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}bind("windowOption_", &wnd_option_);bind("windowConstant_", &wnd_const_);...bind("off_ip_", &off_ip_);bind("off_tcp_", &off_tcp_);...The Agent constructor (~ns/agent.cc):Agent::Agent(int pkttype) :addr_(-1), dst_(-1), size_(0), type_(pkttype), fid_(-1),prio_(-1), flags_(0){memset(pending_, 0, sizeof(pending_)); /* timers */// this is really an IP agent, so set up// for generating the appropriate IP fields. . .bind("addr_", (int*)&addr_);bind("dst_", (int*)&dst_);bind("fid_", (int*)&fid_);bind("prio_", (int*)&prio_);bind("flags_", (int*)&flags_);...}These code fragments illustrate the common case where an agent’s constructor passes a packet type identifier to the Agentconstructor. The values for the various packet types are used by the packet tracing facility (Section 26.5) and are defined in~ns/trace.h. The variables which are bound in the TcpAgent constructor are ordinary instance/member variables for theclass with the exception of the special integer values off_tcp_ and off_ip_. These are needed in order to access a TCPheader and IP header, respectively. Additional details are in the section on packet headers (Section 12.1).Note that the TcpAgent constructor contains initializations for two timers, rtx_timer_ and delsnd_timer_.TimerHandler objects are initialized by providing a pointer (the this pointer) to the relevant agent.10.5.2 Starting the AgentThe TcpAgent agent is started in the example when its FTP source receives the start directive at time 1.2. The startoperation is an instance procedure defined on the class Application/FTP (Section 38.4). It is defined in ~ns/tcl/lib/ns-source.tclas follows:Application/FTP instproc start {} {[$self agent] send -1}In this case, agent refers to our simple TCP agent and send -1 is analogous to sending an arbitrarily large file.The call to send eventually results in the simple TCP sender generating packets. The following function output performsthis:void TcpAgent::output(int seqno, int reason)100