[xcp] Questions about delta_throughput and reaction to packet loss

[Filipe Abrantes]

Hi Leon,

I'm not the XCP expert here but ill try to answer your questions the 
best I can, please correct me if I'm wrong.

Leon Martin wrote:
> Hi all,
> 
> XCP is new to me. So my question may be a bit simple.
> Can someone help me with the following doubts:
> 
> 1. In the original 2002 paper, the feedback is in
> terms of bytes, but in the 2005 draft and the 2.29
> ns-2 implementation, the feedback is in bits/sec
> (i.e., delta_throughput). Is there any reason for this
> change?
>

It's been a while since i last read the 2002 paper, but i think that's 
mainly a matter of simplification of the calculations performed at the 
router.

2002 paper states:

f = alpha*avg_rtt*(c-input_bw)-beta*pqueue

but after, every calculation that involves /f/ is divided by avg_rtt. If 
  you do it already in the calculation of /f/ you save a few divisions 
in the subsequent calculations.

[
(...)
residue_pos_fbk = (max(f,0)+shuffled_traffic)/avg_rtt
residue_neg_fbk = (max(-f,0)+shuffled_traffic)/avg_rtt
(...)
]
(and the same applies for xi_p and xi_n calculations)


> 2. Why does Jacobson's congestion control take over in
> the case there is packet loss? Even if there is real
> congestion in the network, the intermediate routers
> will be allocate the correct bandwidth after at most
> one RTT. 
> 

If the bottleneck is non-XCP, you need to use Jacobson's congestion 
control, if you keep using XCP, the rate of the flow will be regulated 
by a non-bottleneck queue, leading to more losses in the real bottleneck.

The rate of a flow must be regulated by the bottleneck right? If the 
bottleneck doesn't play XCP... you can't use XCP to adjust the flow's rate.

Of course, when losses are caused by some other reason (wireless, ...) 
than a non-XCP bottleneck we might have a problem.

Best Regards

Filipe

> Thanks!
> 
> - Leon
> 
> 
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-- 
Filipe Lameiro Abrantes
INESC Porto
Campus da FEUP
Rua Dr. Roberto Frias, 378
4200-465 Porto
Portugal

Phone: +351 22 209 4266
E-mail: fla at inescporto.pt