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The monitor-queue function is constructed similarly to trace-queue. By calling the link’s init-monitor procedure,it arranges for the creation of objects (SnoopQueue and QueueMonitor objects) which can, in turn, be used toascertain time-aggregated queue statistics.The drop-trace function provides a way to specify a Queue’s drop target without having a direct handle of the queue.26.2 Library support and examplesThe Simulator procedures described above require the trace and init-monitor methods associated with the OTclLink class. Several subclasses of link are defined, the most common of which is called SimpleLink. Thus, the traceand init-monitor methods are actually part of the SimpleLink class rather than the Link base class. The tracefunction is defined as follows (in ns-link.tcl):## Build trace objects for this link and# update the object linkage#SimpleLink instproc trace { ns f } {$self instvar enqT_ deqT_ drpT_ queue_ link_ head_ fromNode_ toNode_$self instvar drophead_set enqT_ [$ns create-trace Enque $f $fromNode_ $toNode_]set deqT_ [$ns create-trace Deque $f $fromNode_ $toNode_]set drpT_ [$ns create-trace Drop $f $fromNode_ $toNode_]$drpT_ target [$drophead_ target]$drophead_ target $drpT_$queue_ drop-target $drpT_$deqT_ target [$queue_ target]$queue_ target $deqT_}if { [$head_ info class] == "networkinterface" } {$enqT_ target [$head_ target]$head_ target $enqT_# puts "head is i/f"} else {$enqT_ target $head_set head_ $enqT_# puts "head is not i/f"}This function establishes Enque, Deque, and Drop traces in the simulator $ns and directs their output to I/O handle $f.The function assumes a queue has been associated with the link. It operates by first creating three new trace objects andinserting the Enque object before the queue, the Deque object after the queue, and the Drop object between the queue andits previous drop target. Note that all trace output is directed to the same I/O handle.This function performs one other additional tasks. It checks to see if a link contains a network interface, and if so, leaves it asthe first object in the chain of objects in the link, but otherwise inserts the Enque object as the first one.232