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guides to TCP for more information.
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</p>
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<ul>
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<li>advanced congestion control</li>
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<li>handling duplicate acknowledgements</li>
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<li>fast retransmit</li>
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<li>fast recovery</li>
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</ul>
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<h2>Protocol Parameters</h2>
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<p>
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The following settings are adequate for a simple simulation. For a more
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advanced exploration, choose different options and click <em>Change
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Settings</em>. This may cause the simulation to restart.
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</p>
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<form name="settings">
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<center>
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<table cellpadding="5">
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<tr>
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<td align="right">User A Message Size (10 to 1000):</td>
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<td>
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<input type="text" name="userAMessageSize" value="800" size="4"/>
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</td>
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</tr>
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<tr>
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<td align="right">User B Message Size (10 to 1000):</td>
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<td><input name="userBMessageSize" size="4" value="800"/> </td>
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</tr>
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<tr>
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<td align="right">Protocol A Receive Window (10 to 1000):</td>
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<td><input type="text" name="windowSizeA" value="1000" size="4"/></td>
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</tr>
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<tr>
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<td align="right">Protocol B Receive Window (10 to 1000):</td>
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<td><input type="text" name="windowSizeB" value="1000" size="4"/></td>
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</tr>
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<tr>
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<td align="right">Medium/Protocol Segment Size (10 to 1000):</td>
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<td>
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<input type="text" name="maxSendPacket" value="200" size="4"/>
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</td>
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</tr>
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<tr>
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<td align="right">Loss Rate (0.0 = lose none, 1.0 = lose any):</td>
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<td><input type="text" name="lossRate" value="0.2" size="4"/> </td>
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</tr>
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<tr>
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<td align="center" colspan="2">
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<input type="button" name="btnSet" value="Change Settings"
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onclick="set()"/>
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</td>
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</tr>
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</table>
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</center>
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</form>
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<script type="text/javascript" language="JavaScript">
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<!--
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// initialise settings from the above and not the default protocol settings
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set();
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//-->
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</script>
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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 two peer service
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users, protocol entities that support them, and a communications medium
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(network) that carries messages. The connection phase is not shown in this
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simulation as it is assumed to have just occurred. Equally, the
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disconnection phase is not shown.
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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="TCP/ss"/>
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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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