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There are two ways to transmit a serialized ADU through transportagents. First, for UDP agent (and all agents derived from there), wecan derive from class UDP and add a new method<TT>send(int nbytes, char *userdata)</TT> to pass user data fromApplication to Agent. To pass data from an Agent to an Application issomewhat trickier: each agent has a pointer to its attachedapplication, we dynamically cast this pointer to an AppConnector andthen call <TT>AppConnector::process_data()</TT>.<P>As an example, we illustrate how class HttpInvalAgent isimplemented. It is based on UDP, and is intended to deliver web cacheinvalidation messages (/webcache/inval-agent.h). It is defined as:<P><PRE> class HttpInvalAgent : public Agent { public: HttpInvalAgent(); virtual void recv(Packet *, Handler *); virtual void send(int realsize, AppData* data); protected: int off_inv_; };</PRE><P>Here <TT>recv(Packet*, Handler*)</TT> overridden to extract user data,and a new <TT>send(int, AppData*)</TT> is provided to include user datain packetes. An application (HttpApp) is attached to an HttpInvalAgentusing <TT>Agent::attachApp()</TT> (a dynamic cast is needed). In<TT>send()</TT>, the following code is used to write user data fromAppData to the user data area in a packet:<P><PRE> Packet *pkt = allocpkt(data-\>size()); hdr_inval *ih = (hdr_inval *)pkt-\>access(off_inv_); ih-\>size() = data-\>size(); char *p = (char *)pkt-\>accessdata(); data-\>pack(p);</PRE><P>In <TT>recv()</TT>, the following code is used to read user data frompacket and to deliver to the attached application:<P><PRE> hdr_inval *ih = (hdr_inval *)pkt-\>access(off_inv_); ((HttpApp*)app_)-\>process_data(ih-\>size(), (char *)pkt-\>accessdata()); Packet::free(pkt);</PRE><P><HR><!--Navigation Panel--><A NAME="tex2html9020" HREF="node495.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node495.html"><IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="file:/usr/share/latex2html/icons/next.png"></A> <A NAME="tex2html9014" HREF="node491.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node491.html"><IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="file:/usr/share/latex2html/icons/up.png"></A> <A NAME="tex2html9008" HREF="node493.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node493.html"><IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="file:/usr/share/latex2html/icons/prev.png"></A> <A NAME="tex2html9016" 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="tex2html9018" 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="tex2html9021" HREF="node495.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node495.html">36.1.4 Transmitting user data</A><B> Up:</B> <A NAME="tex2html9015" HREF="node491.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node491.html">36.1 Using application-level data</A><B> Previous:</B> <A NAME="tex2html9009" HREF="node493.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node493.html">36.1.2 Passing data between</A>   <B> <A NAME="tex2html9017" HREF="node1.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node1.html">Contents</A></B>   <B> <A NAME="tex2html9019" 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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