Re: WG ACTION: 2 weeks to discuss [LL62] Do not space probes randomly

Stuart - can you explain which "collision myth" is referred to in the second
paragraph.  I think there are several.

And, of course, you're right about my statement concerning multiple probes
and randomization.  The randomization of subsequent probes desynchronizes
just the probe traffic...

- Ralph


At 10:46 AM 5/7/2004 -0700, Stuart Cheshire wrote:
>Philip Nye wrote:
>
> >the chance we need to avoid is that you and I
> >both agree on ALL FOUR values in our series.
>
>This is statement is asserted without any supporting evidence, and it is
>false.
>
>This is NOT what we need to avoid.
>
>In order to be a good network citizen, we want to minimize the aggregate
>peak load that a large number of hosts may impose on network
>infrastructure, since peak load is what causes loss. This issue only
>arises when there are a large number of hosts probing simultaneously. Two
>hosts, or three, or four or even ten host probing at *exactly* the same
>time is NOT a problem.
>
>The Ethernet collision loss myth has existed for decades. University
>computer science lecturers teach students that it's a myth, but still the
>myth persists in the larger population.
>
>Collisions do not cause loss on Ethernet. They did not on thick coax,
>thin coax, or on UTP.
>
>Pay particular attention to the second paragraph below.
>
><http://www.postel.org/pipermail/end2end-interest/2001-October/001529.html>
>
>
> >What Vernon says is quite right and I'll only add that Collision sensing
> >and recovery happens in times on the order of 200 uS to a few mS, on
> >even extremely highly contended Ethernet segments (many stations ready
> >with pkts to send all the time).  This is far faster than Token passing
> >(many mS), in any of its forms, as a very pertinent graph from the
> >original IEEE 802 standarization simulations shows (it can be faxed to
> >anyone who wishes a copy).  This graph is particularly telling in that
> >CSMA/CD become better in relation to Token as the number of contending
> >nodes increases -- yes, better.
> >
> >One reason for trying to rid the world of 'collision' myths is that they
> >serve as an alarm for how easily misinformation can arise and how hard
> >it is to gather peoples' intellects together to stamp the myths out.
> >Confining ourselves to networking, we must be aware that this applies in
> >the TCP/IP realm as well.  Back to the LAN realm, which the original IP
> >world had no cognizance of, the collision myth, that started as an IBM
> >marketing weapon, matured into a switch vendors' scare tactic to sell at
> >first cut-through (a bad idea now in disrepute) and later "a segment for
> >every node", so that "collisions would no longer occur" -- just pushing
> >the problem into provisioning of the switch fabric.  "Caveat emptor" is
> >as necessary today as it has ever been.
>
>Ralph Droms wrote:
>
> >If there is no randomization of subsequent probes, there is no point in
> >specifying multiple probes.
>
>No, this is false.
>
>We don't assume the ARP probes are the ONLY traffic on the network.
>
>The reason for trying three times is that there may be OTHER brief bursts
>of traffic on the link that overflow the switch buffers (or cause CSMA/CD
>breakdown, etc.) resulting in one or more probes being lost.
>
>In reality, the amount of traffic generated by ARP probing is too low by
>itself to cause switch buffer overflow -- PROVIDING that it is
>distributed over several seconds. Should all the probes be sent in a
>tight bunch, then the chance of switch buffer overflow could become
>non-zero, which is why the initial random delay is useful to eliminate
>that.
>
>Stuart Cheshire <[email protected]>
>  * Wizard Without Portfolio, Apple Computer, Inc.
>  * www.stuartcheshire.org