[e2e] a new IRTF group on Transport Models
faber at ISI.EDU
Fri Jun 3 12:05:09 PDT 2005
On Thu, Jun 02, 2005 at 10:45:10PM -0700, Cannara wrote:
> Since Sally & I have exchanged a few notes on what I see as the truly serious
> issue that never gets attention, I'll just mention it here once, in case some
> courageous, responsible souls are out there to do for humanity what the IETF &
> crew won't -- messing with TCP (or any transport) is missing the point of
> congestion control. The origin of it in TCP had nothing to do with the "e2e
> principle". It simply created a bandaid to bring the Internet back from the
> edge of Metcalfe's predicted collapse that scared folks in the '80s.
> Why? Because the Internet designers never considered anything but IP and a
> few hosts (ok a few coffee pots too :). DLC? What's that? Unique node
> addresses? Eh? Admission & flow control at the network layer? What's that?
> So, with eyes averted, ears covered and mouths that raised such issues taped
> shut, we got what we have today -- a mess. Whether or not TCP is ever given
> accurate info to distinguish physical loss from true congestion matters
> little. The network layer is responsible for its own congestion management.
> That's where the Internet deserves to finally get a dose of the reality that's
> been faced for decades in more reliable and secure deployed communications
> systems -- real networks, in other words.
It would be easy to get side-tracked into a discussion of what possible
consensus usage of the word "real" you believe doesn't apply to the
Internet, but I'm going to avoid that rathole.
I think one should use some caution in advocating a network-layer-only
approach to congestion control. The new factors that the imaginary
network we call the Internet introduces into the equation is the wide
variety of applications that the network is potentially used for and the
fantastic array of devices used to create the network. The Internet's
strength lies in its diversity and trying to apply a low-level, one-size
fits all congestion control system imperils that diversity.
In the Internet, congestion does not necessarily mean that your Cisco
router's buffers are full. It may mean that your firewall is running
low on memory or CPU cycles, or that your wireless link has encountered
a cloud, or that a new and bursty source is changing the jitter
qualities of your end-to-end stream. *Some* resource is getting scarce;
the Curve of Truth is flattening out.
Notice: the problem here depends both on what resources the devices on
your path are allocating and what resource is critical to your
application. And what resource is critical to the device. A sudden 50%
drop in bandwidth (resulting in a drop in network capacity) is a
non-event to some applications and a congestion moment to others. A
firewall that is running out of memory wants to throttle new
connections; a router running out of buffers wants to throttle sources
sending lots of data.
Now, when we the users of such an imaginary, whack-O network can't agree
on what's being optimized (throughput, jitter, delay, availibility) or
what a reasonable response to a resource becoming scarce (recode your
video, try again tomorrow, slow down transmission rate, smooth your
traffic), it really stretches the imagination to believe that the only
barrier to solving the (ill-defined) problem is a lack of will.
One could, I suppose, force the definition of the problem to be
best-effort bulk-transfer throughput maximization while maintaining
max-min fairness and hammer the fanciful network we have into delivering
that through a unified network-level admission and flow control system.
I don't even deny that such a solution would help the considerable
number of current Internet users who are doing best-effort bulk-transfer
throughput maximization. But it's not going to help the guys who want
to do other things, nor will it help those gruops co-exist.
> If there are such good souls ready to stand up and do what needs be done, I
> applaud you. Remember, courage men/women, God hates a coward.
"God hates me."
"Hate 'im back. It works for me."
-- Danny Glover and Mel Gibson, Lethal Weapon
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