Subject: [AVT] Is dropping flows enough to avoid congestion collapse?

From: Eddie Kohler <[email-masked]>
Date: Tue, 02 Dec 2003 16:47:58 -0800

There have been suggestions that dropping flows is enough to avoid
congestion collapse, and that RTP-over-UDP already does this.

Well, sort of.  Currently, *users* may drop flows when they see high levels
of loss.  On the other hand they may not.  Certainly we can't count on it,
and we can't count on users watching for any particular loss rate.  1%?
30%? 40%?  Depends on the codec and the user.

Carrying through the IAB's suggestion would mean endpoint operating
systems, or perhaps routers or middleboxes, terminating UDP flows, or
treating UDP as less than best effort, when there is congestion.

That would seem a deeply nonsatisfying result, and probably far worse for
user experience than DCCP.  (I'm also not sure DCCP will be that bad --
there are several misunderstandings floating around, as well as some real
issues.)

Of course, UDP-penalization might not ever happen, but some "people working
on the stability of the network" want it to.  (I'm thinking of Sally
Floyd.)

Eddie

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Audio/Video Transport Working Group
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From: John Lazzaro <[email-masked]>
Date: Tue, 2 Dec 2003 19:53:31 -0800 (PST)

> Eddie Kohler <[email-masked]> writes:
>
> Of course, UDP-penalization might not ever happen, but some "people
> working on the stability of the network" want it to.

I found the November 2003 Communications ACM article on TransLight to
be very illuminating on this point (no pun intended).  So much comes
for free if everyone gets (at least) two lamdbas to their endpoint,
one regulated QoS lamdba for media flows, and one legacy lambda for
TCP and other best-effort uses.  If you peek under Layer 2 to Layer 1,
such separation is often there physically -- the wire-pairs of modern
Ethernet, the channels of cable TV plant, the lambdas of PONS.

Of course, the TransLight folks have it even better -- unrouted,
point-to-point, knowable latency lambdas ...

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John Lazzaro    http://www.cs.berkeley.edu/~lazzaro
     lazzaro [at] cs [dot] berkeley [dot] edu
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