# [xcp] Paper on XCP for shared-access links

Fri Jul 14 09:28:40 PDT 2006

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
>