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<html><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"><!--Converted with jLaTeX2HTML 2002 (1.62) JA patch-1.4patched version by: Kenshi Muto, Debian Project.LaTeX2HTML 2002 (1.62),original version by: Nikos Drakos, CBLU, University of Leeds* revised and updated by: Marcus Hennecke, Ross Moore, Herb Swan* with significant contributions from: Jens Lippmann, Marek Rouchal, Martin Wilck and others --><HTML><HEAD><TITLE>31.1.1.0.2 Round-Trip Time Estimation and RTO Timeout Selection</TITLE><META NAME="description" CONTENT="31.1.1.0.2 Round-Trip Time Estimation and RTO Timeout Selection"><META NAME="keywords" CONTENT="everything"><META NAME="resource-type" CONTENT="document"><META NAME="distribution" CONTENT="global"><META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1"><META NAME="Generator" CONTENT="jLaTeX2HTML v2002 JA patch-1.4"><META HTTP-EQUIV="Content-Style-Type" CONTENT="text/css"><LINK REL="STYLESHEET" HREF="everything.css" tppabs="http://www.isi.edu/nsnam/ns/doc/everything.css"><LINK REL="previous" HREF="node367.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node367.html"><LINK REL="up" HREF="node366.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node366.html"><LINK REL="next" HREF="node369.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node369.html"></HEAD><BODY ><!--Navigation Panel--><A NAME="tex2html7066" HREF="node369.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node369.html"><IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="file:/usr/share/latex2html/icons/next.png"></A> <A NAME="tex2html7060" HREF="node366.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node366.html"><IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="file:/usr/share/latex2html/icons/up.png"></A> <A NAME="tex2html7056" HREF="node367.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node367.html"><IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="file:/usr/share/latex2html/icons/prev.png"></A> <A NAME="tex2html7062" HREF="node1.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node1.html"><IMG WIDTH="65" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="contents" SRC="file:/usr/share/latex2html/icons/contents.png"></A> <A NAME="tex2html7064" HREF="node590.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node590.html"><IMG WIDTH="43" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="index" SRC="file:/usr/share/latex2html/icons/index.png"></A> <BR><B> Next:</B> <A NAME="tex2html7067" HREF="node369.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node369.html">31.1.2 Configuration</A><B> Up:</B> <A NAME="tex2html7061" HREF="node366.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node366.html">31.1.1 The Base TCP</A><B> Previous:</B> <A NAME="tex2html7057" HREF="node367.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node367.html">31.1.1.0.1 Responses to Congestion</A>   <B> <A NAME="tex2html7063" HREF="node1.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node1.html">Contents</A></B>   <B> <A NAME="tex2html7065" HREF="node590.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node590.html">Index</A></B> <BR><BR><!--End of Navigation Panel--><H4><A NAME="SECTION06211020000000000000">31.1.1.0.2 Round-Trip Time Estimation and RTO Timeout Selection</A></H4>Four variables are used to estimate the round-trip time andset the retransmission timer: <TT>rtt_, srtt_, rttvar_, tcpTick_,and backoff_</TT>.TCP initializes rttvar to and backoff to 1.When any future retransmission timer is set, it's timeoutis set to the current time plus <!-- MATH $\max(bt(a+4v+1), 64)$ --> seconds,where is the current backoff value, is the value of tcpTick, is the value of srtt, and is the value of rttvar.<P>Round-trip time samples arrive with new ACKs.The RTT sample is computed as the difference between the currenttime and a ``time echo'' field in the ACK packet.When the first sample is taken, its value is used as the initialvalue for <TT>srtt_</TT>. Half the first sample is used as the initialvalue for <TT>rttvar_</TT>.For subsequent samples, the values are updated as follows:<P><BR><P></P><DIV ALIGN="CENTER"><!-- MATH \begin{displaymath}srtt = \frac{7}{8} \times srtt + \frac{1}{8} \times sample\end{displaymath} --></DIV><BR CLEAR="ALL"><P></P><BR><P></P><DIV ALIGN="CENTER"><!-- MATH \begin{displaymath}rttvar = \frac{3}{4} \times rttvar + \frac{1}{4} \times |sample-srtt|\end{displaymath} --></DIV><BR CLEAR="ALL"><P></P><P><HR><!--Navigation Panel--><A NAME="tex2html7066" HREF="node369.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node369.html"><IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="file:/usr/share/latex2html/icons/next.png"></A> <A NAME="tex2html7060" HREF="node366.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node366.html"><IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="file:/usr/share/latex2html/icons/up.png"></A> <A NAME="tex2html7056" HREF="node367.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node367.html"><IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="file:/usr/share/latex2html/icons/prev.png"></A> <A NAME="tex2html7062" HREF="node1.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node1.html"><IMG WIDTH="65" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="contents" SRC="file:/usr/share/latex2html/icons/contents.png"></A> <A NAME="tex2html7064" HREF="node590.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node590.html"><IMG WIDTH="43" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="index" SRC="file:/usr/share/latex2html/icons/index.png"></A> <BR><B> Next:</B> <A NAME="tex2html7067" HREF="node369.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node369.html">31.1.2 Configuration</A><B> Up:</B> <A NAME="tex2html7061" HREF="node366.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node366.html">31.1.1 The Base TCP</A><B> Previous:</B> <A NAME="tex2html7057" HREF="node367.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node367.html">31.1.1.0.1 Responses to Congestion</A>   <B> <A NAME="tex2html7063" HREF="node1.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node1.html">Contents</A></B>   <B> <A NAME="tex2html7065" HREF="node590.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node590.html">Index</A></B> <!--End of Navigation Panel--><ADDRESS>2003-09-23</ADDRESS></BODY></HTML>⌨️ 快捷键说明
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