Subject: Re: PILC ASYM document

From: Sally Floyd <[email-masked]>
Date: Tue, 14 Mar 2000 09:14:36 -0800

>Imagining that TCP has an obligation and capability to control congestion
>remains an engineering mistake.  

Odd.  The lessons that I would take from the last twelve years of
TCP congestion control are that transport-level, end-to-end congestion
control is remarkably robust; that it has successfully prevented
congestion collapse and enabled the evolution of the Internet for
the past twelve years; and that transport-level congestion control
coupled with a minimal network level allows a great heterogeneity
of link-level technologies and higher-level applications as well
as the evolution of other transport-level congestion control
mechanisms for diverse classes of applications (e.g., unreliable
unicast, semi-reliable streaming multimedia, unreliable and reliable
multicast, etc.).

The further lessons that I would take from the last twelve years of
TCP congestion control would be that TCP and other transport
protocols can and should be made even more robust to a variety of
behaviors in the underlying network (e.g., reordered packets due
to link-level transmissions, congestion on the return path); and
that adding some forms of explicit communication between the
transport layer and higher or lower layers (e.g., Explicit Congestion
Notification) can be of significant benefit.

- Sally
--------------------------------
http://www.aciri.org/floyd/
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From: Lloyd Wood <[email-masked]>
Date: Tue, 14 Mar 2000 21:56:14 +0000 (GMT)

On Tue, 14 Mar 2000, Sally Floyd wrote:

> control is remarkably robust

so robust (in the self-perpetuating sense) that improvements to it
such as rate-based pacing cannot be incrementally deployed. (You can
only incrementally deploy mechanisms that are equally or more
aggressive for network capacity - bit of a quandary, that.)

> that it has successfully prevented

To me, the growing emphasis on TCP-friendliness for these mechanisms
indicates the limited scope for evolution available. The mechanisms
need to display the same level of aggression as TCP to coexist well
with TCP.

Transport-level congestion control is certainly a success, but I
suspect that improvements are being boxed into a corner - an
evolutionary dead end, if you will - by the installed
base. Ever-decreasing returns?

(To extend the evolution analogy slightly further: TCP is a very
 very successful species of dinosaur, set to dominate the planet
 for millions of years. There are no global extermination events
 planned; there are currently few evolutionary pressures on TCP in the
 'congestion control' competition space. Now, if there was a reason to
 deploy queuing mechanisms favouring e.g. rate-based pacing, which
 would be equivalent to altering the global temperature...)

> to link-level transmissions, congestion on the return path)

hear, hear.

> and

although TCP seems to have been doing just fine without that thus far.

L.

<[email-masked]>PGP<http://www.ee.surrey.ac.uk/Personal/L.Wood/>