[ns] Questions about RTT exponential backoff in NS-2

[Felix]

Hi all,

I am simulating with NS-2 version 2.26.
I have a question about the implementation on RTT exponential backoff =
mechanism.

As I know, according to the Karn's algorithm, the t_srtt value should be =
doubled after a timeout which has already been implemented in the tcp =
module of NS-2.
However, I found that in NS-2, the t_srtt will be reset to 0 after a few =
consecutive timeouts. What is the reason for this? Does this reset =
mechanism exist in any RFCs ? =20

The following code perform the t_srtt reset:

void TcpAgent::rtt_backoff()
{
 if (t_backoff_ < 64)
  t_backoff_ <<=3D 1;

 if (t_backoff_ > 8) {
  /*
   * If backed off this far, clobber the srtt
   * value, storing it in the mean deviation
   * instead.
   */
  t_rttvar_ +=3D (t_srtt_ >> T_SRTT_BITS);
  t_srtt_ =3D 0;
 }
}


rgs,
Felix