[xcp] Paper on XCP for shared-access links

[Yuri Pradkin]

Hi, Filipe, 

thanks for the link, the paper is very interesting.  A few of comments:

1. It was not very clear from the paper how the upper bound on \chi, 
1/(5-\alpha-\beta) was derived to guarantee that the queue never 
overflows.  I got the impression that it was from a fluid-model 
simulation of a single flow shown in fig 4.  My concern is that this 
bound will not hold in case d != rtt, as might be when the traffic is 
a mix of flows with different RTTs.

2. I thought it might be desirable to preserve continuity in the 
feedback function F in order to avoid oscillations and perhaps make 
analysis more tractable.  Have you considered having F increase 
gradually with time from 0 to something if q=0 && dq/dt=0?  I guess 
Your "Late reaction" idea comes close to that.

3. Re your queue frequency response analysis (fig3), there's been a 
more rigorous analysis of XCP stability in "Stability Analysis of 
Explicit Congestion Control Protocols" by Hamsa Balakrishnan.

4. After we switched to "X" in packet headers in place of throughput 
(and X can be thought of as inter-packet time), I've been on and off 
thinking about recasting the whole XCP calculations in the time 
domain.  Presumably the queue controller will have to use MAC idle 
time instead of (C - input_bw).  I've never got around to finishing 
this line of thought, but it just seems it might be related to the 
whole capacity estimation problem.

  -Yuri

On Monday 10 July 2006 11:52, Filipe Abrantes wrote:
> Hi,
>
> I've been working on an approach to the problem of using XCP in
> links where the capacity is unknown (e.g. 802.11), and , eventually,
> I have put it into paper which is now available to the community in:
>
> http://portal.acm.org/affiliated/citation.cfm?id=1140086.1140091&col
>l=ACM&dl=ACM&idx=1140086&part=periodical&WantType=periodical&title=AC
>M%20SIGCOMM%20Computer%20Communication%20Review&CFID=15151515&CFTOKEN
>=6184618
>
> (pdf direct link)
> http://www.acm.org/sigs/sigcomm/ccr/archive/2006/july/p29-v36n3f-abr
>antes.pdf or
> http://pong.inescn.pt/~fabrantes/xcpb.pdf
>
> I would be happy if this paper would help trigger the discussion on
> this topic, so any comments you may have are most welcome.
>
> Filipe