[xcp] Some remarks on XCP

[Yuri Pryadkin]

On Thursday 11 November 2004 02:14 am, Michael Savoric wrote:
> Yuri Pryadkin wrote:
> > Would you please explain how you propose to change XCP to hand out
> > absolute feedback?
> 
> It's very simple. Avoid dividing the positive and negative feedback
> by H_CWND in the router (I refer to the earlier XCP papers).
> Send this absolute feedback to the sender. The sender changes its CWND
> to the feedback.

If you refer to [1], the pseudo-code there in Appendix A is, I quote:
  On packet departure do:
	pos_fb = xi_p * H_rtt * H_rtt / H_cwnd
	neg_fb = xi_n * H_rtt
Are you suggesting to remove division from the first quoted line and
add a multiplication to the second?  I'd like to be sure we are 
talking about the same thing.
---------
[1] Dina Katabi, Mark Handley, and Charles Rohrs, "Internet 
Congestion Control for High Bandwidth-Delay Product Networks." 
ACM Sigcomm 2002, Pittsburgh, August, 2002.


In the later incarnations of XCP that have packet sizes factored in 
and where the switch has been made from congestion window to 
throughputs, if I understand your proposal correctly, it boils down
to:
	1.  Calculate incremental change in throughput as per XCP
	    (delta_throughput)
	2.  expected_packets_per_ci = old_throughput * CI / packet_size
	3.  new_throughput = old_throughput 
		+ delta_throughput*expected_packets_per_ci
(CI stands for control interval, and Cp and Cn are xi_p and xi_n
adjusted for throughput).

Router per packet calculations do get simpler.  Because 
delta_throughput is calculated as
	Cp * packet_size/throughput - Cn * packet_size
after multiplying by throughput * CI / packet_size you end up with
	new_throughput = old_throughput 
		+ (Cp * CI - Cn * CI * old_throughput)
so there is no division in the per-packet processing.  This is a big
advantage.  Another advantage is that round-off errors do not 
accumulate at the router if you're doing integer arithmetics.

However, there may be some concerns about the stability of this
method.  Especially when flows have widely different RTTs and/or 
traffic is bursty, oscillations may be greater.

  -Yuri