Newsgroups: comp.dcom.lans.ethernet
From: mart@csri.toronto.edu (Mart Molle)
Subject: Re: packet drops on repeaters?
Date: 6 Nov 92 14:57:34 GMT

rpw3@rigden.wpd.sgi.com (Rob Warnock) writes:

>rich@grebyn.com (Richard Lawrence) writes:
>
>| What about "morning boot time" for a workgroup?  Especially in
>| engineering environments after the (shudder) Morning Meeting.

>No problem.

>Try a hard one: Just after the power comes back from a power fail and
>everybody's workstation boots *exactly* at the same time?

>Still no problem.

>Notice I am not talking about lack of load on the net. Certainly any network
>can see load peaks approching 100%. I'm just saying that Ethernet doesn't
>"collapse" under such loads and it *does* continue to more-or-less fairly
>share the bandwidth. *OF COURSE* if 1000 workstations all try to use the
				     ^^^^^^^^^^^^^^^^^
>same 10 Mb/s at the same time, they're gonna see something less than 10 Kb/s
>each, and of course that's going to *feel* like "collapse" to anyone who
>usually uses the net when it's idle. But that's not "collapse"; it's called
>"sharing"...

If you had said >100< workstations instead of >1000< workstations above, I
would have happily agreed with you.  However, the Ethernet backoff algorithm
does NOT do what you expect when there are a lot of stations involved in
a transient overload.  You NEVER reach a state where all 1024 workstations
are randomizing their transmission attempts over 1024 backoff slot-times.
Sometimes a workstation is lucky and acquires the Ether, at which point
it resets its backoff counter to zero and starts beating on the network
much harder than it should.  If not, the unlucky workstation will eventually
fail at all 16 attempts and AGAIN reset its backoff counter to zero before
continuing with the next packet in its transmit queue.

I already posted the math a few months ago, which showed the best you could
hope for in an overload situation was for each workstation to transmit at
the rate of about once every 225 backoff slot-times.  If 1000 workstations
are all doing this, it does indeed result in "collapse" rather than "sharing".

Please don't try to tell me that the DEC measurements described in the
ACM Sigcomm '88 paper: "Measured Performance of an Ethernet: Myths and
Reality" contradict me.  Their experiments were limited to a maximum of
24 workstations.  The effect I'm talking about starts happening when the
number of hosts is an order of magnitude larger (which is certainly
possible in an Ethernet, since the maximum number hosts allowed by the
spec is almost 2 orders of magnitude larger).  If somebody (Rob?) is
willing to repeat the `myths' experiments on a network with, say, 500
or 1000 hosts, I'd be happy to view the results.  However, until that
time I think the most reliable way to determine the behaviour of large
Ethernets is either mathematical analysis or simulation rather than
extrapolation from the behaviour of smaller systems.

At this point, the best results that I know about were obtained by
Speros Armyros using simulation.  (Yes, the simulation model *was*
validated against the measurements presented in the `myths' paper...)
His results are described in CSRI technical report #259, available
by anonymous ftp from the internet host ftp.csri.toronto.edu.

Mart L. Molle
Computer Systems Research Institute
University of Toronto
Toronto, Canada M5S 1A4
(416)978-4928