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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>11.1.2 Example: Tcp retransmission timer</TITLE><META NAME="description" CONTENT="11.1.2 Example: Tcp retransmission timer"><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="node123.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node123.html"><LINK REL="up" HREF="node122.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node122.html"><LINK REL="next" HREF="node125.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node125.html"></HEAD><BODY ><!--Navigation Panel--><A NAME="tex2html3197" HREF="node125.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node125.html"><IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="file:/usr/share/latex2html/icons/next.png"></A> <A NAME="tex2html3191" HREF="node122.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node122.html"><IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="file:/usr/share/latex2html/icons/up.png"></A> <A NAME="tex2html3187" HREF="node123.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node123.html"><IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="file:/usr/share/latex2html/icons/prev.png"></A> <A NAME="tex2html3193" 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="tex2html3195" 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="tex2html3198" HREF="node125.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node125.html">11.2 OTcl Timer class</A><B> Up:</B> <A NAME="tex2html3192" HREF="node122.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node122.html">11.1 C++ abstract base</A><B> Previous:</B> <A NAME="tex2html3188" HREF="node123.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node123.html">11.1.1 Definition of a</A>   <B> <A NAME="tex2html3194" HREF="node1.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node1.html">Contents</A></B>   <B> <A NAME="tex2html3196" HREF="node590.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node590.html">Index</A></B> <BR><BR><!--End of Navigation Panel--><H2><A NAME="SECTION03812000000000000000"></A><A NAME="sec:timerexample"></A><BR>11.1.2 Example: Tcp retransmission timer</H2><P>TCP is an example of an agent which requires timers. There are three timersdefined in the basic Tahoe TCP agent defined in <TT>tcp.cc</TT>:<PRE> rtx_timer_; /* Retransmission timer / delsnd_timer_; /* Delays sending of packets by a small random amount of time, / /* to avoid phase effects / burstsnd_timer_; /* Helps TCP to stagger the transmission of a large window / /* into several smaller bursts /</PRE>In tcp.h, three classes are derived from the base classTimerHandlertcp.h:<PRE> class RtxTimer : public TimerHandler { public: RtxTimer(TcpAgent *a) : TimerHandler() { a_ = a; } protected: virtual void expire(Event *e); TcpAgent *a_; }; class DelSndTimer : public TimerHandler { public: DelSndTimer(TcpAgent *a) : TimerHandler() { a_ = a; } protected: virtual void expire(Event *e); TcpAgent *a_; }; class BurstSndTimer : public TimerHandler { public: BurstSndTimer(TcpAgent *a) : TimerHandler() { a_ = a; } protected: virtual void expire(Event *e); TcpAgent *a_; };</PRE>Inthe constructor for TcpAgent../ns-2/tcp.ccTcpAgent::TcpAgentin <TT>tcp.cc</TT>,each of these timersis initialized with the <TT>this</TT> pointer,which is assigned to the pointer <TT>a_</TT>.<PRE> TcpAgent::TcpAgent() : Agent(PT_TCP), rtt_active_(0), rtt_seq_(-1), \ldots rtx_timer_(this), delsnd_timer_(this), burstsnd_timer_(this) { \ldots }</PRE>In the following, we will focus only on the retransmission timer. Varioushelper methods may be defined to schedule timer events; ,<PRE> /* *Set retransmit timer using current rtt estimate. By calling \fcn[]{resched} *it does not matter whether the timer was already running. */ void TcpAgent::set_rtx_timer() { rtx_timer_.resched(rtt_timeout()); } /* * Set new retransmission timer if not all outstanding * data has been acked. Otherwise, if a timer is still * outstanding, cancel it. */ void TcpAgent::newtimer(Packet* pkt) { hdr_tcp *tcph = (hdr_tcp*)pkt-\>access(off_tcp_); if (t_seqno_ \> tcph-\>seqno()) set_rtx_timer(); else if (rtx_timer_.status() == TIMER_PENDING) rtx_timer_.cancel(); }</PRE>In the above code, the []set_rtx_timer method reschedules the retransmission timer by calling []rtx_timer_.resched. Note that ifit is unclear whether or not the timer is already running, calling[]resched eliminates the need to explicitly cancel the timer. Inthe second function, examples are given of the use of the []statusand cancel methods.<P>Finally, the expire method for class <TT>RtxTimer</TT> must be defined. In this case, expire calls the timeout methodfor <TT>TcpAgent</TT>. This is possible because []timeout is a public member function; if it were not, then <TT>RtxTimer</TT> would havehad to have been declared a friend class of <TT>TcpAgent</TT>.<PRE>void TcpAgent::timeout(int tno){ /* retransmit timer */ if (tno == TCP_TIMER_RTX) { if (highest_ack_ == maxseq_ && !slow_start_restart_) { /* * TCP option: * If no outstanding data, then don't do anything. */ return; }; recover_ = maxseq_; recover_cause_ = 2; closecwnd(0); reset_rtx_timer(0,1); send_much(0, TCP_REASON_TIMEOUT, maxburst_); } else { /* * delayed-send timer, with random overhead * to avoid phase effects */ send_much(1, TCP_REASON_TIMEOUT, maxburst_); } } void RtxTimer::expire(Event *e) { a_-\>timeout(TCP_TIMER_RTX);}</PRE><P>The various TCP agents contain additional examples of timers.<P><HR><!--Navigation Panel--><A NAME="tex2html3197" HREF="node125.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node125.html"><IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="file:/usr/share/latex2html/icons/next.png"></A> <A NAME="tex2html3191" HREF="node122.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node122.html"><IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="file:/usr/share/latex2html/icons/up.png"></A> <A NAME="tex2html3187" HREF="node123.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node123.html"><IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="file:/usr/share/latex2html/icons/prev.png"></A> <A NAME="tex2html3193" 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="tex2html3195" 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="tex2html3198" HREF="node125.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node125.html">11.2 OTcl Timer class</A><B> Up:</B> <A NAME="tex2html3192" HREF="node122.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node122.html">11.1 C++ abstract base</A><B> Previous:</B> <A NAME="tex2html3188" HREF="node123.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node123.html">11.1.1 Definition of a</A>   <B> <A NAME="tex2html3194" HREF="node1.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node1.html">Contents</A></B>   <B> <A NAME="tex2html3196" 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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