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10.5.5 Implementing TimersAs described in the following chapter (Chapter 11), specific timer classes must be derived from an abstract base classTimerHandler defined in ~ns/timer-handler.h. Instances of these subclasses can then be used as various agent timers. Anagent 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 theretransmission timer in TCP (Section 11.1.2) as an example of timer usage.10.6 Creating a New AgentTo create a new agent, one has to do the following:1. decide its inheritance structure (Section 10.6.1), and create the appropriate class definitions,2. define the recv() and timeout() methods (Section 10.6.2),3. define any necessary timer classes,4. define OTcl linkage functions (Section 10.6.3),5. write the necessary OTcl code to access the agent (Section 10.6.4).The action required to create and agent can be illustrated by means of a very simple example. Suppose we wish to constructan agent which performs the ICMP ECHO REQUEST/REPLY (or “ping”) operations.10.6.1 Example: A “ping” requestor (Inheritance Structure)Deciding on the inheritance structure is a matter of personal choice, but is likely to be related to the layer at which the agentwill operate and its assumptions on lower layer functionality. The simplest type of Agent, connectionless datagram-orientedtransport, is the Agent/UDP base class. Traffic generators can easily be connected to UDP Agents. For protocols wishing touse a connection-oriented stream transport (like TCP), the various TCP Agents could be used. Finally, if a new transport or“sub-transport” protocol is to be developed, using Agent as the base class would likely be the best choice. In our example,we’ll use Agent as the base class, given that we are constructing an agent logically belonging to the IP layer (or just above it).We may use the following class definitions:class ECHO_Timer;class ECHO_Agent : public Agent {public:ECHO_Agent();int command(int argc, const char*const* argv);protected:void timeout(int);void sendit();double interval_;ECHO_Timer echo_timer_;};class ECHO_Timer : public TimerHandler {103