CBR-traffic的ptype

 void CBR_Traffic::init()  //(~/tools/cbr_traffic.cc)
{
        // compute inter-packet interval
  interval_ = (double)(size_ << 3)/(double)rate_;
  if (agent_)
  if (agent_->get_pkttype() != PT_TCP &&
       agent_->get_pkttype() != PT_TFRC)
   agent_->set_pkttype(PT_CBR);
}

爲什麼是TCP或者TFRC的時候就不更改pkttype了？