# [e2e] Is a control theoretic approach sound?

Xiaoliang (David) Wei weixl at caltech.edu
Tue Jul 29 15:05:54 PDT 2003

```      Also, Prof. Steven Low gave a talk on FAST recentely. Here's the
slide: http://netlab.caltech.edu/pub/papers/FASTietf0307.ppt
This slide is based on a more detailed paper that will be available online,
soon.
We hope these will further explain the ideas:)

-David

----- Original Message -----
From: "Xiaoliang (David) Wei" <weixl at caltech.edu>
To: "Saad Biaz" <biazsaa at eng.auburn.edu>; <end2end-interest at postel.org>
Sent: Tuesday, July 29, 2003 1:45 AM
Subject: Re: [e2e] Is a control theoretic approach sound?

>
> Not sure if the following may help to explain:
>
> > Consider the following analogy: let a cart being pulled by 50 mice.
>
> This is a very interesting example:)  Let me follow this example:
>
> 1) if the cart is very small and light, and if every mouse pulls very hard
> and brakes very hard to chase the target speed, then the cart goes as what
> you described.
> 2) if the cart is very heavy, and every mouse pulls slightly, then it
takes
> a long time for the cart to gain the target speed, and the system may
> oscillate around the target speed slightly.
> 3) If the mice are a little bit cleverer: they consider the speed of the
> cart. If the speed is far from the target speed, they pull very hard or
> brake very hard; if the speed of the cart is close to the target speed,
they
> pull slightly or brake slightly, or even stop pulling. Then there is a
hope
> to make the system stays around the target speed? :) Actually, even 50
> elephants come to pull the cart, they have to follow the same rule: pull
> or brake slightly when the speed is close to the target.
>
>      I guess we can say that FAST algorithm is doing (3), while Vegas is
> doing (2) since Vegas has a fixed step size delta_cwnd=+/-1 during
> congestion avoidance. Hence Vegas is not so responsive in high speed
> environment.
>      Even by doing (3), there may be some problem: how to set the strength
> according to the gap from target? If the strength is too aggressive, the
> speed of the cart may jump from "much lower than target" to "much higher
> than target", and the system oscillates widely. Here, the control theories
> may come in to help to design the parameters.
>
>      As in (3), the elephants and mice have no difference in the strength
of
> pulling and braking. We don't need to worry about how many elephants or
> how many mice we have.
>
>
> > Are there measurements about multiple FAST TCP sharing a unique
> > bottleneck? If FAST TCP still stable under these conditions? Is it still
> > performant as well?
> >
>      Yes, there were some experiments with 10 FAST flows (the duration
> of the flows >1800sec) of different propagation delays, sharing the same
> dummynet router, showing that the FAST still stay around the target
> speed.:)
>
> > Any feedback will be most appreciated. Is there something I am missing?
> >
> > Assistant Professor
> > Auburn University
> >
>
> -David
>
> Xiaoliang (David) Wei             Graduate Student in CS at Caltech
> http://www.cs.caltech.edu/~weixl
> ====================================================
>
>
Xiaoliang (David) Wei             Graduate Student in CS at Caltech
http://www.cs.caltech.edu/~weixl
====================================================

```