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also used to accumulate packet drop statistics to an associated QueueMonitor object. For init-monitor the sameQueueMonitor object is used in all cases. The C++ definitions of the SnoopQueue and QueueMonitor classes aredescribed below.26.3 The C++ Trace ClassUnderlying C++ objects are created in support of the interface specified in Section 26.3 and are linked into the network topologyas network elements. The single C++ Trace class is used to implement the OTcl classes Trace/Hop, Trace/Enque,Trace/Deque, and Trace/Drop. The type_ field is used to differentiate among the various types of traces any particularTrace object might implement. Currently, this field may contain one of the following symbolic characters: + for enque,- for deque, h for hop, and d for drop. The overall class is defined as follows in ~ns/trace.cc:class Trace : public Connector {protected:int type_;nsaddr_t src_;nsaddr_t dst_;Tcl_Channel channel_;int callback_;char wrk_[256];void format(int tt, int s, int d, Packet* p);void annotate(const char* s);int show_tcphdr_; // bool flags; backward compatpublic:Trace(int type);~Trace();int command(int argc, const char*const* argv);void recv(Packet* p, Handler*);void dump();inline char* buffer() { return (wrk_); }};The src_, and dst_ internal state is used to label trace output and is independent of the corresponding field names in packetheaders. The main recv() method is defined as follows:void Trace::recv(Packet* p, Handler* h){format(type_, src_, dst_, p);dump();/* hack: if trace object not attached to anything free packet */if (target_ == 0)Packet::free(p);elsesend(p, h); /* Connector::send() */}The function merely formats a trace entry using the source, destination, and particular trace type character. The dump functionwrites the formatted entry out to the I/O handle associated with channel_. The format function, in effect, dictates thetrace file format.234