[tcpm] last_max in CUBIC

[Nicolas Kuhn <[e-mail]>]

Hi all, 

We were currently working on CUBIC and LEDBAT in NS-2, using Michael Welzl’s updated TCP Linux for NS-2 
[http://www.ietf.org/mail-archive/web/iccrg/current/msg01160.html]. 
We see some strange behaviour in CUBIC’s window reduction, which we wanted to share with you here.

— Topology — 

The topology is a simple dumbbell topology with a bottleneck of 10Mbps and an RTT of 100ms. The queue size of the router is set 
to 42 packets (with DropTail). There are two non-application limited bulk flows that randomly start within the first second of the simulation. 
Flow#1 is CUBIC (using ns-2.35/tcp/src/tcp_cubic.c version);
Flow#2 is LEDBAT with a target delay of 5 ms or 25ms.

— Problem illustration — 

In cwnd_case_1_default.pdf and cwnd_case_2_default.pdf, we plot the congestion window evaluation and the 
“new last_max_cwnd”, that is the W_last_max in the CUBIC draft [https://datatracker.ietf.org/doc/draft-zimmermann-tcpm-cubic].



We recall here how CUBIC deals with fast convergence: 

      if (W_max < W_last_max){     
          W_last_max = W_max;        
          W_max = W_max*(2-beta)/2;   
      } else {   
          W_last_max = W_max  
      }

In the case with LEDBAT25, at 15s, W_last_max “should” have been reduced, but we have  W_max == W_last_max,
which makes that CUBIC does not reduce W_last_max and “loops”, would not have its expected behaviour.  

— Problem resolution — 

Based on that observation, we have changed the following CUBIC code as follows:
 
      if (W_max < W_last_max){     
-> 
      if (W_max <= W_last_max){    

We plot the updated performance in cwnd_case_1_NK.pdf and cwnd_case_2_NK.pdf.

 


— Discussion — 

We acknowledge that this effect may not occur often in real life and might be a simulation
artefact. However, we have been looking around and are wondering the rationale of having

      if (W_max < W_last_max){   

instead of 

      if (W_max <= W_last_max){   

What do you think about it ?
Why would not we change the ‘<‘ for a ‘<=‘ ?

Kind regards,  

Nicolas KUHN
Emmanuel LOCHIN
Si Quoc Viet TRANG