[ns] Computing TCP retransmission timer in NS2 : problem in rtt_update()

[haifa touati]

according to the rfc 2988 section 2, the tcp's retransmition timer must

be calculated as follows:



====================================================================================



   RTTVAR <- (1 - beta) * RTTVAR + beta * |SRTT - R'| (1)

   SRTT <- (1 - alpha) * SRTT + alpha * R'   (2)

   RTO <- SRTT + max (G, K*RTTVAR)  (3)



The above SHOULD be computed using alpha=1/8 and beta=1/4

=======================================================================================



I checked the implementation of this in ns-2.27 in tcp.cc, it seem to me

that in the implemnenation that there is no multiplaction of parameter alpha by R' eq. (2) (the actual

rtt sample). See the function rtt_update().



==============================================================================================

  if (t_srtt_ != 0) {

  register short delta;

--->>>> delta = t_rtt_ - (t_srtt_ >> T_SRTT_BITS); // d = (m - a0)

  if ((t_srtt_ += delta) <= 0) // a1 = 7/8 a0 + 1/8 m

   t_srtt_ = 1;

  if (delta < 0)

   delta = -delta;

  delta -= (t_rttvar_ >> T_RTTVAR_BITS);

  if ((t_rttvar_ += delta) <= 0) // var1 = 3/4 var0 + 1/4 |d|

   t_rttvar_ = 1;

 }

================================================================================================



in the implemetation T_SRTT_BITS=3, t_rtt_ is the actual sample of rtt,

so delta = t_rtt_  - (t_srtt_ / 8).

and t_srtt_  become after  t_srtt_ += delta , t_srtt_ = (7/8)t_srtt_ -

t_rtt_ 

                                                                     

^    

              |

                                                     here it should be

1/8



And also in the computing of the RTTVAR there is no multiplication by

parameter (1-beta)=3/4 before RTTVAR eq.(1).





I'm not sure, if what i'm saying is true ! 

Can anyone who has experience in the tcp implementation in ns, give me

comments.

Thanks a lot 
   
  
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