As described in the following chapterChapterchap:timers, specific timer classes must be derived from an abstract base TimerHandler../ns-2/timer-handler.h defined in timer-handler.h. Instances of these subclasses can then be used as various agent timers. An agent may wish to override the Agent::timeout method (which does nothing). In the case of the Tahoe TCP agent, two timers are used: a delayed send timer delsnd_timer_ and a retransmission timer rtx_timer_. We describe the retransmission timer in TCPSectionsec:timerexample as an example of timer usage.