Subject: RE: QoS and adaptive end protocols

From: "Manfredi, Albert E" <[email-masked]>
Date: Wed, 21 Jun 2000 15:25:54 -0400

> -----Original Message-----
> From: Dan Grossman [mailto:[email-masked]]

> > It doesn't surprise me, though, that TCP would not do well 
> with another
> > feedback loop under it. Why is that surprising? 

> > It's the same thing that
> > occurs with _any_ control system with layered feedback 
> loops. Unless you
> > know exactly what you're doing, you can easily get into 
> oscillating or
> > unpredictable behavior. For example, if the time constant 
> of the underlying
> > feedback loop is longer than that of the upper layer loop, you're in
> > trouble.

> But of course the opposite is the case here.  ABR with ER has 
> a time constant 
> of 1 RTT across the ATM network.

One round-trip time would be the minimum for ABR, and in large ATM
backbones, how would this differ from TCP's loop? The TCP window size is
adjusted continuously with TCP, so why do you assume the delay to be
significantly greater than that of ABR's OAM cells?

Equal feedback time constants are bad news also, btw. They cause large
amplitude oscillations.

> In order to prove your 
> point, you'd have to 
> come up with a control-theoretical model.  Which I understand 
> would be very 
> hard to do. 

Why not adjust the feedback delay in a simulation? Again, the effect is not
news in control systems, right?

Bert
[email-masked]

---

From: Dan Grossman <[email-masked]>
Date: Wed, 21 Jun 2000 16:05:59 -0400

> significantly greater than that of ABR's OAM cells?
I was trying to avoid getting into this, because I don't know the formula and 
can't easily think where in the literature to go looking. But TCP's time 
constant would necessarily be multiple round trip times, or enough for a 
change in a TCP connection's fair share of the bottleneck bandwidth to cause a 
drop, followed by the time for three dupacks to be received, followed by the 
time for cwnd to ramp back up.  

Also, if we assume a TCP connection would cross multiple subnets,  then by 
definition the additional hops would add length to the TCP control loop.

---

From: "Saverio Mascolo" <[email-masked]>
Date: Sat, 8 Jul 2000 21:08:18 +0200

Hi,

I think that the problem of TCP congestion control over ABR is due to the
fact that proposed ABR control algorithms simply does not prevent cell
losses. Then the reason of bad performance comes from the fact that one cell
loss produces retransmission of  an entire packet. Therefore it is not a
complex issues of interacting control loops or different time constant but
it is that ATM community has not proposed an efficient ABR congestion
control algorithm. In my paper appeared on Automatica Journal, Dec. 1999, I
also show why TCP community did much better work on congestion control
respect to ATM community!

Saverio

---

From: Dan Grossman <[email-masked]>
Date: Mon, 10 Jul 2000 12:04:26 -0400

> losses. 
Are you talking about ER or EFCI?  

Properly implemented ER has an insignificant cell loss ratio, around the same 
order of magnitude as the background cell loss due to things like corrupted 
headers and cosmic ray hits on switch memories.
> Then the reason of bad performance comes from the fact that one cell
> loss produces retransmission of  an entire packet. 
And EPD/PPD takes care of this.  Although since correctly implemented ER is 
does not suffer congestive loss, it would be difficult to prove for ER-based 
ABR.
> Therefore it is not a
I have not read your paper.  Is this a refereed journal?